Plasma emission at the laser tripled frequency

The third harmonic generation in the skin-layer of a dense hot plasma due to inverse bremsstrahlung is investigated for regimes having increasing experimental relevance. New analytical dependencies of the radiation flux density at the frequency 3ω on the plasma and fundamental wave parameters are established.

Electrical and optical low frequency noises in multimodal vertical cavity surface emitting lasers

Experimental investigations of the low frequency noise of multimode 780 nm vertical cavity surface emitting lasers are reported. Electrical noise, optical noise and their correlation have been measured in the frequency range 1 Hz–95 kHz. The results show that the main contribution to the electrical noise is located in the distributed Bragg reflector layers of the laser. The optical power and pump current noise sources are strongly correlated below and around the threshold, while are weakly correlated above threshold. It is argued that the noise in the optical power is due to both free injection carrier noise and optical gain fluctuations.

CONTROL OF RADIATIVE RECOMBINATION BY A STRONG LASER FIELD

Strong-field laser-assisted radiation recombination is investigated: (a) to explore the control possibilities with two laser fields having commensurable frequencies and (b) to get some insight into the influence of the medium (a plasma) in which recombination occurs. It is found that by varying the relative phase of the two fields it is possible to control enhancement, broadening and symmetry properties of the recombination differential emitted power. In the case of an anisotropic two-temperature plasma, an interesting interplay is found between the shape of the laser-modified electron velocity distribution function and the shape of the emitted X-ray photon power spectrum. The novel feature…

Control of multiphoton radiative recombination through the action of two low-frequency fields

Interference effects caused by the simultaneous action of a two low frequency fields with commensurate frequencies during laser assisted radiative recombination (LARR) are investigated theoretically. Varying the relative phase of the two fields the following LARR features may be controlled: a) location of the maxima in the total emitted power; b) enhancement of the total emitted power; c) polar shape of the differential emitted power; d) shape of the emitted power spectra.

Monte Carlo simulation of high‐order harmonics generation in bulk semiconductors and submicron structures

To qualify the feasibility of standard semiconductor materials and Schottky‐barrier diodes (SBDs) for THz high‐order harmonic generation and extraction, the harmonic intensity, intrinsic noise and signal‐to‐noise ratio are calculated by the Monte Carlo method when a periodic high‐frequency large‐amplitude external signal is applied to a semiconductor device. Due to very high signal‐to‐noise ratio heavily doped GaAs SBDs are found to exhibit conditions for frequency mixing and harmonic extraction that are definitively superior to those of bulk materials. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Anisotropic Electron Kinetics and Distribution Functions in High Intensity Laser-Plasma Interactions

Radiation resonant transmission and reflection by a thin layer of an anisotropic plasma

The possibility of radiation resonant transmission and reflection by a thin layer of a plasma possessing an anisotropic electron velocity distribution strongly elongated in the direction of the incident wave polarization is established.

Modeling harmonic generation by a degenerate two-level atom

An analytical theory of the generation of high-order harmonics of laser radiation has been developed on the basis of a two-level model atom with degenerate levels. Among other parameters, onset, width, and cutoff of the plateau in the harmonic spectrum are obtained in simple analytical forms that connect the basic problem parameters and permit a transparent interpretation of the mechanism underlying the spectrum formation for this specific case. Selected numerical calculations are reported to corroborate the analytical findings and to investigate other harmonic-spectrum features.

Radiative recombination in the presence of a few cycle laser pulse.

We have investigated the laser-assisted radiative recombination in the presence of a few-cycle pulse with the aim of demonstrating means of controlling such process. Within the Coulomb-Volkov approach already employed to describe the radiative recombination assisted by a monochromatic laser field, we have found that the emitted photon spectrum is affected by both the cycle number nc and the carrier-envelope relative phase phi . In particular, it has been shown that the minimum and the maximum values of the emitted photon energy may be controlled by varying nc and phi . Finally, it has been found that the enhancement of radiative recombination occurring in the presence of a monochromatic fie…

Multiphoton-ionization transition amplitudes and the Keldysh approximation.

