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Photonic Heat Engines: Science and Applications

Laser cooling under ambient conditions in Yb3+:KYW

Laura B. Andre, Long Cheng, Alexander J. Salkeld, L.H.C. Andrade, S.M. Lima, Junior Reis Silva, Stephen Colby Rand

Published Mar 1, 2019• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2507325

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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.
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.
The role of saturation of background absorption by unintended impurities has been investigated for the first time theoretically and experimentally in the context of laser cooling of crystalline solids and the operation of self-cooled or radiation-balanced lasers. In this thesis it is shown that when saturation of the background takes place at an intensity lower than the saturation intensity of dopant ions responsible for cooling, as in Yb3+:LiYF4, cooling efficiency can be improved by using elevated input intensity. With the same input power, the measured cooling efficiency in 10% Yb3+:LiYF4 crystal was doubled at 1020 nm when the beam radius was reduced by a factor of 10, by partially saturating the background impurity absorption. This is effective not only as a post-growth method of enhancing cooling efficiency but offers a new way of reaching lower temperatures than were previously possible with anti-Stokes fluorescent (ASF) refrigeration. Prior to this work, cooling efficiency was thought to be independent of intensity and crystal purification was considered as the only way to improve the cooling efficiency. It is further demonstrated that background saturation plays a key role in the operation of self-cooled or radiation-balanced lasers. Because radiation-balanced lasers operate under saturated conditions it is essential to include limitations imposed by impurity content of the gain medium. When this is done, quantitative predictions of the required conditions for radiation balance become possible. Laser operation can then be made more efficient and can be extended to new materials which may be relatively impure and uncommon but are well-suited to efficient operation under radiation-balanced conditions by virtue of a special combination of physical characteristics. Radiation-balanced laser operation is achieved in 3% Yb:YAG (1 × 1 × 10 mm3) and is found to result in exceptionally uniform temperature distributions using high resolution thermal imagery. Threshold levels, output power, and input power at the radiation balance point are all shown to be in close agreement with theory. This laser achieved 30.5% laser efficiency (agreed with predictions within 2%), which is the highest efficiency level of any self-cooled laser operated to date. Radiation-balanced lasing is also achieved for the first time in the tungstate crystal 2% Yb3+:KYW (0.9×1.2×10 cm3), despite having very high impurity content as the result of being an uncommon gain medium. The achieved laser efficiency was only 2.2%, as the result of non-optimal optical components, high Yb concentration, high impurity content, crystal polish, excitation wavelength, and output coupling. However, this is the first demonstration of an Yb3+ radiation-balanced laser in the tungstate host. Correction of these deficiencies should enable high quality Gaussian output at the 5 W level with 18% efficiency in existing commercial 1% Yb:KYW samples. Even better performance of this new self-cooled laser could result from improved crystal growth efforts.
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.
For the first time, to the best of our knowledge, optical cooling is demonstrated in a fiber at atmospheric pressure. Using a specialized slow-light fiber Bragg grating temperature sensor, −5.2 mK and −0.65 K were measured in a single-mode (1% YbF3) and multimode (3% YbF3) ZBLAN fiber with respective cooling efficiencies of 2.2% and 0.90%. Fitting a recently reported quantitative model of optical cooling in fibers to the measured temperature change dependence on the pump power per unit length validates the model and allows us to infer the fibers’ absorptive loss and quenching lifetime, key parameters that are scarce in literature. These values are necessary for accurate cooling predictions and will aid in the development of fibers for application in optical coolers and radiation-balanced lasers.
Anti-Stokes cooling has been observed in an Yb3+-doped BaY2F8 single crystal. Single crystals have been grown by the Czochralski technique. The absorption spectra and the emission properties have been measured at room temperature and at 10K. The energy positions of the Stark sublevels of the ground and the excited state manifolds have been determined and separated from the vibronic substructure. The intrinsic decay time of the F5∕22 level has been measured taking care of avoiding the effect of multiple reabsorption processes. The theoretical and experimental cooling efficiencies of Yb:BaY2F8 are evaluated and compared with respect to those of the most frequently investigated materials for laser cooling. A temperature drop of almost 4K was measured by pumping the crystal with 3W of laser radiation at ∼1025nm in single pass configuration with a cooling efficiency of ∼3%.
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.