Transient Optical Reflectivity Research Articles

Comparison of optical and electrical transient response during nanosecond laser pulse-induced phase transition of Ge 2Sb 2Te 5 thin films

The phase transition dynamics of Ge 2Sb 2Te 5 thin films induced by single nanosecond laser pulses were studied by transient optical reflectivity and electrical resistance measurements with nanosecond resolution. It was found that the real-time responses of optical and electrical signals were different under the same pumping condition. It needs much little time to reach equilibrium state for electrical resistance than optical reflectivity. The dependence of crystallization time determined respectively by optical and electrical evolution curve on laser pulse fluence was compared and analyzed. A 2-dimensional percolation model was used to explain the differences between electrical and optical transient responses.

Femtosecond laser generation and detection of high-frequency acoustic phonons in GaAs semiconductors

The experimental detection of short picosecond photoacoustic response induced by femtosecond laser pump radiation deeply penetrating in GaAs and generating long acoustic strain pulse is reported. It is demonstrated that it is possible, in this case, to achieve high-frequency coherent acoustic phonon monitoring in semiconductors as efficiently as in the case of metals where the penetration depth of pump radiation is shorter and the generated acoustic strain pulse is short itself. The physical origin of such monitoring of high-frequency acoustic phonons is discussed thanks to a detailed analysis of the spectral transformation functions of both the generation process (opto-acoustic transformation) and the detection process (acousto-optic transformation). We show that it is possible to tune the detection process from a narrowband to a broadband spectrum detection process. In particular, the broadband detection process is achieved with a proper choice of the probe wavelength permitting efficient coupling between the acoustic field and the probe electromagnetic wave only in a nanometric thin region near semiconductor surface. Efficient broadband acousto-optic detection results in the short pulsed photoacoustic responses in transient optical reflectivity even when long acoustic pulses are generated, because of sufficient sensitivity to high-frequency coherent phonons even weakly contributing to the total acoustic field. These results indicate the opportunity to broaden the range of the substrate materials and pump laser frequencies that can be used in high-frequency coherent phonon spectroscopy of the materials and nanostructures.

Direct Observation of Acoustic Oscillations in InAs Nanowires

Time-resolved X-ray diffraction and optical reflectivity are used to directly measure three different acoustic oscillations of InAs nanowires. The oscillations are excited by a femtosecond laser pulse and evolve at three different time scales. We measure the absolute scale of the initial radial expansion of the fundamental breathing eigenmode and determine the frequency by transient optical reflectivity. For the extensional eigenmode we measure the oscillations of the average radial and axial lattice constants and determine the amplitude of oscillations and the average extension. Finally we observe a bending motion of the nanowires. The frequencies of the eigenmodes are in good agreements with predictions made by continuum elasticity theory and we find no difference in the speed of sound between the wurtzite nanowires and cubic bulk crystals, but the measured strain is influenced by the interaction between different modes. The wurtzite crystal structure of the nanowires however has an anisotropic thermal expansion.

Jones matrix formalism for the theory of picosecond shear acoustic pulse detection

A theoretical analysis of the transient optical reflectivity of a sample by a normalized Jones matrix is presented. The off-diagonal components of the normalized matrix are identified with the complex rotation of the polarization ellipse. Transient optical polarimetry is a relevant technique to detect shear acoustic strain pulses propagating normally to the surface of an optically isotropic sample. Moreover, polarimetry has a selective sensitivity to shear waves, as this technique cannot detect longitudinal waves that propagate normally to the sample surface.

Towards X-ray induced transient grating methods for nanometer scale dynamics: Diffraction on transient structures induced by extreme ultraviolet radiation from FLASH

Using the high brilliance femtosecond soft X-ray pulses from the Free-Electron LASer at Hamburg (FLASH) the X-ray induced transient optical reflectivity change of GaAs has been established as a versatile method for femtosecond X-ray/optical cross-correlation [1]. As the underlying physical mechanism is the X-ray induced dynamics within solids, we present in this work a feasibility study how transient grating methods could be used to study nanometer scale dynamics in materials, such as the radical diffusion parameters in photoresist materials for EUV lithography.

Mechanical contacts probed with picosecond ultrasonics

We investigate the contact between a thin metal film and a ball bearing indented to nanometre depths using picosecond ultrasonics. The area of contact is acoustically imaged to micron spatial resolution using GHz acoustic pulses produced by ultrashort pulsed optical excitation of the film through a transparent substrate. In particular, acoustic echoes are detected through transient optical reflectance changes that are monitored by probe optical pulses. In this way we image the acoustic reflection coefficient at the interface between the film and the indenter. In addition, by imaging the arrival time of the acoustic echoes we determine the penetration profile of the indenter to nanometre precision. Furthermore, imaging the transient thermoreflectance gives a different means for measuring the contact area through spatial variations in thermal diffusion. We thus demonstrate that picosecond ultrasonics and thermoreflectance provide powerful tools for the non-contact evaluation of mechanical contacts. These techniques could be applied to the in situ characterization of contact interfaces between machine elements.

