Avalanche Ionization Research Articles

Mechanism of metastable krypton atom preparation via laser-induced ionization

Preparation of metastable Kr atoms in the 5s[3/2]2 level via laser-induced ionization has been achieved. The temporal evolution of the intensity of Kr atomic spectral lines at 760.15 nm, 811.29 nm, and 431.96 nm was used to elucidate the production mechanisms of metastable Kr atoms. These mechanisms primarily involve two processes: the “excitation + radiation” process, dominated by multiphoton excitation and initial plasma-induced electron impact excitation, and the “ion-electron recombination” process, governed by avalanche ionization. The decay time constants of Kr atomic spectral lines, corresponding to the “excitation + radiation” and “ion-electron recombination” processes respectively, were obtained experimentally under both strong and weak ionization conditions. The experiments revealed delay in preparations of metastable Kr atoms between these two processes. To reduce the loss of metastable Kr atoms and effectively utilize their peak concentration, we drew inspiration from metastable rare gas lasers and proposed the “cycling” idea to keep metastable Kr atoms produced by these two processes as synchronized as possible. We used 811.29 nm laser to excite metastable Kr atoms generated rapidly during the “excitation + radiation” stage to the 5p[5/2]3 level. The Kr atoms returned to the 5s[3/2]2 level through spontaneous radiation, merging with metastable Kr atoms that were slowly produced during the “ion-electron recombination” stage. We hope that the “cycling” idea can shorten the delay in preparations of metastable Kr atoms from both processes and enhance the peak concentration of metastable Kr atoms. However, the experimental results didn't meet expectations, as we observed a decrease in the 811.29 nm fluorescence after laser excitation, attributed to the accumulation of 5p[5/2]3 level Kr atoms. These atoms undergo energy pooling to populate the 4d’[3/2]1 and 5d[7/2]3 levels, followed by absorption of 811.29 nm laser energy leading to photoionization. Reducing the concentration of 5p[5/2]3 level Kr atoms helps mitigate the reionization issue.

Potentiality of graphene as a base material for impact ionization avalanche transit time diode in high-frequency applications

In this paper, the microwave application potential of graphene is studied using a double-drift-region (DDR) impact ionization avalanche transit time (IMPATT) diode. The simulation of this diode is carried out for the very first time at several different atmospheric window frequencies. Because graphene has unique and special properties, it could be used to make electronic gadgets for the next generation. The device is simulated at a variety of millimeter and sub-millimeter wave frequencies using a model called self-consistent drift diffusion (SCDD), which was developed by the author based on current continuity, Poisson’s equation and space charge equation. When compared to traditional IMPATT devices such as Si, GaAs, InP and GaP, the results demonstrate superior performance in terms of efficiency, and RF power across a wide range of operating conditions. Again, the behavior of noise in graphene IMPATT is studied, and it is found that it makes less noise than Si and GaAs IMPATT. The simulation results open up new avenues for IMPATT diode manufacture and design.

Dynamics of femtosecond lasers and induced plasma in non-Kerr nonlinear transparent materials: Competing effects of electron–hole radiative recombinations and single-electron diffusions

Modern laser micromachining utilizes ultrashort optical fields, such as femtosecond lasers, to perform high-precision processings on solid materials, including cutting, drilling, ablation, polishing, and scripturing. Femtosecond laser-based material processings can induce a plasma of free electrons whose density depends on physical phenomena such as single-electron diffusions, multiphoton ionization, and electron–hole radiative recombinations. In this work, we examine the dynamics of femtosecond lasers in transparent materials with non-Kerr nonlinearity, taking into account the generation of an electron plasma. In these specific materials, a balance between the nonlinearity and the group-velocity dispersion of the optical medium can favor the formation of optical filaments propagating with a permanent shape by virtue of their “solitonic” features. We are interested in the effects of the competition between electron–hole radiative recombination and single-electron diffusion processes on the spatiotemporal profiles of the propagating optical field and of the plasma density. The model features a complex Ginzburg–Landau equation with an optical nonlinearity of a general saturable form and a Kth-order nonlinearity term accounting for K-photon ionization processes, coupled to a rate equation for the electron plasma density where the present terms are representing avalanche ionizations, single-electron diffusion, and electron–hole radiative recombination processes. The modulational-instability analysis suggests that the continuous-wave regime will be stabilized by strong electron–hole radiative recombination processes for a fixed value of the single-electron diffusion coefficient, a stability enhanced by an increase in K. In the nonlinear regime, numerical simulations of the model equations for different combinations of the nonlinearity-saturation exponents and different values of the photon number K unveil soliton train structures forming from the laser field propagation and the time evolution of the plasma density. These structures turn out to be either dissipative soliton trains in the absence of electron–hole radiative recombinations or soliton crystals when electron–hole radiative recombination processes are taken into consideration to balance the damping effect caused by single-electron diffusions.

