pump-THz Probe Spectroscopy Research Articles

Optical-pump-terahertz-probe spectroscopy in high magnetic fields with kHz single-shot detection.

We demonstrate optical pump-THz probe (OPTP) spectroscopy with a variable external magnetic field (0-9T), in which the time-dependent THz signal is measured by echelon-based single-shot detection at a repetition rate of 1kHz. The method reduces data acquisition times by more than an order of magnitude compared to conventional electro-optic sampling using a scanning delay stage. The approach illustrates the wide applicability of the single-shot measurement approach to non-equilibrium systems that are studied through OPTP spectroscopy, especially in cases where parameters such as magnetic field strength (B) or other experimental parameters are varied. We demonstrate the capabilities of our measurement by performing cyclotron resonance experiments in bulk silicon, where we observe B-field-dependent carrier relaxation and distinct relaxation rates for different carrier types. We use a pair of economical linear array detectors to measure 500 time points on each shot, offering an equivalent performance to camera-based detection with possibilities for higher repetition rates.

Terahertz spectroscopy of MOFs reveals dynamic structure and contact free ultrafast photoconductivity

Metal-organic frameworks (MOFs) are porous crystalline materials. Their large pores make them particularly interesting for membranes, gas separation, and gas storage. Furthermore, MOFs are ultralight, making them suitable for a large realm of exciting applications ranging from wearable devices to space technology. Optimizing MOFs for these applications demands a detailed understanding of their low energy dynamics and photophysics, which can be provided by terahertz (THz) spectroscopy. MOFs exhibit structural modes, or phonons, with energies in the meV range, which corresponds to the THz spectral range (0.1–10 THz, 0.4–40 meV). Understanding these modes is crucial in determining how a MOF interacts with guest molecules in the process of gas capture and storage. In this perspective, we discuss how gas-MOF interactions alter the MOFs’ spectral fingerprints. We demonstrate that THz spectroscopy can be used for gas adsorption monitoring and explain how density functional theory, together with THz spectra, can illuminate the dynamic structure of MOFs, providing unique insight into their functionality. THz is also a contact free probe for conductivity and allows us to measure short range conductivity within an individual MOF crystal. We will discuss the advantages of THz as a conductivity probe for MOFs as compared to more established direct current techniques. We will then expand our view to incorporate ultrafast photoconductivity in MOFs measured via optical pump-THz probe spectroscopy, in comparison to more established ultrafast spectroscopic tools such as optical transient absorption and photoluminescence. We will supplement this section with a discussion of THz studies on perovskites, which unveiled electron–phonon interactions not yet explored in MOFs.

Charge Carriers Localization Effect Revealed through Terahertz Spectroscopy of MXene: Ti3C2Tx.

The transport properties of charge carriers in MXene, a promising material, have been studied using terahertz time-domain spectroscopy (THz-TDS) to examine its potential applications in optical and electronic devices. However, previous studies have been limited by narrow frequency ranges, which have hindered the understanding of the intrinsic mechanisms of carrier transport in MXenes. To address this issue, ultrabroadband THz-TDS with frequencies of up to 15 THz to investigate the complex photoconductances of MXene (Ti3C2Tx) films with different thicknesses are employed. The findings indicate that the electronic localization is substrate-dependent, and this effect decreases with an increase in the number of layers. This is attributed to the screening effect of the high carrier density in Ti3C2Tx. Additionally, the layer-independent photocarrier relaxations revealed by optical pump THz probe spectroscopy (OPTP) provide evidence of the carrier heating-induced screening effect. These results are significant for practical applications in both scientific research and various industries.

