Abstract
Introduction
In recent years, chemical and biological sensor arrays using photonic crystals have been vigorously studied because of their potential for nanophotonic label-free environmental monitoring and biomedical detection[
With photonic band gaps, PhC can afford us complete control over light propagation by preventing light from propagating in certain direction with specific frequencies. Nevertheless, the actual fabrication of a three-dimension periodic structure remains difficult. So PhC slabs using two-dimensional periodicity combined with vertical index-guiding are proposed. By introducing line and point defects in PhC slabs, one can create waveguides[
In this paper, we present a four-channel mid-infrared silicon PhC sensor working at λ = 3.3μm, spaced by 10nm. The sensor is designed on SOI platform for standard CMOS technique. The optical characteristics of the nanocavity structure are simulated by 3D-FDTD method. The transmittance of each channel is about 39%, with non-uniformity of transmittance across channels less than 0.25 dB. The capability of the sensor is demonstrated by detecting carbon tetrachloride and benzene solutions, and shows a sensitivity of 209.2 nm/RIU.
1 Design and Optimization of Sensor
The schematic of the designed multiport in-plane sensor is depicted in
Figure 1.Schematic structure of the designed multiport in-plane sensor
The band structure for a crystal consisting of air holes in silicon (ε = 11.792) with radius r = 0.4a is plotted in
Figure 2.Band diagram for the PhC slab.
A photonic crystal sensor is composed of a cavity and two waveguides, i.e. the bus waveguide and the drop waveguide. For multi-channel operation, two 60° bend line defects are introduced. We apply a simple optimization method, which makes the bend mode as symmetric as the straight waveguide mode[
Figure 3.The energy distribution of (a) the normal bend and (b) the optimized bend.
We can design a cavity by removing or adjusting some holes around a single point. Resonant phenomena in cavities are dependent on the precise geometric properties, such as size and shape. To excite the whispering gallery mode (WGM) [
Figure 4.The energy distribution of (a) the original cavity and (b) the benzene perturbed cavity.
The six nearest-neighbor holes in each cavity are pushed 49, 55, 61 and 67nm (denoted as Si, i = 1, 2, 3 and 4) away from the cavity center to reach the resonant wavelength λ = 3.29, 3.30, 3.31 and 3.32μm, respectively. When the direct reflected light and the light decaying backwards from the cavity cancel exactly by destructive interference, the energy in the cavity decays into the two waveguides at equal rates and the transmission reaches maximum value. The transmission spectrum of the device is depicted in
Figure 5.The transmission spectrum of the bus waveguide (black line), the bend waveguide (red line) and four channels of the sensor
49 | 3290.6 | 1390.87 |
55 | 3300.02 | 1335.04 |
61 | 3311.41 | 1245.75 |
67 | 3320 | 1189.5 |
Table 1. Resonant wavelengths and quality factor versus Si
The device works as a sensor for solutions with different chemical compositions and concentrations, since the resonant wavelength is sensitive to the refractive index of surrounding environment. We choose the mixture of carbon tetrachloride and benzene as the testing solutions, with the refractive index changing from1.4607(corresponding to pure carbon tetrachloride) to 1.5012(corresponding to pure benzene). As shown in
Figure 6.(a) The shifts in the resonant wavelength of the sensor in pure CCl4 solutions, pure benzene solutions and mixture of these solutions with different concentrations, (b) The linear fits of four channels of the sensor (with center wavelength
By applying the perturbation procedure to equation (1), we obtain a formula for the frequency shift △ω that results from a small perturbation △ε of the dielectric function[
In this equation, ω and
F is a sensitivity function related tothe mode profile for the dielectric function ε, and it is a constant since the change of the mode profile is negligible in this case shown in
The previously reported PhC sensors have achieved sensitivities of 363.8 nm/RIU[
2 Potential Fabrication and Application
The fabrication of the proposed silicon PhC sensor can be realized on an SOI platform using elelctron beam lithography (EBL) and inductively-coupled plasma(ICP) dry eching which includes the following steps. First, SOI wafer with appropriate top-silicon-thickness should be chosed. Then, electron-beam resist (eg, ZEP520) is spin coated on top of an SOI wafer. Then after exposure to electron beam, the developing and fixing steps are carried out. After that, ICP etching in a gas environment of SF6 and C4F8 is used to form the air holes in PhC sensor. Finally, by removing the resist, the fabrication of PhC sensors is completed. As for the mass production in the future, EBL is replaced by ultra-violet (UV) lithography. First, a mask is fabricated according to the design parameters. Then, using UV lithography technique and the folloing ICP etching technique, thousands of PhC sensors are fabricated on an SOI wafer. Finally, slicing, encapsulation and testing of PhC sensors are carried out.
Standard CMOS process can introduce variations to hole size because of optical proximity effects. The variations in hole size can be considered as deviations from the designed lattice, affecting out-of-plane loss and the photon lifetime of the cavity. Different hole radius, ranging from 374 nm to 394 nm, are simulated to evaluate the tolerance of the sensor, and the results are listed in
374 | 3343.14 | 1 404.81 | 203.7 |
379 | 3323.79 | 1 204.23 | 209.9 |
384 | 3300.02 | 1 335.04 | 209.2 |
389 | 3280.28 | 1 220.02 | 209.9 |
394 | 3257.9 | 1 059.84 | 209.9 |
Table 2. Resonant wavelengths and sensitivity function versus R
Before the successful application to label-free environmental monitoring and biomedical detection, further work should be done. In this work, we use mixture of carbon tetrachloride and benzene solutions as an example. We need to broaden the working waveband in order to detect more kinds of chemical compounds and molecules. Furthermore, sensitivity should be raised to detect minor change of concentration. In addition, integration of sensors with sources and analyzers are necessary, which includes broadband mid-IR source, polarizer, input fiber, PhC sensor, output fiber, optical spectrum analyzer. Finally, reliability and portability should be considered in future applications.
3 Conclusion
We have proposed a four-channel SOI based 2-D photonic crystal optical sensor suitable for label-free sensing. First, our simulation shows a transmission of 60% for the linear defect waveguide (i.e. the bus waveguide) at λ = 3.3 μm. Second, we have designed cavities with hexagonally symmetric WGM field distribution reaching a quality factor of about 1.2×103. And then, the output bend waveguide is optimized to obtain 85% transmission compared to normal straight waveguide. Finally, the nonuniformity of transmittance across the four-channel device is less than 0.25 dB, and a sensitivity of 209.2 nm /RIU is presented. The sensitivity function remains stable when the hole diameter varies ±10 nm. This sensor has potential applications in nanophotonic label-free environmental monitoring and biomedical detection.
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