Excitation Pulse Duration Research Articles

Determining the energy balance in barrier-discharge Xe2 excilamp by the pressure jump method

The energy redistribution in barrier-discharge Xe2 excilamp in various excitation regimes is investigated using the pressure jump method. Analytic expressions are derived for calculating power W dissipated in the excilamp discharge plasma in the form of heat and for calculating total discharge heat power PT spent on heating the excilamp. It is shown that the mechanism of the thermal energy dissipation gradually changes upon an increase in the xenon pressure in the excilamp. The conditions for generating the maximal radiation power of the excilamp are determined. It is shown that the maximum of the average radiation power is attained for an excitation pulse duration of 500 ns and the maximal pulse power is attained for a pulse duration of 100 ns. It is found that the optimal operation regime for the excilamp corresponds to the maximal values of the PT–W difference.

Pulsed periodic laser excitation of upconversion luminescence for deep biotissue visualization

Emission spectral properties and quantum efficiency of upconversion particles NaYF4, SrF2, LaF3, BaF2 и CaF2, doped with rare earth ions pair Yb3+–Er3+ were studied using continuous wave (CW) and pulsed periodic excitation modes in the near infrared (NIR) spectral range. Analysis of the obtained results showed that the intensity ratio of upconversion luminescence in green and red spectral ranges depends on excitation pulse duration. Thus, by changing the pulse duration the spectral properties of upconversion luminescence can be controlled. Crystals with higher phonon energy are more sensitive to the change of pumping mode. Interpretation of results was performed on the rate equation model basis. Using numerical methods for all energy levels involved in the upconversion process the population and depopulation dynamics were obtained with respect to the duration of the excitation pulses. It was shown that about 30 ms was required for the complete population of 4F9/2 state, from which the luminescence in the red spectral range occurs. When the pulse duration was less than 30 ms, the 4F9/2 population did not reach a steady state and the intensity of the luminescence in the red part of the spectrum was reduced. The theoretical dependence of the upconversion luminescence intensity in the green and red ranges of the excitation pulse duration for NaYF4:Yb0.2–Er0.02 composition was obtained and demonstrates good agreement with the experimental results.

Indistinguishable single photons with flexible electronic triggering

A key ingredient for quantum photonic technologies is an on-demand source of indistinguishable single photons. State-of-the-art indistinguishable single-photon sources typically employ resonant excitation pulses with fixed repetition rates, creating a string of single photons with predetermined arrival times. However, in future applications, an independent electronic signal from a larger quantum circuit or network will trigger the generation of an indistinguishable photon. Further, operating the photon source up to the limit imposed by its lifetime is desirable. Here, we report on the application of a true on-demand approach in which we can electronically trigger the precise arrival time of a single photon as well as control the excitation pulse duration based on resonance fluorescence from a single InAs/GaAs quantum dot. We investigate in detail the effect of the finite duration of an excitation $\pi$ pulse on the degree of photon antibunching. Finally, we demonstrate that highly indistinguishable single photons can be generated using this on-demand approach, enabling maximum flexibility for future applications.

Sensitivity of a Nanocrystalline Luminescent Thermometer in High and Low Excitation Density Regimes

Two figures of merit are typically taken into account studying rare-earth-doped materials and processes suitable for optical nanothermometry, namely, temperature sensing range and the sensitivity. To optimize the composition of such phosphors and make quantitative comparison between different materials, other factors, such as excitation density and pulse duration, have to been included. Owing to the metastable character of lanthanide excited states, the excitation intensity has a critical importance but has been disregarded so far. Our studies show, based on a new nanocrystalline Er:LiYbP4O12 luminescent thermometer, the influence of the excitation power density on its suitability for temperature sensing. The highest sensitivity was reached for LiYbP4O12:0.1% Er nanocrystals upon pulsed excitation: 2.88%/K at average power below 25 mW/cm2, while the same material displayed lower ∼0.5%/K sensitivity at higher 50–300 mW/cm2 excitation intensities. The mechanism responsible for the observed sensitivity chang...

