Double-pulse Excitation Research Articles

Investigation of the mechanisms involved in formation and decay of laser-produced plasmas

An analysis has been carried out on single and double pulse laser-induced plasma spectroscopy (LIPS) of aluminium samples in air. Spectroscopic techniques were used to retrieve relevant plasma characteristics during the first few microseconds after the plasma ignition. With respect to single laser excitation, dramatic changes in plasma characteristics were observed in the double pulse regime at 1.2 GW cm −2. Above this threshold, the narrowing of both atomic and ionic lines, the increase of the line emission intensity with longer time decays and a higher plasma stability were observed. The different threshold behaviour following single and double pulse excitation is discussed in terms of the optical and physical properties of the laser plasma produced.

Cooperative Photochemical Reaction in Molecular Crystal Induced by Intense Femtosecond Laser Excitation: Photochromism of Spironaphthooxazine

Photochromism of indolinospironaphthooxazine (SNO) microcrystalline powder was investigated by steady state and time-resolved diffuse reflectance spectroscopy using a femtosecond laser as a light source. When laser intensity is weak, the photoinduced ring-opening reaction in a picosecond time region and subsequent thermal ring-closure one in a nanosecond scale take place in the crystal, not leading to permanent photocoloration of the powder. The same sample shows photochromism upon intense femtosecond laser excitation. The absorption spectra after laser excitation are similar to those of the photocolored form in solution. The spectral shape is independent of the laser fluence, whereas the yield increases nonlinearly with the fluence. To reveal the photocoloration mechanism, we examined the excitation fluence dependence of the transient absorption spectra and elucidated the photocoloration behavior by femtosecond double pulse excitation varying the delay of the two pulses. These experimental results show t...

Observation of subpicosecond transient driving force for coherent phonons in GaAs by double-pulse excitation

Temporal variation in the driving force of coherent phonon was studied in n-type GaAs by a double-pulse pump–probe experiment by changing the pulse separation Δ t. Coherent phonons induced by the second pulse corresponding to the LO-phonon mode and the LO-phonon–plasmon coupled mode rapidly decreased in amplitude to their minima in ∼5 ps, then slowly recovered. This attenuation was explained by a screening of the surface space–charge field due to photo-excited carriers, while the recovery was attributed to a weakening of the screening through recombination and diffusion of photo-excited carriers.

Kinetics of Er3+-doped fluorozirconate optical fiber upconversion fluorescence and laser emission under 800 nm excitation

Applying single- and double-pulse excitation at 800 nm, the kinetics of the upconversion fluorescence in the green, as well as the upconversion laser at 543 nm was studied. No significant delay between the pumping pulse and the laser emission was found. In the erbium doped (1000 ppm) optical fiber, the mechanism responsible for the upconversion is purely of the excited state absorption (ESA) type. The double-pulse technique enables also a determination of the lifetime of the intermediate metastable state 4I11/2 (7±0.3 ms). Some other basic properties of the upconverted fluorescence and of the laser itself (fluorescence spectrum, optical gain, laser threshold) are also described.

Study of coherent LO phonon–plasmon-coupled modes by double-pulse excitation

The coherent LO phonon–plasmon-coupled mode in n-GaAs has been studied by using a double-pulse excitation. When the irradiation power of the second pump pulse in the double-pulse excitation is increased, the lower branch (L_ mode) of the LO phonon–plasmon-coupled mode generated by the second pump pulse are shifted to the higher-frequency side than that generated by the first pump pulse. When the separation time between the two pulse components of the double-pulse excitation is varied, the amplitude of the L_ mode after the second pump pulse changes dramatically. These results indicate that the first coherent L_ mode generated by the first pump pulse changes to a new state by the second pump pulse and influences the amplitude of the new coherent L_ mode generated by the second pump pulse.

Generation of coherent THz phonons in GeTe ferroelectrics

We have examined a possibility of light-induced phase transition in GeTe ferroelectrics by using amplified femtosecond laser pulses. The frequency of the A 1g mode corresponding to the soft mode is largely red-shifted and the amplitude of the A 1g mode increases linearly with increasing pumping power densities. The results of double-pulse excitation experiment indicate that the red-shift of the soft mode induced by the second pulse excitation is caused by the electronic weakening of crystal bonding due to the excitation of valence electrons.

Femtosecond Laser Ablation of Liquid Toluene: Molecular Mechanism Studied by Time-Resolved Absorption Spectroscopy

A time-resolved absorption spectroscopic measurement of liquid toluene under femtosecond UV (300−500 fs, 248 nm) laser ablation conditions was carried out, and its molecular mechanism was studied. The lowest excited singlet state of toluene monomer and toluene excimer were clearly observed through the delay time by 19 ns, while benzyl radical was not detected unexpectedly under any condition no matter how high the laser fluence was. This indicates that the femtosecond laser ablation is based on a photothermal mechanism. A femtosecond double-pulse excitation, on the other hand, induced benzyl radical formation, which is consistent with the photochemical mechanism in the case of nanosecond laser ablation. The result that molecular mechanism is dependent on the excitation laser pulse width suggests a possible mechanism control of laser ablation.

