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Register a new userSelf-cooling of a micro-mirror by radiation pressure
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We demonstrate passive feedback cooling of a mechanical resonator based on radiation pressure forces and assisted by photothermal forces in a high-finesse optical cavity. The resonator is a free-standing high-reflectance micro-mirror (of mass m=400ng and mechanical quality factor Q=10^4) that is used as back-mirror in a detuned Fabry-Perot cavity of optical finesse F=500. We observe an increased damping in the dynamics of the mechanical oscillator by a factor of 30 and a corresponding cooling of the oscillator modes below 10 K starting from room temperature. This effect is an important ingredient for recently proposed schemes to prepare quantum entanglement of macroscopic mechanical oscillators.
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Cooling of a 58 MHz micro-mechanical resonator from room temperature to 11 K is demonstrated using cavity enhanced radiation pressure. Detuned pumping of an optical resonance allows enhancement of the blue shifted motional sideband (caused by the oscillators Brownian motion) with respect to the red-shifted sideband leading to cooling of the mechanical oscillator mode. The reported cooling mechanism is a manifestation of the effect of radiation pressure induced dynamical backaction. These results constitute an important step towards achieving ground state cooling of a mechanical oscillator.
We consider the dynamics of a vibrating and rotating end-mirror of an optical Fabry-P{erot} cavity that can sustain Laguerre-Gaussian modes. We demonstrate theoretically that since the intra-cavity field carries linear as well as angular momentum, radiation pressure can create bipartite entanglement between a vibrational and a rotational mode of the mirror. Further we show that the ratio of vibrational and rotational couplings with the radiation field can easily be adjusted experimentally, which makes the generation and detection of entanglement robust to uncertainties in the cavity manufacture. This constitutes the first proposal to demonstrate entanglement between two qualitatively different degrees of freedom of the same macroscopic object.
We present an optical system based on two toroidal mirrors in a Wolter configuration to focus broadband XUV radiation. Optimization of the focusing optics alignment is carried out with the aid of an XUV wavefront sensor. Back-propagation of the optimized wavefront to the focus yields a focal spot of 3.6$times$4.0 $mu$m$^2$ full width at half maximum, which is consistent with ray-tracing simulations that predict a minimum size of 3.0$times$3.2 $mu$m$^2$. This work is important for optimizing the intensity of focused high-order harmonics in order to reach the nonlinear interaction regime.
The technique of passive daytime radiative cooling (PDRC) is used to cool an object down by simultaneously reflecting sunlight and thermally radiating heat to the cold outer space through the Earths atmospheric window. However, for practical applications, current PDRC materials are facing unprecedented challenges such as complicated and expensive fabrication approaches and performance degradation arising from surface contamination. Here, we develop a scalable paper-based material with excellent self-cleaning and self-cooling capabilities, through air-spraying ethanolic polytetrafluoroethylene (PTFE) microparticles suspensions embedded within the micropores of the paper. The formed superhydrophobic PTFE coating not only protects the paper from water wetting and dust contamination for real-life applications but also reinforces its solar reflectance by sunlight backscattering. The paper fibers, when enhanced with PTFE particles, efficiently reflect sunlight and strongly radiate heat through the atmospheric window, resulting in a sub-ambient cooling performance of 5$^{circ}$C and radiative cooling power of 104 W/m$^2$ under direct solar irradiance of 834 W/m$^2$ and 671 W/m$^2$, respectively. The self-cleaning surface of the cooling paper extends its lifespan and keep its good cooling performance for outdoor applications. Additionally, dyed papers are experimentally studied for broad engineering applications. They can absorb appropriate visible wavelengths to display specific colors and effectively reflect near-infrared lights to reduce solar heating, which synchronously achieves effective radiative cooling and aesthetic varieties in a cost-effective, scalable, and energy-efficient way.
We report a direct observation of radiation pressure, exerted on cold rubidium atoms while bouncing on an evanescent-wave atom mirror. We analyze the radiation pressure by imaging the motion of the atoms after the bounce. The number of absorbed photons is measured for laser detunings ranging from {190 MHz} to {1.4 GHz} and for angles from {0.9 mrad} to {24 mrad} above the critical angle of total internal reflection. Depending on these settings, we find velocity changes parallel with the mirror surface, ranging from 1 to {18 cm/s}. This corresponds to 2 to 31 photon recoils per atom. These results are independent of the evanescent-wave optical power.
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