Quantum and Nonlinear Metamaterials for the Optimization of Greenhouse Covers

Abstract

Background: Greenhouses are pivotal to sustainable agriculture as they provide suitable conditions to support the growth of crops in unusable land such as arid areas. However, conventional greenhouse cover materials such as glass, polycarbonate (PC), and polyethylene (PE) sheets are limited in regulating internal conditions in the greenhouses based on environmental changes. Quantum and nonlinear metamaterials are emerging materials with the potential to optimize the covers and ensure appropriate regulation. Objective: This comprehensive review investigated the performance optimization of greenhouse covers through the potential application of nonlinear and quantum metamaterials as nano-additives, examining their effects on electromagnetic radiation management, crop growth enhancement, and temperature regulation within greenhouse systems. Method: The scoping review method was used, where 39 published articles were examined. Results: The review revealed that integrating nano-additives ensured that the greenhouse covers would block harmful near-infrared (NIR) radiation that generated heat while also optimizing for photosynthetically active radiation (PAR) to promote crop yields. Conclusions: The insights also indicated that the high sensitivity of the metamaterials would facilitate the regulation of the internal conditions within the greenhouses. However, challenges such as complex production processes that were not commercially scalable and the recyclability of the metamaterials were identified. Future work should further investigate pathways to produce hybrid greenhouse covers that integrate metamaterials with conventional materials to enhance scalability.

1. Introduction

Despite the diverse measures and innovations implemented to address world hunger, food insecurity remains a pertinent issue. The assertion was supported by findings from the recent 2025 Global Report on Food Crises (GRFC), which revealed that in 2024, about 294 million people, an increase of 13.7 million from 2023, faced increased levels of acute food insecurity in diverse regions [1]. One of the arguments explaining rising food insecurity is the unsustainability of current agricultural practices, coupled with the shrinking available arable land, a decrease in soil nutrients, and large-scale deforestation activities [2]. As such, there is an urgent need to consider alternative production methods to support sustainable agricultural practices and address global hunger.

One such food production method is the adoption of greenhouse systems. Research reveals that greenhouses are effective for the growth of fresh fruits and vegetables in hot and arid areas, as the systems utilize land that is rendered unusable to produce high-value crops [3]. The implication is that fresh foods can be grown in hot and arid areas to support the increasing food demand. Furthermore, greenhouse systems ensure sustainable food production by regulating the conditions in the environment that support optimal growth of plants in controlled and artificial environments [4]. Such conditions also allow agricultural researchers to investigate host–pathogen interactions influenced by climatic factors.

Greenhouse systems also contribute to sustainable food production by ensuring the cultivation of crops all year-round while maximizing resource usage [5]. In such cases, the greenhouse systems reduce water usage while enhancing soil nutrients to promote high yields of fruits and vegetables consumed within urban areas through precise irrigation and nutrient management approaches. A further aspect is that crops grown in greenhouse systems maintain quality aspects due to minimal exposure to contaminants within the environment [6]. Such inferences underscore the value of greenhouse systems in promoting sustainable agriculture and contributing to the reduction in world hunger.

A key aspect within greenhouses is the interaction of electromagnetic radiation from sunlight with the plants. However, solar spectrum (sunlight) radiation can generate both positive and negative effects on plants. In particular, the plant-active spectrum (PAS) (300~800 nm) is absorbed to facilitate photosynthesis [7]. However, the heat-active spectrum (HAS) (800~1500 nm) from the solar spectrum is negative, as it leads to increased heat within the greenhouses and damages plants [7]. In addition to the solar spectrum, there is also an urgent need to balance ultraviolet (UV) light within greenhouse gas systems to increase the beneficial UV-A light while reducing the harmful UV-B variant, which negatively affects plants [8]. This insight underscores the delicate balance between the PAS and HAS and between UV-A and UV-B within greenhouses to ensure maximum crop yields while reducing overheating within systems to avoid damaging crops. The delivery of proper doses of UV-A and UV-B light also promotes optimized growth traits, higher pathogen resistance, and higher flavonoid resistance [8]. Figure 1 illustrates the interaction of different bands of solar energy with plants in a greenhouse.

In Figure 1, the interaction of UV, PAR, and near-infrared radiation (NIR) energy bands with plants in a greenhouse is showcased. The illustration shows that while PAR is beneficial, as it drives photosynthesis and the development of plants in the greenhouse system, NIR and UV radiation generate adverse effects, including damaging crop morphology and affecting the behavior of pests and pollinators [9]. Furthermore, NIR contributes to energetic effects, leading to excessive heat and degradation of the greenhouse cover materials [10]. As such, a delicate balance must be maintained between increasing the positive PAR energy bands and reducing the harmful NIR and UV-B electromagnetic radiation within the greenhouse system.

Greenhouse cover materials are key in regulating conditions of the environment within the greenhouse systems by controlling PAR, NIR, and heat levels. Conventional greenhouse cover materials, including polycarbonate (PC) sheets, are adopted due to their lower costs and high impact resistance [11]. Research showed that PC sheets are favorable for managing night-time heating during cold seasons and are superior in environmental control and energy savings compared to glass and polyethylene (PE) sheets [12]. PE sheets are also inexpensive and easier to install within greenhouse systems, although difficult to use in managing indoor air environments [12]. Glass is also adopted as a greenhouse cover material due to its effectiveness in thermal regulation and longer durability compared to PE and PC sheets [13]. In Figure 2, the different types of materials used as greenhouse covers are showcased.

In Figure 2, the use of plastic film (a), polycarbonate material (b), and glass (c) is showcased as covers for different greenhouses. Factors including affordability, ease of installation, and durability influence the selection of different types of greenhouse covers.

However, despite widespread use in different greenhouses, the cover materials are limited in regulating temperature conditions within such systems by adapting to the dynamic changes in the environment. During the hot weather conditions, such as summer, the conventional materials trap NIR, increasing internal temperatures within the greenhouse, which can lead to plant damage, while in winter, they allow heat to escape [14]. A further limitation is that the materials may block PAR, threatening the growth of plants within the greenhouse system [15]. As such, there is a need to address the limitations of conventional materials to ensure dynamic tuning capabilities to adapt to changes in seasons and maximize PAR energy bands to enhance the growth of plants.

To overcome the limitations of the conventional materials used as greenhouse covers in managing heat and radiation, well-documented solutions have been adopted, including light-blocking films to maximize PAR (85%) while blocking heat-generating radiation [16] and anti-condensation (anti-drip) claddings [17]. However, an emerging novel solution regards the adoption of quantum metamaterials to manage the dynamic conditions within greenhouses [18]. The materials are defined as artificially engineered nanostructures comprising quantum elements whose periodic structures possess qualities not found in conventional materials [19]. Quantum and nonlinear metamaterials have been adopted in diverse applications, including invisibility cloaking, sensing, energy harvesting, and super-resolution [19]. As such, using quantum metamaterials facilitate the control of electromagnetic waves and radiation within greenhouse systems as nano-additives, where they manipulate electromagnetic waves in a manner unlike conventional materials.

The core aim of this review article is to examine the performance optimization of greenhouse covers with the potential use of nonlinear and quantum metamaterials as nano-additives in terms of electromagnetic radiation. The article also examines the effects of quantum nano-additive materials used in greenhouses on crops/PAR, UV limitation, and temperature effects.

