Collinear Laser Beams Research Articles

Lorentz-force shifts in strong-field ionization with mid-infrared laser fields

In the past, the ionization of atoms and molecules by strong, mid-infrared (IR) laser fields has attracted recurrent interest. Measurements with different IR pulses have demonstrated the crucial role of the magnetic field on the electron dynamics, classically known as the Lorentz force ${\mathbf{F}}_{L}=q\phantom{\rule{0.16em}{0ex}}(\mathsc{E}+\mathbf{v}\ifmmode\times\else\texttimes\fi{}\mathsc{B})$, that acts upon all particles with charge $q$ in motion. These measurements also require the advancement of theory beyond the presently applied methods. In particular, the strong-field approximation (SFA) is typically based on the dipole approximation alone and neglects both the magnetic field and the spatial dependence of the driving electric field. Here we show and discuss that several, if not most, observations from strong-field ionization experiments with mid-IR fields can be quantitatively explained within the framework of SFA, if the Lorentz force is taken into account by nondipole Volkov states in the formalism. The details of such a treatment are analyzed for the (peak) shifts of the polar-angle distribution of above-threshold ionization photoelectrons along the laser propagation, the steering of electron momenta by two not quite collinear laser beams, or the enhanced momentum transfer to photoelectrons in standing-light fields. Moreover, the same formalism promises to explain the generation of high harmonics and other strong-field rescattering phenomena when driven by mid-IR laser fields. All these results show how strong-field processes can be understood on equal footings within the SFA, if one goes beyond the commonly applied dipole approximation.

Multiparameter-controlled laser ionization within a plasma wave for wakefield acceleration

We present an optical setup for well-defined ionization inside a plasma such that precisely controlled spots of high electron density can be generated. We propose to use the setup for Trojan Horse Injection (or Plasma Photocathode Emission) where a collinear laser beam is needed to release electrons inside a plasma wakefield. The reflection-based setup allows a suitable manipulation of the laser near field without disturbing the spectral phase of the laser pulses. A required ionization state and volume can be reached by tuning the beam size, pulse duration and pulse energy. The ionization simulations enable a prediction of the ionization spot and are in good agreement with dedicated experiments which measured the number of electrons created during the laser–gas interaction.

The behaviour of sapphire under intense two-colour excitation by picosecond laser

Ultrashort pulse lasers are evidencing their benefits in the processing of transparent materials. Sapphire is one of the most attractive engineering materials today. It is hard and, therefore, difficult to machine mechanically to the required shape. Laser dicing is one of the promising techniques for sapphire separation. Two-pulse two-colour irradiation was applied to initiate free-shape cutting of the material. Two collinear laser beams with wavelengths of 1064 and 355 nm, pulse duration of 10 ps and inter-pulse delay of 0.1 ns were combined to induce intra-volume modifications (directional cracks) in sapphire for wafer separation. The photon energy of both beams is well below the band gap, and various channels of the multi-photon excitation were involved in the process. Significant enhancement in the modification area was experimentally observed when intensities of focused infrared and ultraviolet beams were within narrow ranges. We discuss the resonant laser–sapphire interaction mechanisms, leading to up to four times higher excitation of the material involving multiple photons and energetic levels of intrinsic defects in the band-gap. The energy level schemes of colour centres involved in two-step multi-photon absorption in sapphire under intensive laser irradiation have been prepared.

In-flight sensing of pollen grains via laser scattering and deep learning

The identification and imaging of pollen grains in-flight was performed via illumination of the pollen grains with three collinear laser beams that had central wavelengths of 450 nm, 520 nm and 635 nm. Two neural networks are reported here; the first neural network was able to categorise pollen grain species from their scattering patterns with ∼86% accuracy, while the second neural network generated images of the pollen grains from their scattering patterns. This work demonstrates the potential application of laser scattering and deep learning for real-world in-flight pollen identification.

