Single-crystal silicon is an excellent optical and mechanical material, but its properties are compromised by the incorporation of other materials required for functionality or structural support. Here we describe a monolithic silicon acoustic sensor based on a sensing diaphragm with an integrated Photonic Crystal (PC) mirror. Diaphragm deflection is measured in a Fabry-Perot resonator formed between the PC mirror and a gold coated single-mode fiber. The sensors are fabricated on standard silicon wafers by standard CMOS processing technologies, yielding monolithic, low-stress sensing diaphragms. The packaged sensor exhibits a minimum detectable pressure of 10 μPa/Hz in the 8 kHz to 17 kHz frequency range.

Low-loss photonic crystal (PC) mirrors exhibit positive and negative Goos-Hänchen shift (GHS) due to the strong angular and wavelength dependencies of their reflected phase. This Letter demonstrates the existence of large positive and negative GHS in PC mirrors through theoretical, numerical, and experimental approaches. A simple algebraic relation shows that positive effective thickness yields positive (negative) GHS for resonances that blue (red) shift with angle, while the opposite is true for interfaces with negative effective thickness. Spatiotemporal coupled-mode theory demonstrates the above relation for simple systems with one or two resonance modes, and it also shows the existence of both positive and negative GHS. These effects are numerically and experimentally verified in complex PCs with several resonance modes.

Complex monolithic Si MEMS can be created using a single mask by extending the empty-space-in-silicon (ESS) or silicon-on-nothing (SON) technology. The fabrication combines isotropic and anisotropic etching with selective removal of passivation layers followed by hydrogen annealing. The resulting expanded design space includes multilayer structures and embedded cavities. The MEMS formation by the hydrogen annealing is simulated both at large scales and microscopic scales that together predict the shape of the finished MEMS. We demonstrate the accuracy of our process and simulations by fabricating single-and double-layer evacuated silicon voids that form Fabry-Perot optical pressure sensors with the sealed voids as pressure references. We also create multi-layered sensors with an integrated photonic crystals sensing diaphragm for improved optical readout. The sensors have a calculated 6.7 µV/Pa sensitivity and low noise over a dynamic range of over 70.9 kPa.

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Fabry-Perot cavities made with photonic crystal (PC) mirrors and other resonant structures exhibit nontraditional characteristics due to the strong wavelength dependence of their reflected phase. This Letter describes how engineering the phase of PC mirrors enables sensors that are tolerant to variations in laser center frequency and line width. Reflection spectra measurements of Fabry-Perot cavities made with PC mirrors were collected as a function of wavelength and cavity length, providing experimental verification of theory and simulations.