Photonic–Surface Plasmon Coupling Mode: Experimental Study with a Silver Thin-Film Coating on MPCC

Abstract

In this paper, a silver thin film coating on a monolayer polystyrene colloidal crystal (MPCC) hybrid structure was fabricated, and a photonic–surface plasmon coupling mode was established and experimentally researched. The silver thin film was sputtered onto the MPCC to form Ag-MPCC. The silver film effectively excites surface plasmon polariton (SPP) modes upon the incidence of light, and the MPCC has an intrinsic mode. These two modes couple and result in the extraordinary optical transmission (EOT) phenomenon in the transmission spectrum. Reflection suppression arising from this photon coupling effect was discovered in the reflection spectrum. We etched the single-layer colloidal particles to change the period of the colloidal crystal, thereby forming the MPCC metal hybrid structure with different lattices. We discussed and analyzed the results through experiments. The EOT can be controlled by the incident angle, lattice periodicity, and refractive index distribution of the Ag-MPCC, and the diffraction behavior is determined using the lattice structure and refractive index of the MPCC. The coupling effect of the two models leads to wavelength shifts and intensity variations in the spectral eigenvalues. Reflection suppression is achieved when the reflectivity at a specific wavelength is close to 0.1.

1. Introduction

Colloidal crystals [1,2] composed of nanosized particles benefit from the periodic spatial distribution of their dielectric constants, capable of modulating the propagation of photons at the optical wavelength scale in multiple directions [3]. With the availability of rapid and straightforward fabrication methods for constructing various monolayer colloidal crystal structures (MPCCs), MPCCs have become a focus of attention in optical material research [4,5,6,7]. Consequently, MPCCs are subjects of interest in optical material research, whether for the development of optical properties [8,9] or the application of structural templates [10,11]. The optical transmission behavior at a photonic crystal interface is generally explained using the band theory of photonic crystals, and the specific reflection and transmission behavior of light at such interfaces has been rarely studied [12,13]. Features like the Wood’s anomalies phenomenon [14], which occurs when the wavelength matches the periodicity, and other optical characteristics, are currently under continuous exploration.

Meanwhile, the design and preparation of metal-doped periodic hybrids can efficiently excite surface plasmon polaritons (SPPs), exhibiting the properties of tangential propagation and normal swiftness of electromagnetic waves at the interface of the metal and medium, thus enabling photon resonant transmission. The formation of energy band structure of SPPs and the plasma-assisted absorption of light in regularly shaped metal structures are attractive features for the development of metal–periodic nanotemplate assemblies [15,16,17]. Consequently, hybrids of MPCCs with wavelength-scale lattices and metal thin films exhibit coupling between the eigenmodes of the MPCCs and the SPP resonance modes [18,19]. However, research on the properties of this mixed model at the contact surface is little conclusively determined, which limits its further research and application.

In this study, we innovatively designed and fabricated a hybrid structure of silver and MPCC (i.e., Ag-MPCC)—specifically, an array of polystyrene microspheres coated with a thin silver film. We validate the eigenmodes of the MPCC, exposing its light transmission characteristics in the reverse direction compared to the bare MPCC. Moreover, a thin silver layer was sputtered onto the MPCC, and the resulting periodically corrugated metal layer can effectively excite SPPs, satisfying the matching conditions between these excitations and the Bragg resonance of the eigenmodes, generating a photonic–surface plasmon coupling mode [20]. In order to explore the behavior of light reflection and transmission at the interface, a model with surface plasmon response was constructed to explore the effect of changing the angle of the incident optical wave on the photonic coupling within the hybrid structural array. In the experiment, the structural plasticity of MPCCs was utilized to adjust lattice parameters and refractive index distributions, enabling controlled morphological adjustments to the template beneath the silver film. This allowed us to test and demonstrate its impact on the modulation of photonic–surface plasmon coupling. Our results verify that, in the interface transmission behavior of metal-coated single-layer colloidal crystals, two excitation modes coexist: the surface plasmon excitation mode and the periodic diffraction mode. Furthermore, these modes exhibit coupled behavior.

2. Experiments

2.1. Sample Fabrication

MPCCs with a hexagonal close-packed (HPC) structure were fabricated via the gas–liquid interface self-assembly method [21,22,23]. A suspension of Polystyrene (PS) particles with a diameter of 800 nm was injected onto the surface of deionized water using microfluidic injection, allowing gradual diffusion and the formation of a monolayer array. The assembled monolayer array was then transferred onto a hydrophilic glass substrate to form the MPCCs.

