Strong Picosecond Research Articles

Evolution of picosecond surface electric fields generated by photon-induced charge emission from La0.67 Sr0.33MnO3 films

In recent years, transient electric field is becoming a means for the generation of hidden states or phase transitions. In this study, the spatial-temporal distribution of picosecond surface electric fields, associated with the femtosecond-laser-induced charge emission from ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_{3}$ (LSMO) single-crystal films, were interrogated by subpicosecond electron pulses and reconstructed by a three-layer theoretical model. A strong picosecond surface electric field, on the order of 100 kV/m, was generated on LSMO films. Such self-induced transient electric fields may be a promising means for controlling ultrafast processes in materials.

Superlinear Photoluminescence by Ultrafast Laser Pulses in Dielectric Matrices with Metal Nanoclusters

An intense photoluminescence emission was observed from noble metal nanoclusters (Pt, Ag or Au) embedded in sapphire plates, nucleated by MeV ion-implantation and assisted by an annealing process. In particular, the spectral photoluminescence characteristics, such as range and peak emission, were compared to the behavior observed from Pt nanoclusters embedded in a silica matrix and excited by UV irradiation. Correlation between emission energy, nanoclusters size and metal composition were analyzed by using the scaling energy relation EFermi/N1/3 from the spherical Jellium model. The metal nanocluster luminescent spectra were numerically simulated and correctly fitted using the bulk Fermi energy for each metal and a Gaussian nanoclusters size distribution for the samples. Our results suggest protoplasmonics photoluminescence from metal nanoclusters free of surface state or strain effects at the nanoclusters-matrix interface that can influence over their optical properties. These metal nanoclusters present very promising optical features such as bright visible photoluminescence and photostability under strong picosecond laser excitations. Besides superlinear photoluminescence from metal nanoclusters were also observed under UV high power excitation showing a quadratic dependence on the pump power fluence.

Study of plasma pressure evolution driven by strong picosecond laser pulse

Through one dimensional relativistic particle-in-cell simulation of strong laser interaction with the solid-density plasma, the evolution of the plasma impact pressure behind a thin foil has been investigated in details. An energy-compression mechanism has been proposed to help optimizing the laser and plasma parameters. It has been found that by using a picosecond laser with intensity 1015 W cm–2, an impact pressure as high as several hundreds of GPa order of magnitude can be obtained. The numerical analysis demonstrates that the peak pressure is mainly resulted from the ion contribution. These results are of potential application to the laser loading upon solids in order to study the material properties under extra-high dynamic pressure.

Long plasma channels and high-voltage discharges induced by strong picosecond laser pulses

The formation of long plasma channels and laser-induced high-voltage discharges are demonstrated by focusing infrared picosecond laser pulses in air. Based on measurements of the channel conductivity, the maximum electron density in excess of 1014 cm−3 is estimated. The plasma channels are good conductors, through which long-air-gap high-voltage discharges are triggered. The breakdown voltages show large drops but the discharging paths are not well guided: in this, the plasma spots distributed along the channel might play an important role.

Ta and Au(Pd) alloy metal film transducers for time-domain thermoreflectance at high pressures

We studied the pressure dependence of the thermoreflectance and piezo-optical coefficients of metal film transducers—Al, Ta, and Au(Pd) alloy (≈5 at. % Pd)—at a laser wavelength, 785 nm, commonly used in time-domain thermoreflectance (TDTR) and picosecond acoustics experiments. Al has exceptionally high thermoreflectance at ambient pressure, dR/dT ≈ 1.3 × 10−4 K−1, but its applicability at high temperatures is limited by the low melting temperature. The thermoreflectance of Al also has an undesirable zero-crossing near 6 GPa. The thermoreflectance values of Ta and Au(Pd) are comparable to that of Al at ambient conditions but independent of pressure in the pressure range 0 < P < 10 GPa. Ta and Au(Pd) thin film transducers also show strong picosecond acoustic echoes at all pressures in this range. We conclude that Ta and Au(Pd) metal film transducers can replace Al in TDTR experiments and thereby facilitate the extension of TDTR methods to high pressures and temperatures.

