Subnanosecond Electron Research Articles

Sub-nanosecond free carrier recombination in an indirectly excited quantum-well heterostructure

Nanometer-thick quantum-well structures are quantum model systems offering a few discrete unoccupied energy states that can be impulsively filled and that relax back to equilibrium predominantly via spontaneous emission of light. Here we report on the response of an indirectly excited quantum-well heterostructure, probed by means of time and frequency resolved photoluminescence spectroscopy. This experiment provides access to the sub-nanosecond evolution of the free electron density, indirectly injected into the quantum wells. In particular, the modeling of the time-dependent photoluminescence spectra unveils the time evolution of the temperature and of the chemical potentials for electrons and holes, from which the sub-nanosecond time-dependent electron density is determined. This information allows to prove that the recombination of excited carriers is mainly radiative and bimolecular at early delays after excitation, while, as the carrier density decreases, a monomolecular and non-radiative recombination channel becomes relevant. Access to the sub-nanosecond chronology of the mechanisms responsible for the relaxation of charge carriers provides a wealth of information for designing novel luminescent devices with engineered spectral and temporal behavior.

Specific Features of Pulsed Cathodoluminescence under Excitation by Nanosecond and Subnanosecond Electron Beams

Specific Features of Pulsed Cathodoluminescence under Excitation by Nanosecond and Subnanosecond Electron Beams

A Three-Section Subnanosecond Electron Accelerator

—The design of a subnanosecond accelerator composed of three sections and a gas-filled diode and results of its testing are presented. The three sections of the accelerator are a double forming line, a ferrite line, and a line with a variable wave impedance. A beam current amplitude of ≈2.7 kA has been measured behind the anode foil at a pulse width at half-maximum of ≈370 ps. Using this accelerator and a standard spectrometer, Cherenkov radiation and pulsed cathodoluminescence have been investigated in KU-1 fused silica, polymethyl methacrylate, and KBr crystal samples.

Vavilov–Cherenkov Radiation and Pulsed Cathodoluminescence in Poly(methyl methacrylate) Excited by a Subnanosecond Electron Beam

Vavilov–Cherenkov Radiation and Pulsed Cathodoluminescence in Poly(methyl methacrylate) Excited by a Subnanosecond Electron Beam

A Determination of the Relationship between Energies of Vavilov–Cherenkov Radiation and Cathodoluminescence Excited by an Electron Beam in Diamond

The ratio between energies of Vavilov–Cherenkov radiation and cathodoluminescence excited by an electron beam in diamond samples is determined taking into account electron scattering in these samples, energy distribution of beam electrons, ionization losses of the electron energy, and dispersion of the diamond refractive index. Experimental results on measuring spectral characteristics of the glow of natural and synthetic diamond samples under the action of a subnanosecond electron beam with electron energy of up to 200 keV are presented. It is shown that most of the radiation of the diamond samples in the region of 240–750 nm under the action of an electron beam with electron energy of up to 200 keV belongs to cathodoluminescence.

Emission of diamonds, leucosapphire, and KU-1 quartz in the range of 200–800 nm excited by electron beams with a pulse duration of 0.5 and 12 ns

The paper presents the results of Cherenkov radiation (CR) studies in various crystals excited by nanosecond and subnanosecond electron beams with an electron energy of up to 400 and 200 keV, respectively. These studies are motivated by the need to develop and create calibrated sensors of runaway electrons for tokamak-type installations. The spectral and amplitude-time characteristics of the radiation of different specimens of diamonds, leucosapphire, and KU-1 quartz excited by the electron beams were obtained and compared. These crystals were chosen on the basis of their transparency in the ultraviolet region of the spectrum. CR was observed with a spectrometer. An increase in the radiation energy with decreasing wavelength was observed in synthetic diamonds of IIa type, leucosapphire, and KU-1 quartz. The amplitude-time characteristics of the radiation of crystals, including CR, were measured with a photodiode having a pulse response time of up to ≈80 ps. In chemical vapor deposited high purity diamonds excited by both nanosecond and subnanosecond beams, an exciton radiation with a maximum at 235 nm was observed. Results show that leucosapphire and quartz can be used to create detectors of electrons with energies above 200 keV.

