Intense laser field effect on impurity states in a semiconductor quantum well: transition from the single to double quantum well potential

In this work are studied the intense laser effects on the impurity states in GaAs-Ga_{1− x} Al _x As quantum wells under applied electric and magnetic fields. The electric field is taken oriented along the growth direction of the quantum well whereas the magnetic field is considered to be in-plane. The calculations are made within the effective mass and parabolic band approximations. The intense laser effects have been included through the Floquet method by modifying the confinement potential associated to the heterostructure. The results are presented for several configurations of the dimensions of the quantum well, the position of the impurity atom, the applied electric and magnetic fields, and the incident intense laser radiation. The results suggest that for fixed geometry setups in the system, the binding energy is a decreasing function of the electric field intensity while a dual monotonic behavior is detected when it varies with the magnitude of an applied magnetic field, according to the intensity of the laser field radiation.     

Intense laser effects on donor impurity in a cylindrical single and vertically coupled quantum dots under combined effects of hydrostatic pressure and applied electric field

Using the effective mass and parabolic band approximations and a variational procedure we have calculated the combined effects of intense laser radiation, hydrostatic pressure, and applied electric field on shallow-donor impurity confined in cylindrical-shaped single and double GaAs-Ga_1−xAl_xAs QD. Several impurity positions and inputs of the heterostructure dimensions, hydrostatic pressure, and applied electric field have been considered. The laser effects have been introduced by a perturbative scheme in which the Coulomb and the barrier potentials are modified to obtain dressed potentials. Our findings suggest that (1) for on-center impurities in single QD the binding energy is a decreasing function of the dressing parameter and for small dot dimensions of the structures (lengths and radius) the binding energy is more sensitive to the dressing parameter, (2) the binding energy is an increasing/decreasing function of the hydrostatic pressure/applied electric field, (3) the effects of the intense laser field and applied electric field on the binding energy are dominant over the hydrostatic pressure effects, (4) in vertically coupled QD the binding energy for donor impurity located in the barrier region is smaller than for impurities in the well regions and can be strongly modified by the laser radiation, and finally (5) in asymmetrical double QD heterostructures the binding energy as a function of the impurity positions follows a similar behavior to the observed for the amplitude of probability of the noncorrelated electron wave function.     

FIELD IONIZATION OF MOLECULES IN AN INTENSE LASER FIELD

In order to predict the field ionization probabilities, the accurate ab initio electrostatic potential of molecules has to be calculated. However, the calculation of the full ab initio electrostatic potential of molecules is complicated, even impossible for some larger molecules with low symmetry. Here, we present a semi-empirical model to treat the field ionization of molecules in an intense laser field. In this model, a modified Coulomb potential is used to take the place of the complicated ab initio electrostatic potential of molecules. The analytic equations of the Keldysh adiabatic parameter using the Coulomb potential and the modified Coulomb potential have first been given. Using our semi-empirical model, we have calculated the field ionization probabilities and the Keldysh adiabatic parameters of O₂, N₂, SO₂, C₂H₄, CH₃CN and C₆H₆ in an intense laser field. The results agree excellently with the calculations using the ab initio electrostatic potential of molecules. As the modified parameter for the Coulomb potential can be found from experimental measurements, the field ionization mechanism of molecules can be immediately predicted with our semi-empirical model.     

Polaronic effects on laser dressed donor impurities in a quantum well

The effect of laser field on the binding energy in a GaAs/Ga1_1−xAl_xAs quantum well within the single band effective mass-approximation is investigated. Exciton binding energy is calculated as a function of well width with the renormalization of the semiconductor gap and conduction valence effective masses. The calculation includes the laser dressing effects on both the impurity Coulomb potential and the confinement potential. The valence-band anisotropy is included in our theoretical model. The 2D Hartree–Fock spatial dielectric function and the polaronic effects have been employed in our calculations. We investigate that reduction of binding energy in a doped quantum well due to screening effect and the intense laser field leads to semiconductor–metal transition.     

The effects of intense laser on nonlinear properties of shallow donor impurities in quantum dots with the Woods-Saxon potential

A theoretical study of the effects of intense laser fields on the nonlinear properties of donor impurities in a quantum dot with Woods-Saxon potential is performed within the matrix diagonalization method with the use of the effective mass approximation. The great advantage of our methodology is that it enables confinement regimes by varying two parameters in the model potential. The intense laser effects are included through the Floquet method, by modifying the confining potential associated to the heterostructure. Based on the computed energies and wave functions, the optical absorption coefficients and the refractive index between the ground state (L=0) and the first excited state (L=1) have been examined. Several configurations of the barrier height, the dot radius, the barrier slope of the confinement potential and the incident intense laser radiation have been considered. The outcome of the calculation suggests that all the factors mentioned above can influence the nonlinear optical properties strongly.     

