A. D. Jameson

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.

Transient optical response of quantum well excitons to intense narrowband terahertz pulses

Intense narrowband terahertz pulses are used to modify excitonic transitions in semiconductor quantum wells and to study the dephasing properties of the optically dark 2p states. Time-resolved terahertz-pump and optical-probe measurements exhibit strong nonlinear optical transients of the 1s heavy-hole and light-hole exciton resonances when the terahertz radiation is tuned near the 1s to 2p intraexciton transition. A microscopic theory attributes the observed nonlinearities to Rabi sidebands showing that the 2p-dephasing time is three times that of the 1s-state.

Direct measurement of light–matter energy exchange inside a microcavity

We demonstrate that one can directly measure the time evolution of light–matter interaction in a semiconductor microcavity by tracking how the optical response is changed by strong single-cycle terahertz (THz) pulses. A short THz pulse transiently interrupts the interaction of the quantum-well microcavity with the light mode and resets the polaritonic light–matter oscillations at THz frequencies. This THz-reset protocol can provide novel insights into the light–matter coupling dynamics in a wide range of photonic materials such as plasmonic and organic systems.

Terahertz transmission ellipsometry of vertically aligned multi-walled carbon nanotubes

We demonstrate time-resolved terahertz transmission ellipsometry of vertically aligned multi-walled carbon nanotubes. The angle-resolved transmission measurements reveal anisotropic characteristics of the terahertz electrodynamics in multi-walled carbon nanotubes. The anisotropy is, however, unexpectedly weak: the ratio of the tube-axis conductivity to the transverse conductivity, σz/σxy≅2.3, is nearly constant over the broad spectral range of 0.4–1.6 THz. The relatively weak anisotropy and the strong transverse electrical conduction indicate that THz fields readily induce electron transport between adjacent shells within multi-walled carbon nanotubes.

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 Imaging and Spectroscopy of Large-Area Single-Layer Graphene

THz imaging and spectroscopy using broadband THz pulses map out the THz carrier dynamics of large-area single-layer graphene. Non-contacting, non-destructive THz probing reveals the local sheet conductivity of the graphene samples.

Terahertz-induced optical modulations in quantum-well microcavity

Strong exciton-photon coupling in a high-Q microcavity leads to the formation of two new eigenstates, called excitonpolaritons. We present the quantum dynamics of exciton-polaritons driven by strong few-cycle THz pulses. Our study focuses on an intriguing question of how THz radiation interacts with the strongly coupled light-matter system. We performed THz-pump and optical-probe experiments to answer the question: we observed the time-resolved optical reflectivity of the lower and higher exciton-polariton (LEP and HEP) modes in a QW microcavity in the presence of strong few-cycle THz pulses. In a previous study with a bare QW, a strong THz field tuned to the 1s-to-2p intraexciton transition induced an excitonic Rabi splitting. Since THz radiation interacts only with the excitonic components of exciton-polaritons and has no impact on cavity modes, it is an interesting question how THz radiation drives the excitonpolariton states to higher energy states in the microcavity system. Our study shows that THz radiation resonantly drives the exciton-polariton polarizations giving rise to LEP-to-2p or HEP-to-2p transitions. LEP-to-HEP transition is forbidden because they have the same symmetry. Our experimental and theoretical investigations demonstrate that LEP, HEP, and 2p-exciton states form a three-level Λ system in an optically excited QW microcavity.

Terahertz and optical frequency mixing in semiconductor quantum-wells

The exciton binding energy in GaAs-based quantum-well (QW) structures is in the range of ~10 meV, which falls in the THz regime. We have conducted a time-resolved study to observe the resonant interactions of strong narrowband THz pulses with coherent excitons in QWs, where the THz radiation is tuned near the 1s-2p intraexciton transition and the THz pulse duration (~3 ps) is comparable with the exciton dephasing time. The system of interest contains ten highquality 12-nm-wide GaAs QWs separated by 16-nm-wide Al 0.3Ga 0.7As barriers. The strong and narrowband THz pulses were generated by two linearly-chirped and orthogonally-polarized optical pulses via type-II difference-frequency generation in a 1-mm ZnTe crystal. The peak amplitude of the THz fields reached ~10 kV/cm. The strong THz fields coupled the 1s and 2p exciton states, producing nonstationary dressed states. An ultrafast optical probe was employed to observe the time-evolution of the dressed states of the 1s exciton level. The experimental observations show clear signs of strong coupling between THz light and excitons and subsequent ultrafast dynamics of excitonic quantum coherence. As a consequence, we demonstrate frequency conversion between optical and THz pulses induced by nonlinear interactions of the THz pulses with excitons in semiconductor QWs.

THz-induced nonlinear optical transients of coherent excitons in semiconductor QWs

We demonstrate the ultrafast coherent dynamics of exciton wavepackets in semiconductor QWs resonantly driven by strong narrowband THz pulses. Time-resolved THz-pump and optical-probe measurements exhibit large-signal nonlinear optical transients of the HH and LH exciton lines in GaAs/AlGaAs QWs.