Pulsed photothermal mirror technique: characterization of opaque materials

Abstract

The time-resolved thermal mirror technique is developed under pulsed laser excitation for quantitative measurement of thermal and mechanical properties of opaque materials. Heat diffusion and thermoelastic equations are solved analytically for pulsed excitation assuming surface absorption and an instantaneous pulse. Analytical results for the temperature change and surface displacement in the sample are compared to all-numerical solutions using finite element method analysis accounting for the laser pulse width and sample geometry. Experiments are performed that validate the theoretical model and regression fitting is performed to obtain the thermal diffusivity and the linear thermal expansion coefficient of the samples. The values obtained for these properties are in agreement with literature data. The technique is shown to be useful for quantitative determinations of the physics properties of metals with high thermal diffusivity.

Laser induced thermoelastic surface displacement in solids detected simultaneously by photothermal mirror and interferometry

“…The spatiotemporal distribution of the temperature change, T i (r, z, t), is solved for the sample (i = s) and for the surrounding fluid (i = f ), which obeys the heat diffusion equation [9]…”

“…The temperature field is used then, as the source to compute the thermoelastic displacement of the sample, by solving the thermoelastic equation of motion with the appropriate initial and boundary conditions. The displacement field u ≡ ì u(r, z, t) of a homogeneous and isotropic solid is given by the solution of [9]…”

“…The photothermal mirror (TM) spectrometry detects the deformation of the illuminated surface by analyzing the focusing or defocusing of the reflected probe beam in the far field region [8][9][10][11][12]. TM measures a convoluted signal arising from the wavefront distortion caused by the probed surface of the sample in addition to the phase shift created by heat-coupling in the fluid surrounding the sample.…”

“…TM measures a convoluted signal arising from the wavefront distortion caused by the probed surface of the sample in addition to the phase shift created by heat-coupling in the fluid surrounding the sample. This method has been recently introduced under pulsed Gaussian laser excitation for the measurement of thermal diffusivity and thermo-optical properties of semi-transparent and opaque solids [8,9] and for the detection of thermoelastic waves launched by a localized heat deposition [13][14][15]. This method has also been used to detect surface displacements induced by radiation pressure at air-liquids interfaces [11,12].…”

Generation and detection of thermoelastic waves in metals by a photothermal mirror method

“…10 Detection of light-induced elastic waves is feasible with many different experimental approaches. [3][4][5][6][7][8][9][10][11][12][13][14][15] Bulk and surface waves can be detected utilizing capacitive, piezoelectric, electromagnetic, and optical transduction mechanisms. 16 The possibility of remote generation and stand-off detection of elastic waves using optical probes makes photothermal methods attractive for material characterization 15 and non-destructive testing.…”