Picosecond Excitation Research Articles

Collective processes in the luminescence that accompanies picosecond excitation

This paper presents experimental results and a theoretical analysis of the features of the absorption-emission kinetics in optically dense media accompanying Stokes and anti-Stokes excitation by picosecond laser pulses. The studies were carried out with a solution of rhodamine 101. It is shown that the observed features of the photodynamics of the solutions are associated with manifestations of collective photoinduced luminescence, light quenching, and two-photon absorption.

Scattering-assisted terahertz gain in semiconductor superlattices in the Wannier-Stark-Ladder regime

Using the second-order perturbation theory we have calculated the scattering assisted gain spectra in GaAs/AlGaAs superlattice under a strong applied electric field in the Wannier-Stark-Ladder (WSL) regime. Nonequilibrium distribution function of quasi-two-dimensional carriers localized in each WSL level and indirect optical transitions between neighboring WSL levels accompanied by the emission or absorption of acoustical phonons are taken into account in our theoretical analysis and numerical calculation. We have shown that the experimentally observed down shift of the zero-gain frequency from the Bloch oscillation frequency is due to the inelastic nature of the phonon scattering and the formation of excitons when electron-hole pairs are photoexcited. Our theoretical results agree well with the experimental data which were obtained from analyzing the THz response of superlattices to the picosecond optical pulse excitation.

Superlinear photoluminescence in silicon nanocrystals: The role of excitation wavelength

We study the influence of the wavelength of picosecond excitation pulses on the properties of photoluminescence (PL) in a series of samples of silicon nanocrystals prepared by ion implantation into silica matrix. We observed a gradual change in the behaviour of the PL fast component (spectral shape, decay times, pump-intensity dependence) when tuning the excitation wavelength from 355 to 532 nm. We interpret the results in terms of an interplay between the PL originating from volume states of nanocrystals containing two photoexcited carrier pairs, and the PL due to the silicon oxide states. We discuss also the role of the implant fluence on the PL properties of samples.

Picosecond excitation energy relaxation processes in a ladder-type π-conjugated polymer

We studied the relaxation dynamics of excitations in the lowest electronically excited singlet state in a ladder-type π-conjugated polymer employing picosecond time-resolved spectroscopy with high spectral resolution. At 1.5 K both the decay dynamics and the rising component of the fluorescence transients from the low-energy tail of the distribution of excited states show a strong dependence on time and on the precise emission wavelength. At room temperature, in contrast, such dependencies are not observed. Finally, the data allow us to retrieve the radiative lifetime of the excited states both at 1.5 K and 296 K.

One- and two-photon photochromism of 3,4-bis-(2,4,5-trimethyl-thiophen-3-yl)furan-2,5-dione

Photochromic properties of the diarylethene derivative 3,4-bis-(2,4,5-trimethyl-thiophen-3-yl)furan-2,5-dione ( 1) were investigated in hexane at room temperature under one-photon (linear) and two-photon (nonlinear) excitation. Results of steady-state excitation anisotropy and quantum–chemical calculations (AM1, ZINDO/S) provide insight into the nature of the broad absorption spectrum of the open-form of 1 as an overlapped mixture of several electronic transitions. The quantum yields of cyclization and cycloreversion reactions of 1 were obtained over a broad spectral range and the photochemical stability of 1 was determined for different excitation wavelengths. The two-photon absorption (2PA) spectrum of the open-form of 1, with maximum cross-section ≈80 GM at 674 nm, was obtained using an up-converted fluorescence method. Two-photon induced cyclization reaction of 1 was accomplished under picosecond excitation and the corresponding reaction quantum yield was estimated, providing a comprehensive investigation of this 2PA photochrome.

Primary Photoexcitations and the Origin of the Photocurrent in Rubrene Single Crystals

By simultaneously measuring the excitation spectra of transient luminescence and transient photoconductivity after picosecond pulsed excitation in rubrene single crystals, we show that free excitons are photoexcited starting at photon energies above 2.0 eV. We observe a competition between photoexcitation of free excitons and photoexcitation into vibronic states that subsequently decays into free carriers, while molecular excitons are instead formed predominantly through the free exciton. At photon energies below 2.25 eV, free charge carriers are created only through a long-lived intermediate state with a lifetime of up to 0.1 ms and no free carriers appear during the exciton lifetime.

