Avalanche Ionization Research Articles

Effect of standing-wave field distribution on femosecond laser-induced damage of HfO2/SiO2 mirror coating

Single-pulse and multi-pulse damage behaviors of standard (with λ/4 stack structure) and modified (with reduced standing-wave field) HfO2/SiO2 mirror coatings are investigated using a commercial 50-fs, 800-nm Ti:sapphire laser system. Precise morphologies of damaged sites display strikingly different features when the samples are subjected to various number of incident pulses, which are explained reasonably by the standing-wave field distribution within the coatings. Meanwhile, the single-pulse laser-induced damage threshold of the standard mirror is improved by about 14% while suppressing the normalized electric field intensity at the outmost interface of the HfO2 and SiO2 layers by 37%. To discuss the damage mechanism, a theoretical model based on photoionization, avalanche ionization, and decays of electrons is adopted to simulate the evolution curves of the conduction-band electron density during pulse duration.

Heating of nonequilibrium electrons by laser radiation in solid transparent dielectrics

A computer simulation of the heating of nonequilibrium electrons by an intense high-frequency electromagnetic field leading to the bulk damage of solid transparent dielectrics under single irradiation has been carried out. The dependences of the avalanche ionization rate on threshold field strength have been derived. Using the Fokker-Planck equation with a flux-doubling boundary condition is shown to lead to noticeable errors even at a ratio of the photon energy to the band gap ∼0.1. The series of dependences of the critical fields on pulse duration have been constructed for various initial lattice temperatures and laser wavelengths, which allow the electron avalanche to be identified as a limiting breakdown mechanism. The ratio of the energy stored in the electron subsystem to the excess (with respect to the equilibrium state) energy of the phonon subsystem by the end of laser pulse action has been calculated both with and without allowance for phonon heating. The influence of phonon heating on the impact avalanche ionization rate is analyzed.

Evaluation of Nonlinear Absorptivity and Absorption Region in Fusion Welding of Glass using Ultrashort Laser Pulses

Thermal conduction model is presented, by which nonlinear absorptivity of ultrashort laser pulses in internal modification of bulk glass is simulated. The simulated nonlinear absorptivity agrees with experimental values with maximum uncertainty of ±3% in a wide range of laser parameters at 10ps pulse duration in borosilicate glass. The nonlinear absorptivity increases with increasing energy and repetition rate of the laser pulse, reaching as high as 90%. The increase in the average absorbed laser power is accompanied by the extension of the laser-absorption region toward the laser source, suggesting the seed electrons for avalanche ionization are provided mainly by thermal excitation at locations apart from the focus.

Laser-induced damage of hafnia coatings as a function of pulse duration in the femtosecond to nanosecond range

Laser-damage thresholds and morphologies of hafnia single layers exposed under femtosecond, picosecond, and nanosecond single pulses (1030/1064 nm) are reported. The samples were made with different deposition parameters in order to study how the damage behavior of the samples evolves with the pulse duration and how it is linked to the deposition process. In the femtosecond to picosecond regime, the scaling law of the laser-induced damage threshold as a function of pulse duration is in good agreement with the models of photo and avalanche ionization based on the rate equation for free electron generation. However, differences in the damage morphologies between samples are shown. No correlation between the nanosecond and femtosecond/picosecond laser-damage resistance of hafnia coatings could be established. We also report evidence of the transition in damage mechanisms for hafnia, from an ablation process linked to intrinsic properties of the material to a defect-induced process, that exists between a few picoseconds and a few tens of picoseconds.

Laser frequency upshift and self-defocusing under avalanche breakdown of air

A theoretical model of avalanche breakdown of air by a Gaussian laser beam and frequency upshift is developed. The laser beam, below the threshold for tunnel ionization, heats the seed electrons to high energy and initiates avalanche ionization of the air. The ensuing plasma density profile that has maximum on axis and falls off radially causes refraction divergence of the beam. The temporal evolution of plasma density causes self-phase modulation of the laser, causing frequency broadening and spectral emission in the visible.

