OSE Dissertation Defense by Mr. Brian Topper on High-power laser cooling and temperature-dependent fluorescence studies of ytterbium doped silica
Departmental News
Posted: June 29, 2023
Friday, 6/30/23 at 1:30 PM at PAIS, Room 2540, and on Zoom
Experimental observation of optical refrigeration using ytterbium doped silica glass in recent years has created a new solution for heat mitigation in high-power laser systems, nonlinear fiber experiments, integrated photonics, and precision metrology. Current efforts of different groups focus on compositional optimization, fiber fabrication, and investigating how much silica can be cooled with a laser. At the start of this work, the best effort in laser cooling ytterbium doped silica saw cooling by 6 K from room temperature. This dissertation follows the experimental efforts that culminated in the increase of this initial record by one order of magnitude. Comprehensive spectroscopic studies were carried out to fully understand the potential of the available samples. The external quantum efficiency was measured to be 99% for two Yb-doped silica rods containing at least 6 times more Al than Yb. Current laser cooling results employing a homemade 100 W fiber amplifier operating at 1032 nm have resulted in laser cooling a 1 mm diameter Yb, Al co-doped silica glass rod in vacuum by 67 K from room temperature. These results indicate that with some additional optimization, silica will soon surpass Yb:ZBLAN as the coolest glass. Detailed temperature-dependent and site-selective fluorescence studies carried out in this work show that obtaining relevant laser cooling parameters for Yb:SiO2 is not straightforward. For example, mean fluorescence wavelength measurements at room temperature vary between 1008 and 1012 nm, depending on the excitation wavelength. Regarding the Yb3+ site environment, the zero-phonon line transition is well-described by two band components spaced approximately 40 cm-1 apart, suggesting the possibility of two main local site environments of the Yb3+ ions.