Optical nonlinearity in glass-embedded silver nanoclusters under ultrafast laser excitation

The nonlinear optical transmission in noble metal clusters excited near the surface plasmon resonance can evolve from saturable absorption to induced absorption on varying the cluster size and applied laser fluence. Here we investigate the role of the laser pulsewidth in effecting this transition. Silver nanoclusters of sizes ranging from 2 to 6 nm, embedded in soda lime glass by the ion-exchange method, are excited with 100 fs and 300 ps laser pulses respectively. From femtosecond pump-probe measurements, the hot electron dynamics is measured in the largest cluster. Results show that thermal effects contribute to the nonlinearity in the picosecond excitation domain. Saturation intensities and effective nonlinear absorption coefficients are numerically evaluated.

Condensation kinetics of microcavity polaritons with scattering by phonons and polaritons

We present numerical studies of the exciton polariton condensation kinetics in microcavities for the combined action of polariton-polariton and polariton-acoustic phonon scattering both for quasi-stationary and picosecond pulse excitation, respectively. For excitations of nearly resonant polaritons mainly the polariton-polariton scattering mechanism results in a condensation on the minimum of the lower polariton branch at relatively low areal polariton densities of the order of ${10}^{9}--{10}^{10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. The simultaneously acting polariton-phonon scattering increases the number of condensed particles considerably by providing a heat dissipation from the polariton gas to the lattice. For nonresonant excitations at large $k$ values the considered scattering mechanisms cannot support a polariton condensation. Above (below) laser threshold there exists a (no) polariton condensate and the polariton distribution of the noncondensed particles can be fitted by a Bose-Einstein (Maxwell-Boltzmann) distribution. Our results for $3\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ pulse excitation are in very good agreement with recent experimental observations of Yamamoto et al.

Yield of photo-adduct formation of LOV domains from Chlamydomonas reinhardtii by picosecond laser excitation

The photo-excitation of flavin mononucleotide (FMN) in the wild-type light, oxygen and voltage sensitive (LOV) domains of the blue-light photoreceptors phototropin causes the formation of an intermediate flavin-C(4a)-cysteinyl adduct. The adduct formation due to picosecond laser pulse exaction (wavelength 400 nm) is studied on wild-type LOV1 and LOV2 domains of the phototropin phot from Chlamydomonas reinhardtii. The efficiency of adduct formation is probed by detection of the fluorescence signals caused by time-separated picosecond excitation pulses since FMN is fluorescent and the formed adduct is non-fluorescent. Quantum yields of adduct formation of ϕ Ad ≈ 0.06 are determined for excitation with intense single-pulses of energy densities, w L, comparable to or larger than the saturation energy density, w sat , S 0 , of ground-state population depletion. Under repetitive picosecond pulse excitation conditions the efficiency of adduct formation rises with decreasing picosecond pulse energy density and approaches for w L < 0.001 w sat , S 0 the continuous blue-light exposure quantum yields of adduct formation in the range from ϕ Ad = 0.5–0.9. Picosecond laser pulse induced energy deposition in the LOV domains causing protein conformational changes is discussed as main origin of the intensity dependent reduction of the efficiency of adduct-formation.

Surface‐enhanced hyper Raman scattering in silver colloidal solutions

AbstractWe report surface‐enhanced hyper Raman scattering (SEHRS) from molecules in silver colloidal solutions using high repetition rate mode‐locked picosecond excitation at 1064 nm. SEHRS was is studied under non‐resonant and resonant conditions by choosing adenine and crystal violet as test molecules. Compared with its SERS spectrum, the SEHRS spectrum of adenine displays strong ‘new’ lines, which, in agreement with theory, in most cases also show up in the infrared absorption spectrum. We also explore the capability of SEHRS for trace detection and demonstrate detection and identification of crystal violet based on its SEHRS signals down to nanomolar concentrations. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Synthesis of Two-Photon Absorbing Unsymmetrical Branched Chromophores through Direct Tris(bromomethylation) of Fluorene

Branched fluorene-based chromophores bearing electron-donating diphenylamino or electron-withdrawing nitro groups were synthesized as well as their linear analogues. An efficient synthetic method was developed via a novel 2,4,7-tris(bromomethyl)-9,9-diethylfluorene intermediate. The bromomethyl groups in this key intermediate were converted to either phosphonate or carboxaldehyde moieties, facilitating preparation of a high functionality branched structure. It was found that the reactivity at position 4 is attenuated in the bromomethyl and phosphorylated derivates, facilitating the selective and systematic functionalization of the fluorenyl system. All compounds were stable up to ca. 350 degrees C, except for a sterically crowded branched derivative. The linear optical properties of the compounds were investigated by UV-visible, steady-state fluorescence, and excitation anisotropy spectroscopic measurements. Fluorescence quantum yields were greater than or equal to 0.84 for symmetric linear and unsymmetric branched derivatives. Very high two-photon absorption (2PA) cross-sections were achieved (5765 GM at 520 nm and 4194 GM at 570 nm), as determined with use of picosecond and femtosecond laser excitation sources, respectively.

