595-nm Pulsed Laser Research Articles

Optical field enhanced nonlinear absorption and optical limiting properties of 1-D dielectric photonic crystal with ZnO defect

We report on optical field enhanced nonlinear absorption and on the optical limiting properties of a 1-D photonic crystal with ZnO defect, fabricated by rf sputtering technique. The structural properties of the photonic crystal are studied using scanning electron microscopy (SEM) images. Light transmission spectroscopy measurement shows a broad photonic band gap with a defect mode. Open aperture Z-scan measurement with 532nm pulsed laser illustrates a four times enhancement in the nonlinear absorption coefficient, due to local field enhanced two-photon absorption in the photonic crystal structure with respect to the single layer of ZnO reference. The enhancement of the nonlinear absorption in the photonic crystal, due to the strong confinement of the optical field around ZnO defect layer, leads to an optical power limiting behavior in the photonic crystal. The limiting threshold of the photonic crystal is found to be 0.74J/cm2 @ 532nm with 6ns pulse width, 10Hz repetition rate.

Pocket milling of carbon fiber-reinforced plastics using 532-nm nanosecond pulsed laser: An experimental investigation

Non-traditional machining of carbon fiber-reinforced plastics, such as laser machining, has great advantages over mechanical machining in the aspect of machinability and flexibility. In this article, a laser milling method using diode pumped and frequency doubled Nd:YVO4 nanosecond pulsed laser system is presented. The effects of processing parameters including laser power, scanning speed, and hatch distance were analyzed. It was found that machining quality and efficiency are seriously influenced by laser power and pulses overlapping rate. The features of micro-pit and chopped fibers on the machined surface were observed with metallographic microscope, and ablation mechanism involved was studied to explain the phenomenon. Depth-controlled milling strategy was realized in the laser focusing environment. Finally, challenges and suggestions are presented for wide application of the method.

The ultrafast coherent control of fine green emission in Er3+ ion system by the shaped ultra-short laser pulses

In this paper, we theoretically demonstrate that the fine green emission in Er3+ ion system can be effectively controlled by the shaped ultra-short laser pulses with a π phase modulation. Our results show that the two photon transition probabilities of two excited states are effectively tuned by the single π phase modulation of 800nm or 1600nm laser pulses. By combiningπ phase modulation of the two exciting pulses, one excited state can be populated at maximum value, while the other state population can be widely tuned from zero to the maximum value. Furthermore, we explain the physical control mechanism of fine green emission by considering the second-order power spectrum of the shaped pulses. These schemes can be applied to realize the selective population of multiple excited states.

Optimal generation of high harmonics in the water-window region by synthesizing 800-nm and mid-infrared laser pulses.

We propose a method to optimally synthesize a strong 800-nm Ti:sapphire laser pulse and a relatively weak mid-infrared laser pulse to enhance harmonic yields in the water-window region. The required wavelength of the mid-infrared laser is varied from about 2.0 to 3.2 μm. The optimized waveforms generate comparable harmonic yields as the waveforms proposed in [Sci. Rep.4, 7067 (2014)], but with much weaker intensity for the mid-infrared laser. This method provides an alternative scheme based on the available laser technology to help realize tabletop light source in the water-window region by high-order harmonic generation.

Spatial distribution on high-order-harmonic generation of anH2+molecule in intense laser fields

High-order-harmonic generation (HHG) for the ${{\mathrm{H}}_{2}}^{+}$ molecule in a 3-fs, 800-nm few-cycle Gaussian laser pulse combined with a static field is investigated by solving the one-dimensional electronic and one-dimensional nuclear time-dependent Schr\odinger equation within the non-Born-Oppenheimer approximation. The spatial distribution in HHG is demonstrated and the results present the recombination process of the electron with the two nuclei, respectively. The spatial distribution of the HHG spectra shows that there is little possibility of the recombination of the electron with the nuclei around the origin $z=0$ a.u. and equilibrium internuclear positions $z=\ifmmode\pm\else\textpm\fi{}1.3$ a.u. This characteristic is irrelevant to laser parameters and is only attributed to the molecular structure. Furthermore, we investigate the time-dependent electron-nuclear wave packet and ionization probability to further explain the underlying physical mechanism.

