Time-resolved Terahertz Research Articles

Ultrafast terahertz conductivity of photoexcited nanocrystalline silicon

The ultrafast transient ac conductivity of nanocrystalline silicon films is investigated using time-resolved terahertz spectroscopy. While epitaxial silicon on sapphire exhibits a free carrier Drude response, silicon nanocrystals embedded in glass show a response that is best described by a classical Drude–Smith model, suitable for disorder-driven metal–insulator transitions. In this work, we explore the time evolution of the frequency dependent complex conductivity after optical injection of carriers on a picosecond time scale. Furthermore, we show the lifetime of photoconductivity in the silicon nanocrystal films is dominated by trapping at the Si/SiO2 interface states, occurring on a 1–100 ps time scale depending on particle size and hydrogen passivation.

Photoinduced Charge Carrier Generation in a Poly(3-hexylthiophene) and Methanofullerene Bulk Heterojunction Investigated by Time-Resolved Terahertz Spectroscopy

We report on the ultrafast photoinduced charge separation processes in varying compositions of poly(3-hexylthiophene) (P3HT) blended with the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Through the use of time-resolved terahertz spectroscopy, the time- and frequency-dependent complex photoconductivity is measured for samples with PCBM weight fractions (WPCBM) of 0, 0.2, 0.5, and 0.8. By analysis of the frequency-dependent complex conductivity, both the charge carrier yield and the average charge carrier mobility have been determined analytically and indicate a short (<0.2 nm) carrier mean free path and a suppressed long-range transport that is characteristic of carrier localization. Studies on pure films of P3HT demonstrate that charge carrier generation is an intrinsic feature of the polymer that occurs on the time scale of the excitation light, and this is attributed to the dissociation of bound polaron pairs that reside on adjacent polymer chains due to interchain charge transfer. Both interchain and interfacial charge transfer contribute to the measured photoconductivity from the blended samples; interfacial charge transfer increases as a function of increasing PCBM. The addition of PCBM to the polymer films surprisingly does not dramatically increase the production of charge carriers within the first 2 ps. However, charge carriers in the 0.2 and 0.5 blended films survive to much longer times than those in the P3HT and 0.8 films.

Time-Resolved Photoconductivity of PbSe Nanocrystal Arrays

We report the sub-picosecond photoconductivity dynamics of chemically treated PbSe nanocrystal arrays utilizing time-resolved terahertz spectroscopy (TRTS). TRTS allows both the degree of interdot electronic coupling and the carrier dynamics to be extracted simultaneously. The following capping ligands bonded to the quantum dot surface were studied: hydrazine, ethylenediamine, butlyamine, and aniline. In addition, the arrays were treated with NaOH. We find that the treatments affect both the degree of electronic coupling and the carrier dynamics.

Conductivity of ZnO Nanowires, Nanoparticles, and Thin Films Using Time-Resolved Terahertz Spectroscopy

The terahertz absorption coefficient, index of refraction, and conductivity of nanostructured ZnO have been determined using time-resolved terahertz spectroscopy, a noncontact optical probe. ZnO properties were measured directly for thin films and were extracted from measurements of nanowire arrays and mesoporous nanoparticle films by applying Bruggeman effective medium theory to the composite samples. Annealing significantly reduces the intrinsic carrier concentration in the ZnO films and nanowires, which were grown by chemical bath deposition. The complex-valued, frequency-dependent photoconductivities for all morphologies were found to be similar at short pump-probe delay times. Fits using the Drude-Smith model show that films have the highest mobility, followed by nanowires and then nanoparticles, and that annealing the ZnO increases its mobility. Time constants for decay of photoinjected electron density in films are twice as long as those in nanowires and more than 5 times those for nanoparticles due to increased electron interaction with interfaces and grain boundaries in the smaller-grained materials. Implications for electron transport in dye-sensitized solar cells are discussed.

Electron mobility in dilute GaAs bismide and nitride alloys measured by time-resolved terahertz spectroscopy

We report time-resolved terahertz spectroscopy measurements of the electronic transport properties of dilute GaAs bismide and nitride alloys. The electron mobility for GaAs1−yBiy (y=0.84%) extracted from Drude fits to the transient complex conductivity was ∼2800cm2∕Vs at a carrier density of 2.7×1018cm−3, close to the mobility of 3300cm2∕Vs measured for GaAs at a similar carrier density. The electron mobility did not decrease significantly for Bi concentrations up to 1.4%. In contrast, the GaNxAs1−x (x=0.84%) and GaNxAs1−x−yBiy (x=0.85%, y=1.4%) films exhibited non-Drude behavior with a highly reduced electron mobility and suppressed conductivity at low frequencies indicative of carrier localization.

Terahertz (THz) spectroscopy of Freon-11 (CCl 3F, CFC-11) at room temperature

The rotational spectrum for the lowest vibrational mode for CCl 3F has been measured in the 0.1–2 THz frequency range using a compact time-resolved terahertz spectrometer. The peak of the population spectrum for the ground state rotational manifold was observed at 0.3 THz. The observed absorption profile was modeled with rotational and vapor pressure parameters. The limits of detectivity of the compact THz system are discussed in context of atmospheric sensing.

