Phonon lasers, which exploit coherent amplifications of phonons, are a means to explore nonlinear phononics, image nanomaterial structures and operate phononic devices. Recently, a phonon laser governed by dispersive optomechanical coupling has been demonstrated by levitating a nanosphere in an optical tweezer. Such levitated optomechanical devices, with minimal noise in high vacuum, can allow flexible control of large-mass objects without any internal discrete energy levels. However, it is challenging to achieve phonon lasing with levitated microscale objects because optical scattering losses are much larger than at the nanoscale. Here we report a nonlinear multi-frequency phonon laser with a micro-size sphere, which is governed by dissipative coupling. The active gain provided by a Yb³⁺-doped system plays a key role. It achieves three orders of magnitude for the amplitude of the fundamental-mode phonon lasing, compared with the passive device. In addition, nonlinear mechanical harmonics can emerge spontaneously above the lasing threshold. Furthermore, we observe coherent correlations of phonons for both the fundamental mode and its harmonics. Our work drives the field of levitated optomechanics into a regime where it becomes feasible to engineer collective motional properties of typical micro-size objects.

A comprehensive dynamics analysis of microsphere has been presented in a dual-beam fiber-optic trap with transverse offset. As the offset distance between two counterpropagating beams increases, the motion type of the microsphere starts with capture, then spiral motion, then orbital rotation, and ends with escape. We analyze the transformation process and mechanism of the four motion types based on ray optics approximation. Dynamic simulations show that the existence of critical offset distances at which different motion types transform. The result is an important step toward explaining physical phenomena in a dual-beam fiber-optic trap with transverse offset, and is generally applicable to achieving controllable motions of microspheres in integrated systems, such as microfluidic systems and lab-on-a-chip systems.

Phonon lasers, as mechanical analogues of optical lasers, are unique tools for not only fundamental studies of the emerging field of phononics but also diverse applications such as deep-ocean monitoring, force sensing, and biomedical ultrasonics. Recently, nonlinear phonon-lasing effects were observed in an opto-levitated micro-sphere, i.e., the spontaneous emerging of weak signals of high-order phonon harmonics in the phonon lasing regime. However, both the strengths and the quality factors of the emerging phonon harmonics are very poor, thus severely hindering their potential applications in making and utilizing nonlinear phonon-laser devices. Here we show that, by applying a single-colour electronic injection to this levitated system, giant enhancement can be achieved for all higher-order phonon harmonics, with more than 3 orders enhanced brightness and 5 orders narrowed linewidth. Such an electronically-enhanced phonon laser is also far more stable, with frequency stability extended from a dozen of minutes to over 1 h. More importantly, higher-order phonon correlations, as an essential lasing feature, are confirmed to be enhanced by the electronic injection as well, which as far as we know, has not been reported in previous works using this technique. This work, providing much stronger and better-quality signals of coherent phonon harmonics, is a key step towards controlling and utilizing nonlinear phonon lasers for applications such as phonon frequency combs, broadband phonon sensors, and ultrasonic bio-medical diagnosis.

The orbital rotation is an important type of motion of trapped particles apart from translation and spin rotation. It could be realized by introducing a transverse offset to the dual-beam fiber-optic trap. The characteristics (e.g. rotation perimeter and frequency) of the orbital rotation have been analyzed in this article. We demonstrate the influences of offset distance, beam waist separation distance, light power, and radius of the microsphere by both experimental and numerical work. The experiment results, i.e. orbital rotation perimeter and frequency as functions of these parameters, are consistent with the theoretical model in the present work. The orbital rotation amplitude and frequency could be exactly controlled by varying these parameters. This controllable orbital rotation can be easily applied to the area where microfluidic mixing is required.

In optical traps the position of a trapped bead is usually determined by measuring the intensity distribution of the forward-scattered light and the back-scattered light. In this paper we demonstrate that this position can be determined using the side-scattered light. A quadrant photodiode is used to monitor the position of an optically trapped object in a dual-beam fiber-optic trap by measurement of intensity shifts in the back focal plane of the objective that is perpendicular to the propagating beam. An approximated model based on ray optics is presented with numerical results that describe the use of the side-scattered light for position detection. The influences of system parameters, including fiber separations, the numerical apertures (NA), and the radii of microspheres, are discussed in details. We find out that the displacement sensitivity of the detector is null for some critical radii and numerical apertures. In addition, the noises in laser powers are analyzed, and one power difference regime is proposed to weaken the influences.