Nanosecond repetitively pulsed discharges in air at atmospheric pressure—the spark regime

Nanosecond repetitively pulsed (NRP) spark discharges have been studied in atmospheric pressure air preheated to 1000 K. Measurements of spark initiation and stability, plasma dynamics, gas temperature and current–voltage characteristics of the spark regime are presented. Using 10 ns pulses applied repetitively at 30 kHz, we find that 2–400 pulses are required to initiate the spark, depending on the applied voltage. Furthermore, about 30–50 pulses are required for the spark discharge to reach steady state, following initiation. Based on space- and time-resolved optical emission spectroscopy, the spark discharge in steady state is found to ignite homogeneously in the discharge gap, without evidence of an initial streamer. Using measured emission from the N2 (C–B) 0–0 band, it is found that the gas temperature rises by several thousand Kelvin in the span of about 30 ns following the application of the high-voltage pulse. Current–voltage measurements show that up to 20–40 A of conduction current is generated, which corresponds to an electron number density of up to 1015 cm−3 towards the end of the high-voltage pulse. The discharge dynamics, gas temperature and electron number density are consistent with a streamer-less spark that develops homogeneously through avalanche ionization in volume. This occurs because the pre-ionization electron number density of about 1011 cm−3 produced by the high frequency train of pulses is above the critical density for streamer-less discharge development, which is shown to be about 108 cm−3.

Quantum Theory for Cold Avalanche Ionization in Solids

A theory of photon-assisted impact ionization in solids is presented. Our theory makes a quantum description of the new impact ionization-cold avalanche ionization recently reported by P. P. Rajeev, M. Gertsvolf, P. B. Corkum, and D. M. Rayner [Phys. Rev. Lett. 102, 083001 (2009)]. The present theory agrees with the experiments and can be reduced to the traditional impact ionization expression in the absence of a laser.

Characteristics of High-Tension-Induced Corona-Discharge Plasma in Ozone Generator Diode

Electric-field-induced oxygen corona in an innovative r-z diode of ozone generator is investigated. The cathode (K) of the diode is made of several sharp-ended nozzles arranged in many radial planes on an axial mast symmetrically located inside an anode cup (A). The resulting high fields over megavolts per meter produced stray field emissions from nozzle pinnacles, which initiated secondary electron emissions that sustained a corona plume encircling the cathode. Electrons in the plume multiplied through avalanche ionizations, which resulted in electron densities over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> in a self-consistent scheme. This, in a cold corona, formed in a wide A-K gap with a reduced field of E/n <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o2</sub> ~ 100 Td in a pressure of P ~ bar and at a temperature of T = 300 K, gave a high ozone formation of ~ 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . Current transported in short gaps, however, rendered Joule heating and inhibition of ozone genesis.

Effective design of multiple hollow cathode electrode to enhance the density of rf capacitively coupled plasma

Multiple-hole electrode rf capacitively coupled plasma is experimentally studied to determine the optimum condition for high-density plasma discharge. The plasma density was measured at various pressures, hole diameters, rf currents, and gas species conditions. The bulk plasma intrusion in the hole and the ionization avalanche in the sheath region facilitated high-density plasma generation when the diameter of the hole is slightly wider than triple the sheath length. The analytic design of the efficient multihole electrode for high-density rf capacitively coupled plasma discharge will be discussed.

Modeling the effect of native and laser-induced states on the dielectric breakdown of wide band gap optical materials by multiple subpicosecond laser pulses

A model for the multiple-pulse laser-induced breakdown behavior of dielectrics is presented. It is based on a critical conduction band (CB) electron density leading to dielectric breakdown. The evolution of the CB electron density during the pulse train is calculated using rate equations involving transitions between band and mid-gap states (native and laser-induced). Using realistic estimations for the trap density and ionization cross-section, the model is able to reproduce the experimentally observed drop in the multiple-pulse damage threshold relative to the single-pulse value, as long as the CB electron density is controlled primarily by avalanche ionization seeded by multiphoton ionization of the traps and the valence band. The model shows that at long pulse duration, the breakdown threshold becomes more sensitive to presence of traps close (within one photon energy) to the CB. The effect of native and laser-induced defects can be distinguished by their saturation behavior. Finally, measurements of the multiple-pulse damage threshold of hafnium oxide films are used to illustrate the application of the model.

Characteristics of blocking voltage for power 4H-SiC BJTs with mesa edge termination

According to the avalanche ionization theory, a computer-based analysis is performed to analyze the structural parameters of single- and multiple-zone junction termination extension (JTE) structures for 4H-SiC bipolar junction transistors (BJTs) with mesa structure. The calculation results show that a single-zone JTE can yield high breakdown voltages if the activated JTE dose and the implantation width are controlled precisely and a multiple-zone JTE method can decrease the peak surface field while still maintaining a high blocking capability. The influences of the positive and negative surface or interface states on the blocking capability are also shown. These conclusions have a realistic meaning in optimizing the design of a mesa power device.

