Redistribution Of Laser Energy Research Articles

Intensity-surged and bandwidth-extended terahertz radiation in two-foci cascading plasmas.

The two-color strong-field mixing in gas medium is a widely used approach to generate bright broadband terahertz (THz) radiation. Here, we present a new, to the best of our knowledge, and counterintuitive method to promote THz performance in the two-color scheme. Beyond our knowledge that the maximum THz generation occurs with two-color foci overlapped, we found that, when the foci of two-color beams are noticeably separated along the propagation axis resulting in cascading plasmas, the THz conversion efficiency is surged by one order of magnitude and the bandwidth is stretched by more than two times, achieving 10-3 conversion efficiency and >100 THz bandwidth under the condition of 800/400 nm, ∼35 fs driving lasers. With the help of the pulse propagation equation and photocurrent model, the observations can be partially understood by the compromise between THz generation and absorption due to the spatial redistribution of laser energy in cascading plasmas. The present method can be extended to a mid-infrared driving laser, and new records of THz peak power and conversion efficiency are expected.

Enhancing laser-nanoparticle interactions by diffused laser beams: efficient size-reduction of nanoparticles

Generally speaking, a laser beam with a good spatial profile such as flat-top or Gaussian (TEM00 mode) shape is considered to be a prerequisite to maximize laser-matter interactions. On the contrary, we show that if the process of interest has a threshold in terms of laser fluence or intensity, a diffused laser beam can do a good job of inducing the process. As an example, we demonstrate the efficient size-reduction of colloidal nanoparticles by a diffused laser beam and identify that the physical origin of this counterintuitive results is a redistribution of laser energy, i.e. formation of speckles through a diffuser where the local laser fluence exceeds the size-reduction threshold. We report the systematic results for silver and gold nanoparticles.

Fabricating low loss waveguides over a large depth in glass by temperature gradient assisted femtosecond laser writing.

We propose a strategy of temperature gradient assisted femtosecond laser writing for elaboration of low loss waveguides (WGs) over a large depth in glass. The matter flow driven by the temperature distribution is responsible for forming a highly densified WG core with tunable size. Importantly, the unique position of the guiding core outside the focus allows for abating the influence of laser energy redistribution and inscribing low loss deep WGs. A low insertion loss (Li) of 0.6 dB at 1550 nm is achieved for WGs at the depth from 300 µm to 900 µm. Establishing strong dependence of Li on the WG size offers a unique route to improve WG performance. These findings highlight that the present method would provide new opportunities for creating low loss WG lattices at large depth.

Micropatterning: Ultrafast Large‐Area Micropattern Generation in Nonabsorbing Polymer Thin Films by Pulsed Laser Diffraction (Small 6/2011)

AbstractAn ultrafast, parallel, and beyond‐the‐master micropatterning technique for ultrathin (30−400 nm) nonabsorbing polymer films by diffraction of laser light through a 2D periodic aperture is reported. The redistribution of laser energy absorbed by the substrate causes self‐organization of polymer thin films in the form of wrinklelike surface relief structures caused by localized melting and freezing of the thin film. Unlike conventional laser ablation and laser writing processes, low laser fluence is employed to only passively swell the polymer as a pre‐ablative process without loss of material, and without absorption/reaction with incident radiation. Self‐organization in the thin polymer film, aided by the diffraction pattern, produces microstructures made up of thin raised lines. These regular microstructures have far more complex morphologies than the mask geometry and very narrow line widths that can be an order of magnitude smaller than the openings in the mask. The microstructure morphology is easily modulated by changing the film thickness, aperture size, and geometry, and by changing the diffraction pattern.

Ultrafast Large‐Area Micropattern Generation in Nonabsorbing Polymer Thin Films by Pulsed Laser Diffraction

An ultrafast, parallel, and beyond-the-master micropatterning technique for ultrathin (30-400 nm) nonabsorbing polymer films by diffraction of laser light through a 2D periodic aperture is reported. The redistribution of laser energy absorbed by the substrate causes self-organization of polymer thin films in the form of wrinklelike surface relief structures caused by localized melting and freezing of the thin film. Unlike conventional laser ablation and laser writing processes, low laser fluence is employed to only passively swell the polymer as a pre-ablative process without loss of material, and without absorption/reaction with incident radiation. Self-organization in the thin polymer film, aided by the diffraction pattern, produces microstructures made up of thin raised lines. These regular microstructures have far more complex morphologies than the mask geometry and very narrow line widths that can be an order of magnitude smaller than the openings in the mask. The microstructure morphology is easily modulated by changing the film thickness, aperture size, and geometry, and by changing the diffraction pattern.

