| Abstract [eng] |
First part of this work was orientated on investigation of new material suitable for THz components activated with longer wavelength (1 and 1.55 μm) laser radiation. Dynamics of photoexcited electrons was investigated from the optically induced THz absorption of LTG GaAs layers grown on AlAs/GaAs Bragg mirror and annealed at moderate temperature. It was shown that electron lifetime in these layers is less than 1 ps. Induced THz absorption increased linearly with increasing laser pulse intensity. The maximum absorption was observed for the layers annealed at 420 °C temperature. LTG GaAs epitaxial layers annealed at this temperature exhibit the highest photoconductivity when illuminated with 1 μm laser, because these layers possess optimum combination of arsenic defect concentration and electron mobility. It was determined, that epitaxial layers annealed at 420 °C are preferred for production of THz components: emitters, detectors and mixers, activated with 1 μm radiation. In the second part of this work, photoconductivity of THz detector dependence on activating femtosecond laser pulse wavelength was measured. Photoconductivity spectra of detectors made from LTG GaAs and LT In0.7Ga0.3As epitaxial layers annealed at moderate temperature at longer than material bandgap wavelength were governed by electron excitation from the deep defects. From the comparison of measured spectra of LT GaAs and LT In0.7Ga0.3As epitaxial layers with theoretical model, position and bandwidth of arsenic antisite defect band was determined for aforementioned materials. For the case of LT GaAs these parameters were equal to εdef = 0.77 eV and Δεdef = 78 meV; for the case of LT In0.7Ga0.3As were equal to εdef = 0.4 eV and Δεdef = 70 meV respectively. Finally, terahertz bursts system pumped with solid-state femtosecond Yb: KGW laser and using an external optical expander was designed and tested. The system employed LT GaAs photomixer and exhibited 40 ps duration THz burst radiation. The linewidth of burst varied from 40 to 120 GHz, with the central frequency reaching up to 1 THz. It was shown that terahertz bursts generation system activated with a picosecond fiber laser is more compact and more attractive for the operation under ambient conditions, than one that uses femtosecond laser pulses and two diffraction grating pulse stretcher. THz bursts generation system was tested in coherent and non-coherent detection modes in THz imaging experiment. Images of various objects in the frequency range from 0.1 THz to 1 THz were obtained. |
