Pump-probe Reflectivity Research Articles

Liquid Carbon Reflectivity at 19 nm

We hereby report on a pump-probe reflectivity experiment conducted on amorphous carbon, using a 780 nm laser as a pump and a 19 nm FEL emission as probe. Measurements were performed at 50 degrees with respect to the surface normal to have an un-pumped reflectivity higher than 0.5%. A sub-10 fs time synchronization error could be obtained exploiting the nearly jitter-free capabilities of FERMI. EUV FEL-based experiments open the way to study the behaviour of a liquid carbon phase being unaffected by plasma screening.

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.

Resonant state due to Bi in the dilute bismide alloyGaAs1−xBix

It has been theoretically predicted that isolated Bi forms a resonant state in the valence band of the dilute bismide alloy, ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}$. We present ultrafast pump-probe reflectivity measurements of this interesting alloy system, which provide experimental evidence for the resonant state. The reflectivity transients for pump/probe wavelengths \ensuremath{\lambda} \ensuremath{\sim} 860--900 nm have negative amplitude, which we attribute to the absorption of the probe pulse by the pump induced carriers that are localized at the Bi-resonant state. Our measurements show that the lifetime of carriers localized at the resonant state is \ensuremath{\sim}200 ps at 10 K.

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.

Ultrafast transient reflectance of epitaxial semiconducting perovskite thin films

Ultrafast pump-probe transient reflectance (TR) spectroscopy was used to study carrier dynamics in an epitaxial perovskite oxide thin film of LaFeO3 (LFO) with a thickness of 40 unit cells (16 nm) grown by molecular beam epitaxy on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT). TR spectroscopy shows two negative transients in reflectance with local maxima at ∼2.5 eV and ∼3.5 eV which correspond to two optical transitions in LFO as determined by ellipsometry. The kinetics at these transients were best fit with an exponential decay model with fast (5–40 ps), medium (∼200 ps), and slow (∼ 3 ns) components that we attribute mainly to recombination of photoexcited carriers. Moreover, these reflectance transients did not completely decay within the observable time window, indicating that ∼10% of photoexcited carriers exist for at least 3 ns. This work illustrates that TR spectroscopy can be performed on thin (<20 nm) epitaxial oxide films to provide a quantitative understanding of recombination lifetimes, which are important parameters for the potential utilization of perovskite films in photovoltaic and photocatalytic applications.

Time-integrated photoluminescence and pump-probe reflection spectroscopy of Si doped InN thin films

Temperature and excitation power dependent time-integrated photoluminescence of Si doped InN thin films are investigated. Photoluminescence (PL) spectra at low temperatures are described by single emission peak ensued due to “free-to-bound” recombination; whereas PL spectra at higher temperatures above 150 K are characterized by both “band-to-band” and “free-to-bound” transition. Carrier dynamics of Si doped InN thin films is studied using pump-probe reflection spectroscopy at room temperature. The hot electron cooling process is well described by electron-electron scattering. The dependence of the hot electron cooling rate on total electron density shows sublinear to linear behavior with increase of background electron density. The variation of the carrier recombination lifetime with total electron density implicates the dominance of the defect-related nonradiative recombination channel over other recombination processes.

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.

GaAsの深さ分解フォノン・プラズモンダイナミクス

Pump-probe reflectivity measurements on n-type GaAs with different probe light wavelengths reveal the phonon-plasmon dynamics in the bulk and at the surface depletion layer. Photoexcited hole plasma screen the coherent LO phonons within a few hundred femtoseconds at the surface, whereas impurity-doped electron plasma do so in the n-doped bulk. The coherent responses demonstrate strong interaction between the lattice and charge carriers under inhomogeneous creation and sub-picosecond transport.

Photoinduced dynamics of spinel MnV2O4

By pump-probe optical reflectivity measurements, we studied the photoinduced dynamics of MnV${}_{2}$O${}_{4}$, which is a ${d}^{2}$ Mott insulator and exhibits a simultaneous spin and orbital ordering accompanied by a structural phase transition at 57 K. We compared the present experimental results with the results of the steady-state optical measurement of the same compound and the steady-state and pump-probe optical measurements of Al-doped MnV${}_{2}$O${}_{4}$, in which the orbital ordering and structural phase transition are absent but the magnetic ordering exists. We found that the photoinduced dynamics of MnV${}_{2}$O${}_{4}$ can be separated into three components: (1) the suppression of the Mott-excitation peak, (2) the melting of the orbital ordering, and (3) the melting of the magnetic ordering. Furthermore, we found that the lattice distortion associated with the photoinduced structural phase transition from tetragonal to cubic induces a strain wave whose propagation velocity is discernibly smaller than that of a conventional shockwave.

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 carrier dynamics in single-crystal In2Se3 thin layers

Carrier dynamics in single-crystal In2Se3 thin layers with various thicknesses was studied by femtosecond optical pump-probe reflectivity and ultrafast photocurrent measurements. The results suggest that, in thinner (thicker) layers, the carrier recombination dynamics is dominated by three-carrier (bimolecular) Auger process. The Auger time constant was found to decrease with deceasing layer thickness. Surface states were suggested to be the origin of the transition between different Auger processes as the layer thickness varies.

Evidence of enhanced electron-phonon coupling in ultrathin epitaxial copper films

Electron phonon (el-ph) coupling is a fundamental quantity that controls the electron transport through a conductor. We experimentally determined the el-ph coupling strength of epitaxial copper (Cu) films ranging from 5 to 1000 nm thick using both ultra-fast, optical pump-probe reflectivity and temperature-dependent resistivity measurements. An enhancement of the el-ph coupling strength was observed when the thickness of the films was reduced to below 50 nm. We suggest that this unexpected enhancement of the el-ph coupling strength is partially responsible for the observed increase of resistivity in the films below 50 nm thick.

