Optimization Of Reflectances In Grating Structure

Published Date: 02 Nov 2017

In this paper, we have investigated the spectral response of single and multilayer grating silicon-on-insulator (SOI) using plane wave expansion method (PWE). The simulation results for reflectance and transmittance of single and multilayer SOI structures are compared with experimental and existing theoretical results in literature. The single layer grating SOI structure is compared with experimental result, while multilayer grating SOI is compared with theoretical result, which is done using Rigorous coupled wave analysis method (RCWA). It is found that our simulation result is better agreement with experimental result for single layer grating SOI structure and almost matches with theoretical result for multilayer SOI grating structure. Finally a new type of grating SOI structure is proposed and optimized for its high transmittance and low reflectance values using PWE method.

Introduction:

Silicon on insulator (SOI) has been significant interest in early 1990’s [1]. In recent years, the race for faster optical communication and data processing has motivated research towards optoelectronic devices with more functional elements directly interconnect on a single chip truly integrated circuits[2].SOI is much more attractive with very mature micro-electrochemical and electrical properties in an uniform manner and with readily available and cost effective mass manufacture facilities[3]. Grating on SOI also plays major role in the recent explosion of photonic crystal structure[4]. Basically grating is made of metal film and dielectric multilayer stacks[5,6]. To control the reflectance and transmittance of grating structure one has to raise the issue of absorption of layer and changing of refractive index and thickness of the layers.

In this paper we have computed the reflectance and transmittance for two different types of grating SOI structure in TE polarization i.e. single layer and multilayer grating structure. Basically we have optimized the above grating structure to find out high reflectance or low transmittance using plane wave expansion method (PWE). Then we have made a comparison between our computed results with experimental result as well as theoretical result[7]. Apart from these, we have proposed a novel type of grating structure, which gives high transmittance and low reflectance at the wavelength of 1550 nm and 1310 nm.

Structure Analysis:

For optimizing high reflectance in TE polarization, we place two competitive SOI structure, which are shown in Figure 1 and 2.

Figure 1 ; Single layer grating SOI structure

Figure 2; Multi-layer grating SOI structure

Figure 1 represents single layer grating is placed on SOI structure. The refractive indices of 1st and 2nd layers are 3.48 and 1.00 respectively. Besides this, thickness of grating layer is 329 nm and 671 nm respectively. Again the refractive index of substrate is taken of 1.46.

The above dimensions of the grating structures ( Figure 1) are chosen as intentionally to compare with experimental as well as theoretical result [7].

Similarly figure 2 represents multilayer’s (SiO2-TiO2) grating placed on SOI structure. It has 21 alternate silicon dioxide (SiO2) and titanium dioxide (TiO2) grating layers. The refractive index SiO2 (nSiO2) and TiO2(nTiO2) are 1.44 and 2.43 respectively. Apart from this the refractive index of substrate is taken of 3.46. The thicknesses of grating layers are 303.8 nm and 180 nm respectively.

The above dimensions of the grating structures( Figure 2) are chosen an intentionally to compare with theoretical result [7].

In reference [8], Ricciardi et al. have investigated the wavelength resonant grating in contrast consists of single layer only. This work is done in NIR region, which is not suitable for optical communication. It is also not suitable for silicon based devices because in NIR, silicon based devices are strongly affected by materials absorption losses. But in reference [9], the authors, Chebens et al. investigated the effect of finite grating size on the reflectivity and present the optical transmission measurements on SOI structure. The authors have made simulation using Rigorous coupled wave analysis (RCWA) method.

3. Result and Interpretation:

The simulation in this paper is done using PWE method found elsewhere [10]. Using this method, we have calculated reflectance and transmittance in TE polarization for single layer grating SOI structure as shown in figure 1. This model structure consists of a single layer of silicon and air. The refractive indices of the grating layers are 3.48 and 1.00 of silicon and air respectively. Also the refractive index of the substrate is taken of 1.46. The thicknesses are 329 nm and 671 nm of first and second layer respectively. The parameters are consistent with design rules discussed in theoretically and experimentally. Taking above parameters and using PWE method the simulation results for reflectance and transmittance with respect to wavelengths is shown in Figure 3.

