Transient Reflectivity Changes Research Articles

Transient reflectance of silicon carbide during laser-induced phase separation

Laser irradiation can induce local modulations of functional material properties, such as a decreased resistivity or a variation in reflectance. Recent studies investigated the laser-induced phase separation of 4 H-SiC into carbon and silicon on top of regrown SiC to customize its electrical conductivity for the application in electronic devices. To understand the physical processes leading to the laser-induced phase separation, time-resolved pump-probe measurements represent a suitable tool. This study advances the state of the art by characterizing the transient reflectance changes in 4 H-SiC upon irradiation by spatially resolved pump-probe reflectometry. Since the laser heating alters the reflectance of the sample, the spatially resolved measurement enables to observe the heat conduction from the irradiated to the non-irradiated areas, which sustains for several milliseconds. Numerical simulations of the temperature evolution reveal a restricted one-dimensional heat conduction into depth due to the broad lateral extent of the irradiated area. The associated sustained increased temperature within the irradiated area most certainly abets the feasibility of the phase separation. These findings offer practical insights for optimizing the applied laser parameters to tailor the material properties via phase separation.

Destabilization of spin-Peierls phase via a charge-spin modulated Floquet state induced by intramolecular vibrational excitation

The electronic state control using a periodic light field is one of the central subjects in photophysics. In molecular solids, intramolecular vibrations sometimes couple to intermolecular electron transfer, thus modulating electron and spin densities of each molecule. Here, we show that in a quasi-one-dimensional molecular solid K-tetracyanoquinodimethane (TCNQ) in which TCNQ molecules are dimerized by the spin-Peierls mechanism, an intramolecular vibrational excitation with a phase-locked mid-infrared pulse induces a charge-spin modulated Floquet state, which destabilizes the spin-Peierls phase. By detecting reflectivity changes of the intramolecular transition band along the mid-infrared electric field with 6.6-fs probe pulses, we detected high-frequency oscillations reflecting electron- and spin-density modulations synchronized with intramolecular vibrations. More significantly, we observed an oscillation of ~110 cm−1 due to a dimeric mode driven by a decrease in spin-Peierls dimerization. This dimerization reduction was confirmed by measuring transient reflectivity changes of the Mott-gap transition band. These results demonstrate the effectiveness of intramolecular vibrational excitation as a method for Floquet engineering in molecular solids.

Spectral analysis of amplitude and phase echoes in picosecond ultrasonics for strain pulse shape determination

We introduce a spectral analysis method in picosecond ultrasonics to derive strain pulse shapes in an opaque sample with known optical properties. The method makes use of both the amplitude and phase of optical transient relative reflectance changes obtained, for example, by interferometry. We demonstrate this method through numerical simulation and by analysis of experimental results for a chromium film.

Optical generation and detection of GHz longitudinal and transverse acoustic waves in solids assisted by two-dimensional metallic grating structure

Absorption of laser pulses with picosecond temporal width in a two-dimensional nano-scale metallic grating deposited on a transparent substrate launches both longitudinal and transverse acoustic waves in to the sample. The periodic metallic structure makes the generated acoustic waves propagate several different directions. The propagating acoustic waves are monitored optically by the delayed light pulses (probe light) as the transient reflectivity change which is known as the Brillouin oscillation. The probe light is also diffracted by the metallic grating, allowing the detection of the acoustic waves propagating in the multiple directions efficiently. The analysis of the Brillouin oscillation frequencies brings the longitudinal and transverse sound velocities and the optical refractive index simultaneously from a single measurement. We verify the above mentioned scheme using a sample with two-dimensional square lattice (period 380 nm) of aluminum square islands of lateral dimension 200 × 200 nm2 and thickness of 50 nm on a fused silica substrate of thickness 1 mm. From the obtained Brillouin oscillation frequencies in several tens GHz, the acoustic and optical properties of the substrate material are successfully retrieved. The technique would be applicable to investigate the acoustic and optical properties of more complicated systems, such as anisotropic and/or inhomogeneous medium.

Detection of Mechanical Deformation Induced by Ultrafast Laser Irradiation Upon a Metallic Cantilever

In this work, we systematically investigated the ultrafast optical properties of aluminum (Al) thin films on silicon cantilevers using a microscopic femtosecond optical pump-probe technique to explore the effect of light irradiation upon cantilevers while considering radiation pressure and photothermal effects. The ultrafast laser pulses used for the study were less than 30 fs in pulse duration and 830 nm in wavelength, and the photon energy (1.49 eV) of the light pulses is close to the interband transition threshold (ITT) of Al. Therefore, the change in ITT due to the strain of the cantilevers induced by the light irradiation is detected through the change in the transient reflectivity, which is dominated by the thermalized (Th) electron signal. We uncovered the position dependency of the transient reflectivity change and the Th electronic signal amplitude of the 100 nm-thick Al films on 160 $\mu$m, 200 $\mu$m and 240 $\mu$m-length cantilevers, and these results are in excellent agreement with two temperature model-based curve fits. Furthermore, to understand the effect of light irradiation, we derived equations for the position-dependent radiation pressure effect and the photothermal effect, and demonstrated that thermal expansion-induced changes in ITT dominate the position dependence of the signal intensity. Our findings offer avenues for exploring strain effects on ultrafast properties and applications for ultrafast scanning probe microscopy.

