A two-dimensional (2D) grating guided-mode resonance (GMR) tunable filter is experimentally demonstrated using a low-cost two-step nanoimprinting technology with a one-dimensional (1D) grating polydimethylsiloxane mold. For the first nanoimprinting, we precisely control the UV LED irradiation dosage and demold the device when the UV glue is partially cured and the 1D grating mold is then rotated by three different angles, 30°, 60°, and 90°, for the second nanoimprinting to obtain 2D grating structures with different crossing angles. A high-refractive-index film ZnO is then coated on the surface of the grating structure to form the GMR filter devices. The simulation and experimental results demonstrate that the passband central wavelength of the filter can be tuned by rotating the device to change azimuth angle of the incident light. We compare these three 2D GMR filters with differential crossing angles and find that the filter device with a crossing angle of 60° exhibits the best performance. The tunable range of its central wavelength is 668−742 nm when the azimuth angle varies from 30° to 90°.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The structure of a guided-mode resonance (GMR) filter comprises a high-refractive-index waveguide layer and a grating structure. The incident light can be coupled into the waveguide layer by the grating structure. When light propagates inside the grating waveguide, the modulation period of the grating will lead to a leakage of the wave energy. When the leakage wave constructively interferes upon reflecting, this is the so-called GMR phenomenon and can generate a reflective passband filtering response . Based on this resonance principle, the central wavelength of the GMR filter can be tuned by altering the angle of incidence . This method has been used to implement tunable color filters . The tunable GMR filters which are sometimes called photonic crystal slabs also have been used for sensing applications . For a reflective-type filter, when the angle of incidence is altered, the position of the reflective light spot will also change. This may cause inconvenience when perform alignment in some optical systems. In this paper, we propose a two-dimensional (2D) grating GMR tunable filter whose passband central wavelength can be tuned by rotating the device to change azimuth angle, rather than altering the angle of incidence. The properties of 2D GMR filters have been theoretically studied and reported [5–7]. Recently, some implementations of 2D GMR filters have also been reported [8, 9]. In these previous 2D GMR filter studies, the unit cell was circular or square-shaped, and E-beam lithography was used in the fabrication process. Another 2D GMR filter is implemented by using atomic layer deposition to improve the optical functionality and its passband wavelength could be tuned by altering the angle of incidence as in the 1D grating cases .
In this paper, we propose a two-step nanoimprinting technology to fabricate three different unit-cell shapes of the 2D grating structure and three kinds of GMR filter devices using these three 2D structures are compared. In our previous work, we used this similar process to precisely control the grating modulation height in the 1D grating GMR filter to achieve a narrower bandwidth . Here, this process is further applied to implement the 2D grating structure on a GMR filter. Theoretically, when the GMR filter devices are rotated to change the azimuth angle of the incident light, the transverse electric (TE) and transverse magnetic (TM) polarization components of the incident light will experience different grating pitches, and the resonance reflection is based on the superposition of these two components. The effect of the varying polarization angle was studied in a 1D grating GMR filter, and this effect can be used to implement a polarization-controlled tunable filter . In this tunable 1D grating GMR filter, when the light is normal incident, the passband wavelength can be tuned by changing the polarization of the incident light . Recently, a tunable 1D grating GMR filter has been reported by R. Yukino et. al., they proposed the passband wavelength can be tuned by azimuthally rotating the GMR filter device when the light is oblique incidence . Here, we extend this concept to 2D grating cases, because 2D grating GMR devices have more degree of freedom in the design of the device structures . A 2D grating resonance structures has been reported to have broad angular tolerance at oblique incidence . Here, we demonstrate three different 2D grating structures by using a two-step nanoimprinting process and compare their performances on tuning passband wavelength by rotating the azimuth angle.
