The ternary compound TlGaSe2 crystals belong to the class of layered crystals of complicated structure and fall into C2/c space group. Recently, such semiconductor crystals received a great deal of attention due to their optical and electrical properties in view of possible optoelectronic devise application. Here, experimental evidence that unusually strong photoacoustic pulses (PAP) can be induced in these crystals is presented.
Samples in a form of platelets of several square centimeters were grown by modified Bridgman method. 2-5 mm thick slabs were carefully polished on both sides of layers cross cut planes. The pump-probe technique with orthogonal geometry of pump and probe beams was used. The front sample face is homogeneously excited with 2.5 ns of a tunable wavelength pulses with phonon energies around band gap energy. The free carrier absorption signal and its changes, caused by PAP propagation through probed area is detected using 1.3 or 1.54 μm focused probe beam with polarization orthogonal to c-axis and directed from the lateral side, i.e. carefully aligned along the crystal layers.
It was found that strong PAP are generated by pump laser excitation. The highest response is obtained when pump laser beam is perpendicular to the laminar layers of the crystal. Low laser fluencies (< 0.01 mJ/cm2) are needed to generate PAP. This quantity, normalized to absorption coefficient of TlGaSe2, is four orders of magnitude less than the excitation laser fluence threshold for other semiconductors. The PAP are generated in close proximity of the sample surface and have very small dispersion. Then PAP travels across the sample thickness with the speed of the acoustic waves reflecting from the back and excited surfaces. Contrary to well-known photo-elastic mechanism when the duration of pulse is proportional to the absorption coefficient, it was shown that PAP can be generated by photons with energy below the band edge. High amplitude and low spatial dispersion of the PAP allows some practical application of the observed phenomenon in optical switching from visible to infrared light in telecommunication wavelengths range. Tentative mechanism of the PAP generation is discussed.
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