Emission properties of femtosecond (fs) laser fabricated microstructures in Polystyrene (PS)

We fabricated microstructures and micro craters in thin films and bulk of PS. To the best of our knowledge, this is the first report of the emission from fs laser modified regions of PS when excited at 337, 400, 458, 488 and 514 nm wavelengths with different emission peaks. We systematically studied the emission in context of formation of optical centers and analyzed the spectra of irradiated PS. Change in the excitation wavelength leads to a shift in the emission peak, whereby, we infer that the emission should be due to a myriad of optical centers. Interestingly these optical centers have a similar excitation spectrum. Diphenylbutadiene (DPBD) is probably the main optical center among other optical centers thus formed in the process of fs laser irradiation of PS.     

Delocalized nature of the E'delta center in amorphous silicon dioxide

We report an experimental study by electron paramagnetic resonance (EPR) of E(')(delta) point defect induced by gamma-ray irradiation in amorphous SiO2. We obtained an estimation of the intensity of the 10 mT doublet characterizing the EPR spectrum of such a defect arising from hyperfine interaction of the unpaired electron with a 29Si (I=1/2) nucleus. Moreover, determining the intensity ratio between this hyperfine doublet and the main resonance line of E(')(delta) center, we pointed out that the unpaired electron wave function of this center is actually delocalized over four nearly equivalent silicon atoms.     

Paramagnetic properties of ion-implanted polymer layers

Films of polyethylene, polyethylene terephthalate, and polyamine-6 implanted with B⁺ and N⁺ ions with an energy of 100 keV are investigated by an EPR method in a dose interval of 1·10¹⁴–1·10¹⁷ cm⁻². It is shown that paramagnetic centers with g=2.0025 formed in the implanted polymers have a nature similar to the nature of paramagnetic centers of pyrolized and initially conducting polymers. Correspondence of the character of the variation in paramagnetic characteristics of the modified polymers to the model proposed earlier for the formation of pyrocarbon “drops” in ion implantation is revealed. The relaxation times for paramagnetic centers in the implanted polymer films are calculated and assumptions are made about the formation of a quasi-two-dimensional electron gas as well as the possibility of magnetic ordering in polymer-film layers modified by high-dosage implantation. The effect of oxygen on the electron states of the implanted polymer specimens is studied. Belarusian State University, 4, Skorina Ave., Minsk, 220050 Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 562–567, July–August, 1998.     

Spectroscopic investigations of femtosecond laser irradiated polystyrene and fabrication of microstructures

We report microfabrication of structures in bulk and thin films of polystyrene (PS) using femtosecond (fs) laser pulses. For the first time to our knowledge, we report emission from the fs laser modified regions of bulk and thin films of PS when excited at 458, 488, and 514 nm. Moreover, we report the existence of peroxide type free radicals, for the first time, in fs laser irradiated bulk PS. We observed the suppression of Raman modes in case of structures fabricated at higher energies and the same effect was noticed in central portion of the structures fabricated. No appreciable broadening was observed in the case of structures fabricated at low energies. Possible applications resulting from such structures are discussed briefly.     

Infrared femtosecond laser-induced great enhancement of ultraviolet luminescence of ZnO two-dimensional nanostructures

Two-dimensional (2D) complex nanostructures on the surface of ZnO crystal are fabricated by the interference of three 800 nm fs laser beams. The 2D nanostructures exhibit a great enhancement of UV emission excited by infrared fs laser with central wavelengths ranging from 1,200 nm to 2,000 nm. We propose that the defect states in the band gap of 2D nanostructures induced by 800 nm fs laser ablation cause the great enhancement of UV emission. We make theoretical calculations and explain well with the experimental results.     