Optical Pump Terahertz Probe (OPTP) and Time Resolved Terahertz Spectroscopy (TRTS) of emerging solar materials

Photoconductivity is the crucial benchmark to assess the potential of any emerging material for future solar applications. Many optical techniques, like transient absorption and photoluminescence, explore bound electron states and provide indirect access to photoconductivity. Direct current (DC) measurements under solar simulation determine the total performance of a novel solar device. While this technique has a clear appeal, it involves electrical contacts, causing contact resistance, which impacts the measured conductivity. Furthermore, DC measurements do not provide any insight into ultrafast effects and the photophysics defining a novel material. Terahertz (THz) spectroscopy presents a contact-free technique to measure photoconductivity on a sub-ps time scale. These measurements can be performed on as-synthesized sample materials, including powders. The ultrafast time resolution informs us of trapping dynamics and reveals what physical processes limit the carrier lifetime in a novel material. Additionally, complex conductivity can be measured at THz frequencies. THz-conductivity and photoconductivity shed light on scattering effects, providing a road map toward minimizing these effects. However, THz spectroscopy is less intuitive than widely used DC measurements, and the interpretation of THz-results is more challenging. This tutorial aims to familiarize the reader with the main THz techniques used to explore emerging materials. We will illustrate how carrier lifetimes can be extracted from optical pump THz probe measurements. We will guide the reader through the process of extracting accurate photoconductivities from time resolved THz spectroscopy measurements and present the most commonly used models to describe the underlying physics. We will then discuss the difference between sample and material parameters and highlight potential pitfalls. The tutorial concludes with a perspective view on the ever evolving field of optical pump-THz probe spectroscopy of emerging materials.

Ultrafast hot-carrier cooling in quasi freestanding bilayer graphene with hydrogen intercalated atoms.

Femtosecond-THz optical pump probe spectroscopy is employed to investigate the cooling dynamics of hot carriers in quasi-free standing bilayer epitaxial graphene with hydrogen interacalation. We observe longer decay time constants, in the range of 2.6 to 6.4 ps, compared to previous studies on monolayer graphene, which increase nonlinearly with excitation intensity. The increased relaxation times are due to the decoupling of the graphene layer from the SiC substrate after hydrogen intercalation which increases the distance between graphene and substrate. Furthermore, our measurements show that the supercollision mechanism is not related to the cooling process of the hot carriers, which is ultimately achieved by electron optical phonon scattering.

Unraveling the Photophysics of Liquid-Phase Exfoliated Two-Dimensional ReS2 Nanoflakes

Few-layered transition metal dichalcogenides (TMDs) are increasingly popular materials for optoelectronics and catalysis. Amongst the various types of TMDs available today, rhenium-chalcogenides (ReX2) stand out due to their remarkable electronic structure, such as the occurrence of anisotropic excitons and potential direct bandgap behavior throughout multi-layered stacks. In this letter, we have analyzed the nature and dynamics of charge carriers in highly crystalline liquid-phase exfoliated ReS2, using a unique combination of optical pump-THz probe and broadband transient absorption spectroscopy. Two distinct time regimes are identified, both of which are dominated by unbound charge carriers despite the high exciton binding energy. In the first time regime, the unbound charge carriers cause an increase and a broadening of the exciton absorption band. In the second time regime, a peculiar narrowing of the excitonic absorption profile is observed, which we assign to the presence of built-in fields and/or charged defects. Our results pave the way to analyze spectrally complex transient absorption measurements on layered TMD materials and indicate the potential for ReS2 to produce mobile free charge carriers, a feat relevant for photovoltaic applications.

Unraveling the degradation process of 2D passivated and Cs stabilized FAPbI3 by optical pump THz probe spectroscopy

The carrier dynamics and time-dependent photo-physical properties in the degradation period of hybrid organic-inorganic lead halide perovskites (HOIPs) are still unclear. Here a non-contact optical pump THz probe measurement was performed to observe the long-term degradation process of 2D passivated and cesium stabilized formamidinium lead iodide perovskite (FAPbI3) films. We find that photoconductivity and carrier mobility of the FA0.9Cs0.1PbI3 film decay steadily, while these of the 2D phenylethylammonium lead iodide (PEA2PbI4) passivated FAPbI3 film show an accelerated decay rate. Moreover, it’s also revealed that the localization of charge carriers increases with the transformation process from the fitted results by the Drude-Smith model. These results indicate that the stabilized effect of Cs incorporation is continuous, while the passivated effect of PEA2PbI4 can be weakened by the newly emerged phase boundaries. This report proposes a new perspective and sheds light on the degradation process of HOIPs.