Optical detection of longitudinal and shear acoustic waves with laser picosecond acoustics

The absorption of picosecond light pulses in a medium can generate sub-THz acoustic waves. These cause a transient optical reflectance change that can be monitored by delayed probe light pulses. This technique, termed laser picosecond acoustics, can be used for the nondestructive evaluation of the physical properties of thin films and substrates. This paper describes a general method for quantitative analysis of such reflectance changes. It is applicable to multiple anisotropic layers that may be opaque or transparent. Longitudinal or shear acoustic waves propagating along the stacking direction of the multilayers modulate the dielectric permittivity anisotropically and inhomogeneously through the photoelastic effect, through local rotation, or through the surface and interface displacements. We describe how the optical reflectance for obliquely incident probe light can be calculated for the modulated medium. We then demonstrate the method with reference to experimental results for a sample consisting of a silica film on a zinc substrate in which both longitudinal and shear acoustic waves are generated and detected. The analysis yields the film thickness, sound velocity, and photoelastic tensor components, for example. The method is also applicable to various light scattering problems involving the inhomogeneous modulation of optical properties such as in photothermal experiments.

Electron dynamics in metallic and spin-glass cuprates

Using femtosecond pump-probe measurements, we present transient optical reflectivity measurements of metallic, superconducting and spin-glass ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ cuprates. The rise time is temperature-independent and resolution-limited $(<50\phantom{\rule{0.3em}{0ex}}\mathrm{fs})$ for metallic sample, and is much faster than spin-glass samples $(\ensuremath{\sim}200\phantom{\rule{0.3em}{0ex}}\mathrm{fs})$ at all temperatures. A divergence in the electron-lattice relaxation $({\ensuremath{\tau}}_{\mathrm{R}})$ time with temperature is observed for the metallic samples, but not for spin-glass samples.

Carrier relaxation time divergence in single and double layer cuprates

We report the transient optical pump-probe reflectivity measurements on single and double layer cuprate single crystals and thin films of ten different stoichiometries. We find that with sufficiently low fluence the relaxation time (τR) of all samples exhibits a power law divergence with temperature (T ): τR ∝ T �3±0.5 . Further, the divergence has an onset temperature above the superconducting transition temperature for all superconducting samples. Possible causes of this divergence are discussed.

Laser-induced lateral epitaxy in fully depleted silicon-on-insulator junctions

Junction formation by laser-induced lateral epitaxy was studied on a fully depleted silicon-on-insulator substrate (25–30 nm Si on 375 nm silicon dioxide). Selective laser melting of amorphous films with a 35 ns 308 nm XeCl laser pulse was characterized in situ using transient conduction and optical reflectance techniques. Lateral epitaxy from a channel edge was observed for 146 nm after a 300 mJ/cm2 irradiation. The initial 28 nm of epitaxy was nearly defect free, followed by an increasing density of twins and ultimately terminating in an amorphous quench. The microstructure is discussed as a lateral equivalent of laser-induced amorphization of bulk Si.

Nanosecond dynamics of a gallium mirror’s light-induced reflectivity change

Transient pump-probe optical reflectivity measurements of the nano/microsecond dynamics of a fully reversible, light-induced, surface-assisted metallization of gallium interfaced with silica are reported. The metallization leads to a considerable increase in the interface's reflectivity when solid a-gallium is on the verge of melting. The reflectivity change was found to be a cumulative effect that grows with light intensity and pulse duration. The reflectivity relaxes back to that of alpha-gallium when the excitation is withdrawn in a time that increases critically at gallium's melting point. The effect is attributed to a non-thermal light-induced structural phase transition.

Solute trapping in aluminum alloys

We report the first measurements of solute trapping in metallic alloys. The nonequilibrium solute partition coefficient k was measured in several dilute aluminum alloys (AlCu, AlGe, AlIn, AlSn). Pulsed laser melting was used to bring about rapid plane-front solidification at velocities of 0.6-5.1 m/s. Time-resolved melt depths and solidification velocities were determined using the transient electrical conductance and optical reflectance techniques. The solute trapping model with the fewest free parameters that accounts for the measured velocity dependence of k is the Continuous Growth Model of Aziz and Kaplan. We have also identified an inverse correlation between the diffusive speed (the only free parameter in that model) and the equilibrium partition coefficient; this correlation might be used to estimate diffusive speeds for systems in which it has not been measured. A possible origin of such a correlation is discussed.

Optical and acoustic study of nucleation and growth of bubbles at a liquid-solid interface induced by nanosecond-pulsed-laser heating

The dynamics of liquid-vapor phase-change in the nanosecond time-scale induced by pulsed-laser heating of a liquid on a solid sample is studied by means of optical reflectance and scattering measurements, and the piezoelectric detection technique. The liquids studied include water, ethanol, methanol, IsoproPropyl Alcohol (IPA), and mixtures of water and IPA. The threshold fluence for nucleation is determined with high accuracy using the optical and acoustic signals. Heat diffusion calculations performed for the threshold fluences indicate that the liquids are sufficiently superheated before nucleation sets on. The transient optical reflectance signal is analyzed by an effective-medium theory to provide bubble-growth kinetics, so that the bubble-growth velocity for the test liquids could be estimated. In addition, it is observed that, following the thermally induced nucleation, repetitive acoustic cavitation at the surface of the solid sample occurs, with a time interval related to the speed of sound in the liquid.

