Distribution Of Photoexcited Carriers Research Articles

Tracking photocarrier-enhanced electron-phonon coupling in nonequilibrium

Light-modulated electron-phonon coupling (EPC) is significant in many intriguing phenomena including light-enhanced superconductivity, polaron formation, and hidden charge orders, which provides a powerful strategy to engineer materials’ functionalities on demand. Here we explore EPC in photoexcited graphene during the ultrafast photocarrier dynamics with a phonon bath. Via analysing energy transport between electrons and phonons, light-induced EPC enhancement by more than one order of magnitude is demonstrated, which originates from the dynamic distribution of photoexcited carriers out of equilibrium. Excellent agreements between theory and experiment have been achieved, justifying the validity of the present approach for extracting excited-state dynamic properties. Our result unravels a crucial impact of photoexcitation on EPC by modulating the density and distribution of photocarriers, and provides a useful strategy for tracking ultrafast EPC in real time.

Ultrafast Relaxation of Symmetry-Breaking Photo-Induced Atomic Forces.

We present a first-principles method for the calculation of the temperature-dependent relaxation of symmetry-breaking atomic driving forces in photoexcited systems. We calculate the phonon-assisted decay of the photoexcited force on the low-symmetry E_{g} mode following absorption of an ultrafast pulse in Bi, Sb, and As. The force decay lifetimes for Bi and Sb are of the order of 10fs and in agreement with recent experiments, demonstrating that electron-phonon scattering is the primary mechanism relaxing the symmetry-breaking forces. Calculations for a range of absorbed photon energies suggest that larger amplitude, symmetry-breaking atomic motion may be induced by choosing a pump photon energy which maximizes the product of the initial E_{g} force and its lifetime. The high-symmetry A_{1g} force undergoes a partial decay to a nonzero constant on similar timescales, which has not yet been measured in experiments. The average imaginary part of the electron self-energy over the photoexcited carrier distribution provides a crude indication of the decay rate of symmetry-breaking forces.

Excitation wavelength-dependent carrier dynamics in n-type and semi-insulating 6H-SiC using ultrafast transient absorption spectroscopy

Transient absorption spectroscopy was utilized to monitor ultrafast carrier dynamics in conductive and semi-insulating 6H-SiC wafers. By tuning the excitation wavelength around the bandgap of 6H-SiC (365–400 nm), the surface and bulk recombination processes could be accordingly distinguished. Simultaneously, the locations of surface and defect states in the bandgap were inferred by investigating the evolution of transient absorption spectra for different photoexcited carrier distributions. Vanadium (V) doping had a notable modulation on transient absorption in semi-insulating 6H-SiC, resulting from an additional decay process induced by carrier capturing of V deep levels. The carrier lifetimes induced by various recombination mechanisms were determined and interpreted unequivocally by global analysis and simplified model. The time constant via bulk capturing (∼400 ps) was three orders of magnitude faster than that via inherent defects. Controlling the lifetime by excitation wavelength and doping conditions is essential for fabricating 6H-SiC-based ultrafast photonic devices.

Ultrafast nonlinear absorption and carrier relaxation in ReS2 and ReSe2 films

As two important members of two-dimensional (2D) transition metal dichalcogenides, ReS2 and ReSe2 have gained interest for optoelectronic and photonic applications. The key to the application of the 2D materials to the optoelectronic devices is to understand the interaction between light and matter. Here, we report that the chemical vapor deposition-grown few-layer ReS2 and ReSe2 display saturable absorption under 400 nm pulse excitation, measured through intensity dependent transmission and confirmed by ΔT/T spectroscopy. ΔT/T spectroscopy substantiates the coexistence of saturable absorption and excited state absorption at 800 nm for both ReS2 and ReSe2. Two time constants are extracted from time-resolved spectroscopy; the short time constant of 10–20 ps is associated with the relaxation of hot carriers and exciton formation, and the long time constant of 70–100 ps is assigned to exciton lifetime. The polarization dependence of ΔT/T reveals that the initial distribution of photoexcited carriers centered at excitation state is anisotropic, and this initial anisotropy loses rapidly with carrier relaxation. The nonequilibrium carriers scattered far away from excitation state are fully isotropic in the entire relaxation process. These findings provide fundamental information for using the two materials in ultrafast optoelectronic and photonic devices.