Speed limit of the insulator–metal transition in magnetite

As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator-metal, or Verwey, transition has long remained inaccessible. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase. Here we investigate the Verwey transition with pump-probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator-metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics.

Ultrafast Real Space Dynamics of Photoexcited State in a Layered Perovskite-Type Spin Crossover Oxide La1.5Sr0.5CoO4

Ultrafast optical responses were investigated using a fs laser system on a layered perovskite-type cobalt oxide, La1.5Sr0.5CoO4 with a checkerboard-type charge ordering. To investigate the spatial variation of the photoexcited state, we measured the transient changes of three independent optical quantities, i.e., transmittance, reflectivity, and backside reflectivity, as well as their fluence dependence at 0.25 eV. Using the numerical calculations based on Maxwell's equations, we calculated not only the dielectric constant but also the decay length of the photoexcited region (\(L\)) and analyzed the real-space dynamics of the photoexcited area in terms of the fluence and time dependence of \(L\). With the above information on the time dependence of \(L\) and Kramers–Kronig analysis, we numerically calculated the transient optical conductivity spectra, which showed instantaneous formation of a polaron-like absorption peak in the mid-infrared region. A 10 fs pump–probe reflection measurement revealed that t...

Ultrafast electron spin dynamics in bulk CdTe investigated by femtosecond pump-probe reflection spectroscopy

The transient spin polarization dynamics in bulk cadmium telluride (CdTe) at 70 K is investigated by the circularly polarized pump-probe reflection technique. A general expression is derived from the rate equations of a two-level system with small signal approximation to describe the light-helicity-dependent reflection spectrum. The initial degree of electron spin polarization in the excited state and the electron spin relaxation time in bulk CdTe at low temperature with different carrier density are analyzed according to this model. Our experimental results reveal that the Dʼyakonov–Perelʼ mechanism based on a fully microscopic kinetic spin Bloch equation (microscopic KSBE) approach dominates in the electron spin relaxation process in bulk CdTe crystal. • The transient spin polarization dynamics in bulk CdTe at 70 K is investigated. • A general expression is derived from the rate equations of a two-level system. • P s 0 and τ s in bulk CdTe are analyzed according to this model. • The DP mechanism based on KSBE dominates the electron spin relaxation process.

Ultrafast sub-threshold photo-induced response in crystalline and amorphous GeSbTe thin films

Pump-probe optical reflectivity and terahertz transmission measurements have been used to investigate time resolved sub-threshold photo-induced effects in crystalline and amorphous GeSbTe films at MHz repetition rates. The reflectivity in both phases exhibits long-lived modulations consistent with the sign of the changes that occur upon switching but of smaller magnitude. These can be understood by the generation of acoustic strains with the crystalline phase response dominated by thermal effects and the amorphous phase response associated with electronically induced changes. Evidence for a photo-induced distortion is observed in the amorphous phase which develops homogeneously within the excited region on few-picosecond time scales.

The effect of n- and p-type doping on coherent phonons in GaN

The effect of doping on the carrier–phonon interaction in wurtzite GaN is investigated by pump–probe reflectivity measurements using 3.1 eV light in near resonance with the fundamental band gap of 3.39 eV. Coherent modulations of the reflectivity due to the E2 and A1(LO) modes, as well as the 2A1(LO) overtone are observed. Doping of acceptor and donor atoms enhances the dephasing of the polar A1(LO) phonon via coupling with plasmons, with the effect of donors being stronger. Doping also enhances the relative amplitude of the coherent A1(LO) phonon with respect to that of the high-frequency E2 phonon, though it does not affect the relative intensity in Raman spectroscopic measurements. We attribute this enhanced coherent amplitude to the transient depletion field screening (TDFS) excitation mechanism, which, in addition to impulsive stimulated Raman scattering (ISRS), contributes to the generation of coherent polar phonons even for sub-band gap excitation. Because the TDFS mechanism requires photoexcitation of carriers, we argue that the interband transition is made possible at a surface with photon energies below the bulk band gap through the Franz–Keldysh effect.

Effect of resonant tunneling on exciton dynamics in coupled dot-well nanostructures

Excitonic dynamics in a hybrid dot-well system composed of InAs quantum dots (QDs) and an InGaAs quantum well (QW) is studied by means of femtosecond pump-probe reflection and continuous wave (cw) photoluminescence (PL) spectroscopy. The system is engineered to bring the QW ground exciton state into resonance with the third QD excited state. The resonant tunneling rate is varied by changing the effective barrier thickness between the QD and QW layers. This strongly affects the exciton dynamics in these hybrid structures as compared to isolated QW or QD systems. Optically measured decay times of the coupled system demonstrate dramatically different response to temperature change depending on the strength of the resonant tunneling or coupling strength. This reflects a competition between purely quantum mechanical and thermodynamical processes.

Femtosecond optical pump–probe reflectivity studies of spin-state photo-switching in the spin-crossover molecular crystals [Fe(PM-AzA)2(NCS)2

We report here on the ultrafast photo-switching dynamics of a Fe(II) molecular material [Fe(PM-AzA)2(NCS)2]. It undergoes a thermal spin-crossover which can be detected by magnetic measurements or by optical reflectivity. We use here femtosecond optical reflectivity to study the ultrafast photo-switching dynamics. Our results indicate that the HS state is reached from the LS state within less than 100fs, through an intermediate MLCT state. This ultrafast relaxation from the electronic excited state towards the structurally relaxed HS state is followed by a vibrational cooling of the hot HS molecules within ≈1ps timescale.

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.