Ultraviolet Femtosecond Pulses Research Articles

Response-time improved hydrothermal-method-grown ZnO scintillator for XFEL timing-observation

An iron doped, hydrothermal method-grown, zinc oxide (ZnO) scintillator was demonstrated to exhibit an over 10 times faster response time of its excitonic fluorescence compared with an undoped-ZnO crystal. For X-ray free electron laser (XFEL) excitation, the fluorescence rise and decay time constants were measured to be 2.1 ps and 70 ps, respectively. Accurate timing control of extreme ultraviolet (EUV) femtosecond pulses necessitate temporal jitters to be evaluated. Results suggest that this scintillator’s response time is sufficient to allow the evaluation of sub 100-ps jitter in XFEL measurement systems and will be crucial in pump and probe experiments in the future.

${\hbox {Nd}}^{3+}{\hbox {:LaF}}_{3}$ as a Step-Wise Excited Scintillator for Femtosecond Ultraviolet Pulses

Vacuum ultraviolet fluorescence from Czochralski method-grown Nd3+:LaF3 excited by 290 nm femtosecond pulses from a Ti:sapphire laser is reported. Peak emission at 175 nm with 7 ns lifetime is observed. This decay time would be one of the fastest among solid-state, inorganic scintillators. Laser emission in the VUV region may also be achievable by pumping with existing ultraviolet solid-state lasers.

Custom-Designed Fast-Response Praseodymium-Doped Lithium 6 Fluoro-Oxide Glass Scintillator With Enhanced Cross-Section for Scattered Neutron Originated From Inertial Confinement Fusion

Scintillation properties of Pr3+-doped 20Al(PO3)3-80 LiF glasses melted in N2 were investigated to seek a candidate for a scattered neutron scintillator in nuclear fusion diagnostics. The fluorescence lifetime of the sample with 217 nm ultraviolet femtosecond pulse excitation was measured to be 19.5 ns. More importantly, the fluorescence lifetime with alpha particles from 241Am radioisotope excitation was determined to be 6.7 ns. Based on our material design strategy, we have successfully developed the fast response time praseodymium-doped 6Li glass scintillator for scattered neutron diagnostics.

High gain broadband amplification of ultraviolet pulses in optical parametric chirped pulse amplifier

We report on a high gain amplification of broadband ultraviolet femtosecond pulses in an optical parametric chirped pulse amplifier. Broadband ultraviolet seed pulses were obtained by an achromatic frequency doubling of the output from a femtosecond Ti:Sapphire oscillator. Stretched seed pulses were amplified in a multipass parametric amplifier with a single BBO crystal pumped by a ns frequency quadrupled Nd:YAG laser. A noncollinear configuration was used for a broadband amplification. The total (after compression) amplification of 2.510(5) was achieved, with compressed pulse energy of 30 microJ and pulse duration of 24 fs. We found that the measured gain was limited by thermal effects induced by the absorption of the pump laser by color centers created in the BBO crystal.

Recent Studies of Laser Science in Paintings Conservation and Research

The removal of aged and deteriorated molecular overlayers from the surface of paintings is a delicate and critical intervention in Cultural Heritage (CH) conservation. This irreversible action gets particularly complicated given the multitude of materials that may be present within a painted work of art (often in ultrathin layers or traces), as well as the exceptional sensitivity of the original surfaces to environmental conditions such as heat, light, and so on. Lasers hold an important role among the available cleaning methodologies, as they enable high control and accuracy, material selectivity, and immediate feedback. Still, prior to their implementation, it is imperative to optimize the cleaning parameters, so to ensure that any potential implications to the remaining materials are minimal and well understood. Toward this aim, research at IESL-FORTH is focused on both refining and continuously updating the laser-cleaning protocols (by introducing novel laser technologies into the field, i.e., ultrashort laser pulses), as well as on investigating and studying the nature and extent of laser-induced physicochemical alterations to the involved materials. In this Account, extended work for the understanding of ultraviolet (UV) laser ablation of polymers is presented. Emphasis is placed on the use of model systems (polymers doped with chromophores of known photochemistry) to examine the in-depth laser-induced modifications at the processed surfaces and thus to illustrate the dependence of their nature and extent on laser parameters and material properties. Furthermore, studies for the potential use of femtosecond UV pulses to overcome certain limitations involved with the nanosecond ablation of molecular overlayers from CH surfaces are highlighted. In particular, it is demonstrated that in the femtosecond regime any chemical modifications are, qualitatively and quantitatively, highly defined, limited and nearly independent of the material properties, such as the absorptivity and the degree of polymerization/molecular weight. Thus, they can be highly potent in the treatment of molecular substrates, enabling new material processing schemes that have not been possible with nanosecond laser technology, as for example, processing of ultrathin varnish layers. Finally, a sensitive indicator is introduced to elucidate the extent of any photochemical or structural modification induced at the substrate on the process of the laser-assisted removal of overpaints. A realistic scenario of an overlayered modern painting is simulated by a sensitive polymer film covered with acrylic paint. The indicator is doped with photosensitizers of known photochemistry and strong fluorescence emission, which allow the employment of laser induced fluorescence (LIF) for the detection of any chemical modifications generated into the substrate during laser cleaning. In addition, nonlinear microscopy techniques are successfully employed to examine the extent of these modifications. The suggested methodology is proven to reliably and accurately detect potential changes, and thus, it can serve as a monitoring tool to fine-tune the cleaning protocol and safeguard the original painting.

