Non-resonant Intense Laser Field Research Articles

Tunneling time correction to the intersubband optical absorption in a THz laser-dressed GaAs/Al x Ga1- x As quantum well

Tunneling effect on the intersubband optical absorption in a GaAs/Al x Ga1- x As quantum well under simultaneous presence of intense non-resonant laser and static electric fields is theoretically investigated. Based on the shooting method the quasi-stationary energy levels and their corresponding linewidths are obtained. By considering the joint action of the two external fields the linear absorption coefficient is calculated by means of Fermi’s golden rule and taking into account the intersubband relaxation. We found that: (i) the linewidth broadening due to the electron tunneling has an appreciable effect on the absorption spectrum; (ii) a constant relaxation time adopted in the previous studies could not be justified even for moderate electric fields, especially in the laser dressed wells. Our model predicts that the number of absorption peaks can be controlled by the external applied fields. While in the high-electric fields the excited states become unbounded due to a significant tunneling of the electrons, for high laser intensities and low/moderate electric fields the absorption spectrum has a richer structure due to the laser-generated resonant states. The possibility of tuning the resonant absorption energies by using the combined effects of the static electric field and the THz coherent radiation field can be useful in designing new optoelectronic devices.

Anisotropic optical absorption in quantum well wires induced by high-frequency laser fields

The subband structure and optical properties of a cylindrical quantum well wire under intense non-resonant laser field are investigated by taking into account the correct dressing effect for the confinement potential. The energy levels and wave functions are calculated within the effective mass- approximation using a finite element method. It is found that the absorption coefficient and the saturation intensity are strongly affected by the laser amplitude and frequency as well as by the incident light polarization. As a key result, a large anisotropy in the linear and nonlinear optical absorptions for very intense laser field is predicted. These effects can be useful for the design of polarization sensitive devices.

THz laser field effect on the optical properties of cylindrical quantum well wires

The oscillator strength and the linear and third order nonlinear refractive index changes of a cylindrical quantum well wire under intense non-resonant laser field have been investigated within the effective mass-approximation by using a finite element method. We found that the laser amplitude, the incident light and the intersubband relaxation time have an important influence on the refractive index changes.

Coherent rotational excitation by intense nonresonant laser fields

The rotation of molecules in the gas phase can be coherently excited by irradiation with strong nonresonant short laser pulses, interacting with the molecular anisotropic polarisability. Such coherent rotational excitation has been attracting much attention because of the intriguing nature of the rotational wave packet thus created, and its wide applicability to dynamical studies and advanced optics. In this review article, we first survey various experimental schemes adopted so far for externally controlling molecular rotation, and then describe a new approach based on a quantum-state resolved spectroscopic probe for investigating coherent rotational excitation by intense nonresonant laser fields. Representative examples are given to show how the method provides detailed information on excitation pathways in wave-packet creation, and how it realises full quantum-state reconstruction of the rotational wave packet in a favourable case. We also describe an advanced wave-packet control, i.e. the creation and characterisation of a unidirectionally rotating wave packet, and discuss a further extension of this approach to explore coherent vibrational excitation.

All-Optical Molecular Orientation

We report clear evidence of all-optical orientation of carbonyl sulfide molecules with an intense nonresonant two-color laser field in the adiabatic regime. The technique relies on the combined effects of anisotropic hyperpolarizability interaction and anisotropic polarizability interaction and does not rely on the permanent dipole interaction with an electrostatic field. It is demonstrated that the molecular orientation can be controlled simply by changing the relative phase between the two wavelength fields. The present technique brings researchers a new steering tool of gaseous molecules and will be quite useful in various fields such as electronic stereodynamics in molecules and ultrafast molecular imaging.

Laser-induced reshaping of the density of impurity states in GaAs/AlGaAs nanowires

The binding energy of shallow-donor impurities in a cylindrical quantum well wire irradiated by an intense non-resonant laser field is calculated within the effective mass approximation by using a variational procedure. Accurate laser-dressing effects are considered for both the confinement potential of the wire and the Coulomb potential of the impurity. The computation of the ground state subband energy eigenfunctions for different laser field intensities is based on a bidimensional finite element method. Important changes of the electron probability density under intense laser field conditions are predicted. The study reveals that the laser field compete with the quantum confinement and breaks down the degeneracy of states for donors symmetrically positioned within the nanostructure. A proper analysis of the density of impurity states is found to be essential for controlling the optical emission related to shallow donors in semiconductor quantum wires.

