Ultraviolet Femtosecond Pulses Research Articles

High-order harmonic generations in intense MIR fields by cascade three-wave mixing in a fractal-poled LiNbO3 photonic crystal.

We report on the generation of harmonic-like photon upconversion in a LiNbO3-based nonlinear photonic crystal by mid-infrared (MIR) femtosecond laser pulses. We study below bandgap harmonics of various driver wavelengths, reaching up to the 11th order at 4μm driver with 13% efficiency. We compare our results to numerical simulations based on two mechanisms: cascade three-wave mixing and non-perturbative harmonic generation, both of which include quasi-phase matching. The cascade model reproduces well the general features of the observed spectrum, including a plateau-like harmonic distribution and the observed efficiency. This has the potential for providing a source of tabletop few femtosecond ultraviolet pulses.

Angular streaking and sideband formation in rotating terahertz and far-infrared fields

Electron spectra in atomic ionization by extreme ultraviolet (XUV) or soft x-ray femtosecond pulses, in the presence of a strong circularly polarized far-infrared or terahertz (THz) field are theoretically investigated. When the XUV pulse duration is much shorter than the period of the THz field, the conditions for circular streaking are fulfilled. The peculiarities of the circular streaking are discussed. In the other extreme case when the duration of the XUV pulse is longer than the THz field period, an analog of the sidebands is formed. Special attention is paid to the intermediate case when the duration of the XUV pulse is similar to the THz period. In this case, a well-developed interference structure appears in the photoelectron spectra. A quasiclassical description of this interference structure is presented and compared with numerical calculations based on a quantum-mechanical approach.

Rainbow Stars—Multi-Color Laser Emission for Science and Art

Rainbow Stars—Multi-Color Laser Emission for Science and Art

Rapid propagation of a Bloch wave packet excited by a femtosecond ultraviolet pulse

This work was supported by the Spanish Ministry of Economy and Competitiveness MINECO (Project No. FIS2013-48286-C2-1-P). E.E.K. acknowledges the Alexander von Humboldt Foundation.

Extended femtosecond laser wavelength range to 330 nm in a high power LBO based optical parametric oscillator.

We experimentally demonstrate a compact tunable, high average power femtosecond laser source in the ultraviolet (UV) regime. The laser source is based on intra-cavity frequency doubling of a temperature-tuned lithium tribotate (LBO) optical parametric oscillator (OPO), synchronously pumped at 520 nm by a frequency-doubled, Yb-fiber femtosecond laser amplifier system. By adjusting crystal temperature, the OPO can provide tunable visible to near-infrared (NIR) signal pulse, which have a wide spectral tuning range from 660 to 884 nm. Using a β-barium borate (BBO) crystal for intra-cavity frequency doubling, tunable femtosecond UV pulse are generated across 330~442 nm with up to 364 mW at 402 nm.

Optical vortex driven charge current loop and optomagnetism in fullerenes

Endohedral molecular magnets, e.g. as realized in fullerenes containing DySc2N, are promising candidates for molecular electronics and quantum information processing. For their functionalization an ultrafast local magnetization control is essential. Using full ab initio quantum chemistry calculations we predict the emergence of charge current loops in fullerenes with an associated orbital magnetic moment upon irradiation with weak light vortex pulses that transfer orbital angular momentum. The generated current is controllable by the frequency, the vortex topological charge, and the intensity of the light. Numerical and analytical results show that an ultraviolet vortex femtosecond pulse with an intensity ∼1013 W/cm2 generates non-invasively nA unidirectional surface current with an associated magnetic field of hundreds μT at the center of the fullerene.

High-speed photorefractive keratectomy with femtosecond ultraviolet pulses.

Femtosecond near-infrared lasers are widely used for a number of ophthalmic procedures, with flap cutting in the laser-assisted in situ keratomileusis (LASIK) surgery being the most frequent one. At the same time, lasers of this type, equipped with harmonic generators, have been shown to deliver enough ultraviolet (UV) power for the second stage of the LASIK procedure, the stromal ablation. However, the speed of the ablation reported so far was well below the currently accepted standards. Our purpose was to perform high-speed photorefractive keratectomy (PRK) with femtosecond UV pulses in rabbits and to evaluate its predictability, reproducibility and healing response. The laser source delivered femtosecond 206 nm pulses with a repetition rate of 50 kHz and an average power of 400 mW. Transepithelial PRK was performed using two different ablation protocols, to a total depth of 110 and 150 μm. The surface temperature was monitored during ablation; haze dynamics and histological samples were evaluated to assess outcomes of the PRK procedure. For comparison, analogous excimer ablation was performed. Increase of the ablation speed up to 1.6 s/diopter for a 6 mm optical zone using femtosecond UV pulses did not significantly impact the healing process.

High power tunable femtosecond ultraviolet laser source based on an Yb-fiber-laser pumped optical parametric oscillator.