Effect of particle size on the UV pulsed-laser scribing in computational fluid dynamics-based simulations

A computational model for UV pulsed-laser scribing of silicon target is presented and compared with experimental results. The experiments were performed with a high-power Q-switched diode-pumped solid state laser which was operated at 355 nm. They were conducted on n-type 500 μm thick silicon wafers. The scribing width and depth were measured using scanning electron microscopy. The model takes into account major physics, such as heat transfer, evaporation, multiple reflections, and Rayleigh scattering. It also considers the attenuation and redistribution of laser energy due to Rayleigh scattering. Especially, the influence of the average particle sizes in the model is mainly investigated. Finally, it is shown that the computational model describing the laser scribing of silicon is valid at an average particle size of about 10 nm.

Enhancement of third-harmonic emission from femtosecond laser filament screened partially by a thin fiber

The third-harmonic (TH) emission characteristics from femtosecond laser filament, the center of which is screened by a thin metallic fiber, are experimentally investigated. The intensity of the TH emission has been enhanced for 1 order of magnitude by comparing with the undisturbed filament. The physical mechanism of the TH enhancement is analyzed to be the diffraction of the TH emission on the fiber and a redistribution of laser energy during the reconstruction of the two-colored filament. These two factors can release a part of the TH energy from the filament core into the background and keep more TH energy after the termination of filament.

Channels of energy redistribution in short-pulse laser interactions with metal targets

The kinetics and channels of laser energy redistribution in a target irradiated by a short, 1 ps, laser pulse is investigated in computer simulations performed with a model that combines molecular dynamics (MD) simulations with a continuum description of the laser excitation and relaxation of the conduction band electrons, based on the two-temperature model (TTM). The energy transferred from the excited electrons to the lattice splits into several parts, namely the energy of the thermal motion of the atoms, the energy of collective atomic motions associated with the relaxation of laser-induced stresses, the energy carried away from the surface region of the target by a stress wave, the energy of quasi-static anisotropic stresses, and, at laser fluences above the melting threshold, the energy transferred to the latent heat of melting and then released upon recrystallization. The presence of the non-thermal channels of energy redistribution (stress wave and quasi-static stresses), not accounted for in the conventional TTM model, can have important implications for interpretation of experimental results on the kinetics of thermal and mechanical relaxation of a target irradiated by a short laser pulse as well as on the characteristics of laser-induced phase transformations. The fraction of the non-thermal energy in the total laser energy partitioning increases with increasing laser fluence.

Thermally induced optical property changes in myocardium at 1.06 μm

Light in the visible and near-infrared region is diffusely scattered in tissues by macromolecules. It was therefore hypothesized that tissue coagulation caused by high-power continuous wave laser irradiation might significantly alter tissue optical properties, resulting in a redistribution of laser energy during the laser ablation process. Infrared transmittance studies confirmed the hypothesis by demonstrating an irreversible decrease in light transmittance (45%) during heating of a 0.75 mm thick slice of tissue. Absorption and scattering coefficients were then determined from transmittance and reflectance measurements on thin slices of raw and coagulated myocardium irradiated with a Nd:YAG laser (1.06 microns). The scattering coefficient was found to increase fourfold (0.427 mm-1----1.74 mm-1) during tissue coagulation, while the absorption coefficient remained relatively unchanged (0.044 mm-1----0.051 mm-1). Calculations indicate that the coagulation-induced changes in tissue optical properties substantially increase surface back-scattering and reduce tissue penetration.

Controlled timewise redistribution of laser energy

An electronic feedback circuit is described which has been used in a Nd:glass high-power laser system for converting the spiky light emission into a smooth near-rectangular output of adjustable intensity and duration. The feedback circuit is also capable of converting the laser emission into a stroboscopic output of frequency of several hundreds of kilohertz or into a Q -switched high-intensity output. The attractive feature of the system is that the change from one type of laser operation into the other does not require any modification to the circuit, but only a simple readjustment of a few control knobs. The feedback system incorporates a high-current fast photodiode, a Pockels-cell shutter, and a pulser. The main innovation of the present electronic feedback circuit over previously proposed systems is the ability to set the Pockels cell at any point of its transmission curve. This feature will be demonstrated to be of decisive importance in smoothing out the laser emission and in converting the spiky light output into a series of periodic light pulses.