Microsecond Range Research Articles

Mapping Long-Lived Dark States in Copper Porphyrin Nanostructures

Long-lived excited states in molecular aggregates are a promising route for efficient energy transfer with potential applications in optoelectronic devices. Spatially resolved optical detection of these states is challenging due to a critical trade-off between sufficiently high photon emission rates and negligible contribution of the luminescence channel to the total lifetime. Here, we report on selective mapping of excited states in copper tetraundecylporphyrin (CuTUP) assemblies on graphite (HOPG) using two-photon photoemission electron microscopy. While the photoemission electron microscopy (PEEM) data are consistent with time-resolved luminescence measurements on a nanosecond time scale, additional long-lived states with lifetimes in the microsecond range are found with nondetectable emission of photons. These dark states serve as initial states in a subsequent photoemission process, giving rise to a high yield and pronounced lateral contrast. In combination with long-range corrected density functiona...

Estimation of biophysical characteristics for Chlamydomonas reinhardtii pigment mutants with an M-PEA-2 fluorometer

Light green pigment mutants with a reduced chlorophyll b content were constructed in the microalga Chlamydomonas reinhardtii Dangeard. A simultaneous recording of the induction curves for prompt and delayed fluorescence and the redox state of P700 in the microsecond range with a M-PEA-2 fluorometer revealed decreases in the quantum yield of electron transport in PS2 (φE 0) and the performance index (PI ABS) and increases in the quantum efficiency of energy dissipation (φD 0) and ΔpH-dependent nonphotochemical quenching (qE and NPQ). The light-dependence curves of the fluorescence parameters confirmed a decrease in the coefficient of maximum utilization of light energy (α) for the mutants. However, the mutants showed an adequate rate of electron transport at a medium light intensity under steady-state conditions. The mutations did not directly affect the oxidation reactions of the PS1 pigment (P700) and the decrease in delayed fluorescence. Experience in using the mutants to test polluted waters of Kazakhstan confirmed that the mutants are promising for use in biomonitoring for mutagens.

Calibration of a silver thin film gauge for short duration convective step heat load

Thin film gauges (TFGs) are a promising candidate for measuring transient heat fluxes in the applications involving very short duration of heating environment. They are basically resistance temperature detectors (RTDs) having the capability of responding in the range of few microseconds. In the present study, a silver thin film gauge (STFG) is fabricated and calibrated in-house with a view to assess the performance of STFGs in dynamic environment. Convective heat load is supplied by a hot-air gun where the heated air jet strikes the gauge and its response is obtained through voltage signal. Subsequently, the surface heat fluxes are estimated by using one dimensional heat conduction modeling. The similar experimental environment is studied to obtain the flow behavior of hot-air jet emanating into atmosphere by using numerical simulations. The self-similar velocities are plotted as well as the interference of outer domain into the experiment parameters has been studied. Ultimately, the surface heat fluxes obtained from various methods are compared to analyse the performance of this hand-made STFG. This study reveals the ability of STFGs to be used in practical short duration transient situations.

Molecular Dynamics at Electrical- and Optical-Driven Phase Transitions: Time-Resolved Infrared Studies Using Fourier-Transform Spectrometers

The time-dependent optical properties of molecular systems are investigated by step-scan Fourier-transform spectroscopy in order to explore the dynamics at phase transitions and molecular orientation in the milli- and microsecond range. The electrical switching of liquid crystals traced by vibrational spectroscopy reveals a rotation of the molecules with a relaxation time of 2 ms. The photo-induced neutral-ionic transition in TTF-CA takes place by a suppression of the dimerization in the ionic phase and creation of neutral domains. The time-dependent infrared spectra depend on temperature and laser pulse intensity; the relaxation of the spectra follows a stretched-exponential decay with relaxation times in the microsecond range strongly dependent on temperature and laser intensity. We present all details of the experimental setups and thoroughly discuss the technical challenges.

Evaluation of number concentration quantification by single-particle inductively coupled plasma mass spectrometry: microsecond vs. millisecond dwell times.

