Pump-probe Transient Reflectivity Research Articles

Correlation of refractive index based and THz streaking arrival time tools for a hard X-ray free-electron laser.

To fully exploit ultra-short X-ray pulse durations routinely available at X-ray free-electron lasers to follow out-of-equilibrium dynamics, inherent arrival time fluctuations of the X-ray pulse with an external perturbing laser pulse need to be measured. In this work, two methods of arrival time measurement were compared to measure the arrival time jitter of hard X-ray pulses. The methods were photoelectron streaking by a THz field and a transient refractive index change of a semiconductor. The methods were validated by shot-to-shot correction of a pump-probe transient reflectivity measurement. An ultimate shot-to-shot full width at half-maximum error between the devices of 19.2 ± 0.1 fs was measured.

Pump-probe reflectivity studies of ultrashort laser-induced acousto-mechanical strains in ZnO films

In the current work we report on the generation of acoustic strains in thin ZnO layers using optoacoustic transduction of ultrashort laser pulses into acoustic waves on an Au thin film transducer. After absorption of energy by the electron system of the metal, energy conversion, thermal expansion and mechanical deformation takes place. The generation and propagation of the induced acoustic strains are monitored in time via a degenerate pump-probe transient reflectivity optical setup at 800 nm, as opposed to most commonly used schemes that employ different wavelengths for the pump and probe beams, mostly in the vicinity of ZnO maximum absorption. The experimental results include energy relaxation times and phonon scattering frequencies and are supported by a thermal vibro-acoustic finite element model. The model is based on the combination of a revised two-temperature approach and elasticity theory, and considers anisotropic properties for the ZnO film and the computation of the elastic wave velocity for the first time. The outcomes are discussed in the context of electron–phonon coupling factors and other material properties. A good agreement between the experimental findings and the results from the numerical simulations has been established, regarding outcomes like the mean velocity of the strain waves, establishing a novel characterization method applicable to a variety of materials and structures.

Efficient ultrafast photoacoustic transduction on Tantalum thin films

Nano-acoustic strain generation in thin metallic films via ultrafast laser excitation is widely used in material science, imaging and medical applications. Recently, it was shown that transition metals, such as titanium, exhibit enhanced photoacoustic transduction properties compared to noble metals, such as silver. This work presents experimental results and simulations that demonstrate that among transition metals tantalum exhibits superior photoacoustic properties. Experiments of nano-acoustic strain generation by femtosecond laser pulses focused on thin tantalum films deposited on Silicon substrates are presented. The nano-acoustic strains are measured via pump-probe transient reflectivity that captures the Brillouin oscillations produced by photon–phonon interactions. The observed Brillouin oscillations are correlated to the photoacoustic transduction efficiency of the tantalum thin film and compared to the performance of titanium thin films, clearly demonstrating the superior photoacoustic transduction efficiency of tantalum. The findings are supported by computational results on the laser-induced strains and their propagation in these thin metal film/substrate systems using a two-temperature model in combination with thermo-mechanical finite element analysis. Finally, the role of the metal transducer-substrate acoustic impedance matching is discussed and the possibility to generate appropriately modulated acoustic pulse trains inside the crystalline substrate structures for the development of crystalline undulators used for γ-ray generation is presented.

High ambipolar mobility in cubic boron arsenide revealed by transient reflectivity microscopy.

Semiconducting cubic boron arsenide (c-BAs) has been predicted to have carrier mobility of 1400 square centimeters per volt-second for electrons and 2100 square centimeters per volt-second for holes at room temperature. Using pump-probe transient reflectivity microscopy, we monitored the diffusion of photoexcited carriers in single-crystal c-BAs to obtain their mobility. With near-bandgap 600-nanometer pump pulses, we found a high ambipolar mobility of 1550 ± 120 square centimeters per volt-second, in good agreement with theoretical prediction. Additional experiments with 400-nanometer pumps on the same spot revealed a mobility of >3000 square centimeters per volt-second, which we attribute to hot electrons. The observation of high carrier mobility, in conjunction with high thermal conductivity, enables an enormous number of device applications for c-BAs in high-performance electronics and optoelectronics.

