High Excitation Power Research Articles

Light diffusion and localization in three-dimensional nonlinear disordered media

Using a 3D Finite-Difference Time-Domain parallel code, we report on the linear and nonlinear propagation of light pulses in a disordered assembly of scatterers, whose spatial distribution is generated by a Molecular Dynamics code; refractive index dispersion is also taken into account. We calculate the static and dynamical diffusion constant of light, while considering a pulsed excitation. Our results are in quantitative agreement with reported experiments, also furnishing evidence of a non-exponential decay of the transmitted pulse in the linear regime and in the presence of localized modes. By using an high power excitation, we numerically demonstrate the ``modulational instability random laser'': at high peak input powers energy is transferred to localized states from the input pulse, via third-order nonlinearity and optical parametric amplification, and this process is signed by a power-dependent non-exponential time-decay of the transmitted pulse.

Dielectric surface flashover at atmospheric conditions under high-power microwave excitation

Due to recent advances in the peak output power densities and pulse widths of high-power microwave (HPM) devices, the ability to radiate this power into the atmosphere is limited by surface plasma formation at the vacuum-air interface. Very little is known about this window flashover under HPM excitation at “air” side pressures from atmospheric down to approximately 90Torr, and this paper reports one such study at 2.85GHz and MW∕cm2 pulsed power densities. Due to the high (∼600GHz at standard temperature and pressure) elastic collision frequencies of the electrons with the neutral gas molecules and added energy-loss channels through molecule excitation, proven concepts of vacuum flashover, such as multipactoring electrons, have to be abandoned. The observed flashover field is roughly a factor 3 higher in SF6 compared to air, which is consistent with unipolar volume breakdown data. Quantitative comparisons of HPM flashover data with results from a recently developed computer code are given.

The Fabrication, Fluorescence Dynamics, and Whispering Gallery Modes of Aluminosilicate Microtube Resonators

AbstractIn this paper a novel technique for the production of aluminosilicate microtubes, which are shown to act as optical cylindrical microresonators, is described. The free‐standing microtubes are fabricated by using vacuum‐assisted wetting and filtration of silica gel through a microchannel glass matrix. The microtubes are studied using scanning electron microscopy, micro‐photoluminescence spectroscopy, and fluorescence lifetime imaging confocal microscopy. In the emission spectra of the microresonators we find very narrow periodic peaks corresponding to the whispering gallery modes of two orthogonal polarizations with quality factors up to 3200. A strong enhancement in photoluminescence decay rates at high excitation power demonstrates the occurrence of amplified spontaneous emission from a single microtube. These microtubes show a large evanescent field extending many micrometers beyond the tube radius. Applications for these novel microresonators will be in the areas of microlasers and microsensors and quantum information processing.

Robust Fluorescent Response of Micropatterned Multilayered Films

We demonstrate that photobleaching can be significantly suppressed in fluorescent‐labeled polyelectrolyte layer‐by‐layer (LbL) films by efficient spatial isolation of the dye molecules using multilayered architecture. The films exhibited excellent temporal stability under high excitation power where the bulk films suffered a significant photobleaching. Quenching of the fluorescence caused by the resonant energy transfer between the dye molecules and gold nanoparticles exhibited a weak distance dependence with significant quenching extending beyond 20 nm distance. The robust, freely suspended, fluorescent LbL films exhibited stable fluorescence response under deformation which can make them attractive for sensitive fluoroimmunoassays. In Commemoration of the Contributions of Professor Valery P. Privalko to Polymer Science.

Temperature Dependent Photoluminescence of Self-Organized InAs Quantum Dots on an InGaAs Strain Buffer Layer Grown by MOCVD

In this article, we investigate the formation and optical properties of 3-monolayer (ML) InAs quantum dots (QDs) on the InGaAs strain buffer layer (SBL) grown by metallorganic chemical vapor deposition (MOCVD). As compared to the InAs QDs directly grown on a GaAs surface, the InAs QDs grown on an InGaAs SBL can increase the dot density from 1.7 to 3.5 X 10 10 cm -2 and improve the uniformity. Using microphotoluminescence (μ-PL) measurements, the power-dependent PL at low temperatures shows the ground, first, and second states of InAs QDs due to the state-filling effect at the highest excitation power density of 11900 W/cm 2 . The temperature-dependent PL spectra shows that the PL peak energy has a redshift following the Varshni relation and an unusual temperature dependence of the linewidth, which first reduces and then increases with temperature for the InAs QDs grown on the InGaAs SBL. The PL thermal quenching is determined by the carrier escape from the QDs to the nonradiative recombination centers. The peak wavelength and full width at half maximum of PL spectra at 280 K for the InAs QDs grown on an InGaAs SBL are 1306 nm and 21.5 meV, respectively.