The research novelty stems from the fact that it is the first review study that undertakes an in-depth analysis of the performance optimization of greenhouse covers using quantum metamaterials and their impact on crops, temperature effects, and UV limitation. The authors note that although disparate, disjointed studies have been undertaken on the performance optimization of greenhouse covers, existing review articles focused on quantum metamaterials are unavailable. In Figure 3, a word cloud showcasing the distribution of studies related to metamaterials and nonlinear optics is presented.

As presented in Figure 3, the word cloud distribution shows that the majority of the current studies focused on either metamaterials or nonlinear optics. A research gap thus exists concerning how metamaterials could be embedded with nonlinear optics.

A further gap was also identified in the lack of studies that focused on greenhouse covers, as presented in Figure 4.

In Figure 4, the world cloud reveals that individual studies have been undertaken related to greenhouse covers. Refer to Appendix C, showcasing some studies on the use of composite/nanocomposite materials to solve greenhouse issues related to electromagnetic radiation.

Subsequently, based on the existing research gap in minimal studies on the use of quantum metamaterials for greenhouse covers, the objectives of this review article are stated below:

i.

To investigate the potential uses of nonlinear and quantum metamaterials as nano-additives for the optimization of greenhouse covers;

ii.

To investigate the impact of quantum metamaterials used as nano-additives in greenhouse covers on electromagnetic radiation (PAR/UV), crop growth, and temperature regulation within the greenhouses.

iii.

To investigate the challenges associated with the applications of the nonlinear and quantum metamaterials as nano-additives for the optimization of greenhouse covers.

The review article is structured into five sections. The first section is the Introduction, where the research context and core focus of the review article are outlined. Thereafter, the second section details the materials and methods and highlights the data collection and analysis techniques adopted. In the third section, the results obtained from the reviewed articles are presented. The fourth section is the discussion and re-evaluates the results to address the objectives of the article. Finally, the conclusion is presented, showcasing the key findings, recommendations, and implications of the generated insights.

2. Materials and Methods

To collect data in the research, the scoping review method was adopted. The rationale for selecting the scoping review method stemmed from its reproducibility, transparency, and rigor in collecting data to address a given research problem by evaluating available research studies [20]. A stepwise process involving five main steps was adopted to conduct the scoping review, as presented in the subsequent section. The first process involved outlining the objectives of the review article, as showcased in the Introduction Section.

2.1. Literature Search

The second step in conducting the scoping review entailed the definition of a search strategy and outlining the parameters guiding the search for relevant data sources. First, it was essential to identify relevant scientific databases where the different studies would be sourced. The authors considered scientific databases such as MDPI, Springer Nature, Scopus, and Elsevier. The justification for the databases was that they provide access to relevant and high-quality peer-reviewed articles [21].

The subsequent step entailed identifying keywords from the formulated research objectives, with the core aim of ensuring that relevant studies would be identified. The keywords included “nonlinear”, “quantum”, “meta-materials”, “nano-additives”, “optimization”, “greenhouse covers”, “electromagnetic radiation”, “crop growth”, “greenhouses”, “agricultural efficiency”, “crops”, “production”, “plant physiology”, “destructive radiation”, “PAR/UV”, and “temperature regulation”. The keywords were further combined using the Boolean operators AND/OR to generate search phrases, including nonlinear AND quantum AND meta-materials AND nano-additives, as well as optimization OR enhancement AND greenhouse covers AND electromagnetic radiation AND crop growth OR crop yield AND PAR/UV AND temperature regulation.

The researchers developed various search phrases to enhance the scope of the search for different relevant articles across various databases [22]. Subsequently, diverse relevant articles were selected and identified, which were evaluated in the research.

2.2. Study Selection

The third phase entailed the selection of the literature. The utilization of the search phrases across the selected databases led to the generation of 1050 articles. To ensure the feasibility of the study, inclusion and exclusion criteria were defined [22]. First, the scope was defined, where only studies focusing on the research topic were considered. The selected studies involved performance optimization of greenhouse covers with the potential use of nonlinear and quantum metamaterials as nano-additives in terms of electromagnetic radiation. The scope also considered studies that examined the effects of quantum nano-additive materials used in greenhouses on crops/PAR, UV limitation, and temperature effects. Second, the publication year was narrowed to the last five years (2020–2025) to ensure that only the most recent and updated studies were examined in the review article. The inclusion criteria further specified studies published in English to eliminate the need for further translation. Finally, diverse studies conducted using diverse methodologies were adopted, including primary studies, secondary reviews, and critical reviews. The guiding inclusion and exclusion criteria adopted in the search are detailed in

Table 1. Inclusion and exclusion criteria.

Focus	Inclusion	Exclusion
Scope	Studies focused on performance optimization of greenhouse covers with the potential use of nonlinear and quantum metamaterials as nano-additives in terms of electromagnetic radiation; studies examining the effects of quantum nano-additive materials used in greenhouses on crops/PAR, UV limitation, and temperature effects.	Studies not focused on the performance optimization of greenhouse covers with the potential use of nonlinear and quantum metamaterials.
Period	2020–2025	Before 2020
Language	English	Non-English languages, including French, Italian, and Chinese
Type	Peer-reviewed journal articles, secondary studies, and review articles	Gray sources and blogs

below.

As specified in Table 1, the inclusion criteria emphasized only primary studies related to the specific research topics to support the findings using empirical research. The authors also focused only on studies published in English to avoid further third-party translation that required more time. A further aspect entailed the selection of full-text articles to ensure detailed findings, and images were obtained to explain different research concepts.

The outlined exclusion criteria eliminated all studies published in non-English languages, including Chinese, French, Spanish, and German. All studies published before 2020 were also excluded to ensure that only current findings were reported. Studies beyond the scope of the research were further eliminated to ensure the findings emphasized performance optimization of greenhouse covers with the potential use of nonlinear and quantum metamaterials. The authors also excluded gray articles and website articles that were considered unreliable.

2.3. Study Screening

The third phase in the SLR entailed screening the collected studies based on the defined inclusion and exclusion criteria. Initially, 1050 studies were obtained from using the search phrases in the selected databases. During the sorting process, 450 duplicates were eliminated. The remaining 600 articles were screened to ensure compliance with the selected publishing period (2020–2025). Studies published before 2020 were identified, and 200 were removed. The authors also screened the remaining 400 studies based on the outlined research scope, and 240 were eliminated as they were beyond the scope of the study. The remaining 160 studies were assessed, and 100 abstract-only studies were eliminated. Twenty-one studies were also removed because they were published in non-English languages. Subsequently, 39 studies were selected for the final review. A summary of key findings is showcased in the literature matrix in Appendix B.

2.4. Reporting and Analysis of the Findings

In the final phase, the selected 37 studies were reported and further synthesized to address the formulated research objectives. The findings were evaluated, where similarities and differences between them and relationships between the data were investigated [23]. Subsequently, a summary of knowledge related to the objectives in the research was presented. The rationale for adopting narrative synthesis arose from its widespread adoption in scoping reviews to evaluate findings from multiple studies using words and text [24]. The findings were presented in alignment with the different topics related to the research objectives.

