Waveguide resonances with selectable polarization in an infrared thermal emitter

Non-dispersive infrared (NDIR) detection system has been widely applied in monitoring the hazardous and harmful substances in the human living environment.^1–4 The target molecules can be detected by measuring the transmission optical energy at specific wavelengths due to particular molecular bond vibration, which is inversely proportional to the molecular concentration in the NDIR system. Because the biomolecules usually exhibit multiple absorption peaks in the infrared regions,⁴ the infrared light source plays an indispensable role in the NDIR system.^5,6 A narrow bandwidth infrared light source with multi-band is especially important. In a conventional NDIR system, the optical filter made by multi-layer film was utilized to select the transmission of light in the absorption bands specific to the target gas. However, a traditional color filter is hard to realize multi-band transmission. A multi-narrow-band infrared light source with polarizations would be a better alternative to a broad band light source with color filters and linear polarizers. Furthermore, this approach could detect all of the absorption wavelengths associated with the target gas, thereby improving detection sensitivity. Numerous schemes have been demonstrated to create narrow bandwidth infrared light sources. Such systems rely on photonic crystals,^7–9 surface plasmonic resonance,^10–16 localized surface plasmon resonance,^17–23 and waveguide thermal emitters.²⁴ Many of the IR light sources with polarization selective characteristics have also been reported, such as one-dimensional metallic grating, metallic gutters and silicide grating.^{11,13–18,21,22,25–28} Waveguide resonant systems are particularly promising because of the narrow bandwidth characteristics. In previous studies, a narrow-bandwidth-polarized waveguide thermal emitter based on a metal/dielectric/metal (MDM) tri-layer structure with additional upper metallic gratings has been investigated.²⁹ However, only the waveguide resonant mode with polarization perpendicular to the direction of upper grating can be passed through and then emitted. In this work, by introducing a periodic metal grating inside the dielectric layer of the MDM structure, a multi-wavelength infrared waveguide thermal emitter with selectable polarization has been achieved. Similar structures had been reported earlier,³⁰ the more detailed discussion and improved analysis are proposed in this paper. It is found that the wavelength of the waveguide resonant mode is not only determined by the refractive index of dielectric materials in the resonant cavity as well as the effective cavity length,²⁴ but also depends on the geometry of the embedded Au grating structure. Multiple resonant modes with orthogonal polarizations were observed in this proposed structure. The reflection spectra, amplitude distribution of the electric field (|E_x| and |E_y|) of the resonance, and thermal emission spectra were presented in this work.

FIG. 1 presents a schematic diagram showing the proposed multi-wavelength infrared waveguide thermal emitter. The fabrication processes are described in the supplementary material. In this study, the period and the diameter of hexagonal-arranged holes on the top Au layer were maintained at a constant of 2 μm and 1 μm, respectively. The total thickness of dielectric SiO₂ layer sandwiched between top and bottom Au layers was maintained at a constant of 2250 nm. The 100 nm thick Au grating with period P and metallic line width W is embedded in the SiO₂ layer at a distance T_bottom from the bottom Au layers, as shown in FIG. 1. The embedded Au grating was laid out in the y-direction. The sample parameters were listed in Table I. A Bruker vertex70 FT-IR spectrometer was adopted to measure the reflection spectra and the thermal radiation spectra of samples. The numerical analysis was performed by using the finite-difference time-domain (FDTD) method.³¹ The relative permittivities of metal and dielectric materials are modeled by the Drude-Lorentz model.³² 