Direct observation of radio-frequency negative differential resistance in GaN-based single drift region IMPATT diodes

We report the direct observation of radio-frequency negative differential resistance, via on-wafer S-parameter measurements, in GaN-based impact ionization avalanche transit time (IMPATT) diodes. Clear signatures of reflection gain are observed from 18.7 to 30.6 GHz. These observations have been made possible by suppressing the reverse leakage current (and thereby parasitic shunt conductance) by optimization of the fabrication process, in conjunction with the use of pulsed measurements to suppress device self-heating. Consistent with avalanche-dominated behavior, the measured DC reverse bias current–voltage measurements show a positive temperature coefficient of breakdown. For the high-frequency on-wafer characterization, pulsed-bias S-parameter measurements with low (0.0067%) duty cycle were used to mitigate thermal effects. The measured avalanche frequency aligns closely with theoretical predictions based on Gilden and Hines' small signal model [Gilden and Hines, IEEE Trans. Electron Devices ED-13(1), 169–175 (1966)], measured impact ionization coefficients [Cao et al., Appl. Phys. Lett. 112(26), 262103 (2018)], and experimental saturation velocity measurements [Bajaj et al., Appl. Phys. Lett. 107(15), 153504 (2015)]; this excellent agreement confirms IMPATT operation and provides insights needed to further optimize device performance.

Application of the Green's function formalism to the interplay between avalanche and multiphoton ionization induced by optical pulses

Application of the Green's function formalism to the interplay between avalanche and multiphoton ionization induced by optical pulses

An experimental and computational study on the ignition process of a pulse modulated dual-RF capacitively coupled plasma operated at various low-frequency voltage amplitudes

The ignition process of a pulse modulated capacitively coupled argon discharge driven simultaneously by two radio frequency voltages [12.5 MHz (high frequency) and 2.5 MHz (low frequency, LF)] is investigated by multifold experimental diagnostics and particle in cell / Monte Carlo collision simulations. In particular, (i) the effects of the LF voltage amplitude measured at the end of the pulse-on period, VL,end , on the spatiotemporal distribution of the electron impact excitation rate determined by phase resolved optical emission spectroscopy, and (ii) the electrical parameters acquired by analyzing the measured waveforms of the plasma current and voltage, are studied. Computed electrical parameters and spatio-temporal excitation maps show a good qualitative agreement with the experimental results. Especially, various breakdown mechanisms are found at different VL,end . At low values of VL,end , the ‘RF-avalanche’ mode (volume process) dominates the electron multiplication process. By increasing VL,end , the ionization caused by the volume electrons is suppressed and the electron loss at the electrodes is enhanced, leading to a delayed ignition. At higher values of VL,end , the avalanche ionization is significantly enhanced by ion-induced secondary electron emission at the electrodes, so that the ignition is significantly advanced.