The quality of the quantitative information in single-particle inductively coupled plasma mass spectrometry (SP-ICP-MS) depends directly on the number concentration of the nanoparticles in the sample analyzed, which is proportional to the flux of nanoparticles through the plasma. Particle number concentrations must be selected in accordance with the data acquisition frequency, to control the precision from counting statistics and the bias, which is produced by the occurrence of multiple-particle events recorded as single-particle events. With quadrupole mass spectrometers, the frequency of data acquisition is directly controlled by the dwell time. The effect of dwell times from milli- to microseconds (10ms, 5ms, 100μs, and 50μs) on the quality of the quantitative data has been studied. Working with dwell times in the millisecond range, precision figures about 5% were achieved, whereas using microsecond dwell times, the suitable fluxes of nanoparticles are higher and precision was reduced down to 1%; this was independent of the dwell time selected. Moreover, due to the lower occurrence of multiple-nanoparticle events, linear ranges are wider when dwell times equal to or shorter than 100μs are used. A calculation tool is provided to determine the optimal concentration for any instrument or experimental conditions selected. On the other hand, the use of dwell times in the microsecond range reduces significantly the contribution of the background and/or the presence of dissolved species, in comparison with the use of millisecond dwell times. Although the use of dwell times equal to or shorter than 100μs offers improved performance working in single-particle mode, the use of conventional dwell times (3-10ms) should not be discarded, once their limitations are known.

(Invited) Long-Lived Charge Separated States Exclusively from the Triplet Excited States in Palladium Porphyrin-Fullerene Conjugates

Molecular engineering of donor-acceptor conjugates to control their photoinduced processes to yield the much desired long-lived charge separated states has been one of the major goals in artificial photosynthesis. The charge separated states of sufficient lifetime and stored energy are key in ultimately driving the photocatalytic processes of either light-to-electricity conversion or light-to-fuel production. In simple donor-acceptor dyads, charge separation from the singlet excited donor or acceptor due to rapid charge recombination process results in a short-lived charge separated state of singlet spin character. Utilization of high energy triplet sensitizers instead of the traditionally employed singlet sensitizers could be an elegant approach to generate long-lived charge separated states. The radical ion-pairs of triplet spin character charge recombine to the ground state slowly due to spin forbidden nature of the process. However, witnessing such a process purely from the triplet excited state without the interference of the singlet excited state and other competing photochemical events such as excitation energy transfer has been challenging to establish. Here, we report our strategy to observe electron transfer exclusively from the triplet excited state of the donor in PdP-C60 dyads (see figure below). Using transient absorption spectral studies operating at different timescales and time-resolved electron paramagnetic resonance (TR-EPR) studies, photoinduced electron transfer from 1PdP* to C60 leading to the formation of PdP•+-C60 •- charge separated state of singlet spin character has been witnessed in 2 (instead of triplet-triplet energy transfer observed in 1 1) due to the close proximity of the donor and acceptor entities. In contrast, by lowering the energy of the [(TPA)3PdP]•+-C60 •- state owing to the presence of electron rich TPA entities (facile oxidation) compared to that of 3C60*, electron transfer from 3(TPA)3PdP* to C60 leading to [(TPA)3PdP]•+-C60 •- charge separated state of triplet spin character with lifetime in the microsecond range has been observed in 3 in polar solvents. Results of these findings and its significance will be highlighted. C. Obondi, G. N. Lim, F. D’Souza, Triplet-Triplet Excitation Transfer in Palladium Porphyrin – Fullerene and Platinum Porphyrin – Fullerene Dyads’ J. Phys. Chem. C 2015, 119, 176-185. Figure 1

Relative contributions of various modes of acoustic radiation force to creating mechanical stress in soft tissues

There are several physical effects responsible for the generation of acoustic radiation force (ARF) in soft tissue. The first effect, which is widely used in elastographic applications of ARF, is the transfer of momentum from ultrasonic wave in an attenuating medium due to absorption and scattering. Two other effects include reflection of sound wave from the boundaries between tissues differing in the acoustic impedance and second, spatial variations of the energy density of the propagating wave as a result of variations in the sound wave velocity in a medium. The latter mode of ARF can be directed both outward and toward the source of the propagating wave. In this study, relative contributions of various modes of acoustic radiation force are investigated both theoretically and experimentally. Using Green function for the acoustic potential near the boundaries between tissue structures, the expression for the motion of tissue interfaces under the action of short ultrasonic pulses is derived. Theoretical estimates of dynamics of the soft tissue response to various modes ARF induced by the microsecond range ultrasonic pulses are in agreement with experimental data. Findings of this study may serve as the basis of new approaches in soft tissue elastography.

