Vanadium Dioxide-Based Terahertz Metamaterials for Non-Contact Temperature Sensor

Abstract

Temperature sensors play important roles in wide-spreading human activities. The non-contact method of using temperature sensors offers significant advantages but faces challenges in detection precision. In this work, a double-layer asymmetric terahertz (THz) metamaterial combined with phase transition oxide was proposed to realize non-contact temperature sensor with high sensitivity. The metamaterial exhibited band-stop filtering effects in the simulated transmission spectra. Temperature changes induced a reversible phase transition in VO₂, resulting in altered conductivity. The numerical results indicated that the S21 parameter increases from −44.33 dB to −4.78 dB at a frequency of 1.22 THz as the conductivity of the VO₂ film increases from 10 to 5000 S/m, achieving a modulation depth of 89%. In addition, the 86 nm thick VO₂ film underwent a phase transition in the temperature range of 54.93 °C to 66.93 °C, achieving a sensitivity of 1.82 dB/°C for temperature sensing. This work provided great insights into the development of metamaterials based on high-precision temperature measurement.

1. Introduction

Temperature sensors are extensively utilized across various fields, including industry, healthcare, environmental monitoring, aerospace, and automotive applications. However, traditional temperature sensors, including thermocouples [1,2], thermistors [3,4], and infrared sensors [5,6], often demonstrate slow response times, limited measurement ranges, insufficient accuracy, and susceptibility to environmental interference, severely restricting their practical applications. Terahertz (THz) spectroscopy serves as a significant complement to infrared spectroscopy, providing several notable advantages. Firstly, THz spectroscopy exhibits high transparency to nonpolar materials such as polymers, wood, paper products, and even smoke, allowing it to operate effectively in environments where infrared spectroscopy may be limited by absorption or scattering effects [7,8,9]. Secondly, the non-contact nature of THz measurements allows for high-precision temperature sensing without the risk of thermal interference or damage to sensitive materials, which is particularly beneficial for fragile samples or high-temperature environments [10,11,12]. Despite these advantages, the weak response of THz waves to natural materials significantly hinders their development. Metamaterials, composed of periodically arranged sub-wavelength unit structures, exhibit exceptional physical properties that are not present in natural materials [13,14,15,16]. By designing suitable metamaterial structures, a strong response to THz waves can be achieved, thereby enhancing the performance of sensors [9,17,18,19,20,21,22,23,24]. For example, H. Zhou et al. [25] developed a bilayer cross-shaped plate-hole metamaterial absorber that significantly enhances sensitivity for terahertz biosensing, achieving a theoretical sensitivity improvement of about 7 and 18 times compared to conventional absorbers and experimentally demonstrating a sensitivity of 153 GHz/μM for biotin detection. Y. W. Liu et al. [26] presented a design of electrothermally controllable THz metamaterial based on a split-ring resonator structure, achieving tunable triple-resonance with tuning ranges of 24 GHz, 40 GHz, and 32 GHz by applying a direct current voltage, and the maximum sensitivity is 42 GHz/volt.

In THz metamaterial temperature sensors, temperature-sensitive materials are typically chosen as the modulation medium, including strontium titanate based on the damped resonator model [27,28], graphene based on Falkovsky’s formula [29,30], and InSb based on Drude’s model [31,32], Ge₂Sb₂Te₅ (GST) [33], etc. Changes in ambient temperature induce variations in dielectric properties, which subsequently affect THz wave transmission performance. For instance, D. Li et al. [28] investigated the potential of using strontium titanate for temperature sensing based on a perfect absorber of terahertz metamaterials, which consists of two split-ring resonators of different sizes, a silicon substrate, and a bottom metallic layer. The frequency of the absorption peaks can be blue-shifted from 1.41 to 1.52 THz as the temperature increases from 200 to 400 K. This setup achieves a temperature sensitivity of 0.00055 THz/K. Utilizing the temperature-sensitive properties of InSb materials, T.T. Lang et al. [34] developed a temperature sensor with a sensitivity of 21.9 GHz/K, employing a perfect absorber of terahertz metamaterials. A. Keshavarz et al. [30] reported a nanoscale terahertz metamaterial composed of a graphene H-shape situated on an InSb substrate. Given that the optical properties of graphene and InSb are highly temperature-dependent, the resonance frequency shifts with temperature changes, allowing for temperature sensing with a sensitivity of 0.027 THz/K. Y. L. Guo et al. [33] demonstrated a tunable terahertz (THz) metamaterial based on GST particles that maintain its original THz wave resolution even as the temperature increases, allowing for the active “on” and “off” control of its absorption window by changing the temperature. Despite existing studies on THz metamaterial temperature sensors, challenges such as low sensitivity and measurement accuracy persist. Furthermore, achieving unique resonant responses requires complex designs of metamaterial unit structures, resulting in challenges in analysis, design, and fabrication.

