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Nonlinear Optics

Optical refrigeration of ytterbium doped silica by 67 K starting at room temperature

Brian Topper, Stefan Kühn, Alexander Neumann, Alexander R. Albrecht, Angel S. Flores, Denny Häoner, Sigrun Hein, Christian Hupel, Johannes Nold, Nicoletta Haarlammert, Thomas Schreiber, Mansoor Sheik‐Bahae, Arash Mafi

Published Jan 1, 2023• 0 citations
Publisher

Abstract

Laser cooling of ytterbium doped silica to an absolute temperature of 229 K has been achieved. The experimental results strongly support the utility of optically cooling silica as a method of mitigating thermal nonlinearities.
DOI
10.1364/nlo.2023.m3b.6

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An Yb3+{\mathrm{Yb}}^{3+}Yb3+-doped optical fiber is laser cooled in vacuo from 298 to 282 K. Cooling results from anti-Stokes fluorescence of the ytterbium ions after optical pumping at a wavelength of 1015 nm. The sample temperature is deduced from the emission spectrum, which is sensitive to the populations in the excited-state multiplet of the ions. The temperature change is limited by the coupling between the fiber and ambient blackbody radiation, as confirmed when samples suddenly exposed to the pump laser are found to exponentially relax towards thermal steady state with the expected time constants.
A 250 μm diameter fibre of ytterbium-doped ZBLAN was cooled by 13 K from room temperature. The cooling was performed in vacuum to limit the thermal load on the fibre. 0.85 W of laser light at 1015 nm was coupled into the fibre. The ytterbium ions absorbed this light, and the excited atoms thermalized phononically and on average emitted light at a wavelength of 996 nm. Since the quantum efficiency of the transition was high, this resulted in a net loss of energy from the glass, producing net bulk cooling.
A strongly focused laser beam with high power density employed in optical tweezers typically results in photothermal damage and instability to the trapped nanoparticles or bio-targets. Here, we report an effective laser cooling of trapped Yb3+-Er3+ co-doped rare-earth nanoparticle by employing a new cooling channel provided by Er3+ and utilizing an enhancement of the anti-Stokes emission through energy transfer between Yb3+ and Er3+. Especially in high temperature conditions, the co-doped system demonstrates a high laser cooling performance, that enable the temperature to decrease from 500 K to around room temperature. But for the initial temperature of around room temperature, the cooling temperature does not drop below freezing, which can avoid not only thermal but also cold damage to bio-targets in optical tweezers. This work provides experimental evidence of the Yb3+-Er3+ co-doping rare-earth nanoparticle for laser cooling, which can hold a potential to address the challenge of precise measurements in optical tweezers. The high-power trapping lasers in optical tweezers generate damaging heat. Here, the authors demonstrate efficient laser cooling using an erbium-ytterbium co-doped nanoparticle, which cools targets from high temperatures to near room temperature while avoiding sub-zero cooling from ambient conditions, thus enabling safe trapping of biological specimens.
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We report the first observation of laser cooling in 1%Yb3+:KYW and discuss factors that limit the cooling efficiency. Cooling by 10 K from room temperature at atmospheric pressure was achieved in this crystal at a wavelength of 1025 nm using 8 W from a seeded fiber amplifier. The temperature of the sample was measured using a calibrated differential luminescence thermometry method and was verified with a thermal camera. Infrared imagery and 3-D modeling were used to analyze the impact of thermal conduction, thermal convection, black-body radiation, and background impurities. The simulated results agree with experimental measurements confirming that the chief limitation of laser cooling at room temperature and atmospheric pressure is thermal conduction from sample supports. Best results were obtained for samples mounted on silica aerogel. Theoretical improvement of cooling efficiency in vacuum by exploiting impurity absorption saturation is also discussed.