Оценка соотношения энергий излучения Вавилова--Черенкова и катодолюминесценции, возбуждаемых электронным пучком в алмазе

AbstractThe ratio between energies of Vavilov–Cherenkov radiation and cathodoluminescence excited by an electron beam in diamond samples is determined taking into account electron scattering in these samples, energy distribution of beam electrons, ionization losses of the electron energy, and dispersion of the diamond refractive index. Experimental results on measuring spectral characteristics of the glow of natural and synthetic diamond samples under the action of a subnanosecond electron beam with electron energy of up to 200 keV are presented. It is shown that most of the radiation of the diamond samples in the region of 240–750 nm under the action of an electron beam with electron energy of up to 200 keV belongs to cathodoluminescence.

Formation of a Train of Picosecond Laser Pulses with a Maximum Power Density to 107 W/cm2 upon Electron-Beam Excitation of a Semiconductor Target

The dependence of the pulse shape of laser radiation of a CdS semiconductor target on the parameters of the exciting subnanosecond electron beam (EB) is studied. The effect of the time to reach the threshold excess carrier density (ECD) on the laser radiation pulse shape is considered. A train of laser radiation pulses with a maximum intensity to 107 W/cm2 is obtained at modulating EB pulses by a frequency of 5 GHz. It is shown that an increase in the ECD during the lifetime can be used to further increase the intensity and to reduce the laser radiation duration in the picosecond range.

Single-shot real-time sub-nanosecond electron imaging aided by compressed sensing: Analytical modeling and simulation

Bringing ultrafast (nanosecond and below) temporal resolution to transmission electron microscopy (TEM) has historically been challenging. Despite significant recent progress in this direction, it remains difficult to achieve sub-nanosecond temporal resolution with a single electron pulse, in real-time (i.e., duration in which the event occurs) imaging. To address this limitation, here, we propose a methodology that combines laser-assisted TEM with computational imaging methodologies based on compressed sensing (CS). In this technique, a two-dimensional (2D) transient event [i.e. (x,y) frames that vary in time] is recorded through a CS paradigm, which consists of spatial encoding, temporal shearing via streaking, and spatiotemporal integration of an electron pulse. The 2D image generated on a camera is used to reconstruct the datacube of the ultrafast event, with two spatial and one temporal dimensions, via a CS-based image reconstruction algorithm. Using numerical simulation, we find that the reconstructed results are in good agreement with the ground truth, which demonstrates the applicability of CS-based computational imaging methodologies to laser-assisted TEM. Our proposed method, complementing the existing ultrafast stroboscopic and nanosecond single-shot techniques, opens up the possibility for single-shot, real-time, spatiotemporal imaging of irreversible structural phenomena with sub-nanosecond temporal resolution.

Spectral and Kinetic Characteristics of the Luminescence of Ga2O3 Crystals Excited by Nano- and Subnanosecond Electron Beams

The spectral and kinetic characteristics of the emission of Sn- and Fe-doped Ga2O3 crystals excited by a nanosecond electron beam are studied. A new cathodoluminescence band of the Fe-doped crystal is revealed with a maximum at λm = 315 nm. Comparison with the data obtained with a subnanosecond duration of the beam current pulse is performed. It is shown that, with increasing duration of the beam current pulse, the main contribution is made by cathodoluminescence in the range of 300–450 nm.