Optical Control of Excitons in a Pair of Quantum Dots Coupled by the Dipole-Dipole Interaction

We demonstrate coherent nonlinear-optical control of excitons in a pair of quantum dots (QDs) coupled via dipolar interaction. The single-exciton population in the first QD is controlled by resonant picosecond excitation, giving rise to Rabi oscillations. As a result, the exciton transition in the second QD is spectrally shifted and concomitant Rabi oscillations are observed. We identify coupling between permanent excitonic dipole moments as the dominant interaction mechanism, whereas quasiresonant (Förster) energy transfer is weak. Such control schemes based on dipolar interaction are a prerequisite for realizing scalable quantum logic gates.

Photo-physical and lasing characterisation of a polyparaphenylenevinylene (PPV) neat film

The wave-guided travelling-wave laser action (amplified spontaneous emission) of a neat film of poly(p-phenylenevinylene) (PPV) on a quartz glass substrate prepared by a sulfinyl precursor technique is studied. The samples are transversally pumped with picosecond excitation pulses (wavelength 347.15 nm, duration 35 ps). Lasing occurs at 550 nm. The optical constants of the neat films are determined by transmittance measurements exploiting the multiple beam interference in the transparency region. A fluorescence spectroscopic characterisation is carried out determining the fluorescence quantum distribution, fluorescence quantum yield, degree of fluorescence polarisation, and fluorescence lifetime. The emitting chromophore size (emitting singlet exciton extension) is determined by the ratio of exciton radiative lifetime to repeat-unit based radiative lifetime. The obtained size of about two repeat units is discussed in a disordered solid-state polymer model.

Theoretical and experimental investigation of the second hyperpolarizabilities of methyl orange

We describe experimental and theoretical studies of the third-order nonlinear optical coefficients of methyl orange solutions under different pH conditions. A combination of semiempirical and ab initio methods was adopted to investigate the most stable geometrical structures possible for this molecule. The experimental data obtained using the Z-scan technique for the third-order nonlinear optical properties of this compound has allowed the determination of the nonlinear refractive index and nonlinear absorption coefficient under picosecond excitation in the visible (532 nm) spectral region. From those experimental results, the second hyperpolarizability of methyl orange was inferred both for acidic and alkaline solutions. Comparison of these values to the results predicted by semiempirical methods suggests that even at low pH, when the probability of cis-trans isomerization is increased, the trans conformation of the methyl orange molecule should dominate the nonlinear spectra of this compound. The theoretical results were used as an auxiliary tool to identify possible trends on the nonlinear properties changes as a function of the distinct molecular conformations adopted by the methyl orange molecule under different pH conditions.

Two-photon LIF imaging of atomic oxygen in flames with picosecond excitation

Interference-free planar laser-induced fluorescence measurements of atomic oxygen are demonstrated in steady and unsteady laminar premixed methane flames. Two-dimensional LIF measurements of atomic oxygen present a challenge because of the relatively weak two-photon absorption cross-sections and the nonlinear dependence of the LIF signal on laser intensity. Previous two-photon LIF measurements of atomic oxygen in hydrocarbon flames using nanosecond laser excitation were complicated by significant interference from photolytically produced oxygen atoms. Recent results from line-imaging studies in our laboratory indicate that CO 2 is the dominant photolytic precursor. Here, we use picosecond excitation to image atomic oxygen with negligible photolytic interference. We demonstrate this capability in premixed CH 4/O 2/N 2 ( ϕ = 1.08 , X N 2 = 0.63 ) flames with an adiabatic flame temperature of 2480 K and an estimated peak O-atom mole fraction of 0.5%. Peak single-shot signal-to-noise ratios of 9 are achieved, and the estimated detection limit is approximately 80 ppm. Quantitative O-atom LIF measurements also require knowledge of the temperature-dependent quenching cross-sections, which are not currently available. The effects of collisional quenching on the O-atom LIF are predicted using two models for the temperature dependence of quenching cross-sections, and the results indicate that O-atom LIF signals can provide accurate measurements of O-atom profiles in flames. Measurements in an acoustically forced Bunsen flame illustrate the feasibility of using O-atom LIF to investigate flow–flame interactions.

