Time-resolved Terahertz Spectroscopy Research Articles

Size and surface effects on transient photoconductivity in CdS nanobelts probed by time-resolved terahertz spectroscopy

Ultrafast optical-pump terahertz probe spectroscopy was performed over a graduated size distribution of CdS nanobelts to investigate the size and surface effects on the transient photoconductivity. It was found that the nanobelt size has a profound influence on the carrier localization and photoconductivity dynamics, brought about by the carrier trapping at surface defects. The strong carrier localization in the nanobelt is ascribed to the internal surface boundaries arising from the surface depletion layer. The increased thickness of surface depletion layer due to a continuous trapping of photocarriers at surface defects results in more pronounced carrier localization after photoexcitation.

Electron and Hole Contributions to the Terahertz Photoconductivity of a Conjugated Polymer:Fullerene Blend Identified.

Time-resolved terahertz spectroscopy was employed for the investigation of charge-transport dynamics in benzothiadiazolo-dithiophene polyfluorene ([2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]) (APFO-3) polymers with various chain lengths and in its monomer form, all blended with an electron acceptor ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM). Upon photoexcitation, charged polaron pairs are created, negative charges are transferred to fullerenes, while positive polarons remain on polymers/monomers. Vastly different hole mobility in polymer and monomer blends allows us to distinguish the hole and electron contributions to the carrier mobility.

Electron Transfer in Quantum-Dot-Sensitized ZnO Nanowires: Ultrafast Time-Resolved Absorption and Terahertz Study

Photoinduced electron injection dynamics from CdSe quantum dots to ZnO nanowires is studied by transient absorption and time-resolved terahertz spectroscopy measurements. Ultrafast electron transfer from the CdSe quantum dots to ZnO is proven to be efficient already on a picoseconds time scale (τ = 3-12 ps). The measured kinetics was found to have a two-component character, whose origin is discussed in detail. The obtained results suggest that electrons are injected into ZnO via an intermediate charge transfer state.

(Invited) Ultrafast Carrier Dynamics in Silicon Nanocrystal Films

We have applied time-resolved THz spectroscopy to probe the transient photoexcited carrier dynamics and THz conductivity in Si nanocrystal films with varying silicon volume filling fractions and nanocrystal sizes on picosecond time scales. The THz conductivity reveals microscopic carrier motion with significant interface scattering within nanocrystals as well as percolative transport between nanocrystals. The time variation of the THz conductivity is analyzed within the framework of the Drude-Smith model, an extension of the Drude model that characterizes carrier localization in nanostructured materials. Below the percolation threshold, transport between nanocrystals is inhibited and photoexcited carriers are localized within individual nanocrystals. These films also exhibit efficient optical emission. In films with Si filling fractions above the percolation threshold, photoluminescence is suppressed and a transition from long-range inter-nanocrystal transport immediately after photoexcitation to increased carrier localization over a 50 ps time scale due to accumulation of charges at interface defect sites is observed.

Thermalization of photoexcited carriers in bismuth investigated by time-resolved terahertz spectroscopy andab initiocalculations

The charge carrier dynamics of photoexcited bismuth generates a Drude response that evolves over time. Our data show that the plasma frequency of bismuth displays an initial increase and a subsequent decay. We have performed ab initio calculations on bulk bismuth within the density functional theory and show that this peculiar behavior is due to local extrema in the valence and conduction bands. It follows that most of the carriers first accumulate in these extrema and reach the Fermi level only 0.6 ps after the photoexcitation.

Phase diagram of the ultrafast photoinduced insulator-metal transition in vanadium dioxide

We use time-resolved terahertz spectroscopy to probe the ultrafast dynamics of the insulator-metal phase transition induced by femtosecond laser pulses in a nanogranular vanadium dioxide (VO${}_{2}$) film. Based on the observed thresholds for characteristic transient terahertz dynamics, a phase diagram of critical pump fluence versus temperature for the insulator-metal phase transition in VO${}_{2}$ is established for the first time over a broad range of temperatures down to 17 K. We find that both Mott and Peierls mechanisms are present in the insulating state and that the photoinduced transition is nonthermal. We propose a critical-threshold model for the ultrafast photoinduced transition based on a critical density of electrons and a critical density of coherently excited phonons necessary for the structural transition to the metallic state. As a result, evidence is found at low temperatures for an intermediate metallic state wherein the Mott state is melted but the Peierls distortion remains intact, consistent with recent theoretical predictions. Finally, the observed terahertz conductivity dynamics above the photoinduced transition threshold reveal nucleation and growth of metallic nanodomains over picosecond time scales.

