High Pump Fluence Research Articles

Ultrafast Photoemission Spectroscopy of the Uranium DioxideUO2Mott Insulator: Evidence for a Robust Energy Gap Structure

Time-resolved photoemission spectroscopy utilizing a probe energy of 32.55 eV and a pump energy of 3.1 and 4.65 eV with 30 fs temporal resolution is used to study the carrier dynamics in the $5f$ Mott insulator uranium dioxide (${\mathrm{UO}}_{2}$). The Mott gap and on-site Coulomb interaction energies are measured directly as ${E}_{\text{gap}}=2.5\text{ }\text{ }\mathrm{eV}$ and ${U}_{C}=5\text{ }\text{ }\mathrm{eV}$, respectively, and the dynamics of the upper Hubbard band is mapped. The $f\text{\ensuremath{-}}f$Mott-Hubbard dynamics involves subpicosecond fluence-dependent relaxation, followed by decay via coupling to the lattice upon formation of excitonic polarons. Instead of an expected metallic transition, we observe a robust Mott gap structure, even at high pump fluences.

Glass-like recovery of antiferromagnetic spin ordering in a photo-excited manganite Pr0.7Ca0.3MnO3

Electronic orderings of charges, orbitals and spins are observed in many strongly correlated electron materials, and revealing their dynamics is a critical step toward undertsanding the underlying physics of important emergent phenomena. Here we use time-resolved resonant soft x-ray scattering spectroscopy to probe the dynamics of antiferromagnetic spin ordering in the manganite Pr0.7Ca0.3MnO3 following ultrafast photo-exitation. Our studies reveal a glass-like recovery of the spin ordering and a crossover in the dimensionality of the restoring interaction from quasi-1D at low pump fluence to 3D at high pump fluence. This behavior arises from the metastable state created by photo-excitation, a state characterized by spin disordered metallic droplets within the larger charge- and spin-ordered insulating domains. Comparison with time-resolved resistivity measurements suggests that the collapse of spin ordering is correlated with the insulator-to-metal transition, but the recovery of the insulating phase does not depend on the re-establishment of the spin ordering.

Effects of pump pulse propagation and spatial distribution of bias fields on terahertz generation from photoconductive antennas

Through theoretical analysis and numerical simulation, we show that bipolar terahertz pulses generated from photo-excited small-gap or mid-gap photoconductive (PC) antennas actually stem from a unique combination of spatially inhomogeneous bias field and propagation effects of pump pulses through the PC if the influence of fast carrier recombination on terahertz radiation is excluded. This finding provides new insights on the formation of bipolar terahertz pulses, instead of the traditional view based on the Drude-Lorentz model and the space-charge screening effect. We find large-aperture PC antennas always emit unipolar terahertz pulses rather than bipolar ones because the bias fields in this case can be considered homogeneous. We also show that the saturation of the peak amplitude of terahertz pulses at high pump fluence can be correctly predicted using our model without invoking space-charge screening.

Unified Study of Recombination in Polymer:Fullerene Solar Cells Using Transient Absorption and Charge-Extraction Measurements.

Recombination in the well-performing bulk heterojunction solar cell blend between the conjugated polymer TQ-1 and the substituted fullerene PCBM has been investigated with pump-probe transient absorption and charge extraction of photogenerated carriers (photo-CELIV). Both methods are shown to generate identical and overlapping data under appropriate experimental conditions. The dominant type of recombination is bimolecular with a rate constant of 7 × 10(-12) cm(-3) s(-1). This recombination rate is shown to be fully consistent with solar cell performance. Deviations from an ideal bimolecular recombination process, in this material system only observable at high pump fluences, are explained with a time-dependent charge-carrier mobility, and the implications of such a behavior for device development are discussed.

Ultrafast dynamics of hot electrons and phonons in chemical vapor deposited graphene

The relaxation dynamics of photoexcited carriers in a chemical vapor deposited graphene transferred on quartz substrate has been investigated by using ultrafast optical-pump terahertz (THz)-probe spectroscopy. Terahertz transmission through graphene sample is reduced by optical pumping. The change of transmission decays exponentially after the optical pulse. We find the relaxation time is insensitive to the substrate temperature from 10 K to 300 K but increases sublinearly with pump fluence. We model the relaxation process involving electron-phonon coupling together with a set of rate equations to describe the transient responses of quasi-particles and optical phonons. The increases of the extracted carrier temperature and the measured relaxation time with pump fluence are associated with the fact that high pump fluence significantly increases the carrier temperature and broadens the carrier distribution. As a result, it leads to the reduction of optical phonon emission efficiency and the decrease of cooling rate as well.