Radiation Trapping in a Cold and Dense Atomic Ensemble in a Magnetic Field

We report a theoretical investigation of cooperative spontaneous decay of atomic excitation in cold ensemble under condition that average interatomic separation is comparable with resonant light wavelength. We show that magnetostatic field causes some acceleration of decay at shot time interval upon turn off the excitation pulse and essential decreasing of decay rate of long-lived sub-radiant states. The dependence of this effect on duration of excitation pulse as well as on its carrier frequency is considered.

Rotational spectroscopy and three-wave mixing of 4-carvomenthenol: A technical guide to measuring chirality in the microwave regime.

We apply chirality sensitive microwave three-wave mixing to 4-carvomenthenol, a molecule previously uncharacterized with rotational spectroscopy. We measure its rotational spectrum in the 2-8.5 GHz range and observe three molecular conformers. We describe our method in detail, from the initial step of spectral acquisition and assignment to the final step of determining absolute configuration and enantiomeric excess. Combining fitted rotational constants with dipole moment components derived from quantum chemical calculations, we identify candidate three-wave mixing cycles which were further tested using a double resonance method. Initial optimization of the three-wave mixing signal is done by varying the duration of the second excitation pulse. With known transition dipole matrix elements, absolute configuration can be directly determined from a single measurement.

CHARGE RELAXATION BASED INTEGRAL–DIFFERENTIAL METHOD OF SEMICONDUCTOR ENERGY LEVEL TEMPERATURE SPECTROSCOPY

A new method of measuring the parameters of shallow and medium−depth levels in semiconductor band gaps has been presented. The method is based on temperature scanning, hardware integration and subsequent differentiation by the duration of the relaxation charge excitation pulse of the energy levels during the application of a small amplitude displacement meander to the barrier structure. Experimental results of research AlGaN/ InGaN/GaN of the structures are resulted.

20 THz broadband generation using semi-insulating GaAs interdigitated photoconductive antennas.

We demonstrate broadband (20 THz), high electric field, terahertz generation using large area interdigitated antennas fabricated on semi-insulating GaAs. The bandwidth is characterized as a function of incident pulse duration (15-35 fs) and pump energy (2-30 nJ). Broadband spectroscopy of PTFE is shown. Numerical Drude-Lorentz simulations of the generated THz pulses are performed as a function of the excitation pulse duration, showing good agreement with the experimental data.

Excitation of oscillations of an immiscible liquid interface by a pulsed ultrasound beam parallel to the interface

For several pairs of immiscible liquids, a new opportunity to excite oscillations of their interface by ultrasound pulses propagating parallel to the interface has been discovered experimentally. A plane ultrasound transducer is placed so that the interface between liquids halves its aperture. The evolution of the shape of the interface oscillations under the variation of the amplitude and duration of excitation pulses, as well as of the distance from the transducer, has been analyzed. The possibility of the excitation of various modes of the interface oscillations in a bounded volume has been revealed.

Fluence-Dependent Singlet Exciton Dynamics in Length-Sorted Chirality-Enriched Single-Walled Carbon Nanotubes

