Michael J. Paul

Terahertz imaging and spectroscopy of large-area single-layer graphene.

THz imaging and spectroscopy using broadband THz pulses map out the THz carrier dynamics of a large-area graphene-on-Si sample, showing that the local sheet-conductivity varies across the sample from σ<inf>s</inf>, = 1.7×10⁻³ to 2.4×10⁻³ Ω⁻¹.

High-field terahertz response of graphene

We investigate the response of multi-layer epitaxial graphene and chemical vapor deposition (CVD)-grown single-layer graphene to strong terahertz (THz) fields. Contrary to theoretical predictions of strong nonlinear response, the transmitted fields exhibit no harmonic generation, indicating that the nonlinear response is limited by fast electron thermalization due to carrier-carrier scattering. The fast electron heating gives rise to large THz transmission enhancement (>15%) in single-layer CVD graphene at high THz fields (ETHz > 10kVcm 1 ). The nonlinear effects exhibit non-Drude behavior in the THz conductivity, where THz fields induce extreme non-equilibrium electron distributions.

Terahertz-Triggered Phase Transition and Hysteresis Narrowing in a Nanoantenna Patterned Vanadium Dioxide Film.

We demonstrate that high-field terahertz (THz) pulses trigger transient insulator-to-metal transition in a nanoantenna patterned vanadium dioxide thin film. THz transmission of vanadium dioxide instantaneously decreases in the presence of strong THz fields. The transient THz absorption indicates that strong THz fields induce electronic insulator-to-metal transition without causing a structural transformation. The transient phase transition is activated on the subcycle time scale during which the THz pulse drives the electron distribution of vanadium dioxide far from equilibrium and disturb the electron correlation. The strong THz fields lower the activation energy in the insulating phase. The THz-triggered insulator-to-metal transition gives rise to hysteresis loop narrowing, while lowering the transition temperature both for heating and cooling sequences. THz nanoantennas enhance the field-induced phase transition by intensifying the field strength and improve the detection sensitivity via antenna resonance. The experimental results demonstrate a potential that plasmonic nanostructures incorporating vanadium dioxide can be the basis for ultrafast, energy-efficient electronic and photonic devices.

Large enhancement of nonlinear terahertz absorption in intrinsic GaAs by plasmonic nano antennas

Strong THz fields produce pronounced changes in the THz transmission of a nano-antenna-array-patterned GaAs film. The nonlinear absorption of the nano-antenna/GaAs composite is significantly larger than that of bare GaAs due to field enhancement by the nano-antennas.

Terahertz induced transparency in single-layer graphene

We demonstrate THz-induced transparency in two types of single-layer CVD graphene samples utilizing high-field THz pulses. The nonlinear THz transmission depends on the local conductivity of the samples and dynamically varies in the time domain.

Anisotropic high-field terahertz response of free-standing carbon nanotubes

We demonstrate that unidirectionally aligned, free-standing multi-walled carbon nanotubes (CNTs) exhibit highly anisotropic linear and nonlinear terahertz (THz) responses. For the polarization parallel to the CNT axis, strong THz pulses induce nonlinear absorption in the quasi-one-dimensional conducting media, while no nonlinear effect is observed in the perpendicular polarization configuration. Time-resolved measurements of transmitted THz pulses and a theoretical analysis of the data reveal that intense THz fields enhance permittivity in carbon nanotubes by generating charge carriers.

Terahertz spectroscopy of Ni–Ti alloy thin films

We investigate the carrier dynamics in nickel–titanium (Ni–Ti) alloy thin films using terahertz transmission spectroscopy. Analyzing the power transmission data and the transmitted waveforms, we obtained the alloy resistivity as a function of Ti concentration. Sharp changes in the resistivity were observed at the Ti fractions of 22%, 44%, and 62%, indicating that structural disorder is greatly enhanced when the alloy undergoes a phase transition.

Terahertz ellipsometry of vertically grown carbon nanotubes

THz ellipsometry with broadband THz pulses reveals anisotropic THz responses from closely packed, vertically grown CNTs. Non-negligible conductivity in a direction normal to the CNT axis indicates carrier transport between adjacent CNTs.

Terahertz Induced Transparency in Single-Layer Graphene

We demonstrate THz-induced transparency in two types of single-layer CVD graphene samples utilizing high-field THz pulses. The nonlinear THz transmission depends on the local conductivity of the samples and dynamically varies in the time domain.

Large Enhancement of Nonlinear Terahertz Absorption in Intrinsic GaAs by Plasmonic Nano-Antennas

We demonstrate remarkably strong nonlinear THz effects in an intrinsic GaAs wafer patterned with a nanometer-width slot antenna array. The antenna near-field reaches 20 MV/cm due to the huge field enhancement in the plasmonic nano-structure.