Formation of Extended Tubular Plasma in Argon at Low Pressure and in a Weak Longitudinal Magnetic Field

The results of experimental studies on the formation and subsequent evolution of extended (l = 300 mm) and thin-walled (Δr ≈ 10 mm) tubular (2r ≈ 110 mm) plasma in a weak longitudinal magnetic field (B = 175 G) without the use of a thermionic cathode are presented. The cylindrical chamber in which the tubular plasma was formed was pumped with high purity argon (99.998%) at an average velocity of about 1 m/s at a pressure of P = 10–3–10–2 Torr. Two methods of creating seed electrons initiating the development of ionization avalanches were used. The difference inherent to these methods has been established in the dynamics of breakdown, completing in the formation of a tubular discharge. In the first of them, a pulsed discharge preceding the high voltage supply of the main discharge created gas preionization in a small area around the sectioned cathodes. In the second method, seed electrons were created in the entire working area of the discharge chamber by an RF discharge with a frequency of 85 kHz and duration of about 1 s. High-speed shooting with a 4-frame ICCD camera allowed us to establish the dynamics of tubular discharge formation at all its stages. Measurements of the longitudinal and radial discharge current were carried out. The results we obtained showed the possibility of spatial isolation of an extended tubular plasma from the close located metal wall of the discharge chamber by using a weak longitudinal magnetic field.

Hemispherical Rutile Solid Immersion Lens for Terahertz Microscopy with Superior 0.06–0.11λ Resolution

AbstractSolid immersion microscopy is a near‐field imaging modality that overcomes the Abbe diffraction limit by focusing the light beam behind a high refractive index lens. It offers high energy efficiency, thanks to the absence of any sub‐wavelength probes or apertures in the optical path. A favorable combination of superresolution and high optical throughput opens up a variety of imaging applications in different branches of science and technology. The spatial resolution of solid immersion microscopy is mostly limited by the refractive index value of the lens, with optically denser lenses offering higher resolutions. In this paper, bulk rutile (TiO2) crystal is used as a material for the solid immersion lens, which offers an impressive refractive index of ≈10 in the terahertz range. This is the highest value of refractive index ever used in solid immersion microscopy. A continuous wave impact ionization avalanche transit‐time diode‐emitter at the 0.2 THz frequency (the λ = 1.5 mm wavelength) and a Golay detector are used for building a solid immersion microscope. Numerical and experimental studies reveal 0.06–0.11λ resolution of the developed microscope. This is the highest normalized resolution ever reported for any solid immersion imaging systems.

The latest developments of microwave diagnostics for high temperature plasma in ELVA-1 company

For nearly 30 years, we have been designing and supplying instruments for microwave diagnostics of high temperature plasma. This report provides a description of the mm-wave components we utilize to make diagnostics within the frequency range of 26–330 GHz. While most of these components are standard and readily available on the market, we have also developed a few specific devices that simplify the architecture of our instruments. The article includes descriptions of these devices: Backward Wave Oscillators (BWO), Impact Ionization Avalanche Transit-Time diode (IMPATT) sources, IMPATT Active Frequency Multipliers (AFM), Noise Sources, and Electronically Controlled Attenuators. Furthermore, we offer an overview of the microwave plasma diagnostics we have supplied, including ECE radiometers operating at 50–220 GHz, as well as heterodyne interferometers operating at fixed frequency 94 GHz, 140 GHz, or 300 GHz. We also discuss methods employed to ensure measurement stability and present the achieved results. The advent of the new era of modern Monolithic Microwave Integrated Circuit (MMIC) based devices has brought forth exciting possibilities. As an example, we discuss the upgrade of the low noise receiver for the Collective Thomson Scattering (CTS) diagnostic at Wendelstein 7-X, which enables ion temperature measurements in the plasma core [1]. Lastly, we provide a list of MMIC-based devices that are currently available and have garnered the attention of the plasma diagnostics community.

Junction Diameter Dependence of Oscillation Frequency of GaN IMPATT Diode Up to 21 GHz

An experimental study on the effects of junction capacitance and current density on the oscillation characteristics of GaN single-drift-region (SDR) impact ionization avalanche transit-time (IMPATT) diodes were carried out using GaN p+-n abrupt junction diodes of various diameters, 200, 150, and 100 μm, with a depletion layer width of 2 μm. The fabricated diodes showed a clear avalanche breakdown at 315 V and a pulsed microwave oscillation with a peak output power exceeding 30 dBm. The oscillation frequency depended on junction diameter and current density. It was widely modulated from 8.56 to 21.1 GHz with decreasing junction diameter and increasing current density. The highest oscillation frequency was obtained with a current density of 13.8 kA/cm2 and a junction diameter of 100 μm. A numerical calculation based on Read-type small-signal theory was carried out and found to well explain the experimental results.

