Photoexcitation Density Research Articles

Amplitude Saturation of Coherent Phonon Excited by Field Screening in CdTe

Femtosecond dynamics of 5.00-THz coherent longitudinal optical phonons in CdTe is reported. Polarization dependence indicates that, as in III–V semiconductors, field screening in the depletion layer is the main generation mechanism. The amplitude of the coherent phonons shows saturation with increasing photoexcitation density. This counterintuitive behavior is interpreted as saturation in the screening of surface potential.

Subpicosecond time-resolved Raman studies of LO phonons in GaN: Dependence on photoexcited carrier density

Subpicosecond time-resolved Raman spectroscopy has been used to measure the lifetime of the LO phonon mode in GaN for photoexcited electron-hole pair density ranging from 1016to2×1019cm−3. The lifetime has been found to decrease from 2.5ps, at low density, to 0.35ps, at the highest density. The experimental findings should help resolve the recent controversy over the lifetime of LO phonon mode in GaN.

Interchain coupling effects on photoinduced phase transitions between neutral and ionic phases in an extended Hubbard model with alternating potentials and an electron-lattice coupling

Dynamics of ionic-to-neutral and neutral-to-ionic phase transitions induced by intrachain charge-transfer photoexcitations are studied in a quasi-one-dimensional extended Hubbard model with alternating potentials and an electron-lattice coupling for mixed-stack charge-transfer complexes. For interchain couplings, we use electron-electron interactions previously estimated for TTF-CA (TTF=tetrathiafulvalene, CA=chloranil). Photoexcitation is introduced by a pulse of oscillating electric field. The time-dependent Hartree-Fock approximation is used for the electronic part, and the classical approximation for the lattice part. In the ionic-to-neutral transition, the transferred charge density is a strongly nonlinear function of the photoexcitation density, which is characterized by the presence of a threshold. With substantial interchain couplings comparable to those in TTF-CA, the interchain correlation is strong during the transition. Neutral domains in nearby chains simultaneously grow even if their nucleation is delayed by reducing the amplitude of the electric field. With weaker interchain couplings, the growing processes are in phase only when the amplitude of the electric field is large. Thus, the experimentally observed, coherent motion of a macroscopic neutral-ionic domain boundary is allowed to emerge by such substantial interchain couplings. In the neutral-to-ionic transition, by contrast, the transferred charge density is almost a linear function of the photoexcitation density. Interchain electron-electron interactions make the function slightly nonlinear, but the uncooperative situation is almost unchanged and consistent with the experimental findings.

Temperature dependence of the electron diffusion coefficient in electrolyte-filledTiO2nanoparticle films: Evidence against multiple trapping in exponential conduction-band tails

The temperature and photoexcitation density dependences of the electron transport dynamics in electrolyte-filled mesoporous ${\mathrm{TiO}}_{2}$ nanoparticle films were investigated by transient photocurrent measurements. The thermal activation energy of the diffusion coefficient of photogenerated electrons ranged from 0.19--0.27 eV, depending on the specific sample studied. The diffusion coefficient also depends strongly on the photoexcitation density; however, the activation energy has little, if any, dependence on the photoexcitation density. The light intensity dependence can be used to infer temperature-independent dispersion parameters in the range 0.3--0.5. These results are inconsistent with the widely used transport model that assumes multiple trapping of electrons in an exponential conduction-band tail. We can also exclude a model allowing for widening of a band tail with increased temperature. Our results suggest that structural, not energetic, disorder limits electron transport in mesoporous ${\mathrm{TiO}}_{2}$. The analogy between this material and others in which charge transport is limited by structural disorder is discussed.

Ultrafast carrier dynamics in ZnO nanorods

Free exciton and above-band-gap free carrier dynamics in ZnO nanorods have been investigated at room temperature with a femtosecond transient transmission measurement. Following the photoexcitation of above-band-gap free carriers, an extremely fast external thermalization time on the order of 200 fs can be observed. Under high excitation, hot phonon effects were found to delay the carrier cooling process. While the photoexcitation energy was tuned to match the free exciton transition, stable exciton formation can be uncovered while no evident exciton ionization process can be found unless the photoexcited exciton density exceeded the Mott density.

Photoluminescence study of a current density filament during an impact ionization avalanche in n-GaAs

Photoluminescence (PL) spatial patterns and the PL spectra inside a current density filament have been investigated in n-GaAs at 4.2 K during impact ionization avalanche under an applied pulse voltage. The bright photoluminescence (PL)-pattern observed by applying a pulse voltage characterizes the formation of a current density filament well. The PL brightening was observed only with weak photoexcitation density ( J p ≅ 218 μW / cm 2 ) under the applied pulse voltage with repetition frequency of f 0 = 8 kHz . The observations of the bright PL spectra inside the current density filament provide a relatively new result with regard to the partial Auger recombination process of the ( D 0 , X ) complex, leaving the neutral donor in the excited states.

