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.