We report a high average power tunable 51 MHz femtosecond ultraviolet (UV) laser source based on an intra-cavity sum frequency mixing optical parametric oscillator (OPO) pumped by a fiber laser. The UV laser is generated by sum frequency generation (SFG) between the second harmonic of a mode-locked Yb-fiber laser and the signal of the OPO. A non-collinear configuration is used in the SFG to compensate the group velocity mismatch, and to increase the SFG conversion efficiency dramatically. Tunable ultraviolet pulses within the wavelength range from 385 to 400 nm have been produced with a maximum average power of 402 mW and a pulse width of 286 fs at 2 W Yb-fiber laser pump, corresponding to 20.1% near-infrared to UV conversion efficiency at 387 nm. To our knowledge, this is the first demonstration of tunable femtosecond UV pulse generation from a fiber laser pumped OPO, and is also the highest average power tunable UV femtosecond pulses from an OPO.

Core-level transient absorption spectroscopy as a probe of electron hole relaxation in photoionized H+(H2O)n.

There is fundamental interest in understanding the coupled nuclear and electronic dynamics associated with charge transfer processes in complex molecules and materials, which are often mediated by electron, electron hole or proton motion. With dramatic improvements in the techniques to generate extreme ultraviolet (XUV) and X-ray femtosecond pulses, it now becomes possible to trigger and probe these kinds of processes in real time. Here we study the dynamics of an electron hole created by photoionization in the valence shell of protonated water clusters H(+)(H(2)O)(n). We demonstrate that the electron hole is strongly correlated with the protons forming the hydrogen bond network. We show that it is possible to probe key aspects of the valence electron hole dynamics and the coupled nuclear motion with femtosecond time resolution by resonantly exciting K-shell 1s electrons to fill the electron hole. This represents an opportunity for X-ray transient absorption spectroscopy.

Towards Understanding Photodegradation Pathways in Lignins: The Role of Intramolecular Hydrogen Bonding in Excited States.

The photoinduced dynamics of the lignin building blocks syringol, guaiacol, and phenol were studied using time-resolved ion yield spectroscopy and velocity map ion imaging. Following irradiation of syringol and guaiacol with a broad-band femtosecond ultraviolet laser pulse, a coherent superposition of out-of-plane OH torsion and/or OMe torsion/flapping motions is created in the first excited (1)ππ* (S1) state, resulting in a vibrational wavepacket, which is probed by virtue of a dramatic nonplanar → planar geometry change upon photoionization from S1 to the ground state of the cation (D0). Any similar quantum beat pattern is absent in phenol. In syringol, the nonplanar geometry in S1 is pronounced enough to reduce the degree of intramolecular H bonding (between OH and OMe groups), enabling H atom elimination from the OH group. For guaiacol, H bonding is preserved after excitation, despite the nonplanar geometry in S1, and prevents O-H bond fission. This behavior affects the propensities for forming undesired phenoxyl radical sites in these three lignin chromophores and provides important insight into their relative "photostabilities" within the larger biopolymer.

Molecular alignment induced ultraviolet femtosecond pulse modulation

Ultrashort ultraviolet pulses at 267 nm were generated through four-wave mixing processes in dual-color femtosecond filaments. The generated ultrashort ultraviolet pulses experienced intensity modulation due to molecular-alignment-induced spatial (de)focusing effects, and the output beam profiles were shown to vary observably at different molecular alignment revivals, facilitating all-optical field-free and ultrafast spatial shaping and profile optimization of the ultraviolet pulses.

Revealing the ultrafast process behind the photoreduction of graphene oxide

Effective techniques to reduce graphene oxide are in demand owing to the multitude of potential applications of this two-dimensional material. A very promising green method to do so is by exposure to ultraviolet irradiation. Unfortunately, the dynamics behind this reduction remain unclear. Here we perform a series of transient absorption experiments in an effort to develop and understand this process on a fundamental level. An ultrafast photoinduced chain reaction is observed to be responsible for the graphene oxide reduction. The reaction is initiated using a femtosecond ultraviolet pulse that photoionizes the solvent, liberating solvated electrons, which trigger the reduction. The present study reaches the fundamental time scale of the ultraviolet photoreduction in solution, which is revealed to be in the picosecond regime.

Optimal Dynamic Discrimination in Tryptophan-Containing Dipeptides

Optimal Dynamic Discrimination based on the phase-shaping of deep ultraviolet femtosecond pulses was applied to selectively modulate the time-resolved fluorescence depletion of pairs of tryptophan-containing dipeptides. Our results indicate that phase-sensitive excitation allows their differential identification, beyond the limits of linear and time-resolved spectroscopy.