We utilize individualized, length-sorted (6,5)-chirality enriched single-walled carbon nanotubes (SWNTs) having dimensions of 200 and 800 nm, femtosecond transient absorption spectroscopy, and variable excitation fluences that modulate the exciton density per nanotube unit length, to interrogate nanotube exciton/biexciton dynamics. For pump fluences below 30 μJ/cm(2), transient absorption (TA) spectra of (6,5) SWNTs reveal the instantaneous emergence of the exciton to biexciton transition (E11 → E11,BX) at 1100 nm; in contrast, under excitation fluences exceeding 100 μJ/cm(2), this TA signal manifests a rise time (τ rise ∼ 250 fs), indicating that E11 state repopulation is required to produce this signal. Femtosecond transient absorption spectroscopic data acquired over the 900-1400 nm spectral region of the near-infrared (NIR) region for (6,5) SWNTs, as a function of nanotube length and exciton density, reveal that over time delays that exceed 200 fs exciton-exciton interactions do not occur over spatial domains larger than 200 nm. Furthermore, the excitation fluence dependence of the E11 → E11,BX transient absorption signal demonstrates that relaxation of the E11 biexciton state (E11,BX) gives rise to a substantial E11 state population, as increasing delay times result in a concomitant increase of E11 → E11,BX transition oscillator strength. Numerical simulations based on a three-state model are consistent with a mechanism whereby biexcitons are generated at high excitation fluences via sequential SWNT ground- and E11-state excitation that occurs within the 980 nm excitation pulse duration. These studies that investigate fluence-dependent TA spectral evolution show that SWNT ground → E11 and E11 → E11,BX excitations are coresonant and provide evidence that E11,BX → E11 relaxation constitutes a significant decay channel for the SWNT biexciton state over delay times that exceed 200 fs, a finding that runs counter to assumptions made in previous analyses of SWNT biexciton dynamical data where exciton-exciton annihilation has been assumed to play a dominant role.

Modifying the size distribution of microbubble contrast agents for high-frequency subharmonic imaging

Subharmonic imaging is of interest for high frequency (>10 MHz) nonlinear imaging, because it can specifically detect the response of ultrasound contrast agents (UCA). However, conventional UCA produce a weak subharmonic response at high frequencies, which limits the sensitivity of subharmonic imaging. We hypothesized that modifying the size distribution of the agent can enhance its high-frequency subharmonic response. The overall goal of this study was to investigate size-manipulated populations of the agent to determine the range of sizes that produce the strongest subharmonic response at high frequencies (in this case, 20 MHz). A secondary goal was to assess whether the number or the volume-weighted size distribution better represents the efficacy of the agent for high-frequency subharmonic imaging. The authors created six distinct agent size distributions from the native distribution of a commercially available UCA (Targestar-P®). The median (number-weighted) diameter of the native agent was 1.63 μm, while the median diameters of the size-manipulated populations ranged from 1.35 to 2.99 μm. The authors conducted acoustic measurements with native and size-manipulated agent populations to assess their subharmonic response to 20 MHz excitation (pulse duration 1.5 μs, pressure amplitudes 100-398 kPa). The results showed a considerable difference between the subharmonic response of the agent populations that were investigated. The subharmonic response peaked for the agent population with a median diameter of 2.15 μm, which demonstrated a subharmonic signal that was 8 dB higher than the native agent. Comparing the subharmonic response of different UCA populations indicated that microbubbles with diameters between 1.3 and 3 μm are the dominant contributors to the subharmonic response at 20 MHz. Additionally, a better correlation was observed between the subharmonic response of the agent and the number-weighted size-distribution (R2=0.98) than with the volume-weighted size distribution (R2=0.53). Modifying the size distribution of the agent appears to be a viable strategy to improve the sensitivity of high-frequency subharmonic imaging. In addition, when the size distribution of the UCA has not been suitably modified, the number-weighted size distribution is a useful parameter to accurately describe the efficacy of the agent for high-frequency subharmonic imaging.

Trapping of a mie sphere by acoustic pulses: effects of pulse length.

The acoustic counterpart of optical tweezers shows great promise as a single-particle manipulator using a highly focused acoustic beam. Understanding the dependence of the trapping performance of the acoustic beam on the acoustic pulse length may facilitate its development and extend the applications. Herein, we propose a ray-based model for the time-course simulation of instantaneous forces exerted on single Mie spheres by highly focused acoustic pulses of arbitrary lengths. The simulations considered single fat/lipid spheres with a density of 950 kg/m3 and speed of sound of 1450 m/s, suspended in water and located on the beam axis. Simulation was used to establish the spatial and temporal pressure data of pulsed acoustic fields transmitted from a 100-MHz transducer with a half-power bandwidth of 50% and an f-number of 1. The instantaneous intensity vectors were calculated to represent rays for estimating forces exerted by consecutive wave-particle interactions. The results suggest that short acoustic pulses can exert negative forces pulling spheres beyond the focus in the direction opposite to that of wave propagation. Varying the excitation pulse duration has no effect on the region where the exerted forces are averagely negative. Lengthening the excitation pulse duration rapidly increases the amplitude of the average force. A smaller sphere experiences a greater average force when the spatial length of a transmitted acoustic pulse is comparable to the sphere diameter. The amplitude of the instantaneous force can be maximized as long as the acoustic pulse length is longer than the sphere diameter. Regulating the relation between acoustic pulse length and sphere size may be advantageous in particle sorting applications.