Nematic electron and phonon dynamics in SnS crystals

Tin sulfide (SnS) is one of the promising materials for the applications of optoelectronics and photovoltaics. This study determines the nematic dynamics of photoexcited electrons and phonons in SnS single crystals using polarization-dependent pump–probe spectroscopy at various temperatures. As well as the fast (0.21–1.38 ps) and slow (>5 ps) relaxation processes, a 36–41 GHz coherent acoustic phonon with a sound velocity of 4883 m/s that is generated by the thermoelastic effect is also observed in the transient reflectivity change (ΔR/R) spectra. Electrons and coherent acoustic phonons show significant in-plane anisotropy from 330 to 430 K due to strong electron–phonon coupling. However, this in-plane anisotropy weakens dramatically in the low-temperature (<330 K) and high-temperature (>430 K) phases. These results add to the knowledge about the anisotropy of electrons and coherent acoustic phonons that give SnS applications in photovoltaic or optoelectronic devices.

Nonthermal melting of charge density wave order via nucleation in VTe2

Ultrafast optical control of phase change materials is of great importance from fundamental and practical points of view. Transition-metal dichalcogenides are of significant interest in this research field because of the photoinduced phase transition originating from the nonthermal melting of their charge density wave (CDW) orders. In this work, we investigated the ultrafast optical response of ${\mathrm{VTe}}_{2}$ by using broadband coherent optical phonon spectroscopy with subpicosecond time resolution. With an increase of laser-excitation fluence, a characteristic oscillation around 4.4 THz appeared in the transient reflectivity change. This suggests photoinduction of the high-temperature phase, i.e., a CDW-melt state, of ${\mathrm{VTe}}_{2}$ within a picosecond. Such an ultrafast transition would be induced by a purely electronic effect, but as with thermodynamic melting, it is strongly suggested that the phase transition dynamics induced spatially by laser irradiation proceeds inhomogeneously via a nucleation mode.

Photoelectron transportation dynamics in GaAs photocathodes

We report here a general theory describing photoelectron transportation dynamics in GaAs semiconductor photocathodes. Gradient doping is incorporated into the model through the inclusion of directional carrier drift. The time-evolution of electron concentration in the active layer upon the injection of an excitation pulse is solved both numerically and analytically. The predictions of the model are compared with experiments via a carrier-induced transient reflectivity change, which is measured for gradient-doped and uniform-doped photocathodes using femtosecond pump–probe reflectometry. An excellent agreement is found between the experiments and the theory, leading to the characterization of key device parameters, such as diffusion constant and electron decay rates. Comparisons are also made between uniform doping and gradient doping for their characteristics in photoelectron transportation. Doping gradient is found to be able to accelerate electron accumulation on the device surface. These results offer new insights into the dynamics of III–V photocathodes and potentially open a new avenue toward experimental characterization of device parameters.

Time-resolved broadband impulsive stimulated Brillouin scattering in single crystal hematite

Impulsive stimulated Brillouin scattering (ISBS) has emerged as a noninvasive means to determine the elastic properties of transparent materials. Here, we report on time-resolved broadband ISBS reflectivity measurements in single crystal hematite, α-Fe2O3. We found that the observed transient reflectivity changes are best described by the known strain propagation model (SPM) and introduced a simple derivation of the ISBS-SPM formula based on ray tracing, which accounts for the presence of the interface. Measurements at different incident probe beam angles illustrate a plausible approach toward determining the speed of sound in transparent media without any prior knowledge of their dielectric properties and vice versa.

Transient carrier dynamics of GaAs at room temperature

We use time-resolved optical reflectivity with femtosecond resolution to study the ultrafast dynamics of photoexcited carriers in bulk GaAs. We observe an emergence of short-lived excitons at room temperature right after photoexcitation, in terms of a sharp dip and sub-ps lifetime in the transient reflectivity changes, as long as the pump fluence increases above a critical value Fc. Our findings help understand ultrafast physics in this material.