Supercontinuum generation in aqueous core/shell structured nanoparticles

We report the observation of a supercontinuum (SC) in aqueous CdTe/ZnSe quantum dots (QDs) pumped by 800-nm femtosecond laser pulses based on simulations and experiments. For an average nanocrystal of 5.5 nm in size and a weight concentration of 5 mg/mL, the SC generated with a low-input energy range exhibits high optical to optical conversion efficiency. The measured SC spectrum exhibits spectral broadening that is significantly influenced by the nonlinear coefficient of the QDs. This nanoparticles-based SC effect may have special applications for optical coherence imaging and ultrashort pulse compression.

Free-space air molecular lasing from highly excited vibrational states pumped by circularly-polarized femtosecond laser pulses

We experimentally investigate N2+ lasing from highly excited vibrational states with ultrashort, intense 800-nm laser pulses. We observe laser emissions at multiple wavelengths (353.3, 353.8, and 354.9 nm) corresponding to B2Σu+(v′ = 5, 4, 3) → X2Σg+(v = 4, 3, 2) transitions. Specifically, it is found that two strong signals corresponding to the (5–4) transition at 353.3 nm and the (4–3) transition at 353.8 nm, which can be achieved only at relatively low pressures, show a strong dependence on the ellipticity of the pump pulses. This observation suggests that the ions could be populated to high vibrational energy levels of the B2Σu+ state by electron impact excitation.

Influence of rapid laser heating on the optical properties of in-flame soot

To understand the effect of rapid heating on the optical properties of in-flame soot and its potential influence on the laser-induced incandescence (LII) signal, the time-resolved extinction coefficient of soot is measured in diffusion and premixed flames during laser heating. Heating is performed using a 1064-nm pulsed laser with fluences ranging from 0.2 to 6.2 mJ/mm2. Extinction measurements are carried out using continuous-wave lasers at four different wavelengths. A rapid enhancement of extinction, by up to 10 % in the diffusion flame and 18 % in the premixed flame, occurs during laser heating most likely as a result of temperature-dependent optical properties and laser-induced thermal annealing of soot. The thermal expansion of flame gases causes a gradual decline of soot concentration for about 2 μs after the laser pulse. Significant loss of soot material by sublimation is observed at fluences as low as 1.03 and 2.06 mJ/mm2 for the diffusion and premixed flames, respectively. A secondary rise in extinction coefficient is observed from about 50 to 800 ns after the laser pulse at low monitoring wavelengths, attributed to the formation of light-absorbing gaseous species from the sublimated soot material. These effects may impact the LII signal and should be accounted for in LII analysis.

Proton beam writing of dye doped polymer microlasers

Proton beam writing is employed to fabricate smooth sidewall whispering gallery mode microcavities in dye-doped polymer. These microcavities acts as microlasers under optical excitation in ambient atmosphere. Different cavity designs are implemented to obtain directional laser emission from the whispering gallery mode lasers. The microcavities are fabricated in Rhodamine B doped SU-8 polymer and are optically pumped with 532nm pulsed laser. These microlasers emit light within the emission band of Rhodamine B with operational wavelength around 600nm and the required pumping laser threshold is lower than 3μJ/mm2 for all the micro-lasers.

Nonsequential double ionization with time-dependent renormalized-natural-orbital theory

Recently introduced time-dependent renormalized natural orbital theory (TDRNOT) is tested on non-sequential double ionization (NSDI) of a numerically exactly solvable one-dimensional model He atom subject to few-cycle, 800-nm laser pulses. NSDI of atoms in strong laser fields is a prime example of non-perturbative, highly correlated electron dynamics. As such, NSDI is an important "worst-case" benchmark for any time-dependent few and many-body technique beyond linear response. It is found that TDRNOT reproduces the celebrated NSDI "knee," i.e., a many-order-of-magnitude enhancement of the double ionization yield (as compared to purely sequential ionization) with only the ten most significant natural orbitals (NOs) per spin. Correlated photoelectron spectra - as "more differential" observables - require more NOs.

