Hot Electron Transport Research Articles

Transport and Energy Deposition of hot Electrons through the Shock Ignition pre Compressed Target

AbstractShock ignition as an alternative scheme of the laser fusion has the potential of achieving efficient implosion. However, hot electrons produced in result of ignitor‐corona interaction may penetrate deep into the fuel making the compression less effective. Transport and energy deposition of hot electron beam into the dense pre compressed of HiPER target by means of Monte Carlo approach are discussed considering the influence of real density and electron beam characteristics. The target parameters before igniting the hot spot have been extracted from a fluid code and used as the initial profile for Monte Carlo simulations. In comparison with simplified step like density profile, electrons penetrate slightly deeper in the case of real shaped density profile. In addition, deposition zone of a broad spectrum electron beam is wider while, monoenergetic electrons depose their energy locally resulting more maximum energy deposition value. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Sub GV/cm terahertz radiation from relativistic laser-solid interactions via coherent transition radiation

Broadband terahertz (THz) radiation with extremely high peak power, generated by the interaction of a femtosecond laser with a thin solid target, has been investigated via particle-in-cell simulations. The spatial (angular) and temporal profiles of the THz radiation reveal that it is caused by the coherent transition radiation emitted when laser-produced hot electrons pass through the front or rear surface of the target. Dependence of the THz radiation on laser and target parameters is studied; it is shown to have a strong correlation with hot electron production. The THz radiation conversion efficiency can be as high as a few times 10^{-3}. This radiation is not only a potentially high power THz source, but may also be used as a unique diagnostic of hot electron generation and transport in relativistic laser-solid interactions.

A simple method to prevent hard X-ray-induced preheating effects inside the cone tip in indirect-drive fast ignition implosions

During fast-ignition implosions, preheating of inside the cone tip caused by hard X-rays can strongly affect the generation and transport of hot electrons in the cone. Although indirect-drive implosions have a higher implosion symmetry, they cause stronger preheating effects than direct-drive implosions. To control the preheating of the cone tip, we propose the use of indirect-drive fast-ignition targets with thicker tips. Experiments carried out at the ShenGuang-III prototype laser facility confirmed that thicker tips are effective for controlling preheating. Moreover, these results were consistent with those of 1D radiation hydrodynamic simulations.

Ballistic electron and photocurrent transport in Au/organic/Si(001) diodes with PDI8-CN2 interlayers

The authors use ballistic electron emission microscopy (BEEM) to probe hot-electron and photocurrent transport in Au/organic/n-Si(001) diodes incorporating the n-type perylene diimide semiconductor PDI8-CN2. For the case of an ultrathin organic interlayer, hot-electron injection is weak and can be detected only at randomly distributed nanosized domains, where BEEM provides electronic barrier heights of ∼0.67 and ∼0.94 eV, respectively. No ballistic transport is detected for devices with a 10 nm-thick interlayer. Regardless of the organic layer thickness, BEEM reveals laterally uniform contributions due to scanning tunneling microscopy-induced photocurrent (STM-PC), with a characteristic energy onset at ∼1.2 eV and a broad intensity peak in the 2–4 eV range. The authors give insight on such spectroscopic features by examination of temperature-dependent spectra and of literature data. This study shows that PDI8-CN2 limits the penetration of Au toward Si, likely due to stiff intermolecular interactions and reactivity of the cyano groups. Moreover, ballistic transmittance is remarkably suppressed and photocurrent transport takes place via defects or recombination centers. Our analysis of electronic and STM-PC fingerprints appears useful for the characterization of several organic-on-inorganic interfaces of interest for heterostructures and devices.

Current gain above 10 in sub-10 nm base III-Nitride tunneling hot electron transistors with GaN/AlN emitter

We report on a tunneling hot electron transistor amplifier with common-emitter current gain greater than 10 at a collector current density in excess of 40 kA/cm2. The use of a wide-bandgap GaN/AlN (111 nm/2.5 nm) emitter was found to greatly improve injection efficiency of the emitter and reduce cold electron leakage. With an ultra-thin (8 nm) base, 93% of the injected hot electrons were collected, enabling a common-emitter current gain up to 14.5. This work improves understanding of the quasi-ballistic hot electron transport and may impact the development of high speed devices based on unipolar hot electron transport.