Controlling the Alignment and Orientation of Gaseous Molecules with Laser Electric Fields

Since most molecules are anisotropic quantum systems, alignment or orientation dependence called steric effects is ubiquitous nature in various phenomena where molecules are involved. Therefore, not only in stereodynamics of chemical reactions but also in electronic stereodynamics in molecules, alignment or orientation dependence is always a matter of central concern and the importance of molecular alignment and orientation techniques has been more and more rising. Here various molecular alignment and orientation techniques with intense nonresonant laser fields are reviewed mainly in chronological order. They include one- and three-dimensional molecular alignment both in the adiabatic regime and in the nonadiabatic regime, and one- and three-dimensional molecular orientation in the adiabatic regime. More recent progresses are the demonstrations of laser-field-free molecular orientation and all-optical molecular orientation. Finally future subjects and perspectives on the applications with a sample of aligned or oriented molecules are discussed.

Coherent rotational dynamics of molecules induced by intense ultrafast laser fields

Coherent rotational dynamics of gas-phase molecules induced by the excitation with an intense nonresonant ultrafast laser field is discussed. In particular, utilities of quantum-state resolved spectroscopic probe after the nonadiabatic rotational excitation (NAREX) by the ultrafast pulse(s) are described with an example of double-pulse excitation experiments on NO, which provide detailed information on excitation pathways in NAREX. Future prospects of the present method are briefly discussed.

Role of highest occupied molecular orbitals in molecular field-free alignment by a femtosecond pulse

This paper studies the molecular rotational excitation and field-free spatial alignment in a nonresonant intense laser field numerically and analytically by using the time-dependent Schrödinger equation. The broad rotational wave packets excited by the femtosecond pulse are defined in conjugate angle space, and their coefficients are obtained by solving a set of coupled linear equations. Both single molecule orientation angles and an ensemble of O2 and CO molecule angular distributions are calculated in detail. The numerical results show that, for single molecule highest occupied molecular orbital (HOMO) symmetry σ tends to have a molecular orientation along the laser polarization direction and the permanent dipole moment diminishes the mean of the orientation angles; for an ensemble of molecules, angular distributions provide more complex and additional information at times where there are no revivals in the single molecule plot. In particular, at the revival peak instant, with the increase of temperature of the molecular ensemble, the anisotropic angular distributions with respect to the laser polarization direction of the πg orbital gradually transform to the symmetrical distributions regarding the laser polarization vector and for two HOMO configurations angular distributions of all directions are confined within a smaller angle when the temperature of the molecular ensemble is higher.

Field-free molecular orientation by an intense nonresonant two-color laser field with a slow turn on and rapid turn off

We propose a practical and versatile technique to achieve completely field-free molecular orientation with an intense, nonresonant, two-color laser field with a slow turn on and rapid turn off. The technique is based on the combined effects of both anisotropic polarizability interaction and anisotropic hyperpolarizability interaction. Using a FCN molecule as a sample, we show that the orientation achieved adiabatically by the peak of the laser pulse can be successfully revived at the rotational period of the molecule with the same degree of orientation. The crucial importance of the sufficiently slow turn on of the laser pulse is emphasized to achieve the highest possible degree of orientation.

Quantum State Reconstruction of a Rotational Wave Packet Created by a Nonresonant Intense Femtosecond Laser Field

We have experimentally determined the amplitudes and phases of a rotational wave packet in an adiabatically cooled benzene molecule, created by a nonresonant intense femtosecond laser field. In this wave-packet reconstruction, the initial wave packet is further interfered by a replica of the first laser pulse, and the resultant modulation in population is observed in a state-resolved manner. Though several states with different nuclear-spin modifications are populated in the initial condition, a single wave packet created from one of them (with J=0) is specifically reconstructed. Phase shifts characteristic of stepwise Raman excitation beyond the perturbative regime are experimentally identified.

Nonadiabatic rotational excitation of benzene by nonresonant intense femtosecond laser fields

We investigated the nonadiabatic rotational excitation of the oblate symmetric-top molecule, benzene, induced by nonresonant intense ultrashort laser fields. By employing a quantum-state resolved probe using ns laser pulses, rotational excitation up to J = 10 was observed for the irradiation of a molecular ensemble, initially cooled to 0.5 K in an adiabatic expansion, by the femtosecond laser pulse with the intensity of 2.2 TW/cm 2. The observed excitation was analyzed by the aid of quantum mechanical calculations. These calculations show the systematic change in the excitation pathways for different K, which is characteristic of nonadiabatic rotational excitation in symmetric-top molecules.

Decoding the state distribution in a nonadiabatic rotational excitation by a nonresonant intense laser field

We investigated the quantum-state distribution in nonadiabatic rotational excitation induced by a nonresonant intense laser field. NO molecules, initially prepared in the lowest $J=0.5$ state, were rotationally excited to states with up to $J=8.5$ at a laser intensity of $2.5\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$. The characteristics in the observed state distribution were quantitatively reproduced by a quantum dynamical calculation, and excitation pathways typical to molecules in a doubly degenerate state were identified.

Molecular alignment in a liquid induced by a nonresonant laser field: Molecular dynamics simulation.

