Linearly-polarized Laser Research Articles

A complete photoionization experiment with polarized atoms using magnetic dichroism and phase tilt measurements

The study of the $5p$ photoemission from laser polarized (aligned or oriented) Eu atoms was used to deduce the ratio of the magnitude of the electric dipole amplitudes and the phase difference between the two outgoing $\ensuremath{\epsilon}s$ and $\ensuremath{\epsilon}d$ electron waves. The combination of magnetic dichroism experiments, the measurement of the difference spectra for two atomic polarizations, and phase tilt experiments, the observation of the modulation of the photoelectron signal on the angle \ensuremath{\eta} of linear laser polarization, is a very promising method for performing a ``complete'' photoionization experiment.

Charge transfer in He++-He collisions at keV energies in an intense laser field: A theoretical close-coupled study using a diabatic molecular representation

We carry out a theoretical study of the effect of a strong laser field on charge transfer in collisions of doubly charged helium ions with helium at keV energies, using a close-coupled treatment with a diabatic molecular wave function basis in the semiclassical impact parameter approximation. Our results show significant effects of the laser intensity, wavelength and the orientation of the (linear) laser polarization on the charge transfer cross-sections. Our results also indicate the possibility of an experimental test for differentiating between the effects of the minimal coupling and the multipolar matter-radiation interactions on ion-atom collision processes.

Optical Gratings Fabricated by Deposition of Liquid Crystalline Polymer Films with Linearly Polarized UV Laser

Polyimide layer on a glass substrate was exposed with a linearly-polarized UV laser through a photo-mask having line-and-space patterns of 2.2 μm. UV laser chemical vapor deposition of liquid crystalline polymer was carried out on the treated substrate. The deposited films showed optical diffraction due to periodic change of molecular orientation of liquid crystalline polymer. The efficiency of optical diffraction was changed with polarization of incident light.

Micron-sized control of molecular orientation of liquid crystalline polymer films by using linearly-polarized UV laser.

Micron-sized control of molecular orientation of liquid crystalline polymer films by using linearly-polarized UV laser.

Raman Lidar Observations: Simultaneous Measurements of Water Vapor, Temperature and Aerosol Vertical Profiles, Part I.

A Raman lidar was developed to observe vertical profiles of water vapor, temperature and aerosols. The lidar system can detect signal from 5 detectors simultaneously. Two vibrational Raman backscattering echoes from H 2 O and N 2 (or O 2 ) excited by Nd:YAG 3rd harmonics wavelength (355 nm) are used to observe the profiles of water vapor mixing ratio and atmospheric temperature (or density). Nd:YAG fundamental and 2nd harmonics wavelengths (1064 and 532 nm) are used to observe backscattering coefficient profiles of aerosols and clouds at these wavelengths. The scattering at 532 nm is observed in parallel and perpendicular components corresponding to the linear polarization plane of the transmitted linearly-polarized laser pulse to observe depolarization ratio profiles. Observed water vapor and temperature distributions coincide with the simultaneously observed radiosonde sounding data excellently. The Raman lidar makes it possible to observe the vertical profiles of water vapor, temperature, aerosols, and clouds simultaneously.

Two-photon resonantly-enhanced negative nonlinear refractive index in xenon at 248 nm

We report measurements and ab initio calculations of a large negative nonlinear refractive index in xenon for linearly-polarized KrF laser light, and describe a simple experiment that demonstrates xenon compensation of both self-focusing and self-phase modulation. This effect arises because KrF laser frequencies lie just above a two-photon resonance with the Xe 6p[ 1 2 ] 0 state at 249.6 nm. The negative nonlinear index resulted in self-defocusing of 10 ps linearly-polarized 248.4 nm pulses in a 4.6 m cell at Xe pressures up to 1000 Torr. Numerical simulations of this defocusing at pressures up to 266 Torr yield a nonlinear index n 2 = − (2.06 ± 0.14) × 10 −14 esu for 760 Torr and 300 K, but the simulations fail to adequately reproduce the shapes of the measured profiles at pressures significantly above 266 Torr. In the compensation experiment, the cell was first filled up to 1260 Torr with pure carbon dioxide, whose large positive n 2 resulted in self-focusing of the beam. A small amount of Xe was then added to the cell and allowed to mix with the CO 2. Less than 130 Torr of Xe completely compensated the 1260 Torr of CO 2, allowing both the spectrum and beam profile to be recovered. Numerical simulations of the self-focusing measurements yield a nonlinear index of (1.60 ± 0.07) × 10 −15 esu for CO 2, and the compensation experiment gives n 2(Xe) = − (1.82 ± 0.08) × 10 −14esu at 760 Torr, 300 K, and 248.4 nm. The combined defocusing and compensation measurements give n 2(Xe)=−(1.97±0.23)×10 −14 esu, in good agreement with our ab initio calculation of − (1.54 ± 0.30) × 10 −14 esu.