The size modification of PS microspheres was achieved using reactive ion etching (RIE) technology (NANO-MASTER NRE-4000). By controlling the oxygen flow rate, generator power, and etching time in the etching chamber, we could manipulate the diameter of the PS microspheres while maintaining the lattice periodicity. The samples were prepared under constant chamber pressure of 2 Pa, with a target substrate spacing of 10 mm, substrate rotation at 20 revolutions per minute, etching power set to 15 W, and reaction gas flow rate set to 30 sccm. The reduction in microsphere diameter was controlled by varying the etching time.

A metal film was deposited on the surface of the MPCCs using JSD450-III magnetron sputtering technology. This process aimed to deposit a layer of approximately 40 nm-thick silver film with high uniformity and density onto the MPCC supported on a glass substrate. The reaction chamber was set to a vacuum pressure of 10⁻¹ Pa, constant current was set to 0.04 A, and the reaction gas flow rate was set to 2.5 sccm.

The surface morphology of the Ag-MPCCs was observed using field-emission scanning electron microscopy (SEM, JEOL7600). The top view of the Ag-MPCCs is shown in Figure 1a, revealing a highly ordered periodic distribution of colloidal particles. However, due to slight deviations in particle size, some arrangement defects are still present. The side view depicted in Figure 1b reveals that the MPCC formed by microspheres on the substrate is a very flat monolayer structure. The upper half of the PS microspheres is covered by an Ag film, forming bright silver caps, while the lower half remains unchanged. Figure 2a–e present top views of Ag-MPCCs subjected to etching for 1–5 min under the same technical parameters and covered with the same thickness of silver film. They illustrate a regular decrease in colloidal particle diameter with increasing etching time while still maintaining a relatively smooth spherical appearance.

2.2. Optical Property Measurement

Transmittance and reflectance spectra were measured using the R1 angle-resolved measurement system from Idea Optics, comprising a PG2000-Pro spectrometer, R1-A-UV holder, and a halogen-deuterium light source. In this experimental setup, the beam is emitted and received via the optical fiber; The system specifications are as follows: wavelength range: 200–1100 nm, optical resolution: 0.035 nm (FWHM), dynamic range: 12,000:1, and signal-to-noise ratio 450:1. The angle resolution depends on the grating line density (18 optional gratings available) and slit width (5–200 μm). Using an 1800-line grating a +10 μm slit, the angular resolution is less than 0.005°, and polarization control function is implemented, with the TM mode used in this experiment.

As depicted in Figure 3, natural light with a wavelength range of 500–1000 nm is incident on the sample surface, with the angle between the incident light and the normal to the sample surface denoted as θ. Normalization was performed by subtracting the influence of the base medium to represent the actual results after passing through the sample.

3. Results and Discussion

Before presenting the experimental results, we discuss SPPs and grating diffraction. The SPP excitation on the metal satisfies the following conditions: surface plasmons (SPPs) are electromagnetic modes formed by the coupling of light with free electrons on metal surfaces. When the incident light frequency matches the resonance frequency of the SPPs, light energy propagates through SPPs along the metal–dielectric interface, enhancing transmission efficiency. The momentum-matching conditions of periodic pore arrays (achieved through grating coupling) convert incident light into SPPs:

$$K_{Spp}=K_0\mathbf{sin}\theta \pm n{\displaystyle \frac{2\pi }{a}},$$

(1)

where K₀ is the wave vector of the incident light, θ is the angle of incidence, a is the period of the hole array, n is an integer, and K_SPP is the wave vector of the resulting SPP.

In the case of a periodic structure at a reflection interface, if there is incident light in free space, the behavior of light at the interface is similar to that of light diffracted by a grating. The diffraction light should satisfy the following formula:

$$K_{\mathrm{d}}-K_{\mathrm{i}}=\pm \mathrm{m}{\displaystyle \frac{2\pi }{\mathrm{d}}},$$

(2)

where the wave vector of the incident light is K_i, the period of diffracted light wave vector is K_d, and the period of grating is d.

In the case of the Ag-MPCC structure, in Equations (1) and (2), K₀ and K_i are the same parameter, while a and d are the periods of the MPCCs.

$$K_{Spp}+K_{\mathrm{d}}=K_0(1+\mathbf{sin}\theta )\pm \mathrm{T}{\displaystyle \frac{2\pi }{a}}$$

(3)

where T equals m n, which is also a positive integer. Therefore, the diffraction spectrum is determined by the above two behaviors. According to our observation of spatial light, when K_SPP is at its maximum, absorption is maximum, and the corresponding wavelength spectrum is depressed; when K_d is at its maximum, diffraction enhancement occurs, and the spatial spectral crossing wavelength is enhanced. We conducted quantitative calculations by incorporating the spatial dielectric constant, lattice periodicity, wavelength, and angular parameters of the Ag-MPCC structure into Equation (3). The established parameters were as follows: colloidal particles: PS colloidal particle with diameter d = 800 nm and refractive index n = 1.46; hexagonal packed lattice (lattice constant a = d = 800 nm); silver film thickness of 40 nm; dielectric constant modeled using the Johnson–Christy model (ε = −15.87 + 1.08i). When incident light is incident perpendicularly, the resonant wavelength of the coupled mode reaches approximately 923.2 nm.