We carried out molecular dynamics (MD) simulations for a dilute aqueous solution of pyrimidine in order to investigate the mechanisms of field-induced molecular alignment in a liquid phase. An anisotopically polarizable molecule can be aligned in a liquid phase by the interaction with a nonresonant intense laser field. We derived the effective forces induced by a nonresonant field on the basis of the concept of the average of the total potential over one optical cycle. The results of MD simulations show that a pyrimidine molecule is aligned in an aqueous solution by a linearly polarized field of light intensity I approximately 10(13) W/cm2 and wavelength lambda = 800 nm. The temporal behavior of field-induced alignment is adequately reproduced by the solution of the Fokker-Planck equation for a model system in which environmental fluctuations are represented by Gaussian white noise. From this analysis, we have revealed that the time required for alignment in a liquid phase is in the order of the reciprocals of rotational diffusion coefficients of a solute molecule. The degree of alignment is determined by the anisotropy of the polarizability of a molecule, light intensity, and temperature. We also discuss differences between the mechanisms of optical alignment in a gas phase and a liquid phase.

Polarizability anisotropies of rare gas van der Waals dimers studied by laser-induced molecular alignment

The molecular alignment technique utilizing the interaction between the intense nonresonant laser field and the induced dipole moment is applied to the homonuclear rare gas dimers Rg2 (Rg=Ar, Kr, and Xe). The degree of alignment is investigated by Coulomb exploding Rg2 and by measuring the angular distributions of the fragment ions. At the same peak intensity of the laser field, the degree of alignment ≪cos2 θ ≫ becomes larger in order of Ar2, Kr2, and Xe2, reflecting the order of magnitudes of their polarizability anisotropy Δα. By taking I2 molecules as a reference, Δα of Ar2, Kr2, and Xe2 are estimated to be 0.5, 0.7, and 1.3 Å3, respectively.

Real-time probing of alignment and structural deformation of CS 2 in intense nanosecond laser fields

A time evolution of the alignment and structural deformation processes of neutral CS 2 in intense non-resonant nanosecond laser fields (1.9×10 12 W/ cm 2) is derived by ionizing CS 2 using intense circularly polarized femtosecond laser fields (5.5×10 14 W/ cm 2) . From the real-time probing of the momentum distribution of the resultant S 3+ and C 2+ fragment ions produced through the Coulomb explosion induced by the intense femtosecond laser fields, it is identified that the alignment and the structural deformation proceed simultaneously in the nanosecond laser fields.

Aligning molecules with intense nonresonant laser fields

Molecules in a seeded supersonic beam are aligned by the interaction between an intense nonresonant linearly polarized laser field and the molecular polarizability. We demonstrate the general applicability of the scheme by aligning I2, ICl, CS2, CH3I, and C6H5I molecules. The alignment is probed by mass selective two dimensional imaging of the photofragment ions produced by femtosecond laser pulses. Calculations on the degree of alignment of I2 are in good agreement with the experiments. We discuss some future applications of laser aligned molecules.

Enhanced orientation of polar molecules by combined electrostatic and nonresonant induced dipole forces

Recent experiments have demonstrated the efficacy of orienting low rotational states of a linear polar molecule in a static electric field, εS, or aligning a molecule (polar or not) in an intense nonresonant laser field, εL. We present theoretical results showing that the combined action of εS and εL can markedly sharpen orientation, particularly by introducing a pseudo-first-order Stark effect for tunneling doublets created by the polarizability interaction. Also, if εS and εL are not collinear, the molecular axis can be localized with respect to φ as well as θ, since M states as well as J states undergo hybridization. Another benefit is a means to eliminate “wrong way orientation” which otherwise occurs for “low-field seeking” states.

Alignment enhanced spectra of molecules in intense non-resonant laser fields

The dipole moment induced in a polarizable molecule by an intense non-resonant laser field interacts with the field to create aligned pendular states of definite parity, hybrids of the field-free rotational states. Electric dipole transitions between these states, driven either by a permanent moment (if the molecule is polar) or the induced moment (whether polar or non-polar), give rise to characteristic spectral features. For perpendicular transitions, at high fields the librational period of the molecular axis becomes directly observable. For parallel transitions in Raman spectra, the axis alignment can markedly enhance the transition probability by ≈ 10 3 or more.

Polarization of Molecules Induced by Intense Nonresonant Laser Fields

The anisotropic interaction of the electric field vector of intense laser radiation with the dipole moment induced in apolarizuble molecule by the laser field creates aligned pendular states. These are directional superpositions of field-free states, governed by a cos2 8 potential (with 8 the angle between the molecular axis and the field vector). We show that the spatial alignment and other eigenproperties can be derived from spheroidal wave functions, give explicit expressions for low- and high-field limits, contrast the induced pendular states with those for permanent dipoles subject to static fields, and present calculations demonstrating the utility of the induced states for rotational spectroscopy, laser alignment, and spatial trapping of molecules.