Optomechanics, or the interaction of electromagnetic radiation with degrees of mechanical freedom, has become one of the frontiers of physics over the past decade [1]. One of the recent and more spectacular demonstrations of optomechanical physics was the detection of gravitational waves predicted by Einstein’s theory of general relativity [2]. This signal was received by the Laser Interferometer Gravitational Wave Observatory (LIGO), which is an optomechanical displacement sensor where mechanically pliant mirrors in an optical interferometer probe the gravitational wave and the optical field is used as a readout.

Other important achievements in the field include laser cooling to the ground state of [3], and observation of quantum measurement backaction on [4], a mesoscopic object. Light–matter entanglement has also been achieved in these systems [5]. In all of these, and many other demonstrations, the oscillating component is mechanically clamped to a substrate or support, thus unavoidably coupling its motion to thermal disturbances. These thermal fluctuations place limits on the optomechanical sensitivity, and introduce decoherence which is deleterious if the quantum regime is being targeted.

An effective solution to these problems is to levitate the oscillating element, removing all mechanical contact with the surrounding environment [6]. This can be accomplished using a variety of techniques, using optical tweezers, quasi-electrostatic ion traps, magneto-gravitational potentials, or acoustic fields, for example. Levitated systems can be used to replicate cavity optomechanics in conjunction with optical resonators which serve to confine electromagnetic modes. They can also be used to realize optomechanics without a cavity via interaction with freely propagating electromagnetic modes.

The prototypical experimental setup for the optical levitation involves a nanosphere confined by a focused Gaussian beam. This arrangement presents three distinct degrees of mechanical oscillation, along the three Cartesian directions in space. For small amplitudes, these oscillations are uncoupled. In their feature article, Frimmer and colleagues from ETH Zurich and Harvard University show how a suitable modulation of the optical trapping potential can couple two of these oscillators, and make the system behave analogous to a two-level atom [7]. Additional degrees of freedom can also be combined with mechanical oscillation for levitated nanospheres, and Pettit et al. from the University of Rochester and Los Alamos National Laboratory demonstrate in their article the first observation of electron spin transients, and measurement of the transverse spin coherence time, in nitrogen-vacancy centers contained within optically trapped nanodiamonds [8]. Levitated micron-scale spheres can also carry whispering gallery modes, whose resonance frequencies can be tuned using in situ temperature control, as demonstrated in the paper by Minowa and coworkers from Osaka University [9]. In contrast to optical levitation, Coppock et al. from the University of Maryland and the Laboratory for Physical Sciences have discussed the design and implementation of a quadrupole ion trap for confining graphene nanoplatelets [10]. This setup is aimed at investigating the material properties of graphene, in which there is great ongoing interest, and which are otherwise strongly affected by the substrate to which the platelet is attached.

Recent interest has also arisen in the coupling of various types of modes of mechanical freedom in levitated systems. In their feature article, Liu and colleagues from Tsinghua University and Purdue University describe how orientational and translational motions of a levitated nanoparticle in a cavity can be coupled, leading to the possibility of quantum state transfer between them [11]. Schrinski et al. from the University of Duisberg-Essen show that for an anisotropic levitated particle, orientational localization can carry signatures of a postulated modification to the Schrödinger equation that restores realism at the macroscale [12]. In his paper on radiation pressure effects on diffractive sailcraft, Swartzlander from the Rochester Institute of Technology analyzes the problem of optical levitation in outer space, where cavities, high-gradient force optical fields, and feedback mechanisms are not readily available [13].

Optomechanics based on the exchange of angular momentum between light and matter is also a currently expanding field. Arita et al., from the University of St. Andrews, Chiba University, and the University of Arizona present the dynamics of a levitated nanoparticle trapped in a complex optical potential carrying orbital angular momentum, and are able to reconstruct the potential by following the three-dimensional dynamics of the particle [14]. Bhattacharya and his team from the Rochester Institute of Technology and the Indian Institute of Technology, Kanpur investigate the effect of optical scattering torque on the torsional dynamics of a levitated nanoparticle trapped inside an optical cavity, and show theoretically how the torque can be used to tune the linear versus quadratic optomechanical coupling, which can in turn be detected by homodyning the cavity output field at the appropriate frequency [15].

We hope that this feature issue supplies a perspective on the emerging topic of levitated optomechanics, the variety of platforms being used, and the range of physical problems being addressed. We also hope that this collection of articles will lead to new and interesting ideas that will in turn advance the field. We would like to thank all the authors and reviewers for their contributions. We also thank the OSA staff for their outstanding work throughout the review and production processes.