Studies of high energy density physics and laboratory astrophysics driven by intense lasers

Laser plasmas are capable of creating unique physical conditions with extreme high energy density, which are not only closely relevant to inertial fusion energy studies, but also to laboratory simulation of some astrophysical processes. In this paper, we highlight some recent progress made by our research teams. The first part is about directional hot electron beam generation and transport for fast ignition of inertial confinement fusion, as well as a new scheme of fast ignition by use of a strong external DC magnetic field. The second part concerns laboratory modeling of some astrophysical phenomena, including 1) studies of the topological structure of magnetic reconnection/annihilation that relates closely to geomagnetic substorms, loop-top X-ray source and mass ejection in solar flares, and 2) magnetic field generation and evolution in collisionless shock formation.

Magnetized Fast ignition (MFI) and Laser Plasma Interactions in Strong Magnetic Field

In this paper, magnetized fast ignition (MFI) is proposed for improving the coupling efficiency of a heating laser to a core plasma. In the MFI, the external magnetic field is applied to reduce the hot electron energy and focus the dense hot electron flux to the core. The external magnetic field higher than 100T is generated by the laser driven coil and it is amplified by the implosion. The magnetic field at the tip of the cone is expected to reach higher than 10kT and the laser plasma interaction and the hot electron transport are modified. As the results of applying the external magnetic field, hot electron energy is reduced to less than 5MeV for the laser intensity of 1020W/ cm2 and the Weibel instability is suppressed to collimate the hot electron beam to the core.

Integrated modelling framework for short pulse high energy density physics experiments

Modelling experimental campaigns on the Orion laser at AWE, and developing a viable point-design for fast ignition (FI), calls for a multi-scale approach; a complete description of the problem would require an extensive range of physics which cannot realistically be included in a single code. For modelling the laser-plasma interaction (LPI) we need a fine mesh which can capture the dispersion of electromagnetic waves, and a kinetic model for each plasma species. In the dense material of the bulk target, away from the LPI region, collisional physics dominates. The transport of hot particles generated by the action of the laser is dependent on their slowing and stopping in the dense material and their need to draw a return current. These effects will heat the target, which in turn influences transport. On longer timescales, the hydrodynamic response of the target will begin to play a role as the pressure generated from isochoric heating begins to take effect. Recent effort at AWE [1] has focussed on the development of an integrated code suite based on: the particle in cell code EPOCH, to model LPI; the Monte-Carlo electron transport code THOR, to model the onward transport of hot electrons; and the radiation hydrodynamics code CORVUS, to model the hydrodynamic response of the target. We outline the methodology adopted, elucidate on the advantages of a robustly integrated code suite compared to a single code approach, demonstrate the integrated code suite's application to modelling the heating of buried layers on Orion, and assess the potential of such experiments for the validation of modelling capability in advance of more ambitious HEDP experiments, as a step towards a predictive modelling capability for FI.

Ultrafast and Efficient Transport of Hot Plasmonic Electrons by Graphene for Pt Free, Highly Efficient Visible-Light Responsive Photocatalyst.

We report that reduced graphene-coated gold nanoparticles (r-GO-AuNPs) are excellent visible-light-responsive photocatalysts for the photoconversion of CO2 into formic acid (HCOOH). The wavelength-dependent quantum and chemical yields of HCOOH shows a significant contribution of plasmon-induced hot electrons for CO2 photoconversion. Furthermore, the presence and reduced state of the graphene layers are critical parameters for the efficient CO2 photoconversion because of the electron mobility of graphene. With an excellent selectivity toward HCOOH (>90%), the quantum yield of HCOOH using r-GO-AuNPs is 1.52%, superior to that of Pt-coated AuNPs (quantum yield: 1.14%). This indicates that r-GO is a viable alternative to platinum metal. The excellent colloidal stability and photocatalytic stability of r-GO-AuNPs enables CO2 photoconversion under more desirable reaction conditions. These results highlight the role of reduced graphene layers as highly efficient electron acceptors and transporters to facilitate the use of hot electrons for plasmonic photocatalysts. The femtosecond transient spectroscopic analysis also shows 8.7 times higher transport efficiency of hot plasmonic electrons in r-GO-AuNPs compared with AuNPs.

Luminescence mechanism for Er3+ ions in a silicon-rich nitride host under electrical pumping

