High Power THz Generation in a GaP waveguide and the THz Carrier Dynamics in Epitaxial Graphene.

The generation and detection of ultrafast time domain (TD) THz pulse trains is an active area of research, with recent developments pushing sources to higher power and greater bandwidth. This thesis presents research in two frontiers of the science and technology of THz radiation; the generation of high power TD-THz pulses and the dynamic THz spectroscopy of an emerging new material, epitaxial graphene. To increase the SNR of conventional time domain (TD) THz sources, a novel method is proposed for high average power, high repetition rate, TD-THz generation based on an ultrafast fiber laser and optical rectification inside a GaP waveguide. A model for the THz generation is developed by combining a finite-difference frequency-domain mode solver with the 1D generation equation. The measured 150-µW average power and 3 THz bandwidth represent nearly a two order of magnitude increase over conventional TD-THz systems, and are in good agreement with the theoretical model. Since the demonstration of the isolation of single atomic sheets of graphite, graphene has received tremendous attention due to its unique mechanical and electrical properties. These unique properties indicate graphene is a highly promising material for high-speed (THz-bandwidth) electronic devices. This thesis presents TD-THz spectroscopy of multilayer epitaxial graphene samples, with the goals of identifying the presence of a possible bandgap opening at low energies and of measuring the hot carrier recovery dynamics on picosecond timescales. The graphene transmission spectrum is shown to be remarkably flat and is used to verify the absence of a bandgap at meV energies. Optical pump – THz probe measurements of the temperature-dependent recovery dynamics show a biexponential recovery with which is compared with theoretical predictions. Lastly, THz detection of coherent controlled photocurrents is demonstrated for the first time in epitaxial graphene. Optical coherent control provides a method for contactless injection of ultrafast current bursts into semiconductor materials. The associated radiated THz pulse is used to verify the unique polarization independence and power scaling with theoretical predictions. The effect of background hot carriers on the coherent generation process is explored and the dephasing of the coherent current injection is observed for the first time.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/64643/1/cdivin_1.pd

Aspects of laser absorption spectroscopy in the mid-infrared and visible

Laser absorption spectroscopy can be used to identify and quantify gas analytes. The sponsor company’s present systems operate in the mid-infrared using room temperature pulsed quantum cascade lasers (pulsed-QCL’s). These systems use the noise reduction / sensitivity enhancing technique of sweep integration (SI). In this work, an extension of measurement capabilities is sought in two ways. Firstly, sensitivity enhancement is pursued. The noise reduction technique of wavelength modulation spectroscopy (WMS) is applied using a room temperature continuous wave (cw) QCL spectrometer. Secondly, molecular oxygen is added to the list of measurable analytes. This molecule’s near-infrared and visible transitions are addressed with a wavenumber prototype semiconductor diode laser. The sensitivities of the SI and WMS techniques are compared for the cw-QCL spectrometer, and compared to the SI sensitivity of a typical company pulsed-QCL system. New analysis and modeling software was written to facilitate the thesis work and to carry it forward. A thorough analysis of a pulsed-QCL CT3000 analyzer is undertaken to minimize a reduction in capability - should an oxygen measuring laser replace one of its pulsed-QCL’s. The experimental work was constrained by time and budget - particularly with regard to the cw-QCL spectrometer’s AC-coupled detection. Using AC-coupled detection had cost and integration advantages, but posed a number of problems - including electronic incompatibility issues. Nevertheless, the outlook is positive, and a modest sensitivity improvement was found for WMS over sweep integration (0.017 absorbance units (a.u.) in 102.4s compared to 0.080 a.u. in 51ms). Both sensitivities are some way behind the present sweep integration performance of the company’s pulsed spectrometers (0.004 a.u. in 10ms). However, the sensitivities are comparable to earlier stages of development. In the case of oxygen spectroscopy, the prototype diode laser’s thermal stability was an issue, but several spectral regions were found to be suitable for single or multimode spectroscopy.Laser absorption spectroscopy can be used to identify and quantify gas analytes. The sponsor company’s present systems operate in the mid-infrared using room temperature pulsed quantum cascade lasers (pulsed-QCL’s). These systems use the noise reduction / sensitivity enhancing technique of sweep integration (SI). In this work, an extension of measurement capabilities is sought in two ways. Firstly, sensitivity enhancement is pursued. The noise reduction technique of wavelength modulation spectroscopy (WMS) is applied using a room temperature continuous wave (cw) QCL spectrometer. Secondly, molecular oxygen is added to the list of measurable analytes. This molecule’s near-infrared and visible transitions are addressed with a wavenumber prototype semiconductor diode laser. The sensitivities of the SI and WMS techniques are compared for the cw-QCL spectrometer, and compared to the SI sensitivity of a typical company pulsed-QCL system. New analysis and modeling software was written to facilitate the thesis work and to carry it forward. A thorough analysis of a pulsed-QCL CT3000 analyzer is undertaken to minimize a reduction in capability - should an oxygen measuring laser replace one of its pulsed-QCL’s. The experimental work was constrained by time and budget - particularly with regard to the cw-QCL spectrometer’s AC-coupled detection. Using AC-coupled detection had cost and integration advantages, but posed a number of problems - including electronic incompatibility issues. Nevertheless, the outlook is positive, and a modest sensitivity improvement was found for WMS over sweep integration (0.017 absorbance units (a.u.) in 102.4s compared to 0.080 a.u. in 51ms). Both sensitivities are some way behind the present sweep integration performance of the company’s pulsed spectrometers (0.004 a.u. in 10ms). However, the sensitivities are comparable to earlier stages of development. In the case of oxygen spectroscopy, the prototype diode laser’s thermal stability was an issue, but several spectral regions were found to be suitable for single or multimode spectroscopy

Femtosecond optical parametric oscillator frequency combs for coherent pulse synthesis

Coherent pulse synthesis takes as its objective the piecewise assembly of a sequence of identical broadband pulses from two or more mutually-coherent sequences of narrowband pulses. The requirements for pulse synthesis are that the parent pulses share the same repetition frequency, are phase coherent and have low mutual timing jitter over the required observation time. The work carried out in this thesis explored the requirements for broadband coherent pulse synthesis between the multiple visible outputs of a synchronously pumped femtosecond optical parametric oscillator. A femtosecond Ti:sapphire laser was characterised and used to pump a PPKTP-based OPO that produced a number of second-harmonic and sum-frequency mixing outputs across the visible region. Using a novel lock-to-zero CEO stabilisation technique, broadband phase coherence was established between all the pulses on the optical bench, producing the broadest zero-offset frequency comb to date. Employing a common optical path for all the pulses provided common-mode rejection of noise, ensuring less than 150 attoseconds of timing jitter between the pulses over a 1 second observation window. The parent pulses were compressed and their relative delays altered in a quasi-common path prism delay line, allowing pulse synthesis at a desired reference plane