Carrier dynamics in microcrystalline silicon studied by time-resolved terahertz spectroscopy

We present results of optical pump – terahertz probe experiments applied to a set of thin film silicon samples on sapphire substrates. Structure of the films varied from amorphous to fully microcrystalline. Picosecond time scale evolution of electrical transport properties of photoexcited samples is investigated and discussed. Three different mechanisms are found to contribute to the dynamical conductivity at terahertz frequencies.

Transient terahertz conductivity in photoexcited silicon nanocrystal films

Time-resolved terahertz spectroscopy is used to probe ultrafast carrier dynamics and terahertz conductivity in photoexcited thin films of silicon nanocrystals, polynanocrystalline silicon, and epitaxial silicon-on-sapphire. We show that a Drude-Smith model provides an excellent fit to the observed transient terahertz conductivity in all of our samples, revealing a transition from a Drude-like response with low carrier backscatter in bulk silicon-on-sapphire to a non-Drude-like, localized behavior with high carrier backscatter in the silicon nanocrystal films. Evidence for long-range conduction between nanocrystals is observed, and we show that the photoconductive lifetime of the silicon nanocrystals is dominated by trapping at $\mathrm{Si}∕\mathrm{Si}{\mathrm{O}}_{2}$ interface states.

Direct Observation of Electron-to-Hole Energy Transfer in CdSe Quantum Dots

We independently determine the subpicosecond cooling rates for holes and electrons in CdSe quantum dots. Time-resolved luminescence and terahertz spectroscopy reveal that the rate of hole cooling, following photoexcitation of the quantum dots, depends critically on the electron excess energy. This constitutes the first direct, quantitative measurement of electron-to-hole energy transfer, the hypothesis behind the Auger cooling mechanism proposed in quantum dots, which is found to occur on a 1 +/- 0.15 ps time scale.

Strongly Coupled Optical Phonons in the Ultrafast Dynamics of the Electronic Energy and Current Relaxation in Graphite

Ultrafast charge carrier dynamics in graphite has been investigated by time-resolved terahertz spectroscopy. Analysis of the transient dielectric function and model calculations show that more than 90% of the initially deposited excitation energy is transferred to a few strongly coupled lattice vibrations within 500 fs. These hot optical phonons also substantially contribute to the striking increase of the Drude relaxation rate observed during the first picosecond after photoexcitation. The subsequent cooling of the hot phonons yields a lifetime estimate of 7 ps for these modes.

Nonresonant ionization of oxygen molecules by femtosecond pulses: Plasma dynamics studied by time-resolved terahertz spectroscopy

We show that optical pump-terahertz probe spectroscopy is a direct experimental tool for exploring laser-induced ionization and plasma formation in gases. Plasma was produced in gaseous oxygen by focused amplified femtosecond pulses. The ionization mechanisms at 400- and 800-nm excitation wavelengths differ significantly being primarily of a multiphoton character in the former case and a strong-field process in the latter case. The generation of the plasma in the focal volume of the laser and its expansion on subnanosecond time scale is directly monitored through its density-dependent susceptibility. A Drude model used to evaluate the plasma densities and electron-scattering rates successfully captures the observations for a wide range of pump intensities. In addition, rotational fingerprints of molecular and ionic species were also observed in the spectra.

Optical and transient photoconductive properties of pentacene and functionalized pentacene thin films: Dependence on film morphology

We present a comprehensive study of the optical and transient photoconductive properties of pentacene and functionalized pentacene thin films grown by evaporation or from solution onto a variety of substrates. The transient photoconductivity was studied over picosecond time scales using time-resolved terahertz pulse spectroscopy. The structure and morphology of the films were assessed using x-ray diffraction, atomic force microscopy, and scanning electron microscopy. Regular pentacene films grown by evaporation under similar conditions but on different substrates yielded polycrystalline films with similar morphology and similar optical and transient photoconductive properties. Single exponential or biexponential decay dynamics was observed in all of the regular pentacene films studied. Functionalized pentacene films grown by evaporation at two different substrate temperatures (as well as from solution) yielded significant variations in morphology, resulting in different optical-absorption spectra and transient photoconductivities that could be correlated with film structure. The lower limit of the charge-carrier mobility, estimated from the amplitude of the transient photoconductive response, was ∼0.02–0.04cm2∕Vs in the case of regular pentacene films and ∼0.01–0.06cm2∕Vs in the case of functionalized pentacene films, depending on the film morphology. The best functionalized pentacene films exhibited transient photoconductivity values reaching ∼30%–40% of those obtained in functionalized pentacene single-crystal samples, and showed similar power-law decay dynamics. We also report on terahertz pulse generation from voltage-biased pentacene thin films.