Femtosecond time-evolution of mid-infrared spectral line shapes of Dirac fermions in topological insulators

Mid-infrared (MIR) light sources have much potential in the study of Dirac-fermions (DFs) in graphene and topological insulators (TIs) because they have a low photon energy. However, the topological surface state transitions (SSTs) in Dirac cones are veiled by the free carrier absorption (FCA) with same spectral line shape that is always seen in static MIR spectra. Therefore, it is difficult to distinguish the SST from the FCA, especially in TIs. Here, we disclose the abnormal MIR spectrum feature of transient reflectivity changes (ΔR/R) for the non-equilibrium states in TIs, and further distinguish FCA and spin-momentum locked SST using time-resolved and linearly polarized ultra-broadband MIR spectroscopy with no environmental perturbation. Although both effects produce similar features in the reflection spectra, they produce completely different variations in the ΔR/R to show their intrinsic ultrafast dynamics.

Ultrafast reflectivity change of vanadium dioxide induced by THz field enhanced by a metallic structure

We studied the transient reflectivity change of vanadium dioxide that is induced by terahertz (THz) pulse excitation, whose electric field is enhanced by a metallic dipole antenna. The tightly focused probe pulse in the near-infrared region allows one to monitor the reflectivity change between the antenna gap and also quantitative evaluation of the field dependence of reflectivity change. The observed steep rising of the dependence shows that the insulator-to-metal transition occurs with the threshold electric field strength at around 2 MV cm−1. Our result is explained by the model taking into account the carrier generation via the Poole–Frenkel effect induced by THz electric field.

Arrival timing diagnostics at a soft X-ray free-electron laser beamline of SACLA BL1.

An arrival timing monitor for the soft X-ray free-electron laser (XFEL) beamline of SACLA BL1 has been developed. A small portion of the soft XFEL pulse is branched using the wavefront-splitting method. The branched FEL pulse is one-dimensionally focused onto a GaAs wafer to induce a transient reflectivity change. The beam branching method enables the simultaneous operation of the arrival timing diagnostics and experiments. The temporal resolution evaluated from the imaging system is ∼22 fs in full width at half-maximum, which is sufficient considering the temporal durations of the soft XFEL and the optical laser pulses.

3D visualizations of nanoscale phase separation and ultrafast dynamic correlation between phases in (Na0.32K0.68)0.95Fe1.75Se2

Phase separation of metallic and antiferromagnetic (AFM) insulating phases in alkaline iron selenides $({\mathrm{A}}_{x}\mathrm{F}{\mathrm{e}}_{2\ensuremath{-}y}\mathrm{S}{\mathrm{e}}_{2})$ continues to attract intense interest because the relationship between two peculiar features probably is a key to clarifying the pairing mechanism of ${\mathrm{A}}_{x}\mathrm{F}{\mathrm{e}}_{2\ensuremath{-}y}\mathrm{S}{\mathrm{e}}_{2}$ superconductors. Here we report that the three-dimensional (3D) visualizations of nanoscale phase separation in ${(\mathrm{N}{\mathrm{a}}_{0.32}{\mathrm{K}}_{0.68})}_{0.95}\mathrm{F}{\mathrm{e}}_{1.75}\mathrm{S}{\mathrm{e}}_{2}$ single crystals are revealed by hybrid focused-ion-beam scanning electron microscopy and the superconducting paths are fully percolative in 3D. Moreover, the phase-related ultrafast dynamics in ${(\mathrm{N}{\mathrm{a}}_{0.32}{\mathrm{K}}_{0.68})}_{0.95}\mathrm{F}{\mathrm{e}}_{1.75}\mathrm{S}{\mathrm{e}}_{2}$ is studied by dual-color pump-probe spectroscopy. The anomalous changes in the electron and acoustic phonon components of transient reflectivity change $(\mathrm{\ensuremath{\Delta}}R/R)$ identify two characteristic temperatures ${T}^{*}\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ (the onset temperature of coupling between the nanometallic and AFM phases) and ${T}_{\mathrm{H}}\ensuremath{\sim}230\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ (the onset temperature of the metallic-interface phase). An energy-transfer channel between the nanometallic and AFM phases is inferred. This proposed channel provides insight into the pairing mechanism of alkaline iron selenide superconductors.

Femtosecond ultrafast dynamics study on the photoemission performance of reflection-mode GaAlAs photocathode

The photoemission characteristic of reflection-mode GaAlAs photocathode has been investigated, while a convenient and nondestructive method for evaluating the photocathode performances will be introduced. In this paper, two same reflection-mode photocathode structures with different research methods are adopted. One way is using multi-information measurement system to measure the photoemission characteristic curve of GaAlAs photocathode. Another method is the femtosecond transient reflection spectroscopy. The ultrafast dynamics property of the GaAlAs carrier is studied with the femtosecond transient reflectivity change, and the quantitative calculating is also shown. According to the results of two experimental methods, we analyze some photoemission performance parameters, such as carrier lifetime, diffusion length etc. The results show that the surface electron escape probability P and the drift length LDE are different, while are the same of the back recombination velocity Sv by the two method. In a word, compared with multi-information measurement system, the femtosecond ultrafast dynamics method is more suitable to research on the photoemission characteristic of reflection-mode GaAlAs photocathode which is no damage and accurate.