The Keldysh approximation to treat the multiphoton ionization of atoms is reconsidered. It is shown that, if one consistently uses the hypothesis under which the approximation should be valid (essentially, that of a weak, short-range binding potential), a Keldysh-like term results as an approximation to the first term of a uniformly convergent series in powers of the binding potential. No cancellation occurs when higher-order terms are taken into account. This result allows one to consider the Keldysh approximation as a well-defined theoretical model, without implying, however, that it is adequate to describe multiphoton ionization of real atoms.

Plasma Electron Kinetics and Distribution Functions in Laser Fields

A concise review of the properties of electron distribution functions in a fully ionized plasma in the presence of a high-frequency laser field is presented. In detail is discussed the physical origin of most of the reported results in the case of strong fields. The presence of a laser field, through the inverse bremsstrahlung absorption, alters dynamically the roles of and the interplay between electron-ion and electron-electron collisions shaping the distribution function. Special attention is paid to the role of e-e collisions in the process of laser-plasma interaction.

Radiative recombination in a strong laser field: low-frequency approximation

A theoretical treatment of the laser-assisted radiative recombination (LARR) is presented in which the low-frequency (LF) assumption is exploited. The merit of the proposed LF approximation is twofold. First, the LF approximation considerably simplifies the calculations of the transition rates, whereas the results obtained within this approximation are only slightly different from those obtained without resorting to it. Second, the LF approximation gives more insight into the physical picture of the process, which may be viewed as a two-step process. In the first step, the free electron propagates toward the ion, and its motion is described classically with motion changes ascribed mainly to…

Probing Rydberg atoms through collisions with helium in the presence of static electric and magnetic fields

We report on field induced inelasticity effects in state-to-state transitions caused by collisions of helium with Rydberg atoms in the presence of parallel static electric and magnetic fields. Due to the phases accumulated by the wavefunctions of the states involved into the collision events, the transition cross sections plotted as a function of the external fields exhibit modulations. When the relative velocity of the colliding atoms is high enough, these modulations are wiped out, while sizable modifications of the cross sections may take place due to the alteration of the wavefunctions' spatial localization. The possibility of using the field-assisted collisions as a probe giving inform…

Laser even harmonics generation by a plasma embedded in a static electric field

Even order harmonics generation of the laser radiation due to electron-ion collisions in a plasma embedded in a constant electric field is investigated theoretically. Even harmonics are generated because the presence of a static electric field removes the invariance of the electron distribution function under the symmetry operation of velocity direction inversion. Efficiency generation dependencies are investigatedvsdifferent significant parameters as: harmonics number; the ratio of the electron quiver velocity to the thermal velocity; the orientation of the constant electric field with respect to the laser radiation electric field and its wavevector. It is shown that in the general case th…

Langevin Approach to Understand the Noise of Microwave Transistors

A Langevin approach to understand the noise of microwave devices is presented. The device is represented by its equivalent circuit with the internal noise sources included as stochastic processes. From the circuit network analysis, a stochastic integral equation for the output voltage is derived and from its power spectrum the noise figure as a function of the operating frequency is obtained. The theoretical results have been compared with experimental data obtained by the characterization of an HEMT transistor series (NE20283A, by NEC) from 6 to 18 GHz at a low noise bias point. The reported procedure exhibits good accuracy, within the typical uncertainty range of any experimental determin…

Infrared radiation harmonic generation in degenerate semiconductors

The basic properties of infrared laser radiation odd harmonic generation due to electron-charged impurity collisions in degenerate semiconductors are investigated. It is found that in the case of relatively weak fields, the electron Fermi distribution is the cause of an anomalous suppression of harmonic radiation. In the case of strong fields, the effect of the selective suppression of single harmonics is established.

Analytical evaluation of integrals occurring in bound-free transitions.

A class of three-dimensional integrals on spatial coordinates of the type occurring in treatments of multiphoton ionization of atoms and of other bound-free transitions is evaluated in closed form for both hydrogenic and Slater-type wave functions. The reported method of evaluation is illustrated with a number of examples, including the multiphoton ionization of hydrogen by a very intense laser field, when the ejected electrons have absorbed more photons than the minimum required to reach the continuum.

