Phonon laser with an ultra-low threshold
A scheme of an ultra-low-threshold phonon laser with coupled microtoroid resonators in vacuum. With a novel method of sample design, the coupled microtoroid system, consisting of one invert-mounted microtoroid and one ultra-thin-silicon-pillar-supported microtoroid, simultaneously achieves high optical and mechanical quality factors.
With the rapid development of cavity optomechanics, phonon lasers (as a phonon analogue to the optical lasers) have been achieved recently in several platforms such as single or coupled whispering-gallery mode microcavities. In comparison with the single-cavity phonon lasers, the coupled microcavity phonon laser can exhibit some more impressive features including lower lasing threshold and insensitivity to environmental noises. Furthermore, it does not require to significantly enlarge the dimensions of the device to create/tune the finely spaced levels, which results in a minimal effect on enhancing the optomechanical interaction strength.
In a previous work, the coupled microcavity phonon laser was demonstrated with two microtoroids placed at the edges of separate silicon chips. However, such sample preparation procedure suffers from a major difficulty in achieving a centrosymmetric pillar with a small diameter, which limits the achievable mechanical quality factor. Since it is challenging to guarantee homogeneous etching of the silicon, any inhomogeneous etching will reduce the achievable mechanical quality factor of the microtorid. Besides, the defects in the silicon pillar arising from inhomogeneous etching also severely affect the mechanical modes of the microtoroid by adding harmonic peaks to the mechanical noise spectrum.
To achieve a coupled microcavity system with both excellent optical and mechanical performances, the research group, led by Profs. Xiaoshun Jiang and Min Xiao at the National Laboratory of Solid State Microstructures, Nanjing University, recently demonstrated an ultra-low-threshold phonon laser by introducing a novel method of sample design, which enables to simultaneously achieve high optical and mechanical quality factors in the system composed of coupled microtoroid cavities. This work has just been published in Photonics Research, Vol. 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 microtoroid system consists of one invert-mounted microtoroid and one ultra-thin-silicon-pillar-supported microtoroid. The invert-mounted microtoroid was fabricated at the corner of a silicon chip while the second one was fabricated with additional XeF2 dry etching to form an ultra-thin silicon pillar. By placing this coupled microtoroid system in an ultra-high vacuum chamber, a mechanical quality factor of up to 18,000 has been obtained for a radial breathing mode at 59.2 MHz. Also, by carefully tuning the system parameters, such as the microtoroids’ relative positions and their individual temperatures, the surpermode splitting of the coupled microcavities can be made equal to the mechanical frequency of the ultra-thin-silicon-pillar-supported microtoroid. To excite the phononic lasing, the pump laser is frequency locked at the blue supermode using a wavelength meter. The measured phononic lasing threshold is as low as 1.2 µW.
Such tunable chip-based coupled-microcavity scheme may find potential applications in multimode optomechanical cooling, multimode optomechanically induced transparency, as well as addressable quantum information processing.
超低阈值声子激光
图片说明：真空环境中基于耦合微腔系统的超低阈值声子激光方案。通过利用新的样品制备和耦合的方案，该耦合微腔系统可同时具有高的光学品质因子和力学品质因子，并实现超低阈值的声子激光。
随着腔光力学的快速发展，近来声子激光（作为光学激光在声学上的一种类比）已经在一些平台（包括单个或耦合回音壁模光学微腔）中得以实现。与单腔声子激光相比，耦合微腔声子激光表现出许多突出的特点，如低阈值、对环境噪声不敏感等。此外，耦合微腔也不需要显著地增加器件的尺寸来产生或调节其精细的能级间隔，同时对光力耦合强度几乎没有影响。
之前的工作通过将微环芯腔分别制备在两片硅片的边沿上来构建可调的耦合微腔系统并实现了声子激光。然而这样的一种样品制备方案很难得到极细且中心对称的支撑硅柱，因为它很难保证硅的各向同性刻蚀。而各项异性刻蚀不但会降低微环芯腔的力学品质因子，其所导致的硅柱的缺陷还将严重影响到微环芯腔的力学模式，从而在其力学模式的噪声谱引入一些谐波峰。
为了实现同时具有光学和力学优异性能的耦合微腔系统，最近南京大学固体微结构国家实验室的姜校顺副教授和肖敏教授所领导的研究组通过利用一种新的样品制备和耦合的方案，演示了同时具有高的光学品质因子和力学品质因子的耦合微腔系统，并基于此系统实现了超低阈值的声子激光。相关研究成果发表在Photonics Research 2017年第5卷第2期上（G. Wang, et al., Demonstration of an ultra-low-threshold phonon laser with coupled microtoroid resonators in vacuum）。
在此工作中，耦合微腔系统包含一个倒置的微环芯腔和一个由极细硅柱支撑的微环芯腔。 倒置的微环芯腔被制备在硅片的角上，另一个微环芯腔则经过了一次额外的XeF2刻蚀以得到极细的硅柱。将整个耦合微腔系统放置到超高真空系统中，实验获得了力学品质因子高达18,000的径向呼吸模（频率为59.2 MHz）。通过精细地调节系统的各项参数，如微腔的相对位置、各自的温度等，可以得到一个模式劈裂等于径向呼吸模式力学频率的超模，将抽运激光通过波长计锁频在蓝失谐超模上从而激发声子激光。实验测得的声子激光阈值低至1.2 µW。
这样一个片上的耦合微腔系统在很多方面有着应用潜力，如多模腔光力学冷却、多模光力诱导透明、可寻址的量子信息处理等。