Vanadium dioxide (VO₂) is a material that undergoes a reversible insulator-metal phase transition at approximately 68 °C [35]. During this transition, VO₂ undergoes a first-order phase change from a high-temperature metallic state to a low-temperature insulating state, accompanied by a dramatic change in conductivity of four to five orders of magnitude [36,37]. This characteristic positions it as a promising candidate for temperature sensor applications. This paper systematically investigated the performance of THz metamaterial temperature sensors modulated by VO₂ thin films, utilizing a double-layer asymmetric structure (DLAS) composed of two copper strips, highlighting the advantages of a simple structure and high sensitivity. The results indicated that when the conductivity of the VO₂ thin film varies within the range of 10 to 5000 S/m, the S21 parameter varies from −44.33 to −4.78 dB at 1.22 THz, achieving a modulation depth of up to 89%. In addition, the 86 nm thick VO₂ film underwent a phase transition in the temperature range of 54.93 °C to 66.93 °C, achieving a sensitivity of 1.82 dB/°C for temperature sensing.

2. Experiments and Structural Design

The VO₂ films were prepared using a conventional inorganic sol–gel method: 5 g of V₂O₅ powder was melted at around 850 °C, poured into 150 mL deionized water, and then stirred for 2 h. A sol was then obtained by filtering the solution and leaving it to incubate for 2 days. The substrates were pretreated with ethyl alcohol and the mixture of H₂SO₄ and H₂O₂ with a ratio of 2:1, which can endow the substrate hydrophilicity. The sol was dip-coated on the substrate, dried at around 100 °C, and then annealed at 520 °C for 2 h in a static nitrogen atmosphere. The film thickness can be controlled by repeating the dip coating times. And the film thickness was measured using a stylus profiler (XP-2, Ambios Technology, Genova, Italy).

The crystalline structure of the films was characterized using an X-ray diffraction (XRD) at a grazing angle of 1.5° (X’ Pert, Philips). And the surface morphology of the films was detected using a scanning electron microscopy (SEM, Inspect F). The electricity was measured using a conventional four-probe measuring instrument.

Figure 1 illustrates the structure of the THz metamaterial temperature sensor along with its electromagnetic spectral characteristics. As shown in Figure 1a, the proposed sensor consisted of multiple unit structures arranged periodically. Each unit structure was composed of two identical copper strips and a VO₂ film, separated by a SiO₂ substrate, with a Si protective layer on the surface of the VO₂ film. Figure 1b presents the front view of the unit structure, which includes two copper strips that function as a plasmonic superlattice. The symmetry breaking of the structure arose from the center-to-center distance (δ) between the copper strips in the unit structure. The width (w) of the copper strips was 30 µm, and their thickness (h₁) was 2 µm. An 86 nm thick (h₂) VO₂ film served as the temperature-sensitive layer, regulating the transmittance of THz waves. The periodicity of the unit structure was maintained at (px = 300) µm, which was chosen to provide the necessary momentum for the copper strips to excite waveguide modes through coherent plasmonic scattering in the THz frequency range. Consequently, for a fixed waveguide thickness of (H = 6) µm, the frequency of the waveguide modes was determined by the period. The simulation results were obtained using the well-established 3D full-wave solver, which is based on the finite integral method. The x and y directions were set as magnetic and electric boundaries, respectively, while the z direction was an open boundary, allowing THz waves to propagate along the z-axis. Waveguide ports 1 and 2 were positioned on the negative and positive orientations of the z-axis, respectively, with an accuracy of −80 dB. The dielectric constant of SiO₂ was 3.9, while that of Si was 11.9. The conductivity of copper was 5.96 × 10⁷ S/m.

3. Results and Discussion

As shown in Figure 2, the films with thickness of 28 nm, 59 nm, 86 nm, and 130 nm exhibit high-purified monoclinic VO₂ phase (JCPDS card no. 72-0514). Although the films present polycrystalline characteristics, a strong diffraction peak at around 27.98° indicates that a (011) preferred growth orientation can be observed. The surface morphology of the films with different thicknesses were collected, as shown in Figure 3, which shows that the film became more compact by increasing the thickness from 28 nm to 59 nm. But, an obvious hybrid-growth mode can be observed when the film thickness increased further. This referred to an island-growth mode when the film thickness increased from 59 nm to 86 nm, but a layer-growth mode played more important role when the film thickness increased from 86 nm to 130 nm. What is more important, by increasing the film thickness gradually, VO₂ with compact morphology can be obtained. And the compact structure can lead to enhanced phase transition amplitude, as illustrated in previous report [38].

