Short-pulse Laser Excitation Research Articles

Absorption and emission spectroscopic characterization of Ir(ppy)3

Abstract The absorption and emission spectroscopic behaviour of cyclometalated fac -tris(2-phenylpyridine) iridium(III) [Ir(ppy) 3 ] is studied at room temperature. Liquid solutions, doped films, and neat films are investigated. The absorption cross-section spectra including singlet–triplet absorption, the triplet–singlet stimulated emission cross-section spectra, the phosphorescence quantum distributions, the phosphorescence quantum yields and the phosphorescence signal decays are determined. In neat films fluorescence self-quenching occurs, in diluted solid solution (polystyrene and dicarbazole-biphenyl films) as well as deaerated liquid solution (toluene) high phosphorescence quantum yields are obtained, and in air-saturated liquid solutions (chloroform, toluene, tetrahydrofuran) the phosphorescence efficiency is reduced by triplet oxygen quenching. At intense short-pulse laser excitation the phosphorescence lifetime is shortened by triplet–triplet annihilation. No amplification of spontaneous emission in the phosphorescence spectral region was observed indicating higher excited-state absorption than stimulated emission.

Probing Two-Electron Dynamics of an Atom

Coherent short-pulse laser excitation has been used to control the approximate energy and relative proximity of two valence electrons within the same alkaline-earth atom, thereby providing insight into the dynamical evolution of a three-body Coulomb system. Our time-domain experiments enable direct experimental study of the electron dynamics at the classical limit of a two-electron atom. As an example, we look at the mechanism of autoionization for one two-electron configuration class and find that the doubly excited atom decays through a single violent electron-electron collision rather than a gradual exchange of energy between the electrons.

Investigation of the multiphoton excitation process of the 4f 25d configuration in LiYF4 and LiLuF4 crystals doped with trivalent neodymium ion

Ultraviolet (UV) fluorescence of Nd3+ ions induced by multistep laser excitation was investigated in Nd-doped LiYF4 and LiLuF4 crystals using a technique of time-resolved spectroscopy. The observed UV luminescence was due to transitions between the bottom of 4f25d configuration and the 4f3 states of Nd3+ ions. The first absorption band of 4f25d configuration, which starts around 56 700 cm−1, was excited by three stepwise absorptions of photons in the green (500–535 nm) from a short pulse laser excitation leading to broad emission bands in UV range (180–280 nm). An excitation band in the blue (468–486 nm) was observed due to the excitation of the second absorption band of 4f25d configuration around 63 000 cm−1 according to the photon absorption sequence: I9/24+hν(480 nm)→2G(1)9/2+hν(480 nm)→2F(2)7/2+hν(480 nm)→4f25d(second). The observed UV emissions (180–280 nm) from the bottom of the 4f25d configuration (first state) have a lifetime of 35 ns (parity allowed) and are broadband in contrast to UV emissions from 4f3 configuration, which are also present in the luminescence investigation but having a longer lifetime (8.5 μs) and structures composed of narrow lines. The excitation spectrum of fast UV luminescence exhibited different structures depending on the excitation geometry (σ or π) with respect to the c axis of the crystal. We observed two emissions from the first state of 4f25d configuration with peaks at 535 and 595 nm modifying the luminescence branching ratio of the bottom of the 4f25d configuration around 55 500 cm−1. The equivalent cross section of three and two excitation processes was estimated at 510 nm by solving the rate equations of the system under short laser excitation, which shows that is possible to have laser action under pulsed laser pumping with intensity below the crystal damage threshold.

Determination of triplet formation efficiency from kinetic profiles of the ground state recovery.

A procedure is presented which allows determination of the triplet formation efficiency from the analysis of transient ground state bleaching curves obtained after short laser pulse excitation. Crucial factors that control the applicability and the accuracy of the method are discussed, such as a proper choice of the detection wavelength, which allows avoidance of interference from transient absorption. The technique is tested on porphycene and octaethylporphyrin and applied for several porphyrin derivatives used as photodynamic therapy agents.

A microscope for imaging, spectroscopy, and lithography at the nanometer scale: Combination of a two-photon laser scanning microscope and an atomic force microscope

We designed and built a unique instrument that combines a two-photon laser scanning microscope (LSM) with an inverted atomic force microscope (AFM). Local photoluminescence (PL) spectroscopy and three-dimensional lithography are demonstrated using the two-photon LSM. High spatial resolution topographic images from the AFM can be recorded simultaneously with the PL images of the same region, allowing us to correlate PL variation and surface features of the sample. The wavelength of the short-pulse laser excitation can be varied continuously from 700 to 800 nm while the detection setup is optimized for signals between 350 and 650 nm. We demonstrate the performance of this instrument by examining the spatial variation of PL signals in GaN samples and by fabricating photonic crystal structures in polymer films.

