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Journal of Modern Optics

Laser cooling of a solid from ambient temperature

A. Rayner, M. E. J. Friese, Andrew G. Truscott, N. R. Heckenberg, Halina H. Rubinsztein-Dunlop

Published Jan 1, 2001• 47 citations
Publisher

Abstract

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.

DOI
10.1080/09500340108235158

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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.
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.
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.
We present a theoretical scheme for laser cooling of rare-earth-doped solids with optical superradiance (SR), which is the coherent, sharply directed spontaneous emission of photons by a system of laser-excited rare-earth ions in the solid-state host (glass or crystal). We consider an Yb+{}^{+}+-doped ZnF4{}_{4}4​-BaF2{}_{2}2​-LaF3{}_{3}3​-AlF3{}_{3}3​-NaF (ZBLAN) sample pumped at a wavelength 1015 nm, with a rectangular pulsed source with a power of \ensuremath{\sim}433 W and a duration of 10 ns. The intensity of the SR is proportional to the square of the number of excited ions. This unique feature of SR permits an increase in the rate of the cooling process in comparison with the traditional laser cooling of the rare-earth-doped solids with anti-Stokes spontaneous incoherent radiation (fluorescence). This scheme overcomes the limitation of using only low phonon energy glasses for laser cooling.
A new approach to cool rare earth doped solids with optical super-radiance (SR) is presented. SR is the coherent, sharply directed spontaneous emission of photons by a system excited with a pulsed laser. We consider an Yb3+ doped ZBLAN sample pumped at the wavelength 1015nm with a rectangular pulsed source with a power of ~700W and duration of 20ns. The intensity of the SR is proportional to the square of the number of excited ions. This unique feature of SR permits an increase in the rate of the cooling process in comparison with the traditional laser cooling of the rare earth doped solids with anti-Stokes spontaneous incoherent radiation (fluorescence). This scheme overcomes the limitation of using only low phonon energy glasses for laser cooling.
Theoretical schemes for laser cooling with nanoparticles have been presented and comprehensively investigated. It is shown that specially designed samples based on nanoparticles can be used to improve the process of laser cooling of solids. One of the proposed schemes is based on lead salt colloidal quantum dots (QDs) doped in a glass host. The second one is based on Tm3+ doped oxy fluoride glass ceramic. It has been shown that lead salt colloidal QDs doped in a glass host can operate as artificial atoms. Very short (microsecond range) radiative lifetimes of the excited 1Sh level of PbSe QDs in comparison with the relatively long (millisecond) radiative lifetime of rare-earth (RE) ions allows the cooling process to be accelerated and to use new hosts with relatively high maximum phonon energy, which have so far been considered not suitable for cooling with RE ions. It has been shown that the second sample, which is based on Tm3+ doped oxy fluoride glass ceramic provides the unique combination of high chemical and mechanical stability of the oxide glass, which is important for a number of applications, and the low phonon energy of the fluoride nano-crystals, which trap a majority of Tm3+ ions participating in the cooling process. This is highly beneficial for laser cooling of solids, since the effective embedding of rare-earth ions in the crystalline phase with low phonon energy provides a high quantum efficiency for the 3F4 → 3H6 transition involved in the cooling cycle in the Tm3+ ions, which is a key parameter for laser cooling of solids.