Characteristic research of uniform-doping and exponential-doping Ga1-xAlxAs/GaAs photocathode with femtosecond laser illumination

The properties and characteristics of graded Al component Ga1-xAlxAs/GaAs photocathode are investigated in this paper, especially the transient reflectivity of Ga0.37Al0.63As under the condition of femtosecond laser illumination. Using software CASTEP, we calculate and analyze the optoelectronic property of varied Al composition Ga1-xAlxAs photocathode based on first principle theory. Ga0.37Al0.63As is emphasized studied because its lattice constant is very close to GaAs, reducing the probability of lattice mismatch and the back interface recombination velocity on the interface of substrate and emission layer. In this work, we design two reflection mode GaAs photocathode samples with different doping method: uniform-doping and exponential-doping used in emission layer grown by MBE. The impacts of different doping method on photocathode are analyzed by comparing some typical performance parameters. Transient reflectivity rates of different doping method Ga1-xAlxAs/GaAs photocathode are investigated by the device of pump-probe transient reflectivity system in the paper and we found that the way of exponential-doping is better for improving the performance of Ga1-xAlxAs /GaAs photocathode. The potential value of transient property of photocathode is enormous in future.

Chemical nature of the anion antisite in dilute phosphide GaAs1−xPx alloy grown at low temperature

While nonstoichiometric binary III-V compounds are known to contain group-V antisites, the growth of ternary alloys consisting of two group-V elements might give additional degrees of freedom in the chemical nature of these antisites. Using cross-sectional scanning tunneling microscopy (STM), we investigate low-temperature-grown dilute GaAs1−xPx alloys. High concentrations of negatively charged point defects are found. Combined with transmission electron microscopy and pump-probe transient reflectivity, this study shows that the defects have a behavior similar to the group-V antisites. Further analyses with x-ray diffraction point to the preferential incorporation of arsenic antisites, consistent with ab initio calculations, that yield a formation energy 0.83 eV lower than for phosphorus antisites. Although the negative charge carried by the arsenic antisites in the STM images is shown to be induced by the proximity of the STM tip, the arsenic antisites are not randomly distributed in the alloy, providing insight into the evolution of their charge state during the growth.

Ultrafast laser pulse chirp effects on laser-generated nanoacoustic strains in Silicon

Nanoacoustic strains are generated in Silicon by chirped femtosecond laser pulses using thin Titanium films as transducers. We investigate the effect that the generating laser pulse chirp has on the amplitude of the induced strains, manifested as Brillouin oscillations observed in degenerate femtosecond pump-probe transient reflectivity measurements. The strain amplitude is larger when negatively chirped pulses are used, which is attributed to the more efficient conversion of laser pulse light into acoustic strain in the Titanium transducer. Our present studies clearly show that the dependence of the Brillouin amplitude and the lattice strain is a non-monotonous function of the laser chirp parameter. An optimum negative laser pulse chirp is found for which the strain amplitude is maximized. A detailed thermomechanical model satisfactorily supports the experimental findings. In such a way, it is possible to suppress or enhance the induced nanoacoustic strain amplitude, thus all-optically controlling it by at least a factor of two.

Ultrafast carrier dynamics in LT-GaAs doped with Si delta layers

We characterized the ultrafast properties of LT-GaAs doped with silicon [Formula: see text]-layers and introduced delta-doping ([Formula: see text]-doping) as efficient method for enhancing the properties of GaAs-based structures which can be useful for terahertz (THz) antenna, ultrafast switches and other high frequency applications. Low temperature grown GaAs (LT-GaAs) became one of the most promising materials for ultrafast optical and THz devices due to its short carrier lifetime and high carrier mobility. Low temperature growth leads to a large number of point defects and an excess of arsenic. Annealing of LT-GaAs creates high resistivity through the formation of As-clusters, which appear due to the excess of arsenic. High resistivity is very important for THz antennas so that voltage can be applied without the risk of breakdown. With [Formula: see text]-Si doping, control of As-clusters is possible, since after annealing, clusters align in the plane where the [Formula: see text]-doping occurs. In this paper, we compare the properties of LT-GaAs-based planar structures with and without [Formula: see text]-Si doping and subsequent annealing. We used pump-probe transient reflectivity as a probe for ultrafast carrier dynamics in LT-GaAs. The results of the experiment were interpreted using the Ortiz model and show that the [Formula: see text]-Si doping increases deep donor and acceptor concentrations and decreases the photoinduced carrier lifetime as compared with LT-GaAs with same growth and annealing temperatures, but without doping.

Controlling nanoscale acoustic strains in silicon using chirped femtosecond laser pulses

The influence of femtosecond laser pulse chirp on laser-generated longitudinal acoustic strains in Si (100) monocrystal substrates is studied. Degenerate femtosecond pump-probe transient reflectivity measurements are performed using a layered structure of thin Ti transducer film on an Si substrate. Experimental results show that acoustic strains, manifested as strong Brillouin oscillations, are more effectively induced when negatively chirped femtosecond laser pulses pump the transducer. These results are theoretically supported by a modified thermo-mechanical model based on the combination of a revised two-temperature model and elasticity theory that takes into account the instantaneous frequency of the chirped femtosecond laser pump pulses.