2.5. Limitations of the Methodology

The adopted scoping review methodology was limited by several issues. First, the authors collected data only from previously published secondary studies to address the objectives of the research. As such, the aspect limited the study, where empirical findings from experimental data were not assessed in the research. The researcher justified the choice of including the secondary literature due to their appropriateness in review articles. The second limitation was linked to researcher bias during the selection and analysis of the obtained studies. Researcher bias is linked to the misinterpretation of data and also distorts the evidence base [25]. Therefore, to mitigate researcher bias during the selection of the studies, comprehensive inclusion and exclusion criteria were specified to ensure only eligible articles were adopted in the study. To address analysis bias, a transparent narrative synthesis process was also adopted in the review article.

3. Results

3.1. Properties of Nonlinear Metamaterials

The synthesis of the studies elaborated on the properties of nonlinear metamaterials. Nonlinear metamaterials, as discussed in the findings, are materials whose response to external stimuli, such as electromagnetic fields, depends nonlinearly on the magnitude of the stimulus. A key example identified in the reviewed studies is elastic metamaterials. These materials, as indicated in [26], exhibit extraordinary mechanical properties compared to conventional solid mechanics and are characterized by band gaps—specific frequency ranges where wave propagation is forbidden. In Bae et al. [26], it was highlighted that these bandgaps in elastic metamaterials can be manipulated by varying the amplitude of incident waves, leading to new bandgaps that are not explainable by traditional models like Bragg or resonance gaps. In Figure 5, the schematics of luminescent QD film technology, involving the absorption of short-to-long-wavelength radiation via photoluminescence of a quantum dot fluorophore, is showcased.

The illustration in Figure 5 indicates the effectiveness of QD film in greenhouse covers to convert wavelengths from short to longer, which are suitable in controlled agricultural environments.

Further, the work of Du [28] identified metamaterials with cubic–quintic nonlinearity and third-order dispersion and used the perturbed Schrödinger equation to model them. The perturbed nonlinear Schrödinger equation (NLSE) adopted for modeling the dynamics of ultrashort optical solitons in the nonlinear metamaterials was detailed as showcased in Appendix A. The analysis further highlighted other categories of nonlinear metamaterials. In particular, the work by Gao et al. [29] identified nonlinear acoustic metamaterials (NAMs) as a category of artificial materials with periodic structures, boundary conditions, material constituents, and microstructures supporting resonant motions. While the study focused on examining the propagation of solitary waves in nonlinear metamaterials, the insights also introduced the NAM type of material. The study by Wu et al. [30] proposed a nonlinear metamaterial based on the integration of metamaterial structures and a semiconductor on the same wafer. The insights revealed nonlinear behavior of the electromagnetic field energy in the microwave band. The study by Wu et al. [30] also indicated that the designed nonlinear metamaterial (NLMM) was transparent to low-density electromagnetic radiation fields and adaptively became opaque to high-density fields. In another study, Lou et al. [31] demonstrated how a nonlinear metamaterial could block the propagation of low-frequency Rayleigh waves. The reported findings showed that coupling the motion of an elastic substrate with the dynamics of attached masses led to a linear metamaterial with serial-connected resonators, achieving two band gaps. The close inspection of Du [28], Gao et al. [29], Wu et al. [30], and Lou et al. [31] indicated how the different classes of nonlinear metamaterials behaved when waves were propagated within them. The synthesis further introduced concepts such as the perturbed NLSE to model wave patterns within the different nonlinear materials. Additionally, the potential applications of nonlinear materials as electromagnetic wave blockers were also demonstrated.

The evaluation also highlighted nonlinear metamaterial beams as a different class of materials. In the work by Shen et al. [32], an examination of the effects of beam-bending curvature and local resonators’ linearity on dispersion relations and the ensuing stop behavior was undertaken. The insights indicated that although more studies had focused on linear metamaterial beams, the nonlinearity of the material was also increasingly important. In Figure 6, a nonlinear metamaterial beam model hosting arrays of equally spaced nonlinear resonators with a spacing is showcased.

The illustration in Figure 6 demonstrates a nonlinear metamaterial beam model with different nonlinear resonators. The work by Shen et al. [32] also highlighted various equations, demonstrating bending curvature based on the assumption that nonlinear plane beam behavior is purely flexible. Equation (A2), for bending curvature, is illustrated, as shown in Appendix A. In this context, the equation demonstrated nonlinear beam behavior based on the assumption of pure flexibility.

The work by Xue et al. [33] identified locally resonant nonlinear metamaterials as a different category of materials, showing that they exhibited a wider variety of unusual dynamic behavior. Such insights showed that the nonlinear metamaterials had a potential to be applied in wave control mechanisms in nonlinear dynamics. The work by Xue et al. [33] also revealed that linear metamaterials were locally resonant and comprised sub-wavelength resonators in a host structure, in a periodic manner to suppress wave propagation in challenging low-frequency ranges. The close inspection of Xue et al. [33] indicated that the locally resonant nonlinear type of metamaterials was applicable in diverse settings, including nonlinear dynamics, where applications ranged from noise cancelation, wave guiding, and vibration suppression to wave filtering and impact mitigation. As such, Xue et al. [33] added to the findings of Du [28] and Gao et al. [29], revealing the nonlinear locally resonant type of nonlinear metamaterials, in addition to the elastic and acoustic. These insights showed that nonlinear materials could be employed in diverse settings due to their precisely engineered unit cells that allowed the manipulation of the mechanical properties by changing the cell design. In this context, the inferences indicated that the metamaterials were suitable in other areas where cell manipulation could be harnessed in application areas such as smart greenhouses.

The analysis also revealed optically tunable nonlinear metamaterials that were adopted for light absorption. The synthesis implied that the optically tunable metamaterials could be leveraged in applications involving light propagation such as greenhouse covers. In particular, the work by Lari et al. [34] introduced a nonlinear metamaterial structure design comprising a sandwich structure based on absorption analysis for sensing nonlinear optical liquids. The inference from the study showed that the metamaterial could be manipulated as an absorber of magnetic and electric wave radiation to minimize transmission and reflection while improving absorption [34]. The study also revealed that the metamaterial was based on a nanostructure and displayed high absorbance over a triple band at infrared frequencies, thereby demonstrating large absorbance at different frequencies. The generated numerical simulations also indicated that the nonlinear metamaterial was effectively absorbing light with near-unity absorbance on all bands that were tunable designed. Subsequently, Lari et al. [34] argued that the proposed sensor was insensitive to polarization, and absorbance was high even where large incidence angles were recorded. Pertsch et al. [35] aligned with Lari et al. [34], revealing how the nonlinear properties of metasurfaces could be engineered for diverse application areas, including multipolar interferences, and improved local and collective resonances to drive nonlinear light generation from different nanoscale elements. The discussion also showed that the nonlinear processes in the metamaterials were empowered by the excitation from localized solutions of Maxwell equations for subwavelength particles. Figure 7 showcases the different functionalities of nonlinear metasurfaces.

The illustration in Figure 7 demonstrates the diverse application areas of nonlinear metasurfaces, which could be engineered for holograms, beam deflection, mixing frequencies, and as metalens. The insight showed that the nonlinear processes within the resonant metamaterial surfaces were effective in enhancing the performance and functionalities of photonic devices [35]. Further work by Yesharim et al. [36], supported Pertsch et al. [35], revealed that nonlinear processes of spontaneous downconversion (SPDC) were a key method adopted for generating different types of quantum light. Yesharim et al. [36] reported that the use of different quantum states of light, such as the entangled, squeezed, and fixed photon number (Fock), generated diverse advantages compared to classical light states, including thermal and coherent states. In this case, the adoption of nonlinear spontaneous conversion processes ensured that the metamaterials could be employed in applications such as the realization of light with higher resolution and sensitivity. Refer to Figure 8, illustrating the different types of entanglement used in quantum light generation.