Analysis of adiabatic passage by light-induced potentials with chirped laser pulses in three- and four-level diatomic systems

This paper describes an investigation into the process of adiabatic passage by light-induced potentials (APLIP), which was previously suggested as a method for employing two strong picosecond laser pulses to transfer the population between two electronic states. We have extended earlier numerical studies in order to assess the feasibility of an experimental implementation of the APLIP concept. APLIP has been modeled in a three-level model system based on Na2 with chirped pulses, using laser parameters available from a typical chirped pulse amplified Ti:sapphire laser. The model showed that the APLIP process remains essentially unchanged for chirped pulses of equal magnitude and the opposite, or equal and positive sign of chirp as compared to the transform-limited case. We also examined the case of additional electronic states by introduction of a fourth state that lies close to the "target," i.e., final, state. The investigation showed that there are circumstances in which a significant fraction of the population gets transferred to this state which will disrupt the APLIP process. However, by switching to this fourth state as the target state in an experiment, good transfer efficiency is recovered. The results of the extension of the original APLIP modeling to chirped pulses and additional electronic states indicate that an APLIP experimental realization should be feasible in Na2.

Ionization/dissociation processes in some alkyl iodides induced by strong picosecond laser beam

The interaction of 1-, 2-iodopropane, 1-, 2-iodobutane and 1-iodopentane with strong (2 × 10 15 W cm −2) picosecond laser fields at 1064, 532, 355 and 266 nm is studied by means of time-of-flight (TOF) mass spectrometry. The experimental findings are compared with those reported, for the same molecules, from fs experiments at similar laser intensities. The pertaining molecular ionization mechanism (multiphoton and/or field ionization) is found to depend on the laser wavelength, while the recorded multiply charged atomic ions are generated via field ionization processes in all cases. The identification of these ionization mechanisms has been based on the dependence of the signal intensity and the peak profiles on laser polarization. The recorded mass spectra are analyzed vis-à-vis those reported for methyl iodide. The observed similarities and differences are discussed in detail.

Dynamic alignment of CH 3I by strong picosecond laser pulses

The angular distributions of the fragment ions of methyl iodide (CH 3I) irradiated by strong 35 ps laser pulses have been recorded at different wavelengths using a time-of-flight mass spectrometer. From the analysis of the angular distributions it is concluded that the singly and the transient doubly and triply charged parent ions are aligned by the laser beam (dynamic alignment). Under the present experimental conditions the CH 3I is adiabatically aligned. Molecular alignment has been observed for all the laser wavelengths (1064, 532, 355 and 266 nm) used and it is found to be more efficient at the longer one, while reaches saturation for laser intensities higher than 10 14 W/cm 2.

Strong picosecond ultrasonic responses of semiconductors probed close to interband transitions

We present experimental and numerical results of picosecond ultrasonic experiments performed in different semiconductors at different laser wavelengths. We first come back on the strong oscillations which appear in the reflectivity curves of silicon as the probe pulse is tuned in the blue range. We show that they come from an effect of the electronic transitions on the piezo-optic couplings. We then present similar results obtained in germanium which displays three interband transitions which fall in our laser source tuning range: 3.11, 2.11 and 0.8 eV (399, 587 and 1540 nm). Here again we have noticed strong responses in the reflectivity curves. We then make a comparison between silicon and germanium in respect with the laser wavelength effects. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low‐density polyethylene films doped with europium(III) complex: their properties and applications

AbstractLight‐converting polyethylene film containing europium(III) complex with phenanthroline was manufactured under an extrusion process. The film was characterized first by means of time‐of‐flight secondary‐ion‐mass spectrometry (TOF‐SIMS), ultraviolet‐visible‐infrared (UV‐vis‐IR) spectroscopy, then a study of excitation and luminescence spectra, quantum yield and lifetime of luminescence. The quantum yield of luminescence was measured as 5±2%, and the lifetime of luminescence was determined about 470 μsec. TOF‐SIMS surface mapping showed the uniform distribution of europium(III) over the film. Visualization of strong picosecond UV laser beams through the use of the developed film has been demonstrated. Copyright © 2004 John Wiley & Sons, Ltd.