Coupling 3D Monte Carlo light transport in optically heterogeneous tissues to photoacoustic signal generation

The generation of photoacoustic signals for imaging objects embedded within tissues is dependent on how well light can penetrate to and deposit energy within an optically absorbing object, such as a blood vessel. This report couples a 3D Monte Carlo simulation of light transport to stress wave generation to predict the acoustic signals received by a detector at the tissue surface. The Monte Carlo simulation allows modeling of optically heterogeneous tissues, and a simple MATLAB™ acoustic algorithm predicts signals reaching a surface detector. An example simulation considers a skin with a pigmented epidermis, a dermis with a background blood perfusion, and a 500-μm-dia. blood vessel centered at a 1-mm depth in the skin. The simulation yields acoustic signals received by a surface detector, which are generated by a pulsed 532-nm laser exposure before and after inserting the blood vessel. A MATLAB™ version of the acoustic algorithm and a link to the 3D Monte Carlo website are provided.

Filamentation of ultrashort laser pulses in silica glass and KDP crystals: A comparative study

Ionizing 800-nm femtosecond laser pulses propagating in silica glass and in potassium dihydrogen phosphate (KDP) crystal are investigated by means of a unidirectional pulse propagation code. Filamentation in fused silica is compared with the self-channeling of light in KDP accounting for the presence of defect states and electron-hole dynamics. In KDP, laser pulses produce intense filaments with higher clamping intensities up to 200 TW/cm$^2$ and longer plasma channels with electron densities above $10^{16}$ cm$^{-3}$. Despite these differences, the propagation dynamics in silica and KDP are almost identical at equivalent ratios of input power over the critical power for self-focusing.

Molecular dynamics of methanol cation (CH3OH+) in strong fields: Comparison of 800 nm and 7 μm laser fields

Fragmentation and isomerization of methanol cation by short, intense laser pulses were studied by ab initio classical trajectory simulations using CAM-B3LYP/6-31G(d,p) calculations. Compared to random orientation, CH3OH+ with the CO bond aligned with the laser polarization gained nearly twice as much energy from the laser field, in accordance with the higher vibrational intensities in the mid-IR range for aligned CH3OH+. The energy gained by CH3OH+ from 7μm and 800nm laser pulses with intensities of 0.88×1014 and 1.7×1014W/cm2 is proportional to the intensity and the wavelength squared of the laser field.

In-cylinder soot precursor growth in a low-temperature combustion diesel engine: Laser-induced fluorescence of polycyclic aromatic hydrocarbons

The growth of poly-cyclic aromatic hydrocarbon (PAH) soot precursors are observed using a two-laser technique combining laser-induced fluorescence (LIF) of PAH with laser-induced incandescence (LII) of soot in a diesel engine under low-temperature combustion (LTC) conditions. The broad mixture distributions and slowed chemical kinetics of LTC “stretch out” soot-formation processes in both space and time, thereby facilitating their study. Imaging PAH–LIF from pulsed-laser excitation at three discrete wavelengths (266, 532, and 633nm) reveals the temporal growth of PAH molecules, while soot-LII from a 1064-nm pulsed laser indicates inception to soot. The distribution of PAH–LIF also grows spatially within the combustion chamber before soot-LII is first detected. The PAH–LIF signals have broad spectra, much like LII, but typically with spectral profile that is inconsistent with laser-heated soot. Quantitative natural-emission spectroscopy also shows a broad emission spectrum, presumably from PAH chemiluminescence, temporally coinciding with of the PAH–LIF.

Multiphotonk-resolved photoemission from gold surface states with 800-nm femtosecond laser pulses

We measure direct multiphoton photoemission of the Au(111) surface state with 800-nm laser pulses. We observe the parabolic dispersion in the angular distribution of photoelectrons having absorbed between four and seven photons. The ${\mathbf{k}}_{\ensuremath{\parallel}}$ dispersion we measure can be explained in terms of Shockley-state replicas, with a nascent hot electrons distribution at ${\mathbf{k}}_{\ensuremath{\parallel}}$ above the Fermi level. Moderate laser power densities, of the order of $100\phantom{\rule{0.3em}{0ex}}\mathrm{GW}/\mathrm{cm}{}^{2}$, resulted in large electron yields, indicating the importance of multiphoton excitations to define the electronic and magnetic properties of matter in the first hundred femtoseconds after laser excitation.