Nonlinear and time-resolved optical study of the 112-type iron-based superconductor parentCa1−xLaxFeAs2across its structural phase transition

The newly discovered 112-type ferropnictide superconductors contain chains of As atoms that break the tetragonal symmetry between the $a$ and $b$ axes. This feature eliminates the need for uniaxial strain that is usually required to stabilize large single domains in the electronic nematic state that exists in the vicinity of magnetic order in the iron-based superconductors. We report detailed structural symmetry measurements of 112-type Ca$_{0.73}$La$_{0.27}$FeAs$_{2}$ using rotational anisotropy optical second harmonic generation. This technique is complementary to diffraction experiments and enables a precise determination of the point group symmetry of a crystal. By combining our measurements with density functional theory calculations, we uncover a strong optical second harmonic response of bulk electric dipole origin from the Fe and Ca $3d$-derived states that enables us to assign $C_2$ as the crystallographic point group. This makes the 112-type materials high-temperature superconductors without a center of inversion, allowing for the possible mixing of singlet and triplet Cooper pairs in the superconducting state. We also perform pump-probe transient reflectivity experiments that reveal a 4.6 THz phonon mode associated with the out-of-plane motion of As atoms in the FeAs layers. We do not observe any suppression of the optical second harmonic response or shift in the phonon frequency upon cooling through the reported monoclinic-to-triclinic transition at 58 K. This allows us to identify $C_1$ as the low-temperature crystallographic point group but suggests that structural changes induced by long-range magnetic order are subtle and do not significantly affect electronic states near the Fermi level.

High acoustic strains in Si through ultrafast laser excitation of Ti thin-film transducers.

The role of thin-film metal transducers in ultrafast laser-generated longitudinal acoustic phonons in Si (100) monocrystal substrates is investigated. For this purpose degenerate femtosecond pump-probe transient reflectivity measurements are performed probing the Brillouin scattering of laser photons from phonons. The influence of the metallic electron-phonon coupling factor, acoustical impedance and film thickness is examined. An optical transfer matrix method for thin films is applied to extract the net acoustic strain relative strength for the various transducer cases, taking into account the experimental probing efficiency. In addition, a theoretical thermo-mechanical approach based on the combination of a revised two-temperature model and elasticity theory is applied and supports the experimental findings. The results show highly efficient generation of acoustic phonons in Si when Ti transducers are used. This demonstrates the crucial role of the transducer's high electron-phonon coupling constant and high compressive yield strength, as well as strong acoustical impedance matching with the semiconductor substrate.

Influence of Fe Buffer Layer on Co-Doped BaFe2As2Superconducting Thin Films

A systematic characterization of Co-doped BaFe2As2(Ba-122) thin films has been carried out. Two samples were available, one grown on CaF2substrate and the other on MgO with an Fe buffer layer. The goal was to investigate films’ magnetic and superconducting properties, their reciprocal interplay, and the role played by the Fe buffer layer in modifying them. Morphological characterization and Energy Dispersive X-ray analyses on the Fe-buffered sample demonstrate the presence of diffused Fe close to the Co-doped Ba-122 outer surface as well as irregular holes in the overlying superconducting film. These results account for hysteresis loops obtained with magneto-optic Kerr effect measurements and observed at both room and low temperatures. The magnetic pattern was visualized by magneto-optical imaging with an indicator film. Moreover, we investigated the onset of superconductivity through a measure of the superconducting energy gap. The latter is strictly related to the decay time of the excitation produced by an ultrashort laser pulse and has been determined in a pump-probe transient reflectivity experiment. A comparison of results relative to Co-doped Ba-122 thin films with and without Fe buffer layer is finally reported.

Transient reflectivity as a probe of ultrafast carrier dynamics in semiconductors: A revised model for low-temperature grown GaAs

We revisit pump-probe transient reflectivity (PPTR) as a probe of ultrafast carrier dynamics in photoconductive materials, using low-temperature grown GaAs (LT-GaAs) as an exemplar. The carrier dynamics in a series of annealed LT-GaAs wafers were measured by PPTR. The wafer growth and anneal conditions were tailored to produce a material system with a sub-picosecond carrier lifetime. The PPTR signals from these wafers are bipolar with time constants on the order of 100 fs and 1 ps, consistent with previous literature reports on LT-GaAs. We examined the utility of numerical simulations of the pump-probe transients described in [V. Ortiz et al., J. Appl. Phys. 102, 043515 (2007)] to model our experimental results. We discovered a discrepancy between the model's predictions and experiment with respect to the scaling of the PPTR response with injected carrier density, and show that this discrepancy is rooted in how the model accounts for the index of refraction change due to band filling (BF) and band gap renormalization (BGR). We demonstrate that any model that includes a non-negligible BGR effect is inconsistent with our experimental observations of LT-GaAs. We present a revised model to simulate PPTR signals that account for BF and incorporate optical absorption from mid-gap states. This model can reproduce the experimental results on LT-GaAs and enables comparative assessments of alternate trapping and recombination hypotheses. For LT-GaAs, we compared point defects and nanoparticles as sites for Shockley-Read-Hall recombination, with the result that nanoparticle trapping and recombination centers most accurately reproduce the PPTR probe of carrier dynamics in LT-GaAs.

