Abstract
The physics of short-pulse terahertz radiation has recently become an active research field, with interest both in fundamental aspects of generation and detection of tera-hertz (THz) radiation and in spectroscopic applications after many pioneering researches done in the past twenty years. Many methods have been exploited to develop different sources of pulsed THz. One simple method to generate THz radiation is to create the ultrafast current transients in a semiconductor switch illuminated with femtosecond laser pulses. It is also expected that the THz radiation should be emitted from superconductors if the supercurrent is modulated at a sufficiently high speed by femtosecond laser pulses. However, the ultrafast electromagnetic properties of radiated THz pulses from superconductors are generally quite complicated and not well understood. This is particularly true when high power of THz is generated at the condition of high bias current or pumping power. In this dissertation, we observed the ultrashort electromagnetic pulse radiation from a current-biased bow-tie structure of YBa2Cu3O7-δ thin film dipole antenna on MgO substrate by using 100 fs, 750 nm laser pulses. With the scheme of electro-optic detection, we obtained the THz pulses with 1.0 ps full width at half maximum, containing frequency components up to 1.0 THz. The parallel polarization of THz radiation emitted from a current biased superconductive antenna to the direction of the bias current exhibits that the ultrashort pulses were due to a straight current flowing along the bridge, not the current loop at the luminous area of the bridge. The observed radiation waveforms emitted from a superconductor differ slightly from that radiated by a conventional semiconductor antenna. Moreover, the radiation waveforms for superconductors obtained by other researches are also dissimilar to each other. Almost all other researchers focused on the dependence of THz radiation amplitude for superconductors, they did not address the radiated pulse shape. Namely, no theories or models have been developed to describe the transient carrier dynamics in the nonequilibrium state of superconductors. In order to explain the shape of radiation waveforms emitted from the superconductor and describe the transient carrier dynamics, first we discuss the experimental results in relation to the radiation pattern, polarization, and capacitor effect to confirm the origin of ultrashort electromagnetic pulses radiated from a superconducting antenna. Then, we construct a more complete model by using the concept of carrier acceleration based on the classical electrodynamics. For a constant-current biased superconducting antenna, we propose that the total constant current density consists of time-varying suppercarriers and normal carriers in the two-fluid model when the superconducting antenna is excited by femtosecond laser pulses. The transient carrier dynamics in this nonequilibrium state can be obtained by using the Rothwarf and Taylor rate equations. We have solved successfully the time transient quasiparticles density in the nonequilibrium state of superconductors and the calculated far-field radiation fits well for the radiation waveforms with some fitting parameters, such as the recombination rate, phonon pair breaking time and the mobility of the normal carriers. Through the simulated fitting with our observed waveforms, we can estimate that the product of quasipartcle recombination rate and phonon pair breaking time for the YBCO superconductor at low temperatures is five-time larger than the metallic superconductor Pb. It implies that the YBCO superconductor presents a stronger electron-phonon interaction than the metallic superconductor. The effective mobility for normal carriers also obtained from the fitting shows the impurity dominated characteristics of the YBCO thin film and the mobility in real units is estimated to be μq = 660 (cm2 / V-s), comparable to the hole mobility of Si at room temperature. We also investigated widely the THz emission properties of YBCO antenna by measuring the pumping power, bias current and ambient temperature dependences on the radiation amplitudes. The THz peak amplitude dependence shows the saturation and a nonlinear behavior with a higher excitation pumping power and with the applied bias currents. The saturation on the dependence with the excitation powers exhibits the bolometric heating in nature. However, the measured nonlinear dependence on the applied bias current deviated from our linear simulated results on the effective current density especially at moderate and high pump powers or low temperatures. It indicates that our model should be considered more accurately describing the deviation. Finally, we discussed the possibilities to eliminate the deviation by replacing a nonlinear term in the supercarrier density for our model or taking the screen effects into account with the simulated results.