Enhancement of the Laser Breakdown of Simple Gaseous and Liquid Materials under Intense Picosecond Double-pulse Excitation

We report the breakdown behaviors of simple gaseous and liquid materials under double-pulse excitation from an intense picosecond Nd:YAG laser. With an increasing in the separation time of the pulse pair, a sharp increase in the intensity and decay time of a white-light emission from the breakdown plasma was observed. The further excitation of the preformed plasma by the second pulse explains the origin of the phenomenon.

Multiquantum triple-pulse spin-echo signals from quadrupole nuclei in magnetically ordered substances

We analyze theoretically the formation of NMR pulse responses from a quadrupole spin system in which the inhomogeneous broadening of a spectral line is due to both magnetic and electric quadrupole interactions. We derive formulas for the moments of formation of multiquantum echo signals in the case of three exciting pulses. For the first time we detected in experiments multiquantum spin-echo signals from copper nuclei in ferromagnetic copper sulfochromite in the cases of double-pulse and triple-pulse excitations. We find that there is good agreement between the calculated and experimentally observed moments of echo signal formation.

Two-pulse nondegenerate excitation of electron–hole wave packets in quantum wells:?tunable terahertz emission

It was shown recently that driving an optically excited quantum well with a terahertz (THz) frequency electric field can lead to higher-harmonic generation in the THz regime: the THz analog of the dynamic Franz-Keldysh effect. We propose a new double-pulse excitation method that not only increases the efficiency of the harmonic generation but is also able to control somewhat the dominant upshifted harmonic of the THz radiation by means of selective excitation of spatially chosen electron-hole relative motion wave packets.

Direct determination of transient molar absorption coefficient by double pulse excitation

A new method has been devised to determine the molar absorption coefficient of short-lived photochemical species formed by laser irradiation. The technique consists of a first pump excitation pulse that significantly modifies the concentration of the initial species in its ground state, enough that the variation of the absorbance at maximum absorption of the formed species depends on the initial species concentration. A new absorbance variation is created by a second excitation pulse occurring with a time delay after the first pulse that is small compared to the lifetime of the formed species. The molar absorption coefficient is calculated using the ratio of these two variations in absorbance, assuming that the initial species does not absorb in the spectral range where the absorption variation of the formed species is observed.

Laser-induced incandescence: excitation intensity

Assumptions of theoretical laser-induced incandescence (LII) models along with possible effects of high-intensity laser light on soot aggregates and the constituent primary particles are discussed in relation to selection of excitation laser fluence. Ex situ visualization of laser-heated soot by use of transmission electron microscopy reveals significant morphological changes (graphitization) induced by pulsed laser heating. Pulsed laser transmission measurements within a premixed laminar sooting flame suggest that soot vaporization occurs for laser fluences greater than 0.5 J/cm(2) at 1064 nm. Radial LII intensity profiles at different axial heights in a laminar ethylene gas jet diffusion flame reveal a wide range of signal levels depending on the laser fluence that is varied over an eight fold range. Results of double-pulse excitation experiments in which a second laser pulse heats in situ the same soot that was heated by a prior laser pulse are detailed. These two-pulse measurements suggest varying degrees of soot structural change for fluences below and above a vaporization threshold of 0.5 J/cm(2) at 1064 nm. Normalization of the radial-resolved LII signals based on integrated intensities, however, yields self-similar profiles. The self-similarity suggests robustness of LII for accurate relative measurement of soot volume fraction despite the morphological changes induced in the soot, variations in soot aggregate and primary particle size, and local gas temperature. Comparison of LII intensity profiles with soot volume fractions (f(v)) derived by light extinction validates LII for quantitative determination of f(v) upon calibration for laser fluences ranging from 0.09 to 0.73 J/cm(2).

Femtosecond Double-Pulse Excitation Study of α-Sexithienyl Film

Relaxation dynamics from a higher triplet excited state in α-sexithienyl (hereafter abbreviated as 6T) film was investigated by double-pulse excitation spectroscopy. Very rapid formation (within 1 ps) of the lowest singlet excited state and an intermolecular ion-pair state were observed. The mechanism of this rapid conversion process from a higher electronic excited state was discussed.

