High Excitation Intensities Research Articles

Origin of Free Charge Carrier Generation of the Second-order Process Photocurrent in Conjugated Polymer Studied by Two-Correlated-Pulse Technique

Kinetics of decay of polaron pairs as precursors of free charge carriers at photoexcitation of poly(3-octylthiophene) under applied electrical field have been observed directly within a picosecond time domain by using the two-correlated-pulse laser technique. Experiments have shown that polaron pairs were generated without any noticeable delay after the light absorption. At high enough excitation intensity, free charge carriers have been shown to take part in the second-order processes photocurrent produced by the recombination of two neighboring polaron pairs. It has been clarified that polaron pairs with two kinds of lifetime τ fast , τ delay exists, and τ delay decreases by high electrical field strength, and that polaron pair is the main origin of the second-order photocurrent. The direct observation of photoluminescence within a picosecond time domain has been also discussed.

Carrier capture and relaxation in InAs quantum dots

We have investigated the carrier capture and relaxation processes inInAs/GaAs self-assembled quantum dots at room temperature bytime-resolved photoluminescence techniques with a high time resolution of∼200 fs. Following the initial fast relaxation in GaAs barriers, we have observed rising processesin time-resolved PL intensity at the energies of quantum dot confined states and thewetting layer. The rising processes are assigned to the carrier capture from the barriers intothe wetting layer and confined states in InAs dots and subsequent relaxationin each detected energy level. We found that the carrier capture rate is fasterthan the intra-dot relaxation within the range of excitation densities that weinvestigated. Under high excitation intensity, the electronic states in the dots werepopulated mainly by carriers directly captured from the barrier. However, at lowexcitation densities, the PL rise times were influenced by the carrier diffusion.

Development of a novel DNA detection system for real-time detection of DNA hybridization

Abstract With the recent development in DNA hybridization technology, it is now possible to directly analyze oligonucleotide hybridization events with a variety of DNA detection systems in a rapid and simple way. Hybridization of the immobilized oligonucleotides with target oligonucleotides in solution will lead to changes in the mass, electrical charge, or optical properties of the immobilized oligonucleotide layer, which can be detected by microgravimetric, potentiometric, amperometric or optical transducers. In this study, we propose a new DNA detection system of the probe oligonucleotide hybridization based on evanescent field method. First, we have fabricated a DNA chip microarray. The 5′-biotinylated probe oligonucleotides were immobilized on the surface of DNA chip microarray. The particles with immobilized oligonucleotides were arranged by random fluidic self-assembly on the pattern chips through hydrophobic interaction. Second, we have detected DNA hybridization using evanescent field microscopy. The hybridization with fluorescence conjugated target oligonucleotides was elicited by interaction with the evanescent field microscopy. With the evanescent field excitation and real-time detection method described here, we suggest that a very sharp discrimination of bulk fluorescence against surface excitation in combination with high excitation intensities can be achieved. Moreover, monitoring DNA hybridization in a large analytic concentration range and various mismatching conditions were also performed in a rapid and simple procedure.

High-bandwidth terahertz radiation from ponderomotively accelerated carriers using Bessel–Gauss femtosecond pulses

Terahertz generation from high intensity excitation of a semiconductor plasma by a radially polarized Bessel–Gauss femtosecond pulse is modeled. The results are compared with Gaussian pulses of equivalent fluence. Due to carrier confinement, a radially polarized Bessel–Gauss pulse exerts a significantly stronger ponderomotive force on photocarriers than a Gaussian pulse, resulting in an order of magnitude increase in the THz emission power.

Photoluminescence and stimulated emission from microcrystallineCsPbCl3films prepared by amorphous-to-crystalline transformation

Highly excited photoluminescence of $\mathrm{Cs}\mathrm{Pb}{\mathrm{Cl}}_{3}$, which is known to be one of the most photoluminescent semiconductors, has been measured for thin films prepared by crystallization from the amorphous phase into microcrystalline/polycrystalline states. With the increase of excitation intensity, the microcrystalline state shows successive jumps of the dominant emission band, from a free-exciton band to its phonon replica and finally to a lowest-energy band originating from exciton-exciton inelastic collision. For the exciton-exciton porcess stimulated emission occurs at very low threshold excitation intensities of the order of $10\phantom{\rule{0.3em}{0ex}}\mathrm{kW}∕{\mathrm{cm}}^{2}$ at $77\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At higher excitation intensities above $50\phantom{\rule{0.3em}{0ex}}\mathrm{kW}∕{\mathrm{cm}}^{2}$, single-path-light-amplification stimulated emission across the film thickness is observed suggesting a very large optical gain. The large-gain mechanism is attributable to giant oscillator strength effect characteristic of excitonic superradiance recently reported for films prepared in the same way.

