Enhancement of rotational vacuum friction by surface photon tunneling

Abstract

When a neutral sphere is rotating near a surface in vacuum, it will experience a frictional torque due to quantum and thermal electromagnetic fluctuations. Such vacuum friction has attracted many interests but has been too weak to be observed. Here we investigate the vacuum frictional torque on a barium strontium titanate (BST) nanosphere near a BST surface. BST is a perovskite ferroelectric ceramic that can have large dielectric responses at GHz frequencies. At resonant rotating frequencies, the mechanical energy of motion can be converted to electromagnetic energy through resonant photon tunneling, leading to a large enhancement of the vacuum friction. The calculated vacuum frictional torques at resonances at sub-GHz and GHz frequencies are several orders larger than the minimum torque measured by an optically levitated nanorotor recently, and are thus promising to be observed experimentally. Moreover, we calculate the vacuum friction on a rotating sphere near a layered surface for the first time. By optimizing the thickness of the thin-film coating, the frictional torque can be further enhanced by several times.

Nonequilibrium Casimir effects of nonreciprocal surface waves

“…We note that the nonreciprocity-induced nonequilibrium torque was recently analyzed for a single gyrotropic particle [27] and for two finite-size gyrotropic particles separated by small distance [28]. The magnitude of torque in our work is few orders of magnitude larger than the values reported in the previous studies due to the large density of surface polariton states in the near field of a gyrotropic susbtrate [48]. We also note that the magnitudes of the lateral nonequilibrium Casimir force and torque can be further enhanced using Weyl semimetals [49] which provide much stronger gyrotropy compared to InSb considered here.…”

Thermal equilibrium spin torque: Near-field radiative angular momentum transfer in magneto-optical media

“…At global thermal equilibrium, the torques on two cubes have the same magnitude but opposite signs, and the net torque on the combined system is zero, satisfying the detailed balance of AM exchange between the combined system and the environment at equilibrium. Some recent works predicted that the torques can be induced by vacuum friction of a rotating object made of reciprocal isotropic media [57][58][59][60]. These predictions are of good interest in the context of nanophotonics [16,[61][62][63][64][65][66] and Casimir physics [14,67,68], as they are fundamentally originated from quantum and thermal fluctuations.…”

“…However, such rotational vacuum frictions still remain at the theoretical level and, so far, no experimental observation has been done due to the extremely small magnitudes of the torques which are far below the sitivity that can be achieved by the torque sensor [69]. Many theoretical works tried to enhance the vacuum friction to make it more measurable than a single rotating particle [57], such as using surface plasmon resonance [58] and surface photon tunneling [60]. Recently, an experimental demonstration of the most sensitive torque measurement has been done with levitated nanoparticle, improving the torque sensitivity by a few orders [63] and showing the feasibility for detecting the rotational vacuum friction near a surface.…”

On-chip optical levitation with a metalens in vacuum

“…A levitated dielectric particle in vacuum is one of the most promising systems for studying macroscopic quantum effects since it is well isolated from the thermal environment. Several schemes have been proposed for utilizing optical levitation systems to study Casimir physics [13], quantum aspects of gravity [14], and search for dark matter and dark energy [15]. Typically, optical trapping of a nanoparticle in vacuum uses a bulky objective lens with a large numerical aperture (NA) to tightly focus a laser beam [3,6,7].…”

“…Metalens levitation in vacuum can open more opportunities to study fundamental physics and applied science. For example, we can precisely control the distance between a surface and a trapped particle for studying surface interactions [13]. In addition, a high-NA metalens can potentially replace specially-designed bulky objective lenses [24] to create tightly-focused optical tweezers for trapping ultracold atoms and molecules in vacuum for quantum simulation [25] and ultracold chemistry [26].…”