Laser electron acceleration in vacuum

This scheme involves pairs of intersecting linearly-polarized Gaussian-profile laser beams that are focused at crossover in vacuum and phased such that transverse field components cancel. The E z components accelerate properly-phased z-axis relativistic electrons throughout a half-wave slip distance centered at crossover. The slip length is approximately 90λ for the case considered. The E z components vanish rapidly before and after this slip region, thus providing conditions for a special case that is not included in Palmer's general acceleration theorem. Electron energy-gain gradients of several GeV/m appear to be possible.

Automated measurement of polarized bidirectional reflectance

An Automated Bidirectional Reflectance Acquisition Measurement System (ABRAMS) has been constructed to facilitate measurement of bidirectional reflectance from soil and vegetative samples in the laboratory. The system illuminates a sample with linearly-polarized laser light, λ = 632.8 nm, and measures the like- and cross-polarized scattered intensities over half a hemisphere. System design and polarized bidirectional reflectance measurements from a soil sample and SiO 2 spherical particles are discussed in this work. It is shown that polarization information in the plane of incidence is useful for identifying certain scattering mechanisms associated with soil reflectance. This is because the like-polarized intensity, I vv, is influenced by single-scattered light and the cross-polarized intensity, I HV, is strongly influenced by multiple-scattered light. For example, comparable levels of I VV and I HV indicated that the reflectance of soils, especially those made up of translucent particles, is dominated by significant multiple scattering because single scattering causes minimal depolarization in the plane of incidence. Furthermore, I HV is less dependent on the illumination angle (hence, more Lambertian) than I VV, indicating that the scattering mechanisms which caused them are not the same.

IMPROVED KELDYSH-FAISAL-REISS THEORY

In the framework of Keldysh-Faisal-Reiss (KFR) theory, with taking the coupling bet-ween the atomic potential and the applied light fields into account, the original KFR theory is improved and a new expression of photoionization probability of atoms is obtained.From this new expression, photoelectron spectra of H-atom ATI by strong linear-polarized laser in the short-pulse regime are got. This calulated photoelectron spectra are rather similar to the experimental ones. A new explanation of the experimental spectra is given.

Coherent excitation of N=3 Stark states of hydrogen atoms

The authors describe a method for producing a fast atomic hydrogen beam with a large amount of population selected in 3lj, mod mj mod levels. This is realized by a one-step tunable laser excitation of the metastable component of the atomic beam inside a nonhomogeneous static electric field. The linear laser polarization is taken either parallel or perpendicular with respect to the external field. It allows the selective excitation of all the states of the n=3 Stark manifold (with the exception of the mod mj mod =5/2 state). The electric field strength vanishes immediately after the laser excitation. At this point the population of the excited states are better referred to the atomic basis. The shapes of the resonant lines show that two transient effects are leading the excitation process. The first is of an optical kind (i.e. Rabi oscillation and rapid adiabatic passage) and the second is the intra Stark manifold state mixing due to the nonadiabatic time evolution of the external field strength. The internal state of the atoms crossing these fields is completely described by the density matrix evolution calculated independently on two bases (the atomic basis and the Stark instantaneous basis).

Zeeman spectra of the Cs-D 2 line

Zeeman spectra of the Cs- D 2 line are studied as a function of laser frequency for a static magnetic field. The spectral profiles change drastically with the angle between the plane of polarization of the incident, linearly-polarized laser beam and that of the analyzer. The theoretical spectra, allowing for polarizability, are in good agreement with the experimentally determined profiles for arbitrary angles.

Resonance enhanced multiphoton ionization (2+1) in NH 3 using linearly and circularly polarized light

A pulsed tunable dye laser is used for (2 + 1) REMPI in NH 3 via its C′ state with ν 2 = 2. The spectra, in the 3040-3062 Å range, originate in 300 K NH 3. Both circular and linear laser polarization is used. The results are interpreted via the formalism of McClain and Harris. Because (a) we prepare pure light polarization, (b) the transition symmetry is known, and (c) individual rotational lines are resolved, the interpretation is clear and analysis yields molecular invariants that are otherwise difficult to obtain. Comparison is made with a similar treatment for alkyl iodides in which individual lines were not resolved. Alternatively, the same NH 3 transitions are also observed via the optogalvanic effect in a discharge. Because the REMPI process occurs mainly at the focus of the laser, spatially resolved diagnostics are possible.