From amorphous to crystalline silicon nanoclusters: structural effects on exciton properties

Synchrotron x-ray absorption spectroscopy (XAS) and electron spin resonance (ESR) experiments were performed to determine, in combination with Raman spectroscopy and x-ray diffraction (XRD) data from previous reports, the structure and paramagnetic defect status of Si-nanoclusters (ncls) at various intermediate formation stages in Si-rich Si oxide films having different Si concentrations (y = 0.36-0.42 in Si(y)O(1-y)), fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition and isochronally (2 h) annealed at various temperatures (T(a) = 900-1100 °C) under either Ar or (Ar + 5%H(2)) atmospheres. The corresponding emission properties were studied by stationary and time dependent photoluminescence (PL) spectroscopy in correlation with the structural and defect properties. To explain the experimental data, we propose crystallization by nucleation within already existing amorphous Si-ncls as the mechanism for the formation of the Si nanocrystals in the oxide matrix. The cluster-size dependent partial crystallization of Si-ncls at intermediate T(a) can be qualitatively understood in terms of a 'crystalline core-amorphous shell' Si-ncl model. The amorphous shell, which is invisible in most diffraction and electron microscopy experiments, is found to have an important impact on light emission. As the crystalline core grows at the expense of a thinning amorphous shell with increasing T(a), the PL undergoes a transition from a regime dominated by disorder-induced effects to a situation where quantum confinement of excitons prevails.     

Recombination emissions and spectral blueshift of pump radiation from ultrafast laser induced plasma in a planar water microjet

We report spectroscopic investigations of an ultrafast laser induced plasma generated in a planar water microjet. Plasma recombination emissions along with the spectral blueshift and broadening of the pump laser pulse contribute to the total emission. The laser pulses are of 100 fs duration, and the incident intensity is around 10¹⁵ W/cm². The dominant mechanisms leading to plasma formation are optical tunnel ionization and collisional ionization. Spectrally resolved polarization measurements show that the high frequency region of the emission is unpolarized whereas the low frequency region is polarized. Results indicate that at lower input intensities the emission arises mainly from plasma recombinations, which is accompanied by a weak blueshift of the incident laser pulse. At higher input intensities strong recombination emissions are seen, along with a broadening and asymmetric spectral blueshift of the pump laser pulse. From the nature of the blueshifted laser pulse it is possible to deduce whether the rate of change of free electron density is a constant or variable within the pulse lifetime. Two input laser intensity regimes, in which collisional and tunnel ionizations are dominant respectively, have been thus identified.     

Reaching the resolved tunnel regime for a femtosecond oscillator driven field emission electron source

We report on electron emission from tungsten tips with the help of 800 nm-8 fs laser pulses from a Ti:sapphire laser oscillator. We have measured autocorrelation traces of the exciting laser pulse in the photoelectron current, which allows a measurement of the non-linearity as a function of DC voltage applied to the tip. These data are well described by a numerical integration of the one-dimensional Schrödinger equation. The simulation shows us that electron emission resolves the electric field structure of the driving laser pulse, a regime which is currently an extremely fruitful area of research with atoms in the gas phase. For an 8 fs laser pulse with the correct carrier-envelope phase setting the emission duration is as short as 800 nm.     

Hydrostatic pressure effects on oxygen-related irradiation-produced defects in silicon

Abstract

Hydrostatic pressure has been used as a variable to investigate the E_c-0.164 eV acceptor level for the oxygen-vacancy (O—V) defect in γ-ray irradiated Si, and the annealing/formation of oxygen-related defects in neutron-irradiated Si. The acceptor level is found by deep level transient spectroscopy to move closer to the conduction band and away from the valence band at rates of 3.9 meV/kbar and 2.4 meV/kbar, respectively, i.e., the level moves higher in the gap. There is also a relatively large inward (outward) breathing mode lattice relaxation (4.6±1.2 ų/electron) accompanying electron emission (capture) from this level. Both results reflect the antibonding nature of the level and are qualitatively consistent with the Watkins—Corbett model for the O—V defect. The annealing rate was found by infrared absorption to increase with pressure for the O—V defect at 350°C with a derived activation volume of ?4.5 ų/defect, where the negative sign implies inward relaxation (contraction) on annealing. Pressure has relatively little effect on annealing of the C—Si—O C(3) defect which is interstitial in nature, but strongly favors the formation of the dioxygen (2 oxygen atoms per vacancy, i.e., O₂—V) defect. The intensity of the O₂—V band after annealing at 20 kbar is 5 times higher than that following similar annealing at 0 kbar. Additionally, this intensity at 20 kbar is higher than that achievable by any isochronal or isothermal annealing steps at 0 kbar. These annealing/formation results are discussed qualitatively in terms of models for the various defects.     

