In this work, we present an ultra-high Q cavity at Terahertz (THz) frequencies. The designed cavity is built on a low loss suspended silicon (Si) waveguide. The substrate removal under the waveguide and the use of optimized deep reactive ion etching processing is the main reason for observing very low losses of this design α< 0.09 dB/mm. This very low loss behavior of this designed platform is also demonstrated by the measurement of a 1D photonic wire crystal cavity with Q>18000. Different cavity layouts are adjusted in order to maximize the transmittance while maintaining high Q. A design with reduced number of etched crystal holes achieve a Q>1500 and high transmittance T>70%. These structures are presented at sub-mm waves (around 600GHz) for the design of a gas sensor in this frequency region, but the principles can be scaled and redesigned for other frequencies in the THz band.
A design and multiphysical model is presented for an on-chip gas sensor that transduces terahertz gas absorption through sound generation into a mechanical motion that can be read out externally. The signal is triply enhanced by designing a structure that functions simultaneously as an optical, an acoustical and a mechanical resonator. The structure is made in high-resistivity silicon and can be fabricated using CMOS and MEMS fabrication technologies. The sensor is a purely passive element, so an external THz source and read-out are required. The chip has a footprint of 3 mm 2. A detection limit of 234 ppb of methanol for a source power of 1 mW and an integration time of 1 ms is predicted.
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