STUDY OF OXYGEN RECOMBINATION WITH A DOUBLE PULSE DISCHARGE

Oxygen atoms recombination at the wall is studied by time-resolved Optical Emission Spectroscopy in a pulsed discharge. In order to observe the atomic oxygen emission after discharge pulse, it is indispensable to re-excite O atoms because the lifetime of O* atoms is short. Here is proposed a technique based on a double pulse excitation. Atomic oxygen, created during the main discharge pulse, is probed in a time after-glow with a diagnostic pulse. This diagnostic pulse is as short as possible to only repopulate the atomic excited states, without significant production of supplementary atoms. The intensities of the emission lines O(3P-3S) at 844.6 nm and O(5P-5S) at 777.4 nm, measured during diagnostic pulse, are observed in the whole time after-glow as a function of delay after main pulse. The decrease of line intensities is related to the recombination of O atoms at the wall. For a “Pyrex” wall at 300 K, the obtained recombination probability γ is 10−3. This value is in agreement with previous results.

Coherent Phonons in Solids.

Coherent phonons can be generated in solids by ultra-short pulse laser excitation. Here we report observation of coherent phonon oscillations in the time domain using reflection type pump-probe techniques and discuss the phonon dynamics. An A1g phonon mode was observed for Bi, showing an exponentially decaying oscillation. Temperature dependence of the phonon frequency and the decay rate agrees with the result of Raman measurements. This indicates that the decay process of the coherent phonon is dominated by an anharmonic decay route of energy relaxation. The coherence of phonons in time domain was examined using a double-pulse excitation technique. The oscillation amplitude of the A1g mode in Bi enhances when the pulse separation time is equal to the period of the phonon oscillation, and vanishes when the separation time is adjusted to half integral multiples of the period. We present furthermore the results on coherent folded acoustic modes in GaAs/AlAs superlattices.

Design of a fluorescence technique using double laser pulse excitation for the measurement of molecular Brownian dynamics

A complete analysis of a novel method to evaluate the rotational diffusion time of fluorescent molecules is presented. It is based on the study of the total fluorescence produced after excitation with two consecutive short laser pulses. Expressions for the total fluorescence energy emitted in this case are derived, which would allow the evaluation of the rotational diffusion time. The main advantages of the proposed method are its simple setup, a time resolution only limited by the exciting laser pulsewidth, and the fact that it does not require fast time response detectors.

Nanosecond Electron Transfer Dynamics between Traps in Zeolites Studied by Double-Pulse Excitation Diffuse Reflectance Spectroscopy

The 248 nm excitation of zeolites results in the formation of trapped electrons which are very sensitive to photoexcitation and the amount of water contained in zeolites. Na43+, Na32+, and Na2+, which are prepared by 248 nm single-photon excitation, are directly confirmed as electron trapping states by nanosecond diffuse reflectance spectroscopy. Their stability decreases in the order of Na43+ > Na32+ < Na2+ in terms of the observed lifetimes of these trapped species. Water accelerates recombination between trapped electrons and the electron-ejected sites in the zeolite frameworks. Photoinduced conversion processes between these ion clusters were measured directly by double-pulse excitation experiments.

Triplet quantum yield determination by picosecond laser double-pulse fluorescence excitation

The quantum yields of S 1 → T 1 triplet formation of some organic dyes are determined by picosecond double-pulse laser excitation and time-resolved fluorescence detection. The second pulse fluorescence signal is reduced due to first pulse induced singlet-triplet intersystem crossing. Disturbances due to higher excited state triplet-singlet back transfer are minimized by proper laser pulse duration and pulse energy density adjustment. The saturation energy density of triplet ground state depopulation has to be higher than the saturation energy density of singlet ground state depopulation, and the energy density of the second excitation pulse should be comparable to or less than the saturation energy density of ground state depopulation.

Higher excited-state triplet-singlet intersystem crossing of some organic dyes

Time resolved fluorescence studies of some organic dyes (erythrosin B, rose bengal, tetraphenylporphyrin) by intense picosecond double and multiple pulse excitation (duration 6 ps, wavelength 527 nm, pulse separation 10 ns) reveal an efficient higher excited-state triplet-singlet intersystem crossing. The absorption and emission dynamics is analysed. The quantum yields of the higher excited-state triplet-singlet transfer are determined.

Determination of space-charge dynamics and transport parameters in hydrogenated carbon layers by time-resolved electroluminescence measurements

A new way is presented for deriving space-charge dynamics and transport parameters in thin films sandwiched between two metallic electrodes. The measurement method uses the detection of electroluminescence excited by square voltage pulses. The space-charge buildup time is derived from the electroluminescence time lag when the structure is excited by a single square voltage pulse. The relaxation time of the space charge is investigated by using a double pulse excitation. Carrier mobility is determined by an optical transit-time technique. Measurements are carried out on hydrogenated amorphous carbon layers with different composition and electrical properties. The films display space-charge limited conduction which is due to the existence of trap states. Their concentration is derived from the current–voltage characteristics and the mobility measurements. The trap concentration increases with a decreasing graphite content in the films. Film properties are weakly temperature dependent in graphite-rich layers as opposed to the behavior of layers with a lower graphite concentration. In this later case, the structural features of the films change at low temperature as suggested by the evolution of the trap concentration which decreases by three orders of magnitude, going from an exponential to a uniform distribution.