Yb-doped Y–Al–O thin films with a self-organized columnar structure and their anti-Stokes photoluminescence properties

Y Nakayama,Y Harada,T Ishihara,Y Matsuo,T Kaizu,N Nakagawa,T Kita

doi:10.1063/5.0079632

Y Nakayama, Y Harada + Show 5 more

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https://doi.org/10.1063/5.0079632

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Abstract

We fabricated ytterbium-doped yttrium aluminum oxide (Yb:Y–Al–O) thin films by radio-frequency magnetron sputtering and evaluated their crystallinity and anti-Stokes photoluminescence (PL) properties for optical refrigeration. The Yb:Y–Al–O films that were grown on c-sapphire substrates had better transparency than the films deposited on fused-quartz substrates. The better transparency is considered to be a result of the smaller mismatch between the thermal expansion coefficients of Yb:Y–Al–O and c-sapphire. We found that the thin films on the c-sapphire substrates consist of densely packed sub-micron columnar crystals that are aligned perpendicular to the substrate. In these films, we confirmed the existence of perovskite, garnet, and monoclinic phases despite using a single-phase sputtering target. The excitation wavelength dependence of anti-Stokes PL is used to investigate the energy transfer process between trivalent Yb ions in neighboring columnar crystals. The data indicate that the resonant energy transfer from Yb3+ ions at a specific seven-coordinated site of the monoclinic phase to Yb3+ ions in neighboring columnar crystals is faster than the radiative relaxation at the energy-donor site.

Highlights

In the range of 16–40 degrees shown in Fig. 1(a) for the annealed Yb:Y–Al–O film on the fused-quartz substrate, we can confirm a halo pattern that originates from an amorphous structure of the substrate
From the Rietveld refinement of the data of Yb:Y–Al–O/c-sapphire samples with different film thicknesses (Table I), we found that the degree of phase separation is stronger for thinner films
Yb:Y–Al–O films were grown by radio-frequency magnetron sputtering using a single-phase yttrium aluminum polycrystal sputtering target, and we confirmed perovskite, garnet, and monoclinic phases in the annealed films

Summary

Introduction

Laser cooling in solids can be realized if phonon-assisted anti-Stokes photoluminescence (PL) is efficiently extracted from a luminescent material and heat generation processes are sufficiently suppressed in this material. Such optical cryocoolers are able to refrigerate an object without heat generation near the cooling target. In 2018, an all-solid-state optical cryocooler that can cool an HgCdTe sensor of a Fourier transform infrared spectrometer to 135 K was demonstrated for the first time. The advantages of optical cryocoolers, such as no vibration, no electromagnetic perturbations near the cooling target, and a smaller cryostat, are considered to be even useful for small low-earth-orbiting satellites. Radiation balanced lasers (RBLs) are another interesting application area where the laser cooling process is used to overcome the thermal degradation problem: In conventional laser light sources, thermal degradation is unavoidable (due to the operating principle), but at the radiation-balanced point of an RBL, the heat generated by Stokes and other non-radiative processes is carried away by anti-Stokes PL. Laser cooling in solids can be realized if phonon-assisted anti-Stokes photoluminescence (PL) is efficiently extracted from a luminescent material and heat generation processes are sufficiently suppressed in this material.. Laser cooling in solids can be realized if phonon-assisted anti-Stokes photoluminescence (PL) is efficiently extracted from a luminescent material and heat generation processes are sufficiently suppressed in this material.1 Such optical cryocoolers are able to refrigerate an object without heat generation near the cooling target.. Radiation balanced lasers (RBLs) are another interesting application area where the laser cooling process is used to overcome the thermal degradation problem: In conventional laser light sources, thermal degradation is unavoidable (due to the operating principle), but at the radiation-balanced point of an RBL, the heat generated by Stokes and other non-radiative processes is carried away by anti-. If the heat reduction by anti-Stokes PL is stronger than the heat generation, net-cooling occurs, and a self-cooling laser can be realized

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