Fluorescence Enhancement of CdSe Q-Dots with Intense Femtosecond Laser Irradiation

Effects of intense femtosecond (fs) laser irradiation on the optical properties of cadmium selenide (CdSe) nanocrystals are studied. We present the changes in emission and absorption of laser (800 nm, 110 fs, Ti–Sapphire) irradiated CdSe nanocrystals dispersed in dimethylformamide (DMF). It is observed that the absorbance of CdSe nanocrystals capped with trioctylphosphine (TOP) increases with the number of laser pulses. The trap state luminescence intensity of these crystals degrades, whereas the band edge luminescence intensity shows an increase as a function of the fs laser irradiation. We also report strong two photon absorption and reduction in the trap state luminescence intensity after irradiation with the laser pulses.     

Dynamics of laser-induced desorption from dielectric surfaces on a sub-picosecond timescale

The impact of ultra-short laser pulses induces intensity-dependent non-equilibrium processes in the surface region of dielectric targets, resulting in desorption of surface constituents. We report time-resolved studies on particle ejection from CaF₂ and BaF₂ targets.Pump-probe time-of-flight mass spectrometry (ToF MS) was used to measure the particle yields as a function of the delay time between pairs of sub-damage threshold laser pulses, thus obtaining the temporal dynamics of the laser-excited charged particle emission.In a correlative manner, the positive ion, electron and negative ion desorption yields dependence on the pump-probe delay time reveal a coherence peak around zero delay (similar to a two-pulse autocorrelation), accounting for the increase of the ion yield with laser intensity. Additionally, the measurements reveal a delayed peaks at ∼900 fs (BaF₂) and ∼300 fs (CaF₂). For comparison, we present time-resolved studies on electron emission from aluminum targets with pump and probe pulses below the damage threshold. The measurements show, again, the autocorrelation peak in the coherence region and an additional increase in the electron yield when the pulses are several picoseconds apart.The autocorrelation could also stand for the dephasing time of the electronic coherence, while the delayed peaks may reveal for the time needed for the collisional energy to be transferred to the lattice. The pump pulse induces a new unstable phase, which is further destabilized by the probe pulse.A corresponding qualitative picture for temporal dynamics of femtosecond (fs) laser-induced particle emission is proposed.     

Defect-Dipole Formation in Copper-Doped PbTiO3 Ferroelectrics

The defect structure of hard copper-modified polycrystalline PbTiO3 ferroelectrics is investigated by means of electron paramagnetic resonance and hyperfine sublevel correlation spectroscopy, as well as density functional theory calculations. Special emphasis is put on the 207Pb-hyperfine couplings, which are resolved up to the third coordination sphere. The results prove that copper is incorporated at the octahedrally coordinated Ti site, acting as an acceptor. Because of charge compensation the formation of Cu impurity-oxygen vacancy pairs is energetically very favorable. The corresponding (CuTi'-VO)x defect dipole is found to be orientated along the [001] axis.     

Modelling the formation of nanostructures on metal surface induced by femtosecond laser ablation

We employ the particle-in-cell method to simulate the mechanisms of femtosecond (fs) laser interactions with a metallic target. The theoretical approach considers the solid as a gas of free electrons in a lattice of immobile ions and the laser fluences close to the ablation threshold. At first moments of the interaction, our simulations mapped out different nanostructures. We carefully characterized the rippling phase and found that its morphology is dependent on the distribution of the electron density and the period of the ripples depends on the laser intensity. The simulation method provides new insights into the mechanisms that are responsible for surface grating formation.     

Coherent control of optical emission from a conjugated polymer

We have observed that resonant Rayleigh scattering dominates the emission from poly(p-phenylene vinylene) excited with photons at energies below the threshold at which excitonic migration is reduced. The intensity of the resonant emission decays exponentially with a lifetime of up to 450 fs after pulsed excitation. The coherent nature of the emission was confirmed by angular variations in the far-field emission intensity-bright and dark speckles. Persistence of a coherent polarization was demonstrated by coherent control using phase-locked pulses.     