In-situ thermochemical analysis of hybrid rocket fuel oxidation via laser absorption tomography of $$\text {CO}$$, $$\text {CO}_{2}$$, and $$\text {H}_{2}\text {O}$$

A laser absorption tomography (LAT) technique was developed for investigating the thermochemical structure of a solid fuel oxidation layer in a hybrid rocket geometry. The measurement strategy utilizes tunable infrared lasers to target rovibrational transitions of three major combustion species: carbon monoxide ( $$\text {CO}$$ ), carbon dioxide ( $${\text{CO}}_{2}$$ ), and water ( $$\text {H}_{2}\text {O}$$ ). Species-specific molecular absorption was measured using a quantum cascade laser (QCL) near 4.98 $$\mu$$ m for $$\text {CO}$$ , an interband cascade laser (ICL) near 4.19 $$\mu$$ m for $$\text {CO}_{2}$$ , and a diode laser near 2.48 $$\mu$$ m for $$\text {H}_{2}\text {O}$$ . Spectrally- and spatially-resolved absorption data was collected by translating collinear laser beams across the exit plane of a fuel cylinder with an oxidizer core flow at various fuel-grain lengths. Under an assumption of azimuthal symmetry, Tikhonov-regularized Abel inversion was performed to yield radially-resolved absorption coefficient, from which a two-line method was used to infer temperature and species mole fraction. Planar measurements at different axial distances were compiled to form two-dimensional images, spatially-resolving the thermochemical structure downstream of the oxygen injector. The method is demonstrated to visualize the oxidation of two fuels, poly(methyl methacrylate) (PMMA) and high-density polyethylene (HDPE), with two injector geometries, highlighting the capability to discern variations in hybrid rocket motor design.

Terahertz generation via laser coupling to anharmonic carbon nanotube array

A scheme of terahertz radiation generation employing a matrix of anharmonic carbon nanotubes (CNTs) embedded in silica is proposed. The matrix is irradiated by two collinear laser beams that induce large excursions on CNT electrons and exert a nonlinear force at the beat frequency ω = ω1−ω2. The force derives a nonlinear current producing THz radiation. The THz field is resonantly enhanced at the plasmon resource, ω=ωp(1+β)/2, where ωp is the plasma frequency and β is a characteristic parameter. Collisions are a limiting factor, suppressing the plasmon resonance. For typical values of plasma parameters, we obtain power conversion efficiency of the order of 10−6.

Characterization of single particle aerosols by elastic light scattering at multiple wavelengths

We describe a system to characterize individual aerosol particles using stable and repeatable measurement of elastic light scattering. The method employs a linear electrodynamic quadrupole (LEQ) particle trap. Charged particles, continuously injected by electrospray into this system, are confined to move vertically along the stability line in the center of the LEQ past a point where they are optically interrogated. Light scattered in the near forward direction was measured at three different wavelengths using time-division multiplexed collinear laser beams. We validated our method by comparing measured silica microsphere data for four selected diameters (0.7, 1.0, 1.5 and 2.0 µm) to a model of collected scattered light intensities based upon Lorenz-Mie scattering theory. Scattered light measurements at the different wavelengths are correlated, allowing us to distinguish and classify inhomogeneous particles.

Laser beat frequency heating of a rippled density plasma

Two collinear laser beams propagating through a rippled density plasma, with their frequency difference close to plasma frequency, resonantly excite a large amplitude plasma wave. The density ripple of suitable wavenumber slows down the plasma wave very significantly, leading to strong electron heating via the Landau damping of the plasma wave. An analytical framework of the process is developed and the electron temperature scaling with plasma density, laser power and laser frequency have been obtained. Its relevance to recent experiments on intense short pulse laser plasma interaction has been discussed.

Multireflection sum frequency generation vibrational spectroscopy.

We developed a multireflection data collection method in order to improve the signal-to-noise ratio (SNR) and sensitivity of sum frequency generation (SFG) spectroscopy, which we refer to as multireflection SFG, or MRSFG for short. To achieve MRSFG, a collinear laser beam propagation geometry was adopted and trapezoidal Dove prisms were used as sample substrates. An in-depth discussion on the signal and SNR in MRSFG was performed. We showed experimentally, with "m" total internal reflections in a Dove prism, MRSFG signal is ∼m times that of conventional SFG; SNR of the SFG signal-to-background is improved by a factor of >m(1/2) and <m. MRSFG also improved the SFG sensitivity to resolve weak vibrational signals. Surface molecular structures of adsorbed ethanol molecules, polymer films, and a lipid monolayer were characterized using both MRSFG and conventional SFG. Molecular orientation information on lipid molecules with a 9% composition in a mixed monolayer was measured using MRSFG, which showed a good agreement with that derived from 100% lipid surface coverage using conventional SFG. MRSFG can both improve the spectral quality and detection limit of SFG spectroscopy and is expected to have important applications in surface science for studying structures of molecules with a low surface coverage or less ordered molecular moieties.