Ultrafast damage dynamics and ablation mechanism of GaSe induced by femtosecond laser irradiation

With the development of high-power infrared lasers and optical devices based on GaSe, more attention has been paid to its optical damage under laser irradiation. Understanding the interactions at the surface of GaSe during femtosecond laser irradiation is essential for its applications as infrared nonlinear optical and optoelectronic devices. Here, ultrafast damage dynamic process and ablation mechanism of GaSe under femtosecond laser irradiation are studied. Two types of damage/ablation mechanisms are proposed according to two distinct types of damaged morphologies and damaged products, and further confirmed by ultrafast pump–probe spectroscopy. At lower fluences, the laser-induced damage of GaSe can be attributed to the formation of new products by chemical bonds broken and phase transitions, changing the stoichiometry and spectra while barely destroy surface morphologies. At higher fluences, surface plasma is formed with massive hot free electrons in the conduction band from the multi-photon ionization and avalanche ionization. The plasma heats the surface and forms shock wave, which removes the surface layer by vaporization, causing a damage pit on the surface. Our results gain insight into damage dynamics of GaSe under laser irradiation, which can help to have a deeper comprehension of the laser-materials interaction.

Long-lasting XUV activation of helium nanodroplets for avalanche ionization

We study the dynamics of avalanche ionization of pure helium nanodroplets activated by a weak extreme-ultraviolet (XUV) pulse and driven by an intense near-infrared (NIR) pulse. In addition to a transient enhancement of ignition of a nanoplasma at short delay times fs, long-term activation of the nanodroplets lasting up to a few nanoseconds is observed. Molecular dynamics simulations suggest that the short-term activation is caused by the injection of seed electrons into the droplets by XUV photoemission. Long-term activation appears due to electrons remaining loosely bound to photoions which form stable ‘snowball’ structures in the droplets. Thus, we show that XUV irradiation can induce long-lasting changes of the strong-field optical properties of nanoparticles, potentially opening new routes to controlling avalanche-ionization phenomena in nanostructures and condensed-phase systems.

An Experimental Study of Multiphoton Ionization in Fused Silica at IR and Visible Wavelengths

We present the results of an experimental study of multiphoton ionization in fused silica, using a linearly polarized femtosecond Satsuma fiber laser with an active medium based on Yb+3 ions, at 515 -nm and 1030 -nm wavelengths. The radiation transmission in the fused silica was measured as a function of the laser intensity and wavelength and the data were analyzed using a theoretical model based on the Keldysh theory. We determined the multiphoton absorption cross-sections in the fused silica in the case of four- and eight-photon ionization and analyzed the contribution of avalanche ionization. The obtained results provide insight into the fundamental processes involved in multiphoton ionization and have implications for its applications, such as laser micromachining and material processing.

Wavelength-Independent Performance of Femtosecond Laser Dielectric Ablation Spanning Over Three Octaves

Ultrafast laser breakdown of wide bandgap dielectrics is today a key for major technologies ranging from 3D material processing in optical materials to nanosurgery. However, a contradiction persists between the strongly nonlinear character of energy absorption and the robustness of processes to the changes of the bandgap/wavelength ratio depending on applications. While various materials and bandgaps have been studied, we concentrate here the investigations on the spectral domain with experiments performed with wavelength drivers varied from deep-ultraviolet (258 nm) to mid-infrared (3.5 $\mu$m). The measured fluence thresholds for single shot ablation in dielectrics using 200-fs pulses exhibit a plateau extending from the visible domain up to 3.5-$\mu$m wavelength. This is accompanied, after ablation crater analysis, by a remarkable invariance of the observed ablation precision and efficiency. Only at the shortest tested wavelength of 258 nm, a twofold decrease of the ablation threshold and significant changes of the machining depths are detected. This defines a lower spectral limit of the wavelength-independence of the ablation process. By comparison with simulations, avalanche ionization coefficients are extracted and compared with those predicted with the Drude model. This must be beneficial to improve predictive models and process engineering developments exploiting the new high-power ultrafast laser technologies emitting in various spectral domains.