High-frequency even harmonics generation in a plasma with electron heat flux

It is shown that in a weakly nonuniform plasma with electron heat flux the generation of odd and even harmonics of high frequency radiation is possible. The even harmonics properties qualitatively differ as compared to those of odd harmonics and depend in an essential way on the heat flux value and orientation with respect to the fundamental wave polarization and propagation directions.

Field-assisted photodetachment process to observe the ponderomotive shift

Two-color photodetachment of a negative ion in the presence of a constant magnetic field is proposed as a process in which to observe the ponderomotive shift. Appropriate calculations are reported of the process under consideration based on a simple one-electron model and with the basic physical parameters well within the experimental state of the art.

Generation of even harmonics of sub-THz radiation in bulk GaAs in the presence of a static electric field

The static electric field effects on nonlinear carrier dynamics in low-doped GaAs bulk under the influence of an intense sub-terahertz field are studied by a three-dimensional multivalleys Monte Carlo simulation. The conversion efficiency is calculated by using the appropriate Maxwell equation for the propagation of an electromagnetic wave along a given direction in the medium. Production of odd and even harmonics due to the nonlinearity of the velocity-field relation is investigated.

Strong-field high-frequency approximation to the multiphoton ionization of hydrogen

The strong-field multiphoton ionization of atoms is considered and a theoretical approach dealing nonperturbatively with the radiation field formulated. The general computational scheme is the conventional perturbation theory, but the intermediate states are dressed by the field. We present in detail a method to dress the continuum states and to study the dipole transitions within the continuum. In the high-frequency domain, the proposed procedure rapidly converges over a wide range of field intensity and offers an interesting framework for calculating ionization rates for arbitrary numbers of absorbed (above-threshold) photons and field polarization.

Radiative Recombination in a Strong Laser Field

Recent advances of radiative recombination in the presence of strong laser fields are reported. The intense laser radiation is found to introduce new relevant features, among which enhancement and control of the emitted X-ray spectra are the most important. The influence of the plasma medium in which the process generally takes place is considered as well. The results of the recent investigations shed new light on the laser assisted radiative recombination physics and give relevant indications concerning the possibilities to have effective slow electrons and to balance the plasma heating, as needed in important applications.

Influence of a plasma medium on laser assisted radiative recombination

The problem of laser assisted radiation recombination, taking into account the influence of the plasma in which the elementary process takes place, is investigated theoretically. Among the reported results, remarkable broadening and enhancement of the X-ray photon emission spectrum is found, provided some conditions are met. First, a sufficiently intense laser field must assist the process. For enhancement, the intensity must be such that the amplitude of the quiver electron velocity be comparable or slightly larger than the electron thermal velocity. For broadening it is important that at the level of the elementary act of recombination several multiphoton channels be open. A judicious com…

DC and 1/f noise characterization of cryogenically cooled pseudomorphic HEMT's

Pseudomorphic (AlGaAs/InGaAs/GaAs) HEMT's have exhibited the best noise performance over the entire LF-to-microwave frequency range if compared to MESFET's and conventional GaAs HEMT's, due to either a reduced flicker noise, a lower G/R contribution and a smaller high-field diffusion noise. We have recently investigated the microwave (up to 18 GHz) noise properties of packaged pseudomorphic HEMT's from 290 K down to cryogenic temperature values. The current experimental work is aimed at extending such analysis to the LF noise range and at low temperatures. Cryogenic noise spectra (1 Hz to 100 KHz) and DC characteristics have therefore been recorded and the relevant observations on the devic…

Hydrodynamic Modeling of Transport and Noise Phenomena in Bipolar Two-Terminal Silicon Structures

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Even harmonics generation in plasma as a new tool of current evolution diagnostics

It is shown that using even harmonics of a test wave it is possible to diagnose the fast time evolution of the current density.

