Acoustic waves monochromatic

Figure 4 An acousto-optic tunable filter [a, tellurium dioxide (Te02) crystal b, incident or input beam c, acoustic transducer d, rf input e, monochromatic light (ordinary beam) f, nonscattered light beam g, monochromatic light (extraordinary beam) h, acoustic wave absorber].

Chromatic aberrations do not arise in the acoustic microscope because in its usual mode of operation it may be considered essentially monochromatic. Even when it is necessary to take the spread of frequencies in the acoustic pulses into account, the media through which the waves pass are essentially non-dispersive in solids over the frequency range of interest the phonons are very near the centre of the first Brillouin zone where the dispersion relationship is linear, especially for sapphire. 

There have been a few other experimental set-ups developed for the IR characterization of surfaces. Photoacoustic (PAS), or, more generally, photothermal IR spectroscopy relies on temperature fluctuations caused by irradiating the sample with a modulated monochromatic beam the acoustic pressure wave created in the gas layer adjacent to the solid by the adsorption of light is measured as a function of photon wavelength... 

As shown in Fig. 19 for solid samples, monochromatic light, chopped at a frequency in the order of magnitude of 10-1000 cps which is low compared with the velocity of deactivation, strikes the solid sample contained in a sample holder. After excitation and relaxation the released heat diffuses to the surface, passes into the gas phase and acts as an acoustic piston which generates a pressure wave detected by the microphone and amplified by a phase-sensitive amplifier locked to the chopping frequency co. Solution of the heat diffusion equation proves that after a distance x from their starting point the heat waves are damped by ... 

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