Time-resolved Terahertz Spectroscopy Research Articles

Electronic Coupling in InP Nanoparticle Arrays

Arrays and lattices formed from nanoparticles (NPs) present unique opportunities for new optoelectronic materials whose properties can be tuned by controlling the size of the individual NPs and their interparticle separation to effect strong inter-NP electronic coupling. Characterization of the interdot coupling as a function of interdot distance is essential. Using time-resolved THz spectroscopy, we report a six-fold increase in the transient photoconductivity in disordered arrays of 3.2 nm diameter InP NPs separated by 0.9 nm compared to arrays with 1.8 nm separation. Photoconductivity in the arrays is compared to that of isolated NPs and InP epilayers. The epilayer samples exhibit bulk transport behavior while the NP samples do not.

Terahertz magnetospectroscopy of transient plasmas in semiconductors

Using synchronized near-infrared (NIR) and terahertz (THz) lasers, we have performed picosecond time-resolved THz spectroscopy of transient carriers in semiconductors. Specifically, we measured the temporal evolution of THz transmission and reflectivity after NIR excitation. We systematically investigated transient carrier relaxation in GaAs and InSb with varying NIR intensities and magnetic fields. Using this information, we were able to determine the evolution of the THz absorption to study the dynamics of photocreated carriers. We developed a theory based on a Drude conductivity with time-dependent density and density-dependent scattering lifetime, which reproduced the observed plasma dynamics. Detailed comparison between experimental and theoretical results revealed a linear dependence of the scattering frequency on density, which suggests that electron–electron scattering is the dominant scattering mechanism for determining the scattering time. In InSb, plasma dynamics was dramatically modified by the application of a magnetic field, showing rich magnetoreflection spectra, while GaAs did not show any significant magnetic field dependence. We attribute this to the small effective masses of the carriers in InSb compared to GaAs, which made the plasma, cyclotron, and photon energies all comparable in the density, magnetic field, and wavelength ranges of the current study.

Critical phonons and R- zero-phonon emission line of Cr3+ in Mg-doped SrTiO3

Dielectric permittivity ε and a “dielectric related” temperature shift of a zero-phonon emission R line (2E→4A2) of the octahedral Cr3+ impurity centers was studied for nominally pure SrTiO3 (STO), Sr0.99Mg0.01TiO3 (SMT) and SrTi0.99Mg0.01O3 (STM) ceramics. In STO, as in crystals, ε increases at cooling revealing only a weak dispersion in the 100Hz–1MHz region at lower temperatures and the R line shifts at cooling to lower energies, peaking at ≈12605cm−1 at 25K. In SMT, the permittivity reveals qualitatively the same behavior as STO. But the spectral position of the chromium R line appeared to be at lower energies (12575cm−1, 70K) and furthermore the R- line shifts to lower energies at heating. In STM the permittivity reveals a wide maximum at ∼36K with pronounced relaxation-type dispersion, hinting a possibility of a low-temperature phase transition (PT). The spectral position of the R line in STM is ≈12614cm−1 at 70K (close to that one in STO). As in STO, at cooling the R line shifts to lower energies, but at ∼28K the lineshift changes in sign. Such behavior could be associated with a low-temperature soft mode driven PT in STM. However, time resolved THz spectroscopy revealed no ε (T) anomaly in STM, which means that at least below 28K the temperature dependence of the R line shift is not controlled by the TO1 soft mode.

Measurement of propagation constant in waveguides with wideband coherent terahertz spectroscopy

A quasi-optical technique for characterizing micromachined waveguides is demonstrated with wideband time-resolved terahertz spectroscopy. A transfer-function representation is adopted for the description of the relation between the signals in the input and output port of the waveguides. The time-domain responses were discretized, and the waveguide transfer function was obtained through a parametric approach in the z domain after describing the system with an autoregressive with exogenous input model. The a priori assumption of the number of modes propagating in the structure was inferred from comparisons of the theoretical with the measured characteristic impedance as well as with parsimony arguments. Measurements for a precision WR-8 waveguide-adjustable short as well as for G-band reduced-height micromachined waveguides are presented.

Dephasing in modulation-doped quantum structures probed by THz time-domain spectroscopy

The dephasing of intersubband transitions in semiconductor heterostructures was investigated by time-resolved THz spectroscopy. Single quantum structures show dephasing rates, which are nearly identical to scattering rates obtained by conventional Hall measurements and allow insight into the dephasing dynamics. In multiple quantum wells, inhomogeneous broadening of the density of states is the main dephasing mechanism.

Carrier Localization and Cooling in Dye-Sensitized Nanocrystalline Titanium Dioxide

The transient photoconductivity of dye-sensitized titanium dioxide has been measured using time-resolved terahertz spectroscopy, a noncontact electrical probe with sub-picosecond temporal resolution. The photoconductivity deviates strongly from Drude behavior and is explained by disorder-induced carrier localization and/or backscattering of the photogenerated carriers. In addition, the carriers are found to thermally equilibrate with the lattice in roughly 300 fs. Ramifications for understanding the function of DSSCs and electrical transport in disordered media are discussed.