Intense laser effects on nonlinear optical absorption and optical rectification in single quantum wells under applied electric and magnetic field

In this work the effects of intense laser on the electron-related nonlinear optical absorption and nonlinear optical rectification in GaAs-Ga_1−xAl_xAs quantum wells are studied under, applied electric and magnetic field. The electric field is applied along the growth direction of the quantum well whereas the magnetic field has been considered to be in-plane. The calculations were performed within the density matrix formalism with the use of the effective mass and parabolic band approximations. The intense laser effects are included through the Floquet method, by modifying the confining potential associated to the heterostructure. Results are presented for the nonlinear optical absorption, the nonlinear optical rectification and the resonant peak of these two optical processes. Several configurations of the dimensions of the quantum well, the applied electric and magnetic fields, and the incident intense laser radiation have been considered. The outcome of the calculation suggests that the nonlinear optical absorption and optical rectification are non-monotonic functions of the dimensions of the heterostructure and of the external perturbations considered in this work.     

Effect of an intense laser field on donor impurities in spherical quantum dots

The effect of an intense laser field on the binding energy of hydrogenic impurity states with an impurity atom located at the center of a spherical quantum dot confined by an infinite barrier potential are studied as a function of the dot radius and of the intensity and frequency of the laser field. Accurate binding energies are obtained for the 1s, 2s and 2p states by numerical integration of the Schrödinger equation. The binding energies are found to increase with decrease in the dot radius, and decrease with increase in the value of the laser field amplitude λ in all cases.     

Linear and nonlinear optical properties in a semiconductor quantum well under intense laser radiation: Effects of applied electromagnetic fields

In this work we are studying the intense laser effects on the electron-related linear and nonlinear optical properties in GaAs–Ga_1?xAl_xAs quantum wells under applied electric and magnetic fields. The calculated quantities include linear optical absorption coefficient and relative change of the refractive index, as well as their corresponding third-order nonlinear corrections. The nonlinear optical rectification and the second and third harmonic generation coefficients are also reported. The DC applied electric field is oriented along the hererostructure growth direction whereas the magnetic field is taken in-plane. The calculations make use of the density matrix formalism to express the different orders of the dielectric susceptibility. Additionally, the model includes the effective mass and parabolic band approximations. The intense laser effects upon the system enter through the Floquet method that modifies the confinement potential associated to the heterostructure. The results correspond to several configurations of the dimensions of the quantum well, the applied electric and magnetic fields, and the incident intense laser radiation. They suggest that the nonlinear optical absorption and optical rectification are nonmonotone functions of the dimensions of the heterostructure and of the external perturbations considered in this work.     

The effects of hydrostatic pressure and intense laser field on the linear and nonlinear optical properties of a square quantum well

In this paper, the effects of hydrostatic pressure, temperature and intense laser field on the linear and nonlinear optical processes in the conduction band of a square quantum well are numerically investigated in the effective mass approximation. The analytical expressions of optical properties are obtained by using the compact density-matrix approach. The numerical results are presented for typical square GaAs/Al_xGa_1?xAs single quantum well system. The nonlinear optical absorption and refractive index changes depending on the hydrostatic pressure and intense laser field are investigated for two different temperature values. The results show that the intense laser field, the hydrostatic pressure and the temperature have a significant effect on the optical characteristics of these structures.     

High-intensity molecular harmonic generation without ionization

We theoretically investigate high-order harmonic generation from H + 2 in an infrared laser field.Our numerical simulations show that a highly efficient plateau structure exists in the molecular harmonic spectrum.Under the action of the infrared laser pulse,the bound electronic wave packet in a potential well has enough time to tunnel through the effective potential barrier,which is formed by the molecular potential and the infrared laser field,and then recombine with the neighboring nucleus emitting a harmonic photon.During the entire dynamic process,because the wave packet is mainly located in the effective potential,the diffusion effect is of no significance,and thus a highly efficient harmonic plateau can be achieved.Specifically,the cut-off frequency of the plateau is linearly scaled with the peak amplitude of the infrared laser electric field,which may open another route to examine the internuclear distance of the molecule.Furthermore,one may detect the molecular bond lengths using the harmonic plateau.     

