Laser refrigeration of gas filled hollow-core fibres

N Y Joly,C Sommer,H Ritsch,C Genes

doi:10.1063/1.5121491

Abstract

We evaluate prospects, performance and temperature limits of a new approach to macroscopic scale laser refrigeration. The considered refrigeration device is based on exciplex-mediated frequency up-conversion inside hollow-core fibers pressurized with a dopant - buffer gas mixture. Exciplexes are excited molecular states formed by two atoms (dopant and buffer) which do not form a molecule in the ground state but exhibit bound states for electronically excited states. The cooling cycle consists of absorption of laser photons during atomic collisions inducing light assisted exciplex formation followed by blue-shifted spontaneous emission on the atomic line of the bare dopant atoms after molecular separation. This process, closely related to reversing the gain mechanism in excimer lasers, allows for a large fraction of collision energy to be extracted in each cycle. The hollow-core fiber plays a crucial role as it allows for strong light-matter interactions over a long distance, which maximizes the cooling rate per unit volume and the cooling efficiency per injected photon while limiting re-absorption of spontaneously emitted photons channeled into unguided radiation modes. Using quantum optical rate equations and refined dynamical simulations we derive general conditions for efficient cooling of both the gas and subsequently of the surrounding solid state environment. Our analytical approach is applicable to any specific exciplex system considered and reveals the shape of the exciplex potential landscapes as well as the density of the dopant as crucial tuning knobs. The derived scaling laws allow for the identification of optimal exciplex characteristics that help to choose suitable gas mixtures that maximize the refrigeration efficiency for specific applications.

Highlights

Lasers are sources of energy highly concentrated in space and momentum at almost zero entropy.1 This effective extremely low temperature is very successfully employed to cool dilute atomic gases to almost arbitrarily close to the absolute zero
Its transmission window is center around the laser frequency ωL and is larger than the bandwidth of the laser δω. This can be realized in several types of hollow-core photonic crystal fibres (HC-PCF), where light is guided without diffraction in the empty central region by either a photonic bandgap or anti-resonance guidance such as in a single-ring fibre
We have developed a general theory for collision-assisted cooling of fibre-embedded dopant-buffer gas mixtures of exciplexes

Summary

Introduction

Lasers are sources of energy highly concentrated in space and momentum at almost zero entropy. This effective extremely low temperature is very successfully employed to cool dilute atomic gases to almost arbitrarily close to the absolute zero. Lasers are sources of energy highly concentrated in space and momentum at almost zero entropy.1 This effective extremely low temperature is very successfully employed to cool dilute atomic gases to almost arbitrarily close to the absolute zero. In the following generations of experiments, the use of isotope purified small crystals and improved environmental shielding eventually allowed to reach cryogenic temperatures of macroscopic objects, significantly beating the temperature limits achieved via thermo-electric cooling.. In the following generations of experiments, the use of isotope purified small crystals and improved environmental shielding eventually allowed to reach cryogenic temperatures of macroscopic objects, significantly beating the temperature limits achieved via thermo-electric cooling.14 Despite these significant improvements over the last two decades, the efficiency of the cooling process remained rather low, reaching only a couple of miliwatts of cooling power from 50 W of laser power. One needs thousands of successful cycles per background absorption with non-radiative decay

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