Carrier dynamics and conductivity of SnO2 nanowires investigated by time-resolved terahertz spectroscopy

THz spectroscopy has been applied to investigate the photo-induced and intrinsic conductivity in SnO2 nanowires using the Drude-Smith model. The refractive index of the nanowires was found to decrease from 2.4 to 2.1 with increasing THz frequency and the dc mobility of the non-excited nanowires was determined to be 72 ± 10 cm2/Vs. Measurements reveal that scattering times are carrier density dependent, while a strong suppression of long transport is evident. Intensity-dependent measurements provided an estimate of the Auger coefficient found to be γ = (7.2 ± 2.0) × 10−31 cm6/s.

Photoexcitation Dynamics in Films ofC60and Zn Phthalocyanine with a Layered Nanostructure

We elucidate photoexcitation dynamics in C(60) and zinc phthalocyanine (ZnPc) from picoseconds to milliseconds by transient absorption and time-resolved terahertz spectroscopy. Autoionization of C(60) is a precursor to photocarrier generation. Decay of the terahertz signal is due to decreasing photocarrier mobility over the first 20 ps and thereafter reflects recombination dynamics. Singlet diffusion rates in C(60) are determined by modeling the rise of ground state bleaching of ZnPc absorption following C(60) excitation. Recombination dynamics transform from bimolecular to monomolecular as the layer thickness is reduced, revealing a metastable exciplex at the C(60)/ZnPc interface with a lifetime of 150 μs.

Energy-Gap Dynamics of Superconducting NbN Thin Films Studied by Time-Resolved Terahertz Spectroscopy

Using time-domain terahertz spectroscopy we performed direct studies of the photoinduced suppression and recovery of the superconducting gap in a conventional BCS superconductor NbN. Both processes are found to be strongly temperature and excitation density dependent. The analysis of the data with the established phenomenological Rothwarf-Taylor model enabled us to determine the bare quasiparticle recombination rate, the Cooper pair-breaking rate and the electron-phonon coupling constant, λ=1.1±0.1, which is in excellent agreement with theoretical estimates.

Morphology effects on charge generation and recombination dynamics at ZnPc:C60 bulk hetero-junctions using time-resolved terahertz spectroscopy

The influence of growth temperature induced phase segregation and crystallinity in ZnPc:C60 blend films on the charge generation and recombination dynamics is investigated with optical-pump terahertz-probe spectroscopy. While an ultrafast photo-induced charge generation process is observed for all morphologies, a subsequent sub-nanosecond photoconductivity rise depends on crystallinity and phase segregation. For higher intensities, the signal is dominated by a morphology-dependent bimolecular recombination process. High local mobilities of minimal μ ∼ 0.3 cm2/Vs are found. The increase of photoconductivity with film growth temperature correlates with formerly observed device photocurrent improvements.

Polaron dynamics in molecular polysilane wires

Intrachain charge carrier transport was studied by means of theoretical computing on the model polymer poly[methyl(phenyl)silylene]. The Marcus theory, in which a dynamic electron–phonon coupling term was inserted into the reorganisation energy, was used for the modelling. The on chain charge carrier mobility was determined as 0·06 cm2 V−1 s−1 for 16 mer. For 20 mer, the lower value was found (i.e. 0·005 cm2 V−1 s−1). The experimental value obtained by microwave photoconductivity and time resolved terahertz spectroscopy (pump and probe method) was reported as 0·02 cm2 V−1 s−1.

Effect of Copper on the Carrier Lifetime in Black Silicon

Black silicon is produced by laser annealing of a-Si:H films. During annealing, silicon microstructures are formed on the surface. We use time-resolved terahertz spectroscopy to study the photoconductivity dynamics in black silicon. We find that when a copper film is deposited on top of the a-Si:H layer prior to laser annealing, the carrier lifetime of black silicon is significantly reduced.

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequilibrium time evolution of carriers and low-energy excitations with sub-ps time resolution. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the conductivity of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.

Finite-difference time-domain analysis of time-resolved terahertz spectroscopy experiments

In this paper we report on the numerical analysis of a time-resolved terahertz (THz) spectroscopy experiment using a modified finite-difference time-domain method. Using this method, we show that ultrafast carrier dynamics can be extracted with a time resolution smaller than the duration of the THz probe pulse and can be determined solely by the pump pulse duration. Our method is found to reproduce complicated two-dimensional transient conductivity maps exceedingly well, demonstrating the power of the time-domain numerical method for extracting ultrafast and dynamic transport parameters from time-resolved THz spectroscopy experiments. The numerical implementation is available online.