The illustration in Figure 8 shows that with the bi-photon entanglement (a), the correlation between the two photons was computed to obtain an entangled source in different degrees of freedom (DOF) of the light, including spectrum, polarization, time, and shape [36]. However, with the heralding photon (b), one of the photons, such as the idler, was detected and its presence heralded in a different path [34]. Finally, the squeezed light option entailed increasing the pump power or inserting the nonlinear crystal into a resonant cavity to generate the squeezed light (c) [36]. The key inference from Yesharim et al. [36] was that the nonlinear bulk crystal materials simulated and designed spontaneous parametric downconversion (SPDC) processes based on the nonlinear capabilities of the ferroelectric crystal materials.

In another study, Abuzaid and Asma [37] added to Yesharim et al. [36], where they examined the propagation dynamics of spatiotemporal elliptical super-Gaussian bullets in a Kerr nonlinear metamaterial waveguide. The study adopted the Lagrangian variational method and numerical analysis using an appropriate trial function for input elliptical super-Gaussian beams and the evaluation of self-trapping and deformation of propagating beams of the metamaterials [37]. However, the obtained insights revealed various unusual electromagnetic propagation not observed in conventional media, such as the negative index regime of the nonlinear metamaterials. Further work by Kim [38] highlighted organic π-conjugated crystals with second-order optical linearity as effective materials for diverse terahertz (THz) wave photonics. The inferences showed that nonlinear optical (NLO) crystals could be applied in diverse photonic and photoinduced electronic applications, including linear, nonlinear, electro-optics, fluorescence, and photodetection. Vabishchevich and Kivshar [39] aligned with Kim [38], showing that nonlinear optics was a field of study that traditionally relied on the interaction of light with macroscopic media over distances longer than light wavelengths. The findings in the study by Vabishchevich and Kivshar [39] indicated that nonlinear metamaterials allowed nonlinear responses to be tailored through engineering constituent elements and through the provision of degrees of freedom in the design of artificial materials with nontrivial nonlinear optical properties. As such, the nonlinear photonics of metasurfaces were influenced by the resonances within dielectric materials, which also impacted the physics of nonlinear metasurfaces. The inference from Vabishchevich and Kivshar [39] underscored the role of optically induced multipolar electric and magnetic Mie resonances, as well as and guided-mode resonances, in boosting effects in nonlinear photonics and providing opportunities for the subwavelength control of light at the nanoscale.

Another study by Genchi et al. [40] added to Vabishchevich and Kivshar [39], revealing the potential applications of linear and nonlinear metamaterials in quantum optics and nanophotonics. In their study, Genchi et al. [40] conducted an in-depth spectral investigation of third-order nonlinear optical properties, specifically the nonlinear refractive index and nonlinear absorption coefficient. The investigation considered ϵ-near-zero Au/Al₂O₃ multilayer metamaterials in a broad range of the visible spectrum across the ϵ-near-zero (ENZ) wavelength and at different incidence angles with TM- and TE-polarized light [40]. The results from the experiment showed that a continuous modulation of the linear and nonlinear optical parameters of the materials could be obtained as a function of the angle of incidence, where the peak nonlinear coefficients were close to the ENZ wavelength [40]. In another study, Huang et al. [41] revealed that the efficient nonlinear optical processes led to the fabrication of optical and nano-devices used in areas such as the conversion of broadband frequency and ultrafast optical switching. The findings also highlighted plasmonic excitation as an effective approach adopted for the enhancement of nonlinear optical processes based on its induced enhancement of a strong local electromagnetic field [41]. Such insights from Huang et al. [41] mirrored Genchi et al. [40] and Vabishchevich and Kivshar [39] by showing how metasurfaces were increasingly adopted in applications involving light absorption at the nanoscale. Konishi et al. [42] also aligned with these views in their study, which examined how nonlinear metamaterials with rotational symmetry facilitated circular polarization control. The inferences indicated that nonlinear optical responses in metamaterials were adopted in circular polarization control across application areas such as circular dichroism spectroscopy.

3.2. Properties of Quantum Metamaterials

The synthesis of the different articles also showcased the different properties of quantum metamaterials. Such materials are artificial structures that combine the exotic qualities of metamaterials with the quantum behavior of building blocks. The insights from the studies showed that quantum metamaterials could maintain coherent quantum states, facilitating quantum processes such as superpositioning and entanglement [43]. As such, the quantum metamaterials exhibited qualities such that their particles demonstrated similar qualities upon entanglement, regardless of how far they were separated. The study by Zagoskin et al. [44] also argued that quantum metamaterials, as artificial optical media, were characterized by quantum coherence as an essential feature of their unit elements. The inference showed that at the foundational level, the structures comprised solid-state quantum qubits that demonstrated global quantum coherence [44]. In Figure 9, a quantum metamaterial prototype is showcased, illustrating quantum coherence of the solid-state qubits.

The study by Zagoskin et al. [44] also further outlined three key characteristics of quantum metamaterials: (i) they are made up of quantum-coherent unit elements with engineered parameters, (ii) the quantum states of some of the elements could be directly controlled, and (iii) they maintain global coherence for a time duration exceeding the traversal time of the electromagnetic signal. A close inspection of the insights showed that at the basic level, quantum metamaterials exhibited properties such as entanglement and superposition as the constituent elements (e.g., superconducting qubits, quantum dots) maintained quantum coherence. Similar insights were reported by Chow et al. [45] who examined the theory of quantum coherence phenomena in semiconductor quantum dots and showed the occurrence of inversionless gain, electromagnetically induced transparency, and the refractive index enhancement in the transient regime for dephasing rates under room temperature and conditions of high excitation. As such, similarly to semiconductor quantum dots, quantum metamaterials exhibited features of quantum coherence even in scenarios where electromagnetic signals were integrated.

The quantum metamaterials also exhibited quantum nonlinearity at the level of single or few photons. The implication was that the structures could display interactions between light and matter that were orders of magnitude stronger than in classical materials. The study by Grimsmo [46] demonstrated how quantum metamaterials could be adopted for the broadband detection of single microwave photons in application areas such as quantum information processing, quantum computing, and metrology in the microwave frequency domain. The results of applying many-body numerical simulations indicated that the fidelity of single-photon detection increased with the length of the metamaterial, implying that it was appropriate in the detection of large bandwidths [45]. Another study by Greco et al. [47] added to Grimsmo [46], showing how a quantum model for Josephson-based metamaterials could be generated using three-wave mixing (3 WM) and four-wave mixing (4 WM) regimes at the single-photon layer. The analysis of the study demonstrated that nonlinear processes such as 4 WM could be adopted in quantum metamaterials to enhance their applications, where four waves interact through an energy exchange process, generating new frequencies [47]. Further work by Lin et al. [48] proposed a magnetic-free nonreciprocal scheme based on the 4 WM effect in semiconductor quantum dots and revealed the nonreciprocal transmission window with an isolation higher than 12 dB and an insertion loss lower than 0.08 dB. The insights from Greco et al. [47] and Lin et al. [48] underlined the strong nonlinear properties of quantum metamaterials at the photon level, where processes such as 4 WM could be employed to develop nonreciprocal devices applicable in light refraction areas such as greenhouses.

