Low Laser Intensity Research Articles

Diode-Laser Induced Fluorescence Spectroscopy of an Optically Thick Plasma in Combination with Laser Absorption Spectroscopy

Distortion of laser-induced fluorescence profiles attributable to optical absorption and saturation broadening was corrected in combination with laser absorption spectroscopy in argon plasma flow. At high probe-laser intensity, saturated absorption profiles were measured to correct probe-laser absorption. At low laser intensity, nonsaturated absorption profiles were measured to correct fluorescence reabsorption. Saturation broadening at the measurement point was corrected using a ratio of saturated to non-saturated broadening. Observed LIF broadening and corresponding translational temperature without correction were, respectively,2.20±0.05 GHz and2510±100 K and corrected broadening and temperature were, respectively,1.96±0.07 GHz and1990±150 K. Although this correction is applicable only at the center of symmetry, the deduced temperature agreed well with that obtained by LAS with Abel inversion.

Double ionization effect in electron accelerations by high-intensity laser pulse interaction with a neutral gas

We study the effect of laser-induced double-ionization of a helium gas (with inhomogeneous density profile) on vacuum electron acceleration. For enough laser intensity, helium gas can be found doubly ionized and it strengthens the divergence of the pulse. The double ionization of helium gas can defocus the laser pulse significantly, and electrons are accelerated by the front of the laser pulse in vacuum and then decelerated by the defocused trail part of the laser pulse. It is observed that the electrons experience a very low laser-intensity at the trailing part of the laser pulse. Hence, there is not much electron deceleration at the trailing part of the pulse. We found that the inhomogeneity of the neutral gas reduced the rate of tunnel ionization causing less defocusing of the laser pulse and thus the electron energy gain is reduced.

Simulation of cross-correlation method for temporal characterization of VUV free-electron-lasers

The cross-correlation method for temporal characterization is investigated using simulations of the twocolor above threshold ionization (ATI) on He induced by a vacuum ultraviolet (VUV) free-electron laser (FEL) in the presence of an infrared (IR) field. Non-linear dependencies of the sideband structure produced in the two-color ATI process are expressed as a function of IR laser intensity by considering the spatial distributions and temporal jitter of both lasers. The temporal properties of the FEL pulse can be characterized accurately using the cross-correlation method at a low IR laser intensity of ~3 \times 10^{10} W/cm2 but with low cross-correlation signals. When the dynamic range of sidebands is increased to high IR intensity, the accuracy of the cross-correlation method becomes crucially dependent on the actual nonlinear index. An approach of determining this index is proposed here to improve the accuracy of temporal characterizations.

Non-sequential double ionization of argon by elliptically polarized laser pulses

With a classical ensemble model, we investigated non-sequential double ionization (NSDI) of argon by elliptically polarized laser pulses. The results show that the correlation behaviors of two electrons depend strongly on the laser intensity. At relatively high laser intensity, the momentum spectra of two electrons along the long axis of the laser polarization plane are mainly distributed in the first and third quadrants and display V-like structures. However, at relatively low laser intensity, the momentum spectra of two electrons are mainly distributed in the second and fourth quadrants. By back analyzing the classical trajectories of NSDI, we find that all of the successful NSDI events still come from recollision in the cases of elliptically polarized laser pulses, and the final-state electron repulsion plays a decisive role for the V-like structure along the long axis of the laser polarization plane. In addition, we find that the initial velocity of the first electron at ionization along the short axis of the laser polarization plane are essential for the recollision, and the time delay between the first ionization and recollision depends on the ellipticity strongly.

