Transient Terahertz Spectroscopy Research Articles

Probing photocarrier dynamics of pressurized graphene using time-resolved terahertz spectroscopy

Graphene hosts intriguing photocarrier dynamics such as negative transient terahertz (THz) photoconductivity, high electron temperature, benefiting from the unique linear Dirac dispersion. In this work, the pressure effects of photocarrier dynamics of graphene have been investigated using in situ time-resolved THz spectroscopy in combination with diamond anvil cell exceeding 9 GPa. We find that the negative THz conductivity maintains in our studied pressure range both for monolayer and bilayer graphene. In particular, the amplitude of THz photoconductivity in monolayer graphene manifests an extraordinary dropping with pressure, compared with that from the counterparts such as bulk silicon and bilayer graphene. Concomitantly, the time constant is reduced with increasing pressure, highlighting the pressure-induced hot carrier cooling. The pressure dependence of photocarrier dynamics in monolayer graphene is likely related with the enhancement of the interfacial coupling between diamond surface and sample, allowing for the activity of new electron–phonon scattering. Our work is expected to provide an impetus for the studies of high-pressure THz spectroscopy of two-dimensional materials.

Ultrafast Photocarrier Dynamics in Vertically Aligned SnS2 Nanoflakes Probing with Transient Terahertz Spectroscopy.

By employing optical pump Terahertz (THz) probe spectroscopy, ultrafast photocarrier dynamics of a two-dimensional (2D) semiconductor, SnS2 nanoflake film, has been investigated systematically at room temperature. The dynamics of photoexcitation is strongly related to the density of edge sites and defects in the SnS2 nanoflakes, which is controllable by adjusting the height of vertically aligned SnS2 during chemical vapor deposition growth. After photoexcitation at 400 nm, the transient THz photoconductivity response of the films can be well fitted with bi-exponential decay function. The fast and slow processes are shorter in the thinner film than in the thicker sample, and both components are independent on the pump fluence. Hereby, we propose that edge-site trapping as well as defect-assisted electron-hole recombination are responsible for the fast and slow decay progress, respectively. Our experimental results demonstrate that the edge sites and defects in SnS2 nanoflakes play a dominant role in photocarrier relaxation, which is crucial in understanding the photoelectrochemical performance of SnS2 nanoflakes.

Detection of living cervical cancer cells by transient terahertz spectroscopy.

The photonic energy of terahertz wave is in the same order of magnitude as the rotational and vibrational energy levels of organic and biological macromolecules, so it has unique advantages in detecting cells and biological macromolecules. However, in the life environment, the dynamic time scale of cell-environment interaction and structural conformation change of biological macromolecules are within picosecond to millisecond, and water has strong absorption to terahertz wave, which has become the bottleneck problem for the detection of cells and biological macromolecules by terahertz technology. In this article, we developed a set of terahertz single measurement system based on the tilt wave front of grating pulse technique. The system was employed for the terahertz detection of trace living cervical cancer cells. We achieved transient detection of the terahertz pulse time-domain waveform of the living HeLa cells. The characteristic absorption peaks were identified by Lambert-Beer law, respectively, at 0.49, 0.71, 1.04, 1.07, 1.26 and 1.37 THz. The absorbance is proportional to the cell concentration.

High resolution terahertz spectroscopy for analytical applications

The latest achievements in the area of terahertz spectrometers based on transient effects are presented. The potential to use these spectrometers in various applications demanding the composition analysis of multicomponent gas mixtures simultaneously with a high sensitivity (at the ppb level) and resolving power is considered. The results obtained with the use of transient terahertz spectroscopy for medical diagnostics, security systems, etc. are shown.

Evaluating intrinsic mobility from transient terahertz conductivity spectra of microcrystal samples of organic molecular semiconductors

In microcrystal samples of the prototypical organic molecular semiconductors rubrene and C10-DNTT (2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene), we measured spectra of complex optical conductivity caused by photocarriers with optical-pump terahertz-probe spectroscopy. We analyzed the σ̃ω spectra using the Drude-Smith model and evaluated the DC mobility μDC, which includes extrinsic effects, and the intrinsic mobility μint. The resulting μDC and μint values are comparable to the mobility values obtained using transport measurements of polycrystalline and single-crystalline field-effect transistors, respectively. This correspondence demonstrates that the intrinsic mobility values of organic molecular semiconductors can be evaluated from the measurements of microcrystal samples using transient terahertz spectroscopy.

