Carl E. Mungan

Optical cooling in Er3+:KPb2Cl5.

For the first time, optical cooling has been observed in the (4)I(13/2) excited state of erbium(III), using the low phonon energy host materal, potassium lead chloride (KPb(2)Cl(2)). Cooling was observed when samples were pumped at wavelengths longer than 1557 nm, 17 nm longer than the mean fluorescence wavelength of 1540 nm, which implies a nonradiative heat load of 1.1% for the (4)I(13/2)-->(4)I(15/2) transition. When pumped at 1568 nm, the total cooling efficiency was 0.38% of the absorbed power. These results highlight the potential of Er(3+):KPb(2)Cl(5) as a material for lasers operating in an eye safe spectral region.

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.

Radiation thermodynamics with applications to lasing and fluorescent cooling

Laser cooling of bulk matter uses thermally assisted fluorescence to convert heat into light and can be interpreted as an optically pumped laser running in reverse. Optical pumping in such devices drives the level populations out of equilibrium. Nonthermal radiative energy transfers are thereby central to the operation of both lasers and luminescent coolers. A thermodynamic treatment of their limiting efficiencies requires a careful development of the entropy and effective temperatures of radiation, valid for the entire range of light from the blackbody to the ideal laser limiting cases. In particular, the distinct meaning and utility of the brightness and flux temperatures should be borne in mind. Numerical examples help illustrate these concepts at a level suitable for undergraduate physics majors.

Suppression of stimulated Brillouin scattering in optical fibers using a linearly chirped diode laser

The output of high power fiber amplifiers is typically limited by stimulated Brillouin scattering (SBS). An analysis of SBS with a chirped pump laser indicates that a chirp of 2.5 × 10(15) Hz/s could raise, by an order of magnitude, the SBS threshold of a 20-m fiber. A diode laser with a constant output power and a linear chirp of 5 × 10(15) Hz/s has been previously demonstrated. In a low-power proof-of-concept experiment, the threshold for SBS in a 6-km fiber is increased by a factor of 100 with a chirp of 5 × 10(14) Hz/s. A linear chirp will enable straightforward coherent combination of multiple fiber amplifiers, with electronic compensation of path length differences on the order of 0.2 m.

A Primer on Work-Energy Relationships for Introductory Physics

There has been and continues to be considerable discussion in the educational community about different ways of relating the concepts of work and energy in introductory physics.1 The present article reviews a consistent and streamlined treatment of the subject, drawing particular attention to aspects seldom covered in textbooks. The paper is intended to clarify the central equations for introductory courses and to put the wider literature in context. It is specifically designed to tie closely in terminology and order of presentation to standard texts, so that it complements rather than supplants them. In brief, the key point is that there are two major categories of work, center-of-mass work and particle work.2 After an overview of these two approaches, I illustrate them with a couple of instructive examples that can be used in group problem-solving sessions in class.

Irreversible Adiabatic Compression of an Ideal Gas

Consider the following problem.1 A frictionless, massive piston partitions an insulated box vertically into two parts. The upper compartment is evacuated, while the lower contains an equilibrated ideal gas. A weight is suddenly placed on the piston. How much is the gas compressed when equilibrium is again restored, neglecting the heat capacity of the cylinder and piston?

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.

A classic chase problem solved from a physics perspective

The trajectory, travel time and relative approach velocity of a pursuer tracking a prey along a simple curve of pursuit are deduced using basic principles of two-dimensional kinematics. While such curves are well known in the mathematics literature, little attention has been paid to this problem by the physics educational community, despite the fact that it has abundant physical applications. It also makes an interesting alternative to the traditional problems of introductory kinematics.

Internal ballistics of a pneumatic potato cannon

Basic laws of thermodynamics and mechanics are used to analyse an air gun. Such devices are often employed in outdoor physics demonstrations to launch potatoes using compressed gas that is here assumed to expand reversibly and adiabatically. Reasonable agreement is found with reported muzzle speeds for such homebuilt cannons. The treatment is accessible to undergraduate students who have taken calculus-based introductory physics.