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Recent experiment-theory investigations have demonstrated that the 2p-exciton state and the polariton resonances of a semiconductor quantum-well (QW) microcavity form a lambda (Λ) system where a terahertz (THz) field can selectively couple to either of the transitionallowed arms. However, semiconductors are open systems because seemingly isolated states are coupled to the continuum of states, either directly or via Coulomb or phonon scattering. A systematic theory is presented to treat these effects in a Λ system and in a two-level system (TLS) described by the 1s-to-2p exciton transition in a semiconductor QW, without the microcavity. Excitonic occupations of polarization are analyzed as function of THz-field frequency and intensity with (Λ system) and without (TLS) cavity. Even though the existence of Λ and TLS resonances is verified, significant deviations are observed at higher THz intensities, as higher ionized exciton states are generated via multi-THz-photon absorption.

We demonstrate terahertz (THz) imaging and spectroscopy of single-layer graphene deposited on an intrinsic Si substrate using THz time-domain spectroscopy. A single-cycle THz pulse undergoes multiple internal reflections within the Si substrate, and the THz absorption by the graphene layer accumulates through the multiple interactions with the graphene/Si interface. We exploit the large absorption of the multiply reflected THz pulses to acquire high-contrast THz images of graphene. We obtain local sheet conductivity of the graphene layer analyzing the transmission data with thin-film Fresnel formula based on the Drude model.

The exciton binding energy in GaAs-based quantum-well (QW) structures is in the range of ~10 meV, which falls in the terahertz (THz) regime. THz-induced nonlinear optical effects of excitons in QWs are of great interest because they provide new insights into the quantum coherence and wavepacket dynamics in semiconductors. For example, an experiment/theory study demonstrated that strong single-cycle THz pulses induce coherent extreme-nonlinear optical transients in semiconductor QWs, where the rotating wave approximation breaks down and the ponderomotive effects become prominent [1]. 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 excitonic transition and the THz pulse duration (~3 ps) is comparable with the exciton dephasing time.

Strong THz pulses induce pronounced nonlinear optical effects in a QW microcavity, resonantly driving exction-polariton polarizations coupled to an optically dark 2p-exciton polarization. The coherent coupling between the polarizations dephases within a few picoseconds.

Interactions of few-cycle terahertz pulses with the induced optical polarization in a quantum-well microcavity reveal that the lower and higher exciton-polariton modes together with the optically forbidden 2p-exciton state form a unique Λ-type three-level system. Pronounced nonlinearities are observed via time-resolved strong-terahertz and weak-optical excitation spectroscopy and explained with a fully microscopic theory. The results show that the terahertz pulses strongly couple the exciton-polariton states to the 2p-exciton state while no resonant transition between the two polariton levels is observed.

We investigate the charge transport in nickel-titanium (Ni-Ti) alloy thin films using terahertz (THz) transmission spectroscopy. Ni-Ti alloys have peculiar mechanical properties such as shape memory effects. Electrical conductivity can be a good measure to characterize the alloy phase transitions, yet the carrier transport properties of this material are relatively unexplored in the thin film regime. We grew 60-80-nm Ni-Ti alloy films of various Ti concentrations (0- 100%) on intrinsic Si substrates by Ar plasma sputtering. We carried out THz transmission spectroscopy of the samples using broadband THz pulses. The broadband THz pulses were generated by optical rectification of femtosecond laser pulses in a 1-mm ZnTe crystal. The light source was a 1-kHz Ti:sapphire amplifier producing 800-nm femtosecond pulses (pulse energy, 1 mJ; pulse duration, 90 fs). The transmitted THz pulses were measured by either a L-He-cooled Si:Bolometer (sensitive to time-averaged THz power) or by electro-optic (EO) sampling using a 1-mm ZnTe crystal. 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.