Size-Dependent Photoconductivity in CdSe Nanoparticles as Measured by Time-Resolved Terahertz Spectroscopy

The size-dependent transient photoconductivity in CdSe nanoparticles (NPs) has been investigated using time-resolved THz spectroscopy (TRTS). We find that the mobility increases with an r4 dependence for NPs in the strong confinement regime and is governed by a restricted mean free path for NPs in the weak confinement regime. The photoexcited carriers undergo ballistic transport to the surface of the NPs. The average number of electron−hole pairs generated in the NPs falls into three distinct groupings based on the NP radius.

Ultrafast carrier-relaxation dynamics in self-assembled InAs/GaAs quantum dots

We apply the recently developed technique of ultrafast scanning tunneling microscopy to study carrier dynamics in InAs/GaAs self-assembled quantum-dot samples. The results obtained with this new technique are compared with standard ensemble-averaging ultrafast optoelectronic techniques such as femtosecond optical pump/probe reflectivity measurements and time-resolved terahertz spectroscopy. These measurements reveal a unified picture of the relaxation dynamics in InAs/GaAs self-assembled quantum-dot samples at T=300 K. The initial carrier relaxation proceeds by Auger carrier capture from the InAs wetting layer on a time scale of 1–2 ps, followed by recombination of carriers in the wetting layer, GaAs substrate, and quantum dots on time scales of 350 ps, 2.3 ns, and 900 ps, respectively. The consistency of these three experimental techniques demonstrates ultrafast scanning tunneling microscopy as a reliable tool for probing the local dynamics of nanostructures.

Subpicosecond carrier dynamics in low-temperature grown GaAs as measured by time-resolved terahertz spectroscopy

The transient photoconductivity in a 1 μm layer of low temperature grown GaAs (LT-GaAs) on a GaAs substrate was measured using time-resolved terahertz spectroscopy. When photoexcitation occurs at 400 nm we find a time-dependent mobility that increases from 400±100 to 1100±100 cm2 V−1 s−1 with a time constant of 2 ps. Photoexcitation at 800 nm produces a time-independent mobility of 3000±500 cm2 V−1 s−1. We determine the carrier lifetime in LT-GaAs to be 1.1 ± 0.1 ps.

Time-resolved THz spectroscopy of proton-bombarded InP

We investigate the recombination dynamics of photoexcited carriers in proton-bombarded InP crystals using near-infrared pump–THz-probe spectroscopy. The carrier lifetime is directly related to the proton dose and hence to the induced trap density: In weakly damaged samples we observe a saturation of the trap states for high-excitation densities. For highly damaged samples the time-dependent THz transmission can be explained by taking into account an Auger-assisted trapping process.

Terahertz Emission due to Miniband Transport in GaAs/AlGaAs Superlattices

We have investigated terahertz (THz) emission due to miniband transport of electrons in biased GaAs/AlGaAs superlattice diodes by time-resolved THz spectroscopy. Quasi-autocorrelation measurements were performed on the emitted THz electromagnetic wave using a bolometer as a wideband detector. We found that the THz emission due to miniband transport is strongly dependent on the photon energy of the pump laser pulses. A clear oscillation in the THz autocorrelation signal due to Bloch oscillation has been observed by exciting the sample at a photon energy slightly lower than the center of the Wannier-Stark ladder even for a superlattice with a miniband width larger than the longitudinal optical (LO) phonon energy. Furthermore, transient velocities of electrons in the miniband have been estimated from the measured THz autocorrelation signal.

Using the finite-difference time-domain pulse propagation method to simulate time-resolved THz experiments

The finite-difference time-domain (FDTD) method has been applied to time-resolved THz spectroscopy (TRTS) experiments. Time-resolved THz spectroscopy utilizes an optical pump pulse to excite the sample, followed by a far-infrared (FIR) probe pulse with frequency components that span from 10 to 100 cm−1. The subpicosecond evolution of the FIR spectrum is obtained as a function of time after the visible photoexcitation event. Significant challenges arise in interpreting these experimental results due to the very different frequencies of the pump and probe pulses. Therefore, it is essential to simulate the experiment. The method described entails numerically propagating both the THz probe pulse and the visible pump pulse simultaneously, keeping track of the transiently induced polarization from absorption of the visible pulse. Group velocity mismatch between the visible and THz pulse and a transiently changing response function are completely accounted for in the calculation. Furthermore, a spatially varying polarization can be included to account for a nonuniform excited region of the sample under investigation. The response function of the material is described as a multimode Brownian oscillator that can describe dispersive media in a very general sense. In particular, the overdamped, underdamped, and critically damped cases are all included, as well as special cases such as a Debye or Drude response. As a specific example, we present results of modeling a TRTS experiment of photoexcitation of a dye in solution, namely, 2,11,20,29-tetra-tert-butyl-2,3-napthalocyanine, dissolved in toluene. We carry out a nonlinear least squares fit of a parameterized model to the measured data to show that the FDTD–TRTS method is able to accurately reproduce the features observed in the measured data set.