A combined experimental and theoretical study on the electroluminescent excitation mechanism for trivalent erbium (Er3+) ions in a silicon-rich nitride (SiNx) host is presented. Direct impact by hot electrons is demonstrated to be the fundamental excitation mechanism. The Er3+ excitation by energy transfer from silicon nanostructures and/or defects is shown to be marginal under electrical pumping. A bilayer structure made of a SiO2 electron-accelerating layer and an Er-implanted SiNx layer has been sandwiched between a metal–insulator–semiconductor structure with a highly doped N-type silicon substrate and an indium–tin–oxide window functioning as a transparent electrode. Monte Carlo (MC) simulations are used to model hot electron transport in the proposed device structure. Acoustic, polar and non-polar optical electron–phonon scattering mechanisms are considered as well as a new scattering process related to the trapping/detrapping on energetically shallow traps in the band gap of silicon nitride. For SiO2 layers around 20 nm-thick and beyond, the number and kinetic energy of hot electrons before entering the SiNx layer are maximal. A significant enhancement of the 1.54 μm electroluminescence power efficiency of two orders of magnitude is observed in devices composed of a 20 nm-thick SiO2 layer compared to those composed of 10 nm-thick SiO2. We demonstrate by MC simulations that such a difference, in terms of power efficiency, is ascribed to the high-energy tail of the hot electron energy distribution, which becomes more pronounced as the SiO2 electron-accelerating layer thickness increases. It is also unveiled that direct excitation of the 1.54 μm Er3+ main radiative transition requiring an excitation energy of only 0.8 eV is inefficient, and that the major part of the Er3+ ions are excited via higher level energy states. The obtained results are sufficiently consistent to be extended to other trivalent rare-earth ions inside similar insulating material environments.

Ionization state of ultra-thin carbon film irradiated by ultra-short intense laser pulse

Ion acceleration is of interest for applications in fast ignition, compact particle sources, medical science, and others. The formation of plasma is of fundamental importance for understanding ion acceleration driven by intense laser. In order to further understand the solid dense material ionization dynamics under ultra-strong field, we use two-dimensional particle-in-cell code to study the ionization process of ultra-thin carbon film, driven by ultra-short intense laser pulse, particularly to see the plasma generation and distribution during the interaction. When an ultra-intense short pulse laser irradiates a solid dense nm-thick film target, the collisional ionization can be ignored for such a thin film target. If the target thickness is larger than laser pulse skin depth, the formation of plasma is contributed from laser field direct ionization and the ionization of electrostatic field inside the target, both of which are discussed and compared by the simulation results in this work. The ionization directly stimulated by laser field happens only near the laser-target interaction surface. After the generation of plasma on the target surface, electrons are accelerated into the target because of laser ponderomotive force. A huge electrostatic field is formed inside the target as a result of hot electron transport in it, and ionizes the target far from the interaction surface. It is found that a bigger fraction of ionization is contributed from electrostatic field ionization inside the target. The effect of laser pulse intensity on ionization is studied in detail, in which the laser pulse intensity is changed from 11018 W/cm2 to 11020 W/cm2. Comparing the results obtained under different intensities, we can see that higher intensity results in higher ionization speed, and much higher-order ions can be generated. At an intensity of 11020 W/cm2, although the intensity much higher than the threshold can generate C+6, only a small part of ions can be ionized into C+6. The reason is that the C+6 ions can be generated directly only by laser field, and the total number of C+6 ions is determined by laser pulse skin depth and spot size. We also consider the effect of laser pulse duration from 30 fs to 120 fs at an intensity of 11020 W/cm2. It is found that higher ionization speed can be obtained, while much less higher-order ions can be generated under shorter laser pulse duration. This description of the generation of solid density plasma driven by intense laser interacting with nm-thick target helps us to further understand the material characteristic under ultra-strong field. This work also benefits the numerical model of plasma in application, namely laser driven ultra-thin film ion acceleration.

Impact of proton implantation parameters on the photoconductivity of photorefractive multiple quantum wells

Semi-insulating GaAs/AlGaAs multiple quantum wells are photorefractive materials with high sensitivity and a short response time. Semi-insulation of these structures is commonly obtained by proton implantation. We present the measurements of photoconductivity in the samples with different proton doses. The results were compared to the theoretically predicted relationship between photoconductivity and the donor to acceptor concentration ratio. This allows to estimate the impact of proton dose on deep donor concentration. Full Text: PDF References D.D. Nolte, "Semi-insulating semiconductor heterostructures: Optoelectronic properties and applications", J. Appl. Phys. 85, 6259 (1999). CrossRef D.D. Nolte, M.R. Melloch, in Photorefractive effects and Materials, ed. D.D. Nolte (Kluwer, Dordrecht, 1995). CrossRef Q. Wang, R.M. Brubaker, D.D. Nolte, M.R. Meloch, "Photorefractive quantum wells: transverse Franz–Keldysh geometry", J. Opt. Soc. Am. B 9, 1626 (1992). CrossRef Q. Wang, RM. Brubaker, D.D. Nolte, "Photorefractive phase shift induced by hot-electron transport: multiple-quantum-well structures", J. Opt. Soc. Am. B 11, 1773 (1994). CrossRef M. Wichtowski, A. Ziolkowski, E. Weinert-Rączka, "A general approach to the space?charge field solution in photorefractive materials in a TWM geometry", J. Opt. 12, 065201 (2010). CrossRef A. Ziółkowski, "Self-bending of light in photorefractive semiconductors with hot-electron effect", Opt. Expr. 22, 4599 (2014). CrossRef K. Seeger, Semiconductors Physics, fifth ed., Chapter 1 (Springer-Verlag, Berlin, 1991). CrossRef S.M. Sze, Physics of Semiconductors Devices, Second ed., Chapter 1 (Wiley, New York, 1981). S. Balasubramanian, I. Lahiri, Y. Ding, M.R. Melloch, D.D. Nolte, "Two-wave-mixing dynamics and nonlinear hot-electron transport in transverse-geometry photorefractive quantum wells studied by moving gratings", Appl. Phys. B 68, 863 (1999). CrossRef P. Yeh in Introduction to Photorefractive Nonlinear Optics, (Wiley, New York, 1993). CrossRef

