Phonon focusing in [001] germanium

Phonon focusing in [001]-oriented Ge crystals has been studied at temperatures near 2.0K using electron-beam scanning for phonon generation and thin-film bolometers with an effective area of 10 m×10 m or smaller for phonon detection. The thickness of the Ge crystals was 2 or 3 mm. In our experiments the angular resolution appeared to be dominated by effective diameter of the local region heated by the beam and acting as phonon source. Whereas metallic overlay films did not appreciably effect the angular resolution, a distinct increase in resolution due to high-frequency beam modulation has been observed which can be understood in terms of the thermal skin effect. From a comparison between theoretical calculations and our experimental results the effective diameter of the phonon source has been found to be 20–30 m. Our focusing images show appreciable deviations from theoretical calculations in the low-frequency limit. However, excellent agreement is obtained, if dispersion is included and if the dominant phonon frequencies are place in the range 300–400 GHz.