Ultra-Compact On-Chip Photonic Crystal Interferometers
We have utilized the unique dispersive properties of photonic crystals (PCs) to make functional devices.. One of these novel devices that we have proposed is the “two-wave PC interferometer” shown in Fig. 1. It consists of a 45°-rotated square lattice slab PC fabricated by etching holes in silicon on SOI wafers. The two output interfaces are aligned with the holes at 45 degrees and provide the refracted waves that will interfere on the detection plane, which consists of an array of detectors.
Figure
1. (a) The structure of the two-wave PC-based interferometer. The incident lightwave excites the appropriate mode inside the PC region, the propagating wave refracts at the output interfaces and the resulting output refracted waves interfere on the detection plane. The length of the structure is L, the width is d, the PC lattice constant is a, and the radius of the holes is r. (b) The SEM image of the structure.
The proposed interferometer is very compact (40 microns x 70 microns), and can be easily integrated with other photonic devices on a chip. By properly engineering the dispersion of the PC through appropriate design of its dimensions (radius of holes and lattice constant), ultra-high sensitivities can be achieved, i.e. the output interference pattern changes dramatically with a slight change of input wavelength. Therefore, it can be used as a micro-spectrometer. Also, it can be used as a highly sensitive sensor, where one arm of the interferometer is exposed to the materials of interest and the relative phase change between the two arms translates as a change of the interference pattern at the output.
In many resonance-based sensing scenarios, it is necessary to keep track of a high-Q resonance feature in response to the change of analyte, which requires a precise spectrometer. We have optimized our design for the micro-spectrometer application over a range of 30nm bandwidth. We have calibrated the device by using a precise tunable laser. Then we have tested the accuracy of the proposed spectrometer in finding an unknown spectral single resonance feature. The result is shown in Fig. 2 as the estimation error in the spectral location of the input resonance wavelength versus the noise level ratio at the detection plane. It can be seen that the location of the resonance feature can be determined with an accuracy better than 15pm at a noise level ratio of 0.1.
Figure
2. (a) On-chip LSPR sensing using the hybrid plasmonic-photonic waveguide structure. (b) Resonance shift due to the introduction of an analyte. (c) Sensitivity measurement results.