Second sound spectroscopy: A new method for studying optical absorption in solids

Volume 56A, number 3

PHYSICS LETTERS

22 March 1976

SECOND SOUND SPECTROSCOPY: A NEW METHOD FOR STUDYING OPTICAL ABSORPTION IN SOLIDS* J.B. SMITH Department of Chemistry, Harvard University, Cambridge, Ma. 02138, USA

GA. LAGUNA* Aeronautics and Applied Physics, California Institute of Technology, Pasadena, Ca. 91125, USA Received 28 January 1976 Second sound in liquid Hell is used to measure the heat evolved by a solid sample illuminated with chopped monochromatic light. Preliminary experiments are presented.

We

report preliminary

experiments demonstrating

a new at low method temperature. for studying The solid optical is immersed absorption in liquid in solids helium maintained below the lambda transition ternperature and illuminated with chopped monochromatic light. The periodic heating of the solid excites a second sound wave [1] (a propagating temperature wave) in

Second-Sound Cell / / L He

Quartz Windows ‘Tunable________ Light Source

_______

I

Sample

BOth

placed out of the path of the light beam. The method the helium that is detected by a sensitive thermometer solids, is similar in which in concept the absorption to photo-acoustic of chopped spectroscopy light by a of solid produces sound waves in the surrounding gas [2]. This effect has been attributed to the heating of the layer of gas near the solid surface [3]. We reasoned that for low temperature studies, it would be advantageous to use second sound to measure the heating of the solid more directly. A schematic of the experiment is shown in fig. 1. The tunable light source consisted of a half-meter scanning monochromator with a 150 watt xenon short-arc lamp. The beam emerging from the monochromator passed through a slotted-wheel chopper, which also provided the reference signal to the lock-in amplifier, The chopped light was focused to pass through the quartz windows of the dewar and second-sound cell and uniformly illuminate the sample. During some experiments 25% of the light was removed with a *

*

Supported by U.S. Air Force Office of Scientific Research Contract #F44620-75-C-0038 at the Graduate Aeronautical Laboratories, California Institute of Technology. Present address: Dept. of Engineering and Applied Science Yale University, New Haven, Ct.06520, USA.

Lock Recording I I Electronics I I ~I

I

Detector

II Amp

Dewar

-.

Preamp

Fig. 1. Schematic of an experiment to measure optical absorption via second-sound in liquid helium.

beam splitter so that the lamp characteristics could be monitored. The second-sound cell had a cylindrical shape with a 1.25 cm inner diameterand a 7 cm length. One end wall was a quartz window and the other end wall consisted of the sample. The second-sound detector was mounted on an indentation in the sidewall of the cavity at a position corresponding to the antinode of the second resonance. Since the amplitude at an antinode is increased in proportion to the Q-factor of the cavity, a significant amplification is obtained when the cavity is operated in a resonant mode. This is possible at acoustic frequencies because of the low phase velocity of second sound. The cell used in these experiments has a Q of approximately 100 in the second mode, but Q’s of 1000 or more are really attainable [4]. 223

Volume 56A, number 3 1.0

PHYSICS LETTERS

ment due to the resonant cavity make the secondsound method potentially capable of detecting very weak absorptions.

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Fig. 2 is the absorption spectrum of a doped selenium edge filter measured by the above method. This is not to be taken as a direct measure of the absorption coefficient; the quantitative relationship between second sound amplitude and optical absorption coefficient needs further exploration. Preliminary results indicate that it may be possible to measure absorption coefficients of l0—~cm’ more directly by the second sound method. Thus we believe it can be developed into a useful technique for the study of solids, surfaces and the solid-liquid interface at low temperatures.

0.8

.2 0.6

—

Q4

—

0.2

—

\ \

0 400

500

I

I

600

700

Wavelength In ml Fig. 2. Second-sound generated by optical absorption of a doped selenium edge filter.

The second-sound detector consisted of a superconducting thin film [5]. This type of detector provides the needed sensitivity as well as fast response time. When the film is biased on its superconducting transition, the resistivity is a sensitive function of tempera. ture. The second-sound oscillations cause a sinusoidally varying resistance which is converted to a voltage and amplified bya low-noise preamp and lock-in amplifier With proper impedance matching this arrangement is capable of detecting amplitudes of 3 X 10~ degrees [6]. Such a temperature sensitivity plus the enhance-

224

22 March 1976

We are very grateful to H.W. Liepmann for support and encouragement, to T.C. McGill for many helpful discussions and to other too numerous to mention for the loan of various pieces of equipment. We would also like to thank Mr. Jack Wise for his help in the construction of the experiments. One of us (J.B.S.) wishes to thank the National Science Foundation for an Energy-Related Postdoctoral Fellowship. References [11 L.D. ~

Landau and E.M. Lifschitz, Fluid mechanics (Pergamon Press, New York, 1959) p. 517. Today 28(1975) 23.

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[4] H. Snyder, Phys. Fluids 6 (1963) 755. [5] G. Laguna, Cryogenics, to be published. [6] G. Laguna, Phys. Rev. B12 (1975) 4874.