Ultrafast Terahertz Spectroscopy And Nonlinear Optics Of Semiconductor Nanostructures

This manuscript presents 2 individual experimental studies in the field of ultrafast carrier dynamics of semiconductor nanostructures:1.The development of Terahertz time domain spectroscopy(THz-TDS)and its application in gain measurements of GaAs/AlGaAs quantum cascade lasers.2.The 3^rdorder nonlinear optical properties of tellurite niobic glasses doped with AgCl nanocrystalss with Z-scan and degenerate four waves mixing(DFWM).In order to accomplish this goal,we developed a THz-TDS system with different radiation sources: optical rectification in a ZnTe crystal and interdigitated semi-insulating gallium arsenide photoconductive antennas.First,we studied the competition between Tera-Hertz(THz)generation by optical rectification in〈110〉ZnTe crystals,two-photon absorption,second harmonic generation and free-carrier absorption. The two-photon absorption coefficient,second harmonic generation efficiency and free-carrier absorption coefficient in the THz range are measured independently.The incident pump field is shown to be depleted by two-photon absorption and the THz radiation is shown to be reduced,upon focusing, by free-carrier absorption.The reduction of the generated THz radiation upon tight focusing is explained,provided that one also takes into account diffraction effects from the sub-wavelength THz source.Second,we investigated the temporal and spectral properties of the THz emission of a novel semi-insulating GaAs interdigitated photoconductive antenna with different excited carrier densities,in a temperature range from 4.2K to 270K.The THz emission properties are shown to be linear at low bias fields but saturated at large fields due to the intervalley scattering from theΓband to the L band.The THz performances also saturate for high laser excitation due to the space charge screening.The carrier dynamics at different temperatures was experimentally investigated by recording the temporal and spectral shape of the THz wave.The emitted THz spectrum shifts to the lower frequency with increasing temperature.The impact of electron-hole mobility in this problem is discussed.Third,we compared the aforementioned two THz generation methods and decided to use photoconductive antennas as the excitation source of our THz-TDS to investigate the gain and losses of a THz quantum cascade laser(QCL)operating at 2.9THz.We investigated the THz gain variation with different injection currents and different temperatures.We measured THz amplitude and phase spectra,allowing the direct determination of the gain.At the emission frequency of the QCL,a value of 6.5cm^-1is found.Effects such as gain clamping and spectral narrowing are also observed.Finally,We investigated the non-resonant ultrafast 3^rdorder nonlinear optical properties of niobic tellurite glass(80TeO₂-20Nb₂O₅)doped with silver chloride(AgCl)nanocrystals.Our samples were synthesized by melting-quenching and thermal treatment techniques.We control the grain-size of AgCl nanocrystals by varying the thermal treatment time.Their structures and AgCl nanocrystalss were characterized by spectroscopy and electronic microscopy.The longer thermal treatment time can enlarge the grain-size and increase the AgCl nanocrystalss density,and further results in forming more defect centers and stronger lattice deformation at the interface of AgCl nanocrystals,which gives rise to the bandgap redshift and formation of trapped state excitons.The larger two-photon absorption and local state carrier absorption result in a decrease of the optical limiting threshold,and the third order nonresonant nonlinear susceptibility x⁽³⁾can be enhanced by the transient polarization of trapped state electrons on the surface of AgCl nanocrytstals.