Mechanism of avalanche charge domain transport for nonlinear mode of GaAs photoconductive semiconductor switches

Photoconductive semiconductor switch is of significance in the fields of ultafast electronics, high-repetition rate and high-power pulse power system, and THz radiation. The mechanism of the nonlinear mode of the switch is an important area of study. In this work, stable nonlinear wave forms are obtained by a semi-insulating GaAs photoconductive semiconductor switch triggered by a 5-ns laser pulse with pulsed energy of 1 mJ at a wavelength of 1064 nm under a bias of 2750 V. Based on two-photon absorption model, the photogenerated carrier concentration is calculated. The theory analysis and calculation result show that the photogenerated carrier can compensate for the lack of intrinsic carrier, and lead to the nucleation of photo-activated charge domain. According to transferred-electron effect principium, the electric field inside and outside the domain are calculated, indicating that the electric field within the domain can reach the electric field which is much larger than intrinsic breakdown electric field of GaAs material, and results in strong impact avalanche ionization in the bulk of the GaAs switch. According to the avalanche space charge domain, the typical experimental phenomena of nonlinear mode for GaAs switch are analyzed and calculated, the analysis and calculations are in excellent agreement with the experimental results. Based on drift-diffusion model and negative differential conductivity effect, the transient electric field in the bulk of the switch is simulated numerically under the optical triggering condition. The simulation results show that there are moving multiple charge domains with a peak electric filed as high as the intrinsic breakdown electric field of GaAs within the switch. This work provides the experimental evidence and theoretical support for studying the generation mechanism of the nonlinear photoconductive semiconductor switch and the improvement of the photo-activated charge domain theory.

Electron temperature relaxation in the clusterized ultracold plasmas

Ultracold plasmas are a promising candidate for the creation of strongly coupled Coulomb systems. Unfortunately, the values of the coupling parameter Γe actually achieved after photoionization of the neutral atoms remain relatively small because of the considerable intrinsic heating of the electrons. A conceivable way to get around this obstacle might be to utilize a spontaneous ionization of the ultracold Rydberg gas, where the initial kinetic energies could be much less. However, the spontaneous avalanche ionization will result in a very inhomogeneous distribution (clusterization) of the ions, which can change the efficiency of the electron relaxation in the vicinity of such clusters substantially. In the present work, this hypothesis is tested by an extensive set of numerical simulations. As a result, it is found that despite a less initial kinetic energy, the subsequent relaxation of the electron velocities in the clusterized plasmas proceeds much more violently than in the case of the statistically uniform ionic distribution. The electron temperature, first, experiences a sharp initial jump (presumably, caused by the “virialization” of energies of the charged particles) and, second, exhibits a gradual subsequent increase (presumably, associated with a multi-particle recombination of the electrons at the ionic clusters). As a possible tool to reduce the anomalous temperature increase, we also considered a two-step plasma formation, involving the blockaded Rydberg states. This leads to a suppression of the clusterization due to a quasi-regular distribution of ions. In such a case, according to the numerical simulations, the subsequent evolution of the electron temperature proceeds more gently, approximately with the same rate as in the statistically uniform ionic distribution.

O2 based resonantly ionized photoemission thermometry analysis of supersonic flows.

Characterization of the thermal gradients within supersonic and hypersonic flows is essential for understanding transition, turbulence, and aerodynamic heating. Developments in novel, impactful non-intrusive techniques are key for enabling flow characterizations of sufficient detail that provide experimental validation datasets for computational simulations. In this work, Resonantly Ionized Photoemission Thermometry (RIPT) signals are directly imaged using an ICCD camera to realize the techniques 1D measurement capability for the first time. The direct imaging scheme presented for oxygen-based RIPT (O2 RIPT) uses the previously established calibration data to direct excite various resonant rotational peaks within the S-branch of the C3Π, (v = 2) ← X3Σ(v' = 0) absorption band of O2. The efficient ionization of O2 liberates electrons that induce electron avalanche ionization of local N2 molecules generating N2 +, which primarily deexcites via photoemissions of the first negative band of N2+(B 2 Σ u+-X 2 Σ g+). When sufficient lasing energy is used, the ionization region and subsequent photoemission signal is achieved along a 1D line thus, if directly imaged can allow for gas temperature assignments along said line; demonstrated here of up to five centimeters in length. The temperature gradients present within the ensuing shock train of a supersonic under expanded free jet serves as a basis of characterization for this new RIPT imaging scheme. The O2 RIPT results are extensively compared and validated against well-known and established techniques (i.e., CARS and CFD). The direct imaging capability fully realizes the technique's fundamental potential and is expected to be the standard of implementation going forward. The direct imaging capability can play instrumental roles in future scientific studies that rely upon acute characterization of thermal gradients within a medium that cannot be easily resolved by a point. Furthermore, the removal of the spectrometer greatly reduces the cost, complexity, and optical alignment associated with prior RIPT measurements.

