摘要:

The history of the theoretical discovery of the Brillouin effect is presented along with the experimental discovery of this phenomenon many years after the theoretical prediction. A fairly complete resume of the work accomplished on the Brillouin effect in liquids in the era prior to the invention of the laser is given in outline.

ISSN:0001-4966    

DOI:10.1121/1.1912452

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Large‐angle Rayleigh scattering of light provides a tool for studying fluctuations in dilute gases where the wavelength is comparable to the mean free path. Interpretation of such an experiment requires a molecular calculation of the spectrum of the density fluctuation. The initial value problem can be formulated using the linearized Boltzmann equation. Solutions are obtained for two types of repulsive intermolecular potential, Maxwell molecule and rigid sphere, and the results are used in a quantitative analysis of high‐resolution measurement. The problem of studying molecular gases in a similar way is also discussed.

ISSN:0001-4966    

DOI:10.1121/1.1912453

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

A high‐sensitivity high‐resolution system for the study of light scattered from thermal fluctuations in gases has been constructed. A Bendix Channeltron photomultiplier tube with a dark noise of three‐tenths of a photoelectron/second is the basis of an experimental setup which includes a Fabry‐Perot interferometer having a bandwidth of 10 MHz with a piezoelectric scan and a frequency stabilized single‐mode He‐Ne laser. Resolution of 10 MHz is, therefore, attainable, and signal levels as low as six‐tenths of a photoelectron/second yield reliable data. With this apparatus the density‐density correlation function,G(r,t), of several gases has been investigated for a range of pressures. Whether the form ofG(r,t) is derived from a kinetic or hydrodynamic view depends on the value of the Knudsen number, i.e., the ratio of λs, the wavelength of the scattering fluctuation, to the mean free path. Previous studies have concentrated on varying the angle of scattering (changing λs) while this study varies the gas pressure itself (changing the mean free path), allowing a more detailed investigation of the transition from the kinetic to the hydrodynamic view.

ISSN:0001-4966    

DOI:10.1121/1.1912454

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

By making use of stimulated Brillouin scattering, hypersonic absorption and velocity measurements in quartz glass were carried out at 24 GHz. Absorption data for longitudinal sound waves are presented below 8°K and velocity data between 4° and 300°K. The hypersonic absorption is observed to decrease strongly on cooling corresponding to a mean free path of the hypersonic phonons larger than 2 mm below 3.5°K. This value exceeds the mean free path calculated from heat conductivity data at 1.5°K for thermal phonons of similar energy by three orders of magnitude. The absorption data are discussed in connection with previous lower‐frequency measurements.

ISSN:0001-4966    

DOI:10.1121/1.1912455

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Intense stimulated Brillouin scattering produced by high‐power (greater than 107W/cm2) light beams is discussed in this paper. It is shown theoretically that the Brillouin intensity increases exponentially with the incident light intensity, with the interaction length, and with a gain factor depending on the medium properties. Quantitative estimates of the stimulated Brillouin scattering have been obtained from experiments using traveling waves and amplifying systems.

ISSN:0001-4966    

DOI:10.1121/1.1912456

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

A summary of important results obtained by the authors on stimulated Brillouin scattering is presented. The paper deals with three topics: (1) fine structure of light scattering by the Rayleigh line wing, (2) stimulated Mandel'stam‐Brillouin scattering and temperature light scattering, and (3) stimulated Mandel'stam‐Brillouin scattering and glass fracture.

ISSN:0001-4966    

DOI:10.1121/1.1912457

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Two methods are presented for generating sound at microwave frequencies with aQ‐switched ruby laser. They are based on the modulation of the laser due to its mode structure and on the nonlinear properties of the solid. Sound is created at the laser modulation frequency. (1) Electrostrictive mixing in aLiNbO3crystal, using a forward Brillouin scattering geometry, produces sound pulses at 800 MHz inside the crystal. Acoustic power densities were around 0.1 mW/mm2. (2) We then used surface heating of a metal film deposited on the surface of a dielectric crystal. At microwave frequencies, good efficiencies can be achieved; at 840 MHz, we generated in a gold film nearly 10 mW/mm2of sound in 30‐nsec pulses.

ISSN:0001-4966    

DOI:10.1121/1.1912458

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Brillouin spectra of liquidCCl4(at 20.0°C) have been investigated 20 scattering angles, from 40° to 145°. The spectra were excited by 6328‐Å radiation from a He‐Ne laser and analyzed with a pressure‐scanned Fabry‐Perot spectrometer. The measured frequency shifts and linewidths of the Brillouin component were used to determine velocities and attenuations of thermal waves in the frequency range 1.6–4.7 Gc/sec. A large dispersion (>10%) and corresponding change in attenuation were found, characteristic of a thermal relaxation process in this region. A broad new component in the spectrum of isotropic scattering was observed, which is also characteristic of a relaxation process. Analyses of these data were carried out in two ways: one is based on a simple extension of acoustic equations to light scattering, and the other, on Mountain's theory of light scattering in the region of a thermal relaxation. It is shown that the observed spectra and derived data agree with Mountain's theory in all respects. Also, from these data it was possible to evaluate the total internal specific heatCI = 11.6±0.3 cal/mole°C. Within experimental error, this relaxes in a single relaxation timeτ = (6.5±0.5) × 10−11 sec.

ISSN:0001-4966    

DOI:10.1121/1.1912459

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Brillouin spectra of ethyl ether and carbon disulfide (at 20.0°C) were investigated for scattering angles of 20°–145°. A single‐mode laser was used for excitation; scattered light was collected in a solid angle of 1.5×10−4sr; and the total instrumental width was ∼100 Mc/sec. Linewidths of 10±5 to 215±10 Mc/sec were measured and velocities determined to an accuracy of ±0.5%. In ethyl ether the phonon velocity and attenuation were constant in the frequency range 0.8–4.1 Gc/sec, and the values agreed with acoustic measurements at 100 Mc/sec. InCS2a considerable decrease in attenuation was observed in the range 1.7–6.1 Gc/sec, with the value at high frequency approaching the classical attenuation attributed to shear viscosity. This variation in attenuation is in good agreement with the theory of a single relaxation time for the total vibrational specific heat, as established by acoustic measurements in the frequency range 2–200 Mc/sec.

ISSN:0001-4966    

DOI:10.1121/1.1912460

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Formulas for the spectrum of light scattered by the fluctuations of density and temperature, or pressure and entropy, in a thermally conducting viscous fluid are derived by applying the fluctuation‐dissipation theorem directly to the scattering configuration. The language of simple circuits and familiar logic of Nyquist are used, and particular attention is paid to the phasing between the fluctuations of different variables. Spectral profiles and the correlation between the fluctuations of the variables indicated by these rather complicated formulas are also discussed in terms of physical concepts (normal modes, degrees of freedom, and the relaxation due to thermal conduction). Contact is made in this way with both classical and recent theories of light scattering. Limitations in these other theories due to inadequate phasing of fluctuations are described and extensions are proposed. It is established that different sets of approximate normal coordinates are required for an adequate simple description of spectra, depending on the ratio of the frequencies of relaxation and acoustic resonance. It is also shown that the traditional neglect of the contributions of temperature fluctuations may lead to serious error in the analysis of Brillouin spectra of scattered light. The extension of the new theory to fluids with relaxing heat capacity and compressibility or a different structure factor is sketched briefly.

ISSN:0001-4966    

DOI:10.1121/1.1912442

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP