Time-resolved Terahertz Spectroscopy Research Articles

Optical parameters of Zn1−xMgxO nanowires in THz regime

We report experimental characterization of optical and dielectric constants of ZnO and ZnMgO nanowires (NWs) with Mg mole fraction of 9%, 18%, 29% and volume fraction 43.5% in terahertz (THz) region using terahertz time domain spectroscopy (THz-TDS). Measured index of refraction and absorption coefficient for ZnO nanowires are 1.12 and 10.7cm−1 at 1.5THz which increases by 32% and 132% for ZnMgO nanowires with Mg mole fraction of 29%. Real and imaginary dielectric constants for ZnO NWs are 1.122 and 0.01 at 1.5THz which increases by 32% and 58% for Zn0.71Mg0.29O nanowires. Time resolved terahertz spectroscopy (TRTS) of ZnO nanowires provides real and imaginary conductivity of 0.1 and 0.008 (S/cm), respectively which increased by 294% and 58% for Zn0.71Mg0.29O nanowires. The measured parameters are extrapolated to obtain characteristics of MgO nanowires in the terahertz regime. Observed theoretical values of dielectric constant of MgO nanowires (2.25) are smaller than those of bulk MgO (6.6) which is attributed to an air–nanowire composite in the nanowire samples. Bruggeman effective medium theory was implemented to extract permittivity of bulk MgO from composite which is in excellent agreement with reported data. With the increase of nanowire density or volume fraction, optical properties of nanowires more resemble those of bulk.

Employing time-resolved terahertz spectroscopy to analyze carrier dynamics in thin-film Cu2ZnSn(S,Se)4 absorber layers

We report the application of time-resolved terahertz spectroscopy (TRTS) to measure photoexcited carrier lifetimes and mobility, and to determine recombination mechanisms in Cu2ZnSn(S,Se)4 (CZTSSe) thin films fabricated from nanocrystal inks. Ultrafast time resolution permits tracking the evolution of carrier density to determine recombination rates and mechanisms. The carrier generation profile was manipulated by varying the photoexcitation wavelength and fluence to distinguish between surface, Shockley-Read-Hall (SRH), radiative, and Auger recombination mechanisms and determine rate constants. Surface and SRH recombination are the dominant mechanisms for the air/CZTSSe/SiO2/Si film stack. Diffusion to, and then recombination at, the air-CZTSSe interface occurred on the order of 100 picoseconds, while SRH recombination lifetimes were 1–2 nanoseconds. TRTS measurements can provide information that is complementary to conventional time-resolved photoluminescence measurements and can direct the design of efficient thin film photovoltaics.

Ultrafast Electrical Measurements of Isolated Silicon Nanowires and Nanocrystals.

We simultaneously determined the charge carrier mobility and picosecond to nanosecond carrier dynamics of isolated silicon nanowires (Si NWs) and nanocrystals (Si NCs) using time-resolved terahertz spectroscopy. We then compared these results to data measured on bulk c-Si as a function of excitation fluence. We find >1 ns carrier lifetimes in Si NWs that are dominated by surface recombination with surface recombination velocities (SRV) between ∼1100-1700 cm s(-1) depending on process conditions. The Si NCs have markedly different decay dynamics. Initially, free-carriers are produced, but relax within ∼1.5 ps to form bound excitons. Subsequently, the excitons decay with lifetimes >7 ns, similar to free carriers produced in bulk Si. The isolated Si NWs exhibit bulk-like mobilities that decrease with increasing excitation density, while the hot-carrier mobilities in the Si NCs are lower than bulk mobilities and could only be measured within the initial 1.5 ps decay. We discuss the implications of our measurements on the utilization of Si NWs and NCs in macroscopic optoelectronic applications.

Ultrafast terahertz dynamics of hot Dirac-electron surface scattering in the topological insulatorBi2Se3

Two-dimensional surface-scattering dynamics are central in the physics of topological insulators. Numerous electrical and optical measurements have evidenced that the origins of novel optoelectronic response can be traced back to Dirac surface-electron dynamics. Intrinsic surface dynamics, however, remain elusive because these experiments cannot access the frequencies of the surface-scattering rate. Time-resolved terahertz spectroscopy is the only apparatus for directly probing the collective response of low-energy electronic transitions. Here, by utilizing ultrafast optical-pump terahertz-probe spectroscopy, we discovered anomalous characteristics of the surface-scattering dynamics. Upon photoexcitation, the surface-scattering rate is increased and results in negative dynamic conductance at low temperature. Surprisingly, the differential changes of the surface-scattering rate are strongly reduced by photoexcited electrons at elevated temperature. We find that this nontrivial surface-electron dynamics is due to opening a carrier-relaxation channel from bulk to the surface state---one distinct characteristic of topological insulators. Our observations reveal unexpected surface dynamics of hot Dirac electrons, providing experimental a priori knowledge toward ultrafast optoelectronic operations.

Ultrafast terahertz response of multilayer graphene in the nonperturbative regime

The nonlinear dynamics of electrons in multilayer epitaxial graphene is investigated by time-resolved terahertz (THz) spectroscopy in a regime where the interaction of electrons with the external field dominates over scattering processes. The predominantly coherent electron response to the THz field involves both intra- and interband currents, leading to coherently driven interband transitions of carriers and to the generation of higher harmonics of the THz carrier frequency. The overall behavior of the graphene layers is always absorptive, even after generation of an initial electron-hole distribution by femtosecond midinfrared excitation. The results are in agreement with theoretical calculations of the nonperturbative THz response.

Terahertz Depolarization Effects in Colloidal TiO2 Films Reveal Particle Morphology

Films of colloidal TiO2 nanoparticles are widely used in photovoltaic and photocatalytic applications, and the nature of electrical conductivity in such materials is therefore of both fundamental and practical interest. The conductive properties of colloid TiO2 films depend strongly on their morphology and deviate greatly from the properties of the bulk material. We report ultrafast photoconductivity studies of films consisting of sintered TiO2 particles of very different sizes performed using time-resolved terahertz spectroscopy. Remarkably, identical photoconductivity spectra are observed for films of particles with diameters of tens and hundreds of nanometers, respectively. The independence of photoconductivity on particle size directly demonstrates that the terahertz photoconductive response of colloidal TiO2 films is not affected by carrier backscattering at particle boundaries as has previously been concluded, but rather by depolarization fields resulting from the spatial inhomogeneities in the dielectric function inherent to these types of films. Modeling of the influence of depolarization fields on the terahertz conductivity allows us to explain the measured data and gain insights into the morphology of the film. Specifically, we show that the observed photoconductivity spectra reflect percolated pathways in the colloidal TiO2 nanoparticles films, through which charge carrier diffusion can occur over macroscopic length scales.

Photoinduced insulator-metal phase transition and the metallic phase propagation in VO2 films investigated by time-resolved terahertz spectroscopy

The particle size and film thickness dependence of the photoinduced insulator-metal phase transition in VO2 films has been studied systematically by time-resolved terahertz spectroscopy at room temperature. It is found that the dynamical photoinduced phase transition from insulator to metal consists of two processes: a 1.7 ps fast process and a slow process with a typical time constant of 40 ps. Both of the two processes show particle size independence. The 40 ps slow process is revealed to arise from the longitudinal propagation of the metallic phase from the photoexcited surface to the interior of the VO2 film. A phase boundary propagation speed with a magnitude of ∼2400 m/s is obtained, which is close to the velocity of sound in solid materials and coincides with the prediction of diffusionless phase transformation. Our experimental results clearly establish the entire procedure of photoinduced phase change in the VO2 film.

Efficiency of Interfacial Electron Transfer from Zn-Porphyrin Dyes into TiO2 Correlated to the Linker Single Molecule Conductance

High performance dye-sensitized solar cells (DSSCs) rely upon molecular linkers that allow efficient electron transport from the photoexcited dye into the conduction band of the semiconductor host substrate. We have studied photoinduced electron injection efficiencies from modular assemblies of a Zn-porphyrin dye and a series of linker molecules which are axially bound to the Zn-porphyrin complex and covalently bound to TiO2 nanoparticles. Experimental measurements based on terahertz spectroscopy are compared to the calculated molecular conductance of the linker molecules. We find a linear relationship between measured electron injection efficiency and calculated single-molecule conductance of the linker employed. Since the same chromophore is used in all cases, variations in the absorptivities of the adsorbate complexes are quite small and cannot account for the large variations in observed injection efficiencies. These results suggest that the linker single-molecule conductance is a key factor that should be optimized for maximum electron injection efficiencies in DSSCs. In addition, our findings demonstrate for the first time the possibility of inferring values of single molecule conductance for bridging molecules at semiconductor interfaces by using time-resolved THz spectroscopy.

Ultrafast Photoconductivity of Graphene Nanoribbons and Carbon Nanotubes

We present a comparative study of the ultrafast photoconductivity in two different forms of one-dimensional (1D) quantum-confined graphene nanostructures: structurally well-defined semiconducting graphene nanoribbons (GNRs) fabricated by a "bottom-up" chemical synthesis approach and semiconducting carbon nanotubes (CNTs) with a similar bandgap energy. Transient photoconductivities of both materials were measured using time-resolved terahertz spectroscopy, allowing for contact-free measurements of complex-valued photoconductivity spectra with subpicosecond time-resolution. We show that, while the THz photoresponse seems very different for the two systems, a single model of free carriers experiencing backscattering when moving along the long axis of the CNTs or GNRs provides a quantitative description of both sets of results, revealing significantly longer carrier scattering times for CNTs (ca. 150 fs) than for GNRs (ca. 30 fs) and in turn higher carrier mobilities. This difference can be explained by differences in band structures and phonon scattering and the greater structural rigidity of CNTs as compared to GNRs, minimizing the influence of bending and/or torsional defects on the electron transport.

Electron Injection Dynamics from Photoexcited Porphyrin Dyes into SnO2and TiO2Nanoparticles

The photoexcited electron injection dynamics of free-base and metallo-derivatives of tris(pentafluorophenyl)porphyrins bound to TiO2 and SnO2 nanoparticle surfaces have been investigated using time-resolved terahertz spectroscopy (TRTS). The metallo-derivatives include Zn(II), Cu(II), Ni(II), and Pd(II). For the TiO2–porphyrin assemblies, electron injection from the photoexcited dye to the semiconductor occurs only when using the zinc derivative as the sensitizer because it is the only dye studied in this report with long-lived excited states higher in energy than the TiO2 conduction band edge. All of the dyes, however, have excited-state energies above the SnO2 conduction band edge, and the electron injection rates vary widely from 0.4 to 200 ps depending on the sensitizer. For the SnO2–porphyrin assemblies, electron injection is strongly influenced by competition with alternate deactivation routes that are accessible following Soret band excitation. These results offer thermodynamic and kinetic considerations for designing improved high-potential porphyrin photoanodes with applications to solar-powered water oxidation.

Accessing Terahertz Complex Conductivity Dynamics in the Time-Domain

Time-resolved terahertz (THz) spectroscopy is frequently utilized to measure conductivity dynamics on ultrafast time scales. Typically photo-induced changes in the transmission of the peak of the THz waveform are recorded and thought to represent the real conductivity dynamics. Here we evaluate a time-domain scheme that accesses both the real and imaginary conductivity dynamics by recording the photo-induced change in the transmitted THz pulse at its peak and its zero-crossing, respectively. The applicability and limitations of this method are investigated by modeling the THz response of material systems with known conductivity dynamics. The relationship between measured time-domain dynamics and the actual charge carrier dynamics is discussed.

Transient Photoconductivity of Ternary CdSSe Nanobelts As Measured by Time-Resolved Terahertz Spectroscopy

The frequency- and fluence-dependent transient photoconductivity in ternary CdSSe nanobelts is investigated using time-resolved terahertz spectroscopy. The carrier density and mobility are extracte...

The Applications of Terahertz Spectroscopy in Functional Optical Materials Researches

Terahertz radiation refers to the electromagnetic wave whose frequency is usually defined between 0.1 and 10 THz (1 THz=1012Hz). With the development of the emission and detection technologies of THz radiation, terahertz time-domain spectroscopy (THz-TDS) has been widely used in medical diagnosis, security inspection and materials characterization. In this paper, we introduced briefly the progress of terahertz measurement technologies, and then reviewed the applications of THz spectra in functional materials researches. As two important functional optical materials, TiO2nanoparticles and yttrium aluminum garnet (YAG) crystal have been investigated with THz-TDS. We introduced the electron injection process in TiO2studied by time resolved THz spectroscopy which is reported in the literature, and then presented our own work, the THz optical constants of undoped and Tm3+doped YAG crystals.

Charge Transport in ${\hbox{TiO}}_{2}$ Films With Complex Percolation Pathways Investigated by Time-Resolved Terahertz Spectroscopy

The depolarization fields play an important role in terahertz experiments on nanostructured samples with complex nanoparticle morphologies and percolation pathways. Namely, their effects can hide or distort peculiarities of nanoscopic charge transport in the spectra measured on these structures. We calculate the local fields for a large number of percolated and non-percolated two-dimensional model structures by numerical solving of Maxwell equations in the quasi-static limit. The results strongly suggest that in a broad family of structures a simple effective medium approximation model can be applied to characterize the effective response. The model consists in an equivalent circuit composed of a resistance accounting for the percolated chains with an additional parallel RC-branch describing the non-percolated part. The physical meaning of this model is discussed in the frame of the Bergman spectral representation of effective medium. We show a recipe for the retrieval of a response connected to the depolarization fields and to the nanoscale transport mechanisms from transient terahertz spectra. Finally, we use the model to interpret our THz photoconductivity spectra in various <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\hbox{TiO}}_{2}$</tex> </formula> films with nanofabricated percolation pathways.

On Ultrafast Photoconductivity Dynamics and Crystallinity of Black Silicon

We investigate the carrier dynamics of thin films of black silicon, amorphous hydrogenated silicon which under laser annealing forms a microstructured surface with extremely high broadband optical absorption. We use Raman spectroscopy to determine the degree of crystallinity of the annealed surfaces, and investigate the dependence on crystallinity and fabrication method of the photoconductivity. Time-resolved THz spectroscopy is used to determine the evolution of the carrier scattering time and confinement of carriers on the picosecond time scale. We conclude that a fabrication method with high energy leading edge of the annealing laser results in black silicon with the largest photon-to-electron conversion efficiency, largest mobility, and longest carrier lifetime.

Electron transfer in hydrogenated nanocrystalline silicon observed by time-resolved terahertz spectroscopy

We report on the ultrafast carrier dynamics in hydrogenated nanocrystalline silicon (nc$\ensuremath{-}$Si:H) using time-resolved terahertz spectroscopy. Photoexcitation at 407 nm primarily produces charge carriers in the $a\ensuremath{-}$Si phase, but they undergo a rapid electron transfer to the $c\ensuremath{-}$Si phase prior to complete thermalization into the band-tail states of $a\ensuremath{-}$Si. We studied the carrier dynamics on a range of nc$\ensuremath{-}$Si:H samples with varying crystalline volume fractions (${X}_{c}$) and mapped out the carrier dynamics with sub-ps resolution. Our measurements are consistent with a model in which electrons are first trapped at interface states at the $a\ensuremath{-}$Si--$c\ensuremath{-}$Si boundary prior to being thermally emitted into the $c\ensuremath{-}$Si phase. Wavelength and temperature dependent measurements are consistent with our model. The phenomena observed here have implications toward solar cell structures that utilize an amorphous material as an absorber layer, previously thought to have a mobility value too low to attain effective charge transport in a device.

Quantum Physics With Ultrabroadband and Intense Terahertz Pulses

We present recent advances in the generation of highly intense multiterahertz transients and their application to nonlinear spectroscopy of bulk semiconductors. An optimized scheme of parametric amplification results in broadband single- or few-cycle terahertz transients with peak electric fields up to 10 or 25 MV/cm, respectively. Time-resolved four-wave mixing terahertz spectroscopy of InSb far away from the interband resonance demonstrates clear signatures of a nonperturbative regime of Rabi flopping. We qualitatively explain the observed behavior within a model of a quantum two-level system. In addition, we demonstrate the dynamical Franz-Keldysh effect in InP resolved on a subcycle timescale. The field-induced modulation of the interband optical absorption at the second harmonic of the driving terahertz field is observed in full agreement with theoretical predictions.

Insights into the Charge Carrier Terahertz Mobility in Polyfluorenes from Large-Scale Atomistic Simulations and Time-Resolved Terahertz Spectroscopy

Multiscale atomistic simulations were used to understand the factors that determine the charge carrier mobility spectrum obtained from time-resolved terahertz (THz) spectroscopy in conjugated polymers. The simulation approach combines classical molecular dynamics simulations of atomic structure, large-scale electronic structure calculations, and the evaluation of the THz mobility using Kubo’s linear response formula. We found that THz radiation probes a single carrier hop at low temperatures and high frequencies, while the transport over somewhat longer distances is probed in the opposite cases. Our results indicate that charge carrier transport at THz frequencies is thermally activated but with a much smaller activation energy compared to the dc case. The shape of the real and imaginary part of the mobility spectral curve reveals the presence of above THz hopping rates that are relevant for charge carrier transport. Strong differences of the mobilities in the polymer and the corresponding monomer material are largely caused by stronger energetic disorder of the monomer material.

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.