Electronic and vibrational population transfer in diatomic molecules as a function of chirp for different pulse bandwidths

We study the dynamics of two-photon nonresonant electronic excitation of diatomic molecules driven by chirped pulses. While the majority of the experimental results address the role of the chirp for fixed pulse bandwidth, we analyze the possibility of selective excitation for fixed time, as a function of the pulse bandwidth, depending on the sign of the chirp. With strong picosecond pulses and positive chirp it is shown that the dynamics always prepare the molecule in the ground vibrational level of the excited electronic state. The robustness of the dynamics inherits the properties of an effective Landau–Zener crossing. For negative chirp the final state is very sensitive to the specific pulse bandwidth. The dynamics of the system follow a complex convoluted behavior, and the final state alternates between low vibrational levels of the excited electronic state and excited vibrational levels of the ground potential, which become increasingly more excited with increasing bandwidth. The final electronic populations follow a double-period oscillatory behavior. We present a model based on sequential independent crossings which correlates the long-oscillation period with changes in the final vibrational state selected. We show that the short-oscillation period is related with nonadiabatic effects that give rise to fast dynamic Rabi flipping between the electronic states, providing only information of the field–molecule effective coupling. Although the short-oscillation period partially masks the expected results of the final populations, we show that it is still possible to retrieve information from the long-oscillation period regarding the frequencies of the electronic potentials. In order to do so, or in order to control the outcome of the dynamics, it is necessary to perform experiments scanning very different pulse bandwidths, and we propose a possible experimental implementation. All the numerical results of the paper are calculated for a model of the Na2 dimer.

Multielectron dissociative ionization of CH 3I under strong picosecond laser irradiation

The multielectron dissociative ionization (MEDI) processes of methyl iodide (CH 3I) induced by strong ( I=10 15 W cm −2) picosecond laser light at 532 and 1064 nm are studied by means of a time-of-flight mass spectrometer. The experimental results are compared to those reported from femtosecond experiments at similar laser intensities. The influence of the laser pulse duration is clearly reflected in the kinetic energy values of the ejected fragments, which were found to be significantly lower in the picosecond experiments. Multiple charged atomic fragments (up to I 6+) have been recorded in the mass spectra. It is suggested that the higher charged atomic ions (I n+ , n≥3) are produced by field ionization processes on atomic ions which have been liberated via Coulomb explosion within unstable multiple charged parent ion in the rising time of the laser pulse. The laser intensity thresholds for the appearance of the multiple charged atomic ions have been determined and found to be reasonably close to those predicted by the barrier-suppression ionization model. Nevertheless, the observed variation of the appearance intensity thresholds with the laser wavelengths used implies that there is a contribution to the ionization by electron tunneling processes, especially at 1064 nm.

Corrections to "Strong picosecond optical pulse propagation in semiconductor optical amplifiers at transparency"

Corrections to "Strong picosecond optical pulse propagation in semiconductor optical amplifiers at transparency"

Four-wave mixing of strong picosecond optical pulses in passive semiconductor waveguides

Four-wave mixing (FWM) of strong picosecond optical pulses in passive semiconductor waveguides is investigated numerically, taking into account two-photon absorption (TPA), ultrafast nonlinear refraction, probe depletion, and cross-gain modulation, as well as interband and intraband carrier processes. Simulations show good agreement with published experimental measurements, and indicate that interband and intraband nonlinear mixing and TPA-induced pulse reshaping play significant roles in determining conjugate pulse behaviors for strong pulse energy cases. In particular, for increased pulse energy, an increase in FWM conversion efficiency is predicted, and an optimum pulse width is identified.

Transformation of strong picosecond pulses in radiation with an extended quasirotational spectrum during self-focusing in high-pressure hydrogen

Radiation with a spectrum representing a discrete analog of the extended spectrum observed in the generation of a supercontinuum in gases is generated in the self-focusing of 30-ps pulses with a wavelength of 1.06 μ]m in hydrogen at pressures up to 120 atm. The spectrum contains lines with similar intensities, an average frequency spacing approximately equal to the rotational transition frequency in hydrogen (587 cm−1), and a smooth spatial profile. The lines consist of several vibrational-rotational components. As the pressure is increased, the spectral lines are transformed so that at a pressure above 60 atm each line in the spectrum contains one or two components formed as a result of the smaller number of cascade (rotational and vibrational) processes. Self-focusing is manifested in the occurrence of a radiating channel up to 12 cm in length. The formation of a channel of this length is associated mainly with the variation of the refractive index in vibrational excitation of the hydrogen molecules by electrons heated in the pump field.