Influence of excitation pulse duration of dielectric barrier discharges on biomedical applications

Two dielectric barrier discharges created in atmospheric pressure air were compared to investigate influences of excitation pulse duration on plasma parameters. A plasma source with a pulsed excitation and pulse durations in the µs range as well as a source with pulse durations in the ns range were investigated. An aluminum plate with skin lipids of the stratum corneum on the one hand and an aluminum needle without lipids for operating in the single filamentary mode on the other hand were used as opposite electrodes.The optical emission spectroscopy was arranged to determine the rotational and vibrational temperatures by comparing experimental with simulated spectra. Vibrational temperatures were calculated in a range 2200–2600 K, rotational temperatures were measured from 300 up to 600 K. In addition, the electron temperatures (7–15 eV) and the reduced electric fields (280–800 Td) were estimated. Electric parameters were detected by both current and voltage measurements with a resulting range 200–500 mW of dissipated power.

X-ray induced optical reflectivity

The change in optical reflectivity induced by intense x-ray pulses can now be used to study ultrafast many body responses in solids in the femtosecond time domain. X-ray absorption creates photoelectrons and core level holes subsequently filled by Auger or fluorescence processes, and these excitations ultimately add conduction and valence band carriers that perturb optical reflectivity. Optical absorption associated with band filling and band gap narrowing is shown to explain the basic features found in recent measurements on an insulator (silicon nitride, Si3N4), a semiconductor (gallium arsenide, GaAs), and a metal (gold, Au), obtained with ∼100 fs x-ray pulses at 500-2000 eV and probed with 800 nm laser pulses. In particular GaAs exhibits an abrupt drop in reflectivity, persisting only for a time comparable to the x-ray excitation pulse duration, consistent with prompt band gap narrowing.

Pulsed cathodoluminescence of calcite crystals of various origins

We have studied the luminescence of calcites from phlogopite-calcite veins, marbles, leucogranites, apatite-calcite ores, and carbonatites in the spectral range 300–800 nm under excitation with nanosecond electron pulses at an electron beam current density of ∼10 A/cm2. All of the calcite varieties have a fast emission component, with a broad spectrum in the near-UV and visible spectral regions, whose decay time is comparable to the excitation pulse duration, and a slow emission component, with a peak-emission wavelength of 610–620 nm and decay time of tens of milliseconds. The spectral composition of the emission depends on the nature of the calcite sample. We analyze the evolution of the spectrum after excitation and discuss the luminescence excitation mechanisms and the nature of the emission centers.

In vivo free induction decay based 3D multivoxel longitudinal hadamard spectroscopic imaging in the human brain at 3 T.

We propose and demonstrate a full 3D longitudinal Hadamard spectroscopic imaging scheme for obtaining chemical shift maps, using adiabatic inversion pulses to encode the spins' positions. The approach offers several advantages over conventional Fourier-based encoding methods, including a localized point spread function; no aliasing, allowing for volumes of interest smaller than the object being imaged; an option for acquiring noncontiguous voxels; and inherent outer volume rejection. The latter allows for doing away with conventional outer volume suppression schemes, such as point resolved spectroscopy (PRESS) and stimulated echo acquisition mode (STEAM), and acquiring non-spin-echo spectra with short acquisition delay times, limited only by the excitation pulse's duration. This, in turn, minimizes T2 decay, maximizes the signal-to-noise ratio, and reduces J-coupling induced signal decay. Results are presented for both a phantom and an in vivo healthy volunteer at 3 T.