Pure orange color emitting Sm3+ doped BaNb2O6 phosphor for solid - state lighting applications

Single phase pure orange light emitting BaNb2O6:xSm3+ (0.5≤x≤3.0mol%) phosphors with orthorhombic structure have been synthesized via high temperature solid state reaction method. Crystal structure, morphology, luminescent and decay properties were studied extensively for the synthesized phosphor. The luminescence spectrum of Sm3+ doped BaNb2O6 phosphor exhibits three intense peaks centered at 565, 599 and 645nm corresponding to the 4G5/2→6H5/2, 4G5/2→6H7/2 and 4G5/2→6H9/2 transitions, respectively. The optimum concentration for Sm3+ doped BaNb2O6 is observed as 1.5mol% and the probable energy transfer mechanisms between Sm3+ ions caused concentration quenching have been explained in detail. The calculated Commission Internationale de L’Eclairage (CIE) chromaticity coordinates have been found to be (0.557, 0.440) lies in the pure orange region, which is in the close proximity to the Nichia corporation developed amber LED. The luminescence decay lifetime for 4G5/2 excited level was found to be in the microseconds (µs) range. The Sm3+ doped BaNb2O6 phosphor could be a potential candidate for use in the display and solid - state lighting applications.

Dynamics of high quantum efficiency photoluminescence from N-Si-O bonding states in oxygenated amorphous silicon nitride films

We have reported high internal quantum efficiency (IQE) (∼60%) of photoluminescence (PL) at 470 nm wavelength from oxygenated amorphous silicon nitride (a-SiNx:O) films. In this work, we explored the dynamics of high PL IQE from luminescent N-Si-O bonding states in a-SiNx:O films by using a combination of time resolved PL (TRPL) and temperature dependent PL (TDPL) measurements. The TRPL measurements include time integrated PL, microsecond range PL, and nanosecond range PL measurement modes. The a-SiNx:O films exhibit ns PL decay dynamics that is independent of pumping fluence (WPF) and uniform across the PL spectrum, which is different from the PL decay behavior in a-SiNx films. Particularly, we precisely monitored the temporal evolution of the PL spectra profile to verify that the luminescent N-Si-O bonding states are responsible for the observed blue PL with a radiative recombination rate of ∼108 s−1. Such very fast radiative recombination rate can be compared with that in direct band gap CdSe nanocrystals and can also help us to understand the high PL IQE in a-SiNx:O films. Moreover, by combining the TD-PL lifetimes with the PL IQE values, the temperature dependence of radiative and nonradiative lifetime can be determined.

Design and application of a new control system for tokamak ECRH power supply

ABSTRACTThe biggest challenge of designing and building tokamak electron cyclotron resonance heating (ECRH) pulse step modulation (PSM) power supply is satisfying its required output voltage rising time to be less than 100 µs while suppressing the voltage overshoot to be no more than 1%. To fulfill the two requirements, a new control strategy with startup time in microsecond range is proposed in this paper, and a new control system to realize the control strategy is introduced. The control system was built and tested on 60 kV/50 A ECRH power supply. The experimental results indicate that the control system can restrain the overshoot effectively, increase response speed, and obviously improve the dynamic characteristics of the PSM power supply system. Thus, the proposed control system helps the PSM power supply to meet the design specifications.