Exciton-seeded multiphoton ionization in bulkSiO2

In a femtosecond pump-probe experiment the pump pulse injects a modest density of free carriers by multiphoton absorption. Measuring the nonlinear absorption of the probe pulse we observe exciton-seeded multiphoton ionization. The excitons self-trap in ${\text{SiO}}_{2}$ in $&lt;300\text{ }\text{fs}$ following free-carrier injection and decay biexponentially with lifetimes of $34\ifmmode\pm\else\textpm\fi{}8$ and $338\ifmmode\pm\else\textpm\fi{}67\text{ }\text{ps}$ at room temperature. The extent of the probe-pulse absorption provides a model-independent demonstration that avalanche ionization plays a significant role in free-carrier generation by laser pulses as short as 45 fs. Free-carriers injected into a dielectric from extreme-ultraviolet (XUV) sources created by high harmonic or attosecond pulse generation and then avalanched with a perfectly synchronized infrared (fundamental) probe pulse, can provide a route to nanoscale laser machining.

Visible nonlinear band-edge luminescence in ZnSe and CdS excited by a mid-infrared free-electron laser

Visible nonlinear band-edge luminescence in ZnSe and CdS bulk crystals was observed upon excitation by a mid-infrared free-electron laser (mid-IR FEL) at approximately 9 mm. The emission intensity is proportional to the 74th and 45th powers of the excitation intensity for ZnSe and CdS, respectively. For ZnSe, the temporal profile of the emission intensity does not follow the profile of the excitation macropulse of the FEL, but sharply rises and decays only at the maximum of the macropulse profile. These features are in marked contrast to those of a previous report, where the emission profile follows that of the macropulse, and the emission intensity scales with the 4th power of the excitation intensity. The experimental observations were reproduced by a numerical simulation based on impact ionization and avalanche ionization by electrons accelerated by the optical electric field of the FEL. The large nonlinearity in the bandedge emission comes from the macropulse temporal structure, which consists of micropulses densely spaced to allow excited carriers to survive when the next micropulse arrives. They work as seed carriers in the next carrier multiplication step.

α-SiC nanoscale transit-time diodes: performance of the photo-irradiated terahertz sources at elevated temperature

The effects of elevated junction temperature on the terahertz (THz) frequency characteristics of α-(hexagonal, 4H and 6H) silicon carbide (SiC) based double-drift region (DDR, p++ p n n++ type) impact ionization avalanche transit-time (IMPATT) devices are studied and compared for the first time through simulation experiments. This study reveals that at 300 K < T < 600 K, a 4H-SiC IMPATT diode may yield 3.5 W of output power (efficiency (η) ∼ 8.6%) at 1.3 THz, while its 6H-SiC counterpart can deliver 3 W of output power (η = 6.3%) at 1.2 THz. It is interesting to observe that at elevated temperature, the performance of a 6H-SiC IMPATT diode degrades more in comparison with its 4H-SiC counterpart. These comparative analyses reveal the superiority of 4H-SiC diodes over their 6H-SiC counterparts, and thus establish the potential of the former as a high-power THz IMPATT oscillator even in harsh environments. Mobile space charge effects and the effect of positive series resistance on the high-temperature performance of the THz devices are also simulated, and it is found that series resistance reduces the output power level of the diodes by at least 15.0%. Moreover, the effects of increased junction temperature on the photo-sensitivity of top mounted (TM) and flip chip (FC) α-SiC IMPATTs are also investigated using a modified simulation technique. The device operating frequencies, under TM illumination configuration, shifts upward by at least 40.0 GHz, whereas the operating frequency shifts upward by at least 100.0 GHz under FC illumination configuration. The simulation results and the proposed experimental methodology presented here may be used for realizing optically controlled α-SiC transit-time devices for application in THz communication.