Dual wavelength InGaN/GaN multi-quantum well LEDs grown by metalorganic vapor phase epitaxy

InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with different well widths and barrier widths were grown on sapphire substrates using metalorganic vapor phase epitaxy (MOVPE). The designed emission wavelengths were in the blue and green regions. The blue MQWs and green MQWs were deposited sequentially and the growth parameters were separately optimized for single color blue and green emission LEDs. Room-temperature photoluminescence (PL) spectra of the dual wavelength samples under high-intensity laser excitation showed dual emission peaks. Electroluminescence (EL) of fabricated LEDs at the typical driving current of 20 mA was predominantly single color, depending on the sequence and the number of blue and green MQWs. The second emission could be enhanced with larger driving current and the relative intensity of the two emission peaks could thus be tuned. Total optical power of more than 7 mW was obtained from an unpackaged 300 μm die when over driven.

Power broadening of the exciton linewidth in a single InGaAs∕GaAs quantum dot

We use high-resolution photocurrent spectroscopy to investigate the ground state of a single quantum dot. In the limit of low optical excitation power, we observe a ground state linewidth down to 4μeV. With increasing excitation intensities, the linewidth shows a characteristic power broadening. This effect is a direct consequence of the saturation of the absorption in a two-level system under conditions of high excitation intensities. From a comparison of both effects, we conclude that power-dependent dephasing is negligible in our system.

Absorption and emission spectroscopic characterisation of the LOV2-His domain of phot from Chlamydomonas reinhardtii

The absorption and emission behaviour of flavin mononucleotide (FMN) in the wild-type light, oxygen and voltage sensitive domain LOV2 of the photoreceptor phot from the green alga Chlamydomonas reinhardtii is studied. Actually a LOV2-His protein (LOV2 domain bound at N-terminal to 15 His aminoacids via a Gly aminoacid) expressed in an Escherichia coli strain is investigated. For fresh samples stored in the dark an initial fluorescence quantum yield of ϕ F = 0.12 ± 0.01 and an effective fluorescence lifetime of τ F = 2.4 ± 0.1 ns are determined. Blue-light photo-excitation generates an intermediate photoproduct (flavin-C(4a)-cysteinyl adduct with absorption peak at 390 nm) resulting in an intensity-dependent fluorescence quenching. In the aqueous solutions at pH 8 approximately 3.8% of the FMN molecules are not bound to the protein binding pocket, whereas 96.2% are non-covalently bound. Even at high-intensity light excitation at 428 nm a fraction of about 7% of the non-covalently bound FMN remains non-converted to an FMN-Cys adduct because of photo-induced back-relaxation of the adduct to non-covalently bound FMN. Two holo-LOV2-His conformations with different adduct recovery time constants are revealed by spectrally and temporally resolved fluorescence and absorption measurements: A fraction of about 48% forms FMN-Cys adducts with a fast recovery time constant of τ Ad,f = 19 ± 2 s in the dark, and the rest forms adducts with a slow recovery time constant of τ F,s = 5.5 ± 1 min. Prolonged blue light irradiation of the flavin-C(4a)-cysteinyl adducts reduces their ability to recover back in the dark to non-covalently bound FMN (photo-induced permanent adduct formation). Numerical simulations of the intensity-dependent absorption depletion reveals a quantum yield of intermediate photo-adduct formation of ϕ Ad = 0.9 ± 0.1. Simulation of the adduct absorption dynamics gives a quantum yield of photo-induced adduct back-relaxation of ϕ Ad,b = 0.15 ± 0.01 and a quantum yield of photo-induced permanent adduct formation of ϕ Ad,p = (2.6 ± 0.5) × 10 −4 .

Directional emission from asymmetric microlaser resonators of π-conjugated polymers

A π-conjugated polymer film was fabricated into an asymmetric microlaser resonator having a quadrapole deformation with irregular boundaries and a Q factor of 600. At high excitation intensities above the threshold for lasing, we observed multimode laser emission spectra and directional emission at four different angles. Chaotic ray dynamics explains the observed emission pattern.

Nonlinear optical properties of 75TeO 2 –20Nb 2 O 5 –5ZnO glasses doped with CeO 2

Nonlinear optical properties of 75TeO2–20Nb2O5–5ZnO glasses doped with CeO2 have been investigated with a self-diffracted time-resolved degenerate four-wave mixing (DFWM) technique at different excitation intensities and lattice temperatures. The DFWM signal exhibits three peaks at higher excitation intensities, where a main peak appears at zero delay time and two rather weak side peaks are located symmetrically at the negative and positive time delay. Due to destructive interferences between the fifth- and third-order polarizations, the line-shape of the main peak around the zero time delay evolves from single peak into a double-peak structure with increasing excitation intensity. Two side peaks emerge at the positive and negative time delay and gradually intensify with increasing excitation intensity or lattice temperature and their positions are independent of the pulse duration, temperature and excitation intensity, which are attributed to the many-body Coulomb interaction.