Generation of an extreme ultraviolet supercontinuum with a multicycle chirped laser and a static electric field

We theoretically present a method for generating an ultrabroad extreme ultraviolet (XUV) supercontinuum by using the combination of a multicycle chirped laser and a static electric field. At a low laser intensity, the spectral cutoff is extended to the 495th order harmonic, and the bandwidth of the supercontinuum spectrum is broadened to 535 eV. At a high laser intensity, the harmonic cutoff is enlarged to the 667th order, and a supercontinuum covering a bandwidth of 1035 eV is generated. In these two cases, the long quantum path is removed, and the short quantum path is selected. Especially for the relatively high laser intensity, an isolated 23-attosecond pulse with a bandwidth of about 170.6 eV is directly obtained. Finally, we also analyze the influences of the chirp parameter and the duration of the chirped pulse as well as the static field strength on the supercontinuum.

Influence of lateral target size on hot electron production and electromagnetic pulse emission from laser-irradiated metallic targets

The influences of lateral target size on hot electron production and electromagnetic pulse emission from laser interaction with metallic targets have been investigated. Particle-in-cell simulations at high laser intensities show that the yield of hot electrons tends to increase with lateral target size, because the larger surface area reduces the electrostatic field on the target, owing to its expansion along the target surface. At lower laser intensities and longer time scales, experimental data characterizing electromagnetic pulse emission as a function of lateral target size also show target-size effects. Charge separation and a larger target tending to have a lower target potential have both been observed. The increase in radiation strength and downshift in radiation frequency with increasing lateral target size can be interpreted using a simple model of the electrical capacity of the target.

Resonant effects in nonlinear photon absorption during femtosecond laser ablation of Nd-doped silicate glass

This study investigates the resonant effects in nonlinear photon absorption in femtosecond laser ablation of Nd-doped silicate glass (Nd:glass). During the femtosecond laser ablation process, the resonant ablation threshold fluence is decreased by up to 40% compared with that of ordinary ablation. However, it is found that the resonant effect is closely related with laser intensity, and lower laser intensities are required to achieve a significant enhancement. When the intensity is lower than 2.28×10(14) W/cm(2) at which multiphoton ionization dominates, resonant effect is enhanced by a factor of 1.4 to 4.4. When the intensity is higher than 2.28×10(14) W/cm(2), at which intensity tunnel ionization dominates, the resonant effect becomes weak and gradually fades away. It is shown that the resonant effect is still important for multiphoton ionization yet insignificant for tunnel ionization.

Overview of non-invasive factors (low level laser and low intensity pulsed ultrasound) accelerating tooth movement during orthodontic treatment

The need for orthodontic treatment is increasing all the time. As the treatment is time consuming ranging from a year to several years, any method of reducing the period of treatment and increasing the quality of the tissue will be beneficial to patients. The use of non-invasive techniques such as low level laser therapy and low intensity pulsed ultasound in accelerating orthodontic tooth movement are promising. Thus, this overview study will help to generate more understanding about the background information and the possible applications of them in daily orthodontics, depending on previous literature searching for reviews and original research articles.

Conditions for extracting photoionization cross sections from laser-induced high-order-harmonic spectra

We show that photoionization cross sections of atoms can be accurately extracted from the high-order-harmonic spectra generated by intense infrared lasers only if the degree of ionization in the gas medium is small, implying that for this purpose the high-order-harmonic generation spectra should be taken at low gas pressure and at low laser intensity experimentally.

Electron rescattering at metal nanotips induced by ultrashort laser pulses

We investigate plateau and cutoff structures in photoelectron spectra from nanoscale metal tips interacting with few-cycle near-infrared laser pulses. These hallmarks of electron rescattering, well-known from atom-laser interaction in the strong-field regime, appear at remarkably low laser intensities with nominal Keldysh parameters of the order of $\ensuremath{\gtrsim}\phantom{\rule{-0.16em}{0ex}}\phantom{\rule{-0.16em}{0ex}}10$. Quantum and quasiclassical simulations reveal that a large field enhancement near the tip and the increased backscattering probability at a solid-state target play a key role. Plateau electrons are by an order of magnitude more abundant than in comparable atomic spectra, reflecting the high density of target atoms at the surface. The position of the cutoff serves as an in situ probe for the locally enhanced electric field at the tip apex.

