The optical measurement and control of mechanical vibration are two of the main cores of many technological and basic advancements in physics and engineering. The realization of the nanoscale cavity optical mechanical system can localize the light at the subwavelength volume scale, thereby enhancing the strong optical coupling between the photons and phonons of the nano-mechanical structure. This coupling can be used to achieve a very wide range of applications, such as highly sensitive force and displacement measurement, microcavity cooling, biosensing, optical force induced transparency, etc. Lithium niobate (LN) crystal has became one of the most widely used optical materials due to its superior nonlinear optical properties. With the widespread use and maturity of semiconductor manufacturing processes on commercial optical-grade lithium niobate films, the mechanical, thermal, and acoustic properties of this material have also attracted more and more research interests.

We fabricated a 40 um-long one-dimensional lithium niobate photonic crystal nanocavity on a 300 nm thick lithium niobate thin film, and studied its optical coupling characteristics. In the experiment, with the increase of optical power in the lithium niobate photonic crystal nanocavity, we observed the frequency shift of the mechanical vibration and the high-order nonlinear mechanical oscillation. When the power in the cavity is 430 uW, we observed the highest 14th order mechanical resonance. In addition, a strong positive temperature dependence is achieved around room temperature, which is significantly different from the negative intrinsic temperature coefficient of lithium niobate crystals. The LN photonic crystal nanocavity turns out to be a good platform for the potential sensitive displacement sensing, the temperature-to-mechanical sensing, or the nonlinear mechanical oscillation generator, which may function as a mechanical frequency comb.

FIG. 1. (a) Scanning electron micrograph (SEM) of the photonic crystal nanocavity. Inset: two unit cells of the device with dimensions of w = 750 nm and lattice constant a = 545 nm. (b) Displacement fields of dispersive coupling of the device modes. (c) Changes of dispersive coupling of the resonance line shape, respectively. (d) Amplitude of the optomechanical signal almost goes to zero at peak point.

FIG. 2. Frequency spectrum obtained from the real-time spectrum analyzer, with the frequency range of 0–40 MHz. The intracavity powers of blue, red, orange, and purple curves are 0 uW, 43 uW, 136 uW, and 430 uW, respectively. The inset upper figure shows that the more detailed high harmonic optomechanical oscillations observed in the LN device. The bottom inset shows the mechanical displacement profile of the labeled mode (27.2 MHz), simulated by the finite element method.

This work was done by Prof. Yuping Chen, Prof. Xianfeng Chen, and Prof. Qiang Lin from University of Rochester in collaboration. The result was published in “Haowei Jiang, Xiongshuo Yan, Hanxiao Liang, Rui Luo, Xianfeng Chen, Yuping Chen* and Qiang Lin*, High harmonic optomechanical oscillations in the lithium niobate photonic crystal nanocavity, Appl. Phys. Lett. 117, 081102 (2020)”.

Link：https://doi.org/10.1063/5.0016334