Short-wavelength experiments on laser pulse interaction with extended pre-plasma at the PALS-installation

AbstractThe paper is a continuation of research carried out at Prague Asterix Laser System (PALS) related to the shock ignition (SI) approach in inertial fusion, which was carried out with use of 1ω main laser beam as the main beam generating a shock wave. Two-layer targets were used, consisting of Cu massive planar target coated with a thin polyethylene layer, which, in the case of two-beam irradiation geometry, simulate conditions related to the SI scenario. The investigations presented in this paper are related to the use of 3ω to create ablation pressure for high-power shock wave generation. The interferometric studies of the ablative plasma expansion, complemented by measurements of crater volumes andKαemission, clearly demonstrate the effect of changing the incident laser intensity due to changing the focal radius on efficiency of laser energy transfer to a shock wave and fast electron emission. The efficiency of the energy transfer increases with the radius of the focused laser beam. The pre-plasma does not significantly change the character of this effect. However, it unambiguously results in the increasing temperature of fast electrons, the total energy of which remains very small (<0.1% of the laser energy). This study shows that the optimal radius from the point of view of 3ω radiation energy transfer to the shock wave is the maximal one used in these experiments and equal to 200 µm that corresponds to the minimal effect of two-dimensional (2D)-expansion. Such a result is typical for the ablation process determined by electron conductivity energy transfer under the conditions of one-dimensional or 2D matter expansion without any appreciable effect due to energy transfer by fast electrons. The 2D simulations based on application of the ALANT-HE code and an analytical model that includes generation and transport of hot electrons has been used to support of experimental data.

Coupled hydrodynamic model for laser-plasma interaction and hot electron generation

We present a formulation of the model of laser-plasma interaction (LPI) at hydrodynamical scales that couples the plasma dynamics with linear and nonlinear LPI processes, including the creation and propagation of high-energy electrons excited by parametric instabilities and collective effects. This formulation accounts for laser beam refraction and diffraction, energy absorption due to collisional and resonant processes, and hot electron generation due to the stimulated Raman scattering, two-plasmon decay, and resonant absorption processes. Hot electron (HE) transport and absorption are described within the multigroup angular scattering approximation, adapted for transversally Gaussian electron beams. This multiscale inline LPI-HE model is used to interpret several shock ignition experiments, highlighting the importance of target preheating by HEs and the shortcomings of standard geometrical optics when modeling the propagation and absorption of intense laser pulses. It is found that HEs from parametric instabilities significantly increase the shock pressure and velocity in the target, while decreasing its strength and the overall ablation pressure.

Time-resolved Kα spectroscopy measurements of hot-electron equilibration dynamics in thin-foil solid targets: collisional and collective effects

Time-resolved Kα spectroscopy measurements from high-intensity laser interactions with thin-foil solid targets are reviewed. Thin Cu foils were irradiated with 1−10 J, 1 ps pulses at focused intensities from 1018 to 1019 W cm−2. The experimental data show Kα-emission pulse widths from 3 to 6 ps, increasing with laser intensity. The time-resolved Kα-emission data are compared to a hot-electron transport and Kα-production model that includes collisional electron–energy coupling, resistive heating, and electromagnetic field effects. The experimental data show good agreement with the model when a reduced ponderomotive scaling is used to describe the initial mean hot-electron energy over the relevant intensity range.