Microwave excited CO2 laser with multiple units using orthogonal electric fields

To realize a CO2 laser oscillator using a fast-axial-flow high-power microwave discharge excitation, we devised a method of making microwave discharge uniform by varying the oscillation direction of an electric field with time and verified the effectiveness of this method. When five orthogonal electric fields units were arranged in series for one laser tube, a maximum laser output power of 609 W and a laser efficiency of 17.0% were achieved for a microwave input power of 3590 W. In addition, using eight laser tubes, namely, 40 orthogonal electric fields units, we achieved a maximum laser output power of 3.6 kW and a laser efficiency of 13% for a microwave input power of 27 kW. Thus, a high-power microwave-discharge-excited CO2 laser has become feasible.

Recent and future developments in low power total reflection X-ray fluorescence spectroscopy

Today, energy-dispersive X-ray fluorescence (EDXRF) spectroscopy with low power X-ray tubes and thermoelectrically cooled detectors provide analytical performance which in the past was restricted to systems with kW power X-ray tubes and liquid nitrogen cooled Si(Li) detectors. Especially for low power TXRF spectrometers, the sensitivity could be improved by an order of one magnitude in the last 5 years. This progress was mainly based on improvements in quantum efficiency of all components and less on high power excitation sources. Recent developments caused further improvements in the analytical sensitivity as well as in the analytical performance. The introduction of a micro-focus X-ray tube increased the excitation power by a factor of 1.3. An additional improvement could be achieved by optimizing the detector window design. By optimizing the detector entrance geometry, the solid detection angle of a silicon drift detector (SDD) was increased by a factor of 1.8. In addition, the recent development of a new generation of silicon drift detectors increased the active detector area by a factor of three enhancing the peak to background ratio by a factor of two. Furthermore, the high-energy efficiency of this new detector type was significantly improved. As a result of all these improvements the detection limit for nickel could be decreased to a value of 1 pg.

Carrier Lifetimes in GaN Revealed by Studying Photoluminescence Decay in Time and Frequency Domains

The carrier dynamics in GaN epilayer, grown using migration enhanced metalorganic chemical vapor deposition (MEMOCVD), is investigated in a wide range of photoexcitation power density of twelve orders of magnitude. Study of photoluminescence decay lifetimes at extremely low excitation power densities of 0.5 mW/cm2 was performed using frequency-domain lifetime measurements with a UV LED (280 nm) as an excitation source. The carrier lifetimes at high excitation power densities was studied using time-resolved photoluminescence (TR PL) spectroscopy and light-induced transient grating (LITG) technique. The measurements were carried out in the temperature range from 10 to 300 K. At high carrier densities, which are typical for device applications, the room- temperature carrier lifetime of 2 ns was obtained, while a variety of decay components was revealed at low excitation intensities. Trapping and detrapping of the carriers/excitons is shown to play an important role in the carrier dynamics at low carrier densities.

Confined optical modes and amplified spontaneous emission from a microtube cavity formed by vacuum assisted filtration

The authors demonstrate a new route to the fabrication of individual aluminosilicate microtubes that can act as micron-scale optical cylindrical resonators. The microtubes were prepared using a simple vacuum assisted wetting and filtration through a microchannel glass matrix. Microphotoluminescence spectra of the microtube cavity show sharp resonant modes with quality factors up to 3200. A strong reduction of the emission decay time at high excitation power confirms the occurrence of amplified spontaneous emission from a single microtube.