Numerical study of laser-assisted transmission and reflection coefficients of a wave packet by an attractive impurity

We study the transmission and reflection probabilities for a wave packet that collides with an attractive impurity (e.g. a quantum dot, a perturbation on a waveguide, etc.) in the presence of an intense, ultra-fast Gaussian laser pulse with intensity in the range I=1012–1014 W/cm2, wavelength of 780 nm and FWHM of 10 fs. The study is based on a numerical solution to the time-dependent Schrödinger equation by means of a finite difference approach. The attractive impurity is modeled by different 1-D potential shape (square, harmonic, triangular, circular, and double harmonic) wells. We find a strong effect of the laser intensity on the reflection and transmission coefficients as a function of energy of the incoming wave-packet, with the strongest effect in the low collisions energy range and high laser intensity. Furthermore, we find that contrary to the laser-free case where there is no trapping probability by the impurity, in the presence of the laser pulse appears a trapping probability for low collision energies. The probability of trapping increases as the laser intensity increases. We present different scenarios where these results can be of interest.

Comparing light-induced colloidal quasicrystals with different rotational symmetries

Quasicrystals are aperiodic structures with long-range orientational order. Unlike crystals, quasicrystals can, in principle, possess any non-crystallographic rotational symmetry. However, only a few of these rotational symmetries have been observed. By using Monte Carlo simulations of colloidal particles in laser interference patterns with quasicrystalline symmetry, we compare the onset of quasicrystalline order for different rotational symmetries in two dimensions. We find that quasicrystals with 5-, 8-, 10-, and 12-fold rotational axes can be induced with lower laser intensities than quasicrystals with other non-crystallographic rotational symmetries. We relate this finding to the number of local symmetry centers in the respective interference patterns.

Trapping of Nano-Particles Using a Near-Field Optical Fiber Probe

By employing a generalization of the conservation law for momentum using the finite difference time domain (FDTD) method, the feasibility of using a near-field optical fibre probe to create near-field optical trapping is investigated. Numerical results indicate that the scheme is able to trap nanoparticles with diameters of tens of nanometres in a circular shape with lower laser intensity. Using the built system with a tapered metal-coated fibre probe, 120 nm polystyrene particles are trapped in a multi-circular shape with a minimum size of 400 nm. They are at a resolution of λ/7 (λ: laser wavelength) and d (d: tip diameter of fiber probe), respectively.

Angular-resolved Photoelectron Spectroscopy of Superatom Orbitals of Fullerenes

Photoelectron angular distributions from both C(60) and C(70) were recorded for low laser intensity femtosecond and picosecond pulses. Rich structure is seen for electron kinetic energies that lie below the photon energy. Strong, broad peaks are observed for photoelectron energies corresponding to single-photon ionization of so-called superatom molecular orbitals (SAMOs). The very simple angular distributions measured for these peaks, the close similarity of the spectra observed from C(60) and C(70), and the comparison with time-dependent density functional theory provide strong support for the SAMO hypothesis.

Ultrashort Pulsed Laser Micromachining of Polycrystalline Diamond

A femtosecond pulsed Nd:YAG laser was used to micromachine polycrystalline diamonds (PCD) and study the ablated microstructure characteristics under different irradiation parameters. With high laser intensity and low traverse speed, a micro cone with very smooth surface and clear edge can be obtained. There was almost no recast layer around the cone and no phase transition on the ablated surfaces was found. By contrast, the use of low laser intensity and high traverse speed could ablate only the grain boundaries without any damage to the diamond grits. These results suggest a need for further research in femtosecond laser micromachining of PCD for potential applications in fabricating new types of micro grinding wheels or diamond pencils.