Spatial modes in a PCF fiber generated continuum

We measure the propagation properties of a highly nonlinear photonic crystal fiber (PCF). The spatial, temporal and frequency dependent properties of the propagating modes are measured under conditions of high power, seven picosecond excitation, white light continuum generation. The experimentally determined multi-mode nature of the white light continuum is found to be in good agreement with numerical simulations.

Dynamics of Re(2,2′-bipyridine)(CO)3Cl MLCT formation and decay after picosecond pulsed X-ray excitation and femtosecond UV excitation

The dynamics of Re(2,2'-bipyridine)(CO)3Cl MLCT state formation and decay were determined after femtosecond UV laser excitation and picosecond pulsed X-ray excitation, in an N,N-dimethylformamide (DMF) solution as well as in its solid form. At room temperature, after UV excitation, this MLCT excited state emits both in DMF solution and in the solid form. Transient absorption spectra were measured in solution at various delay times following excitation by a 160 fs, 390 nm laser pulse. There was a prompt absorption increase at around 460 nm occurring within the pump probe convolution (<1 ps), which was assigned to the formation of the 3MLCT state. This transient absorbance was constant over 100 ps. In contrast to the solution state, in the solid state, the emission maximum slightly red-shifts with increasing time after laser excitation. In both solid and solution the emission rises within the system response time. The solid sample exhibited a 1.4 ns emission decay that was not observed for the solution sample. The emission rise from a solid sample after 20 ps pulsed X-ray excitation was significantly slower than the system's time resolution. It is proposed that kinetically energetic electrons are ejected following X-ray induced ionisation, creating ionised tracks in which energetic cations and electrons take time to recombine yielding delayed 3MLCT states that emit.

Investigation of Surface Passivation in InAs/GaSb Strained-Layer-Superlattices Using Picosecond Excitation Correlation Measurement and Variable-Area Diode Array Surface Recombination Velocity Measurement

ABSTRACTThe effect of ammonium sulfide passivation on InAs/GaSb superlattice infrared detectors was investigated using two complementary techniques, namely, picosecond excitation correlation (PEC) measurement and variable-area diode array (VADA) surface recombination velocity (SRV) measurement. PEC measurements were conducted on etched InAs/GaSb superlattice mesas, which were passivated in aqueous ammonium sulfide solutions of various strengths for several durations. The PEC signal's decay time constant (DTC) is proportional to carrier lifetimes. At 77 K the PEC signal's DTC of the as-grown InAs/GaSb superlattice sample was 2.0 ns, while that of the unpassivated etched sample was reduced to 1.2 ns by the surface states at the mesa sidewalls. The most effective ammonium sulfide passivation process increased the PEC signal's DTC to 10.4 ns. However it is difficult to isolate surface recombination from other processes that contribute to the lifetime using the PEC data, therefore a VADA SRV measurement was undertaken to determine the effect of passivation on surface recombination. The obtained SRV in the depletion region of the InAs/GaSb superlattice and GaSb junction was 1.1×106 cm/s for the unpassivated sample and 4.6×105 cm/s for the passivated sample. At 77 K the highest R0A value measured in our passivated devices was 2540 W cm2 versus 0.22 W cm2 for the unpassivated diodes. The results of the lifetime, the SRV and the R0A measurements indicate that ammonium sulfide passivation will improve the performance of InAs/GaSb superlattice infrared detectors.

Stimulated Raman scattering in crystals pumped by phase-modulated picosecond pulses

The appearance of hysteresis in the dependence of the efficiency of transient SRS on the energy of picosecond laser excitation is studied. The hysteresis appears due to the frequency-phase modulation of laser radiation which changes the SRS gain with changing the degree of modulation.

Electronic coupling effect on carrier dynamics in InAs/GaAs vertically stacked QD layers

We report on the electronic coupling effect on carrier dynamics in InAs/GaAs vertically stacked quantum dot (QD) layers. For this purpose, both pico-second and continuous-wave excitation techniques have been used. We show that for large numbers of QD deposition cycles (≥10 planes), lateral coupling effects between the neighbouring QDs of the same InAs layer influence remarkably the electronic structure. Lateral coupling of vertically-aligned QDs results in the appearance of a new photoluminescence (PL) band. This new PL line is associated to the radiative recombination of excitons via electronic states induced by laterally coupled QDs (LCQDs). In the 20-layer sample, this effect is shining by dominating the PL spectrum. A strong increase of the radiative lifetime has been obtained for the laterally coupled QDs. Moreover, strong temperature and power excitation dependencies of the PL decay time have been found.