Figure 3 . Spectral response diagram single layer grating SOI structure with variation of wavelength

Figure 3, represents the spectral response (reflectance and transmittance) for the zeroth order normal incidence. It is seen from the above figure that, the reflectance is slightly more than 0.2 at the wavelength of 1400 nm. Further it increases with the increase of wavelength and then decreases. The above result is also good agreement with experimental result. The simulation is also done by K.J. Lee et. al. for same structure using RCWA method. But our simulation is closely vicinity with experimental result [7].

Apart from this, we have used PWE method in TE polarization to calculate the reflectance and transmittance of multilayer grating SOI structure, figure 2. The model structure consists of SiO2 and TiO2 alternate, 21 grating layers. To find out the reflectance and transmittances of this structure, the refractive indices and thickness are considered as model input parameters. The refractive index SiO2 (nSiO2) and TiO2(nTiO2) are 1.44 and 2.43 respectively. Apart from this the refractive index of substrate is taken of 3.46. Here the thicknesses of grating layers are 303.8 nm and 180 nm respectively. The parameters are consistent with design rules discussed in theoretically [7]. For a plane wave with a free space having a wavelength incident parallel to the grating, figure 2. The result for the reflectance and transmittance of the grating structure with respect to wavelength is shown in Figure 4.

Figure 4; Spectral response diagram of multilayer grating SOI structure with varying wavelength

Figure 4 represents the calculated spectral response (reflectance and transmittance) of multilayer grating SOI structure. From this figure, is found that the high reflectance and low transmittance is found between 1450 nm to 2160 nm wavelength range. This simulation result, which is done using PWE method is good agreement with the simulation work is done by K.J.et. al. using RCWA technique [7].

4. Proposed grating structure:

In similar to single layer grating structure, figure 1 , here we have chosen different materials of the silicon grating layers having refractive indies of 1.46 and 1.48. Considering different combinations of refractive indices (1.46-1.48 and 1.48-1.46), we have proposed to design two types of silicon grating structure. The thicknesses of first and second layers are taken of 890 nm and 860 nm for both structures. The schematic diagram of proposed grating SOI structure is shown in figure 5.

Figure 5. Proposed grating SOI structure

5. Result analysis of proposed grating structure:

We have optimized the proposed grating structure as shown in figure 5 by PWE method. Choosing refractive indices of 1.46 - 1.48 for first - second layers of silicon grating structure, the simulation result is shown in Figure 6.

Figure 6, simulation result for grating structure having refractive indices of 1.46 and 1.48. The thicknesses of grating layers are 890 nm and 860 nm

Form figure 6 , it is seen that the reflectance is less than 0.01% and transmittance is more than 99.99% at the wavelength of 1550 nm. Similarly reflectance is less than 1.2% and more than 98.80% at the wavelength of 1310 nm.

Similarly considering other combinations of refractive indices of grating layers i.e. 1.48-1.46 for first-second layers, the simulation is done and the result is shown in Figure 7.

Figure 7, simulation result for grating structure having refractive indices of 1.48 and 1.46. The thicknesses of grating layers are 890 nm and 860 nm

Again it is seen from figure 7 that the reflectance is less than 0.01% and transmittance is more than 99.99% at the wavelength of 1550 nm. Similarly reflectance is less than 0.03% and more than 99.70% at the wavelength of 1310 nm.

6. Conclusion:

The spectral response using PWE method for single layer grating SOI and multi layer grating SOI structure is thoroughly investigated. Although the simulation results using RCWA method is suitable technique to find out the reflectance and transmittance of SOI grating structure, PWE method also gives computation results, which is better agreement with experiments. So PWE technique is suitable to find out the reflectance and transmittance of single and multi layer grating SOI structure. Besides these, the proposed silicon grating structures having different combinations of refractive indices (1.46 - 1.48 and 1.78-1.46) , thickness of 890 nm and 860 nm give high transmittance at the wavelength of 1550 nm and 1310 nm.