A third feature of the quantum metamaterials was their tunable electromagnetic response. In this context, their optical or electromagnetic properties such as permittivity and permeability could be dynamically tuned using external controls such as electric or magnetic fields and microwave signals [49]. The work by Mazhorin [49] conducted an experiment to explore a quantum metamaterial based on a superconducting chip with 25 frequency-tunable transmon qubits coupled to a common coplanar resonator. The insights from the experiment indicated that the frequency tunability of the quantum metamaterial allowed the number N of resonantly coupled qubits to change and the introduction of a disorder in their excitation frequencies with preassigned distributions [49]. The key insight from [49] was the tunable electromagnetic response of the quantum metamaterial, where the increase in N led to the increase in the N^1/2 scaling law for the energy gap (Rabi splitting) between the bright modes around the cavity frequency. The argument was that when controllable disorder was introduced in the quantum metamaterial, computing an average of the transmission amplitude over a large number of realizations, this led to a decay of mesoscopic fluctuations, which mimicked an approach towards the thermodynamic limit. In this context, Mazhorin [49] showed that the electromagnetic/optical properties of the quantum metamaterials could be tuned through the introduction of controllable disorder. Fernández-Fernández et al. [50] aligned with Mazhorin [49] in their study, which demonstrated that the subradiant excitations generated in quantum metamaterials could be harnessed to access tunable directional emission patterns and collective dissipative couplings in scenarios where additional atoms were placed nearby the atomic array. The study by Fernández-Fernández et al. [50] demonstrated that the directionality of the emission patterns could be controlled through the relative dipole orientation between the auxiliary atoms and that of the arrays. In Figure 10, an illustration of the tunable electromagnetic responses of the quantum metamaterials applied in greenhouse cover materials is showcased.

In Figure 10, the emission patterns of anisotropic metamaterials that would be applicable in greenhouse covers are detailed. A further close inspection of Mazhorin [49] and Fernández-Fernández et al. [50] underscored the tunability of optical or electromagnetic properties of the quantum metamaterials using different controls, such as integrating controllable disorders or relative dipole orientation. Another study by Kim et al. [51] also demonstrated a tunable superconducting cavity based on superconducting quantum interference device materials. In their study, Kim et al. [51] considered a design where an array of radio-frequency superconducting quantum interference devices (rf SQUIDs) was used inside a superconducting cavity to create a tunable quantum metamaterial that coupled to the cavity through its magnetic plasma frequency. The study by Kim et al. [51] showed that when the resonant frequency of the metamaterial was tuned through a magnetic flux, this facilitated tuning the cavity mode profile and detuning it from the initial 5.593 GHz by over 200 MHz. The synthesis underscored how quantum metamaterials could be tuned using magnetic flux, adding to the works of Mazhorin [49] and Fernández-Fernández et al. [50]. Upon closer inspection, such inferences indicated the appropriateness of quantum metamaterials in optimizing greenhouse covers, where their tunable properties would ensure rapid adaptation to seasonal changes more effectively than classical materials.

The quantum metamaterials were also characterized by discrete energy levels, where the metamaterials’ response could be influenced by quantum transitions between these discrete energy levels, similar to atoms or quantum dots. In their study, Belousov [52] demonstrated an approach for describing nonstationary quantum systems with a discrete energy spectrum on a time-dependent basis using eigenstates of the time-dependent Hamiltonian defined at the specific current time. In another study, Doser [53] added to the work of Belousov [52], showing that due to the discrete energy levels in quantum metamaterials, potential application areas include low-dimensional systems such as quantum dots and the manipulation of the ensemble of quantum systems, such as detection systems and single or polyatomic systems. The analysis of Doser [53] suggested that due to the discrete energy levels of the quantum metamaterials, they could be applied as high-energy particle physics detectors in avenues such as timing, tracking, and calorimetry. Directly, such insights indicated the relevance of quantum metamaterials in greenhouse covers, where their discrete high-energy levels would be adopted in absorption or emission application areas. Furthermore, based on the insights from Mazhorin [49], Fernández-Fernández et al. [50], and Kim et al. [51], which showed that the optical and electromagnetic properties of the quantum metamaterials were tunable, this indicated that the discrete energy levels could be harnessed to facilitate the tuning features. In this context, tuning at the photon layer would facilitate the transmission or blocking of harmful UV rays within greenhouse systems.

Low loss or high sensitivity was a further property of the quantum metamaterials, where in their superconducting versions, the metamaterials had very low dissipation. As such, they were adopted in quantum information processing and quantum sensing application areas. The study by Uriri et al. [54] revealed that due to the low-loss features in quantum metamaterials, they were appropriate in quantum information processing and quantum optics. The inferences further indicated that the high sensitivity of the materials also made them appropriate in quantum sensing applications such as holography and quantum optics, where they enhanced optical encryption and information capacity. Another study by Yu et al. [55] explored ultra (coupling) as a method of manipulating the optical responses of metamaterials based on the ensemble of individual constituent units in the coupling regime. By implementing a framework based on linear response for quantum electrodynamical systems to examine the optical response of a two-level emitter linked to the single-cavity mode, results showed the tunability of the optical behavior of the material. As such, Yu et al. [55] argued that the provided framework was effective in the design of quantum metamaterials with low loss, highly confined modes, and tunable (single-photon) nonlinearities. Further work by Jeremy et al. [56] also highlighted that nanophotonics and plasmonics had led to the emergence of significant innovations in quantum metamaterials, which led to breakthroughs in control over light–matter interactions at the nanoscale. Inferences from the synthesis of Uriri et al. [54], Yu et al. [55], and Jeremy et al. [56] showed that due to the high sensitivity of quantum metamaterials and their low dissipation qualities, they were applicable in quantum sensing, where they offered advanced possibilities for functionality, efficiency, and miniaturization. Such insights are relevant to the topic of the review article, as sensors are applicable in greenhouse covers to ensure the efficiency and tunability of electromagnetic and optical light waves.

The synthesis of the different studies also revealed further unique properties of the quantum metamaterials, including their artificial structure, where the quantum metamaterials were engineered on sub-wavelength scales, similar to classical metamaterials. In their study, Uriri et al. [54] postulated that quantum metamaterials were artificially engineered nanostructures containing quantum coherent states that could support controllable quantum states while ensuring global coherence. As such, based on the artificial structure of the quantum metamaterials, their bulk behavior maintained global coherence. Additionally, the synthesis also indicated that the behavior of the quantum metamaterials was not the collective sum of the individual quantum elements, and collective phenomena such as quantum phase transitions or macroscopic entanglement could emerge [57]. The study by Lamberto et al. [57] explored the existing controversy that a potential phase transition could occur between a normal ground state and a photon-condensed state in many-dipole light–matter systems by assessing two specific atom arrangements, including the three-dimensional cubic lattice and cavity-embedded square lattice layer. The results obtained showed that a ferroelectric phase transition could still occur in principle, while the description of the abnormal phase beyond the critical point required including nonlinear terms in the Holstein–Primakoff mapping [57]. Another study by Zhao et al. [58] aligned with Lamberto et al. [57], demonstrating that quantum phase transition could be strategically exploited to design a novel, robust light state. The study Zhao et al. [58] used an interface between parity–time (PT) symmetric media with different quantum phases and further used the complex Berry phase to demonstrate the quantum phase transition. The implication was that the discovery of the robust light state by the quantum phase transition could promise fault-tolerant light transport in computing and different optical communications [58]. The inference from Lamberto et al. [57] and Zhao et al. [58] was that the behavior of quantum metamaterials was not the collective sum of the individual components, as quantum phase transitions were reported.