Vuv fluorescence from selective high-order multiphoton excitation ofN2

Recent fluorescence studies suggest that ultrashort pulse laser excitation may be highly selective. Selective high-intensity laser excitation holds important consequences for the physics of multiphoton processes. To establish the extent of this selectivity, we performed a detailed comparative study of the vacuum ultraviolet fluorescence resulting from the interaction of ${\mathrm{N}}_{2}$ and $\text{Ar}$ with high-intensity infrared ultrashort laser pulses. Both ${\mathrm{N}}_{2}$ and $\text{Ar}$ reveal two classes of transitions, inner-valence $ns\ensuremath{\leftarrow}np$ and Rydberg $np\ensuremath{\leftarrow}{n}^{\ensuremath{'}}{\ensuremath{\ell}}^{\ensuremath{'}}$. From their pressure dependence, we associate each transition with either plasma or direct laser excitation. Furthermore, we qualitatively confirm such associations with the time dependence of the fluorescence signal. Remarkably, only ${\mathrm{N}}_{2}$ presents evidence of direct laser excitation. This direct excitation produces ionic nitrogen fragments with inner-valence $(2s)$ holes, two unidentified transitions, and one molecular transition, the $\mathrm{N}_{2}{}^{+}:X\phantom{\rule{0.3em}{0ex}}^{2}\ensuremath{\Sigma}_{g}^{+}\ensuremath{\leftarrow}\mathrm{C}\phantom{\rule{0.3em}{0ex}}^{2}\ensuremath{\Sigma}_{u}^{+}$. We discuss these results in the light of a recently proposed model for multiphoton excitation.

Hierarchical Self-Assembly ofGaAs/AlGaAsQuantum Dots

A novel structure containing self-assembled, unstrained GaAs quantum dots is obtained by combining solid-source molecular beam epitaxy and atomic-layer precise in situ etching. Photo-luminescence (PL) spectroscopy reveals light emission with very narrow inhomogeneous broadening and clearly resolved excited states at high excitation intensity. The dot morphology is determined by scanning probe microscopy and, combined with single band and eight-band k.p theory calculations, is used to interpret PL and single-dot spectra with no adjustable structural parameter.

Delayed emission from recombination of charge-separated pairs on polyfluorene chains in dilute solution

We report the observation of delayed emission (approximately 400 ps with respect to the excitation pulse) from polyfluorene in dilute solution at high excitation intensities. Photoinduced absorption (with initial amplitude superlinearly dependent on the excitation intensity) is also observed with almost instantaneous onset and subpicosecond initial decay, followed by a slow component that decays on the time scale of 200--400 ps. The results are interpreted in terms of generation of charge-separated pairs via dissociation of a higher-energy excited state (created by sequential absorption within the pump pulse) and subsequent recombination of the charged-separated pairs into neutral excitons, which are responsible for the delayed emission. The insensitivity of these features to the concentration and to the use of different solvents confirms their intrachain origin.

Degenerate two-photon-absorption spectral studies of highly two-photon active organic chromophores.

Degenerate two-photon absorption (TPA) spectral properties of five AFX chromophore solutions have been studied using a single and spectrally dispersed sub-picosecond white-light continuum beam. In a specially designed optical configuration, optical pathways inside the sample solution for different spectral components of the focused continuum beam were spatially separated from each other. Thus, the nondegenerate TPA processes coming from different spectral components can be eliminated, and the direct nonlinear absorption spectrum attributed to degenerate TPA processes can be readily obtained. Using this new technique, the complete TPA spectra for these five highly two-photon-active compounds (AF-380, AF-350, AF-295, AF-270, and AF-50) were obtained in the spectral range from 600 to 950 nm on an absolute scale of TPA cross section. The relationship between the molecular structures and their TPA spectral behaviors are discussed. In general the measured TPA spectra are not identical with the linear absorption spectra on the scale of absorbed photon(s) energy. Moreover, for some sample (such as AF-380), the TPA spectrum is totally different from the linear spectrum, which implies the difference of molecular transition pathways and selection rules for one- and two-photon excitation processes. At high excitation intensity levels (>or=15 GW/cm(2)), the saturation behavior of TPA transition can be observed obviously in AF-350 and AF-380 solutions that exhibit much higher nonlinear absorptivity than the other chromophores investigated.

