Dynamics of quantum materials at the nanoscale

Programming the properties of quantum materials on demand is a central goal of condensed matter physics with the potential to usher in a new technological era. Photoexcitation has proven to be an exceptionally capable means of resonant and non-resonant control over matter offering coveted routes to selectively control the electronic, lattice, interband or valley optical and excitonic properties of quantum materials. One major limitation of probing the rich class of phenomena enabled by photoexcitation is the diffraction limit. The properties of quantum materials are often sensitive to the microscopic details of the environment at phase transition boundaries: which naturally leads drastic inhomogeneity at the nanoscale. In other cases, the media may transiently support high-momentum “nano-light” or host topologically protected conductive channels that are localized to one-dimensional physical edges. All of these phenomena demand a probe with the spatial resolution that is commensurate with the emergent behavior. To address these demands the author contributed to the development of time-resolved scattering near-field optical microscopy (Tr-SNOM). Utilizing the principles developed as part of this thesis amplified laser technology was combined with a commercial near-field optical microscope to produce a state-of-the-art time-resolved nanoscope. The custom apparatus operates with twenty nanometer spatial resolution with unprecedented spectral coverage spanning visible to mid-infrared all with (30-300) femtosecond temporal resolution. The experimental apparatus was, first, applied to investigate the photo-induced insulator-to-metal transition in Vanadium Dioxide. We observe nanoscale inhomogeneity of the transient conductivity. Our data reveals that local nanoscopic variations of the strain exist in our particular VO2 thin film at equilibrium. Regions of compressive strain are, furthermore, found to correlate with regions where a high degree of transient conductivity is attained. Our systematic study of the local fluence dependence and dynamics reveal that the fluence threshold, Fc, for the monoclinic-insulator to rutile-metal transition is inhomogeneous in real-space. A second growth process is identified, even at excitations fluences well below Fc, which operates on a longer timescale with an inhomogeneous rise time, tau-1. Together Fc and tau-1 govern the inhomogeneous nano-texturing of the transient conductivity. Secondly, we uncover that crystals of van-der Waals (vdW) semiconductors behave as optical waveguides with broadly tunable properties at femto-second time scales. We detect giant optical phase shifts of waveguided photons under strong photo-excitation devoid of any unwanted added losses in the vdW crystal, WSe2. Our results firmly implicate bound excitons in the observed behavior. Our transient spatio-temporal maps reveal two concomitant effects: i) photo-generation of electron-hole plasma that drives the WSe2 crystal towards a Mott transition where excitons dissociate and ii) a coherent interaction between the waveguide material and pump light, known as the optical Stark effect, that alters the phase velocity of guided photons on the femtosecond timescale

PT-symmetry in honeycomb photonic lattices

We apply gain/loss to honeycomb photonic lattices and show that the dispersion relation is identical to tachyons - particles with imaginary mass that travel faster than the speed of light. This is accompanied by PT-symmetry breaking in this structure. We further show that the PT-symmetry can be restored by deforming the lattice

Social Work Practice as Collective Experience

This is an account of four workers in the human services who have developed an ongoing collective experience. The four of us, all white, professional social workers, drew together in the late Spring, 1972. We were all involved in academic life, primarily as social work professors, although one of us was detaching himself from academic life at that time

Nonlinear terahertz metamaterials via field-enhanced carrier dynamics in GaAs

We demonstrate nonlinear metamaterial split ring resonators (SRRs) on GaAs at terahertz frequencies. For SRRs on doped GaAs films, incident terahertz radiation with peak fields of ~20 - 160 kV/cm drives intervalley scattering. This reduces the carrier mobility and enhances the SRR LC response due to a conductivity decrease in the doped thin film. Above ~160 kV/cm, electric field enhancement within the SRR gaps leads to efficient impact ionization, increasing the carrier density and the conductivity which, in turn, suppresses the SRR resonance. We demonstrate an increase of up to 10 orders of magnitude in the carrier density in the SRR gaps on semi-insulating GaAs substrate. Furthermore, we show that the effective permittivity can be swept from negative to positive values with increasing terahertz field strength in the impact ionization regime, enabling new possibilities for nonlinear metamaterials.Comment: 5 pages, 4 figure

Phase transition in bulk single crystals and thin films of VO2 by nanoscale infrared spectroscopy and imaging

We have systematically studied a variety of vanadium dioxide (VO2) crystalline forms, including bulk single crystals and oriented thin films, using infrared (IR) near-field spectroscopic imaging techniques. By measuring the IR spectroscopic responses of electrons and phonons in VO2 with sub-grain-size spatial resolution (similar to 20 nm), we show that epitaxial strain in VO2 thin films not only triggers spontaneous local phase separations, but also leads to intermediate electronic and lattice states that are intrinsically different from those found in bulk. Generalized rules of strain-and symmetry-dependent mesoscopic phase inhomogeneity are also discussed. These results set the stage for a comprehensive understanding of complex energy landscapes that may not be readily determined by macroscopic approaches

Somatosensory phenomena in Huntington's disease

Sensory symptons are generally not associated with Huntington's disease (HD). We describe two patients with HD who had painful sompatosensory symptons. One patient also had auditory hallucinations. No other cause was found for these symptoms. Both patients also had significant depression and one patient committed suicide. Somatosensory symptons may be a marker for depression in HD.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/50396/1/870030411_ftp.pd