Femtosecond optical reflectivity measurements of lattice-mediated spin repulsions in photoexcited LaCoO3thin films

We present results on the temperature dependence of ultrafast electron and lattice dynamics, measured with pump-probe transient reflectivity experiments, of an epitaxially grown LaCoO3 thin film under tensile strain. Probing spin-polarized transitions into the antibonding e(g) band provides a measure of the low-spin fraction, both as a function of temperature and time after photoexcitation. It is observed that femtosecond laser pulses destabilize the constant low-spin fraction (similar to 63%-64%) in equilibrium into a thermally activated state, driven by a subpicosecond change in spin gap Delta. From the time evolution of the low-spin fraction, it is possible to disentangle the thermal and lattice contributions to the spin state. A lattice mediated spin repulsion, identified as the governing factor determining the equilibrium spin state in thin-film LaCoO3, is observed. These results suggests that time-resolved spectroscopy is a sensitive probe of the spin state in LaCoO3 thin films, with the potential to bring forward quantitative insight into the complicated interplay between structure and spin state in LaCoO3.

Coherent nanoscale optical-phonon wave packet in graphene layers

Coherent large-wave-vector phonons in graphene layers were excited by using a 7.5-fs ultrashort laser, and observed by wavelength-resolved pump-probe transient reflectivity spectroscopy. Inter-Dirac-cone electron scattering mediated by edges and defects in graphene layers drives large-wave-vector coherent $D$-mode phonons near the Dirac point ($K$ point) of the Brillouin zone. In contrast to the normal coherent phonons generated with first-order Raman process, phonon chirp is observed, and amplitude and frequency strongly change depending on the probed wavelength. The results are discussed on the basis of doubly resonant Raman scattering process, demonstrating that generated $D$-mode coherent phonons can propagate as nanoscale optical-phonon wave packets.

Ultrafast Evolution of the Excited-State Potential Energy Surface ofTiO2Single Crystals Induced by Carrier Cooling

We investigate the influence of carrier cooling dynamics in TiO(2) on the excited-state potential energy surface along the A(1g) optical phonon coordinate after above band-gap excitation using ultrashort ultraviolet pulses. The large amplitude coherent oscillation observed in a pump-probe transient reflectivity measurement shows a phase shift of -0.2π with respect to a purely instantaneous displacive excitation. The dynamic evolution of the potential energy surface minimum of the coherent phonon coordinate is explored using accurate density functional theory calculations, which confirm a shift of the potential energy surface minimum upon resonant laser excitation and reveal a significant positive contribution to the displacive force due to the cooling of the excited hot electron-hole plasma. We show that this noninstantaneous effect can quantitatively explain the experimentally observed phase using reasonable assumptions for the parameters characterizing the excited carriers. Our work demonstrates that the fast equilibration dynamics of laser-excited nonequilibrium carrier populations can have a pronounced effect on the initial structural response of crystalline solids.

Picosecond photoexcitation of acoustic waves in locally canted gold films

The laser photoacoustic technique is used to generate and detect picosecond coherent acoustic vibrations in gold film media deposited on Si substrates. As a consequence of the gold crystallites’ canted orientation, the pump-probe picosecond transient reflectivity shows oscillations at the fundamental shear mode frequency. The shear character of the mode is suggested by its dispersion, by the dependence of the signal on the probe laser wavelength, and by x-ray texture analysis.

Does femtosecond time-resolved second-harmonic generation probe electron temperatures at surfaces?

Femtosecond pump-probe second-harmonic generation (SHG) and transient linear reflectivity measurements were carried out on polycrystalline Cu, Ag and An in air to analyze whether the electron temperature affects Fresnel factors or nonlinear susceptibilities, or both. Sensitivity to electron temperatures was attained by using photon energies near the interband transition threshold. We find that the nonlinear susceptibility carries the electron temperature dependence in case of Ag and Au, while for Cu the dependence is in the Fresnel factors. This contrasting behavior emphasizes that SHG is nota priori sensitive to electron dynamics at surfaces or interfaces, notwithstanding its cause.