Charge transport and localization in nanocrystalline CdS films: A time-resolved terahertz spectroscopy study

Assessment of characteristic length and time scales of the charge localization in nanostructured semiconductors is a key point for understanding the initial stage of carrier transport after photoexcitation. A concerted use of time-resolved terahertz spectroscopy and Monte Carlo simulations of the motion of confined electrons allow us to obtain this information and develop a quantitative microscopic model of the electron transport in a nanocrystalline CdS film. A weak localization is observed inside individual nanocrystals while much stronger localization stems from the existence of nanocrystal clusters partially surrounded by voids. The efficiency of the short-range transport is controlled by the excess energy of electrons: Its increase enhances the conductive coupling between adjacent nanocrystals and clusters. Relaxation of electrons with high excess energy then leads to a decrease of their mobility on a subpicosecond time scale. Filling of conduction-band states by increasing the optical pump fluence allows us to maintain a high level of conductive coupling even at later times.

Ultrafast percolative transport dynamics in silicon nanocrystal films

We have applied time-resolved terahertz (THz) spectroscopy to probe ultrafast conduction dynamics of photoexcited carriers in silicon nanocrystal films with a wide range of nanocrystal sizes and concentrations. The picosecond THz conductivity reveals microscopic photocarrier motion with significant interface scattering within the nanocrystals, as well as percolative transport between nanocrystals. In films with silicon filling fractions above the percolation threshold, we observe a transition from long-range internanocrystal transport immediately after photoexcitation to increased carrier localization over a 50-ps time scale due to accumulation of charges at interface defect sites. However, in films with silicon filling fractions below the percolation threshold, transport between nanocrystals is strongly suppressed at all times. Finally, we estimate effective carrier diffusion lengths of 60 to 130 nm for the silicon nanocrystal composites with silicon filling fractions above the percolation threshold, making such films promising candidates for active layers in photovoltaic devices.

Picosecond Electron Injection Dynamics in Dye-Sensitized Oxides in the Presence of Electrolyte

We employ time-resolved terahertz (THz) spectroscopy (TRTS) to directly monitor the picosecond dynamics of electron transfer in dye-sensitized oxides in the presence of an electrolyte phase. Understanding the time scale on which electrons are injected from the dye into the oxide phase in the presence of electrolyte is important for optimization of the solar cell efficiency. We quantify injection dynamics from two different dyes into both mesoporous TiO2 and SnO2 films. Measurements are performed in inert media (air, acetonitrile), in the presence of two different electrolytes (the conventional iodine/iodide couple and the recently reported disulfide/thiolate redox couple), and in the presence of two different electrolyte additives (Li+ ions and tert-butyl pyridine). Electron injection dynamics in TiO2 is found to occur on two time scales: sub-150 fs and ∼10 ps, attributed to injection from the singlet and lower-lying triplet state, respectively. For SnO2, injection is slower, despite the lower energy of the band edge. The slow injection observed for SnO2 is attributed to the reduced density of electronic states in the material. We observe that for both oxides electron injection can be strongly retarded by changing the composition of the medium in which the sensitized oxide film is immersed. In particular, our results indicate that injection dynamics can be significantly slowed down in the presence of the disulfide/thiolate redox couple and/or tert-butyl pyridine.

Charge carrier mobility in poly[methyl(phenyl)silylene] studied by time-resolved terahertz spectroscopy and molecular modelling

Time-resolved terahertz spectroscopy and combination of quantum chemistry modeling and molecular dynamics simulations were used for the determination of charge carrier mobility in poly[methyl(phenyl)silylene]. Using time-resolved THz spectroscopy we established the on-chain charge carrier drift mobility in PMPSi as 0.02 cm(2) V(-1) s(-1). This value is low due to the formation of polarons: the hole is self-trapped in a potential formed by local chain distortion and the transient THz conductivity spectra show signatures of its oscillations within this potential well. This view is supported by the agreement between experimental and calculated values of the on-chain charge carrier mobility.

Applications of time-resolved terahertz spectroscopy in ultrafast carrier dynamics (Invited Paper)

Three time-resolved terahertz (THz) spectroscopy methods (optical-pump/THz-probe spectroscopy, THz-pump/THz-probe spectroscopy, and THz-pump/optical-probe spectroscopy) are reviewed. These are used to characterize ultrafast dynamics in photo- or THz-excited semiconductors, superconductors, nanomaterials, and other materials. In particular, the optical-pump/THz-probe spectroscopy is utilized to investigate carrier dynamics and the related intervalley scattering phenomena in semiconductors. The recent development of intense pulsed THz sources is expected to affect the research in nonlinear THz responses of various materials.

Thin-sample limit for time-resolved terahertz spectroscopy

The retrieval of the primary (subpicosecond) photoproducts in small-band two-dimensional time-domain terahertz spectroscopy has succeeded only in a few model systems, due to the complicated correlation between the two time parameters. We show that the nature of the primary photoproducts is revealed upon differentiating the measured time-domain spectra with respect to the pump-probe delay. An analytical treatment is given in the thin-sample limit, which distinguishes unambiguously between excitons and free charge carriers. This treatment gives a fair description of the data even for thick samples, including the subpicosecond timescale.