X-ray He-like ion diagnostics: New computations for photoionized plasmas

In a previous article, with a new code designed for a photoionized thick hot medium using a full transfer treatment of both lines and continuum, we have studied the He-like emission of a medium photoionized by an X-ray source. We considered the influence of density, ionization parameter and column density. We stressed that photoionized media are not uniform in temperature and we showed that photoexcitation and large column densities should be considered to avoid misinterpretation of the properties of the medium. To complete the possible situations observed in photoionized plasmas, we consider here the influence of microturbulence due for instance to a velocity gradient, of different element abundance, and of different ionizing spectral distributions. All new situations are computed using the same code, but with an improved version of our He-like atomic model. We give values of the G and R ratios, the equivalent width, and the ionic column densities with a greater accuracy than previously achieved. A large microturbulence results in a decrease of the G ratios for large column densities, for which the He-triplet resonant lines are optically thick. On the contrary, the R ratio is almost independent of the dispersion velocity and the element abundances, as long as the intercombination lines are optically thin. Through two 'universal diagrams' not depending on any parameter, we also present a way to determine the ionization parameter and the total column density of the emitting regions from the temperature of the observed radiative recombination continuum of a given ion and its column density.

Charge carrier photogeneration in oligo(phenylenevinylene) thin films: A quantitative study

We analyze the generation and recombination of charged states in oligo(phenylenevinylene)s at high photoexcitation densities, using femtosecond pump-probe spectroscopy as a direct probe for the time-dependent density of photogenerated singlet, triplet, and doublet states. Pairs of charged states are formed during the exciting laser pulse, partly by a two-photon process. Geminate recombination of these pairs is found in the first ps after formation, after which a nongeminate dispersive mechanism prevails. The application of high electric field strengths opens up an additional channel for charge carrier formation, involving field-induced electron transfer between two singlet excitons. This process is competing with bimolecular singlet exciton annihilation. Field-induced formation of triplet states is governed by nongeminate recombination of field-induced charged states. We give values for cross sections of the photogenerated species, the efficiency of field-induced charge carrier photogeneration and for the early charge carrier mobility.

Ultrafast optical switching to a metallic state by photoinduced mott transition in a halogen-bridged nickel-chain compound.

We demonstrate the ultrafast photoinduced Mott transition from a charge transfer insulator to a metal in a halogen-bridged Ni-chain compound by pump-probe reflection spectroscopy. Upon the irradiation of a 130-femtosecond laser pulse, the spectral weight of the gap transition is transferred to the inner-gap region. When the photoexcitation density exceeds 0.1/Ni site, the Drude-like high-reflection band appears in the infrared region, signaling the formation of a metallic state. The photogeneration of the metallic state and the subsequent recovery to the original gapped state occur within a few picoseconds.

EVOLUTION OF ELECTRON-HOLE PLASMA AND EXCITONS IN ZnO EPITAXIAL THIN FILMS STUDIED BY FEMTOSECOND SPECTROSCOPY

Time evolution of a high density electron-hole plasma (EHP) and excitons in ZnO epitaxial thin films was studied under resonant excitation of the excitonic state using pump-probe absorption and optical Kerr gate luminescence techniques with sub-picosecond time resolution in order to elucidate dynamics of the photo-excited high density carriers. The bleaching of the exciton absorption occurs without any measurable delay time, suggesting that the excitonic state instantaneously becomes unstable because of the dissociation of the high density excitons into the EHP due to the screening of Coulomb interaction. This leads to an instantaneous bandgap reduction. Then the recovery of the renormalized bandgap as well as that of the exciton bleaching takes place with decreasing the carrier density due to radiative recombination of electrons with holes. From the time-resolved absorption and luminescence experiments, the recovery time is determined to be 25 v ps.

Interface Properties and in-Plane Linear Photoluminescence Polarization in Highly Excited Type-II ZnSe/BeTe Heterostructures

Properties of interfaces with no-common atom in type-II ZnSe/BeTe heterostructures are studied with polarization spectroscopy. Radiative recombination and its polarization anisotropy have been found to depend crucially on the interface configuration and excitation power. Effects of external and internal electric fields on the in-plane polarization are studied both numerically and experimentally at low and high levels of photoexcitation density.

Many-body effects on excitonic optical properties of photoexcited semiconductor quantum wire structures

We study carrier-interaction-induced many-body effects on the excitonic optical properties of highly photoexcited one-dimensional semiconductor quantum wire systems by solving the dynamically screened Bethe-Salpeter equation using realistic Coulomb interaction between carriers. Including dynamical screening effects in the electron-hole self-energy and in the electron-hole interaction vertex function, we find that the excitonic absorption is essentially peaked at a constant energy for a large range of photoexcitation density $(n=0--6\ifmmode\times\else\texttimes\fi{}{10}^{5} {\mathrm{cm}}^{\ensuremath{-}1}),$ above which the absorption peak disappears without appreciable gain; i.e., no exciton to free electron-hole plasma Mott transition is observed, in contrast to previous theoretical results but in agreement with recent experimental findings. This absence of gain (or the nonexistence of a Mott transition) arises from the strong inelastic scattering by one-dimensional plasmons or charge density excitations, closely related to the non-Fermi-liquid nature of one-dimensional systems. Our theoretical work demonstrates a transition or a crossover in one-dimensional photoexcited electron-hole systems from an effective Fermi liquid behavior associated with a dilute gas of noninteracting excitons in the low-density region $(n<{10}^{5} {\mathrm{cm}}^{\ensuremath{-}1})$ to a non-Fermi liquid in the high-density region $(n>{10}^{5} {\mathrm{cm}}^{\ensuremath{-}1}).$ The conventional quasistatic approximation for this problem is also carried out to compare with the full dynamical results. Numerical results for exciton binding energy and absorption spectra are given as functions of carrier density and temperature.