Terahertz Emission and Ultrafast Carrier Dynamics of Ar-Ion Implanted Cu(In,Ga)Se2 Thin Films

We investigated THz emission from Ar-ion-implanted Cu(In,Ga)Se2 (CIGS) films. THz radiation from the CIGS films increases as the density of implanted Ar ions increases. This is because Ar ions contribute to an increase in the surface surge current density. The effect of Ar-ion implantation on the carrier dynamics of CIGS films was also investigated using optical pump THz probe spectroscopy. The fitted results imply that implanted Ar ions increase the charge transition of intra-and carrier–carrier scattering lifetimes and decrease the bandgap transition lifetime.

Terahertz probe of nonequilibrium carrier dynamics and ultrafast photocurrents in the topological insulator Sb2Te3

In this study, we report the ultrafast excitation and dynamics of the carrier and photocurrent in a 3D typical topological insulator Sb2Te3. We utilize time-resolved optical pump-THz probe spectroscopy to explore the nonequilibrium carrier dynamics of Sb2Te3. The electron system undergoes an ultrafast relaxation and releases through intra-band and inter-band scattering. Additionally, THz emission spectroscopy is employed to investigate the different ultrafast photocurrents in Sb2Te3 through tuning the polarization of excitation pulses and rotating the sample's azimuthal angle. We distinguish the different ultrafast photocurrents driven by the linear photogalvanic effect, circular photogalvanic effect, and thermoelectric effect. Our results potentially enable an all-optical modulation of THz emission without any external bias field, which could play an important role in the development of topological insulator-based high-speed THz optoelectronic and opto-spintronic devices.

Transient transmission of THz metamaterial antennas by impact ionization in a silicon substrate.

The picosecond dynamics of excited charge carriers in the silicon substrate of THz metamaterial antennas was studied at different wavelengths. Time-resolved THz pump-THz probe spectroscopy was performed with light from a tunable free electron laser in the 9.3-16.7 THz frequency range using fluences of 2-12 J/m2. Depending on the excitation wavelength with respect to the resonance center, transient transmission increase, decrease, or a combination of both was observed. The transient transmission changes can be explained by local electric field enhancement, which induces impact ionization in the silicon substrate, increasing the local number of charge carriers by several orders of magnitude, and their subsequent diffusion and recombination. The studied metamaterials can be integrated with common semiconductor devices and can potentially be used in sensing applications and THz energy harvesting.

Ultrafast Photoexcited-Carrier Behavior Induced by Hydrogen Ion Irradiation of a Cu(In,Ga)Se2 Thin Film in the Terahertz Region

We investigated the effects of H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ion irradiation on a chalcogenide Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGS) thin film as a function of the dose concentration (10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> ∼ 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) at 200 keV by using time-resolved THz spectroscopy. The optical conductivity of these films was derived from THz time-domain spectroscopy (THz-TDS) in the THz region. THz emission spectroscopy revealed the emission of a strong-intensity THz pulse from the H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ion-irradiated CIGS thin film. This is explained by considering the ultrafast behavior of photocarriers, resulting in strong surface band bending induced by the built-in potential with a conversion of the CIGS thin film from <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> - to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -type. Finally, we used optical pump THz probe spectroscopy to examine the ultrafast photocarrier dynamics determined by the time of capture at the increased number of surface and bulk defect states. This clearly showed that irradiation with H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ions could generate a number of defect states while simultaneously pacifying the Cu vacancy defects.