Even harmonics generation of high frequency radiation in current-carrying plasmas

Generation of high frequency radiation harmonics in a current-carrying plasma is studied. The physical mechanism responsible for harmonics generation is provided by electron-ion collisions. The current in the plasma is sustained by a constant electric field. It is shown that the electron distribution function anisotropy due to the static field yields generation of even harmonics. As a result, the radiation spectrum emitted by the current-carrying plasma contains both even and odd harmonics, the latter being attributed to currentless plasma. For a broad range of plasma and high frequency radiation parameters, a detailed analysis of the even harmonics properties is reported.

Radiative Recombination in strong laser field

A theoretical treatment of the laser-assisted radiative recombination (LARR) is presented in which the low-frequency (LF) assumption is exploited. The merit of the proposed LF approximation is twofold. First, the LF approximation considerably simplifies the calculations of the transition rates, whereas the results obtained within this approximation are only slightly different from those obtained without resorting to it. Second, the LF approximation gives more insight into the physical picture of the process, which may be viewed as a two-step process. In the first step, the free electron propagates toward the ion, and its motion is described classically with motion changes ascribed mainly to…

Nonlinear inverse bremsstrahlung and highly anisotropic electron distributions

A procedure is proposed to deal with the approximate solution of the kinetic equation for the velocity distribution function of electrons in a fully ionized plasma in the presence of strong, high frequency radiation. The Legendre polynomial expansion is applied after the kinetic equation has been written in an oscillating frame, where some directions are appropriately scaled, with the aim of making approximately isotropic, on the average, distributions that are otherwise anisotropic. The equations are derived for the isotropic part of the electron distribution in the scaled frame and for the scaling factor. The procedure is meant to display its potential in cases where the electron distribu…

control and enhancement of radiative recombination by a strong laser field

Anomalous properties of hot dense nonequilibrium plasmas

A concise overview of a number of anomalous properties of hot dense nonequilibrium plasmas is given. The possibility of quasistationary megagauss magnetic field generation due to Weibel instability is discussed for plasmas created in atom tunnel ionization. The collisionless absorption and reflection of a test electromagnetic wave normally impinging on the plasma with two-temperature bi-maxwellian electron velocity distribution function are studied. Due to the wave magnetic field influence on the electron kinetics in the skin layer the wave absorption and reflection significantly depend on the degree of the electron temperature anisotropy. The linearly polarized impinging wave during reflec…

Multichannel Multiphoton Ionization of the Hydrogen Atom by a Chaotic Field: Role of the Polarization

This contribution is concerned with the theory of ionization of atoms by strong laser fields, when ionization channels of different photon multiplicity are simultaneously open. Presently, this process is attracting a lot of attention, from both the experimental and theoretical sides.

Third-harmonic generation in the skin layer of a hot dense plasma

The third-harmonic generation of a pump wave, resulting from the electron-ion collision frequency dependence on the electric field in the skin layer of a hot dense plasma is investigated. The relation of the current third harmonic with the high-frequency field in the skin layer is established for arbitrary ratios of the electron-ion collision frequency to the field frequency. For arbitrary ratios of these two frequencies, the field structure inside the skin layer is determined, and the field of the wave irradiated by the plasma at tripled frequency, too, is calculated. It has permitted us to find the explicit dependencies of the third-harmonic generation efficiency on the plasma and pump fi…

Nonperturbative treatments of nonresonant multiphoton ionization of the hydrogen atom: weak-field limit

A nonperturbative treatment of the multiphoton ionization of the hydrogen atom based on the S matrix and devised for nonresonant strong-field situations is analyzed in the weak-field limit. Comparisons are presented with other S matrices as well as other nonperturbative approaches. Our treatment is found to perform generally better than similar S-matrix treatments. The usual perturbative results are recovered provided that the photon wavelengths are sufficiently short and are off resonance with the atomic transitions. Important indications are obtained as to the role of the atomic structure, the relevance of the gauge consistency, and the reliability and improvement of the present nonpertur…

Unusual optical properties of dense non-equilibrium plasmas

Third harmonic generation in the skin layer of a hot dense plasma

The third harmonic generation of a pump wave, resulting from the electron-ion collision frequency dependence on the electric field in the skin-layer of a hot dense plasma is investigated. The relation of the current third harmonic with the high-frequency field in the skin-layer is established for arbitrary ratios of the electron-ion collision frequency to the field frequency. For arbitrary ratios of these two frequencies the field structure inside the skin-layer is determined, and the field of the wave irradiated by the plasma at tripled frequency too is calculated. It has permitted to find the explicit dependencies of the third harmonic generation efficiency on the plasma and pump field

