An experimental study of diffraction-induced effects on single-cycle terahertz pulses.

摘要

This doctoral thesis documents a study of diffraction-induced effects uniquely characteristic of broadband, single-cycle electromagnetic pulses. A general introduction of terahertz (THz) optoelectronics is first presented to properly place this study in context. Models detailing a basic means of generation and detection of THz radiation is also provided. Following this, the spatiotemporal effect on a single-cycle pulse due to the Gouy phase shift is then introduced. This experimental study considers the phase effects imparted on these broadband pulses via transmissive optics (or quadratic phase media), in addition to a direct observation of the predicted on-axis polarity reversal imposed by the Gouy phase shift after passing through a focus. Next, an application exploiting the space-time symmetry of solutions to the wave equation is presented. Here, the transmission function of one and two-dimensional objects is interpreted by time-reversing and back-propagating electric fields diffracted from the object. After deriving a time-reversed form of the Huygens-Fresnel diffraction integral, we demonstrate through simulation and experimentation, the reconstruction of one- and two-dimensional objects by numerically back-propagating measured scattered terahertz transients. The spatial resolution determined by a time-domain adaptation of the Sparrow criterion is found to correspond to approximately 30% of the peak wavelength and 85% of the mean wavelength of the power spectrum associated with the single-cycle waveforms. Finally, the modulation transfer function is simulated and is shown to be nearly diffraction-limited when compared to an ideal imaging system.