Melvin I. Buchwald

Development of the Los Alamos solid-state optical refrigerator

Laser-induced cooling of a solid by net anti-Stokes fluorescence, first experimentally demonstrated in 1995, can be the basis of a new type of cryocooler, an optical refrigerator. This article describes the physics and design issues of a practical optical refrigerator for operation at 77 K. In particular, the Los Alamos Solid-State Optical Refrigerator (LASSOR) which we are developing would have an operating efficiency comparable to commercial small cryocoolers, be completely vibration-free and operate for years without maintenance.

Spectroscopic evaluation of Yb/sup 3+/-doped glasses for optical refrigeration

The absorption and emission properties of Yb/sup 3+/-doped ZBLANP, BIGaZYT and QX/Yb phosphate glasses are studied to evaluate their potential for laser-induced fluorescent cooling or optical refrigeration. The efficiency of optical refrigeration increases with pump wavelength in the anti-Stokes region. The cooling efficiencies of the three glasses as a function of temperature are evaluated at the wavelength /spl lambda/p corresponding to the absorption coefficient of 10/sup -3/ cm/sup -1/. For temperatures <110 K, the cooling efficiency of the BIGaZYT glass may be more than twice that of the ZBLANP.

INTERNAL LASER COOLING OF YB3+-DOPED GLASS MEASURED BETWEEN 100 AND 300 K

Laser cooling of a solid can occur when it emits photons of higher mean energy than those it absorbs. Photothermal deflection spectra of a fluorozirconate glass (ZBLANP) doped with 1 wt% Yb3+ show cooling in an internal volume of the sample at temperatures between 100 and 300 K. A cooling efficiency of ∼1% relative to the absorbed laser power at 1015 nm is maintained at all temperatures. The results show promise for solid-state cooling of bulk materials at temperatures below 150 K.

Interaction of far‐infrared and mid‐infrared laser transitions in the NH3 laser

Laser emission on the conventional 12.8‐μm NH3 laser transition, optically pumped by the CO2 9R16 line, can be completely quenched and replaced by emission at 12.2μm. Cascading far‐infrared laser transitions in the excited ammonia ν2 state are shown to be responsible for the shift in wavelength.

Measurement and analysis of the ν2 and ν4 infrared bands of 13CD4

Abstract A Fourier transform infrared spectrum of 91% 13CD4 has been recorded between 885 and 1193 cm−1 with a resolution of 0.04 cm−1. The frequencies of 600 lines were measured with an accuracy of ±0.003 cm−1. Of these, approximately 368 are assigned as allowed transitions in ν4, 95 are forbidden ν4 transitions, and 137 belong to ν2; maximum upper state J values are 20 for ν4 and 19 for ν2. The ground state tensor constants Dt, H4t, and H6t were obtained by fitting them to four rotational transitions observed by Kreiner and Opferkuch in the infrared-microwave double-resonance spectrum. An interacting-band analysis of the ν 2 ν 4 diad then yields 22 spectroscopic constants for these Coriolis-coupled fundamentals and fits the experimental frequencies with an rms deviation of 0.0055 cm−1 for 432 unblended lines that were assigned unit weight. A ν4 P+(12) transition at 943.3812 cm−1, nearly coincident with the 10P(22) emission of the 12C16O2 laser, has been investigated by heterodyne spectroscopy and its detuning (−64 MHz) and absorption coefficient have been determined. Such coincidences may lead to the development of laser analytical techniques for 13CD4, which is a useful nonradioactive atmospheric tracer. 13CD4 transitions that are within 300 MHz of isotopic CO2 laser lines are tabulated for this purpose and for use in double-resonance experiments.

The Los Alamos Solid-State Optical Refrigerator

Optical refrigeration may provide the physical basis for solid-state cryocooling. Devices based on this physics would be vibrationless, compact, and free of electromagnetic interference. Having no moving parts and being pumped by a diode lasers, optical refrigerators would be rugged with operating lifetimes of years. Experiments at Los Alamos National Laboratory have demonstrated the basic physical principles of optical refrigeration.1 Design studies suggest that optical refrigerators with efficiencies comparable to other small cryocoolers should be realizable with existing technologies.2

A Novel 15.9-Micron Source

Laser emission has been generated in 15ND3 by pumping at 860.4 cm-1. Strong laser action has been observed at 123 cm-1, 109 cm-1 and, with up to 10 millijoules extracted, at 628.1 cm -1. Spectroscopic analysis indicates that the pumping arises from, and the 628.1-cm-1 emission terminates on, the same rotational state. Analysis of the time histories of the three laser emissions as well as studies of laser output energies indicates that 15.9-micron output arises from a 4-wave type process. This system, in addition, clarifies the interpretation of earlier studies of CO2-laser-pumped ammonia lasers.

THE LOS ALAMOS SOLID-STATE OPTICAL REFRIGERATOR (LASSOR) PROGRAM

Recent work at Los Alamos National Laboratory has demonstrated the physical principles for a new type of solidstate cryocooler based on anti-Stokes fluorescence1. From our laboratory work and computer simulations we estimate that a practical, first-generation, all-solid-state fluorescent cryocooler will have the following properties2: • no vibrations • will not produce and is not susceptible to electromagnetic interference • cool to 77 K • be ~1% efficient (DC power to cooling power) • weigh less than 3 kg/Watt • have a lifetime of 10 years continuous operation. This first-generation cryocooler will employ material with demonstrated fluorescent cooling capability (ytterbium doped ZBLANP). In this paper we will present the current status of our work including the measured temperature dependence of the fluorescence and absorption for Ytterbium in ZBLANP, the model predictions for the temperature dependence for several hosts, and the current prototype design and component analysis.