3.3. Potential Applications of Quantum and Nonlinear Metamaterials in Greenhouse Covers

One of the potential application areas identified from the evaluation of the diverse studies regarded the optimization of photosynthetically active radiation (PAR) transmission through the adoption of nano-additives, such as quantum dots, to absorb harmful UV rays and further down-convert them to promote crop yields. The work by Hebert [27] revealed that CuInS₂/ZnS quantum dots used in greenhouse films were a novel and emission-tunable luminescent material adopted for the optimization of the sunlight spectrum. The quantum dots absorbed and converted harmful UV and blue photons from sunlight to a photoluminescent emission at 600 nm, translating to improved productive yield, fruiting, total yield, and vegetative growth rate. Such insights from Hebert [27] aligned with research by Yesharim et al. [36] on quantum light generation through nonlinear processes like spontaneous parametric down-conversion (SPDC), which showed that quantum metamaterials could manipulate light in ways that traditional materials could not. Another study by Parrish [59] supported Hebert [27], where they demonstrated the effectiveness of CuInS₂/ZnS quantum dot (QD) films in down-converting ultraviolet/blue photons to red emissions, centered between 600 and 650 nm, thereby increasing biomass accumulation in red romaine lettuce. The study by Parrish [59] showed that when lettuce was grown under QD films ranging from 600 nm, an increase in edible dry mass (13–19%), edible fresh mass (11%), and total leaf area was observed compared to control films that did not contain any QD films. The study by Lakshminarayanan et al. [18] aligned with Parrish [59], demonstrating that optical nonlinear metasurfaces could also be adopted to promote the performance of low-emission glazing designed to enhance energy efficiency within agricultural greenhouses in cold climates. The findings from Lakshminarayanan et al. [18] showed that incorporation of the metamaterials ensured that solar energy was captured in the mid-infrared spectrum, thereby improving the energy efficiency of the glazing. The insights from Parrish [59] and Lakshminarayanan et al. [18] indicated that integrating metamaterials as additives in greenhouse cover films led to modification of the incoming solar spectrum through down-conversion of UV energy and capturing the mid-infrared spectrum to promote crop yields in cold climates. As such, the technology enhanced the productivity within greenhouses while also lowering the costs of energy.

Therefore, integrating quantum metamaterials as nano-additives in greenhouse covers offers a novel approach to controlling electromagnetic radiation more precisely and dynamically. This ability to generate and control quantum states of light, such as entangled and squeezed states, could be exploited to optimize light delivery to plants within greenhouses. By controlling the intensity, polarization, and wavelength of light, these metamaterials can help ensure that plants receive the optimal conditions for photosynthesis [37]. The ability to tune light at various frequencies, as explored by [37], is especially important for greenhouse applications where light quality and intensity need to be precisely controlled. Furthermore, Lakshminarayanan et al. [18] demonstrated that using metamaterials as additives ensured that solar energy was captured in the mid-infrared spectrum, thereby enhancing crop growth in cold climates. The studies by [27,60] also revealed the importance of CuInS₂/ZnS quantum dot (QD) films in down-converting ultraviolet/blue photons to red emissions, thereby improving overall crop growth and yield in the greenhouses.

Quantum metamaterials could allow real-time adjustments to the light environment inside greenhouses, ensuring that crops receive the correct spectrum of light for growth while minimizing energy waste. In addition to enhancing light conditions, quantum metamaterials can help manage harmful radiation. Studies like Du [28] have shown that ultraviolet (UV) radiation in greenhouses can be damaging to crops. However, quantum metamaterials can be engineered to selectively block harmful UV-B radiation while allowing beneficial UV-A radiation to pass through, thus promoting plant growth without causing damage [29]. This selective filtering of UV light could be a significant advancement over traditional materials, which often fail to provide such precise control. Furthermore, insights from Mazhorin [49] also revealed that the optical or electromagnetic properties of quantum metamaterials, such as permittivity and permeability, could be dynamically tuned using external controls such as electric or magnetic fields or microwave signals. Based on the tunability aspects, the nonlinear quantum materials could be tuned to match and absorb harmful UV waves that could damage crops in the greenhouses. The study by Elsharabasy et al. [60] aligned with Mazhorin [49], demonstrating a wide-band metamaterial perfect absorber (MPA) effective in absorbing the harmful infrared (IR) radiation at 10 µm in greenhouses above 99%. The insights from Gao et al. [29], Mazhorin [49], and Elsharabasy et al. [60] indicated that integrating quantum and nonlinear metamaterials as nano-additives in greenhouse covers blocked harmful IR radiation and protected crops within the greenhouses. As such, harmful UV photons would be blocked from damaging the crops while also extending the lifespan of the greenhouse cover materials.

The analysis further indicated the potential adoption of quantum and nonlinear metamaterials in the dynamic regulation of temperature within the greenhouses. The results from [49] showed that the electromagnetic/optical properties of the quantum metamaterials could be tuned through the introduction of controllable disorder. Similarly, an analysis of the results indicated that quantum metamaterials could be tuned using a relative dipole orientation between the auxiliary atoms and one of the arrays [50] and magnetic flux [51]. Such insights underscored the appropriateness of the metamaterials as additives in greenhouse covers, leading to higher effectiveness in the rapid adaptation of greenhouse cover films to seasonal changes compared to classical materials. A further mechanism highlighted by Elsharabasy et al. [60] was that the metamaterials blocked harmful IR, which led to increased heat within greenhouses. As such, the inferences from the studies indicated that the potential applications of quantum and nonlinear metamaterials in enhancing greenhouse covers stemmed from their tunable characteristics. By blocking harmful NIR, the temperatures within the greenhouses were easily regulated.

The results also identified the potential application of quantum and nonlinear metamaterials in smart control and advanced sensing within greenhouses. The study by Uriri et al. [54] indicated that due to the high sensitivity of the quantum and nonlinear metamaterials, they could be adopted in sensing applications. The study by Jeremy et al. [56] also demonstrated breakthroughs in their control over light–matter interactions at the nanoscale. The analysis of Uriri et al. [54] and Jeremy et al. [56] indicated that metamaterials were applicable in sensing areas due to their high sensitivity and low-loss features. As such, the materials had the potential to optimize greenhouse covers for smart control application areas.

The review of further studies identified the applications of the quantum metamaterials in dynamic temperature regulation. In Yang et al. [61], composite phase changing materials (PCMs) in greenhouse covers were identified for thermal energy storage applications. The inferences also indicated that combining the quantum metamaterials with PCMs enhanced thermal regulation to increase emissivity and radiate excess heat away from the greenhouse. Another study [62] supported [61], revealing that PCMs were suitable in optimizing greenhouse covers based on their high thermal storage density and excellent thermal regulation performance. The inspection of [61,62] identified the potential application of integrating PCMs with quantum metamaterials to enhance tunable thermal emissivity. During the hot periods, the greenhouse covers were tuned to radiate excess heat away from the greenhouse, while during the cold weather, they were tuned to lower emissivity and trap more heat inside. In another study, ref. [63] revealed that composite materials comprising quantum fluorescent polymer and PCM exhibited excellent thermal energy storage density, rapid heat transfer, and negligible temperature changes. These insights underscored the potential application of PCM–quantum metamaterial composites in greenhouse covers to develop tunable thermal regulators.

