Pair Of Laser Pulses Research Articles

Observation of Intrinsic Inverse Spin Hall Effect

We report observation of intrinsic inverse spin Hall effect in undoped GaAs multiple quantum wells with a sample temperature of 10 K. A transient ballistic pure spin current is injected by a pair of laser pulses through quantum interference. By time resolving the dynamics of the pure spin current, the momentum relaxation time is deduced, which sets the lower limit of the scattering time between electrons and holes. The transverse charge current generated by the pure spin current via the inverse spin Hall effect is simultaneously resolved. We find that the charge current is generated well before the first electron-hole scattering event. Generation of the transverse current in the scattering-free ballistic transport regime provides unambiguous evidence for the intrinsic inverse spin Hall effect.

Optical manipulation of coherent phonons in superconducting YBa2Cu3O7−δ thin films

The coherent phonons of YBa2Cu3O7-delta are believed to be strongly coupled to its superconductivity. Controlling the phonons below its transition temperature, therefore, may serve as a promising scheme of the control of superconductivity. Here we demonstrate optical manipulation of the Ba-O and Cu-O vibrations in a thin-film YBa2Cu3O7-delta below its transition temperature using a pair of femtosecond laser pulses. The interpulse delay is tuned to integral and half-integral multiples of the oscillation period of a specific phonon mode (Ba-O or Cu-O vibration) to enhance and suppress its amplitude, respectively.

Tuning the shape anisotropy of silver nanoparticles using time-delayed laser pulse pair irradiations

Glass-embedded spherical silver nanoparticles were irradiated by pairs of delayed femtosecond laser pulses. The influence of intensity, relative polarization (parallel or orthogonal) and time delay between the pulses was investigated. We found a delay-dependent reversal of the orientation of prolate nanoparticles produced in the low-intensity regime: at very short time delays up to 10 ps between pulse pairs the polarization direction of the second-hitting pulse defines the particles’ symmetry axes; in an intermediate regime between 10 and 20 ps no optical dichroism is found at all; at more than 20 ps delay between the pulses, finally, the transformed nanoparticles are oriented along the polarization direction of the first-hitting pulse. Also, in the quite different situation of the high-intensity regime using parallel-polarized pulse pairs, where normally oblate particles are created, isotropic spectral changes (i.e., no dichroism) after irradiation were observed at delay times around 20 ps. The possible physical background of this apparently very special inter-pulse delay of around 20 ps is discussed.

Transient phenomena in the dielectric breakdown of HfO2 optical films probed by ultrafast laser pulse pairs

The laser induced breakdown threshold of HfO2 films is studied with single pairs of pulses of variable delay and 50 fs and 1 ps pulse duration. Two distinct transient regimes are observed that can be related to the relaxation of the electron density from the conduction band via an intermediate state to the valence band. The experimental results are in good agreement with a theoretical model that assumes occupation of mid gap states after the first pulse on a time scale of several tens of picoseconds and subsequent decay of this population via recombination with holes in the valence band on a time scale of several tens of milliseconds.

S-matrix theory of high harmonic generation and ionization of coherently ro-vibrating linear molecules by intense ultrashort laser pulses

A theory of high harmonic generation and ionization of coherently rotating and vibrating linear molecules by a delayed pair of intense ultrashort laser pulses is presented. It correlates the nuclear motions in real time with the modulation of the harmonic emission and ionization signals as a function of the time-delay between the exciting and the probing pulses. An illustrative analytical example of the excitation and detection of the clock-motion associated with the “0–1” vibrational oscillation of a diatomic molecule is also given.

Plume composition control in double pulse ultrafast laser ablation of metals

Ultrafast laser ablation of a metallic target induced by a pair of identical laser pulses temporally delayed from ≈1 to 2000 ps was studied by optical emission spectroscopy, imaging, and ion probe. Our experimental results demonstrate that plume excitation/ionization enhancement or nanoparticles reduction is achieved by properly delaying the two pulses. This possibility of controlling plume composition via an efficient coupling of the energy of the second pulse to the various ablation components produced by the first pulse is of particular interest in the process of material deposition and film growth.

