Silica glass is an extremely useful material with excellent optical and mechanical properties, that is easily shaped into a wide variety of forms. Unfortunately, due to the high phonon energy of silica, it has been a challenge to use it for laser cooling applications as it requires ultrahigh purity to avoid parasitic heating adding to the non-radiative pathways, and which act as a severe heat load in any cooling event. Our research over the years has focused on trying to incorporate rare earth materials in silica glass for example, in the form of nanocrystals of fluoride through heat treatment, which provide a low phonon environment, thus shielding it from the high phonon energy silica host. As opposed to the untreated glasses, which showed significant heating, this approach resulted in near zero temperature rise. However, this route as well, is an arduous one as purification of the starting materials is still an issue for the casting technique used for glass manufacture. Recently, our work has focused on inducing phase separation of active rare earth oxides in an environment of a non-active rare earth oxides, such as yttrium oxide to form a unique glass. We have shown that through temperature control, the phase separation of RE: yttria can be either enhanced or reversed, transitioning from clear to turbid to clear glass states using a high purity MCVD process. Our work led to the largest successful cooling reported to date of GAYY-PS oxide glass by with a temperature drop of 4 K from the ambient. This presentation reviews these developments and subsequent progress of laser cooling and other applications in these novel and highly promising glasses.
We recently demonstrated laser induced cooling in all oxide silica glass [1], a proof of principle of our materials engineering approach. This technique has the potential of significantly impacting silica photonics by not only improving laser cooling with the preferred rare earth ion, Yb3+, but for the first time, also with different rare earths. The higher rare earth concentration possible in silica without affecting its optical properties, indicates that new amplifiers and laser may be possible. This talk will review our engineering perspective to mitigating serious materials shortcoming in silica and elaborate what may be possible for new applications in photonics.
1. J. Thomas, T. Meyneng, N. Gregoroire, F. Monet, A. Tehranchi, D. Seletskiy, Y. Messaddeq, Raman Kashyap, “Laser Cooling of a Novel GAYY Glass at Atmospheric Pressure”, Advanced Photonics Congress, Maastricht, Holland, Post Deadline paper JTH4A.5, Optica (28 July 2022).
We report optical cooling of a novel rare earth doped oxide-only silica glass fabricated using the modified chemical vapour deposition (MCVD) technique. An ytterbium concentration in silica glass of up to 6.55 x 10^26 ions/m^3 was achieved without compromising its optical properties. Samples were cooled to -0.8 K from room temperature (RT) at atmospheric pressure with pump power of 7 W at 1029 nm. We report experimental results on other RE dopants in oxide-only silica glass. Our approach opens possibilities not only for high efficiency laser cooling, but also in radiation balanced lasers, and compact high-power lasers and amplifiers.
The optimization of the fiber geometry, core composition & rare earth ion concentration in optical fibers are of great importance in obtaining high photoluminescence quantum yield (PLQY) for optical refrigeration applications. Presented herein are the important advancements in the development of a Y2O3: Yb (5 molars percent) nanoparticle doped fibers embedded in glass matrix with composition 95SiO2-5GeO2, with an in-depth investigation on structural and optical properties for laser cooling applications. The impurity absorption was minimized by using high-purity precursors to fabricate the fiber using modified chemical vapor deposition method (MCVD). Structural characterizations have evidenced the presence of Y2O3 nanoparticles ranging from 25 to 50 nm. The optical properties such as transmission, refractive index, photoluminescence (PL) emission and lifetime were studied in detail. Finally, the background absorption as well as temperature change of the fibers using 1030 nm laser are measured using fiber Bragg grating (FBG) sensor.
Rare earth doped oxyfluoride glass-ceramics (GCs) have recently attracted much attention as a novel material candidate for solid state laser cooling application. In this study, highly transparent (~ 90 % in the infrared region) ytterbium doped aluminosilicate oxyfluoride glasses and glass ceramics containing YF3 nanocrystals prepared by the conventional melt quenching process have been investigated to determine and compare their potential to obtain high photoluminescence quantum yield (PLQY). The highest ASF emission intensity was observed in the GC composition with near-infrared PL emission centered at ~1010 nm under a laser excitation at 1020 nm. The PL spectra at different temperatures (25 °C – 200 °C) were measured using different excitation wavelengths varying from 920 nm to 1030 nm in order to understand the nature of Stokes and anti-Stokes emission in glass ceramics. The glass-ceramic has a net heating near to zero with an excitation between 1020 nm and 1030 nm showing its potential for optically induced heat-management applications. The optical properties such as refractive index, quantum efficiency and lifetime of GC and the precursor glass were also studied in detail in order to explore these properties for laser cooling applications.
Oxyfluoride glass-ceramics are considered to be one of the promising materials for anti-Stokes laser cooling. Our previous results showed that glass-ceramics are more appropriate than glassy materials for laser cooling applications. In the present work, we describe the role of optimization of the oxide: fluoride ratio in oxyfluoride glass ceramics with the composition (SiO2-Al2O3)(100-x)(YLiF4)x: (YbF3)1 (x = 35 and 40; in mol.%) in order to obtain high photoluminescence quantum yield (PLQY) and low background absorption focusing on optical refrigeration applications. Glass-ceramics with high transparency (~ 90 % in the infrared region) were synthesized by the conventional melt-quenching process followed by heat-treatment. Near-infrared (NIR) photoluminescence (PL) emission due to the 2F5/2 – 2F7/2 Yb3+ transition, centered at ~1010 nm was observed. An improvement in the quantum efficiency is observed for all the samples, which varies between 64 % and 99 %, depending on the oxide: fluoride ratio. A decrease in the background absorption of the samples was investigated by calorimetry. The enhanced radiative (radiative emission) quantum efficiency is achieved due to the YLiF4 crystals (low phonon energy ~450 cm-1) which minimize non-radiative relaxations. At a laser wavelength of 1020 nm, the glass-ceramics show anti-Stokes PL emission, essential to achieve laser cooling. The proposed composition is an ideal candidate for laser cooling considering the low phonon energy and low background absorption compared to the other oxyfluoride glasses previously investigated.
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