A two-dimensional comprehensive hydrodynamic model for femtosecond laser pulse interaction with metals

Femtosecond laser–metal interaction in air and the resultant early plasma evolution are investigated by a two-dimensional comprehensive hydrodynamic model in this paper. The model comprises a two-temperature model and a hydrodynamic model supplemented with a quotidian equation of state model, considering the relevant multiphysical phenomena during the laser–metal interaction. The experimental measurements for plasma expansion were carried out to validate the simulation results, using a shadowgraph technique and direct fluorescence measurement. The evolution of both the early plasma and plume plasma is investigated by the model. The early plasma is proved to be generated by electron emission and ambient gas ionization and splits into several portions during its expansion due to different mechanisms. The plume plasma comes from the target material ejection. The photoelectric emission is revealed to be the dominant electron emission mechanism at high laser intensities, while thermal emission is more important at low laser intensities. The electron emission process and early stage plasma are critical to ultrashort laser–metal interaction, especially at high laser intensities. Without considering this, the electron temperature can be overestimated by as much as 70%.

New methods for high current fast ion beam production by laser-driven acceleration

An overview of the last experimental campaigns on laser-driven ion acceleration performed at the PALS facility in Prague is given. Both the 2 TW, sub-nanosecond iodine laser system and the 20 TW, femtosecond Ti:sapphire laser, recently installed at PALS, are used along our experiments performed in the intensity range 10(16)-10(19) W∕cm(2). The main goal of our studies was to generate high energy, high current ion streams at relatively low laser intensities. The discussed experimental investigations show promising results in terms of maximum ion energy and current density, which make the laser-accelerated ion beams a candidate for new-generation ion sources to be employed in medicine, nuclear physics, matter physics, and industry.

Measurement of heavy-hole spin dephasing in (InGa)As quantum dots

We measure the spin dephasing of holes localized in self-assembled (InGa)As quantum dots by spin noise spectroscopy. The localized holes show a distinct hyperfine interaction with the nuclear spin bath despite the p-type symmetry of the valence band states. The experiments reveal a short spin relaxation time {\tau}_{fast}^{hh} of 27 ns and a second, long spin relaxation time {\tau}_{slow}^{hh} which exceeds the latter by more than one order of magnitude. The two times are attributed to heavy hole spins aligned perpendicular and parallel to the stochastic nuclear magnetic field. Intensity dependent measurements and numerical simulations reveal that the long relaxation time is still obscured by light absorption, despite low laser intensity and large detuning. Off-resonant light absorption causes a suppression of the spin noise signal due to the creation of a second hole entailing a vanishing hole spin polarization.

Acceleration and distribution of laser-ablated carbon ions near the target surface

The plasma plume induced during laser ablation of graphite is studied both experimentally and theoretically. The hydrodynamic model applied shows that the high velocities of the plume observed in the experiment result from the large pressure gradient built up during laser–plume interaction. This means that at a low laser intensity of ∼109 W cm−2 the acceleration mechanism is rather gasdynamical than electrostatic. The calculated temporal profiles of plasma temperature and electron density in the early phase of expansion are in reasonable agreement with experimental profiles obtained from emission spectroscopy. The effect of evolution of the plasma temperature on time-of-flight signals is presented. It has been shown that the appearance of ions with higher charge prior to ions with lower charge often ascribed to the effect of the induced electric field can be explained by the evolution of the plume temperature.

Proton beams emission from laser-generated plasmas

An alternative method employing large dimension ion accelerator systems to generate proton beams can be the production of ions by using a power pulsed laser, operating at high intensity, irradiating in vacuum hydrogenated targets and by extracting the ions of interest from the generated plasma. The choice of the adequate target, of the laser pulse intensity regime and of the ion current obtainable, is strongly dependent of the type of experimental apparatus, as will be discussed in the article. Proton beam emission from experiments conduced at low and high laser intensities are presented, in order to generate protons from about 100 eV, as that prepared at the Physics Department of Messina University, up to about 100 keV, as prepared at INFN-LNS of Catania, and up to about 1 MeV obtained at the international PALS Laboratory of Prague, Czech Republic.