Table 1. Change in resistivity of the VO₂ films with different thicknesses before and after phase transition.

VO2 Films with Different Thicknesses	Resistivity (S/m)
	Room Temperature (25 °C)	85 °C
28 nm	9.08	357.15
59 nm	9.29	1564.10
86 nm	10.79	6209.23
130 nm	11.17	11,518.15

illustrates the resistivity of the films before (at room temperature) and after phase transition (at 85 °C). It further confirmed that the phase transition amplitude of the VO₂ film can be enhanced with compact morphology. And the film with thickness of 130 nm can reach around three orders of change in the resistivity across the phase transition.

Figure 1c compares the S21 parameters of the SLS, DLSS, and DLAS. For the SLS and DLSS, transmission dips occurred at frequencies of 1.28 THz and 1.26 THz, with amplitudes of −11.72 dB and −13.12 dB, respectively. The amplitude of the transmission dip for the DLAS was −44.33 dB, with the resonance frequency red-shifting to 1.22 THz.

To further elucidate the formation mechanism of this transmission dip, Figure 1d–f present the corresponding electric field distributions. The formation mechanisms of the transmission dips for the SLS, DLSS, and DLAS were similar. The interaction between the THz waves and the Cu strips excited plasmonic modes, which interact with transverse waveguide modes, leading to the formation of the transmission dips. Specifically, the transmission dip originated from the spectral overlap between the narrow transmission band of the waveguide mode and the broad plasmonic scattering band associated with the Cu strips. The narrow transmission band was generated by the scattering of localized plasmonic modes within the Cu strips, which excite the waveguide mode (i.e., the bright-dark mode). In contrast, the broad plasmonic band was directly excited by the localized plasmonic modes present in the Cu strips. Therefore, the spectral position and linewidth of the transmission dip were determined by the coupling strength between these two modes.

Figure 4 illustrates the optimization process of the parameters of the unit structure. The impact of the unit structure period on the transmission spectrum is shown in Figure 4a. When the unit structure period was 100 µm, it failed to provide sufficient momentum, preventing the Cu strips from exciting waveguide modes through coherent plasmonic scattering, which resulted in no resonance formation, as indicated by the black line in Figure 4a. As the unit structure period increased to 200 µm, there was enough momentum to form a transmission dip at a frequency 1.64 THz, with an amplitude of −47.87 dB. Further increasing the period caused the resonance frequency to shift to lower frequencies. For instance, at a unit structure period of 300 µm, the resonance frequency was 1.25 THz, with a corresponding amplitude of −40.97 dB. Continuing to increase the unit structure period excites additional resonances, as shown by the green and purple lines in Figure 4a.

The effect of the SiO₂ substrate thickness on the transmission dip is depicted in Figure 4b. As the substrate thickness increased, the frequency of the transmission dip shifted to lower frequencies, while the amplitude first decreased and then increased. At substrate thicknesses of 2, 4, 6, 8, and 10 µm, the transmission dip frequencies were 1.25, 1.24, 1.22, 1.21, and 1.20 THz, with corresponding amplitudes of −40.97, −42.23, −44.33, −43.52, and −36.06 dB.

Figure 4c demonstrates the influence of the width of the Cu strips on the transmission spectrum. As the width of the Cu strips increased, the frequency of the transmission dip shifted to lower frequencies, the amplitude decreased, and the bandwidth increased. When the width of the Cu strips increased from 20 µm to 90 µm, the resonance frequency shifted from 1.29 THz to 1.06 THz, and the amplitude decreased from −18.21 dB to −58.69 dB.

Figure 4d illustrates the effect of the asymmetry of the unit structure on the transmission spectrum. As the asymmetry increased, the resonance frequency initially shifted to lower frequencies and then to higher frequencies. When the asymmetry increased from 0 µm to 30 µm, the resonance frequency shifted from 1.26 THz to 1.22 THz, with the amplitude decreasing from −13.15 dB to −44.33 dB. As the asymmetry continued to increase to 60 µm, the resonance frequency blue shifted to 1.25 THz, and the amplitude increased to −30.43 dB. This behavior occurred because, at higher asymmetry, the interaction between the two Cu strips gradually weakened, leading to a reduction in the ultimately excited resonance. This resulted in the blue shift in the transmission dip frequency and an increase in amplitude.

To further explain the impact of asymmetry in the unit structure on the transmission dip, we calculated the electric field distributions at different symmetry levels, as shown in Figure 5. The results indicated that as the degree of asymmetry increases, the overall electric field initially weakened and then strengthened, which is consistent with the findings in Figure 4d. This behavior can be attributed to the gradual weakening of the plasmonic modes excited by the copper strips as the asymmetry increased. When the degree of asymmetry reached 30 μm, the waveguide modes began to strengthen (as evidenced by the increasing electric field within the substrate between the copper strips). Consequently, with further increases in asymmetry, the electric field continued to enhance, leading to a transmission dip amplitude that first decreased and then increased.