2. Two-step nanoimprinting process and the 2D GMR device
The two-step nanoimprinting fabrication process for implementing the 2D grating structure is illustrated in Fig. 1. First, UV glue (GN969-62, Everwide, Taiwan) is dropped on the grating surface of the polydimethylsiloxane (PDMS) mold with the 1D grating structure replicated from a commercially available holographic grating with 1800 lines/mm, and a glass substrate is covered on the top. It is then placed under a 365-nm UV LED lamp at 3W for 3.5 seconds to partially cure the UV glue. After that, we demold and rotate the PDMS mold by three different angles, 30°, 60°, and 90° for the second nanoimprinting. In the second imprinting, a pressure of 0.25 kgf/cm2 is applied on the mold. Finally, the grating is completely cured by UV lamp and is demoded. A homemade nanoimprinting machine adapted from a commercial hot pressing machine (Yiulih Machinery Co., Ltd., Taiwan) is shown in Fig. 2. It is equipped with a UV LED and a pressure sensor system adapted from a common electronic scale with a maximum measurement mass of 3 kg and resolution of 1 × 10−3 kg. After completing the 2D grating structures, a high-refractive-index ZnO film with a thickness of 250 nm is coated on the grating surface by a sputtering machine.
Only the schematic of the GMR filter device with the PDMS mold rotation angle of 90° in the second imprinting (90°-crossing structure) and cross section view of the xz-plane as the coordinate defined in the figure are shown in Fig. 3, as it is simple and easy to understand. Its shape is similar to a four-sided cone. The symbols in the figure are defined as follows: Λ is the pitch, d is the ZnO film thickness, θ is the angle of incidence, ϕ is the azimuth angle, and h is the modulation height. The refractive indexes used in our simulation are as follows: nair = 1.0 (air), nZnO = 2.0 (ZnO), ng = 1.475 (UV glue), and nsub = 1.515 (glass substrate). Because the light is incident on the top (air side), not the bottom (glass substrate side), of the grating waveguide structure on the GMR filter device, the effects of the glass substrate and the distance from the grating to the substrate have weak effects on the characteristic of the filter device. It shows that when a wave propagates inside the waveguide in different directions, it will experience different grating pitches. In this schematic, because the pitch of the original PDMS mold is 555 nm (1800 lines/mm), and the pitch along the x-axis will be 784.8 nm (ϕ = 0°) and that along the 45° direction with respect to the x-axis will be 555 nm (ϕ = 45°). AFM images of the devices with three different crossing angles, 30°, 60°, and 90°, are shown in Figs. 4(a)-4(c), respectively. The profile curves along the two scanning lines, as indicated in their corresponding AFM image (while solid line), are shown on the right of each AFM image. It can be seen that the pitches of the two scanning lines are different for both the 30°- and 60°-crossing images but are the similar for 90°-crossing image. We also find that the average grating depths for these three AFM images are different; their approximate values for 30°, 60°, and 90°-crossing structures are 80, 70, and 60 nm, respectively. Finally, a high refractive index ZnO film was deposited on the surface of the devices by a sputter and the film thickness was chosen to be 250 nm. Thought, the resonance wavelength of the GMR filter can be affected by the film thickness, we still can vary the incident angle to select the passband wavelength. Besides, the reflectivity of the GMR filter device also can be affected by the film thickness. According to our simulation result, a 1D grating GMR filter device with a minimum ZnO film thickness of 250 nm can achieve a reflectivity of better than 90%. Therefore, we choose this thickness to save the coating time.
3. Measurement results
To define the azimuth angles for these three GMR filter devices, three-dimensional and 2D (top view) schematics of these three device structures are illustrated in Fig. 5. The rhombus as indicated in each 2D schematic (red dash line) is basis of the four-side cone structures of its 3D schematic. The shorter diagonal direction of the rhombus is defined as the x-axis of the coordinate for each 2D schematic. The azimuth angle of each GMR filter device can be obtained as illustrated in Fig. 3. Their ideal and measured pitches of the scanning lines along different azimuth angles, 0°, 15°, 30°, 45°, and 90° are compared and listed in Table 1. The smallest and largest pitches of the 30°-crossing GMR device are found to be near the azimuth angles of 15° and 90°. If the input light is horizontally polarized, when this GMR filter device is rotated near 15°and 90°, their corresponding reflective resonance wavelength will be the shortest and the longest, respectively. Those of the 60°-crossing GMR device are found to be near 30°, and 90°, while those of the 90°-crossing GMR device are 45° and 90°. Therefore, the reflective resonance wavelength (passband) can be tuned by rotating the GMR filter devices to change the azimuth angle.
