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

Spectroscopic Measurement of Interlayer Screening in Multilayer Epitaxial Graphene

International audienceThe substrate-induced charge-density profile in carbon face epitaxial graphene is determined using nondegenerate ultrafast midinfrared pump-probe spectroscopy. Distinct zero crossings in the differential transmission spectra are used to identify the Fermi levels of layers within the multilayer stack. Probing within the transmission window of the SiC substrate, we find the Fermi levels of the first four heavily doped layers to be, respectively, 360, 215, 140, and 93 meV above the Dirac point. The charge screening length is determined to be one graphene layer, in good agreement with theoretical predictions

Electronic Cooling via Interlayer Coulomb Coupling in Multilayer Epitaxial Graphene

In van der Waals bonded or rotationally disordered multilayer stacks of two-dimensional (2D) materials, the electronic states remain tightly confined within individual 2D layers. As a result, electron-phonon interactions occur primarily within layers and interlayer electrical conductivities are low. In addition, strong covalent in-plane intralayer bonding combined with weak van der Waals interlayer bonding results in weak phonon-mediated thermal coupling between the layers. We demonstrate here, however, that Coulomb interactions between electrons in different layers of multilayer epitaxial graphene provide an important mechanism for interlayer thermal transport even though all electronic states are strongly confined within individual 2D layers. This effect is manifested in the relaxation dynamics of hot carriers in ultrafast time-resolved terahertz spectroscopy. We develop a theory of interlayer Coulomb coupling containing no free parameters that accounts for the experimentally observed trends in hot-carrier dynamics as temperature and the number of layers is varied.Comment: 54 pages, 15 figures, uses documentclass{achemso}, M.T.M. and J.R.T. contributed equally to this wor

Time-Reversal and Model-Based Imaging in a THz Waveguide

We investigate two approaches to improving the resolution of time-reversal based THz imaging systems. First, we show that a substantial improvement in the reconstruction of time-reversed THz fields is achieved by increasing the system’s numerical aperture via a waveguide technique adapted from ultrasound imaging. Second, a model-based reconstruction algorithm is developed as an alternative to time-reversal THz imaging and its performance is demonstrated for cases with and without a waveguide.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85920/1/Fessler15.pd

Hot carrier cooling by acoustic phonons in epitaxial graphene by ultrafast pump-probe spectroscopy

We report the application nondegenerate ultrafast mid-infrared spectroscopy to investigate the acoustic phonon cooling process in epitaxial graphene. We show that the power-dependent experimental results match theoretical predictions of the low temperature acoustic cooling process. The hot phonon effect in acoustic phonon cooling is observed experimentally, and a deformation potential of 30 eV can be determined from fitting the data. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/83459/1/1194_ftp.pd

Coherent Control of Ballistic Photocurrents in Multilayer Epitaxial Graphene Using Quantum Interference

International audienceWe report generation of ballistic electric currents in unbiased epitaxial graphene at 300 K via quantum interference between phase-controlled cross-polarized fundamental and second harmonic 220 fs pulses. The transient currents are detected via the emitted terahertz radiation. Because of graphene's special structure symmetry, the injected current direction can be well controlled by the polarization of the pump beam in epitaxial graphene. This all optical injection of current provides not only a noncontact way of injecting directional current in graphene but also new insight into optical and transport process in epitaxial graphene

Ultrafast dynamics and interlayer thermal coupling of hot carriers in epitaxial graphene

We report the first application of nondegenerate ultrafast pump-probe spectroscopy to investigate the dynamics of hot Dirac Fermions in epitaxial graphene. The DT spectra can be understood in terms of the effect of hot thermal carrier distributions on interband transitions with no electron-hole interaction. We also investigate the thermal coupling between carriers of doped and undoped layers. The coupling time is found to be below 500fs. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/61906/1/470_ftp.pd

Current relaxation due to hot carrier scattering in graphene

In this paper, we present direct time-domain investigations of the relaxation of electric currents in graphene due to hot carrier scattering. We use coherent control with ultrashort optical pulses to photoinject a current and detect the terahertz (THz) radiation emitted by the resulting current surge. We pre-inject a background of hot carriers using a separate pump pulse, with a variable delay between the pump and current-injection pulses. We find the effect of the hot carrier background is to reduce the current and hence the emitted THz radiation. The current damping is determined simply by the density (or temperature) of the thermal carriers. The experimental behavior is accurately reproduced in a microscopic theory, which correctly incorporates the nonconservation of velocity in scattering between Dirac fermions. The results indicate that hot carriers are effective in damping the current, and are expected to be important for understanding the operation of high-speed graphene electronic devices.DFG, 130170629, SPP 1459: Graphen

Mapping the availability of bushmeat for consumption in Central African cities

The trade of bushmeat from rural areas to supply burgeoning cities is a major conservation and livelihood concern. Using a whole-city sampling strategy we mapped the distribution and numbers of meat outlets in the Kinshasa-Brazzaville metropolitan area, two neighboring capital cities in Central Africa. We show that both cities differ in the number and density of meat outlets, with more in Brazzaville per area sampled and inhabitants. The number of meat outlets is related to human population densities and primarily concentrated along the banks of the Congo River, in the more affluent areas of the cities. Across the two cities, roughly 22% of all sampled markets (50% in Brazzaville and 19% in Kinshasa) and 24% of all visited restaurants (24% in each city) were selling bushmeat during our survey. Despite the relatively low number of establishments offering bushmeat for sale, extrapolated to the entire area and population of both cities, we expect the overall amounts of wild animal meats consumed per annum to be significantly high. We suggest that the supply of such amounts of wild animal meat will strongly impact the animal populations sourcing these cities. Our data also indicate that the number of domestic meat outlets may be adequate to supply urban dwellers with sufficient animal protein