3.4. Challenges of Potential Applications of Quantum Metamaterials as Greenhouse Covers

The review article further examined the challenges associated with the applications of the nonlinear and quantum metamaterials as nano-additives for the optimization of greenhouse covers. The inferences from the review of diverse studies showed that despite the promising potential of quantum metamaterials (QMMs), several challenges remain in their application for greenhouse optimization. One significant challenge is the cost and complexity associated with manufacturing these materials [28]. Unlike conventional materials such as polyethylene (PE), polycarbonate (PC), and glass, QMMs require advanced fabrication techniques that are currently more expensive and less scalable [31]. The production of nanostructured metamaterials—such as those used in light absorption and wave manipulation—requires highly specialized equipment and expertise [29,31]. As such, it makes widespread adoption in commercial greenhouse systems a slow process, as the production techniques are still not cost-effective for large-scale implementation.

Additionally, integrating QMMs into existing greenhouse infrastructure presents logistical and economic hurdles [52]. Traditional greenhouse covers are optimized for cost and ease of installation, and retrofitting them with QMM-based systems could be technically and financially challenging. Thus, it would require developing new materials and creating compatible systems for their efficient deployment [42,53]. The cost factor also emerges in the scalability pathways for the QMMs, where methods such as nano- and microfabrication techniques that would be adopted are highly expensive and less feasible in large-scale applications. The comparison of QMMs [48,49] to PCMs [62,63] in relation to scalability applications implied that pathways for PCMs such as form stabilization and microencapsulation were more economically feasible relative to nano- and micro-fabrication used for QMMs. Furthermore, the synthesis of Cui et al. [62] and Shen et al. [63] showed that PCMs were a more economical and feasible option compared to QMMs in large-scale greenhouse applications due to cost and durability aspects. The synthesis of Mazhorin [49] and Fernández-Fernández et al. [50] indicated that QMMs based on quantum dots were difficult to recycle due to the complexity of separating the nanoparticles from the host polymers. However, as Shen et al. [63] showed, PCMs are more easily recycled as they are encapsulated within the host material, making it easier to simplify their recovery and separation. Despite such differences, maintenance was an advantage in PCMs and QMMs, where minimal maintenance was required because they did not include electrical or moving parts [62]. Conversely, the applications of QMMs and PCMs in large-scale greenhouse covers imply that regular maintenance operations are required to ensure the integrity and stability of the quantum dots and PCM-infused covers. Another obstacle is the lack of extensive experimental data on the long-term effects of quantum metamaterials on crop growth [32]. While many studies have demonstrated the theoretical benefits of these materials, more empirical research is needed to evaluate their performance in real-world greenhouse environments.

The current body of research primarily focuses on the theoretical properties of QMMs, such as light manipulation and wave propagation, using models like the nonlinear Schrödinger equation (NLSE) and studying the effects of beam-bending curvature in nonlinear metamaterial beams [29,32]. These theoretical models show significant promise, but more data is needed to determine the actual impact of these materials on crop productivity in a practical setting. Furthermore, the environmental stability of QMMs under typical greenhouse conditions—exposure to UV radiation, fluctuating temperatures, and humidity—remains an open question. As these materials degrade over time, especially under the intense UV radiation found in greenhouses, their long-term effectiveness and durability must be carefully assessed to ensure their viability in such demanding environments [34]. While QMMs hold immense promise for enhancing greenhouse optimization, such as improving light management for photosynthesis and energy efficiency, addressing these challenges will be crucial for future implementation. Rigorous testing and further development of scalable, durable QMMs are essential for realizing their potential in agricultural applications.

4. Discussion

4.1. Impact of Nonlinear and Quantum Metamaterials in Optimizing Greenhouse Covers

This study examined the potential uses of nonlinear and quantum metamaterials as nano-additives for the optimization of greenhouse covers and their impact on electromagnetic radiation (PAR/UV), crop growth, and temperature regulation within the greenhouses. The analysis of the findings revealed various application areas of the metamaterials in the greenhouse covers, with the first involving the optimization of the PAR transmission through the adoption of nano-additives. The study by Hebert [27] revealed that CuInS₂/ZnS quantum dots used in greenhouse films were a novel and emission-tunable luminescent material adopted for the optimization of the sunlight spectrum. Parrish [59] also demonstrated the effectiveness of CuInS₂/ZnS quantum dot (QD) films in down-converting ultraviolet/blue photons to red emissions centered between 600 and 600 nm, thereby increasing biomass accumulation in red romaine lettuce. Lakshminarayanan et al. [18] further showed that incorporation of the metamaterials ensured that solar energy was captured in the mid-infrared spectrum, thereby improving the energy efficiency of the glazing. Therefore, insights from Hebert [27], Parrish [59], and Lakshminarayanan et al. [18] indicated that the metamaterials were appropriate in optimizing PAR transmission to enhance crop yields.

A further application area regarded real-time adjustments to the light environment inside greenhouses, ensuring that crops receive the correct spectrum of light for growth while minimizing energy waste. In studies such as those of Mazhorin [49] and Elsharabasy et al. [61], insights also revealed that the optical or electromagnetic properties of quantum metamaterials, such as permittivity and permeability, could be dynamically tuned using external controls such as electric or magnetic fields or microwave signals. Subsequently, the tunability would ensure that greenhouse covers blocked harmful IR radiation and protected crops. The results from [50,51,52] further indicated the potential adoption of quantum and nonlinear metamaterials in the dynamic regulation of temperature within greenhouses, especially by blocking harmful NIR that leads to heat generation.

Finally, the insights from Uriri et al. [54] and Jeremy et al. [56] indicated that the metamaterials were applicable in sensing areas due to their high sensitivity and low-loss capabilities. As such, the synthesis of the diverse studies showed the four application areas of nonlinear and quantum metamaterials as nano-additives for the optimization of greenhouse covers: (i) optimization of PAR admission to promote crop yields, (ii) blocking harmful NIR, and (iii) as smart sensors to ensure real-time adjustments to the light environment inside greenhouses. These insights align with past work, such as [18], which emphasized that using quantum metamaterials to manage the dynamic conditions within greenhouses was an emerging application area. The findings also align with previous work by Alaee et al. [19], which indicated that quantum metamaterials were appropriate for invisibility cloaking, sensing, energy harvesting, and super-resolution. As such, the different applications ensure optimization of greenhouse covers to manage the dynamic conditions within the greenhouses and positively impact crop yields.

However, contrasting perspectives emerged regarding environmental control mechanisms in greenhouse systems. In the Introduction Chapter, insights revealed that the traditional cover materials used in greenhouses, such as polycarbonate (PC) sheets, polyethylene (PE) sheets, and glass, could hardly regulate the temperature conditions dynamically [11,12,13]. More precisely, these materials could capture near-infrared (NIR) radiation when it is hot outside, reduce internal temperatures, and possibly harm plants, while during winter, they may release heat [14]. The Introduction also pointed out that traditional materials can interfere with photosynthetically active radiation (PAR), which can endanger plant growth in greenhouse systems [15]. However, the results of the present study offer a sharp contrast to these drawbacks and show that quantum metamaterials can offer dynamic tuning capabilities that accommodate seasonal variations or other environmental fluctuations. This inconsistency is especially evident when referring to the fact that the traditional materials are rigid in their nature, whereas quantum metamaterials can respond to the outside world in an adaptive manner.