VO₂ is a material that exhibits a reversible insulator-metal phase transition, during which its conductivity experiences a dramatic change of 4–5 orders of magnitude. This characteristic makes it suitable as a sensitive layer for temperature sensors, enabling high-sensitivity temperature measurements by monitoring its phase transition properties. Figure 6a illustrates the variation in electrical conductivity of an 86 nm thick VO₂ film during heating and cooling processes. It can be observed that the electrical conductivity of the VO₂ film varied between 10 and 6209 S/m before and after the phase transition. Figure 6b shows the transmission spectrum of VO₂ films at different conductivities. As the conductivity of the VO₂ film increased from 10 S/m to 5000 S/m, there was no significant change in the resonance frequency of the transmission dip, while the amplitude gradually increased from −44.33 dB to −4.78 dB, achieving a modulation depth of up to 89%, as illustrated in Figure 6c. Figure 6d presents the variation in transmission dip amplitude corresponding to different temperatures of the VO₂ film during heating and cooling processes. During heating, as the temperature increased from 59 °C to 72.16 °C, the corresponding amplitude rose from −26.60 dB to −4.77 dB, resulting in a sensitivity of 1.66 dB/°C (where the sensitivity of the temperature sensor is defined as S = ΔT/Δt, with ΔT representing the change in amplitude and Δt representing the temperature change). Within the same amplitude change range, during cooling, the temperature can vary between 66.93 °C and 54.93 °C, yielding a sensor sensitivity of 1.82 dB/°C.

Table 2. Performance comparison of different types of temperature sensors.

Modulating Medium Materials	Working Temperature (K)	Sensitivity	Reference
Graphene	397–304	0.0271 THz/°C	[30]
InSb	260–310	0.0219 THz/K	[34]
InSb	190–240	0.0126 THz/°C	[39]
Strontium Titanate	200–400	0.00055 THz/K	[28]
Boron Nitride/Barium Titanate	296–473	0.000000462 THz/°C	[40]
Metal Thermal Sensing Branch	1025	0.0097 dB/K	[41]
VO2	328–345	1.82 dB/°C	This Work

compares the sensitivities of various types of temperature sensors.

Figure 7a–d present the electric field distribution maps at the resonance frequency for different conductivities. In low-temperature environments, the VO₂ film remained in the insulating phase with low conductivity. At this stage, the asymmetric Cu strips within the unit structure excited a plasmonic mode that interacts with the transverse waveguide mode, forming a very narrow transmission dip at a frequency of 1.29 THz. As the temperature rose to the phase transition temperature of the VO₂ film, its conductivity gradually increased. Consequently, the plasmonic mode excited by the asymmetric Cu strips weakened, leading to a reduced interaction with the transverse waveguide mode. This weakening of resonance resulted in an increase in the value of the transmission dip, indicating the sensor’s sensitivity to temperature changes. Overall, the unique properties of VO₂, combined with its significant conductivity variation during the phase transition, make it an excellent candidate for high-performance temperature sensing applications.

4. Conclusions

This work proposes a phase transition oxide-based THz metamaterials for non-contact temperature sensors. The systematic structure is composed of two identical Cu strips and a VO₂ film separated by a SiO₂ substrate. This study examined the effects of various parameters, including the unit structure period, the thickness of the SiO₂ substrate, the width of the Cu strips, the degree of asymmetry, and the conductivity of the VO₂ film. The results indicated that the temperature-induced reversible phase transition of the VO₂ film alters its conductivity, thereby controlling the transmission of THz waves and facilitating environmental temperature monitoring. As the conductivity of the VO₂ film varied from 10 to 5000 S/m, the amplitude of the transmission dip changed from −44.33 dB to −4.78 dB, achieving a modulation depth of up to 89%. The 86 nm thick VO₂ film underwent a phase transition in the temperature range of 54.93 °C to 66.93 °C, achieving a sensitivity of 1.82 dB/°C for temperature sensing. Additionally, potential fabrication methods such as photolithography and thin-film deposition could be employed to realize the proposed ultra-thin structure in practical applications. The designed THz metamaterial functions as a highly sensitive non-contact temperature senso, showing great potential for applications in biomedical monitoring and smart building systems. Future research can focus on further optimizing the structure of the metamaterial to enhance sensitivity and response time, as well as exploring scalable manufacturing methods. Furthermore, integrating this technology with other sensing modalities, such as programmable metasurface technology, could lead to the development of multifunctional sensors capable of monitoring various environmental parameters.