Simulation on different orientation hexagonal SiC-based hetero-polytype IMPATT terahertz diodes

SiC hetero-polytypes that could be fabricated by chemical vapor deposition etc. methods can be employed to design the power devices. Herein, the 〈0 0 0 1〉(〈1 1 2¯ 0〉)4H(6H)-SiC-based hetero-polytypes are adopted to construct the n+/n/i/p/p+-type impact ionization avalanche transit time diodes (IMPATTDs) and mixed tunneling avalanche transit time diodes (MITATTDs). The DC, large-signal and noise performance of the proposed diodes operating at 0.85 THz are numerically simulated based on the fundamental equations for semiconductor devices. The results indicate that, the diodes with 〈0 0 0 1〉4H(6H)-SiC in each of the i-type, n-type and p-type regions can render higher break-down voltage and conversion efficiency while lower quality factor comparing to those with 〈1 1 2¯ 0〉4H(6H)-SiC in the three regions, the latter devices involve lower noise. The diodes contained 3C-SiC show lower break-down voltage and quality factor while higher noise than those without 3C-SiC. Little difference of DC, large-signal and noise properties between IMPATTDs and MITATTDs is implied due to very weak tunneling through thick intrinsic-region. The present diodes show higher conversion efficiency comparing to the n+/n/p/p+-type devices. In addition, the fabricating feasibility of the proposed devices is discussed. This paper can guide to design the reasonable structures by using suitable anisotropic semiconductors to achieve high-performance devices.

Avalanche ionization during UV filamentation in fused silica: suppression of blueshifted spectra extent

In general, ionization-induced free-electron plasma is considered to increase spectral blueshift during femtosecond laser filamentation. Here we theoretically show that the enhancement of plasma density via avalanche ionization decreases the blueshift of supercontinuum (SC) spectra associated with a ultraviolet filament in fused silica. By numerically solving the forward Maxwell equations, our simulations show that the arrest of beam collapse is ascribed to multiphoton absorption rather than plasma defocusing. In addition, SC spectral broadening is mainly dominated by Kerr self-phase modulation (SPM), while the plasma generated by ionization plays a more significant role in absorbing the laser pulse energy, which would suppress the Kerr SPM effect, than in reducing the refractive index. Our work provides a clearer understanding of ultraviolet laser propagation dynamics in condensed media.

The key role of the semiconductor property of water in femtosecond laser-induced plasma spectroscopy

We investigate femtosecond laser-induced plasma spectroscopy (fs-LIPS) of water, and find that at different input laser energies the measured characteristic fs-LIPS emissions from trace metal elements in water exhibit different dependences on the chirped pulse duration. As the pulse is temporally stretched from 50 fs to longer durations up to 500 fs, the emission intensity from Al I on the 3s24s 2S1/2 → 3s23p 2S1/2, 3/2 transitions experiences three typical variation regions, i.e., the increasing, plateau, and decreasing regions, respectively, but their temporal range varies as a function of the input laser energy. By comparing with the optical emissions of fs-LIPS of different materials including ambient air, solid metal Aluminum, and the two semiconductor materials with different bandgaps, i.e., silicon and sapphire, we ascribe the observed dependence of fs-LIPS emission of trace Al element in water on the pulse duration to the tradeoff between the strong field ionization (SFI) and the subsequent avalanche ionization (AI), both of which are determined by the wide bandgap semiconductor property of water.