4.2. Challenges of Nonlinear and Quantum Metamaterials in Optimizing Greenhouse Covers

The review article also investigated the challenges associated with the applications of nonlinear and quantum metamaterials as nano-additives for the optimization of greenhouse covers. The synthesis of the results showed that, unlike conventional materials such as polyethylene (PE), polycarbonate (PC), and glass, QMMs require advanced fabrication techniques that are currently more expensive and less scalable [31]. The insights also indicated that highly specialized equipment and expertise are needed for the production of the nonlinear and quantum metamaterials [29,31]. Such inferences showed that the widespread adoption of commercial applications would be a slow process, as production techniques are not cost-effective for large-scale implementation. Further synthesis also indicated that modeling of the metamaterials is mainly theoretical, using models like the nonlinear Schrödinger equation (NLSE) and studying the effects of beam-bending curvature in nonlinear metamaterial beams [29,32]. The direct implication is that more research is required to determine the actual impact of these materials on crop productivity in real-world settings. A further concern is the recyclability of the nano-metamaterials used in the greenhouse covers, where difficulties are anticipated.

Upon comparing these findings to the discussion of conventional greenhouse materials and the pattern of adoption as highlighted in the Introductory Chapter, a number of significant differences emerged. The insights showed that conventional materials, including polycarbonate (PC) sheets, are used mainly due to the lower prices and impact resistance [11], and research studies have found that they are better at controlling the environment and saving energy than glass and polyethylene sheets [12]. It was also noted in the Introduction that polyethylene (PE) sheets are preferable because they are cheaper and simpler to install in the greenhouse systems [12], and glass is used because it is more efficient in thermal control and has a longer lifespan [13]. The results of the present study are the direct opposite of the findings of the above-mentioned researchers, who state that quantum metamaterials are faced with much higher costs and complexity of manufacturing and installation.

The Introduction Chapter’s emphasis on affordability, ease of installation, and durability as primary factors influencing greenhouse cover selection [13] presents a significant contradiction to the current study’s findings regarding the economic challenges associated with quantum metamaterial implementation. The implications of these economic and practical challenges are profound for the agricultural industry and the adoption of advanced greenhouse technologies. The contradiction between the cost-effectiveness of conventional materials and the high costs associated with quantum metamaterials suggests that the transition to advanced greenhouse systems may be gradual and selective rather than widespread. This economic disparity has significant implications for agricultural equity, as advanced quantum metamaterial technologies may initially be accessible only to large-scale commercial operations with substantial capital resources, potentially widening the gap between industrial and small-scale farming operations. However, the long-term benefits demonstrated by quantum metamaterials, including improved energy efficiency, enhanced crop yields, and reduced operational costs, may justify the initial investment over time. The Introduction Chapter’s discussion of existing solutions, such as light-blocking films and anti-condensation claddings [16,17], represents intermediate approaches that bridge the gap between conventional materials and quantum metamaterials, suggesting that incremental improvements may be necessary before full-scale quantum metamaterial adoption becomes economically viable.

The synthesis of Yang et al. [61], Cui et al. [62], and Shen et al. [63] further introduced the use of composites comprising PCMs and quantum metamaterials in tunable thermal regulation applications in greenhouse covers. The discussion further compared PCMs and quantum metamaterials in application areas in greenhouse covers based on adaptability, cost, and durability. Based on adaptability, the evaluation indicated that quantum metamaterials offer a high degree of spectral adaptability and can be engineered to interact with specific radiation wavelengths [44]. However, as demonstrated in Cui et al. [63], PCMs are restricted to a single transition temperature and cannot be tuned to interact with specific light wavelengths. As such, using a composite of PCMs and quantum metamaterials would enhance the dynamic responsiveness of the greenhouse covers to external temperature.

The discussion also considered how PCMs compare to quantum metamaterials based on their costs. The arguments advanced in Yang et al. [61], Cui et al. [62], and Shen et al. [63] highlighted that PCMs are a more mature technology and have a lower cost compared to the emerging quantum metamaterials. Directly, this implied that costs remained the most significant barrier to the adoption of quantum metamaterials. Therefore, based on economic feasibility, PCMs are more likely to be adopted in agricultural applications. A further discussion considered how PCMs compared to quantum metamaterials based on durability. Studies such as Mazhorin [49] and Fernández-Fernández et al. [50] showed that although quantum metamaterials had a tunable electromagnetic response, quantum dots were more susceptible to degradation from moisture, UV exposure, and oxidation. However, Cui et al. [62] and Shen et al. [63] showed that PCMs were more durable as they existed as encapsulated solids and liquids to prevent leakage and degradation. As such, PCMs offer better economic feasibility based on their higher durability advantages.

5. Conclusions

The core aim of this review article was to investigate the performance optimization of greenhouse covers with the potential use of nonlinear and quantum metamaterials as nano-additives in terms of electromagnetic radiation. The article also examines the effects of quantum nano-additive materials used in greenhouses on crops/PAR, UV limitation, and temperature effects. Based on the examination of diverse studies, inferences showed that nonlinear and quantum metamaterials have potential as nano-additives to optimize greenhouse covers in applications, such as the following: (i) optimization of PAR admission to promote crop yields, (ii) blocking harmful NIR, and (iii) as smart sensors to ensure real-time adjustments to the light environment inside greenhouses. The discussion also indicated that the quantum nano-additive materials used in greenhouse covers positively impact PAR and lead to increased crop growth within the greenhouses. Furthermore, due to the high sensitivity of the quantum and nonlinear metamaterials, they block harmful NIR, regulating internal temperature and preventing harm to the crops.

The recommendations from this review article are that more scholars ought to focus on investigating how quantum and nonlinear metamaterials can be adopted within real-world settings. Similarly, industry practitioners should adopt metamaterials as nano-additives within the greenhouse covers to promote overall crop yields in the greenhouses. Agricultural practitioners in extreme climates, such as cold and hot areas, are further recommended to adopt quantum and nonlinear metamaterials for advanced sensing to block harmful UV (NIR) in controlled environments.

In future research, it is necessary to investigate the long-term consequences of quantum-enhanced radiation management on the genetics of the plant, the health of the soil, and the dynamics of the ecosystem. Moreover, future studies should be dedicated to understanding how it is possible to combine quantum metamaterials with other precision agriculture technologies (e.g., sensors, artificial intelligence systems, and automated irrigation) to build complete smart greenhouse systems. A further avenue regards the development of cost-effective manufacturing processes for quantum metamaterials that can compete with conventional materials in terms of initial investment and lifecycle costs.

Additionally, comprehensive economic impact studies are needed to quantify the long-term benefits of quantum metamaterial implementation, including energy savings, yield improvements, and operational cost reductions, to provide clear return-on-investment calculations for greenhouse operators. Research should also explore hybrid approaches that combine quantum metamaterials with conventional materials to achieve optimal performance while maintaining economic feasibility. The development of standardized installation and maintenance protocols for quantum metamaterial-based greenhouse systems is another critical gap that requires investigation.