Emission from Crystals Irradiated with a Beam of Runaway Electrons

An investigation of the spectral and amplitude-temporal characteristics of emission from different crystals, promising in terms of their application as detectors of runaway electrons, is performed. This emission is excited by subnanosecond electron beams generated in a gas diode. It is found out that at the electron energies of tens–hundreds of kiloelectronvolts, the main contribution into the emission from CsI, ZnS, type IIa artificial and natural diamonds, sapphire, CaF2, ZrO2, Ga2O3, CaCO3, CdS, and ZnSe crystals comes from the cathodoluminescence; the radiation pulse duration depends on the crystal used and sufficiently exceeds the Cherenkov radiation pulse duration. It is demonstrated that the latter radiation exhibits low intensity and can be detected in the short-wave region of the spectrum in the cases where a monochromator and a high-sensitivity photomultiplier tube (PMT) are used.

A high-current subnanosecond electron accelerator with a gas-filled former

A subnanosecond electron accelerator prototype based on the ARSA small-size accelerator with a gas-filled former (nitrogen ~4 MPa) has been developed and studied. The operation principle of the former involves charging of a short storage line and its discharge to a stepped line with an accelerating tube that generates electrons. The recorded electron-beam current pulse length was t0.5 = 0.3 ns, the current amplitude was at least 1.5 kA, and the maximum electron energy was ~850 keV.

Luminescence of crystals excited by a runaway electron beam and by excilamp radiation with a peak wavelength of 222 nm

This paper presents research data on cathodoluminescence, photoluminescence, and Cherenkov radiation at 200–800 nm excited in crystals with different refractive indices by a subnanosecond runaway electron beam and by KrCl excilamp radiation with a peak wavelength of 222 nm. The data include spectral and amplitude-time characteristics measured with a resolution of up to ∼100 ps for natural and synthetic diamonds of type IIa, sapphire, CsI, ZnS, CaF2, ZrO2, Ga2O3, CaCO3, CdS, and ZnSe. The research suggests that cathodoluminescence and photoluminescence should be accounted for in Cherenkov-type detectors of runaway electrons. The results can be useful for detecting high-energy electrons in tokamaks.

Excitation of high-intensity laser radiation of semiconductor targets by a subnanosecond electron beam

The results of the excitation of СdS semiconductor targets by a subnanosecond electron beam (EB) with an electron energy of 60–230 keV are presented. The maximum intensity of laser radiation from targets for a 1-mm EB diameter exceeded 107 W/cm2 at an efficiency of ~10%. Lasing was initiated at the leading edge of the EB current; laser radiation then reproduced the shape of the excitation pulse. At low excitation levels, a single-mode lasing regime with the wavelength λ = 522 nm was observed. The maximum power of laser radiation (10 MW) was achieved on a multielement CdS semiconductor target. The duration of laser pulses changed in the range of 100–500 ps.

Effects of Post-Assembly Molecular and Atomic Passivation of Sensitized Titania Surface: Dynamics of Electron Transfer Measured from Femtoseconds to Seconds.

The dynamics of electron transfer at the dye-titania and titania-electrolyte interfaces is investigated in two post-sensitization processes: (i) atomic layer deposition of blocking alumina coating and (ii) hierarchical molecular multicapping. To measure the electron transfer dynamics, time-resolved spectroscopic methods (femtosecond transient absorption on the time scale from femtoseconds to nanoseconds and electrochemical impedance spectroscopy on the time scale from milliseconds to seconds) are applied to the complete dye-sensitized solar cells with cobalt-based electrolyte and champion ADEKA-1 dye (with silyl-anchor unit) or its popular carboxyl-anchor analogue, MK-2 dye. Both molecular capping and alumina blocking layers slow down the electron injection process (the average rate constant decreases from 1.1 ps-1 to 0.4 ps-1) and partial sub-nanosecond back electron transfer from titania to the dye (from ca. 10 ns-1 to 5 ns-1). Very small alumina layers (of 0.1 nm thickness) have the highest impact on reducing the rate constants of these electron transfer processes, and for the thicknesses greater than 0.3 nm the rate constants hardly change. In contrast, the electron recombination between titania and electrolyte, occurring on the millisecond time scale, starts to be significantly suppressed for the blocking layers of 0.3 nm or more in thickness (up to ca. 20 times for 0.5 nm thickness with respect to that for untreated sample), improving open circuit voltage and fill factor of the cells. The amplitude of the relative photocurrent (short circuit current per number of absorbed photons) is found to depend almost exclusively on the ultrafast and fast processes taking place in the first nanoseconds after dye excitation. The positive impact of coadsorbents on the solar cells performance for both ADEKA-1 and MK-2 is also studied.