Time-resolved investigation of laser-induced shape transformation of silver nanoparticles

Laser-induced permanent shape transformation dynamics of glass-embedded silver nanoparticles is investigated by a pulse-pair irradiation technique, where the nanoparticles are irradiated by pairs of time-delayed femtosecond laser pulses. The temporal evolution of surface plasmon extinction bands is obtained up to a time delay of 1 ns. We find that the aspect ratio of the modified nanoparticles, i.e., the observed dichroism, reduces rapidly to a minimum level within 20 ps after the laser irradiation. From that interpulse delay onward, the resulting aspect ratio stays at that minimum level until 100 ps. Subsequently, for longer time delays, the aspect ratio increases again until 1 ns. Temperature modeling of the nanoparticle-glass system gives evidence that the intermediately reduced dichroism is due to the high diffusion mobility that emitted silver ions can possess within a shell of heated glass around the nanoparticle, in a time window of 100 ps. For longer time delays the size of the heated glass shell gets reduced, thereby limiting the range of ionic diffusion. The theoretical estimations confirm the experimental results and suggest that the nanoparticle shape changes depend strongly on the thermal situation induced to the surrounding glass.

Laser-driven clockwise molecular rotation for a transient spinning waveplate

Our simulations show a copropagating pair of laser pulses polarized in two different directions can selectively excite clockwise or counterclockwise molecular rotation in a gas of linear molecules. The resulting birefringence of the gas rotates on a femtosecond timescale and shows a periodic revival structure. The total duration of the pulse pair can be subpicosecond, allowing molecular alignment at the high densities and temperatures necessary to create a transient spinning waveplate.

Actively Tailored Spatiotemporal Images of Quantum Interference on the Picometer and Femtosecond Scales

Interference fringes of quantum waves weave highly regular space-time images, which could be seen in various wave systems such as wave packets in atoms and molecules, Bose-Einstein condensates, and fermions in a box potential. We have experimentally designed and visualized spatiotemporal images of dynamical quantum interferences of two counterpropagating nuclear wave packets in the iodine molecule; the wave packets are generated with a pair of femtosecond laser pulses whose relative phase is locked within the attosecond time scale. The design of the image has picometer and femtosecond resolutions, and changes drastically as we change the relative phase of the laser pulses, providing a direct spatiotemporal control of quantum interferences.

Polarization-resolved laser-induced breakdown spectroscopy

It is shown that plasma polarization measurements can be used to enhance the sensitivity of laser-induced breakdown spectroscopy (LIBS). The polarization of the plasma emission is used to suppress the continuum with only slight attenuation of the discrete atomic and ionic spectra. The method is demonstrated for LIBS detection of copper and carbon samples ablated by pairs of femtosecond laser pulses.

Raman coupling of Zeeman sublevels in an alkali-metal Bose-Einstein condensate

We investigate amplitude and phase control of the components of the spinor order parameter of a $^{87}\mathrm{Rb}$ Bose-Einstein condensate. By modeling the interaction of the multilevel atomic system with a pair of Raman-detuned laser pulses, we show that it is possible to construct a pulse-sequence protocol for producing a desired state change within a single Zeeman manifold. We present several successful elementary tests of this protocol in both the $F=1$ and $F=2$ Zeeman manifolds of $^{87}\mathrm{Rb}$ using the ${D}_{1}$ transitions. We describe specific features of the interaction, which are important for multimode, spatially varying field configurations, including the role of state-dependent, light-induced potentials.

Ultrafast laser ablation of metals with a pair of collinear laser pulses

We investigated the process of ultrafast laser ablation of metallic targets induced by a pair of identical laser pulses, with either p or s polarizations, temporally delayed from ≈1 ps to a few nanoseconds. We used fast ion probe diagnostics to characterize the ion plume at the moderate laser intensity (≈1012 W/cm2) typically employed in ultrafast laser deposition and material processing. We observed a consistent time-correlated enhancement of the ion yield and velocity, which lends itself to an interesting and useful method for manipulating ablation plasma characteristics. The mechanisms producing this feature are also discussed.

Efficient generation of relativistic electrons in a target irradiated by a pair of femtosecond laser pulses with a nanosecond delay

It is shown experimentally that the mean energy of accelerated electrons generated by irradiating a dense target with a pair of 13-ns-delay femtosecond laser pulses forming radiation with an energy contrast of 100–500 and an intensity of about 1018 W/cm2 increases significantly in comparison with the case of high contrast ∼106.

Observation of near total polarization in the ultrafast laser ablation of Si

We report nearly completely polarized emission from the plasma produced in the femtosecond ablation of Si(111). Pairs of ultrashort laser pulses were focused onto the target in air, and the polarization spectrum was measured as a function of energy, pulse delay, and polarization state of the laser. When the laser was focused on the surface, the fluorescence continuum was strongly polarized, whereas discrete lines appeared as minima in the polarization spectrum. Under this focusing condition, the continuum polarization increased with pulse delay and decreased with pulse energy and fluorescence wavelength, with >95% polarization in the ultraviolet.