Rotational Raman Scattering Using Molecular Nitrogen Gas for Calibration of Thomson-Scattering Apparatus

Anti-Stokes rotational Raman lines in molecular nitrogen gas were used for the calibration of Thomson-scattering apparatus. It was found that molecular nitrogen gas is suitable for a vessel having strong stray light. The polarization ratio was 0.16 using linear-polarized laser light.

Experimental Raman Cross-Section of Hydrogen and Deuterium Molecules for Absolute Density Calibration of Thomson Scattering Device

The Raman scattered light is polarized only 14% by linearly-polarized ruby laser light incident in D2. The total Raman cross-sections of H2 and D2 for the ruby-laser wavelength of 6943 Å were estimated from an experiment. The Raman cross-section σH2(J) of the J=2 →0 pure rotational transition for H2 was found to be 2.5·10-30 cm2, while σD2(J) of the J=3 →1 pure rotational transition for D2 was 2.3·10-30 cm2.

Resonant photon-catalyzed predissociation and autoionization: Fragment yield, rate constant, and rotational line strengths

General theory and analytic results are given for calculating the fragment yield and the rate constant of bound-to-continuum decompositions of atoms or molecules induced by a multiphoton effect called the resonant photon-as-catalyst effect (PCE). In this effect the rate constant of the reaction is increased due to the presence of the photons while the total number $N$ of photons in a given mode of the electromagnetic field is conserved. The decomposition processes include (1) photon-catalyzed (PC) predissociation from excited states of molecules in general, as well as from ground states in some cases of polyatomic molecules: $AB+N\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\rightarrow}A+B+N\ensuremath{\hbar}\ensuremath{\omega}$; and (2) PC autoionization of atoms or molecules: $A+N\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\rightarrow}{A}^{+}+{e}^{\ensuremath{-}}+N\ensuremath{\hbar}\ensuremath{\omega}$. Both pulsed and continuous-wave laser irradiation are studied. Optimization of the yield and the rate constant by the lifetimes of the molecular states are studied for the first time. Resonant enhancement [Lau, Phys. Rev. Lett. 43, 1009 (1979)] and coherent saturation [Lau, Phys. Rev. A 22, 614 (1980)] in this effect are further elaborated. Laser-stimulated decays and optical Stark shifts due to the continua and other nonresonant discrete states are included in the theory. The rotational line strengths for symmetric top molecules predissociated by resonant PCE using linear-polarized laser radiation are also given.

Experimental evidence for light-shift induced zero-field level crossing (“optical hanle effect”)

The optical analogue of the magnetic Hanle effect where the magnetic field and the Zeeman effect are replaced respectively by a powerful circularly-polarized laser beam detuned from resonance and by the dynamic Stark shift has been observed in a J = 0 ↔ J = 1 two-level system. The experiment has been performed with a barium atomic beam resonantly excited by a weak linearly-polarized laser beam in both narrow-band and broad-band cases.

Multiphoton ionization and two-photon fluorescence excitation spectroscopy of triethylenediamine

The multiphoton ionization (MPI) and two-photon fluorescence excitation (TPFE) spectra of triethylenediamine (DABCO) are obtained and analyzed. A low-lying state (0–0 at 35 795 cm−1) is observed as a two-photon resonance in the four photon ionization spectrum and also in the TPFE spectrum. Circular vs linear laser polarization studies give an A′1 symmetry for this state. The state results from a 3s (+) ←n (+) orbital promotion. The vibrational progressions and sequences observed are also analyzed. The lifetime of the fluorescence resulting from two-photon 1A′1←1A′1 excitation compares well with the lifetime observed upon one-photon ?←? excitation,suggesting that the 1A′1 state is the emitting state. This 1A′1← 1A′1 one photon forbidden, two-photon allowed, transition is most probably responsible for the very weak long wavelength absorption reported in previous conventional spectroscopic studies. The MPI spectra and laser polarization characteristics of the strong (0–0 at 39 825 cm−1) transition of DABCO are also investigated. The transition is found to be both one- and two-photon allowed and is assigned as 1E″[3px,y (+)]←1A1[n (+)].