Radiation pressure-induced nonlinearity in a micro-droplet

Engineered entropic forces allow ultrastrong dynamical backaction

When confined within an optical cavity light can exert strong radiation pressure forces. Combined with dynamical backaction, this enables important processes, such as laser cooling, and applications ranging from precision sensors to quantum memories and interfaces. However, the magnitude of radiation pressure forces is constrained by the energy mismatch between photons and phonons. Here, we overcome this barrier using entropic forces arising from the absorption of light. We show that entropic forces can exceed the radiation pressure force by eight orders of magnitude and demonstrate this using a superfluid helium third-sound resonator. We develop a framework to engineer the dynamical backaction from entropic forces, applying it to achieve phonon lasing with a threshold three orders of magnitude lower than previous work. Our results present a pathway to exploit entropic forces in quantum devices and to study nonlinear fluid phenomena such as turbulence and solitons.

Surface Charge Density Gradient Printing To Drive Droplet Transport: A Numerical Study

Traditional strategies, such as morphological or chemical gradients, struggle to realize the high-velocity and long-distance transport for droplets on a solid surface because of the pinning hydrodynamic equilibrium. Thus, there is a continuing challenge for practical technology to drive droplet transport over the last decades. The surface charge density (SCD) gradient printing method overcame the theoretical limit of traditional strategies and tackled this challenge [Nat. Mater. 2019, 18: 936], which utilized the asymmetric electric force to realize the high-velocity and long-distance droplet transport along a preprinted SCD gradient pathway. In the present work, by coupling the electrostatics and the hydrodynamics, we developed an unexplored numerical model for the water droplet transporting along the charged superhydrophobic surface. Subsequently, the effects of SCD gradients on the droplet transport were systematically discussed, and an optimized method for SCD gradient printing was proposed according to the numerical results. The present approach can provide early guidance for the SCD gradient printing to drive droplet transport on a solid surface.

Spectrally tunable liquid resonator based on electrowetting

We present a tunable on-chip liquid resonator in conjunction with a tapered fiber coupling scheme. The resonator consists of a glycerol droplet submerged within an immiscible liquid bath, which mitigates the effects of environmental fluctuations. The platform is fabricated using standard semiconductor techniques, which enable the future integration of photonic components for an on-chip liquid resonator device. The liquid resonator maintains its high Q-factor on chip (10⁵) due to surface tension forming an atomically smooth liquid-liquid interface. Higher Q-factor resonance modes experienced linewidth broadening due to the random excitation of thermal capillary vibrations. Spectral tuning is demonstrated using the electrowetting effect, increasing the surface's wettability and an expansion in the droplet diameter. A maximum spectral tuning of 1.44 nm ± 5 pm is observed by applying 35 V. The tuning range is twice the free spectral range (FSR) of 0.679 nm measured at a pumping wavelength range of 770-775 nm. A 2D axisymmetric finite-element simulation shows resonance modes in good agreement with experimentally measured spectra and with predicted tuning speeds of 20 nm/s.

3D printed mounts for microdroplet resonators

Liquid microdroplet resonators provide an excellent tool for optical studies due to their innate smoothness and high quality factors, but precise control over their geometries can be difficult. In contrast, three dimensional (3D) printed components are highly customizable but suffer from roughness and pixelation. We present 3D printed structures which leverage the versatility of 3D printing with the smoothness of microdroplets. Our devices enable the reliable creation of microdroplet resonators of varying shapes and sizes in an ambient environment, and our coupling scheme allows for high control over droplet position.

Light pressure in droplet micro-resonators excited by free-space scattering

A droplet optical resonator is a unique environment to investigate light-matter interaction and optomechanics in liquids. Here, we report on light pressure effects derived from whispering gallery modes excited in a liquid-polymer droplet micro-resonator by free-space laser scattering. From the nonlinear resonance spectrum observed in the visible, we provide evidence of photon pressure exerted at the liquid-air boundary and quantify it with a coherent physical model. Our findings pave the way to studies on micro-rheology and nonlinear optofluidics, where droplets serve as miniature liquid laboratories.

Quantitative measurement and mechanism analysis of the high-efficiency laser propulsion of a graphene sponge

Laser propulsion of a graphene sponge shows tremendous potential in propellant-free flight, photoresponsive actuators and micro opto-electro mechanical systems. However, the mechanism is still in dispute and the propulsion force hasn't been accurately measured, seriously hindering its development. This work develops a quantitative method to measure the propulsion force. It is found that the characteristics of the force agree qualitatively with the Knudsen force due to laser-induced thermal nonequilibrium in rarefied gas, which might be another possible mechanism of laser propulsion of a graphene sponge. Also, this kind of laser propulsion is highly efficient, stable and sustainable.