Terahertz High Q One-dimensional Photonic Crystal Cavity And Sensing Applications

Terahertz technology has attracted a lot of research interest during the past decades, due to its potential applications in nondestructive sensing, imaging, spectroscopy, and communications etc. The rapid progress in the development of new terahertz generation and detection techniques has made it possible to realize many novel applications in the terahertz regime. Especially the demands for high performance devices such as terahertz filters, optically or electrically controlled switches and modulators, or high sensitive biochemical sensors are increasing. A device with high Q-factor is critical for these solutions. In this paper, we presented a terahertz one-dimensional photonic crystal cavity with high Q-factor and studied the applications of the cavity in terahertz tunable filter, optically controllable switch and gas concentration refractive index sensing. Our original work includes three parts:(1) We proposed and fabricated a terahertz one-dimensional photonic crystal cavity with high Q-factor. The cavity consists of two parallel distributed Bragg mirrors with one middle air gap between them as the defect layer. Both Bragg mirrors are made of several silicon layers inserted with air layers and the parameters of each dielectric layer have been studied and optimized theoretically. The transmission characteristic of the photonic crystal cavity is calculated using transfer matrix method and measured experimentally. By increasing the length of the defect layer or the number of silicon wafers that comprise the one-dimensional photonic crystal cavity, the Q-factor of the photonic crystal cavity can be improved. A Q-factor over 1.1×104 is achieved experimentally which is the highest value currently reported in the similar devices. Controlling the length of the defect layer using ultra-precision linear motor stage, the terahertz narrowband filter is established based on the photonic crystal cavity and the tunabe range of the filter is 299 ~ 355 GHz in the entire PBG.(2) We demonstrated an optically controllable THz switch based on our proposed one-dimensional photonic crystal structure. When the silicon wafer is optically excited, large free carriers are generated that the transmission of the terahertz wave is attenuated. In our experiment, the light beam is irradiated on one of the middle silicon wafer of the one-dimensional photonic crystal cavity easily because the length of the defect layer is relatively long. The optical beam power needed for the switch is only 0.16 W/cm2, which is nearly 50 times smaller than that for a THz switch using a single silicon wafer.(3) We presented firstly the one dimensional photonic crystal cavities with very high quality factors for gas concentration refractive index sensing at room temperature and atmospheric pressure in the terahertz regime. Benefiting from the high quality factor of the cavity, a theoretically detectable refractive index change of 1.4×10-5 and an experimental detection of 6% change of the hydrogen concentration in air can be achieved. And the performance could be at least doubled through further optimization of the photonic crystal cavity. Besides the ease of manufacturing compared to other components with similar functionality working at optical frequencies and the higher sensitivity than other reported gas sensors in terahertz regime, our device has long-term reliability and measurement repeatability.