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Based on the analogy between resistor–capacitor (RC) filters and the thermoelastic component of a photoacoustic signal, the complex thermoelastic response of an open photoacoustic cell is described as a simple linear time-invariant system of a low-pass RC filter. This description is done by finding a linear relation between the thermoelastic cut-off frequency and sample material thickness within the range of 10–1000 μm. Based on the theory of a composite piston, we run numerical simulations of the proposed method for n-type silicon, described as either a surface or volume absorber. Theoretical predictions are experimentally validated by using an open photoacoustic cell setup to record the signal of an 850-μm-thick n-type silicon wafer illuminated by a blue light source modulated with frequencies from 20 Hz to 20 kHz. The obtained experimental results confirm the linear dependence of the thermoelastic cut-off frequency on the sample thickness, and, therefore, they lay the foundation of a new method for the thermal characterization of materials. [ABSTRACT FROM AUTHOR]

The acoustic attenuation inside the bandgaps is, together with the bandgap width, a fundamental design parameter for phononic-crystal-based systems. We discuss approximate expressions for the maximum attenuation inside the bandgaps of one-dimensional longitudinal phononic crystals and its dependence on the acoustic contrast and the fractional bandwidth. We provide different approximations at small and large fractional bandwidths, computed from the trace of the transmission matrix of the crystal elementary cell. We show that, for relatively small gaps, the attenuation is roughly proportional to the fractional bandwidth, in analogy with the flexural case. For larger gaps, a large attenuation can be obtained only for high (and possibly impractical) acoustic contrasts. Approximate expressions are validated through comparison with FEM results. We also derive asymptotic upper limits for the bandgap borders and show that high contrasts do not necessarily lead to wide bandgaps, a fact connected to geometrical phase inversion for the acoustic wave in the crystal. We finally compare the attenuation of flexural and longitudinal waves at a fixed fractional bandwidth and derive regions of optimum attenuation for the two propagation modes. [ABSTRACT FROM AUTHOR]

We theoretically investigate the potentiality of dual phononic-photonic (the so-called phoxonic) crystals for liquid sensing applications. We study the transmission through a two-dimensional (2D) crystal made of infinite cylindrical holes in a silicon substrate, where one row of holes oriented perpendicular to the propagation direction is filled with a liquid. The infiltrated holes may have a different radius than the regular holes. We show, in the defect structure, the existence of welldefined features (peaks or dips) in the transmission spectra of acoustic and optical waves and estimate their sensitivity to the sound and light velocity of the analyte. Some of the geometrical requirements behave in opposite directions when searching for an efficient sensing of either sound or light velocities. Hence, a compromise in the choice of the parameters may become necessary in making the phoxonic sensor. © 2014 AIP Publishing LLC. [ABSTRACT FROM AUTHOR]