Observation of intra- and inter-band transitions in the transient optical response of graphene

The transient optical conductivity of freely suspended graphene was examined by femtosecond time-resolved spectroscopy using pump excitation at 400 nm and probe radiation at 800 nm. The optical conductivity (or, equivalently, absorption) changes abruptly upon excitation and subsequently relaxes to its initial value on the time scale of 1 ps. The form of the induced change in the optical conductivity varies strongly with excitation conditions, exhibiting a crossover from enhanced to decreased optical conductivity with increasing pump fluence. We describe the graphene response in terms of transient heating of the electrons, with the characteristic relaxation time of the transient conductivity reflecting the cooling of the electron system and the strongly coupled optical phonons through emission of lower energy phonons. The change in the optical conductivity is attributed to a combination of induced absorption from intra-band transitions of the photo-generated carriers and bleaching of the inter-band transitions by Pauli blocking. The former effect, which corresponds to the high-frequency wing of the Drude response, dominates at low pump fluence. In this regime of a limited rise in the electron temperature, an increase in the optical conductivity is observed. At high pump fluence, elevated electron temperatures are achieved. The decrease in the inter-band bleaching then dominates the transient response, the intra-band contribution being overwhelmed despite an increase in the Drude scattering rate with temperature. The temporal evolution of the optical conductivity in all the regimes can be described within a model including the intra- and inter-band contributions with a time-varying electronic temperature. An increased Drude scattering rate is inferred for high electron temperature and mechanisms for this enhancement are considered. The calculated scattering rate for interactions of the carriers with zone-center and zone-edge optical phonons agrees well with the rates obtained from experiment.

Precessional switching by ultrashort pulse laser: Beyond room temperature ferromagnetic resonance limit

Conventional pure magnetic recording schemes have a serious and unavoidable problem known as the ferromagnetic resonance (FMR) limit. Here we demonstrate a fast precessional switching via interplay of ultrafast heating by ultrashort pulse laser and large temperature dependency of magnetic resonance. In ferrimagnetic GdFeCo, when the temperature approaches the angular momentum compensation point TA, both frequency of FMR mode and the Gilbert damping parameter increase significantly. High-speed and strongly damped precessional switching was triggered with ultrafast heating of a Gd24.5Fe66.1Co9.4 across its magnetization compensation point TM up to TA, under a static applied magnetic field. To initiate and investigate the magnetization reversal, we have used an all-optical pump-probe technique employing an amplified Ti:sapphire laser system with 90 fs pulses. In particular, following the laser excitation with low pump fluence (0.9 mJ/cm2) the magnetic system relaxes back toward initial state while the rather high pump fluence (3.3 mJ/cm2) excitation induces a metastable opposite magnetic state. After the sudden heating which causes just 30% reduction of magnetization at first breakdown, consequently magnetization was started to rotate across Mz = 0 within 6 ps during first precession and finished the high damped precessional motion within few cycles into opposite direction.

Efficient terahertz generation by optical rectification at 1035 nm

We demonstrate efficient generation of THz pulses by optical rectification of 1.03 um wavelength laser pulses in LiNbO3 using tilted pulse front excitation for velocity matching between the optical and THz fields. Pulse energies of 100 nJ with a spectral bandwidth of up to 2.5 THz were obtained at a pump energy of 400 uJ and 300 fs pulse duration. This conversion efficiency of 2.5x10(-4) was an order of magnitude higher than that obtained with collinear optical recitification in GaP, and far higher still than that measured using ZnTe in an optimized geometry. Using a simple model we demonstrate that two- and three-photon absorption strongly limit the THz generation efficiency at high pump fluences in ZnTe and GaP.

Terahertz radiation from heavy-ion-irradiated In0.53Ga0.47As photoconductive antenna excited at 1.55μm

We investigate terahertz (THz) emission from heavy-ion-irradiated In0.53Ga0.47As photoconductive antennas excited at 1550 nm. The carrier lifetime in the highly irradiated In0.53Ga0.47As layer is less than 200 fs, the steady-state mobility is 490cm2V−1s−1, and the dark resistivity is 3Ωcm. The spectrum of the electric field radiating from the Br+-irradiated In0.53Ga0.47As antenna extends beyond 2 THz. The THz electric field magnitude is shown to saturate at high optical pump fluence, and the saturation fluence level increases with the irradiation dose, indicating that defect center scattering has a significant contribution to the transient mobility.