Issues in deep ocean collinear double-pulse laser induced breakdown spectroscopy: Dependence of emission intensity and inter-pulse delay on solution pressure

Double-pulse laser-induced breakdown spectroscopy (DP-LIBS) with a collinear laser beam orientation is shown for high-pressure bulk aqueous solutions (up to 50bar) along with bubble and plasma images. These investigations reveal that the emission plasma is quenched much more rapidly in solution requiring much shorter detector gate delays than for typical LIBS measurements in air. Also, the emission is inversely proportional to solution pressure, and the most intense emission at all pressures occurs when the laser-induced vapor bubble is at a maximum diameter. It is also shown that the laser-induced bubble grows initially at the same rate for all solution pressures, collapsing more quickly as the pressure is increased. Intense emission is best obtained for conditions where the laser-induced bubble formed by the first laser pulse is small and spherically shaped.

Development of the collinear laser beam line at TRIUMF

The low-energy collinear laser beam line at ISAC routinely produces highly nuclear-spin polarized beams of 8Li + for depth-controlled studies of thin films and near-surface phenomena. It also is used for nuclear structure and fundamental symmetry studies requiring other beams of polarized lithium and sodium isotopes. Further development of polarized 20F and 11Be beams is described, the latter in some detail. Laser fluorescence spectroscopy at high sensitivity using unpolarized bunched beams is briefly mentioned.

Coherent anti‐Stokes Raman scattering microscopy within a microcavity with parallel mirrors

AbstractCoherent anti‐Stokes Raman scattering (CARS) microscopy, or micro‐CARS, is becoming more and more popular to identify chemical species with notable three‐dimensional resolution. In view of this important application, the present work tries to extend the analysis of cavity effects that are well known in other branches of linear and nonlinear optics. In particular, the CARS process taking place within a Fabry‐Pérot microcavity (with mirror spacing on the order of the anti‐Stokes wavelength) is here examined. Two theoretical cavity models are used and found interchangeable as to the description of the typical phenomenology of enhancement and inhibition expected for the electromagnetic confinement induced by the boundary conditions. These phenomena are responsible of strong alterations in the spatial distribution of micro‐CARS, and instructive examples are shown for spherical Raman scatterers of different sizes. To sum them up, the signal enhancement occurs in correspondence of pronounced narrowing and elongation of the emission lobes surrounding the optical axis (coincident with the propagation direction of the collinear laser beams). Conversely, the inhibition is characterized by the ejection of radiation at large angles (with respect to the optical axis) for those wavelengths that do not respect the cavity tuning adjusted on the given anti‐Stokes wavelength. The different angular properties of spectrally separated components of CARS signals are then proven useful for the amplification of the contrast defining the quality of CARS imaging. In the end, amplifications of one or two orders of magnitude are calculated depending on different experimental conditions. Limits and future developments of the present approach are discussed in the conclusions. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Nuclear Charge Radii ofBe7,9,10and the One-Neutron Halo NucleusBe11

Nuclear charge radii of $^{7,9,10,11}\mathrm{Be}$ have been determined by high-precision laser spectroscopy. On-line measurements were performed with collinear laser spectroscopy in the $2{s}_{1/2}\ensuremath{\rightarrow}2{p}_{1/2}$ transition on a beam of ${\mathrm{Be}}^{+}$ ions. Collinear and anticollinear laser beams were used simultaneously, and the absolute frequency determination using a frequency comb yielded an accuracy in the isotope-shift measurements of about 1 MHz. Combining this with accurate calculations of the mass-dependent isotope shifts yields nuclear charge radii. The charge radius decreases from $^{7}\mathrm{Be}$ to $^{10}\mathrm{Be}$ and then increases for the halo nucleus $^{11}\mathrm{Be}$. When comparing our results with predictions of ab initio nuclear-structure calculations we find good agreement. Additionally, the nuclear magnetic moment of $^{7}\mathrm{Be}$ was determined to be $\ensuremath{-}1.3995(5){\ensuremath{\mu}}_{N}$ and that of $^{11}\mathrm{Be}$ was confirmed with an accuracy similar to previous $\ensuremath{\beta}$-NMR measurements.

Alignment of nanoparticles formed on the surface of 6H-SiC crystals irradiated by two collinear femtosecond laser beams

Nanoripples with periods of 150 and 80nm are formed on the surface of 6H-SiC crystals irradiated by the p-polarized 800nm and the s-polarized 400nm femtosecond lasers, respectively. When both of the two collinear laser beams focus simultaneously on the sample surface, nanoparticles are formed on the whole ablation area, and they array in parallel lines. We propose and confirm that the second harmonics in the sample surface excited by the incident lasers plays an important role in the formation of nanostructures.