Femtosecond dynamics of BSCCO-2212 in the weak and strong excitation limits

We present measurements of the ultrafast optical response of BSCCO-2212 in the weak perturbation limit (photoexcitation density ≤ 10 μJcm −2), where we focus attention on the dynamics of electronic correlations associated with the pseudogap, and also in the strong perturbation limit (≥ 1JJcm −2), where the superconducting condensate is destroyed.

Excitonic and Biexcitonic States in Semiconductor Quantum Dots

We analyze excitonic and biexcitonic effects in the coherent optical spectra of semiconductor quantum dots using a density-matrix approach, that explicitly accounts for exciton–exciton interactions. A consistent description of additional peaks appearing at higher photoexcitation density is given.

Excitonic and biexcitonic effects in the coherent optical response of semiconductor quantum dots

We analyze few-particle effects in the coherent optical spectra of semiconductor quantum dots using a density-matrix approach that explicitly accounts for exciton–exciton interactions. A consistent description of additional peaks appearing at high photoexcitation density is given.

Few-particle effects in the optical spectra of semiconductor quantum dots

We analyze few-particle effects in the optical spectra of semiconductor quantum dots using a density-matrix approach that explicitly accounts for exciton–exciton, as well as exciton–carrier interactions. We give a consistent description of additional peaks appearing at high photoexcitation density, and predict that a strong polarization dependence should be characteristic of few-particle features.

Ultrafast photogeneration mechanisms of triplet states inpara-hexaphenyl

We present femtosecond pump-probe measurements, both conventional and electric field-assisted, on organic light-emitting devices based on para-hexaphenyl. The dominant triplet exciton generation mechanism is assigned to nongeminate bimolecular recombination of photogenerated, spin-$\frac{1}{2}$ polarons. This process is active within a few hundred femtoseconds after photoexcitation and involves about 20% of the initially excited states. At higher photoexcitation densities, we observe an additional triplet generation mechanism, which occurs in the 10-ps time domain, due to fusion of singlet excitons and subsequent fission into correlated triplet pairs. The latter decay on the ${10}^{2}\ensuremath{-}\mathrm{ps}$ time scale by geminate recombination.

Few-Particle Effects in Nonlinear Optical Spectra of Semiconductor Quantum Dots

ABSTRACTWe present a density-matrix approach for the description of nonequilibrium carrier dynamics in optically excited semiconductor quantum dots, that explicitly accounts for exciton-exciton as well as exciton-carrier interactions. Within this framework, we analyze few-particle effects in the optical spectra and provide a consistent description of additional peaks appearing at high photoexcitation density. We discuss possible applications of such optical nonlinearities in future coherent-control experiments.

Excitation Density and Temperature Dependent Photoluminescence of InGaAs Self-Assembled Quantum Dots

In this paper, we shall report on the excitation density and temperature dependent photoluminescence produced by discrete energy levels from InGaAs self-assembled quantum dots. While increasing the photoexcitation density, five peaks originating from discrete energy levels of quantum dot and wetting layer are observed. By deconvoluting these spectra using multiple Gaussian fit, the intensity of each state is saturated following its degeneracy. We describe the lateral confinement of quantum dots using a parabolic potential model. The saturated values are in good agreement with the degeneracy of this potential type. From the temperature dependent photoluminescence, we observed the thermally activated quenching of each state. Our results suggest that the wetting layer acts as a barrier to the carrier thermallization processes offering a two dimensional path for inter-dot coupling.

Recombination of carriers in SiGe/Si heterostructures measured by photomodulated intersubband absorption

We used photoinduced absorption (PIA) spectroscopy of optical transitions between valence subbands of a nominally undoped SiGe/Si quantum wells to study the recombination processes of photogenerated carriers in these heterostructures. We measured the PIA as a function of the ambient temperature, photo-excitation density and modulation frequency. Our measurements show that the recombination kinetics is governed by both an extrinsic mono-molecular and intrinsic bi-molecular terms. We account for this density dependence by a simple rate model which takes into account thermal activation of the photogenerated holes out of the SiGe wells. The model describes very well the measured temperature dependence of the PIA and it provides quite an accurate determination of the intrinsic recombination rate of electrons and holes within the well regions in these heterostructures. The rate that we obtain is faster than the measured recombination rates in bulk Si and Ge. This recombination rate enhancement is possibly due to carriers confinement within the SiGe well regions.