Key signal contributions in photothermal deflection spectroscopy

We report on key signal contributions in photothermal deflection spectroscopy (PDS) of semiconductors at photon energies below the bandgap energy and show how to extract the actual absorption properties from the measurement data. To this end, we establish a rigorous computation scheme for the deflection signal including semianalytic ray tracing to analyze the underlying physical effects. The computation takes into account linear and nonlinear absorption processes affecting the refractive index, thus leading to a deflection of the probe beam. We find that besides the linear mirage effect, nonlinear absorption mechanisms make a substantial contribution to the signal for strongly focused pump beams and sample materials with high two-photon absorption coefficients. For example, the measured quadratic absorption contribution exceeds 5% at a pump beam intensity of about 1.3×105W/cm2 in Si and at 5×104W/cm2 in GaAs. In addition, our method also includes thermal expansion effects as well as spatial gradients of the attenuation properties. We demonstrate that these effects result in an additional deflection contribution that substantially depends on the distance of the photodetector from the readout point. This distance dependent contribution enhances the surface related PDS signal up to two orders of magnitude and may be misinterpreted as surface absorption if not corrected in the analysis of the measurement data. We verify these findings by PDS measurements on crystalline silicon at a wavelength of 1550 nm and provide guidelines on how to extract the actual attenuation coefficient from the PDS signal.