Terahertz probe of photoexcited carrier dynamics in the Dirac semimetal Cd3As2

The relaxation dynamics of photoexcited quasiparticles of three-dimensional (3D) Dirac semimetals are vital towards their application in high performance electronic and optoelectronic devices. In this work, the relaxation dynamics of photoexcited carriers of 3D Dirac semimetal Cd3As2 are investigated by transient terahertz spectroscopy. The visible pump-THz probe spectroscopy measurement shows clear biexponential decays with two characteristic time constants. According to the pump-power and temperature dependence, these two characteristic time constants are attributed to the electron phonon coupling (1-4 ps) and anharmonic decay of hot coupled phonons to electronic uncoupled phonons (2-9 ps), respectively. An anomalous electron-optical phonon coupling reduction and a bottleneck slowing of hot optical phonons relaxation are observed with higher excitation intensities similar to that in graphene. On the other hand, the electron-optical phonon coupling can be enhanced due to the phonon frequency broadening and softening at elevated lattice temperature. Furthermore, the transient THz spectrum response is strongly modified by the phonon assisted intraband absorption of hot carriers from a pure electronic Drude model, which is evidenced by a characteristic THz absorption dip in the transient THz absorption spectrum. This absorption dip is pinned by the discrete optical phonon energy that assists the intraband transition enabled by photoexcitation of hot carriers.

Slow Electron–Hole Recombination in Lead Iodide Perovskites Does Not Require a Molecular Dipole

Hybrid organic/inorganic lead iodide perovskites of the formula APbI3, where A is a molecular cation such as methylammonium, exhibit remarkably slow photoinduced charge carrier recombination rates, for reasons that remain uncertain. Prevalent hypotheses credit this behavior to the unique dipolar nature of the molecular cation. Herein, transient terahertz spectroscopy is applied to solution-processed, all-inorganic, perovskite-phase cesium lead iodide (CsPbI3) thin films, which lack such a dipole. The recombination kinetics are studied as a function of the initial photoinduced carrier concentration and the wavelength of excitation. A kinetic model combining diffusion and recombination is fit to the data, from which the rate constants are determined, revealing a bimolecular recombination rate of 10–10 cm3 s–1, comparable to high-quality, single-crystal, direct-gap semiconductors. This rate, as well as a charge carrier mobility > 30 cm2 V–1 s–1 measured herein for CsPbI3, are similar to values reported for t...

Femtosecond-timescale buildup of electron mobility in GaAs observed via ultrabroadband transient terahertz spectroscopy

The buildup of photoconductivity in an archetypal polar semiconductor GaAs was monitored on the timescale of carrier-carrier and carrier-lattice correlations, with sub-50 fs temporal resolution provided by ultra-broadband transient terahertz spectroscopy. Two relevant timescales are identified: the transition in photoconductivity from a regime of extremely weak screening to a screened Drude plasma was observed on the ∼280 fs timescale, followed by the gradual increase and saturation of electron mobility on a timescale of ∼160 fs. The Drude photoconductivity picture following carrier photoinjection was found to stabilize only on the timescale corresponding to several lattice and plasma oscillations.

Picosecond charge transport in rutile at high carrier densities studied by transient terahertz spectroscopy

We study terahertz photoconductivity of a rutile single crystal between 10 and 300 K under strong photoexcitation by femtosecond pulses at 266 nm. A marked dependence of the carrier mobility on the carrier density is observed leading to highly complex transport phenomena on a picosecond time scale. We develop a general model of carrier photoconductive response in the case of time dependent inhomogeneous distribution of carrier density and mobility. This allows us to assess an important role of both electrons and holes in the response of photoexcited rutile. At low temperatures, the carrier mobility is initially reduced due to the electron-hole scattering and increases by one order of magnitude upon ambipolar diffusion of the carriers into deeper regions of the sample. At room temperature, contributions of transient hot optical phonons and/or of midinfrared polaron excitations with charge-density-dependent dielectric strength emerge in the photoconductivity spectra.