Optical trapping and coherent anti-Stokes Raman scattering (CARS) spectroscopy of submicron-size particles

Optical trapping combined with coherent anti-Stokes Raman scattering (CARS) spectroscopy is demonstrated for the first time as a new technique for the chemical analysis of individual particles over an extended period of time with high temporal resolution. Single submicron-size particles suspended in aqueous media are optically trapped and immobilized using two tightly focused collinear laser beams from two pulsed Ti:Sapphire laser sources. The particles can remain stably trapped at the focus for many tens of minutes. The same lasers generate a CARS vibrational signal from the molecular bonds in the trapped particle when the laser frequencies are tuned to a vibrational mode of interest, providing chemical information about the sample. The technique is characterized using single polystyrene beads and unilamellar phospholipid vesicles as test samples and can be extended to the study of living biological samples. This novel method could potentially be used to monitor rapid dynamics of biological processes in single particles on short time scales that cannot be achieved by using other vibrational spectroscopy techniques.

The collinear laser beam line at ISAC

The collinear laser beam line at ISAC is producing a range of highly polarized Li and Na isotopes with high beam transmission efficiency. Using a Ta production target, a 8 Li rate of more than 10 8 atoms s −1 with 60% polarization at the user experiment is typical. The beam line is also used for laser induced fluorescence spectroscopy on heavy ions and atoms. A preliminary scheme for polarizing 20 F is presented.

Nonlinear magneto-optical rotation produced by atoms near aJ=1→J=0transition

The nonlinear magneto-optical rotation in a medium consisting of $J=1\ensuremath{\rightarrow}J=0$ atoms placed in a static magnetic field is studied. The density matrix approach and irreducible atomic basis are used to describe the state of the atomic system. The stationary propagation equations for two collinear laser beams with perpendicular circular polarizations are derived and analyzed in the case of the magnetic field perpendicular to the light propagation direction. The effect of the linear polarization rotation toward the direction parallel or perpendicular to the magnetic field vector and lossless propagation of the resulting light are predicted. The conversion of the circularly polarized beam into linearly polarized one is shown. The propagation of the leading edges of switched on cw-laser beams and their stationary propagation are analyzed numerically. The dependence of the considered effects on the light detuning and on the additional magnetic field component parallel to the light propagation direction is discussed. The destructive role of the collisional relaxation is demonstrated.

Rotation of a Fine Particle with Circularly Polarized Anti-Parallel Collinear Laser Beams

We have succeeded in the operation of rotating a fine particle which was held without mechanical contact by circularly polarized laser beams with opposite propagation directions. A polystyrene latex particle 11.9 μm in diameter deformed in a disk-like shape was dispersed in water, and was trapped using the anti-parallel collinear laser beams which were set up so that they had the circular polarization rotating in the same direction. It was observed that the trapped particle rotated in the same direction as the rotation of the circular polarization of light. Inversion of the rotational direction of the circular polarization of light reversed the direction of the rotation of the particle. Although the rotational speed of a particle varied significantly from particle to particle, it was found to be proportional to the laser beam intensity. A typical value for the rotational speed per beam intensity was 2.1 Hz/W.

Measurement of the electric quadrupole moments of 26–29Na

The nuclear electric quadrupole moments of the isotopes 26Na, 27Na, 28Na and 29Na were measured by β-NMR spectroscopy in single crystals of LiNbO3 and NaNO3. High degrees of nuclear polarization were produced by optical pumping of the sodium atoms in a fast beam with a collinear laser beam. The polarized nuclei were implanted into the crystals and NMR signals were observed in the β-decay asymmetries. Preparatory measurements also yielded improved values for the magnetic moments of 27–31Na and confirmed the spin I = 3/2 for 31Na. The results are discussed in comparison with large-basis shell model calculations.

Excitation of electrostatic waves at the beat frequency of two laser beams in a semiconductor plasma

A theoretical investigation has been made of the parametric excitation of electrostatic waves at the beat frequency of two high-power, collinear, uniform laser beams in a highly collisional, magneto-active, compensated semiconductor plasma. The gyro-kinetic approach has been employed to find the linear and nonlinear response of electrons and holes in this semiconductor plasma. It is found that for typical plasma parameters in compensated Ge, two collinear laser beams of equal power density, 2.37 MW cm-2, can excite beat waves of power density 1 kW cm-2. The effects of electron-phonon collision frequency (v), transversely applied external magnetic field (B0), plasma density (n0), and variation of the effective mass of electrons and holes on this parametric excitation process in a semiconductor plasma are discussed.