Optimization of the Field Enhancement and Spectral Bandwidth of Single and Coupled Bimetal Core–Shell Nanoparticles for Few-Cycle Laser Applications

We have theoretically studied and optimized the field enhancement and temporal response of single and coupled bimetal Ag/Au core–shell nanoparticles (NPs) with a diameter of 160 nm and compared the results to pure Ag and Au NPs. Very high-field enhancements with an amplitude reaching 100 (with respect to the laser field centered at 800 nm) are found at the center of a 2-nm gap between Ag/Au core–shell dimers. We have explored the excitation of the bimetal core–shell particles by Fourier transform-limited few-cycle optical pulses and identified conditions for an ultrafast plasmonic decay on the order of the excitation pulse duration. The high-field enhancement and ultrafast decay makes bimetal core–shell particles interesting candidates for applications such as the generation of ultrashort extreme ultraviolet radiation pulses via nanoplasmonic field enhancement. Moreover, in first experimental studies, we synthesized small bimetal Ag/Au core–shell NPs and compared their optical response with pure Au and Ag NPs and numerical results.

Optimization of decay kinetics of YAG:Ce single crystal scintillators for S(T)EM electron detectors

To enhance the detective quantum efficiency (DQE) of scintillation electron detectors for scanning electron microscopes or scanning transmission electron microscopes (S(T)EM), the cathodoluminescence (CL) decay kinetics of YAG:Ce single crystal scintillators is studied in this paper. Some possibilities for reductions of decay time and afterglow of the mentioned scintillators are presented. The pulse mode utilizing a blanking system and 10 keV electrons for the excitation and a sampling oscilloscope for the CL detection were used for the measurement of decay characteristics. The CL decay characteristics of YAG:Ce single crystals of different Ce concentrations, pulled by the Czochralski method and cleaned and annealed under specific conditions, were measured in dependence on excitation pulse duration and on the specimen temperature. To interpret the presented results, a kinetic model of cathodoluminescence of the YAG:Ce single crystals was created. The effects of quenching impurities and of defect centers in YAG:Ce were specified.

Effect of excitation level on the photoluminescence of barium thiogallate activated with europium and cerium ions

The photoluminescence of barium thiogallates doped with Eu 2+ , Ce 3+ , and Eu 2+ + Ce 3+ ions is studied over a wide range of excitation levels (10 –3 –10 6 W/cm 2 ). Introduction of 3 at.% Eu and 3 at.% Ce instead of 5 at.% Eu into a BaGa 2 S 4 matrix doubles the luminescence quantum yield of the phosphor. Doped BaGa 2 S 4 exhibits a high linearity in its luminescence intensity as a function of excitation level (a constant efficiency) up to 2·10 4 W/cm 2 for excitation pulse durations of 8 ns, which corresponds to cw pumping at a power density of about 5·10 2 W/cm 2 in terms of the concentration of excited ions. It is shown that using BaGa 2 S 4 :Eu,Ce along with the “yellow” phosphor of a Nichia NS6L083 LED may increase its color rendering index from 0.64 to 0.80 with no reduction in its luminous efficiency.

A pulse nuclear quadrupole resonance spectrometer

A pulse nuclear quadrupole resonance (NQR) spectrometer intended for studying the resonance signal spectra of nuclei of solid substances in a 1- to 30-MHz band is described. The pulse duration of excitation pulses is regulated from 10 to 100 μs at a 5- to 100-Hz pulse repetition rate. The spectrometer ensures a probing pulse power of up to 1.5 kW and is characterized by the availability of an NQR signal simulator, a low-noise preamplifier, and a phase detector with a dynamic range exceeding 100 dB. The efficiency of the spectrometer has been tested by studying the NQR spectrum of 14N nuclei in urotropine. The spectrometer ensured a possibility of detecting 0.2 g of urotropine.