Electromagnetic waves of the terahertz (THz) range now turn to be applied in communication systems. Besides, THz radiation can penetrate through many materials such as plastics, fabrics, cardboard, ets; therefore it is advantageous in imaging of biological objects and in medicine. These applications determine the importance of further development of effective generators and detectors for THz radiation. Among the most promising and powerful techniques for THz generation and detection are optical rectification and up-conversion of femto-second laser pulses in semiconductors. By using semiconductors with nanocanals, we have succeeded in increasing efficiencies of these processes by 4-7 times.
To create the nanostructures, GaAs crystals were irradiated by different ions of 90 MeV energy, up to fluence of 1012 cm-2. AFM measurements demonstrated appearance of defects at the bulk and surface of the crystal. The depth of the induced canals was about 15 microns, the diameter was 20-30 nm. The surface defects of the form of hillocks with height of 7 nm were found. IR absorption spectra measured for irradiated samples demonstrated that there were no significant changes in the range of phonon absorption, but noticeable change of spectra was observed in the range of wave vectors higher than 3000 cm-1.
The augment of the efficiency of THz generation in irradiated semiconductors is explained by the fact that non-linear processes are more efficient in materials with irregular structures; this can be caused by concentration of high-frequency electromagnetic field near the structural defects. The concentration leads to a strong increase of the corresponding non-linear polarization of media.
In order to calculate the main characteristics of non-linear susceptibility of semiconductor crystals in the THz range, all scattering mechanisms were taken into account. The results show that the optimum control of temperature and carriers concentration leads to increasing of non-linear susceptibility in more then two orders of value.
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