Red-emitting manganese-doped aluminum nitride phosphor

We report high efficiency luminescence with a manganese-doped aluminum nitride red-emitting phosphor under 254nm excitation, as well as its excellent lumen maintenance in fluorescent lamp conditions, making it a candidate replacement for the widely deployed europium-doped yttria red phosphor. Solid-state reaction of aluminum nitride powders with manganese metal at 1900°C, 10atmN2 in a reducing environment results in nitrogen deficiency, as revealed diffuse reflectance spectra. When these powders are subsequently annealed in flowing nitrogen at 1650°C, higher nitrogen content is recovered, resulting in white powders. Silicon was added to samples as an oxygen getter to improve emission efficiency. NEXAFS spectra and DFT calculations indicate that the Mn dopant is divalent. From DFT calculations, the UV absorption band is proposed to be due to an aluminum vacancy coupled with oxygen impurity dopants, and Mn2+ is assumed to be closely associated with this site. In contrast with some previous reports, we find that the highest quantum efficiency with 254nm excitation (Q.E.=0.86±0.14) is obtained in aluminum nitride with a low manganese doping level of 0.06mol.%. The principal Mn2+ decay of 1.25ms is assigned to non-interacting Mn sites, while additional components in the microsecond range appear with higher Mn doping, consistent with Mn clustering and resultant exchange coupling. Slower components are present in samples with low Mn doping, as well as strong afterglow, assigned to trapping on shallow traps followed by detrapping and subsequent trapping on Mn.

Effects of millisecond and microsecond pulsed electric fields on red beet cell disintegration and extraction of betanines

The efficacy of PEF treatments—in the range of milliseconds at moderate electric field strengths (0.2–0.6kV/cm) and in the range of microseconds at high electric field strengths (2–6kV/cm)—has been compared in terms of red beet cell disintegration degree (Zp), extraction of betanines and total specific energy requirements.In the range of treatment conditions investigated, the highest Zp value was larger for the treatments applied in the millisecond range than for the treatments applied in the microsecond range. While in the treatment applied at 0.6kV achieved a Zp value higher than 0.95 after 60ms of treatment, the highest Zp value obtained at 6kV/cm in the microsecond range was 0.78 after 75μs of treatment. In the range of 0–20kJ/kg and Zp values among 0–0.75, the disintegration index mainly depended on the total specific energy applied, independently of the electric field strength or the duration of the treatment.The comparison of the maximum betanine extraction yield (BEYmax) in the ms and μs ranges under the conditions in which the maximum extraction was observed revealed that the improvement in the betanine extraction was similar. The more intense treatment conditions applied in the ms range (0.6kV/cm; 40ms) and the μs range (6kV/cm; 150μs) increased the BEYmax 6.6 and 7.2 times, respectively, compared with the control. However, the comparison of the BEYmax in the ms and μs ranges under the same total specific energy revealed that the improvement in the betanine extraction was higher for the treatment applied in the μs range. For example, to obtain a BEYmax of 775μg/g, a total specific energy of 6kJ/kg was required when the treatment was applied in the μs range (75μs; 4kV/cm) and around 20kJ/kg if the treatment was applied in the ms range (40ms, 0.4kV/cm).

Beyond an ‘On-Off’ Activation Model of G-Protein-Coupled Receptors

G-protein-coupled receptors (GPCRs) are the largest family of membrane-bound receptors that transduce the effects of several extracellular stimuli such as hormones, neurotransmitters or light. Structures of inactive as well as active conformations of these seven-transmembrane receptors have been solved recently. To date, fluorescence imaging techniques revealed receptor activation in the millisecond range. However, there is growing evidence of faster time scales in the range of microseconds and beyond (Lohse et al., Curr Opin Cell Biol, 2014, 27, 87-93). As Fluorescence Correlation Spectroscopy (FCS) can capture faster kinetics than common single molecule imaging techniques, it is one method of choice to further investigate GPCR activation.We show FCS experiments of GPCRs in live cells and directly compare confocal FCS with Total Internal Reflection (TIR-) FCS. Our results demonstrate that FCS covers a large range of receptor dynamics and interestingly reveals two distinct diffusion times of the receptor whereas the fast component would hardly be accessible by solely imaging based approaches. These findings—supported by other biophysical studies—suggest that the assumption of a single ‘on-off’ model is too scarce to characterize the GPCR activation mechanism (Manglik & Kobilka, Curr Opin Cell Biol, 2014, 27, 136-43). Complementary scenarios may involve intermediate activation steps on various time scales that are accessible by FCS or related fluorescence fluctuation methods.

