David J. Hilton

Carbon nanotube terahertz polarizer.

We describe a film of highly aligned single-walled carbon nanotubes that acts as an excellent terahertz linear polarizer. There is virtually no attenuation (strong absorption) when the terahertz polarization is perpendicular (parallel) to the nanotube axis. From the data, the reduced linear dichrosim was calculated to be 3, corresponding to a nematic order parameter of 1, which demonstrates nearly perfect alignment as well as intrinsically anisotropic terahertz response of single-walled carbon nanotubes in the film.

Ultrafast Phase Transition via Catastrophic Phonon Collapse Driven by Plasmonic Hot-Electron Injection

Ultrafast photoinduced phase transitions could revolutionize data-storage and telecommunications technologies by modulating signals in integrated nanocircuits at terahertz speeds. In quantum phase-changing materials (PCMs), microscopic charge, lattice, and orbital degrees of freedom interact cooperatively to modify macroscopic electrical and optical properties. Although these interactions are well documented for bulk single crystals and thin films, little is known about the ultrafast dynamics of nanostructured PCMs when interfaced to another class of materials as in this case to active plasmonic elements. Here, we demonstrate how a mesh of gold nanoparticles, acting as a plasmonic photocathode, induces an ultrafast phase transition in nanostructured vanadium dioxide (VO2) when illuminated by a spectrally resonant femtosecond laser pulse. Hot electrons created by optical excitation of the surface-plasmon resonance in the gold nanomesh are injected ballistically across the Au/VO2 interface to induce a subpicosecond phase transformation in VO2. Density functional calculations show that a critical density of injected electrons leads to a catastrophic collapse of the 6 THz phonon mode, which has been linked in different experiments to VO2 phase transition. The demonstration of subpicosecond phase transformations that are triggered by optically induced electron injection opens the possibility of designing hybrid nanostructures with unique nonequilibrium properties as a critical step for all-optical nanophotonic devices with optimizable switching thresholds.

Terahertz emission via ultrashort-pulse excitation of magnetic metal films

We observe terahertz emission by optical rectification of an intense 1.5-eV, 50-fs pulse in single-crystal iron thin films grown by molecular beam epitaxy. The azimuthal dependence of the emission indicates the presence of a magnetic nonlinearity and a nonmagnetic surface nonlinearity.

Carrier dynamics in self-assembled ErAs nanoislands embedded in GaAs measured by optical-pump terahertz-probe spectroscopy

We use optical-pump terahertz (THz)-probe spectroscopy to study carrier dynamics in self-assembled ErAs nanoislands embedded in GaAs and deposited in a superlattice structure. Measurements are performed at several pump fluences on samples with different superlattice periods, enabling a determination of the time-dependent conductivity. Subpicosecond carrier capture times are obtained, indicating the potential of these devices as time-domain THz detectors with performance comparable to low-temperature grown GaAs and superior control of material parameters.

Ultrafast conductivity dynamics in pentacene probed using terahertz spectroscopy

We present measurements of the transient photoconductivity in pentacene single crystals using optical-pump terahertz-probe spectroscopy. We have measured the temperature and fluence dependence of the mobility of the photoexcited charge carriers with picosecond resolution. The pentacene crystals were excited at 3.0 eV, which is above the bandgap of ∼2.2 eV, and the induced change in the far-infrared transmission was measured. At 30 K, the carrier mobility is μ≈0.4 cm2/V s and decreases to μ≈0.2 cm2/V s at room temperature. The transient terahertz signal reveals the presence of free carriers that are trapped on the time scale of a few picoseconds or less, possibly through the formation of excitons, small polarons, or trapping by impurities.

On Photo-Induced Phenomena in Complex Materials: Probing Quasiparticle Dynamics using Infrared and Far-Infrared Pulses

It is now possible to routinely generate and detect subpicosecond pulses in the mid and far-infrared portions of the electromagnetic spectrum enabling novel time-resolved spectroscopic investigations. In the context of complex materials, spectral selectivity from approximately 0.001-1.0eV is especially important since many relevant quasiparticle excitations lie in this range. This includes, as examples, gapped excitations related to superconductivity, charge ordering, and hybridization phenomena, phonon and polaron dynamics, and the coherent Drude response so intimately related to metal-insulator transitions. Temporally resolving spectral changes associated with such excitations in pump-probe-like experiments is proving to be a powerful tool in materials where multiple degrees of freedom (charge, lattice, spin, and orbital) conspire to determine functionality. Quite generally, important insights into ground state properties, coupling parameters, degrees of freedom influencing quasiparticle transport, the nature of phase transitions, and nonequilibrium dynamics can be obtained. Following a brief review of illustrative examples highlighting such possibilities we will present, in some detail, recent experiments on two different ferromagnetic materials. First, we describe terahertz emission experiments on single crystal iron thin films. Following photoexcitation, a burst of coherent THz radiation is emitted which is shown to be consistent with partial demagnetization of the Fe film occurring on a picosecond timescale. As a second example, we describe recent optical-pump infrared-probe measurements on the low carrier density magnetoresistive pyrochlore Tl 2 Mn 2 O 7 . In the ferromagnetic and paramagnetic phases, the recombination of photoexcited carriers is strongly influenced by spin fluctuations.

Quantum control of a Landau-quantized two-dimensional electron gas in a GaAs quantum well using coherent terahertz pulses

T. Arikawa, 2 X. Wang, 2 D. J. Hilton, J. L. Reno, W. Pan, and J. Kono 2, ∗ Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, USA Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA Department of Physics, University of Alabama at Birmingham, Birmingham, AL 35294, USA Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, NM 87123, USA Sandia National Laboratories, Albuquerque, NM 87123, USA (Dated: September 29, 2011)

Terahertz emission via ultrashort pulse excitation of magnetic metal films

We observe terahertz emission via optical rectification of a 1.5 eV, 50 fs pulse in single crystal iron thin films. The azimuthal dependence of the emission indicates the presence of a magnetic and surface nonlinearity.

Direct measurement of cyclotron coherence times of high-mobility two-dimensional electron gases.

We have observed long-lived (approximately 30 ps) coherent oscillations of charge carriers due to cyclotron resonance (CR) in high-mobility two-dimensional electrons in GaAs in perpendicular magnetic fields using time-domain terahertz spectroscopy. The observed coherent oscillations were fitted well by sinusoids with exponentially-decaying amplitudes, through which we were able to provide direct and precise measures for the decay times and oscillation frequencies simultaneously. This method thus overcomes the CR saturation effect, which is known to prevent determination of true CR linewidths in high-mobility electron systems using Fourier-transform infrared spectroscopy.

Ultrafast Spectroscopy of the Uranium(IV) and Thorium(IV) Bis(ketimide) Complexes (C5Me5)2An[-N=C(Ph)(CH2Ph)]2 (An = Th, U)

Ultrafast pump-probe spectroscopic studies have been performed on (C 5Me 5) 2U[- N=C(Ph)(CH 2Ph)] 2 and (C 5Me 5) 2Th[- N=C(Ph)(CH 2Ph)] 2 including, for the uranium complex, the first direct measurement of dynamics of electronic deactivation within a 5f-electron manifold. Evidence has been found for strong coupling between the electronic ground state and the f-electron manifold which dominates the dynamics of the excited states of the bis(ketimide) uranium complex. These also demonstrate strong singlet-f manifold coupling, which assists in the deactivation of the photoexcited state of the uranium complex, and provide information on intersystem crossing and internal conversion processes in both complexes.