Titanium dioxide (TiO₂) photonic crystals can be applied to prepare guided-mode resonance (GMR) filters that allow gratings to have high reflectance at resonance wavelengths. A GMR filter is a photonic crystal resonant reflector with a perfect resonant reflection of incident light with specific polarizations in arbitrarily narrow bands while allowing the light in other wavelengths to pass through. In China, few studies have been conducted on GMR filters prepared from TiO₂ films. The current research on GMR filters mainly focuses on the design of GMR filters, such as sub-wavelength grating GMR filters with tunable resonance wavelengths, but their structures are more complex, which makes device fabrication difficult to be realized. In this work, a GMR filter is prepared by depositing a thin film of TiO₂ on a two-dimensional patterned photonic crystal grating structure as an optical waveguide layer by atomic layer deposition (ALD). The device can produce high reflectance at the resonance wavelengths. The band of resonance wavelengths is sensitive to the refractive index of the filling material in the grating region. By controlling the refractive index of the TiO₂ film, the resonance wavelength of the device can be precisely controlled, and an efficient and controllable narrow linewidth filter can be prepared.

In this present experiment, TiO₂ films are deposited on a two-dimensional patterned quartz glass by the ALD technique, and then the prepared TiO₂ films are annealed at different temperatures to have different refractive indices. Then we simulate the reflection spectrum of the filters by using the rigorous coupled wave algorithm (RCWA). In addition, we use a D8 X-ray diffractometer (XRD) from Bruker for the phase identification and structure analysis, a SIGMA 500 field emission scanning electron microscope (FESEM) from ZEISS for microstructure analysis, an ICON2-SYS atomic force microscope (AFM) from Bruker for surface roughness analysis, and a spectroscopic ellipsometer (SE 850 DUV) from Sentech for thickness and refractive index analysis of the thin film samples.

The prepared TiO₂ thin film samples show tetragonal anatase structure at the anneal temperatures of 200 ℃, 300 ℃, and 400 ℃, and no other characteristic peaks are found (Fig. 1). The grain sizes of the prepared TiO₂ thin film samples at the anneal temperatures of 200 ℃, 300 ℃, and 400 ℃ are calculated to be 380.2 ?, 390.3 ?, and 423.6 ?, respectively, by using Scherrer's formula (Table 1). The intensity of the characteristic peaks of the TiO₂ thin film samples gradually becomes higher with the increase in the annealing temperature, which indicates that the crystalline quality of the prepared samples becomes better. The FESEM and AFM photographs show that the surface of all the samples is flat and smooth, with a surface roughness of less than 0.4 nm, which indicates that the prepared thin film samples have positive denseness and flatness (Fig. 2 and Fig. 3). The thickness of the TiO₂ films is 80.5 nm, 80.7 nm, and 80.8 nm after annealed at 200 ℃, 300 ℃, and 400 ℃, respectively, and the thickness of the films is independent of the annealing temperature. By changing the refractive index of the TiO₂ film, the resonance wavelength of the device can be precisely controlled, and a high reflectance close to 100% at the resonance wavelength can be achieved for the application in high-efficiency narrow linewidth filters (Fig. 4). As the refractive index becomes larger, the shape of the reflection spectrum does not change, and the resonance wavelength is gradually red-shifted with resonance peaks of 946.9 nm, 959.4 nm, and 967.9 nm, respectively, and the position of the resonance peak changes within 22 nm, with full widths at half maximum of 0.74 nm, 0.77 nm, and 0.79 nm, respectively, and there is almost no change in the sideband reflectivity (Fig. 6). Therefore, by changing the refractive index of the waveguide layer TiO₂ film, a narrow linewidth GMR filter can be designed for the required resonance wavelength.

In this paper, high-quality TiO₂ films with different refractive indices are prepared on a two-dimensional patterned quartz glass by the ALD technique and annealed at different temperatures. The XRD and AFM test results show that the TiO₂ films have an anatase structure, and their surface roughness is less than 0.4 nm at an annealing temperature of greater than 200 ℃. Different anneal temperatures change the refractive index of TiO₂ films. The effect of the refractive index on the resonance wavelength is analyzed by the RCWA. By changing the refractive index of the TiO₂ film of the optical waveguide layer, the position of the resonant peak can be effectively controlled. As a practical application example, we design a GMR filter that can control resonance wavelength in the range of 946.9-967.9 nm, and a narrow linewidth (less than 0.8 nm) is always maintained within the scope of usage. By using this method, one can achieve precise control of the resonance wavelength peak, which is beneficial to the practical application of future devices.