Transient photoconductivity in GaAs as measured by time-resolved terahertz spectroscopy

The onset and decay of photoconductivity in bulk GaAs has been measured with 200-fs temporal resolution using time-resolved THz spectroscopy. A low carrier density $(l2\ifmmode\times\else\texttimes\fi{}{10}^{16} {\mathrm{cm}}^{\ensuremath{-}3})$ with less than 100-meV kinetic energy was generated via photoexcitation. The conductivity was monitored in a noncontact fashion through absorption of THz (far-infrared) pulses of several hundred femtosecond duration. The complex-valued conductivity rises nonmonotonically, and displays nearly Drude-like behavior within 3 ps. The electron mobilities obtained from fitting the data to a modified Drude model $(6540 {\mathrm{cm}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1}$ at room temperature with $N=1.6\ifmmode\times\else\texttimes\fi{}{10}^{16}{\mathrm{cm}}^{\ensuremath{-}3},$ and $13600 {\mathrm{cm}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1}$ at 70 K with $N=1.5\ifmmode\times\else\texttimes\fi{}{10}^{16}{\mathrm{cm}}^{\ensuremath{-}3})$ are in good agreement with literature values. There are, however, deviations from Drude-like behavior at the shortest delay times. It is shown that a scalar value for the conductivity will not suffice, and that it is necessary to determine the time-resolved, frequency-dependent conductivity. From 0 to 3 ps a shift to higher mobilities is observed as the electrons relax in the $\ensuremath{\Gamma}$ valley due to LO-phonon-assisted intravalley absorption. At long delay times (5--900 ps), the carrier density decreases due to bulk and surface recombination. The time constant for the bulk recombination is 2.1 ns, and the surface recombination velocity is $8.5\ifmmode\times\else\texttimes\fi{}{10}^{5}\mathrm{c}\mathrm{m}/\mathrm{s}.$

Optical phonon dynamics of GaAs studied with time-resolved terahertz spectroscopy

We investigate the reflection near the reststrahlen band of the optical phonon in bulk GaAs in the time domain, using time-resolved terahertz spectroscopy. We find that the dynamics of the reflection measured for GaAs differs strongly from the reflection dynamics that would be expected for a TO phonon with a frequency-independent dephasing time.

Theory for determination of the low-frequency time-dependent response function in liquids using time-resolved terahertz pulse spectroscopy

With the current rapid improvement in techniques for generating and detecting femtosecond terahertz (fs-THz) pulses, it is now possible to detect time-dependent perturbations to low-frequency intermolecular far-infrared modes during solvation events. The interpretation of these time-resolved terahertz spectroscopy experiments has generally relied on a collapse of the information available in the fs-THz pulse to give a one-dimensional time-dependent function, whose physical significance is not well defined. We describe a method to exploit the full electric field profile of the pulse as a function of pump/probe delay time to obtain a two-dimensional function that describes the system’s time-dependent dielectric response. We discuss the physical significance of this response function in terms of the time evolution of solute/solvent modes following photoexcitation, and give examples to show how its qualitative features relate to the measurable signal.

Time-Resolved Terahertz Spectroscopy of Condensed Phase Reactions

Ultrafast time-resolved visible-pump, far-IR (THz) probe spectroscopy has been developed in our lab and has been applied to study carrier dynamics in photoexcited GaAs and dipole solvation dynamics in betaine and p-nitroaniline. This type of spectroscopy enables us to study for the first time the nonequilibrium interaction between excited electronic states and low frequency vibrational modes.

Terahertz Emission from Quantum Beats in Coupled Quantum Wells

We have investigated the terahertz (THz) emission from quantum beats in coupled quantum wells by time-resolved THz spectroscopy. It is found that the intensity of the THz emission exhibits a clear resonance in the excitation wavelength, suggesting that quantum beats in coupled quantum wells are caused by excitons. It is also found that the dephasing time of quantum beats is almost constant (∼6 ps) up to 50 K and decreases rapidly with increasing temperature, indicating that the coherence of quantum beats is broken by impurity and/or roughness scattering at low temperatures and by optical phonon scattering at high temperatures.

Experimental realization of the Bloch oscillator in a semiconductor superlattice

We detect the coherent submillimetre-wave radiation from electron Bloch oscillations in a biased GaAs/AlGaAs semiconductor superlattice by time-resolved terahertz (THz) spectroscopy. The emitted THz radiation is tunable from 0.3 to 4 THz. The emission from Bloch oscillations is observable for temperatures as high as 100 K.

Detection of Bloch oscillations in a semiconductor superlattice by time-resolved terahertz spectroscopy and degenerate four-wave mixing

We have studied the spatial dynamics of coherently excited wave packets of charge carriers in a semiconductor superlattice structure by both time-resolved THz spectroscopy and degenerate four-wave mixing. In the Wannier-Stark bias regime, we observe electron Bloch oscillations with a frequency that depends linearly on the electric bias field and can be tuned between a few hundred GHz and 5 THz.