Firefly synchronization with phase rate equalization and its experimental analysis in wireless systems

The convergence and precision of synchronization algorithms based on the theory of pulse-coupled oscillators is evaluated on programmable radios. Measurements in different wireless topologies show that such algorithms reach precisions in the low microsecond range. Based on the observation that phase rate deviation among radios is a limiting factor for the achievable precision, we propose a distributed algorithm for automatic phase rate equalization and show by experiments that an improved precision below one microsecond is possible in the given setups. It is also experimentally demonstrated that the stochastic nature of coupling is a key ingredient for convergence to synchrony. The proposed scheme can be applied in wireless systems for distributed synchronization of transmission slots, or sleep cycles.

Advanced optical manipulation of carrier spins in (In,Ga)As quantum dots

Spins in semiconductor quantum dots have been considered as prospective quantum bit excitations. Their coupling to the crystal environment manifests itself in a limitation of the spin coherence times to the microsecond range, both for electron and hole spins. This rather short-lived coherence compared to atomic states asks for manipulations on timescales as short as possible. Due to the huge dipole moment for transitions between the valence and conduction band, pulsed laser systems offer the possibility to perform manipulations within picoseconds or even faster. Here, we report on results that show the potential of optical spin manipulations with currently available pulsed laser systems. Using picosecond laser pulses, we demonstrate optically induced spin rotations of electron and hole spins. We further realize the optical decoupling of the hole spins from the nuclear surrounding at the nanosecond timescales and demonstrate an all-optical spin tomography for interacting electron spin sub-ensembles.

Hybrid Perovskites for Photovoltaics: Charge-Carrier Recombination, Diffusion, and Radiative Efficiencies.

Photovoltaic (PV) devices that harvest the energy provided by the sun have great potential as renewable energy sources, yet uptake has been hampered by the increased cost of solar electricity compared with fossil fuels. Hybrid metal halide perovskites have recently emerged as low-cost active materials in PV cells with power conversion efficiencies now exceeding 20%. Rapid progress has been achieved over only a few years through improvements in materials processing and device design. In addition, hybrid perovskites appear to be good light emitters under certain conditions, raising the prospect of applications in low-cost light-emitting diodes and lasers. Further optimization of such hybrid perovskite devices now needs to be supported by a better understanding of how light is converted into electrical currents and vice versa. This Account provides an overview of charge-carrier recombination and mobility mechanisms encountered in such materials. Optical-pump-terahertz-probe (OPTP) photoconductivity spectroscopy is an ideal tool here, because it allows the dynamics of mobile charge carriers inside the perovskite to be monitored following excitation with a short laser pulse whose photon energy falls into the range of the solar spectrum. We first review our insights gained from transient OPTP and photoluminescence spectroscopy on the mechanisms dominating charge-carrier recombination in these materials. We discuss that mono-molecular charge-recombination predominantly originates from trapping of charges, with trap depths being relatively shallow (tens of millielectronvolts) for hybrid lead iodide perovskites. Bimolecular recombination arises from direct band-to-band electron-hole recombination and is found to be in significant violation of the simple Langevin model. Auger recombination exhibits links with electronic band structure, in accordance with its requirement for energy and momentum conservation for all charges involved. We further discuss charge-carrier mobility values extracted from OPTP measurements and their dependence on perovskite composition and morphology. The significance of the reviewed charge-carrier recombination and mobility parameters is subsequently evaluated in terms of the charge-carrier diffusion lengths and radiative efficiencies that may be obtained for such hybrid perovskites. We particularly focus on calculating such quantities in the limit of ultra-low trap-related recombination, which has not yet been demonstrated but could be reached through further advances in material processing. We find that for thin films of hybrid lead iodide perovskites with typical charge-carrier mobilities of ∼30cm(2)/(V s), charge-carrier diffusion lengths at solar (AM1.5) irradiation are unlikely to exceed ∼10 μm even if all trap-related recombination is eliminated. We further examine the radiative efficiency for hybrid lead halide perovskite films and show that if high efficiencies are to be obtained for intermediate charge-carrier densities (n ≈ 10(14) cm(-3)) trap-related recombination lifetimes will have to be enhanced well into the microsecond range.