Measurement of plasma decay processes in mixture of sodium and argon by coherent microwave scattering

This paper presents the experimental measurement and computational model of sodium plasma decay processes in mixture of sodium and argon by using radar resonance-enhanced multiphoton ionization (REMPI), coherent microwave Rayleigh scattering of REMPI. A single laser beam resonantly ionizes the sodium atoms by means of 2+1 REMPI process. The laser beam can only generate the ionization of the sodium atoms and have negligible ionization of argon. Coherent microwave scattering in situ measures the total electron number in the laser-induced plasma. Since the sodium ions decay by recombination with electrons, microwave scattering directly measures the plasma decay processes of the sodium ions. A theoretical plasma dynamic model, including REMPI of the sodium and electron avalanche ionization (EAI) of sodium and argon in the gas mixture, has been developed. It confirms that the EAI of argon is several orders of magnitude lower than the REMPI of sodium. The theoretical prediction made for the plasma decay process of sodium plasma in the mixture matches the experimental measurement.

Plasma density enhancement in atmospheric-pressure dielectric-barrier discharges by high-voltage nanosecond pulse in the pulse-on period: a PIC simulation

A particle-in-cell (PIC) plus Monte Carlo collision simulation is employed to investigate how a sustainable atmospheric pressure single dielectric-barrier discharge responds to a high-voltage nanosecond pulse (HVNP) further applied to the metal electrode. The results show that the HVNP can significantly increase the plasma density in the pulse-on period. The ion-induced secondary electrons can give rise to avalanche ionization in the positive sheath, which widens the discharge region and enhances the plasma density drastically. However, the plasma density stops increasing as the applied pulse lasts over certain time; therefore, lengthening the pulse duration alone cannot improve the discharge efficiency further. Physical reasons for these phenomena are then discussed.

Ionization oscillations in Hall accelerators

The underlying mechanism of low-frequency oscillations in Hall accelerators is investigated theoretically. It is shown that relaxation oscillations arise from a competition between avalanche ionization and the advective transport of the working gas. The model derived recovers the slow progression and fast recession of the ionization front. Analytical approximations of the shape of current pulses and of the oscillation frequency are provided for the case of large amplitude oscillations.

Calculation of femtosecond pulse laser induced damage threshold for broadband antireflective microstructure arrays

In order to more exactly predict femtosecond pulse laser induced damage threshold, an accurate theoretical model taking into account photoionization, avalanche ionization and decay of electrons is proposed by comparing respectively several combined ionization models with the published experimental measurements. In addition, the transmittance property and the near-field distribution of the 'moth eye' broadband antireflective microstructure directly patterned into the substrate material as a function of the surface structure period and groove depth are performed by a rigorous Fourier model method. It is found that the near-field distribution is strongly dependent on the periodicity of surface structure for TE polarization, but for TM wave it is insensitive to the period. What's more, the femtosecond pulse laser damage threshold of the surface microstructure on the pulse duration taking into account the local maximum electric field enhancement was calculated using the proposed relatively accurate theoretical ionization model. For the longer incident wavelength of 1064 nm, the weak linear damage threshold on the pulse duration is shown, but there is a surprising oscillation peak of breakdown threshold as a function of the pulse duration for the shorter incident wavelength of 532 nm.

Dynamic Analysis of a Si/SiGe-Based Impact Ionization Avalanche Transit Time Photodiode With an Ultrahigh Gain-Bandwidth Product

We investigate the dynamic performance of a Si/SiGe-based impact ionization avalanche transit time photodiode (PD) fabricated on a standard Si substrate that operates at the 830-nm wavelength. The bandwidth-enhancement effect under negative-photoconductance (NPC) operation can greatly relax the internal transit time as well as the tradeoff between the gain and bandwidth performance that characterizes the traditional avalanche PD. Our modeling and measurement results show that the extracted internal resonant frequency increases significantly with the reverse leakage current. By choosing the proper bias voltage in the NPC region, we can simultaneously achieve a wide 3-dB bandwidth (30 GHz), ultrahigh gain-bandwidth product (690 GHz) with a 53.2% external efficiency at unit gain, and clear eye opening at 10 Gb/s.

Nonequilibrium Calculation of High-Temperature Radiating H2-He Flowfield

Chemical kinetics of high-temperature hydrogen-helium gas mixture behind a shock wave is numerically investigated by integrating rate equations for species concentration in time. State-to-state transition rates are used to determine quasi-steady-state rate coefficients for atomic hydrogen ionization. The electron concentrations in front of the shock wave are deduced by solving the radiative heat transfer equation of molecular hydrogen. The computed incubation time of avalanche ionization is compared with the experimental data and those appeared in past studies. It is found that the precursor photoionization and the associative ionization of the molecular hydrogen are important to determine the ionization time behind the shock wave. The present chemical kinetic models are found to reproduce the shock tube experimental data of the ionization time reasonably well.