Ultrafast Carrier Dynamics in Single ZnO Nanowire and Nanoribbon Lasers

Time-resolved second-harmonic generation (TRSHG) and transient photoluminescence (PL) spectroscopy are utilized to probe the ultrafast creation and subsequent relaxation of excited carriers immediately following band-gap excitation in single ZnO nanowire and nanoribbon lasers. The TRSHG signal consists of a 1−5 ps recovery present only during strong lasing and a 10−80 ps intensity-dependent component. The transient PL response from single structures exhibits an 80 ps decay component independent of pump power (free exciton PL), and a < 10 ps power-dependent component (stimulated emission) that shifts to earlier delay by ca. 10 ps at high pump fluence.

Terahertz dynamics of photogenerated carriers in ferromagnetic InGaMnAs

We have measured the picosecond transient carrier response of InGaMnAs/InP by two-color pump–probe spectroscopy, using an intense near-infrared beam as the pump and a 52.5 μm (5.7 THz) beam as the probe. We observed strongly nonexponential decays, especially at high pump fluences and low temperatures, where a pronounced dip developed in the terahertz differential transmission. This dip disappeared at the highest fluence where a transmission plateau versus time delay was observed. Our band structure calculations suggest that this intriguing behavior may be due to carrier dynamics associated with Γ–L intervalley scattering that becomes efficient when ferromagnetism modifies the band structure.

Photothermal analysis of polymeric dye laser materials excited at different pump rates.

The photothermal properties and heat diffusion of polymeric lasers, made up from solutions of Rhodamine 6G in solid matrices of poly(2-hydroxyethyl methacrylate) with different amounts of the cross-linking monomer ethylene glycol dimethacrylate and copolymers of 2-hydroxyethyl methacrylate and methyl methacrylate have been studied through photothermal deflection spectroscopy. The heat load that is due to the pumping process was quantified as a function of the pump excitation repetition frequency (0.25-10 Hz), determining the time-dependent temperature changes at different locations within the laser matrix. A theoretical model, which reproduces these changes with high accuracy, was developed on the basis of the heat-diffusion equation of optically dense fluids. The observed thermal effects became important for impairing the laser stability at pump repetition frequencies higher than 1 Hz. In addition, the irreversible optical changes produced in the laser matrices at high pump fluence values (>1 J/cm2) were also analyzed. These effects originate, most likely, from a two-step photothermal mechanism.

Luminescence of Silicon Nanocrystals in SiO2: Effects of Excitation Spectrum

AbstractSilicon nanocrystals formed by ion implantation and annealing of fused silica wafers show a strong, broad photoluminescence (PL) peak centered at a wavelength between 750 and 900 nm, depending on the processing conditions. This luminescence has been extensively investigated and trial device structures based on these materials have been built. However, relatively few studies also report the optical absorption spectra. In fact, the absorbance of these specimens is quite low (usually &lt; 10%) at wavelengths greater than 450 nm (i.e., at the pump wavelengths typically used for PL studies). This suggests that in numerous studies of Si nanocrystals produced by ion implantation, only a small fraction of the nanocrystals is responsible for the observed PL at the typical pump wavelengths. In this study, we investigated how the PL spectrum and intensity depend on the power and wavelength of the pump laser. We find that the PL intensity approaches saturation at high pump fluences, and that the peak emission wavelength is sensitive to the excitation power. These observations can be attributed to the dynamics of the excitation/recombination processes at different energies, and indicate that considerable care must be taken when comparing the emission spectra of different specimens. Our data are uniformly consistent with a mechanism of light emission involving subgap states (i.e., radiative trap sites) and are not supportive of a “pure” quantum confinement model.

