Semester

Summer

Date of Graduation

2013

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Alan D. Bristow.

Abstract

This dissertation summarizes a first effort to address the suitability of chalcopyrite structured semiconductors for efficient and broadband terahertz (THz) pulse emission by optical rectification. The experiments demonstrate that ZnGeP2, a chalcopyrite semiconductor showing improved growth quality in recent decades [1,2], is a promising source for THz spectroscopy while also addressing general fundamental questions concerning the nonlinear generation process in chalcopyrite semiconductors. Beyond ZnGeP2, the potential of other chalcopyrite structured semiconductors for THz emission is examined in the context of calculated phase-matching characteristics, onset pump wavelength of multiphoton absorption, and tabulated second-order nonlinear coefficients. Based on these, CdGeP2 is determined to show particular promise among semiconductors as a THz source when pumped at or near the fiber laser line of 1.55 im. The orientation dependence of THz optical rectification efficiency is examined in tetragonal birefringent ZnGeP2, compared to that of cubic GaP, and modeled based on the second-order nonlinear tensor, while accounting for birefringence. These results demonstrate the significant effects of birefringence on THz emission efficiency while defining the most efficient orientations for generation and can be generalized to other chalcopyrite semiconductors; the general approach is applicable to other uniaxial birefringent crystals. Experimental mapping of generated THz pulses is presented over a broad pump tuning range (1120-2480 nm) in 3 mm thick ZnGeP2 crystals. This mapping, for two distinct and efficient orientations, demonstrates the moderate angle tunability of phase mismatch achievable in chalcopyrite crystals; the data also continuously demonstrate the effects of phase mismatch on the temporal and spectral THz pulse waveform. Finally, pump-intensity dependence of emission is presented for three crystal lengths, showing the effects of near-infrared nonlinear optical absorption and THz free-carrier absorption. Measurements, modeling, and calculation of characteristic lengths provide insight into the upper limit of pulse intensity and crystal length for generation of intense terahertz pulses without detriment to the bandwidth. Direct comparison of generation efficiency from GaP and ZnGeP2 is also measured for reference.

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