Self-referenced ultra-broadband transient terahertz spectroscopy using air-photonics.

Terahertz (THz) air-photonics employs nonlinear interactions of ultrashort laser pulses in air to generate and detect THz pulses. As air is virtually non-dispersive, the optical-THz phase matching condition is automatically met, thus permitting the generation and detection of ultra-broadband THz pulses covering the entire THz spectral range without any gaps. Air-photonics naturally offers unique opportunities for ultra-broadband transient THz spectroscopy, yet many critical challenges inherent to this technique must first be resolved. Here, we present explicit guidelines for ultra-broadband transient THz spectroscopy with air-photonics, including a novel method for self-referenced signal acquisition minimizing the phase error, and the numerically-accurate approach to the transient reflectance data analysis.

Iron sensitizer converts light to electrons with 92% yield.

Solar energy conversion in photovoltaics or photocatalysis involves light harvesting, or sensitization, of a semiconductor or catalyst as a first step. Rare elements are frequently used for this purpose, but they are obviously not ideal for large-scale implementation. Great efforts have been made to replace the widely used ruthenium with more abundant analogues like iron, but without much success due to the very short-lived excited states of the resulting iron complexes. Here, we describe the development of an iron-nitrogen-heterocyclic-carbene sensitizer with an excited-state lifetime that is nearly a thousand-fold longer than that of traditional iron polypyridyl complexes. By the use of electron paramagnetic resonance, transient absorption spectroscopy, transient terahertz spectroscopy and quantum chemical calculations, we show that the iron complex generates photoelectrons in the conduction band of titanium dioxide with a quantum yield of 92% from the (3)MLCT (metal-to-ligand charge transfer) state. These results open up possibilities to develop solar energy-converting materials based on abundant elements.

Direct observation of the collapse of the delocalized excess electron in water.

It is generally assumed that the hydrated electron occupies a quasi-spherical cavity surrounded by only a few water molecules in its equilibrated state. However, in the very moment of its generation, before water has had time to respond to the extra charge, it is expected to be significantly larger in size. According to a particle-in-a-box picture, the frequency of its absorption spectrum is a sensitive measure of the initial size of the electronic wavefunction. Here, using transient terahertz spectroscopy, we show that the excess electron initially absorbs in the far-infrared at a frequency for which accompanying ab initio molecular dynamics simulations estimate an initial delocalization length of ≈ 40 Å. The electron subsequently shrinks due to solvation and thereby leaves the terahertz observation window very quickly, within ≈ 200 fs.

Direct Observation of Sub-100 fs Mobile Charge Generation in a Polymer-Fullerene Film

The formation of mobile charges in a roll-to-roll processed poly-3-hexylthiophene-fullerene bulk heterojunction film is observed directly by using transient terahertz spectroscopy with sub-100fs temporal resolution. The transient terahertz ac conductivity reveals that 20% of the incident pump photons are converted into highly delocalized charges within the 40fs, 3.1eV pump pulse duration, which then rapidly becomes localized within 120fs. Approximately 2/3 of these carriers subsequently decay, possibly into an exciton, on a 1ps time scale.

Transient terahertz spectroscopy of excitons and unbound carriers in quasi-two-dimensional electron-hole gases

We report a comprehensive experimental study and detailed model analysis of the terahertz dielectric response and density kinetics of excitons and unbound electron-hole pairs in GaAs quantum wells. A compact expression is given, in absolute units, for the complex-valued terahertz dielectric function of intraexcitonic transitions between the $1s$ and higher-energy exciton and continuum levels. It closely describes the terahertz spectra of resonantly generated excitons. Exciton ionization and formation are further explored, where the terahertz response exhibits both intraexcitonic and Drude features. Utilizing a two-component dielectric function, we derive the underlying exciton and unbound pair densities. In the ionized state, excellent agreement is found with the Saha thermodynamic equilibrium, which provides experimental verification of the two-component analysis and density scaling. During exciton formation, in turn, the pair kinetics is quantitatively described by a Saha equilibrium that follows the carrier cooling dynamics. The terahertz-derived kinetics is, moreover, consistent with time-resolved luminescence measured for comparison. Our study establishes a basis for tracking pair densities via transient terahertz spectroscopy of photoexcited quasi-two-dimensional electron-hole gases.