T. R. Gosnell

Room-temperature upconversion fiber laser tunable in the red, orange,green, and blue spectral regions

We report continuous-wave tunable laser operation over the 635–637-, 605–622-, 517–540-, and 491– 493-nm wavelength bands in Pr3+/Yb3+-doped ZrF4 –BaF2 –LaF3 –AlF3 –NaF (ZBLAN) optical fiber. A single Ti:sapphire laser is used as the pump source. With as little as 60 mW of pump power launched into the fiber, laser output is obtained over the entire pump-wavelength tuning range of 780 to 880 nm. At a pump wavelength of 860 nm, we have produced output powers of 300 mW at 635 nm (760 mW of launched pump power), 45 mW at 615 nm (430 mW of launched power), 20 mW at 520 nm (200 mW of launched power), and 4 mW at 493 nm (200 mW of launched power).

Uniform upconversion in high-concentration Er 3+ -doped soda lime silicate and aluminosilicate glasses

Uniform upconversion in erbium-doped silicate glasses is investigated as a function of glass composition, concentration, and fabrication method. Comparisons of upconversion coefficients are made among soda lime silicate and aluminosilicate bulk glasses and soda lime silicate waveguides. Comparisons are also made with studies performed by other researchers. The results indicate that both the composition and the preparation method of the glass affect the value of the upconversion coefficient, with as much as a factor-of-4 variation observed at fixed Er(3+) concentration. Values of the upconversion coefficient are found to be consistent with the Förster-Dexter microscopic model.

Laser Cooling of Solids

Publisher Summary This chapter focuses on the concept of laser cooling of a solid, which may occur when the average energy of the photons emitted by the solid is larger than the energy of the ones it absorbs. More formally, the anti-Stokes emission, which occurs at frequencies larger than that of the pump laser, must dominate the Stokes emission that occurs at smaller frequencies. A crucial additional requirement is that the nonradiative decay rates of the laser-pumped states be negligible in comparison to their radiative decay rates. For the sake of specificity, these states will be taken to be those of a set of isolated ions embedded within an insulating host, although in general they could equally well be those of gas-, liquid-, or solid-phase neutral atoms or molecules, or even those of the energy bands of intrinsic semiconductors. Outside the realm of impurity-doped insulators, recent experiments with semiconductors make net anti-Stokes fluorescence cooling of these materials appear achievable. A long-term idea for improving laser cooling of solids takes advantage of photonic band gap materials. The idea is to fabricate a refractive-index structure within the cooling element that would establish a three-dimensional photonic gap immediately below the pump frequency. In this way, emission at Stokes frequencies falling within the gap would be inhibited, thereby enhancing the cooling power derived from the anti-Stokes emission.

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.

Laser induced dynamic spectral weight transfer in La0.7Ca0.3MnO3

Abstract We report on optical pump-probe measurements at 1.5 eV and 3 eV on the colossal magnetoresistance material La 0.7 Ca 0.3 MnO 3 . The material response is determined in the femtosecond to the nanosecond temporal regimes. By analyzing the absorption and refractive dynamics within the Kramers–Kronig transformations we demonstrate dynamic spectral weight transfer from the intraband Drude component to the interband transitions. This spectral transfer and its strongly temperature dependent dynamics are discussed in the framework of photoinduced demagnetization due to the exchange of energy and thermal equilibration among the electronic, lattice and spin systems. This effect is observed in other manganites with different transition temperatures. The measured dynamics are found to be insensitive to the excitation photon energy.

Electrochemical purification of heavy metal fluoride glasses for laser-induced fluorescent cooling applications

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. An electrochemical purification stage has been incorporated into the conventional fabrication process of heavy metal fluoride glasses. This change was undertaken to reduce the absorption losses associated with residual transition metal impurities, particularly the Fe 2+ band at 1.0 μm. Purified samples doped with ytterbium exhibited net cooling due to anti-stokes fluorescence and a relative cooling efficiency of about 2% was observed via photothermal deflection spectroscopy. Pumping to the lower Stark levels of the 2 F 5/2 manifold, followed by population redistribution across this manifold and fluorescence to the ground state, results in net cooling. This cooling can be achieved only if energy transfer and the extrinsic absorptive component of the host glass are suppressed.

Chirped-pulse amplification in XeCl

By using a 0.25-ps nearly transform-limited pulse and both 16- and 75-ps strongly chirped pulses of the same bandwidth, measurements of the effective saturation fluence (E(sat)) of a XeCl discharge amplifier have been performed. We find that while E(sat) for the 16-ps pulse is comparable with that of the 0.25-ps pulse, pulse stretching to a width of 75 ps increases the effective E(sat) by 35%. However, the measured values of E(sat) for the 16- and 75-ps pulses are significantly less than would be expected for transform-limited pulses of the same pulse widths. A simple rate-equation model, which takes into account variation of the small-signal gain across the gain bandpass and which assumes that E(sat) is enhanced for the longer pulse widths only by population decay of the lower electronic state, predicts the measured values of E(sat) to within the experimental error.

Broadband microwave absorption spectrometer for liquid media

A broadband, continuous‐sweep microwave spectrometer has been constructed for measurements of the absorption coefficient of aqueous solutions and other liquid media. The spectrometer makes use of the phase fluctuation optical heterodyne technique, which provides a direct measure of the microwave power deposited in the sample. Consequently, in contrast to the standard dielectrometric techniques that indirectly determine the absorption coefficient via separate measurements of the real and imaginary parts of the dielectric constant, this spectrometer directly measures the microwave absorption coefficient. Broadband spectra are obtained using a transmission line to couple microwave power into the liquid sample. The absorption spectrum for deionized water in the range 3–20 GHz is presented as an example and shows excellent agreement with calculated values of the absorption coefficient based on previously published dielectric data.

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.

High-intensity subpicosecond XeCl laser: laser physics and x-ray generation

Experiments performed with a terawatt-class laser system which is based on amplification in XeCl discharge amplifiers are described. Energy extraction in XeCl discharge amplifiers is examined experimentally and theoretically using 0.25-ps nearly transform limited pulses and both 16- and 75-ps strongly chirped pulses of the same bandwidth. We find that while the saturation fluence for the 16-ps pulse is comparable to that of the 0.25-ps pulse, stretching the pulse to a width of 75 ps increases the saturation fluence by 35%. A study of the interaction of the terawatt laser pulses with aluminum targets at irradiances exceeding 5 X 1018 W/cm2 is also presented.