Ultrafast dynamics of hard tissue ablation using femtosecond-lasers.

Several studies on hard tissue laser ablation demonstrated that ultrafast lasers enable precise material removal without thermal side effects. Although the principle ablation mechanisms have been thoroughly investigated, there are still open questions regarding the influence of material properties on transient dynamics. In this investigation, we applied pump-probe microscopy to record ablation dynamics of biomaterials with different tensile strengths (dentin, chicken bone, gallstone and kidney stones) at delay times between 1 picosecond and 10 microseconds. Transient reflectivity changes, pressure and shock wave velocities and elastic constants were determined. The result revealed that absorption and excitation show the typical well-known transient behavior of dielectric materials. We observed for all samples a photomechanical laser ablation process, where ultrafast expansion of the excited volume generates pressure waves leading to fragmentation around the excited region. In addition, we identified tensile-strength-related differences in the size of ablated craters and ejected particles. The elastic constants derived were in agreement with literature values. In conclusion, pressure-wave-assisted material removal seems to be a general mechanism for hard tissue ablation with ultrafast lasers. This photomechanical process increases ablation efficiency and removes heated material, thus ultrafast laser ablation is of interest for clinical application where heating of the tissue must be avoided.

Photoexcitation Carrier Kinetics in WSe2 Nanolayers in the Vicinity of the Band Edge

The photoexcitation and relaxation of carriers is studied in WSe2 nanolayers on Si/SiO2 substrate by optical pump–probe technique. The excitation wavelength falls in the vicinity of the band edge. The thickness variation of WSe2 layers acts as tuning of band gap of the layers. In this way, a band edge spectroscopy is carried out using the constant excitation wavelength. It is shown that the relaxation time constants increase with the thickness increase, while the amplitude of transient reflectivity changes nonmonotonically. A model is suggested describing the amplitude behavior.

Nonequilibrium Quasiparticle Dynamics in Bi-Based Superconductors Measured by Modulation Photoexcitation Spectroscopy

We develop an orbital angular momentum (OAM) excitation pump-probe (Pp) spectroscopy and perform experiments on a Bi2Sr2CaCu2O8+δ (Bi2212) superconductor. The OAM modulation for the pump is realized by using a transverse mode conversion technique which allows to modulate the chirality of the optical OAM with a fixed polarization. The result reveals a difference of the OAM-dependent transient reflectivity changes below T c, indicating that the pump OAM can break the chiral symmetry of the superconducting (SC) state. The OAM dependences of the transient signal imply that the quasi-particle (QP) excitation with OAM enhances the SC with opposite chirality and/or suppress the SC with equal chirality. The temperature dependence of the OAM dynamics shows virtually no pairing fluctuation that usually appears in the QP dynamics observed by conventional Pp, suggesting that the nonequilibrium OAM dynamics is sensitive to the delocalized OAM associated with long range order.

Dynamics of phonons in Sr3Ir4Sn13: an experimental study by ultrafast spectroscopy measurements

We report a study of ultrafast dynamics of photoexcited electrons and phonons in Sr3Ir4Sn13 using dual-color transient reflectivity change () measurements. Time resolved optical spectroscopy of collective excitations reveal the marked features near its structural phase transition temperature K. Two distinctive oscillatory timescales in have been clearly resolved. The rapid THz-range oscillations are attributed to the dynamics of the optical phonons which strongly correlate to the structural phase transition. The slow GHz-range oscillatory phenomenon which only occurs below about 150 K is associated with the dynamic response of the longitudinal-acoustic phonons. These low-energy phonons show a softening feature on approaching the transition temperature, also indicating a strong relevance to the structural phase transition. The information that we demonstrated would provide a deeper understanding of the structural phase transition in Sr3Ir4Sn13.

Ultrafast Jahn–Teller polaron dynamics in a bi‐layered manganite single crystal

We report the photoinduced ultrafast quasiparticle recombination dynamics of colossal magnetoresistance single crystal La1.2Sr1.8Mn2O7 by using femtosecond time‐resolved reflection spectroscopy in a wide range of temperatures. Here, we find the transient reflectivity change ΔR exhibits a competition process of a fast negative component (∼ sub‐picosecond) against a slow positive component (∼ nanosecond), as the sample goes from paramagnetic (PM) to ferromagnetic (FM) state with decreasing the temperature. The very fast initial decay of the photoinduced ΔR corresponds to the photoionization of the Jahn–Teller polaronic response, which is unambiguously sensitive to the coupling between electron and lattice in the PM and FM phases in La1.2Sr1.8Mn2O7 single crystal. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)