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AIP Advances

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

Yuta Nakayama, N. Nakagawa, Y. Matsuo, Toshiyuki Kaizu, Yukihiro Harada, Tohru Ishihara, Takashi Kita

Published Feb 1, 2022• 1 citation

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.

DOI: 10.1063/5.0079632

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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.

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Cooling via efficient emission of anti-Stokes photoluminescence can be utilized to realize novel cooling devices. It has been shown that rare earth-doped materials are good candidates for solid-state laser cooling because of their narrow emission spectra and discrete energy levels. We systematically study laser cooling efficiency of rare earth-doped oxide crystals. The photoluminescence spectrum evidences that the phonon-assisted energy transfer from resonant states to inhomogeneously distributed energy states sensitively depends on temperature. We discuss detailed properties of the cooling efficiency of Yb-doped yttrium aluminum oxide crystals and, furthermore, touch emission properties of their crystal thin films.

Successful incorporation of both erbium and ytterbium in alumina by ion implantation is reported. Some evidence for indirect pumping of erbium through the transfer of energy from ytterbium has been observed. Both plasma-enhanced CVD deposited alumina thin films and sapphire crystals were employed as substrates. Yb3+ and Er3+ concentrations ranged from 2.4 At% to 8 At% and from 0.4 At% to 0.8 At%, respectively. The samples show strong, broad, room-temperature photoluminescence at λ = 1.53 µm corresponding to the intra-4f transitions between the 4I13/2 (first excited) and the 4I15/2 (ground) state of Er3+. The full width at half maximum of the emission spectrum is as high as 67 nm for the Al2O3 thin films; for the sapphire crystals it is 45 nm. The fluorescence lifetime of the samples has been measured to be as high as 4.2 ms at 50 mW pump power.

Laser cooling in solids has been demonstrated, and cooling from room temperature to cryogenic temperatures has so far been the main focus of research. However, operation at high temperatures can be advantageous. We propose that laser cooling of Yb-doped yttrium aluminum garnet ceramic discs is enhanced above 300 K. Photoluminescence (PL) spectra is evidence that the phonon-assisted energy transfer from resonant states, which exhibit a narrow PL peak, to inhomogeneously distributed energy states becomes stronger at higher temperatures. This results in an enhanced non-resonant anti-Stokes PL at high temperatures. The ideal cooling efficiency determined from the PL spectrum at 470 K is 1.7 times higher than that at 300 K.

Sputter-deposited thin-film amorphous AlN:Er (1 at. %) emits at 554 and 561 nm as a result of (2)H(11/2)→(4)I(15/2) and (4)S(3/2)→(4)I(15/2) transitions. AlN:Yb (1 at. %) gives a weak emission peak at 966 nm as a result of (2)F(5/2)→(2)F(7/2). The codoping of Er and Yb in AlN results in energy transfer from Er(+3) to Yb(+3) and enhances the Yb(+3) emissions by an order of magnitude. Transfer of electrons occurs from the (4)S(3/2) state of Er(+3) to the (2)F(5/2) state of Yb(+3). The weak emission from Yb(+3), when excited by a 532 nm laser in the absence of Er(+3), confirms that the luminescence enhancement in ytterbium is due to energy transfer and not to direct green light excitation by the erbium emission. A possibility of population inversion and a four-level laser cavity formation exists in the Er(+3)-Yb(+3) system.

A series of Yb3+-doped yttrium aluminum monoclinic Y4Al2O9 (Yb:YAM) single crystals with Yb3+ ion concentrations of 0.1, 1, 5 and 10 at.% was grown by means of micro pulling down method. Blue emission was observed in Yb3+ activated YAM crystals under infrared site-selective laser excitation. Through the excitation and emission spectra, decay time measurements and excitation intensity dependence the process responsible for this anti-Stokes emission was assigned to the cooperative deexcitation of two Yb3+ ions. The results show that at 10% Yb3+ dopant concentration investigated system could be considered as single-site system in which cooperative luminescence is dominated by the energetically lowest Yb3+ site. Theoretical cooperative emission spectrum and the cooperative luminescence rate of Yb3+:YAM were calculated.