Ultrafast far-infrared dynamics probed by terahertz pulses: A frequency domain approach. I. Model systems

Time-resolved terahertz spectroscopy has become a widely used experimental tool for the investigation of ultrafast dynamics of polar systems in the far infrared. We have recently proposed an analytical method for the extraction of a transient two-dimensional susceptibility from the experimental data [Nĕmec, Kadlec, and Kuzel, J. Chem. Phys. 117, 8454 (2002)]. In the present paper the methodology of optical pump-terahertz probe experiments is further developed for direct application in realistic experimental situations. The expected two-dimensional transient response function is calculated for a number of model cases (including Drude dynamics of free carriers, harmonic and anharmonic oscillator modes); these results serve as a basis for the interpretation of experimental results. We discuss also the cases where only partial (one-dimensional) information about the system dynamics can be experimentally obtained.

Ultrafast far-infrared dynamics probed by terahertz pulses: A frequency-domain approach. II. Applications

We present data obtained by time-resolved terahertz spectroscopy in selected semiconducting and molecular systems exhibiting subpicosecond far-infrared dynamics. We use a frequency-domain method which eliminates the influence of instrumental functions and artifacts due to frequency mixing and yields a two-dimensional transient conductivity of the photoexcited sample. This technique enables improving the attainable experimental time resolution and allows a simple qualitative interpretation of the results without a priori modeling. The quantitative interpretation is based on the time-dependent Drude and damped harmonic oscillator models.

Bandlike transport in pentacene and functionalized pentacene thin films revealed by subpicosecond transient photoconductivity measurements

We observe bandlike transport in pentacene and functionalized pentacene thin films using time-resolved terahertz pulse spectroscopy. The measured transient photoconductivity exhibits fast $(&lt;400\phantom{\rule{0.3em}{0ex}}\mathrm{fs})$ photogeneration of mobile charge carriers and reveals a transient carrier mobility that increases as the temperature decreases from 300 K down to 10 K, indicative of bandlike transport over subpicosecond time scales. A wavelength-independent photoconductive signal is observed. The transient photoconductivity in the thin-film samples exhibits a single-exponential decay, whereas a power-law decay is seen in single-crystal samples.

Ultrafast Electron Dynamics in GaAs and InP Studied by Time-Resolved Terahertz Emission Spectroscopy

We studied the ultrafast dynamics of electrons generated by tunable femtosecond optical pulses having positive and negative excess energies in GaAs and InP by observing the temporal waveform of THz radiation emitted from biased photoconductive antennas. Sub-picosecond intraband relaxation was observed when the excess energy was positive. When excited by optical pulses having negative excess energies, it was observed that the THz waveform had a picosecond decay, which was attributed to the transition from the Urbach state to the free carrier state of electrons on the picosecond time scale. This dynamical behavior was found to be very sensitive to the applied electric field in the range of several kV/cm. The largest THz signal was obtained by pumping the emitter at the band-gap energy.

Time-Resolved Terahertz Spectroscopy with Free-SpaceElectro-Optic Sampling

We present a time-resolved ultrafast measurement in terahertz (THz)frequency region by means of the free-space electro-optic sampling. Thefast delay scan technique is used to suppress the noisewith low frequency and to improve the signal-to-noise ratio of thesystem. The transmission spectra of different materials are obtained.The optical properties of these materials in a THz region are shown. Thebroadening of spectrum and chirping phenomena are illustrated. We findthat polystyrene is an excellent material for the THz application.

Anisotropic photoconductivity of InGaAs quantum dot chains measured by terahertz pulse spectroscopy

We report results of time-resolved terahertz (THz) pulse spectroscopy experiments on laterally ordered chains of self-assembled InGaAs quantum dots photoexcited with 400nm, 100fs laser pulses. A large anisotropy in the transient photoconductive response is observed depending on the polarization of the THz probe pulse with respect to the orientation of the dot chains. Fast (3.5–5ps) and efficient carrier capture into the dots and one-dimensional wetting layers underneath the dot chains is observed below 90K. At higher temperatures, thermionic emission into the two-dimensional wetting layers and barriers becomes significant and the anisotropy in the photoconductive signal is reduced.

Comparison of Subspace and ARX Models of a Waveguide's Terahertz Transient Response After Optimal Wavelet Filtering

A quasi-optical deembedding technique for characterizing waveguides is demonstrated using wide-band time-resolved terahertz spectroscopy. A transfer function representation is adopted for the description of the signal 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 (ARX), as well as with a state-space model. Prior to the identification procedure, filtering was performed in the wavelet domain to minimize both signal distortion, as well as the noise propagating in the ARX and subspace models. The optimal filtering procedure used in the wavelet domain for the recorded time-domain signatures is described in detail. The effect of filtering prior to the identification procedures is elucidated with the aid of pole-zero diagrams. Models derived from measurements of terahertz transients in a precision WR-8 waveguide adjustable short are presented.

Optical two-photon absorption in GaAs measured by optical-pump terahertz-probe spectroscopy

Time-resolved terahertz experiments in a novel setup were used for a direct observation of the competition between the single- and two-photon absorption in highly excited GaAs. The experiments were carried out near $800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ where the single photon absorption usually dominates. The crystal surface was excited by femtosecond laser pulses with fluences up to $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕{\mathrm{cm}}^{2}$, for which the single-photon absorption saturates and the two-photon absorption becomes the leading absorption mechanism. The two-photon absorption coefficient $\ensuremath{\beta}$, its anisotropy and the depth profile of the photoexcited carriers were determined.