Analysis of femtosecond laser ionization/dissociation of polyatomic molecule C6H10O from one-colour pump-probe measurement

This paper reports that a one-colour fs pump-probe measurement has been carried out for studying photoionization/photodissociation of cyclohexanone (C6H10O) in intense laser field. Two of the fragments from cyclohexanone, C2H3+ and C3H3+ , are studied under 800 nm laser pump-probe and the results obtained show similar time evolutions. It proposes a feasible model for analysing the experimental observations of the one-colour fs pump-probe measurement. The results demonstrate that as an intermediate product, the excited molecular parent ions play a very important role in photionization/photodissociation processes in intense laser field.     

Quasistatic magnetic fields generated by a nonrelativistic intense laser pulse in uniform underdense plasma

Quasistatic magnetic fields generated by nonrelativistic intense linearly polarized (LP) and circularly polarized (CP) laser pulses in an initially uniform underdense plasma in the collision-dominated limit are investigated analytically. Using a selfconsistent analytical model, we perform a detailed derivation of quasistatic magnetic fields in the laser pulse envelope in the collision-dominated limit to obtain exact analytical expressions for magnetic fields and discuss the dependence of magnetic fields on laser and plasma parameters. Equations for quasistatic magnetic fields including both axial component B_z and the azimuthal one B_θ are derived simultaneously from such a selfconsistent model. The dependence of quasistatic magnetic field on incident laser intensity, transverse focused radius of laser pulse, electron density and electron temperature is discussed.     

Dissociation of molecular hydrogen ions by an IR laser pulse

Dissociation dynamics of the simplest molecular systems, such as H₂ ⁺, D₂ ⁺ and HD⁺ ions, in an intense IR laser field has been investigated by numerical modeling. An n-term approximation has been developed to describe the molecular system dynamics in an intense electromagnetic field. Calculations by the n-term approximation have been compared to an accurate numerical solution of the two-particle problem. The dissociation probability as a function of the frequency and intensity of radiation for different isotopes in a molecular hydrogen ion is discussed. A quasistatic model of molecule dissociation in an IR field has been suggested, and limits of its applicability have been determined. Zh. éksp. Teor. Fiz. 113, 128–143 (January 1998)     

Thermonuclear fusion in a strong laser field

Thermonuclear fusion induced by the irradiation of solid deuterated cluster targets and foils with fields of strong femtosecond and picosecond laser pulses is discussed. The thermonuclear-fusion process D(d, n)³He in a collision of two deuterons at an energy of 50 to 100 keV in a deuterium cluster target irradiated with a strong laser pulse is discussed. A theory of thermonuclear fusion proceeding upon the irradiation of clusters formed by deuterium iodide (DI) molecules with the field of a superintense femtosecond laser pulse is developed. This theory is based on an above-barrier process in which the sequential multiple inner ionization of atomic ions within a cluster is accompanied by field-induced outer ionization. The yield of neutrons from thermonuclear fusion in a deuteron-deuteron collision after the completion of a laser pulse is calculated. The yield of neutrons is determined for the thermonuclear-fusion reaction proceeding in the interaction of an intense picosecond laser pulse with thin TiD₂ foils. A multiple ionization of titanium atoms at the front edge of the laser pulse is considered. The heating of free electron occurs in induced inverse bremsstrahlung in the process of electron scattering on multiply charged titanium ions. The yield of alpha particles in the thermonuclear-fusion reaction involving protons and ¹¹B nuclei that is induced in microdrops by a strong laser field is determined. Experimental data on laser-induced thermonuclear fusion are discussed.     

The effects of the intense laser field on bound states in GaxIn1-xNyAs1-yN/GaAs single quantum well

We have investigated the effects of the intense laser field and nitrogen concentration on bound state energy levels in Ga _x In_{1- x} N _y As_{1- y} N/GaAs quantum well. The results show that both intense laser field and N-incorporation into the GaInNAs have strong influences on carrier localization. We hope that our results can stimulate further investigations of the related physics, as well as device applications of group-III nitrides.     

