# Guided Mode Resonance in a Low-Index Waveguide Layer
## 3 Experiment and Discussion  
To characterize the guided mode resonance experimentally, the present structures were fabricated on a Lithium Niobate (LN) substrate (\(n_{s}\approx\) 2.2-2.3). Electro-beam lithography was used to define the patterns on the substrate. Metal deposition and liftoff processes produced the metal gratings. After the grating fabrication, SUS (\(n=1.6\)) was spin-coated on the gratings with different thicknesses to work as the waveguide layer. Reactive Ion Etching (RIE) was used to etch the SU8 layer to obtain various thicknesses. Figure 1b shows the SEM picture of the fabricated structures before coating. The duty cycle of the fabricated grating is ~0.4 and the thickness of the metal grating array is 30 nm for all samples. We use the optical microscope (Olympus, BX53M) equipped with a spectrometer to characterize the performance of the guided mode resonance. As an example, we measure the transmittance spectra to confirm the existence of the resonance rather than reflectance due to the large field aperture of the microscope which requires a large sample size. The objective with NA = 0.1 was used for light collection. A linear polarizer was inserted between the light source and the sample to control the polarization of the incident light. All the spectra are calibrated with a blank LN substrate. Figure 5 shows the experiment transmittance spectra for TE polarization. In Figure 5a, the thickness of SU8 \(t\) = 205 nm. The periods of the gratings vary from 400 nm to 600 nm. Pronounced resonance dips can be observed in these spectra and these increase with the increase of the period. The simulated spectra for this device are also plotted in the figure with the dashed line. Good agreement can be seen from these spectra which indicates the presence of the guided mode resonances. The small divergence between simulation and measurement is due to two factors. Imperfections, such as roughness and inclusion on the metal gratings, can cause extra absorption which will introduce extra loss. Another factor is the small incident angle in the experiment. Both effects will broaden the resonance and decrease the quality factor (see Figure 4b). Figure 5b shows the transmittance of this device with a thickness of SU8 \(t\) = 172 nm. Compared with Figure 5a, a blueshift of the resonance exists as expected. Like Figure 5a, we simulate the transmittance spectra for this thickness. As can be seen from simulated results, the simulated spectra fit well with the experiment results. We also measured and simulated the spectra of the gratings with no SU8 waveguide layer. As shown in Figure 5c, no obvious resonance occurs. This implies that the resonance is surely caused by the guided mode resonance in the SU8 waveguide layer. As shown by simulation and experiment, we demonstrate that the GMR can exist in the low-index waveguide layer made of SU8 polymer. Pronounced resonance dips occur in the transmittance spectra due to the GMR effect and all these dips follow the behavior  
Figure 5: (**a**) The measured (solid line) and simulated (dash line) transmittance spectra with various periods (\(P\) = 400 nm, \(P\) = 500 nm, and \(P\) = 600 nm) with SU8 as the waveguide layer for TE polarization. The thickness of SU8 \(t\) = 205 nm. (**b**) The measured (solid line) and simulated (dash line) transmittance spectra with various periods (\(P\) = 400 nm, \(P\) = 500 nm and \(P\) = 600 nm) with SU8 as the waveguide layer for TE polarization. The thickness of SU8 \(t\) = 172 nm. (**c**) The measured (solid line) and simulated (dash line) transmittance with various periods (\(P\) = 400 nm, \(P\) = 500 nm and \(P\) = 600 nm) with no SU8 for TE polarization. In all simulations, the duty cycle is set as 0.4. \(n_{s}=2.2\) and \(n\) = 1.6. \(t\) is set as 200 nm and 170 nm. as predicted. These results reveal that, with the help of metal gratings, any dielectric could be used as the core waveguide for GMR regardless of the refractive index of the substrate.