We demonstrate THz imaging and time-domain spectroscopy of a single-layer graphene film. The large-area graphene was grown by chemical vapor deposition on Cu-foil and subsequently transferred to a Si substrate. We took a transmission image of the graphene/Si sample measured by a Si:bolometer (pixel size is 0.4-mm). The graphene film (transmission: 36 - 41%) is clearly resolved against the background of the Si substrate (average transmission: 56.6%). The strong THz absorption by the graphene layer indicates that THz carrier dynamics are dominated by intraband transitions. A theoretical analysis based on the Fresnel coefficients for a metallic thin film shows that the local sheet resistance varies across the sample from 420 to 590 Ohm, consistent with electron mobility ~ 3,000 cm 2 V -1 s -1 . We also measured time-resolved THz waveforms through the Si substrate and the graphene/Si sample. The waveforms consist of a series of single-cycle THz pulses: a directly transmitted pulse, then subsequent "echos" corresponding to multiple reflections from the substrate. The amplitude difference between graphene/Si pulses and Si pulses becomes more pronounced as the pulses undergo more reflections. From these measurements, we obtained spectrally flat transmission spectra of the transmitted pulses and the average sheet resistance 480 Ohm, consistent with the results of the power transmission measurement. The flat spectral responses indicate that the carrier scattering time in our graphene sample is much shorter than the THz pulse duration.

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.

We study graphene/dielectric interfaces using THz imaging and spectroscopy. Non-contacting, non-destructive THz probing reveals the local sheet conductivity of single-layer graphene deposited on a Si substrate and covered by a thin PMMA layer.

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.

Interaction of strong THz pulses with a QW microcavity reveals that exciton-polariton modes and 2p-exciton state form a Λ-type three-level system. THz pulses resonantly drive the transitions from the exciton-polariton states to the 2p-exciton state.

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.

Interaction of strong few-cycle THz pulses with QW excitons produces pronounced nonlinear optical transients. With THz radiation tuned near the 1s-to-2p intraexciton transition, the exciton resonances exhibit Rabi sidebands revealing the 2p-dephasing time.

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.

We investigate the coherent dynamics of excitonic wavepackets in semiconductors driven by intense narrowband THz pulses. Time-resolved THz-pump and optical-probe measurements demonstrate strong nonlinear-optical transients of the light-hole and heavy-hole excitonic resonances in GaAs/AlGaAs QWs.

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.

We investigate electron transport properties in large-area, single-layer graphene embedded in dielectric media, using free-space terahertz (THz) imaging and time-domain spectroscopy. Sandwiched between a thin polymethyl methacrylate (PMMA) layer and a Si substrate, graphene layers of different growth recipes exhibit distinctive spatial inhomogeneity of sheet conductivity. The non-contacting, non-destructive THz probe reveals that the PMMA layer induces a small, yet noticeable reduction in conductivity.

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.

We demonstrate terahertz (THz) imaging and spectroscopy of a 15x15-mm^2 single-layer graphene film on Si using broadband THz pulses. The THz images clearly map out the THz carrier dynamics of the graphene-on-Si sample, allowing us to measure sheet conductivity with sub-mm resolution without fabricating electrodes. The THz carrier dynamics are dominated by intraband transitions and the THz-induced electron motion is characterized by a flat spectral response. A theoretical analysis based on the Fresnel coefficients for a metallic thin film shows that the local sheet conductivity varies across the sample from {\sigma}s = 1.7x10^-3 to 2.4x10^-3 {\Omega}^-1 (sheet resistance, {\rho}s = 420 - 590 {\Omega}/sq)., Comment: 6 pages, 5 figures

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.

We developed a tabletop source of intense, narrow band terahertz pulses via type-II difference-frequency generation in ZnTe crystal using two linearly chirped and orthogonally polarized optical pulses. The pulse energy is in the range of 1–3 nJ depending on the variable pulse duration from 1 to 5 ps. The amplitude of electric field reaches ∼10 kV/cm. The central frequency of the spectrum is continuously tunable from 0.3 to 2.5 THz with the bandwidth at 0.2–0.5 THz.

Using THz spectroscopy, we obtained the resistivity of Ni-Ti alloy thin-films as a function of Ti concentration. The resistivity sharply increases near the phase transition boundaries, Ti concentrations of 22%, 44% and 62%.