Ultrasensitive native fluorescence detection of proteins with miniaturized polyacrylamide gel electrophoresis by laser side‐entry excitation

Direct detection of separated proteins inside polyacrylamide gels has many advantages compared to staining methods. Ultrasensitive native fluorescence detection of proteins with miniaturized 1-D and 2-D PAGE was achieved with laser side-entry excitation. The detection limit for R-phycoerythrin protein spots in 1-D SDS-PAGE with 532 nm excitation was as low as 15 fg, which corresponds to only 40,000 molecules. The average detection limit of six standard native proteins was 5 pg per band with 275 nm excitation. The dynamic range spanned more than three orders of magnitude. By using the same detection setup, approximately 150 protein spots from 30 ng of total Escherichia coli extraction were detected on a 0.8 cm x 1 cm gel in 2-D separation. The significant improvement in sensitivity for laser side-entry excitation comes from higher excitation power and lower background level compared with other excitation modes.

Effect of Nonradiative Recombination on Carrier Dynamics inGaInNAs/GaAs Quantum Wells

The nonradiative recombination effect on carrier dynamics inGaInNAs/GaAs quantum wells is studied by time-resolved photoluminescence(TRPL) and polarization-dependent TRPL at various excitationintensities. It is found that both recombination dynamics and spinrelaxation dynamics strongly depend on the excitation intensity. Undermoderate excitation intensities the PL decay curves exhibit unusualnon-exponential behaviour. This result is well simulated by a rateequation involving both the radiative and non-radiative recombinationsvia the introduction of a new parameter of the effective concentrationof nonradiative recombination centres in the rate equation. In the spindynamics study, the spin relaxation also shows strong excitation powerdependence. Under the high excitation power an increase of spinpolarization degree with time is observed. This new finding provides auseful hint that the spin process can be controlled by excitation powerin GaInNAs systems.

The effect of two-photon absorption and optical excitation area on the generation of THz radiation

It was found experimentally that the THz radiation throughput based on optical rectification is strongly related to the optical excitation size in a ZeTe emitter. The factors affecting the THz radiation throughput are investigated in detail both experimentally and theoretically. By taking into account optical rectification, diffraction and two-photon absorption effects, a theoretical model is established to describe the emitting field intensity of THz radiation. There is excellent agreement between the theoretical results and the experimental data under high excitation power. Both theoretical and experimental results demonstrate that there is a trade-off between these three effects for THz radiation, and that in order to increase the THz radiation throughput based on optical rectification, one should choose suitable excitation size under a given pump power.

Emission characteristics of quantum dots in planar microcavities

The emission properties of single quantum dots in planar microcavities are studied experimentally and theoretically. Fivefold enhanced spontaneous emission outside the microcavity is found for dots in resonance with the cavity mode, relative to detuned dots. Using high-power excitation we obtain the in-plane cavity dispersion. Near-field images of the emission show spatial distributions of several micrometers for resonant dots, which decrease in size with the detuning from resonance. These experimental findings are explained using a simple and intuitive model.

Time-resolved pump probe of 1.55μm InAs∕InP quantum dots under high resonant excitation

We have performed time-resolved resonant pump-probe experiment to study the dynamic response of InAs∕InP quantum dot transitions. A 72-stacked InAs∕InP quantum dot layer sample is grown on (311)B substrate. Photoluminescence at high excitation power reveals ground and excited transitions. Carrier radiative lifetimes and differential transmission are determined under strong excitation powers. The variation of measured carrier radiative lifetimes with increasing excitation powers is attributed to the exciton and biexciton lifetimes difference. The implications of such a difference on differential transmission are discussed, and finally exciton and biexciton lifetimes are measured to be about 1720 and 530ps, respectively.

Ultrafast carrier dynamics in Au∕GaAs interfaces studied by terahertz emission spectroscopy

The measurements of optically generated terahertz emission from Au∕GaAs interfaces are investigated in detail. We observe that, under high laser power excitation, terahertz signals from bare GaAs wafers and Au∕GaAs samples exhibit an opposite polarity. The polarity-flip behaviors are also observed in temperature-dependent and femtosecond pump-generation studies of the Au∕GaAs interfaces.