Experimental investigation of the kink effect and the low frequency noise properties in pseudomorphic HEMT’s

The kink effect in low-noise pseudomorphic (AlGaAs/InGaAs) HEMT's has been examined in detail by investigating the steady-state and pulsed I-V characteristics, the behavior of the output conductance dispersion and the performance of the gate leakage current to understand its origin. No clear evidence of impact ionization occurrence in the InGaAs channel at kink bias conditions (V-DS.kink = 1.5 V) has been found, thus suggesting that the predominant mechanism should be attributed to trap-related phenomena. A significant rise of the gate current has been found at very high drain voltages (far from V-DS.kink) associated with low drain current values which is probably due to impact ionization o…

Langevin approach to collisional bremsstrahlung in a magnetic field

Noise decomposition in random telegraph signals using the wavelet transform

Abstract By using the continuous wavelet transform with Haar basis the second-order properties of the wavelet coefficients are derived for the random telegraph signal (RTS) and for the 1 / f noise which is obtained by summation of many RTSs. The correlation structure of the Haar wavelet coefficients for these processes is found. For the wavelet spectrum of the 1 / f noise some characteristics related to the distribution of the relaxation times of the RTS are derived. A statistical test based on the characterization of the time evolution of the scalogram is developed, which allows to detect non-stationarity in the times τ 's which compose the 1 / f process and to identify the time scales of …

Potential scattering transitions in a strong chaotic non-Markovian radiation field.

Theorie non relativiste de la diffusion par un potentiel en presence d'un champ laser intense, fluctuant en amplitude et en phase, a l'approximation de Born. Calcul des largeurs et profils de raies et des facteurs de coherence en fonction de l'intensite du laser

Nonlinear Current Generation in Degenerate Semiconductors Embedded in Constant and Low-Frequency Electric Fields

The explicit analytical forms of nonlinear currents in degenerate semiconductors serving as sources of coherent radiation in the wavelength millimetric and submillimetric ranges are established.

Radiation resonant transmission and reflection by a thin layer of an anisotropic plasma

2D and 3D Laser Writing for Integrated Optical Elements Creation

Multiphoton One-step Ionization of Helium at 1064 nm

Abstract The multiphoton ionization of helium by a strong radiation field having a wavelength of 1064 nm and a field intensity up to 5 × 1014 W cm −2 is considered within a non-perturbative treatment partially accounting for the electron-electron correlations in the initial bound state and approximately for the Coulomb interaction between the residual ion and the ejected electron in the final state. Only single-electron transitions are considered, the main emphasis being on the field polarization effects. The obtained results are compared with the few results available in the literature in the same regime as well as with very recent experimental findings. In particular, the calculated photo…

Evolution of the electron distribution function in intense laser-plasma interactions

We report a numerical investigation of the time evolution of the electron distribution function (EDF) in a laser-embedded, fully ionized plasma. A distinctive feature of the calculations is removal of the frequently adopted assumption of small anisotropy of the EDF in velocity space. This requires solving a two-dimensional partial differential equation for the EDF. Within the adopted range of parameters, the EDF undergoes significant changes. An initially isotropic EDF transforms rapidly into an anisotropic one characterized by a longitudinal velocity scale larger than the perpendicular one. This longitudinal stretching persists for several cycles of the radiation field, implying the establ…

Electron Angular Distribution in Laser-Assisted X-ray Photoeffect

Calculations are reported of the differential cross section of the laser-assisted X-ray photoeffect from K shells showing that drastic changes occur in the shape of the ejected-electron angular distribution when laser photons are exchanged during the ionisation. This process appears well suited to the study of laser effects and multiphoton exchanges in laser-assisted atomic collisions.