Ultrafast electron pulses from a tungsten tip triggered by low-power femtosecond laser pulses

We present an experimental and numerical study of electron emission from a sharp tungsten tip triggered by sub-8-fs low-power laser pulses. This process is nonlinear in the laser electric field, and the nonlinearity can be tuned via the dc voltage applied to the tip. Numerical simulations of this system show that electron emission takes place within less than one optical period of the exciting laser pulse, so that an 8 fs 800 nm laser pulse is capable of producing a single electron pulse of less than 1 fs duration. Furthermore, we find that the carrier-envelope phase dependence of the emission process is smaller than 0.1% for an 8 fs pulse but is steeply increasing with decreasing laser pulse duration.     

Thermal emission of electrons from highly excited sodium clusters

Positively charged sodium clusters can be easily ionized by a fs laser pulse of relatively low intensity (<10¹⁰ W/cm²), if the laser is in resonance with the plasmon excitation of the cluster. This ionization process was investigated in detail by measuring the kinetic energy distribution of electrons emitted from a size-selected Na₉₃ ⁺ as a function of the fs laser intensity. In all cases pure Boltzmann-like energy distributions were observed. A comparison with statistical theory shows that the emission is a purely thermal process. It is different to normal thermionic emission insofar as the electrons are emitted from a hot electron system which is only weakly coupled to a cold ionic background. The results demonstrate purely statistical behaviour of a small fermionic system even for very high excitation energy. Received: 25 May 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

The control of the electron quantum paths for the generation of isolated attosecond pulse using a three-color laser scheme

We theoretically propose a three-color laser scheme to enhance the high-order harmonic intensity and generate an isolated attosecond pulse. By adding a 3 fs, 1600 nm laser pulse to a synthesized two-color laser field (5 fs, 800 nm and 10 fs, 1200 nm), the harmonic intensity is effectively enhanced and an isolated attosecond pulse with duration 41 as is generated. In this scheme, the short trajectory is suppressed, the selection of the long quantum path can be achieved. We also investigate emission time of harmonics in terms of the time-frequency analysis and the semi-classical three-step model to illustrate the physical mechanism of high-order harmonic generation.     

Spectral analysis of K-shell X-ray emission of magnesium plasma produced by ultrashort high-intensity laser pulse irradiation

Spectral analysis of K-shell X-ray emission of magnesium plasma, produced by laser pulses of 45 fs duration, focussed up to an intensity of ～10¹⁸ W cm^?2, is carried out. The plasma conditions prevalent during the emission of X-ray spectrum were identified by comparing the experimental spectra with the synthetic spectra generated using the spectroscopic code PrismSPECT. It is observed that He-like resonance line emission occurs from the plasma region having sub-critical density, whereas K-α emission arises from the bulk solid heated to a temperature of 10 eV by the impact of hot electrons. K-α line from Be-like ions was used to estimate the hot electron temperature. A power law fit to the electron temperature showed a scaling of I ^0.47 with laser intensity.     

Attosecond correlated dynamics of two electrons passing through a transition state

The strong-field induced decay of a doubly excited, transient Coulomb complex Ar**→Ar(2+)+2e(-) is explored by tracing correlated two-electron emission in nonsequential double ionization of Ar as a function of the carrier-envelope phase. Using <6 fs pulses, electron emission is essentially confined to one optical cycle. Classical model calculations support that the intermediate Coulomb complex has lost memory of its formation dynamics and allows for a consistent, though model-dependent definition of "emission time," empowering us to trace transition-state two-electron decay dynamics with sub-fs resolution. We find a most likely emission time difference of ～200±100 as.     

Native defect changes in cds single crystal platelets induced by vacuum heat treatments at temperatures up to 600°C

Physical properties of selected CdS single crystal platelets as-grown and after vacuum heat treatments at temperatures up to 600°C have been studied using u.v. excited edge emission, mass spectrometry, electrical resistivity and electron paramagnetic resonance (EPR). It was found that sulfur leaves the crystal at temperatures as low as 100°C creating a depletion layer. The native defect changes were monitored by edge emission studies at 4.2°K in combination with etch treatments. The defect structure throughout the crystal is not only dependent upon the temperature and atmosphere of the treatments, but is also strongly dependent upon the cooling rate.