Charge-carrier relaxation in sonochemically fabricated dendronized CaSiO3–SiO2–Si nanoheterostructures

We present systematic studies of charge-carrier relaxation processes in sonochemically nanostructured silicon wafers. Impedance spectroscopy and transient photovoltage techniques are employed. It is found that interface potential in Si wafers remarkably increases upon their exposure to sonochemical treatments in Ca-rich environments. In contrast, the density of fast interface electron states remains almost unchanged. It is found that the initial photovoltage decay, taken before ultrasonic treatments, exhibits the involvement of shorter- and longer time recombination and trapping centers. The decay speeds up remarkably due to cavitation treatments, which is accompanied by a substantial quenching of the photovoltage magnitude. It is also found that, before the treatments, the photovoltage magnitude is markedly non-uniform over the wafer surface, implying the existence of distributed sites affecting distribution of photoexcited carriers. The treatments cause an overall broadening of the photovoltage distribution. Furthermore, impedance measurements monitor the progress in surface structuring relevant to several relaxation processes. We believe that sonochemical nanostructuring of silicon wafers with dendronized CaSiO3 may enable new promising avenue towards low-cost solar energy efficiency multilayered solar cell device structures.

Circular Photogalvanic Effect in the Weyl Semimetal TaAs**Supported by the National Basic Research Program of China under Grant Nos 2015CB921300, the National Key Research and Development Program of China under Grant Nos 2016YFA0300300, 2017YFA0504703 and 2017YFA0302900, the National Natural Science Foundation of China under Grant Nos 11604372, 11474323 and 11774391, the Strategic Priority Research Program (B) of the Chinese Academy

Weyl semimetal (WSM) is expected to be an ideal spintronic material owing to its spin currents carried by the bulk and surface states with spin-momentum locking. The generation of a sizable photocurrent is predicted in non-centrosymmetric WSM arising from the broken inversion symmetry and the linear energy dispersion that is unique to Weyl systems. In our recent measurements, the circular photogalvanic effect (CPGE) is discovered in the TaAs WSM. The CPGE voltage is proportional to the helicity of the incident light, reversing direction if the radiation helicity changes handedness, a periodical oscillation therefore appears following the alteration of light polarization. We herein attribute the CPGE to the asymmetric optical excitation of the Weyl cone, which could result in an asymmetric distribution of photoexcited carriers in momentum space according to an optical spin-related selection rule.

Influence of the pump pulse wavelength on the ultrafast demagnetization of Gd(0 0 0 1) thin films

We studied the magnetization dynamics of gadolinium metal after femtosecond laser excitation recording the x-ray magnetic circular dichroism in reflection (XMCD-R) at the Gd M5 absorption edge. Varying the photon energy of the pump pulse allows us to change the initial energy distribution of photoexcited carriers. The overall similar response for excitation with 0.95, 1.55 and 3.10 eV photons at comparable pump fluences indicates that ultrafast ballistic carrier transport leads to a homogeneous energy distribution on the femtosecond timescale in the probed sample volume. Differences are observed in the initial ultrafast demagnetization magnitude. They are attributed to an enhanced spin-flip probability at higher electron energies characterizing the non-thermal electron distribution.

Detecting the Photoexcited Carrier Distribution Across GaAs/Transition Metal Oxide Interfaces by Coherent Longitudinal Acoustic Phonons.

A prominent architecture for solar energy conversion layers diverse materials, such as traditional semiconductors (Si, III-V) and transition metal oxides (TMOs), into a monolithic device. The efficiency with which photoexcited carriers cross each layer is critical to device performance and dependent on the electronic properties of a heterojunction. Here, by time-resolved changes in the reflectivity after excitation of an n-GaAs/p-GaAs/TMO (Co3O4, IrO2) device, we detect a photoexcited carrier distribution specific to the p-GaAs/TMO interface through its coupling to phonons in both materials. The photoexcited carriers generate two coherent longitudinal acoustic phonons (CLAPs) traveling in opposite directions, one into the TMO and the other into the p-GaAs. This is the first time a CLAP is reported to originate at a semiconductor/TMO heterojunction. Therefore, these experiments seed future modeling of the built-in electric fields, the internal Fermi level, and the photoexcited carrier density of semiconductor/TMO interfaces within multilayered heterostructures.

Terahertz radiation from accelerating charge carriers in graphene under ultrafast photoexcitation

We study the generation of THz radiation from the acceleration of ultrafast photoexcited charge carriers in graphene in the presence of a DC electric field. Our model is based on calculating the transient current density from the time-dependent distribution function which is determined using the Boltzmann transport equation within a relaxation time approximation. We include the time-dependent generation of carriers by the pump pulse by solving for the carrier generation rate using the Bloch equations in the rotating wave approximation (RWA). The linearly polarized pump pulse generates an anisotropic distribution of photoexcited carriers in the $k_x-k_y$ plane. The collision integral in the Boltzmann equation includes a term that leads to the \textit{thermalization} of carriers via carrier-carrier scattering to an effective temperature above the lattice temperature, as well as a \textit{cooling} term which leads to energy relaxation via inelastic carrier-phonon scattering. The radiated signal is proportional to the time derivative of the transient current density. In spite of the fact that the magnitude of the velocity is the same for all the carriers in graphene, there is still emitted radiation from the photoexcited charge carriers with frequency components in the THz range due to a change in the \textit{direction} of velocity of the photoexcited carriers in the external electric field as well as \textit{cooling} of the photoexcited carriers on a sub-picosecond time scale.