A reflective resonance wavelength measurement system is illustrated in Fig. 6. We use a halogen lamp as the light source and an aperture to control the size of the light spot to be small. The output light from the lamp is collimated by lenses and a polarizer is used to generate horizontally polarized light (p-wave). We restrict this filter to be used under the incident p-wave (or TM-polarization) light for simplicity because the TE-polarization light will produce more resonance peaks as that in the general GMR filters. The incident light with a fixed angle of 32° illuminates the GMR filter device, and the reflective light is collected by a spectrometer (DK240 1/4 meter).
In our experiment, each GMR filter device is rotated to change the azimuth angle from 0° to 90° with a step of 10°. The measurement and simulation results of the three devices with different crossing angles, 30°, 60°, and 90° are compared and presented in Figs. 7(a)-7(c), respectively. The simulation results are obtained by a finite-difference time-domain (FDTD) solver. The measured shortest and longest resonance wavelengths obtained by changing the azimuth angle are matched to the prediction in the previous paragraph. The reason why the device does not have 100% reflection efficiency is because there are some incident light energy contribute to high order diffraction modes which are not shown in the results. For the experimental results, the wavelength tuning ranges are 680−865, 665−742, and 645−688 nm, for the 30°-, 60°-, and 90°-crossing structures, respectively. There are some discrepancies between the measurement and simulation results. For the 30°- and 60°-crossing structure, the measured results exhibit a higher efficiency than the simulation results for the shorter wavelength range. The average efficiency of the 60°-crossing structure is higher than that of the 30°-crossing structure in both the measurement and simulation results. For the 90°-crossing structure, because of symmetry, there are only five resonance wavelengths found in simulation results when the azimuth angle is rotated from 0° to 90° with a step of 10°. For example, 0° (40°) and 90° (50°) have the same resonance wavelength of 683 (637) nm. However, in the experimental results, 0° (30°) and 90° (60°) have not the same resonance wavelength and efficiency. The same results are also observed for other azimuth angles. These discrepancies may be attributed to the structure difference between the real and ideal (simulation) appearances.
Our experimental 2D grating structures are implemented by the two-step nanoimprinting process, and the curing status of UV-glue are different during the two-step nanoimprinting process and this may cause the 2D grating structure is not symmetry as the structure in the simulation. Besides, because the devices were rotated by hand in our fabrication process, the crossing angle may also be not so accurate and it is possible to have an angle error. For the 90°-crossing structure, the resonance wavelengths of two azimuth angles, 0° and 90°, should be the same. However, we obtained different responses in our experiment results. This discrepancy may be improved by using a precision rotation stage.Among these devices, the GMR filter with a 60°-crossing structure performs the best if we consider the efficiency and the tuning range. Summarized plots of the resonance wavelengths in the measurement and simulation results for the device with 60°-crossing structure are presented in Fig. 8. They agree well with each other.
In this study, we successfully demonstrate that the passband of a 2D reflective grating GMR filter can be tuned by rotating the device rather than changing the angle of incidence. This tuning method will not change the reflective light spot position. The principle of the tunable GMR filter is based on the change in equivalent pitch as the device is rotated while the input polarization is fixed. We implement this concept by using a low-cost two-step nanoimprinting technology using a 1D PDMS mold. The measurement results agree well with and simulation results. Our experimental results show that 60°-crossing structure can achieve tuning ranges of 665-742 nm. Though, in the primary results, the efficiency of our 2D GMR filter is not good enough. The 2D structure can be further studied to improve the efficiency and more precise fabrication method can achieve a better structure uniformity to improve filter quality. This tunable 2D GMR filter offers a convenience and low-cost approach to alter the passband wavelength of a reflective-type filter and may be a useful tool for some optical systems.
Ministry of Science and Technology (MST) of Taiwan (104-2221-E-150-064-MY3).
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