Coupling between flexural modes in free vibration of single-walled carbon nanotubes

The nonlinear thermal vibration behavior of a single-walled carbon nanotube (SWCNT) is investigated by molecular dynamics simulation and a nonlinear, nonplanar beam model. Whirling motion with energy transfer between flexural motions is found in the free vibration of the SWCNT excited by the thermal motion of atoms where the geometric nonlinearity is significant. A nonlinear, nonplanar beam model considering the coupling in two vertical vibrational directions is presented to explain the whirling motion of the SWCNT. Energy in different vibrational modes is not equal even over a time scale of tens of nanoseconds, which is much larger than the period of fundamental natural vibration of the SWCNT at equilibrium state. The energy of different modes becomes equal when the time scale increases to the microsecond range.

Polarity Effects of Propylene Carbonate on Breakdown Strength in Microsecond Range

We investigate the polarity effects of the propylene carbonate on the breakdown voltage using the needle-plate electrodes with gaps of 0.5, 1.0, and 2.0 mm. The devices used in this study involve a compact capacitive-energy-storage pulse power source with charging time varying from 5 ms to 20 ms and a test cell with the needle-plate electrodes. The breakdown voltage is recorded by a digital oscilloscope for each gap. The results of these three groups indicate that the positive breakdown voltage is higher than the negative one and the breakdown voltage of the PC increases with the ascending electrode gap. In addition, a simulation is conducted to support this experiment. Some explanations about the polarity effect of the PC are also given.

Temperature dependence of the spall strength and equation of state for austenitic chromium–nickel steel 18-10

The spall strength of austenitic steel 12Kh18N10T was measured for times of ~10–7 s in the temperature range of 77–580 K. The feature at ~140 K, which was caused by the mutual influence of spallation and austenite transformation into martensite under tension, was revealed in the temperature–time dependence of the spall strength. A decrease in the spall strength with an increase in temperature was revealed in the temperature range of 300–580 K, which was interpreted in terms of the kinetic strength concept. The published experimental data on the rupture life at times of ~107 s, which agree with the temperature–time dependence of the spall strength in a microsecond range of loading times, were considered. The equation of state of austenitic steel 18-10 was constructed and used to analyze the temperature history of loading in the compression–unloading–tension during spallation.

Anomalously slow spin dynamics and short-range correlations in the quantum spin ice systemsYb2Ti2O7andYb2Sn2O7

We report a positive muon spin relaxation and rotation (\muSR) study of the quantum spin ice materials Yb2Ti2O7 and Yb2Sn2O7 focusing on the low field response. In agreement with earlier reports, data recorded in small longitudinal fields evidence anomalously slow spin dynamics in the microsecond range below the temperature T_c at which the specific heat displays an intense peak, namely T_c = 0.24 K and 0.15 K, respectively, for the two systems. We found that slow dynamics extends above T_c up to at least 0.7 K for both compounds. The conventional dynamical Gaussian Kubo-Toyabe model describes the \muSR spectra recorded above T_c. At lower temperatures a published analytical extension of the Gaussian Kubo-Toyabe model provides a good description, consistent with the existence of short-range magnetic correlations. While the physical response of the two systems is qualitatively the same, Yb2Ti2O7 exhibits a much larger local magnetic susceptibility than Yb2Sn2O7 below T_c. Considering previously reported ac susceptibility, neutron scattering and \muSR results, we suggest the existence of anomalously slow spin dynamics to be a common physical property of pyrochlore magnetic materials. The possibility of molecular spin substructures to be associated to the slow dynamics and therefore the short-range correlations is mentioned. The slow spin dynamics observed under field does not exclude the presence of much faster dynamics detected in extremely low or zero field.