With the rapid development of cavity photodynamics, the recent development of phonon lasers, which are acoustically analogous to optical lasers, has been achieved in some platforms, including single or coupled whispering gallery mode optical microcavities. Compared with the single-cavity phonon laser, the coupling micro-cavity phonon laser exhibits many outstanding features, such as low threshold and is not sensitive to environmental noise. In addition, the coupling microcavity does not need to significantly increase the size of the device to create or adjust its fine energy level spacing, with little effect on the strength of the optical coupling.
Previous work built the tunable coupling microcavity system and realized the phonon laser by preparing the microring core separately on the edge of two wafers. However, such a sample preparation scheme is very difficult to get a very thin and centrosymmetric support silicon column, because it is difficult to ensure isotropic silicon etching. The anisotropic etching will not only reduce the mechanical quality factor of the microcirculation core cavity, but also cause the defects of the silicon pillar will seriously affect the mechanical mode of the microcirculation core cavity, so as to introduce some harmonic in the noise spectrum of the mechanical mode crest.
In order to achieve a coupled microcavity system with excellent optical and mechanical properties, the group led by Professor Jiang Jishun and Xiao Min recently at the National Laboratory of Solid State Microstructures, Nanjing University, through the use of a new sample preparation and coupling scheme , Demonstrates a coupled microcavity system with both high optical quality factor and mechanical quality factor, and based on which a very low threshold phonon laser is realized. Relevant research results have been published in Photonics Research, Volume 5, Issue 2, 2017 (G. Wang, et al., Demonstration of an ultra-low-threshold phonon laser with coupled microtoroid resonators in vacuum).
In this work, the coupled microcavity system consists of an inverted microcirculation core cavity and a microring core cavity supported by an ultra-fine silicon pillar. The inverted microcirculation core was fabricated at the corners of the wafer, and the other microcentrifuge core cavity was subjected to an additional XeF2 etch to obtain a very thin silicon column. The entire coupled microcavity system was placed in an ultra-high vacuum system and experimental radial breathing modes (frequency 59.2 MHz) were obtained with a mechanical quality factor of up to 18,000. By finely adjusting the parameters of the system, such as the relative position of the microcavities, their respective temperatures, etc., a mode split can be obtained which is equal to the super-mode of the radial breathing mode mechanical frequency. The pumping laser is locked in the blue Off-tune supermodel to stimulate phonon laser. Experimental phonon laser threshold as low as 1.2? W.
Such an on-chip, coupled microcavity system has potential applications in many areas, such as multi-cavity photomechanical cooling, multimode optical power-induced transparency, addressable quantum information processing and the like.
Photo Caption: Ultra-low threshold phonon laser scheme based on coupled microcavity system in vacuum environment. By utilizing new approaches for sample preparation and coupling, the coupled microcavity system can have both high optical and mechanical quality factors and ultra-low threshold phonon lasers.
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