Zero Quantum Coherence in a Series of Covalent Spin-Correlated Radical Pairs

Photoinitiated subnanosecond electron transfer within covalently linked electron donor-acceptor molecules can result in the formation of a spin-correlated radical pair (SCRP) with a well-defined initial singlet spin configuration. Subsequent coherent mixing between the SCRP singlet and triplet ms = 0 spin states, the so-called zero quantum coherence (ZQC), is of potential interest in quantum information processing applications because the ZQC can be probed using pulse electron paramagnetic resonance (pulse-EPR) techniques. Here, pulse-EPR spectroscopy is utilized to examine the ZQC oscillation frequencies and ZQC dephasing in three structurally well-defined D-A systems. While transitions between the singlet and triplet ms = 0 spin states are formally forbidden (Δms = 0), they can be addressed using specific microwave pulse turning angles to map information from the ZQC onto observable single quantum coherences. In addition, by using structural variations to tune the singlet-triplet energy gap, the ZQC frequencies determined for this series of molecules indicate a stronger dependence on the electronic g-factor than on electron-nuclear hyperfine interactions.

On the nature of emissions of polymethyl methacrylate excited by an electron beam of subnanosecond or nanosecond duration

The results of studies of the physical nature of emissions produced in polymethyl methacrylate excited by electron beams of a subnanosecond or a nanosecond duration are presented. The spatial, amplitude, and spectral-kinetic properties of emissions have been examined under an electron beam energy density varying from 10–4 to 4 × 10–1 J/cm2. It has been found that cathodoluminescence is the primary type of emission under low energy densities of the electron beam. When the energy density of a nanosecond electron beam and/or the number of pulses of excitation by a subnanosecond electron beam were increased, an electrical breakdown of polymethyl methacrylate occurred in the irradiated region. This process was accompanied by a burst of emission of dense, low-temperature plasma.

Laser emission efficiency of semiconductor target of gas diode in the picosecond range

Laser radiation excited in a cadmiumsulfide semiconductor target (ST) (λ = 522 nm) by a high-intensity subnanosecond electron beam (EB) with an energy of 70–150 keV has a maximum intensity of 3 · 107W/cm2 at an efficiency of~10%. Lasing arose at the EB exciting pulse front. The laser radiation pulse shape reproduced the EB pulse shape.

Polymethyl methacrylate glow under the influence of runaway electron beams generated in a gas diode

Spectral and amplitude–time characteristics of PMMA radiation under the impact of runaway electrons with subnanosecond duration are studied. The PMMA radiation spectra for a subnanosecond electron beam pulse duration are determined for the first time. The studies show that radiation of the band with a maximum at about 490 nm the intensity of which decreases toward the short-wave spectral region is recorded in the glow spectra. The glow intensity of this band varies proportionally to the number of electrons in the beam, which allows the possibility of using this radiation for determination of the number of high energy electrons in electron beams.

Calibration of detectors of ionizing emissions by means of a subnanosecond runaway electron beam generated by discharge in open atmosphere at high overvoltages

The electric discharge in air at atmospheric pressure under the conditions of multiple overvoltages generates an electron beam of subnanosecond duration. By means of measurements of the resolution time of registration lines including different detectors of ionizing emissions, efficiency is demonstrated of the beam application for calibration of the detectors with the subnanosecond resolution. The basic advantages of such a way of calibration is the absence of the evacuated accelerating tube, small dimensions of the source, and the big resource.