Synthesis and characterization of translucent MgO-doped Al2O3 hollow spheres in millimeter-scale

Translucent MgO-doped Al2O3 hollow spheres with millimeter-scale in diameter were prepared by a three-sintering-step method using polystyrene (PS) spheres as template through connection function of chitosan-assistant aggregation. The as-prepared hollow spheres were characterized by Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Field-Emission Scanning Electron Microscope (FE-SEM) and Electron Probe Microanalysis (EPMA). The results show that the obtained translucent MgO-doped Al2O3 hollow spheres has diameter of 1–2mm, wall thickness of 130μm and 532nm pulse laser transmittance of 50%. It is also noted that the formation of PS–Al2O3 core–shell spheres is based on the connection function of –OH and –NH2 groups in chitosan molecules. This method provides a synthesizing method for preparing translucent millimeter-scale hollow spheres which have a great potential application for Inertial Confined Fusion (ICF) required hollow pellet.

Proposed classification system for reporting 532‐nm pulsed potassium titanyl phosphate laser treatment effects on vocal fold lesions

Currently, no standard exists for reporting treatment results for the potassium titanyl phosphate (KTP) laser. The goal of this study was to establish a validated classification schema for reporting immediate tissue effects after laser treatment. Evaluation of KTP laser video sequences by academic laryngologists with use of the rating system. A five-point classification system was developed; this included noncontact angiolysis, epithelial blanching, epithelial disruption, contact epithelial ablation, and contact epithelial ablation with tissue removal. Video recordings were made prospectively for each treatment effect. Ten treatment recordings, with two repeated recordings, were presented to seven academic laryngologists, who were asked to categorize each based on the given classification scheme. Overall accuracy for the combined reviewers in rating the treatments was 82%. Six of seven reviewers showed perfect intrarater reliability. Accuracy in rating clips did not correlate with the previous number of 532-nm KTP or 585-nm pulsed dye laser procedures performed but showed a trend toward correlating with total years in practice. This study reveals that standardized reporting of effects of the KTP laser is feasible. We believe that results of KTP treatment should be reported using a validated classification system of immediate laser effect, along with specific laser settings. This classification system allows for future systematic evaluation of long-term treatment results prospectively from single laser treatments.

Quantum-path control and isolated-attosecond-pulse generation using H2+molecules with moving nuclei in few-cycle laser pulses

We theoretically investigate the high-order harmonic generation (HHG) and isolated-attosecond-pulse generation from a full one-dimensional (1D) model of H${}_{2}$${}^{+}$ molecule in 3-fs, 800-nm laser pulses by using numerical solutions of the non-Born-Oppenheimer time-dependent Schr\"odinger equation (TDSE). The numerical results with moving nuclei and the static nuclei are demonstrated. The harmonic spectrum from the 22nd to the cutoff becomes smooth and fewer modulations and an isolated-attosecond pulse with duration 129 as is generated when we consider the nuclear motion. We investigate the emission time of harmonics in terms of the time-frequency analysis, which shows that, with moving nuclei, the long trajectory is suppressed, and the short trajectories is enhanced. We apply the nuclear and electronic probability density and the simulation of classical electron trajectory to illustrate the physical mechanism of HHG.

Fine structures in the intensity dependence of excitation and ionization probabilities of hydrogen atoms in intense 800-nm laser pulses

We studied the elementary processes of excitation and ionization of atomic hydrogen in an intense 800-nm pulse with intensity in the 1.0 to $2.5\ifmmode\times\else\texttimes\fi{}{10}^{14}$ ${\mathrm{W}/\mathrm{cm}}^{2}$ range. By analyzing excitation as a continuation of above-threshold ionization (ATI) into the below-threshold negative energy region, we show that modulation of excitation probability and the well-known shift of low-energy ATI peaks vs laser intensity share the same origin. Modulation of excitation probability is a general strong field phenomenon and is shown to be a consequence of channel closing in multiphoton ionization processes. Furthermore, the excited states populated in general have large orbital angular momentum and they are stable against ionization by the intense 800-nm laser---they are the underlying reason for population trapping of atoms and molecules in intense laser fields.

Coherent quantum control of green emission inEr3+-doped glass byπ-phase-shaped ultrashort laser pulses

We experimentally and theoretically study the coherent quantum control of green emission in Er${}^{3+}$-doped glass by $\ensuremath{\pi}$-phase-shaped 800-nm femtosecond laser pulses. The experimental results show that the green emission intensity is enhanced over the transform-limited pulse by $\ensuremath{\pi}$-phase-shaped pulses as the laser field increases to several 10${}^{12}$ W/cm${}^{2}$. Coherent control of multiphoton absorption is studied based on the fourth-order perturbation theory, and the theoretical results accord well with the experimental observations.