Ultrafast electron transport in metallic antiferromagnetic Mn2Au thin films probed by terahertz spectroscopy

The ultrafast electron transport is central to the requirement of all-electrically controlled spintronics in metallic collinear antiferromagnetic (AFM) ${\mathrm{Mn}}_{2}\mathrm{Au}$ thin films within the terahertz (THz) range. Here we use the THz time-domain spectroscopy to measure the conductivity spectra of ${\mathrm{Mn}}_{2}\mathrm{Au}$ thin films with different thicknesses and temperatures. We demonstrate an onset of carrier localization in ${\mathrm{Mn}}_{2}\mathrm{Au}$ with decreasing the film thickness from 15 to 5 nm. By using optical pump-THz probe spectroscopy, we found the surface trapping states in the thinnest ${\mathrm{Mn}}_{2}\mathrm{Au}$ film limit its photoconductivity significantly. The DC conductivity and scattering time become larger and longer with decreasing temperature for all samples, which suggests that ${\mathrm{Mn}}_{2}\mathrm{Au}$ behaves in a typical metallic fashion. The THz optical and conductivity properties of ${\mathrm{Mn}}_{2}\mathrm{Au}$ films will advance our understanding of ultrafast electronic transport processes in AFM ${\mathrm{Mn}}_{2}\mathrm{Au}$ thin films.

Ultrafast charge carrier dynamics in quantum confined 2D perovskite.

We studied the charge carrier dynamics in 2D perovskite NBT2PbI4 by ultrafast optical pump-THz probe spectroscopy. We observed a few ps long relaxation dynamics that can be ascribed to the band to band carrier recombination, in the absence of any contribution from many-body and trap assisted processes. The transient conductivity spectra show that the polaron dynamics is strongly modulated by the presence of a rich exciton population. The polarization field resulting from the exciton formation acts as the source of a restoring force that localizes polarons. This is revealed by the presence of a negative imaginary conductivity. Our results show that the dynamics of excitons in 2D perovskites at room temperature can be detected by monitoring their effect on the conductivity of the photoinduced polaronic carrier.

Ultrafast wavelength-dependent carrier dynamics related to metastable defects in Cu(In,Ga)Se2 solar cells with chemically deposited Zn(O,S) buffer layer

The Cu(In,Ga)Se2 (CIGS) solar cells were fabricated with a Zn(O,S) buffer layer via chemical bath deposition method (CBD) using different thiourea (TU) mole concentration. The solar cells showed a substantial increment in the performance after light soaking treatment for 1 h. The performance increment was found to be in proportion to the TU mole concentration used in Z(O,S) layer deposition. To understand the cause of increased efficiency, we examined the metastable defect states that were reversible and reproducible in the CIGS solar cell. Firstly, to understand the film quality and the interface between CBD-Zn(O,S) and the CIGS layer, chemical states and band alignment along the depth direction was studied from the results of X-ray photoemission spectroscopy (XPS). It was observed that as the TU mole concentration increased, the distribution of S atoms in the CBD-Zn(O,S) layer changed from Gaussian to almost straight line due to the existence of numerous Zn–S bonds at the interface. With the actively diffused S atoms toward CIGS, a hollow band region was formed at the interface of the CIGS/CBD-Zn(O,S) layers, suppressing the electron-hole recombination, and thus enhancing the cell efficiency. Further, to measure the metastable defect states, optical pump-THz probe (OPTP) spectroscopy was utilized with two pumps having beam energies of 400 nm and 800 nm, which were sensitive enough to detect the defect states at an ultrafast time scale. We observed a unique and unusual decay curve of a re-excitation process after the initial ultrafast decay. The decay was contributed by “(VSe–VCu) divacancy complex,” which is considered a metastable defect state predominantly found near the surface of CIGS layer. The (VSe–VCu) metastable defect states were filled by photocarriers and then the re-excited photocarriers contributed to the expeditious flow of the photocurrent, giving rise to the increment of the cell efficiency.

Ultrafast THz Probe of Photoinduced Polarons in Lead-Halide Perovskites.

We study the nature of photoexcited charge carriers in CsPbBr_{3} nanocrystal thin films by ultrafast optical pump-THz probe spectroscopy. We observe a deviation from a pure Drude dispersion of the THz dielectric response that is ascribed to the polaronic nature of carriers; a transient blueshift of observed phonon frequencies is indicative of the coupling between photogenerated charges and stretching-bending modes of the deformed inorganic sublattice, as confirmed by DFT calculations.