FIG. 2 displays the experimental reflection spectra in x- (blue solid curve) and y- (red solid curve) polarizations of samples A1, A2, and A3, respectively. The blue and red dashed curves represent the corresponding FDTD simulation results. In these three samples A1-A3, the metallic line width of the embedded Au grating were maintained at a constant width of 2 μm, however, the period of the grating varied from 3 μm to 5 μm. In y-polarization, the resonant wavelength in sample A1 located at 5.2 μm, which was red-shifted to 5.4 μm and 5.6 μm in samples A2 and A3, respectively. In this study, this mode is defined as the y-polarized waveguide resonant mode which is confined between the upper and underlying Au grating layer. For x-polarization, the resonant wavelength of the x-polarized waveguide mode in sample A1 located at 4.9 μm, which was also red-shifted to 5.1 μm and 5.35 μm in samples A2 and A3, respectively. The longer wavelength of x-polarized reflection dip at 7.1 μm of sample A1 is the localized surface plasmon in the gap resonant (gap-LSP) mode which was blue-shifted from 7.1 μm to 6.95 μm and 6.8 μm in samples A2 and A3, respectively. The electric field of gap-LSP mode was locally confined in the gap between embedded Au strips and the bottom Au layer.^21,30 More discussion of the gap-LSP mode is shown in FIG. S3 and discussed in the supplementary material.

FIG. 3(a) to 3(c) presents the simulated amplitude distribution of y-polarized electric field (|E_y|) of the waveguide resonant modes in samples A1 to A3. The embedded Au grating which laid in the y-direction acts as a mirror for the y-polarized light waves. The light waves that resonate back and forth along z-direction are partially blocked by the embedded Au grating. It can reach the bottom of SiO₂ cavity only in the metal-free gap region between two grating strips in x-direction. In sample A1, most of the electric fields are confined between the top Au layer and the embedded Au grating with less metal-free gap region, as shown in FIG. 3(a). When the x-directional period of Au grating was increased from 3 μm (sample A1) to 5 μm (sample A3), the waveguide resonant field, which was originally distributed in the upper Au grating layer, became leaked downward and concentrated within the metal-free gap regions, due to the wider spacing of Au strips. Thus, the red-shift wavelength of the y-polarized waveguide resonant mode in samples A1 to A3 can be attributed to the longer effective optical cavity length along the z-direction, induced by wider metal-free gap regions.

FIG. 3(d) to 3(f) presents the simulated amplitude distribution of x-polarized electric field (|E_x|) of the waveguide resonant modes in samples A1 to A3, respectively. Besides the waveguide type resonance, a localized surface plasmon resonance near the edges (edge-LSP) of each Au strips in x-polarization was induced, as shown in FIG. 3(d) to 3(f). This mode in x-polarization can be attributed to the coupling between the waveguide and the edge-LSP resonances. FIG. 4(a) and 4(b) presents the phase diagram of the y-polarized electric field (|E_y|) of waveguide mode and the x-polarized electric field (|E_x|) of edge-LSP coupled waveguide mode in sample A2, respectively. The fields of waveguide mode are all in-phase in the SiO₂ cavity in y-polarization. For x-polarization, however, the coupled fields of the waveguide and the edge-LSP modes are out-of-phase, as shown in FIG. 4(b). The out-of-phase edge-LSP resonance influences the effective optical cavity length of the coupled waveguide mode along the z-direction. The resonant wavelength of x-polarized waveguide mode is shorter than that mode in y-polarization due to the shorter effective optical cavity length in each sample. When the distance T_bottom between the embedded Au strips and the bottom Au layer were varied while maintaining the total thickness of dielectric SiO₂ layer fixed at 2250 nm, the effect on the resonant modes can be calculated. FIG. 5 presents the reflection spectra in x- and y-polarizations of samples B1, A2 and B2. Because of the effective cavity length is reduced by the upward movement of Au grating, the resonant wavelength of y-polarized waveguide mode was blue-shifts from 5.5 μm to 5.2 μm as T_bottom increases from 250 nm to 450 nm in samples B1 to B2, respectively. In x-polarization, the wavelength position of the edge-LSP coupled waveguide mode was also blue-shift from 5.25 μm to 5 μm in samples B1 to B2, respectively. The angle dependence of the reflection spectra of sample A2 is shown in FIG. S4 and discussed in the supplementary material.