Triple Luminescence Peaks Observed in the InGaAsN/GaAs Single Quantum Well Grown by Metalorganic Vapor Phase Epitaxy

In this paper, we report the observed triple luminescence peaks of InGaAsN/GaAs single quantum well (SQW) grown by metal organic vapor phase epitaxy (MOVPE). The triplet-peak was discovered by the photoluminescence (PL) measurement performed under low temperature. The saturation of PL-peak intensities at high excitation power suggests the transition of the lower-energy peak was dominated by fewer numbers of available states as compared to the higher-energy peak. The position of the higher-energy peaks remained invariant after rapid thermal annealing (RTA) treatments revealed that the responsible transition was attributed to effects not associated with incorporated nitrogen. In addition, a large blue-shift was observed for the lower-energy peak and was likely attributed to local nitrogen bonding transformation. As results of our observation, the PL spectrum of InGaAsN SQW is mainly composed of a cluster-induced emission peak and two planar-feature peaks.

Multiphoton excitation of fluorescent DNA base analogs

Multiphoton excitation was used to investigate properties of the fluorescent DNA base analogs, 2-aminopurine (2AP) and 6-methylisoxanthopterin (6MI). 2-aminopurine, a fluorescent analog of adenine, was excited by three-photon absorption. Fluorescence correlation measurements were attempted to evaluate the feasibility of using three-photon excitation of 2AP for DNA-protein interaction studies. However, high excitation power and long integration times needed to acquire high signal-to-noise fluorescence correlation curves render three-photon excitation FCS of 2AP not very useful for studying DNA base dynamics. The fluorescence properties of 6-methylisoxanthopterin, a guanine analog, were investigated using two-photon excitation. The two-photon absorption cross-section of 6MI was estimated to be about 2.5 x 10(-50) cm(4)s (2.5 GM units) at 700 nm. The two-photon excitation spectrum was measured in the spectral region from 700 to 780 nm; in this region the shape of the two-photon excitation spectrum is very similar to the shape of single-photon excitation spectrum in the near-UV spectral region. Two-photon excitation of 6MI is suitable for fluorescence correlation measurements. Such measurements can be used to study DNA base dynamics and DNA-protein interactions over a broad range of time scales.

High-power subnanosecond beams of runaway electrons and volume discharge formation in gases at atmospheric pressure

A non-local criterion for runaway electrons generation is proposed, which is a universal two-valued dependence of the critical voltage U cr on pd for a certain gas (where p is the pressure and d is the interelectrode distance). This dependence subdivides the (U, pd) plane into an area of efficient electron multiplication and an area in which electrons leave the gas gap without multiplication. Electron beams of subnanosecond pulse duration and an amplitude of hundreds of amperes are generated at atmospheric pressure in various gases. A volume nanosecond discharge with a high specific excitation power was realized without the pre-ionization of the discharge gap by using an additional external source. The role of the electron avalanches propagating from the cathode to the anode in a dense gas was considered.

Time-resolved emission in CuInS 2 under high excitation

Time-resolved photoluminescence of CuInS 2 crystals has been investigated under high excitation power density. A new observed emission at 1.533 eV at low temperature that is between free exciton and a bound exciton. This emission exhibits very fast decay and can be attributed to radiative recombination of biexciton as determined from the results of excitation power density dependence. The binding energy of biexciton is estimated to be 1.6 meV.

Impact of the capping layers on lateral confinement in InAs∕InP quantum dots for 1.55μm laser applications studied by magnetophotoluminescence

We have used magnetophotoluminescence to study the impact of different capping layer material combinations (InP, GaInAsP quaternary alloy, or both InP and quaternary alloy) on lateral confinement in InAs∕InP quantum dots (QDs) grown on (311)B orientated substrates. Exciton effective masses, Bohr radii, and binding energies are measured for these samples. Conclusions regarding the strength of the lateral confinement in the different samples are supported by photoluminescence at high excitation power. Contrary to theoretical predictions, InAs QDs in quaternary alloy are found to have better confinement properties than InAs∕InP QDs. This is attributed to a lack of lateral intermixing with the quaternary alloy, which is present when InP is used to (partially) cap the dots. The implications of the results for reducing the temperature sensitivity of QD lasers are discussed.