Experimental determination of the kurtosis of RF noise in microwave low-noise devices

Abstract The degree of the Gaussian nature of the white noise present in microwave low-noise devices is experimentally investigated. The chosen experimental technique consists of simultaneously digitizing four versions of the noise which are amplified by four parallel independent amplifiers. The four independent signals are then used to compute the second, and, to a good approximation, the fourth moment of the noise. The ratio of the fourth moment to the square of the second moment is the kurtosis of the noise. Gaussian processes are characterized by a kurtosis equal to 3. A deviation from this value gives an indication about the degree of non-Gaussian nature of the noise. By using this tec…

Amplification of nonlinear currents generation at harmonics frequencies of submillimeter radiation

The conditions are found when rapid heating of the electrons of a degenerate semiconductor in the presence of two electric fields, one constant and the other variable, is accompanied by the formation of a distribution function significantly departing from the Fermi one. It is also shown that the newly found modification of the electron distribution yields a relative amplification of nonlinear currents generation.

Skewness and kurtosis of 1/f noise in semiconductor devices

An experimental investigation of the third and fourth moments of the 1/f noise of two different electronic devices is reported. The skewness and the kurtosis of the noise voltage data are estimated. Although the devices under investigation have similar noise power spectral density, the time waveforms are shown to have slightly different statistical properties. In both cases, a small deviation from Gaussian distribution is observed.

Langevin Approach to understand the Noise in Microwave Transistors

A noise analysis procedure for microwave devices based on Langevin approach is presented. The device is represented by its equivalent circuit with the internal noise sources included as stochastic processes. Fromthe circuit network analysis a stochastic integral equation for the output voltage is derived and fromits power spectrumthe noise figure as a function of the operating frequency is obtained. The theoretical results have been compared with experimental data obtained by the characterization of an HEMT transistor series (NE20283A, by NEC) from6 to 18 GHz at a low noise bias point. The reported procedure exhibits good accuracy, within the typical uncertainty range of any experimental de…

Unusual Optical Properties of the Dense Nonequilibrium Plasma

A concise overview of new optical properties of dense nonequilibrium plasma formed on the solid state target boundary is given. In this chapter, we describe phenomena such as the third harmonic generation in the skin layer, collisionless electron heating in the high frequency skin-effect regime, absorption, and reflection and transmission of radiation by a plasma with anisotropic electron distribution.

Coherence functions of the electrical and the optical noises in monomodal packaged vertical cavity surface emitting lasers

Calculations of the frequency noise spectra and of the coherence functions between the electrical and the optical noises for a single-mode packaged vertical surface emitting laser (VCSEL) are reported.A rate-equation model for an index guided VCSEL is used, implemented with the electrical noise model including the intrinsic device, the package and external bias network.The extended model works up to the laser relaxation frequency. The amplitude optical noise and its correlation with the frequency and electrical noises are also analysed. The influence on the calculated quantities of some electrical and laser parameters is evaluated. The results show that generally the frequency noise is corr…

Photoelectric effect from a metal surface: a revisited theoretical model

The Sommerfeld model extended to include radiation–electron interaction in the regime of highly intense fields is taken as the basis for studying theoretically the laser multiphoton photoelectric effect from a metal surface. Numerical analysis is carried out without approximations other than those inherent in the model itself; the study of the multiphoton aspect of the problem is based on a scheme that is nonperturbative in an essential way. The numerical analysis facilitates insight into the potential and the limits of the model in the interpretation of recent experiments and into the similarities and differences between the metal multiphoton effect and atomic multiphoton ionization. The r…

Plasma radiation spectra in the presence of static electric and high-frequency radiation fields

Harmonics generation of high-frequency radiation in a plasma embedded in a constant electric field is investigated theoretically. It is shown that the electron directed motion due to the static electric field yields the appearance in the plasma emission spectrum of high-frequency radiation even harmonics. The conditions are established when the even harmonics generation is as effective as that of the odd ones. At variance with the odd harmonics, the even harmonics polarization plane is found to rotate with respect to that of the fundamental field. The basic dependencies concerning the rotation angle and the generation efficiency on the plasma and field parameters are established.

Even-harmonics generation in a plasma acted upon by a static electric field