Figure 4 displays the transmittance reflectance spectra of the Ag-MPCC under normal incidence conditions. Before testing the sample, we selected a number of different test points on the same sample. The transmission spectra showed high consistency, and the transmittance was less than 0.2% in the range of 500–1100 nm, which shows the testing method and samples have good repeatability and consistency. The dip in transmittance at λ_T1 = 933.1 nm signifies a specific optical response where light couples into the eigenmode of the Ag-MPCC. The result has a small error compared with the theoretically calculated resonance wavelength of 923.2 nm. The main reason for this error is the deviation of the lattice constant and colloidal particle diameter, as well as the roughness of silver surface. To further explore the eigenmode characteristics of the Ag-MPCC, detectors were simultaneously placed in the symmetric reflection direction relative to the incident light on the sample. We discovered a noticeable decrease at λ_R1 = 931.5 nm in the reflectance spectrum as well. This further elucidates the light-confining characteristics of the eigenmode of the Ag-MPCC, confirming that its guided light transmission behavior manifests as a reduction in optical energy in both the transmission and reflection directions outside the interface.

Figure 5a presents the transmittance spectra of the MPCC and the Ag-MPCC measured under normal incident light, with the overall transmittance normalized to that of a flat metal film of the same thickness [24,25,26]. The transmittance spectrum of the Ag-MPCC exhibits the phenomenon of EOT [27], with a peak transmittance observed at λ_T2 = 1003.4 nm. The transmission band results from the coupling between the photonic resonance guided mode and the surface plasmon resonance (SPR) on the metallic coating [28]. According to Equation (3), when light is incident on the metal surface, the reciprocal lattice vectors provided by the periodic array of the MPCC match the momentum of the SPPs, leading to the excitation of SPPs. This localizes the energy of the light field on the metal surface and allows it to tunnel through the gaps between the microspheres to the bottom surface, where it radiates outward, resulting in enhanced transmission. The transmission peak in the spectra of the Ag-MPCC replaces the transmission minimum of the MPCC spectra and appears on the red-shifted side of the wavelength of the guided mode excitation.

In Figure 5b, the wavelength of the reflection valley in the reflectance spectrum of the Ag-MPCC is 924.4 nm, which is very close to the wavelength corresponding to the guided resonance mode of the MPCC. According to Equation (3), a change in the angle leads to a change in the diffraction wave vector. It indicates that the Ag-MPCC primarily restricts the emission of light in the reflection direction due to the effect of the eigenmode of the MPCC, different from the EOT phenomenon, which requires satisfying the periodicity condition of the metal corrugation and causes a significant shift in the wavelength. The Ag-MPCC achieves wavelength control of light reflection and transmission through the eigenmode of its internal medium layer. The addition of the metal layer further enhances the structure’s near-background reflection and light-transmission properties. This characteristic endows the structure with the ability to strongly limit light transmission at specific wavelengths, indicating its potential applications in optical reflectors, optical filters, and optical sensing devices.

We observed the effect of altering the incident wave vector on the photonic–surface plasmon coupling. Figure 6a illustrates the transmission spectra of the Ag-MPCC measured as the incident angle increases from 0° to 20° (in steps of 2°). The main transmission peak is most prominent at normal incidence, and with increasing angle, there is a slight blue shift observed in the EOT peak. Additionally, the peak intensity of the main transmission peak decreases correspondingly. Figure 6b shows a red shift in the wavelength corresponding to the minimum reflection, and the characteristic weakening of reflection becomes less apparent. The resonant response wavelength shifts from 915.4 nm to 982.3 nm. At a fixed wavelength, for example, at 950 nm, the angular range corresponding to the half-height width of the resonance peak is about ±3°, indicating strong angular selectivity. Under normal incidence, all components of natural light have electric field directions perpendicular to the structure’s normal, contributing uniformly to coupled transmission. The dispersion behavior at different incident angles affects the excitation conditions of SPPs, altering the light-wave matching conditions for coupling into the MPCC.