Magneto collective excitations of armchair carbon nanotubes

Magneto single-particle and collective excitations of armchair carbon nanotubes are studied within the RPA. They strongly depend on the angular momentum ( L), the momentum ( q), and the magnitude ( φ) and the direction ( α) of magnetic field. The L=0 mode has lower excitation frequencies, higher excitation intensity, stronger q-dependence, and smaller critical momentum, as compared with the L=1 mode. The plasmon frequency of L=0 mode increases (decreases) with φ (α) , while the strength exhibits the opposite behavior.

Quantum structures in Zn–Se–Te system containing submonolayer quantities of Te

AbstractIn this paper we show that a multilayer Zn‐Se‐Te system, grown by molecular beam epitaxy in such a way as to contain sub‐monolayer quantities of Te (so‐called delta‐doped ZnSe:Te), exhibits optical properties consistent with the presence of quantum size centers. We present results of excitation intensity and temperature dependent cw photoluminescence (PL) as well as time‐resolved PL measurements that explicitly show that the ensemble of type‐II quantum islands is formed in this system. Specifically, for the 2.46–2.51 eV PL we show that (1) its maximum shifts as a cube root of excitation intensity; (2) its full‐width‐at‐half‐maximum decreases with the temperature while its maximum undergoes a red shift in the temperature interval where the integrated PL intensity stays approximately constant; (3) the PL decay is non‐exponential at high excitation intensities and a single exponential decay at low excitation intensities; (4) the PL life‐time initially increases with temperature from of 86 ns at T = 15 K to 143 ns at T = 115 K before deceasing due to non‐radiative processes. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparison of linear and nonlinear optical spectra of various ZnO epitaxial layers and of bulk material obtained by different experimental techniques

AbstractWe investigate ZnO epitaxial layers grown by MBE (Molecular Beam Epitaxy) and MOVPE (Metal Organic Vapor Phase Epitaxy) techniques. The samples show similar optical behavior in temperature dependent photoluminescence measurements, reflection and photoluminescence excitation spectroscopy in the low density regime. High excitation measurements show different behavior. While the MBE sample leads to stimulated emission from the exciton‐exciton‐scattering, an electron hole plasma is formed in the MOVPE sample which leads to stimulated emission at higher excitation intensities. The gain value measured by the variable stripe length method is much higher for the MBE grown sample. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoionization of [Ru(bpy)3]2+: A Catalytic Cycle with Water as Sacrificial Donor

The photoionization of the tris-2,2‘-bipyridyl ruthenium(II) ion [Ru(bpy)3]2+ in water was investigated by laser-flash photolysis at 308, 355, and 532 nm up to very high excitation intensities Iexc (500. . .900 mJ/cm2). By single-pulse absorption and two-pulse luminescence measurements, it was established that the mechanism is cyclic and comprises three steps: excitation of [Ru(bpy)3]2+ to the metal-to-ligand charge-transfer (MLCT) excited state *[Ru(bpy)3]2+, ionization of *[Ru(bpy)3]2+ by a second photon to give a hydrated electron and the oxidized complex [Ru(bpy)3]3+, and photoreduction of [Ru(bpy)3]3+ by water. No other species is involved in the reaction. Experiments on chemically generated [Ru(bpy)3]3+ yield direct evidence for the photoreduction step and show that it is completed in less than 30 ns. The concentrations of [Ru(bpy)3]2+, *[Ru(bpy)3]2+, [Ru(bpy)3]3+, and at the end of the laser pulse were measured as functions of Iexc. Closed-form expressions for these dependences were derived by a k...

Measurement of optical nonlinearities from intersubband transitions in GaN/AlGaN quantum wells at 1.5 μm

AbstractWe measured the resonant nonlinear optical absorption and refraction due to intersubband transitions in GaN/AlGaN multiple quantum well structures at 1.5 μm. The measured values are much smaller than that predicted by theoretical studies. This is attributed to a larger than expected dephasing of excited electrons. At high excitation intensity, the absorption can be saturated. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical gain in self-assembled ZnO microcrysallite thin films

The lasing properties in ZnO thin films grown by laser molecular beam epitaxy on sapphire substrates have been studies. The excitation dependence of the optical gain spectra has been obtained at room temperature using a variable stripe length method. At moderate excitation intensities, the optical gain responsible for lasing is shown as a result of an exciton–exciton scattering process. At higher excitation intensities, the exciton–hole plasma processes become more dominant and exhibit the highest gain reaching value 571 cm −1 at an excitation density of 280 kW/cm 2. An optical loss in ZnO thin film at room temperature was found to be a low value of 68 cm −1. A lasing threshold of 85 kW/cm 2 has been observed at room temperature.