Simulation of Crystallization Processes in Phase Change Optical Discs

ABSTRACTFuture development of erasable phase change recording requires a thorough understanding of the fundamental mechanisms involved in crystallization processes to be able to forecast the behavior of a new material or to guide its optimization when submitted to specific conditions. In this paper, we emphasize that a predictive simulation is valuable only if a good modeling and accurate values of the involved parameters are used. We will present the main characteristics of our phase change model (effect of the short-pulse laser excitation, chemical influence of the materials in contact with the PC layer, stabilization of the amorphous state for very thin film layers) and we will discuss experimental and simulated crystallization behaviors.

Analysis of writing and erasing behaviours in phase change materials

An understanding of the process involved in writing and erasing of phase-change optical recording media is vital to the development of new, and the improvement of existing, products. The present work investigates both experimental and theoretical laser-induced fast structural transformations of GeSbTe thin films. Optical and microstructural changes are correlated using both a static tester and transmission electron microscopy. In the second part of this paper we try to elucidate the physics underlying the amorphous-to-crystalline phase transformation under short-pulse laser excitation. Both thermal and thermodynamical behaviours must be taken into account to illustrate real processes.

Transient photocurrent in sexithiophene-based photovoltaic cells

Evidence for photovoltaic behaviour of sexithiophene sandwich cell is shown through I– V characterisation in dark and under illumination. Then transient measurements were performed under short laser pulse excitation. Transient photocurrent, which vanishes rapidly, could not be accounted for by a simple exponential decay. We used a theoretical model in which we resolved the kinetic equations, which govern the time evolution of carriers in the device. The best agreement of the model with the experiment allows us to reach the life time of the carriers ( τ n ≈1 μs, τ p ≈10 μs) and the interaction coefficient electrons plus holes, R≈2.5×10 −9 cm 3 s −1.

Photoluminescence properties of a GaN0.015As0.985/GaAs single quantum well under short pulse excitation

Under short pulse laser excitation, we have observed an extra high-energy photoluminescence (PL) emission from GaNAs/GaAs single quantum wells (QWs). It dominates the PL spectra under high excitation and/or at high temperature. By measuring the PL dependence on both temperature and excitation power and by analyzing the time-resolved PL results, we have attributed the PL peak to the recombination of delocalized excitons in QWs. Furthermore, a competition process between localized and delocalized excitons is observed in the temperature-dependent PL spectra under the short pulse excitation. This competition is believed to be responsible for the temperature-induced S-shaped PL shift often observed in the disordered alloy semiconductor system under continuous-wave excitation.

High finesse organic microcavities

We have constructed high-finesse light-emitting microcavities from the conjugated polymer poly( p-phenylene-vinylene), PPV, and high reflectivity distributed Bragg reflectors. The modifications to the photoluminescence and electroluminescence of the polymer arising from confinement of the photon field by these mirrors were investigated. Spontaneous emission enhancements of 2 orders of magnitude were found in the forward direction. The total integrated quantum yield for photoluminescence from PPV in such devices was 8.5 ± 1%. Stimulated emission from these devices was also investigated using short-pulse laser excitation. The emission line width narrowed by a factor of 2 above the lasing threshold. Short-term polarisation memory effects were observed as the life time of the excited states were reduced by coupling to stimulated emission. The lasing threshold for pulsed excitation was found to have been reduced by an order of magnitude, to 15 μJ/cm 2, compared to previous studies on microcavities incorporating metal mirrors. Findings were complemented by simulations of the optical fields using multi-layer stack theory and transfer matrices.

Scheme for a coherently controlled pulsed electron gun

When the outer electron of an alkali atom in an electric field is laser excited just above the field-induced ionization threshold, the electron ejection will not be instantaneous. Calculations show there are a number of energy regions where, upon short-pulse laser excitation, the atom will eject a train of electron probability pulses with the pulses being nearly equally spaced in time. This system can be the source of a picosecond pulsed electron gun. We show that it is possible to coherently control the electron pulse frequency while the pulse train is being emitted.

Effects of co-dopant concentrations and excitation conditions on the 2 ?m fluorescence dynamics in Tm, Ho:YLF crystals

Results of spectroscopic studies carried out on Tm, Ho:YLF crystals are reported. In particular, we have investigated the effects of the co-dopant concentrations on the time evolution of the fluorescence at 2 μm emitted in response to short-pulse laser excitation at 792 nm. The fluorescence intensity profiles are analyzed in terms of their temporal and amplitude characteristics over a wide range of excitation conditions.

Pumping-laser-fluence dependence of the time resolved fluorescence at 2.09 μm in Tm: Ho-YAG crystals

Spectroscopic studies of Tm: Ho-YAG crystals were performed, with a detailed analysis of the temporal evolution of the fluorescence at 2.09 μm emitted in response to short laser pulse excitation at 785.5 nm. All the fluorescence intensity temporal profiles showed a strong dependence on pumping laser fluence. A considerable decrease of the effective lifetime and of the build-up time has been observed for increasing values of pumping laser fluence.