Density-functional theory approach to ultrafast laser excitation of semiconductors: Application to theA1phonon in tellurium

Calculations of the ${A}_{1}$ phonon frequency in photoexcited tellurium are presented. The phonon frequency as a function of photoexcited carrier density and phonon amplitude is determined, including anharmonic effects. The sensitivity of the ${A}_{1}$ mode to photoexcitation is related to the Peierls mechanism for stabilizing the \ensuremath{\alpha}-Te structure. The assumptions of slow and fast carrier recombination are investigated and it is found that the two regimes give qualitatively different predictions for the excitation dependence of the phonon frequency. Recent pump-probe experiments are compared with the calculations. The predictions based on fast carrier recombination are not in agreement with experiment. The reflectivity oscillations expected to occur in pump-probe experiments are simulated, including the coupled effects of optical absorption, carrier diffusion, and phonon dynamics. Using the calculated dependence of phonon frequency on carrier density (assuming slow carrier recombination) and experimental values for the optical dielectric constants, the derivative of the frequency peak for reflectivity oscillations with respect to pump fluence is found to be $\ensuremath{-}0.085 \mathrm{}\mathrm{THz}$ per ${\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^{2},$ compared to an experimental value of $\ensuremath{-}0.07 \mathrm{THz}$ per ${\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^{2}$ in the low-fluence regime. The ambipolar diffusion constant for the optically excited carriers is estimated to be 10 ${\mathrm{cm}}^{2}/\mathrm{s},$ substantially smaller than its equilibrium value. The effects of carrier diffusion are found to be more important than phonon anharmonicity in the observed changes of phonon frequency within the first few cycles of motion after laser excitation. Greatly increased damping of the reflectivity oscillations at high pump fluences, which was reported in recent experiments, is not found in the simulations.

Excited-state absorption and up-conversion losses in the Nd-doped glasses for high-power lasers

Excited-state absorption spectra of two Nd-doped glasses, a silicate LSG91H and a phosphate LHG8 from Hoya, are reported. An adequate model based on up-conversion energy transfers is then used to account for the shortening of the infrared fluorescence decays and the losses on the stored energies observed or predicted in this type of materials at high pump fluences.

Spin dynamics inCoPt3alloy films: A magnetic phase transition in the femtosecond time scale

We have measured the magnetization dynamics of an epitaxial ${\mathrm{CoPt}}_{3}$ alloy film using a magneto-optical pump-probe Kerr setup with 120 fs laser pulses. At high pump fluence, the film is driven from the ferro- to the paramagnetic state with a characteristic time comparable to the pulse duration. The associated demagnetization occurs within the thermalization time of the electrons to a Fermi distribution. These results demonstrate the feasibility of ultrafast magneto-optical devices.

Biexcitons in π-conjugated oligomers: Intensity-dependent femtosecond transient-absorption study

We report femtosecond transient-absorption (TA) studies of a five-ring oligomer of poly({ital para}-phenylene vinylene) prepared in two different forms: solid-state films and dilute solutions. At high pump fluences, in both types of samples, we observe generation of two-exciton states, which are detected by the evolution of TA spectra and dynamics with increasing pump intensity. In solutions, double excitation of molecules results in the formation of stable biexcitons with enhanced oscillator strength, leading to an increased efficiency of the radiative decay and a superlinear pump dependence of the stimulated emission. In solid-state samples, the two-exciton states are unstable and decay on the subpicosecond time scale due to ultrafast charge transfer, accompanied by generation of interchain excitons. {copyright} {ital 1998} {ital The American Physical Society}

Electron and Hole Trapping Dynamics in Semiconductor Nanocrystals: Femtosecond Nonlinear Transmission and Photoluminescence Study

AbstractApplication of two complementary femtosecond techniques (time-resolved nonlinear transmission and photoluminescence up-conversion) allows us to observe separately the electron and the hole relaxation paths in CdS nanocrystals. The obtained data indicate that hole relaxation channels are different at low and high pump fluences which is attributed to an Auger-process-assisted hole trapping at surface/interface states activated at high excitation intensities.

Transient excitation and emission in hexagonal ZnS:Mn

Pulsed excitation and emission data are reported for ZnS : Mn using UV wavelenghts below and above the bandgap. The data are interpreted to be consistent with donor-acceptor (DA) recombination proceeding simultaneously with Mn intra-atomic emission. Mn excitation occurs primarily through resonance energy transfer. The temperature dependence of the emission spectra shows dramatic differences depending on the UV photon energy relative to the bandgap. Luminescence decay is not exponential for both DA and Mn emission, and it is observed that DA emission is extinguished well before the Mn emission. The dependence of the emission on pump intensity is found to be nonlinear both below and above the bandgap for high pump fluence, and linear for low pump fluence. With the crystal placed in a resonator, no evidence of stimulated emission was observed. This is interpreted as being primarily due to the low optical conversion efficiency of the pump light to 580 nm emission.