Corneal stromal ablation with femtosecond ultraviolet pulses in rabbits

To determine the effectiveness of femtosecond ultraviolet (UV) pulses in ablating corneal stroma in a rabbit model and to compare the healing response between eyes treated with femtosecond UV pulses and eyes treated with standard excimer photorefractive keratectomy. Laser Research Center, Vilnius University, Vilnius, Lithuania. Experimental study. Myopic photoablation using a femtosecond UV solid-state laser system was applied to corneas of pigmented rabbits. Experiments in 16 eyes were performed for optimization of the laser system parameters (fluence, spot size, pulse repetition rate) and calibration of ablation rate. In 7 rabbits, deep femtosecond UV ablation (∼130 μm) in 1 eye and shallow ablation (∼30 μm) in the contralateral eye were performed. Nine rabbits received an approximately 30 μm ablation with femtosecond UV pulses in 1 eye and with a conventional excimer system in the contralateral eye. Two eyes were used as controls. The ablation process and surface-temperature dynamics were monitored and recorded. Surface quality and haze development were evaluated. Rabbits were humanely killed 0 to 6 months after surgery, and eyes were enucleated for histological examination. Rabbit corneas ablated with femtosecond UV pulses or excimer laser radiation were similar in terms of the corneal wound-healing process, surface quality, and histology. The experiments indicate the feasibility of clinical application of femtosecond UV lasers for stromal ablation. The ability to switch between laser harmonics allows fast changeover from infrared to the UV mode, implying that a wide range of ophthalmic procedures can be performed using a single solid-state laser device. Ms. Gabryte and Mr. Danielius are paid employees of Light Conversion Ltd. Mr. Danielius is a shareholder of Light Conversion Ltd. No other author has a financial or proprietary interest in any material or method mentioned.

Effect of ambient conditions on ultraviolet femtosecond pulse laser induced breakdown spectra

In this work, the results of a study on the influence of buffer gas and pressure on the LIBS spectrum characteristics are reported. Ultraviolet femtosecond pulses were employed in the analysis of several solid samples including brass, copper, aluminum and silicon under different buffer gasses: air, argon and helium. Given a buffer gas, ambient pressure was varied and its effect on the intensity and broadening of the spectral emission lines and on the background of the spectra was evaluated. A maximum emission intensity of the lines was observed at 500hPa in helium or argon and at 30hPa in air. Line broadening, attributed to the Stark effect, was found to be negligible in spectra acquired with helium as a buffer gas compared to spectra recorded under air or argon atmosphere, which showed noticeable line broadening particularly at high ambient pressure.

Circular Dichroism in Laser-Assisted Short-Pulse Photoionization

A remarkable effect of circular dichroism, i.e., a difference in photoelectron spectra produced by right and left circularly polarized light in two-color multiphoton ionization of atoms, is predicted for the case when the atom is ionized by an extreme ultraviolet or x-ray femtosecond pulse in the field of a strong infrared laser pulse, both pulses being circularly polarized. We show that the sidebands formed in the spectra exhibit different circular dichroism often of different signs both in angle-resolved and angle-integrated experimental conditions. The effect can be used for detecting and measuring circular polarization of x rays in a spectral range where other methods are not effective.

Temporal and phase measurements of ultraviolet femtosecond pulses at 200 nm by molecular alignment based frequency resolved optical gating

We demonstrate that the temporal profile and phase of a weak ultraviolet femtosecond pulse around 200 nm can be measured by molecular alignment based cross-correlation frequency resolved optical gating technique. This technique employs the impulsive alignment of gaseous molecules as gate function and exhibits the advantage of no phase-matching constraint and applicability to pulses at various wavelengths. Simultaneous measurements of near-infrared and ultraviolet pulses are demonstrated. Its agreements with the independent spectral phase interferometry for direct electric field reconstruction and second-order auto-correlation measurements confirm that the molecular alignment gating is a robust technique for ultrashort pulse diagnosis.

The formation of an intense filament controlled by interference of ultraviolet femtosecond pulses

We experimentally investigated the formation of a wavelength-scale photonic plasma grating induced by interference-assisted coalescence of two noncollinear ultraviolet femtosecond laser pulses. The period of the created plasma grating decreased with the crossing angle of the interacting laser pulses. For a proper small crossing angle, the noncollinear ultraviolet filaments were coalesced and an intense single ultraviolet filament was formed with a diameter of 5 μm which was below the focused limitation. This may provide a way to control ultraviolet femtosecond filamentation.

Ultraviolet and near-infrared femtosecond temporal pulse shaping with a new high-aspect-ratio one-dimensional micromirror array

We demonstrate the capabilities of a new optical microelectromechanical systems device that we specifically developed for broadband femtosecond pulse shaping. It consists of a one-dimensional array of 100 independently addressable, high-aspect-ratio micromirrors with up to 3 μm stroke. We apply linear and quadratic phase modulations demonstrating the temporal compression of 800 and 400 nm pulses. Because of the device's surface flatness, stroke, and stroke resolution, phase shaping over an unprecedented bandwidth is attainable.

Cross-correlation frequency-resolved optical gating by molecular alignment for ultraviolet femtosecond pulse measurement

We experimentally demonstrate a molecular-alignment-based cross-correlation frequency-resolved optical gating technique for ultraviolet femtosecond pulse measurement, which use laser-induced impulsive alignment of gaseous molecules as the gate function instead of the phase-matched frequency mixing process in a nonlinear crystal. The spectrogram measurements and retrieving results of the intensity and phase of the second and third harmonic pulses centered at 400 and 267 nm are presented.