External control of electron energy distributions in a dual tandem inductively coupled plasma

The control of electron energy probability functions (EEPFs) in low pressure partially ionized plasmas is typically accomplished through the format of the applied power. For example, through the use of pulse power, the EEPF can be modulated to produce shapes not possible under continuous wave excitation. This technique uses internal control. In this paper, we discuss a method for external control of EEPFs by transport of electrons between separately powered inductively coupled plasmas (ICPs). The reactor incorporates dual ICP sources (main and auxiliary) in a tandem geometry whose plasma volumes are separated by a grid. The auxiliary ICP is continuously powered while the main ICP is pulsed. Langmuir probe measurements of the EEPFs during the afterglow of the main ICP suggests that transport of hot electrons from the auxiliary plasma provided what is effectively an external source of energetic electrons. The tail of the EEPF and bulk electron temperature were then elevated in the afterglow of the main ICP by this external source of power. Results from a computer simulation for the evolution of the EEPFs concur with measured trends.

Computational study of magnetic field compression by laser-driven implosion

The compression of an external magnetic field by a laser-driven implosion is studied by using the two-dimensional resistive magneto-radiation hydrodynamic simulation code for electron-beam guiding in fast ignition. The simulation results show that it is possible to compress the magnetic field to 1 kT (107 G); however, the strong magnetic field affects the implosion dynamics because of the suppression of the electron heat flux that crosses the strong magnetic field lines. This result suggests that care must be taken to design a target and the initial conditions for fast ignition with an external magnetic field not only to control the hot electron transport but also to form the high-density plasma.

Effect of Foil Target Thickness in Proton Acceleration Driven by an Ultra-Short Laser**supported by the Key Project of Chinese National Programs for Fundamental Research (973 Program) (No. 2011CB808104) and National Natural Science Foundation of China (Nos. 11335013, 11375276, 11105234)

Proton acceleration experiments were carried out by a 1.2×1018 W/cm2 ultra-short laser interaction with solid foil targets. The peak proton energy observed from an optimum target thickness of 7 μm in our experiments was 2.1 MeV. Peak proton energy and proton yield were investigated for different foil target thicknesses. It was shown that proton energy and conversion efficiency increased as the target became thinner, on one condition that the preplasma generated by the laser prepulse did not have enough shock energy and time to influence or destroy the target rear-surface. The existence of optimum foil thickness is due to the effect of the prepulse and hot electron transportation behavior on the foil target.

Monte Carlo simulation of hot carrier transport in III-N LEDs

Recent measurements aiming to resolve the origin of the efficiency droop in III-Nitride (III-N) light-emitting diodes (LEDs) have given invaluable information on the phenomenon and reinforced the interest for detailed device-level simulations. We have developed a microscopic device-level Monte Carlo model of electronic transport in III-N multi-quantum well (MQW) LEDs to analyze their operation in more detail and to increase the understanding of hot electrons generated by Auger recombination and electron overflow. In this work we apply the model to simulate the LED structure studied experimentally by Lin et al. (IEEE Photonics Technol Lett 24(18):1600, 2012) to compare the results to predictions given by the widely used drift-diffusion model and to investigate the relationship between the efficiency droop, Auger recombination, and the transport of hot electrons. To evaluate the reliability of the results and to enable comparison to other works, we also study how some of the most important uncertainties in the material parameters affect the results. In particular, we study how changing the polarization charges, properties of p-type GaN, and the sidevalley energy offset of the conduction band within the range reported in recent literature affects the results. Based on the results we discuss the difficulty to fit device-level models to measurements of the efficiency droop and the incomplete understanding of current transport in III-N MQW structures that might be limiting the development of next-generation optoelectronic devices.

Low-energy electro- and photo-emission spectroscopy of GaN materials and devices

In hot-electron semiconductor devices, carrier transport extends over a wide range of conduction states, which often includes multiple satellite valleys. Electrical measurements can hardly give access to the transport processes over such a wide range without resorting to models and simulations. An alternative experimental approach however exists which is based on low-energy electron spectroscopy and provides, in a number of cases, very direct and selective information on hot-electron transport mechanisms. Recent results obtained in GaN crystals and devices by electron emission spectroscopy are discussed. Using near-band-gap photoemission, the energy position of the first satellite valley in wurtzite GaN is directly determined. By electro-emission spectroscopy, we show that the measurement of the electron spectrum emitted from a GaN p-n junction and InGaN/GaN light-emitting diodes (LEDs) under electrical injection of carriers provides a direct observation of transport processes in these devices. In particular, at high injected current density, high-energy features appear in the electro-emission spectrum of the LEDs showing that Auger electrons are being generated in the active region. These measurements allow us identifying the microscopic mechanism responsible for droop which represents a major hurdle for widespread adoption of solid-state lighting.