Nonlinear optical properties of asymmetric n-type double <Emphasis Type="Italic">δ</Emphasis>-doped GaAs quantum well under intense laser field

The effect of non-resonant intense laser field on the intersubband-related optical absorption coefficient and refractive index change in the asymmetric n-type double δ-doped GaAs quantum well is theoretically investigated. The confined energy levels and corresponding wave functions of this structure are calculated by solving the Schrödinger equation in the laser-dressed confinement potential within the framework of effective mass approximation. The optical responses are reported as a function of the δ-doped impurities density and the applied non-resonant intense laser field. Additionally, the calculated results also reveal that the non-resonant intense laser field can be used as a way to control the electronic and optical properties of the low dimensional semiconductor nano-structures.     

Ab initio molecular dynamics study of the reactions of allene cation induced by intense 7 micron laser pulses

ABSTRACT

The isomerisation and fragmentation of allene cation (H₂C=C=CH₂⁺) by short, intense laser pulses were simulated by Born-Oppenheimer molecular dynamics (BOMD) on the ground state potential energy surface using the B3LYP/6-31?+?G(d,p) level of theory and a 10 cycle 7?µm cosine squared pulse with a maximum field strength of 0.07?au. Laser fields polarised along the C=C=C axis deposits an average of 150?kcal/mol in the molecule, compared to only 25 and 51?kcal/mol for perpendicular polarisations. Approximately 90% of the trajectories with the field aligned with the C=C=C axis underwent one or more structural rearrangement steps to form H₂C=CH–CH⁺ (15%), H₃CCCH⁺ (4%), cyclopropene cation (6%), and allene cation with rearranged hydrogens and carbons (47%). In addition, a variety of fragments including H₂CCCH⁺?+?H (10%), c-C₃H₃⁺?+?H (7%), and HCCCH⁺?+?H₂ (2%) trajectories were produced after isomerisation. With the same amount of thermal energy, field-free BOMD shows good agreements with the BOMD with the field. However, RRKM calculations favour isomerisation to propyne cation and dissociation to HCCCH⁺?+?H₂. This suggests that for molecules in intense laser fields the energy in the intermediate isomers is not distributed statistically.     

Calculation of multidimensional potential energy surfaces for even-even transuranium nuclei: systematic investigation of the triaxiality effect on the fission barrier

Static fission barriers for 95 even-even transuranium nuclei with charge number Z = 94-118 have been systematically investigated by means of pairing self-consistent Woods-Saxon-Strutinsky calculations using the potential energy surface approach in multidimensional(β_2, γ, β_4) deformation space. Taking the heavier (252)~Cf nucleus(with the available fission barrier from experiment) as an example, the formation of the fission barrier and the influence of macroscopic, shell and pairing correction energies on it are analyzed. The results of the present calculated β_2 values and barrier heights are compared with previous calculations and available experiments. The role of triaxiality in the region of the first saddle is discussed. It is found that the second fission barrier is also considerably affected by the triaxial deformation degree of freedom in some nuclei(e.g., the Z =112-118 isotopes). Based on the potential energy curves, general trends of the evolution of the fission barrier heights and widths as a function of the nucleon numbers are investigated. In addition, the effects of Woods-Saxon potential parameter modifications(e.g.,the strength of the spin-orbit coupling and the nuclear surface diffuseness) on the fission barrier are briefly discussed.     

CO2-laser-induced multiphoton absorption of CF2Cl2

Energy absorption by CF₂Cl₂ from an intense TEA CO₂ laser pulse is measured as a function of the pressure of CF₂Cl₂ and the pressure of Ar bath gas for different laser energy fluences. The fraction of the molecules excited by the laser field and the average energy of the molecular ensemble are determined by a simple experimental method.     

Electron-related optical responses in triple δ-doped quantum wells

A detailed theoretical study on the electron-related optical responses in triple δ-doped GaAs quantum wells in the presence of non-resonant, monochromatic intense laser field is presented. For this purpose, we first obtained the bound subband energy levels and their corresponding envelope wave functions of the structure for different central doping concentrations within the effective-mass approximation. Then, we calculate the effect of the non-resonant intense laser field on the optical properties of this structure using the compact-density-matrix approach via the iterative method. We found that the optical absorption coefficients and refractive index changes in the triple δ-doped GaAs quantum well can be modulated by changing the central doping concentration and the intensity of the non-resonant, monochromatic laser field. In addition, it is shown that a sufficiently intense laser field suppresses the multiple quantum well configuration towards a single potential well one and the optical response becomes practically independent of the δ-doping concentration.