Diffraction-limited THz time-domain spectroscopy to investigate electrodynamics of correlated electron systems

Abstract

Terahertz (THz) spectroscopy has attracted much attention because it is the unique tool for giving information regarding the carrier dynamics. Although THz spectroscopy is now stepping into a mature stage, some of technical difficulties still remain. In this dissertation, we present the limitations of current conventional THz spectroscopy technique and propose the way for overcoming these drawbacks. And, using our proposed the new-technique of THz spectroscopy, we provide the carrier dynamics of n-InAs and Tb-doped Sr2IrO4. First, we explore the propagation properties of THz light itself. We generate the THz wave on the surface of an unbiased GaAs crystal by illuminating femtosecond laser pulses with a 45° incidence angle, and investigate its propagation properties comprehensively both in a near-field and in a far-field zone by performing a knife-edge scan measurement. We observe a crossover from the Gaussian beam in the near-field zone to the anisotropic non-Gaussian beam in the far-field zone. In the near-field zone, i.e. 540 mm away from the generation point, we found that the beam simply takes a Gaussian shape of which width follows well a behavior predicted by a paraxial wave equation. In the far-field zone, on the other hand, it takes a highly anisotropic shape; whereas the beam profile maintains a Gaussian shape along the normal to the plane of incidence, it takes satellite peak structures along the direction in parallel to the plane of incidence. From the comparison with simulation results obtained by using a dipole radiation model, we demonstrated that this irregular beam pattern is attributed to the phase retardation of the transient dipoles which leads to the interference of the waves in the far-field zone. Also, we found that this consideration can be applied for a comprehensive understanding of THz beam profiles obtained in several different configurations. Second, we investigate a free space propagation of a truncated THz beam created by a partial reflection from a small-sized metallic reflector. By using THz time-domain spectroscopy, we obtain both magnitude and phase spectra of a clipped THz beam after its propagation by about 10 cm. Compared to a fully-reflected THz beam, a gradual and a large amount of the reduction are founded in magnitude spectra at the THz frequency region (0.2 – 1.3 THz) as turning into the partially reflected beam. In contrast, drastic change is observed in phase spectra. To model the free space propagation of the truncated Gaussian beam, we decompose it using a super-Gaussian and several other Gaussian beams with phase factors which is determined by the refractive index difference of an interface between the metallic reflector and the just right faced medium. From this, we could successfully reproduce the experimental results, i.e., the modulations observed in both magnitude and phase spectra. We utilize such factors to recover the magnitude and phases spectra which the large-enough sample is supposed to have, and retrieve the absolute values of optical constants reasonably well for the InAs of which size is smaller than the beam size. Third, we investigate temperature-dependent carrier dynamics of the InAs crystal by using reflection-type terahertz time-domain spectroscopy particularly with a recently developed emitter-sample hybrid structure. We successfully obtain the optical conductivity in a THz frequency of the bulk InAs whose dc-conductivity is in the range of 100-150 W-1cm-1.We find that both real and imaginary parts of optical conductivity can be fit well with the simple Drude model, and the free carrier density and the scattering rate obtained from the fitting are in good agreement with corresponding values obtained by using other techniques, such as the Hall measurement and the dc-resistivity measurement. These results clearly demonstrate that the proposed technique adopting the emitter-sample hybrid structure can be exploited to determine temperature-dependent optical constants in a reflection geometry and hence to investigate electrodynamics of bulk metallic systems. Finally, we observe that the temperature dependent carrier dynamics of the bulk iridate Sr2(Ir,Tb)O4 by means of the terahertz time-domain spectroscopy in the reflection geometry that can be corroborated by the bandstructure calculated from the tight-binding method with the adjusted strength of the Jeff=1/2 spin current hidden order. These observations suggest that the quasi-2D Dirac semimetallic phase can emerge in the tiny doped-Sr2IrO4 regardless of the doping type, providing the new knowledge for the high-temperature superconducting phase in connection with the Dirac fermion, which resembles the attempt in the cuprate physics.