Amplification of strong picosecond optical pulses in semiconductor optical amplifiers

Numerical simulations have been undertaken to investigate the amplification of strong picosecond optical pulses in semiconductor optical amplifiers (SOAs), taking into account carrier heating, spectral holeburning, two-photon absorption (TPA) and ultrafast nonlinear refraction (UNR) effects. It is shown that both the TPA and UNR play important roles in determining the output pulse properties if the pulse energy is larger than ~0.5 pJ, and that these effects can be significantly enhanced with increased pulse energy and decreased pulse width as well as with higher small signal gain of the SOA. Results also indicate that, for larger input pulse energy, the symmetry of the amplified optical pulse can be controlled simply by adjusting the injected current.

Influence of the energy transport of electrons by optical phonon emission on the superluminescence and reversible bleaching of a thin GaAs layer excited by a strong picosecond light pulse

The dependence of the superluminescence and optical bleaching of GaAs during the interband absorption of a strong picosecond light pulse on the photon energy ℏωex of the exciting pulse is investigated experimentally. The bleaching (transmission enhancement, i.e., greater transparency) mainly reflects the density of the photogenerated electron-hole plasma. Several events are observed when the distance between the energy level at which electrons are generated and the level from which they undergo stimulated recombination is a multiple of the longitudinal optical phonon energy. These events are: 1) amplification of recombination superluminescence; 2) slowing of the increase in optical bleaching as ℏωex is increased; and 3) an increase in the shift of the edge of the emission spectrum toward longer wavelengths. These phenomena are qualitatively attributed to an increase in the fraction of transitions involving the emission of LO phonons in the energy transport of electrons toward the bottom of the conduction band and to the influence of this process on the heating of the electron-hole plasma and the density of nonequilibrium LO phonons.

Ultrafast carrier dynamics in wide gap hydrogenated amorphous silicon

We report on picosecond and femtosecond pump and probe measurements of the dynamics of photoexcited carriers in wide gap a-Si : H prepared by microwave electron–cyclotron resonance plasma-enhanced chemical-vapour-deposition. We interpret the picosecond dynamics of transient absorption under strong picosecond excitation in terms of a bimolecular recombination process with the rate constant B≈5×10 −10 cm 3 s −1, followed by a slower nanosecond decay. In the femtosecond measurements, we observed an initial decay with effective time constant ≈20 ps. We have not found any change in the picosecond dynamics when tuning the excitation wavelength through photoluminescence (PL) excitation spectrum profile. The ultrafast dynamics do not differ in the samples with PL efficiency differing in more than one order of magnitude, and they agree well with those in standard a-Si : H.

Strong picosecond optical pulse propagation in semiconductor optical amplifiers at transparency

The propagation of strong picosecond optical pulses in semiconductor optical amplifiers (SOAs) is investigated numerically by taking into account carrier heating, spectral hole-burning, and two-photon absorption, as well as ultrafast nonlinear refraction. Very good agreement with published experimental results are observed in both the time and frequency domains. It is shown that the effects of two-photon absorption and ultrafast nonlinear refraction are very important in determining the output pulse properties for pulse energy larger than 1 pJ.

Free induction signal from biexcitons and bound excitons

A theory of the free induction signal from biexcitons and bound excitons is presented. The simultaneous existence of the exciton continuum and a bound state is shown to result in a new type of time dependence of the free induction. The optically detected signal increases in time and oscillates with increasing amplitude until damped by radiative or dephasing processes. Radiative decay is anomalously fast and can result in strong picosecond pulses. The expanding area of a coherent exciton polarization (inflating antenna), produced by the exciting pulse, is the underlying physical mechanism. The developed formalism can be applied to different biexciton transients.