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

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

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

Electronic interactions in Dirac fluids visualized by nano-terahertz spacetime mapping

Ultraclean graphene at charge neutrality hosts a quantum critical Dirac fluid of interacting electrons and holes. Interactions profoundly affect the charge dynamics of graphene, which is encoded in the properties of its collective modes: surface plasmon polaritons (SPPs). The group velocity and lifetime of SPPs have a direct correspondence with the reactive and dissipative parts of the tera-Hertz (THz) conductivity of the Dirac fluid. We succeeded in tracking the propagation of SPPs over sub-micron distances at femto-second (fs) time scales. Our experiments uncovered prominent departures from the predictions of the conventional Fermi-liquid theory. The deviations are particularly strong when the densities of electrons and holes are approximately equal. Our imaging methodology can be used to probe the electromagnetics of quantum materials other than graphene in order to provide fs-scale diagnostics under near-equilibrium conditions

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 (∼20nm), 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

Infrared plasmons propagate through a hyperbolic nodal metal

Metals are canonical plasmonic media at infrared and optical wavelengths, allowing one to guide and manipulate light at the nanoscale. A special form of optical waveguiding is afforded by highly anisotropic crystals revealing the opposite signs of the dielectric functions along orthogonal directions. These media are classified as hyperbolic and include crystalline insulators, semiconductors, and artificial metamaterials. Layered anisotropic metals are also anticipated to support hyperbolic waveguiding. However, this behavior remains elusive, primarily because interband losses arrest the propagation of infrared modes. Here, we report on the observation of propagating hyperbolic waves in a prototypical layered nodal-line semimetal ZrSiSe. The observed waveguiding originates from polaritonic hybridization between near-infrared light and nodal-line plasmons. Unique nodal electronic structures simultaneously suppress interband loss and boost the plasmonic response, ultimately enabling the propagation of infrared modes through the bulk of the crystal