In situ accurate determination of the zero time delay between two independent ultrashort laser pulses by observing the oscillation of an atomic excited wave packet

We propose a novel method that uses the oscillation of an atomic excited wave packet observed through a pump-probe technique to accurately determine the zero time delay between a pair of ultrashort laser pulses. This physically based approach provides an easy fix for the intractable problem of synchronizing two different femtosecond laser pulses in a practical experimental environment, especially where an in situ time zero measurement with high accuracy is required.

Monitoring the buildup of the quantum dot polaron: Pump-probe and four-wave mixing spectra from excitons and biexcitons in semiconductor quantum dots

Pump-probe (PP) and four-wave mixing (FWM) signals from a single quantum dot are presented which are based on a four-level quantum dot model, accounting for the fine structure splitting as well as for the biexciton binding and phonon-induced pure dephasing. We have derived closed form analytical expressions for the PP and FWM polarization components in the time domain after excitation with a pair of ultrafast laser pulses. The solutions are exact within this model. We discuss the dependence of the signals on the polarization of the exciting pulses. We show that for PP spectra after excitation with collinearly polarized pulses and for FWM spectra after cocircular excitation changes in the line shape are entirely due to the polaron formation leading to a fixed shape when this process is completed. For PP spectra after cocircular excitation and FWM spectra after collinear excitation in general periodic modulations of the shape are superimposed which persist even on long time scales and reflect quantum beats due to the fine structure splitting or the biexciton binding energy.

Read and write amplitude and phase information by using high-precision molecular wave-packet interferometry

We demonstrate an experimental approach to read and write populations and relative phases of vibrational eigenstates within a wave packet created in the $B$ state of the iodine molecule by using a pair of phase-locked femtosecond laser pulses. Our highly stabilized optical interferometer provides attosecond stability and resolution in the interpulse delay. The stability and resolution have realized an exquisite tuning of the interference of two vibrational wave packets to manipulate the relative populations and the relative quantum phases among the vibrational eigenstates within the wave packets. These populations and phases have been retrieved by measuring fluorescence from the upper $E$ state induced by another nanosecond (ns) or femtosecond (fs) probe laser pulse. The bandwidth of the ns probe pulse is narrow enough to select only a small portion of the rotational progression of a particular vibrational band of the $E\ensuremath{-}B$ transition. By scanning the probe wavelength, we measure the population distribution of the vibrational eigenstates within the wave packet. The fs probe pulse is used to measure quantum beats arising from the temporal evolution of the wave packet. Combining these two complementary measurements, we can read both population and phase information written and stored in the wave packet.

Inversion recovery of single quantum-dot exciton based qubit

We investigate the coherence decay of a single-exciton-based qubit with electrical readout. Pairs of laser pulses initialize the qubit state and then map the target basis onto the measurement basis, probing the decay of either the population inversion or electronic polarization components of the state vector. A comparison of coherence and population decay confirms that the exciton coherence is lifetime limited by fast voltage-tunable electron tunneling. Optimum coherence times are limited by a heavy-hole tunneling rate slow compared with the repetition rate of the laser. © 2007 The American Physical Society.

Theory for Quantum Interference Signal from an Inhomogeneously Broadened Two-Level System Excited by an Optically Phase-Controlled Laser-Pulse Pair.

A useful expression for the quantum interference (QI) signal was derived for an inhomogeneously broadened two-level system when it was excited by an optically phase-controlled laser-pulse pair. It was shown that the QI signal oscillates as a function of a relative optical phase, with the reduced angular frequency given by the relation ωa = (Γ(2)ω0 + γg(2)Ω)/(γg(2) + Γ(2)), where γg and Γ are standard deviations of the system's absorption and the laser spectra both having a Gaussian line shape, respectively, and ω0 and Ω are the center angular frequency of the system absorption and the carrier angular frequency of the laser, respectively.

Mechanism for the ablation of Si⟨111⟩ with pairs of ultrashort laser pulses

Pairs of ultrafast laser pulses are used to ablate Si⟨111⟩. The fluorescence from Si atoms and ions was observed to increase by an order of magnitude as the delay between the pulses was increased. From the dependence of the fluorescence enhancement on the laser fluence and the pulse delay, it is deduced that the first pulse melts the surface and that the second pulse interacts more strongly with the liquid phase.