Evolution of anisotropic-to-isotropic photoexcited carrier distribution in graphene

Evolution of anisotropic-to-isotropic photoexcited carrier distribution in graphene

Spintronics aided by terahertz exposure

Generation, control, and detection of a non-equilibrium polarization of electron spin is the key challenge in the growing field of spintronics.1, 2 Optical orientation, in which circularly polarized photons transfer their angular momentum to electrons, is a versatile tool to prepare and investigate non-equilibrium spin polarization. In semiconductor quantum wells (QWs), this momentum transfer yields a spin-polarized electric current—a spin photocurrent—and the irradiated QW acts as a ‘spin battery’.3, 4 Because spin photocurrents occur in homogeneous, unbiased samples under uniform irradiation, the effect is very different from photovoltaic currents, which are caused by charge separation at potential barriers. These currents have been studied in various materials over a wide range of temperatures, including the technologically important room temperature. In contrast to higher-energy, inter-band excitations, the terahertz (THz) spectral range has the advantage that it involves only one kind of carrier, electrons or holes, yielding unipolar spin orientation. Two mechanisms by which THz excitation can generate a spin photocurrent have been investigated in detail: the circular photogalvanic effect (CPGE)4 and the optically induced spin-galvanic effect (SGE).5 A characteristic feature of a spin photocurrent, j, is that it reverses direction, j→ − j, when the radiation helicity is changed, for example from right-handed circular to left-handed circular, σ+ → σ−. The photocurrent is proportional to the helicity, Pcirc. 7 This dependence is illustrated in Figure 1 for p-GaAs QWs. The experimental arrangement is shown in the inset. In these low-symmetry, (113)-grown QWs, the spin photocurrent occurs for radiation at normal incidence, but in (001)-oriented samples a helicity-dependent signal is observed only at oblique incidence due to the higher symmetry. The spin photocurrent results from an asymmetric momentum distribution of photoexcited carriers, which in turn reflects Figure 1. Photocurrent, normalized to the incident power, P, of terahertz radiation, for p-GaAs quantum wells grown with (113) crystallographic orientation. The angleφ defines the helicity through Pcirc = sin 2φ. The radiation source was an NH3 molecular laser pumped by a TEA-CO2 laser emitting 100ns long pulses at 76μm at about P = 10kW peak power.

Above-band gap two-photon absorption and its influence on ultrafast carrier dynamics in ZnTe and CdTe

We present measurements of transient carrier dynamics subsequent to intense above-band gap femtosecond excitation of the II–VI compounds ZnTe and CdTe, probed in the far infrared by transient terahertz time-domain spectroscopy. The intensity and temperature dependence of the carrier dynamics illuminates the role of two-photon absorption (TPA) of pump photons. At cold temperatures and high excitation intensities TPA results in a photoexcited carrier distribution which requires several picoseconds to thermalize. As a consequence, the dielectric function of the carrier distribution deviates strongly from the Drude model for as long as 20 ps after excitation.

Weak variband surface layers as an effective means of protection from the surface recombination of photoexcited carriers in infrared threshold CdHgTe photoresistors

It is demonstrated that variband near-surface layers with even a small composition gradient can provide ultimate photomodulation (corresponding to the absence of the surface recombination) of the conductivity of thin CdHgTe samples used for manufacturing threshold photoresistors (including those of the SPRITE type) for the wavelengths λ =8−12 μm and λ=3−5 μm. This is explained by the formation (even in weak variband layers) of a special profile of the photoexcited carrier distribution that inhibits their diffusion to the surface. As is known, the introduction of the near-surface variband layers with a small composition gradient must not lead to any substantial increase in the number of defects in the sample.