The optical transmittance characteristics exhibited by the Ag-MPCC largely depend on the coupling of light into the eigenmode of the MPCC. Therefore, the spectral features can be modulated by altering the structure of the MPCC, which is one of the most noteworthy features of its optical properties [29]. We assembled MPCCs using two different sizes of PS microspheres, 800 nm and 860 nm, as substrates for the metal film. The comparative transmission spectra are shown in Figure 7a, and the comparative reflection spectra are shown in Figure 7b.

As expected, both the characteristic values of transmission and reflection exhibit a red-shift phenomenon with increasing microsphere diameter. With the increase in colloidal microsphere diameter, the transmission peak corresponding to the EOT phenomenon shifts from 1003.4 nm to 1074.9 nm, while the reflection suppression phenomenon corresponding to the reflection valley shifts from 933.8 nm to 979.6 nm. This further confirms that the coupling result of photons entering the metal film and then transmitting through the microsphere array is indeed closely related to the eigenmode of the MPCC. The wavelength selection for the transmission peak in the EOT phenomenon mainly depends on the periodic matching of the metal ripple array. Therefore, changes in the period caused by changes in particle size lead to a shift in the wavelength of the transmission peak. The wavelength selection for the reflection suppression phenomenon, in turn, depends on the wavelength matching of the intrinsic mode of the MPCC layer.

Another method of altering the morphology of MPCC templates involves adjusting the distribution of the medium’s refractive index. Figure 8a displays the transmittance spectra of Ag-MPCCs formed from MPCCs constructed with PS microspheres of 860 nm diameter after different etching degrees. Firstly, it is noted that with increasing etching time, the diameter of the microspheres decreases (as seen in the sample appearance in Figure 2, resulting in a blue shift of the transmission peak). The diffraction enhancement wavelength gradually changes from 900.5 nm to 930.6 nm, 960.3 nm, 1028.6 nm, and 1075.7 nm. Simultaneously, the same shifting pattern is observed in the reflection spectra, as shown in Figure 8b. The excitation absorption wavelength of SPPs gradually changes from 960.5 nm to 1000.5 nm, 1040.7 nm, 1065.9 nm, and 1070.2 nm, with reflectivity close to 0.1. These results demonstrate that good reflection suppression can be achieved by controlling the lattice structure parameters. However, the wavelength shifts of the two are not synchronous, reflecting that the wave vectors and diffraction vectors of the SPPs are not the same due to the periodic lattice changes.

On one hand, the EOT phenomenon relies on the eigenmode of the MPCC and the arrangement of the metal ripples. With an increase in the interstitial spacing of the etched microspheres, there is a corresponding increase in the area of the metal depressions, leading to a change in the wavelength of light that satisfies the transmission condition. The wavelength shift of the reflection valley primarily depends on the eigenmode of the MPCC; hence, it almost synchronously changes with the reflection characteristics of the bare MPCC. On the other hand, increased etching depth weakens the waveguide eigenmode of the MPCC [30]. The enhanced resonance effect of surface plasmons coupled with them becomes less pronounced. Moreover, the reduction in contact between adjacent silver caps may hinder the effective propagation of surface plasmon waves. Consequently, the intensity of the transmission peak associated with the EOT phenomenon diminishes. Conversely, the intensity of the reflection suppression phenomenon remains relatively stable, primarily due to the reflective role of the silver caps.

4. Conclusions

This work investigates the spatial optical transmission properties of Ag-MPCCs and presents experimental research and results analysis aimed at utilizing MPCC to modulate SPR. The silver film successfully excites SPPs coupled with the eigenmodes of the MPCC, exhibiting the EOT phenomenon within the Ag-MPCC structure. This is manifested as an anomalous transmission peak in the transmission spectra, associated with the Wood’s anomalies observed in bare MPCCs. We innovatively characterize the transmission results of SPR coupled with eigenmode in the reflection direction, demonstrating wavelength-selective reflection suppression phenomena where reflectance approaches 0.1 at specific wavelengths.

The structural model of Ag-MPCC holds valuable advantages for development. Notably, the excitation of SPPs in the metal is sensitive to the dispersion behavior of incident light. Hence, by adjusting the incidence angle, we can match the coupling conditions for light waves of different wavelengths. More significantly, by employing HCP MPCCs composed of microspheres with varying diameters, as well as etched Ag-MPCC, we achieved precise control over the distribution patterns between MPCC and metal coatings, enabling effective modulation of both the wavelength selectivity and intensity of the EOT and reflection suppression phenomena. The results of this work provide important support for the mode of MPCC to modulate and generate the behavior of the SPP mode.