Oscillator strengths for resonance transitions in neutral selenium and tellurium derived from time-resolved laser spectroscopy

We have measured radiative lifetimes for selenium and tellurium resonance levels by using short pulse laser excitation in differentially heated resonance cells. We obtained tau(3S1 Se I) = 2.9(5) ns and tau(3S1 Te I) = 3.1(5) ns, which together with branching ratio values given by Ubelis and Berzinsh yield the absorption oscillator strength f(abs)(Se I, 196.0 nm) = 0.074(16) and f(abs)(Te I, 214.3 nm) = 0.098(17).

High-speed photodetectors on InGaAs/GaAs-on-GaAs superlattices

We investigate a new type of metal-semiconductor-metal photodetector (MSM) on an InxGa1−xAs/GaAs strained superlattice whose performances compare to state of the art results achieved with MSMs on lattice matched In0.53Ga0.47As-on-InP material. The superlattices consist of 120 periods of 6-nm GaAs and 6–8 nm InxGa1−xAs with InAs content x between 48% and 66%. The structures were grown by molecular-beam epitaxy on GaAs substrates. Smooth morphologies and defect densities below 500 cm−2 were achieved with a growth temperature of 450–470 °C and with a minimum As to Ga pressure ratio. The cutoff wavelength and the dark current of the detectors increase with InAs content. The dark currents range between 100 nA and 3 μA at 10 V for 2500 μm2 devices and the spectral responses show cutoffs between 1.3 and 1.6 μm. The response time to short laser pulse excitation at 1.3 μm is below 30 ps full width at half maximum, and is limited by the time resolution of the measuring system. The internal quantum efficiency under pulsed excitation achieves maximum values near 60%. The very fast response time and relatively low quantum efficiency are indicative of fast carrier recombination processes in the active region of the detectors.

Wave packets in a semiconductor superlattice

A theoretical study of hole wave packets in a GaAs, Al0.3Ga0.7As superlattice is presented. Formation of the wave packet by short laser pulse excitation and the time-dependent nature of the wave packet are discussed, with particular attention given to the quasi-periodic nature of the time dependence. Phenomena analogous to those displayed in atomic and molecular wave packet systems are predicted. The wave packet formalism provides a powerful tool for studying coherent hole dynamics, including quantum beat phenomena, and could be useful in determining heavy and light hole relaxation rates.

Excited-state absorption in HgBr

The photoabsorption spectrum of the lowest ion pair state (B2Σ+1/2 ) of HgBr has been measured in the visible and ultraviolet (280≤λ≤480 nm) by laser excitation spectroscopy. Several previously unreported bands, peaking at 289, 302, 318, 330, 340, 364–373, and 401 nm, are observed and provide a stringent test for ab initio calculations of the excited state interaction potentials for the diatomic radical. A highly structured band lying between ∼444 and 470 nm confirms the observations of D. P. Greene, K. P. Killeen, and J. G. Eden [J. Chem. Phys. 85, 3188 (1986)] in a laser discharge. Also, the spectra fully resolve the apparent discrepancy between recent, unsuccessful attempts to produce a HgBr laser by flashlamp pumping the B←X band and experiments in which short pulse (8–25 ns) laser excitation of this transition has generated 502 nm coherent radiation with an energy conversion efficiency exceeding 20% [D. P. Greene, K. P. Killeen and J. G. Eden, Appl. Phys. Lett. 48, 1175 (1986)].

Vibrational analysis of the S2→S1 fluorescence of azulene in a naphthalene mixed crystal at 4.2 K

The S2→S1 fluorescence of azulene in a naphthalene mixed crystal has been measured from 740 to 1000 nm at 4.2 K. Of the 25 in-plane fundamentals (exclusive of C–H stretches) of the S1 state, 20 are readily identifiable in emission. A number of other bands are present in the spectrum and these are tentatively identified as S1 out-of-plane vibrations induced by vibronic coupling to σπ* and/or πσ* excited electronic states. An emission previously assigned as T1→S0 phosphorescence is shown to be a combination band of S2→S1 fluorescence. The branching ratio of S2 →S0 fluorescence to S2→S1 fluorescence is approximately 7500 for xenon arc lamp excitation; this is an order of magnitude larger than the value reported for short pulse laser excitation. A summary of photophysical parameters for the low-lying electronic states of azulene, based on the most recent and reliable experimental data, is also presented.

Proceedings of the 1975 International Conference on Luminescence, Tokyo, Japan, 1–5 September 1975

The collection of review and research papers in this volume represents the most up to date perspective on the frontiers of luminescence available and includes both inorganic and organic systems. The use of high intensity, short pulse laser excitation has opened up new fields, notably the study of condensed exciton droplets and the pre-relaxed states of luminescence emission complexes; these developments are well represented in the various papers.