The effect of different metallic line width of embedded Au grating is then discussed. FIG. 6 presents the reflection spectra in x- and y-polarizations of samples C1, A2 and C2. In y-polarization, an increase in the metallic line width of the Au grating with the same period caused a decrease in field leaking downward resonant regions. Then the effective cavity length becomes shorter which results in the resonant wavelength of y-polarized waveguide mode blue-shifts from 5.6 μm to 5.2 μm as W increases from 1 μm to 3 μm in samples C1 to C2, respectively. In contrast to y-polarization, the wavelength of edge-LSP coupled waveguide mode in x-polarization is red-shifts from 4.6 μm to 5.2 μm in sample C1 to C2, respectively. This is because the edge-LSP mode is affected by the change in metallic line width of Au strips that results in the red-shift phenomenon of the coupled waveguide mode. FIG. 7(a) to 7(c) presents the simulated amplitude distribution of x-polarized electric field (|E_x|) of the edge-LSP coupled waveguide resonant modes in samples C1, A2, and C2, respectively. The localized edge-LSP resonance refers to the Fabry–Pérot resonance of spatially confined free electrons oscillating within a metallic cavity. The resonant wavelength is proportional to the metallic line width of Au strips. In x-polarization, the edge-LSP and waveguide resonance modes are coupled to each other. Due to the mismatch of the shorter resonant wavelength in edge-LSP mode and the longer cavity length in waveguide mode, the coupling efficiency is not good in sample C1. The intensity of electric field in the edge of Au strips is much stronger than that in the SiO₂ cavity. In this case, the weak coupling resonance is dominated by the edge-LSP mode rather than waveguide mode. This resulted in a less reflective deep in the x-polarized reflection spectra of sample C1, as shown in FIG. 6. As the metallic line width of Au strips increases, the resonant wavelength of edge-LSP mode red-shifts, and the coupling fields of these two modes becomes stronger due to the closer resonant wavelength. In sample A2, the fields are laterally distributed in the SiO₂ cavity in x-direction. In sample C2, the edge-LSP mode is no longer excited due to the too longer metallic line width which is mismatch the wavelength of waveguide mode. Compare sample C1 to A2, the field is more laterally extended in the SiO₂ cavity in sample C2, as shown in FIG. 7. Thus, the red-shift of the x-polarized waveguide mode in samples C1 - C2 can be attributed to the coupling effect of the edge-LSP in different metallic line width.

FIG. 8 shows the thermal radiation spectra of samples A1 to A3. The device was heated to 200 °C by sending a direct current through the bottom Au layer. The blackbody radiation generated in dielectric layer was coupled into the resonance modes. The emission spectra satisfies Kirchhoff’s law, which states that emissivity E can be expressed as E = 1 - R, where R is the reflectivity of the object. The blue and red curves in FIG. 8 display the radiation spectra with the x- and y-polarizations, achieved by rotating the linear polarizer during the measurement of thermal emission. Emission peaks, located at 5.2 μm, 5.4 μm, and 5.6 μm represent waveguide modes in the y-polarization of samples A1 to A3, respectively. Emission peaks, located at 4.9 μm, 5.1 μm, and 5.35 μm represent waveguide modes in the x-polarization of samples A1 to A3, respectively. The emission peaks have ratios of the FWHM to the peak wavelength of 0.04 and 0.033 for the y- and x-polarizations in sample A2, respectively. The emissions clearly exhibit the characteristics of two orthogonal polarizations. The polarization characteristics appear to be dominated by the geometry of the embedded Au grating in the SiO₂ layer.

In conclusion, a multi-wavelength infrared emission with narrow bandwidth and orthogonal polarizations in a remodeled waveguide thermal emitter was investigated. The embedded Au grating within the dielectric cavity acts as a polarization divider that separates the waveguide resonance into two mutually orthogonal polarization states. The resonant wavelength of waveguide mode is determined by the effective cavity length which is affected by the embedded Au strips. Waveguide resonance in the x-polarization is due to the coupling between edge-LSP and waveguide resonant modes. The ratios of the FWHM to the peak wavelength were achieved 0.04 and 0.034 for the y- and x-polarizations, respectively. These characteristics show the suitability of the device for multi-peaks, narrow bandwidth, and polarization infrared light source applications.

See supplementary material for the fabrication processes, the detailed discussion of the gap-LSP mode in x-polarization, and the angular dependence of the device.

The authors would like to thank the Ministry of Science and Technology in Taiwan for financial support under Contract No. MOST 104-2119-M-002-017.