Exciton dynamics inGaAs/AlxGa1−xAsquantum wells

The changes induced in the optical absorption spectrum of a ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}\mathrm{}$ multiple quantum well due to a photoexcited carrier distribution are reexamined. We use a femtosecond pump-probe technique to excite excitons and free electron-hole pairs. We find that for densities up to ${10}^{11} {\mathrm{cm}}^{\ensuremath{-}2}$ the saturation of exciton absorption is caused both by a reduction of the oscillator strength and by a broadening of the exciton resonance, which exhibit quite different temporal evolutions. The restoring of the initial value of the reduced oscillator strength is determined by the free-carrier lifetime, which we have measured to be 65 ps, whereas the temporal evolution of the broadening is due to the exciton-exciton interaction and determined by the exciton lifetime, which is found to be 410 ps. We have measured the oscillator strength saturation densities and shown that at low temperature the influence of free electron-hole pairs on the exciton oscillator strength is twice as effective as the influence of other excitons.

Temperature-dependent photoluminescence of In0.5Al0.5As/Al0.25Ga0.75As self-organized quantum dots

The photoluminescence spectra of large ensembles of self-organized In0.5Al0.5As/Al0.25Ga0.75As quantum dots were studied. The spectra reveal a number of sharp peaks with energy separation and full width at half maximum values of ∼20 meV for 150 K⩽T⩽275 K. Significant changes are observed in the luminescence characteristics with varying temperature, which are attributed to a changing distribution of photoexcited carriers between dots.

Polarization-dependent efficiency of photoconducting THz transmitters and receivers

The emission and detection efficiencies of photoconducting THz transmitters and receivers are found to be sensitive to the polarization of the optical gating pulse. Signal amplitudes from GaAs coplanar stripline transmitters and silicon-on-sapphire dipole receivers vary by factors of up to 4 and 3, respectively, with rotation of the exciting pulse polarization. In both cases, maximum signal is obtained when the polarization of the normally incident light is perpendicular to the edge of the metal electrodes. This polarization sensitivity, which appears to arise from differences in the spatial distribution of photoexcited carriers in the semiconductor, needs to be considered when optimizing the signal-to-noise ratio in coherent THz spectroscopy.

Response of excitonic absorption spectra to photoexcited carriers in GaAs quantum wells.

The changes induced in the optical-absorption spectrum of a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple-quantum well due to photoexcited carrier distributions are reexamined. We use a picosecond pump-probe technique to excite carriers in a sample having wells of thickness 210 \AA{} and barriers of thickness 100 \AA{}. We find that for densities up to 1\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ the peak of the exciton line of the lowest-energy exciton becomes bleached; however the spectrally integrated area does not change, indicating that the carriers reduce the absorption peak by collision broadening rather than by phase-space filling or exchange. We also observe shifts of the exciton line to higher energy (``blueshift'') or to lower energy (``redshift'') depending on the energy of the pump photons. The shift can be ascribed to competing effects of band-gap renormalization and the change in the binding energy of the exciton, although a detailed theoretical description is still needed.

Thermalization of Photoexcited Carriers in Bulk and Quantum Wire Semiconductors

A theoretical study of the internal thermalization of high-energy carriers created by laser excitation in bulk and quantum wire semiconductors is presented. For the bulk case the results of our Monte Carlo simulation are compared to luminescence up-conversion experiments used to monitor the spectral and temporal evolution of the photoexcited carrier distributions with a time resolution of about 100 fs. The agreement between theory and experiment is very good and shows that the Coulomb interaction among carriers is responsible for the initial ultrafast thermalization. On the contrary, a much slower thermalization is found theoretically in quantum wires, which can be mainly attributed to the reduced efficiency of intersubband processes and to the reduced effect of electron-electron intrasubband scattering. Available experiments seem to confirm such findings.

Picosecond absorption dynamics of photoexcited InGaP epitaxial films

The absorption recovery of a photoexcited InGaP epitaxial film 0.4 μm thick was investigated using the pump-probe laser technique and found to have a time constant of 55 ps at room temperature. Measurements done in the temperature range of 300–50 K show the decay of the photoexcited carrier distribution to be dominated by ambipolar diffusion and surface recombination. The measured absorption recovery time constant corresponds to an ambipolar diffusion coefficient D≳2.8 cm2/s and a surface recombination velocity of S≳4×105 cm/s at room temperature.

Influence of nonradiative and nongeminate processes on a lifetime distribution of photoexcited carriers in amorphous semiconductors at low temperatures

The influence of nonradiative and nongeminate radiative processes on the lifetime distribution of photoexcited carriers is studied in a simple computer simulation. The lifetime distribution was simulated recently for geminate radiative processes, and turned out to be much broader than that observed by experiment. The aim of the present simulation is to clarify whether nonradiative or nongeminate radiative processes could lead to the observed narrowing of the distribution. The results show that nonradiative processes lead to a narrowing only with an appreciable and unrealistic shift of the distribution to shorter times. Nongeminate radiative processes lead to a narrowing of the distribution with a weaker and more reasonable shift.