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.

Ultrafast Terahertz Nonlinear Optics of Landau Level Transitions in a Monolayer Graphene.

We investigated the ultrafast terahertz (THz) nonlinearity in a monolayer graphene under the strong magnetic field using THz pump-THz probe spectroscopy. An ultrafast suppression of the Faraday rotation associated with inter-Landau level (LL) transitions is observed, reflecting the Dirac electron character of nonequidistant LLs with large transition dipole moments. A drastic modulation of electron distribution in LLs is induced by far off-resonant THz pulse excitation in the transparent region. Numerical simulation based on the density matrix formalism without rotating-wave approximation reproduces the experimental results. Our results indicate that the strong light-matter coupling regime is realized in graphene, with the Rabi frequency exceeding the carrier wave frequency and even the relevant energy scale of the inter-LL transition.

Oxidation Mechanism of Si1-xGex Nanowires with Au Catalyst Tip as a Function of Ge Content.

Si1-xGex nanowires (NWs) (0.22 ≤ x ≤ 0.78) were synthesized using a vapor-liquid-solid procedure with a Au catalyst. We measured the intrinsic physical, chemical, and electrical properties of the oxidized Si1-xGex NWs using several techniques, including transmission electron microscopy, X-ray photoemission spectroscopy, and optical pump-THz probe spectroscopy. We suggest two distinct oxidation mechanisms depending on the Ge content in the Si1-xGex NWs: (i) when the Ge content is around 0.22, a Au catalytic effect brings about oxidation in both the axial and lateral directions; and (ii) when the Ge content is greater than 0.22, the Au tip is detached from the NW body and does not act as a catalyst, which is a result of the high degree of Ge-atom participation in the oxidation process. Additionally, we measured the photoconductivity decay time distribution for the Si1-xGex NWs before and after oxidation process; the decay time is significantly shortened in oxidized Si1-xGex NWs (0.22 < x), whereas it is maintained for Si-rich Si1-xGex NWs (x ≈ 0.22) as compared to the as-grown Si1-xGex NWs. It indicates that the number of defect states is generated with the formation of defective Ge oxide at the oxide-shell-layer/Si1-xGex-core-NW interface.

Photoinduced terahertz radiation and negative conductivity dynamics in Heusler alloy Co2MnSn film.

We report the broadband terahertz (THz) radiation in ferromagnetic half-metallic Heusler alloy Co2MnSn thin film upon the irradiation of a femtosecond laser pulse at room temperature. The magnetic-, sample symmetry-, and pump fluence-dependent THz emission reveals that the THz radiation is originated from the magnetic-dipole radiation, i.e., the light-induced subpicosecond demagnetization. In addition, by optical pump-THz probe spectroscopy, we found that the photoexcited increase of the scattering rate of hot carriers thereby leads to the photoinduced negative THz conductivity in Co2MnSn thin film.

A critical review on the carrier dynamics in 2D layered materials investigated using THz spectroscopy

Accurately illustrating the photocarrier dynamics and photoelectrical properties of two dimensional (2D) materials is crucial in the development of 2D material-based optoelectronic devices. Considering this requirement, terahertz (THz) spectroscopy has emerged as a befitting characterization tool to provide deep insights into the carrier dynamics and measurements of the electrical/photoelectrical conductivity of 2D materials. THz spectroscopic measurements would provide information of transient behaviors of carriers with high accuracy in a nondestructive and noncontact manner. In this article, we present a comprehensive review on recent research efforts on investigations of 2D materials of graphene and transition metal dichalcogenides (TMDs) using THz spectroscopy. A brief introduction of THz time-domain spectroscopy (THz-TDS) and optical pump-THz probe spectroscopy (OPTP) is provided. The characterization of the electron transport of graphene at equilibrium state and transient behavior at non-equilibrium state is reviewed. We also review the characterizations of TMDs including MoS2 and WSe2. Finally, we conclude the recent reports and give a prospect on how THz characterizations would guide the design and optimization of 2D material-based optoelectronic devices.