Transient optical reflectivity from bounded nonlocal media: Normal incidence.

The reflection of a finite-duration optical pulse from a semi-infinite nonlocal medium for various additional boundary conditions (ABC's) is investigated theoretically. We have obtained explicit expressions for the amplitude and phase of the transient reflected field (local and nonlocal) and evaluated them numerically for different ABC's. We predict a damped oscillatory decay in transient reflectivity (after the pulse is cut off) due to relaxation processes which cause dephasing of emitted light on a scale longer than the cutoff time of the pulse. The effects of spatial dispersion for the reflected transients associated with the light pulse are important for the laser frequency at the vicinity of an exciton-polariton resonance. For various ABC's, for the cases of CdS and CuCl semiconductors, we find quantitative differences in the magnitude of the amplitude of the transient. This can be used to analyze different ABC's experimentally.

Novel effects in transient optical reflectivity from bounded nonlocal media via exciton polaritons

We report results of a theoretical investigation of time-dependent reflectivity from a semi-infinite, spatially dispersive medium subject to an incident pulse of finite duration. A damped oscillatory decay in the tail of transient reflectivity after the pulse cut-off is predicted, sensitive to the material and to the specific additional boundary conditions (ABC's). Physically the origin is in relaxation processes which dephase the emitted light on a time scale larger than the cut-off time of the pulse and shorter than the spontaneous dipole relaxation time. For laser frequency at the exciton resonance the transients are enhanced; near to resonance a double peak occurs due to dispersion of polariton group velocity.

Quench rate enhancement in pulsed laser melting of Si by processing under water

The thermal quench rate during pulsed laser heating of Si can be enhanced by immersing the sample in a liquid (e.g., water) during irradiation. The liquid in contact with the irradiated surface acts as an additional heat sink increasing the thermal quench rate. The heat transfer processes and phase transformations were studied in real time using transient optical reflectance and electrical conductance techniques. Measurements of the melting and solidification dynamics of the Si reveal that the quench rate may be enhanced by 30% for deep melts. The measurements also indicate that a steam or superheated water phase is formed near the Si surface during the laser pulse. The observed phenomena are analyzed in terms of standard heat flow.

Transient Processes at the SI-Water Interface During Pulsed Laser Irradiation

ABSTRACTHeat transfer processes and phase transitions at the Si-water interface during nanosecond pulsed laser irradiation were studied in real time using transient optical reflectance measurements. Measurements at two probe laser polarizations suggest that during irradiation a steam phase is formed near the Si surface. Relaxation of this phase occurs several hundred nanoseconds after solidification of the Si is completed. Acousto-optical measurements indicate that a shock wave, propagating with the sound velocity in water, is initiated near the Si-water interface.

Measurements of the melt dynamics in laser annealed semiconductors

Combined measurements of the transient electrical conductance, optical reflectance and absorbed energy have been used to determine the dynamics of the annealing process during laser irradiation of bulk silicon and silicon on sapphire. The combined transient conductance and optical reflectance yield the regrowth velocity and melt duration in bulk silicon; in silicon on sapphire the measurements also yield the melt velocity and maximum melt depth. Measurement of the absorbed energy provides energy coupling data used in numerical calculations of the melt dynamics for direct comparison with experiment in bulk silicon and the result indicates annealing proceeds by a purely thermal process.

Transient optical reflectivity from bounded nonlocal media

This paper considers the problem of reflection of a finite-duration optical pulse incident normally on a semi-infinite nonlocal medium. The light frequency is assumed to lie in the vicinity of an exciton-polariton resonance. The reflected pulse is found to have transients associated with its leading and trailing edges. It is shown that spatial dispersion enhances reflected transients when the laser frequency is at resonance with the exciton polariton. For the case of CdS and GaAs semiconductors, the transient intensities are about 10% of the incident intensity at a time 0.1 psec after the trailing edge of the reflected pulse and remain about 1% even after several picoseconds. We have obtained explicit expressions for the transient part of the reflected field and evaluated them numerically under certain simplifying assumptions; analytical results are presented in some limiting cases of interest. The theory predicts a crossover from exponential to inverse power-law decay rate of transient reflectivity; this occurs at a characteristic time of the order of 1 psec for CdS and GaAs crystals.

A transient optical reflectivity study of laser annealing of ion-implanted silicon: Thresholds and kinetics

By probing transient optical reflectivity, we have investigated the liquid phase epitaxial regrowth kinetics of ion-implanted silicon during laser annealing. For annealing of ion-implanted silicon with an 80-ns (FWHM) pulse of various intensities at 1.06 μm, the threshold energy density required for melting is observed to be a constant. Above this threshold value, annealing occurs via an amorphous-to-liquid-to-crystalline transition. Redistribution of As profiles as measured by He+ backscattering spectra is observed only when the incident energy density exceeds the threshold. Below this value, a reduction of dechanneled fraction is observed, but no dopant redistribution occurs. From the observed melting onset, a maximum heating rate of about 2×1010 deg sec−1 is observed.