Theory of the photoacoustic effect with solids

Abstract

Combined optical and acoustical method for determination of thickness and porosity of transparent organic layers below the ultra-thin film limit Rev. Sci. Instrum. 82, 103111 (2011) Spatial anisotropy of the acousto-optical efficiency in lithium niobate crystals J. Appl. Phys. 108, 103118 (2010) Observation of the forbidden doublet optical phonon in Raman spectra of strained Si for stress analysis Appl. Phys. Lett. 97, 041915 (2010) High-resolution laser lithography system based on two-dimensional acousto-optic deflection Rev. Sci. Instrum. 80, 085105 (2009) Additional information on J. Appl. Phys.When chopped light impinges on a solid in an enclosed cell, an acoustic signal is produced within the cell. This effect is the basis of a new spectroscopic technique for the study of solid and semisolid matter. A quantitative derivation is presented for the acoustic signal in a photoacoustic cell in terms of the optical. thermal, and geometric parameters of the system. The theory predicts the dependence of the signal on the absorption coefficient of the solid, thereby giving a theoretical foundation for the technique of photoacoustic spectroscopy. In particular, the theory accounts for the experimental observation that with this technique optical absorption spectra can be obtained for materials that are optically opaque.

Optical Absorption and Photocurrent Spectra of CdSe Quantum Dots Adsorbed on Nanocrystalline TiO2 Electrode Together with Photovoltaic Properties

“…Figure 1 shows the schematic diagram of a photoacoustic spectrometer. Typical gas-microphone method was applied in the PA spectroscopic investigation (Rosencwaig & Gersho, 1977). The PA cell was composed of an aluminum cylinder with a small channel at the periphery in which a microphone (electret condenser type) was inserted (Shen &Toyoda, 2004).…”

“…Modulation frequency of 33 Hz was determined to exclude the saturation effect of the spectrum. In this case (modulation frequency: 33 Hz), the optical absorption length is longer than the thermal diffusion length, indicating that the PA signal intensity is proportional to the optical absorption coefficient (no saturation effect) (Rosencwaig & Gersho, 1977). The PA signal was monitored by first passing the microphone output through a preamplifier and then into a lock-in amplifier.…”

“…Thus, the PT technique is a useful technique for studying the optical absorption spectra of the strongly scattering and/or opaque samples with which we are dealing in our investigations. PA method detects the acoustic energy produced by heat generation through nonradiative processes in materials (Rosencwaig & Gersho, 1977). The PA cell, which is a small gas-tight enclosure with a sensitive acoustic microphone built in one wall, monitors the temperature changes in the sample produced by absorbed photons through nonradiative processes.…”

Photoacoustic spectroscopy as an approach to assess chemical modifications in edible oils

“…5 The heat propagation across the whole chamber is governed by heat diffusion equations of each medium coupled via boundary conditions at the interfaces (T a = T b and k a dT a /dx = k b dT b /dx, a and b are adjacent media, T is temperature and k is thermal conductivity). After all mathematical treatment, the normalized photoacoustic signal (S n ) produced by the microphone is: [5][6][7][8][9][10]17 (2) where r s = b s /s s , b s is the sample optical absorption coefficient, s s = (1 + i)a s with i = (-1) ½ and a s = (p f /a s ) ½ , a s is the sample thermal diffusivity, b nm = k n a n /k m a m (n = g, s, b, that is, g = gas, s = sample, b = backing), and L S is the sample thickness. This equation is a complex mathematical function of the optical, thermal and geometrical parameters of the system, and it governs our results for all the chopper frequencies.…”

“…4 PAS is based on the photoacoustic effect discovered by Alexander Graham Bell in 1880. 5 This effect is obtained in a closed chamber which is filled with a gas (in general, air) and contains a transparent window which permits a modulated (chopped) radiation to reach the sample. If the sample absorbs this modulated radiation then heat is produced due to non-radiative de-excitation processes within it.…”