Enhanced quantum light generation

The use of a specially designed cavity to enhance the intensity of femtosecond ultraviolet pulses dramatically increases the rate at which non-classical states of light can be produced.

Ultraviolet enhancement cavity for ultrafast nonlinear optics and high-rate multiphoton entanglement experiments

The first enhancement cavity for femtosecond ultraviolet pulses is demonstrated. More than 7 W of average ultraviolet power at an 81 MHz repetition rate, available to pump a nonlinear crystal inside the cavity, is exploited in an implementation of a powerful source for high-rate experiments with entangled multiphoton states.

Amplification of Ultraviolet Femtosecond Pulse by a Micro-Pulling Down Method-Grown Ce:LiCAF Crystal in a Prismatic Cell-Type, Side-Pumping Configuration

We propose an efficient side-pumping scheme for ultraviolet (UV) femtosecond pulse amplification in micro-pulling-down (µPD) method-grown Ce:LiCAF using a prismatic cell-type configuration. By uniformly illuminating four sides of a 2-mm-diameter and 30-mm-length Brewster-cut, nearly as-grown crystal, the risk of damage is dramatically reduced. Without using a chirp pulse amplifier scheme, UV femtosecond pulse at 290 nm is amplified 4 times with no significant increase to its pulse duration and B-integral. Our results show that a laser-quality, Ce:LiCAF crystal, efficiently grown by the µPD method and pumped in a prismatic-cell type configuration paves new possibilities in providing high energy UV femtosecond pulses.

Generation of phase-controlled ultraviolet pulses and characterization by a simple autocorrelator setup

A versatile femtosecond ultraviolet (UV) pulse generation, a phase modulation, and a characterization setup for coherent control applications are demonstrated. For high-performance phase control of ultrashort pulses direct in the UV a microelectromechanical-system-based 2D mirror array is applied. Multiple examples for successful phase control of ultrashort UV pulses are given, such as arbitrarily phase tailoring and pulse recompression in open and closed loop schemes. For simple and effective characterization of the generated pulses, a UV autocorrelator based on two-photon absorption in a solar blind photomultiplier is constructed. The effects of space-time coupling on split mirror autocorrelation measurements are addressed and minimized.

Laser-assisted atom probe analysis of zirconia/spinel nanocomposite ceramics

Contrary to the conventional belief that insulators cannot be field evaporated, we have demonstrated that atom probe analysis of bulk insulating ceramics, yttoria-stabilized tetragonal zirconia with spinel dispersoids, is possible by using an ultraviolet femtosecond pulse laser. Needle-like specimens were prepared from spark plasma sintered bulk ceramics by the focused ion beam technique. Three-dimensional elemental distributions from both the zirconia and spinel have clearly demonstrated that laser-assisted field evaporation yields quantitative atom probe tomography.

Complete characterization of weak ultra-short near-UV pulses by spectral interferometry

We present a method for a complete characterization of a femtosecond ultraviolet pulse when a fundamental near-infrared beam is also available. Our approach relies on generation of the second harmonic from the pre-characterized fundamental, which serves as a reference against which an unknown pulse is measured using spectral interferometry (SI). The characterization apparatus is a modified second harmonic frequency resolved optical gating setup which additionally allows for taking SI spectra. The presented method is linear in the unknown field, simple and sensitive. We checked its accuracy using test pulses generated in a thick nonlinear crystal, demonstrating the ability to measure the phase in a broad spectral range, down to 0.1% peak spectral intensity as well as retrieving π leaps in the spectral phase.

Application of adaptive feedback loop for ultra-violet femtosecond pulse shaper control

We apply an adaptive feedback loop to control a ultra-violet (UV) femtosecond pulse shaping apparatus. The adaptive feedback control is implemented by a continuous parameter genetic algorithm. We use the adaptive shaper to compensate for the pulse chirp. The genetic algorithm produces a pulse with a width of 115 fs, identical to that of the transformlimited pulse. We then apply the adaptive shaper to the Stokes pulse in a femtosecond coherent anti-Stokes Raman scattering (CARS) experiment on dipicolinic acid solution. The algorithm maximizes the first CARS beat signal at the probe pulse delay of 650 fs. We confirm that a transformlimited Stokes pulse achieves the best detection sensitivity.

Characterization of ultraviolet femtosecond pulse propagation in aluminum-coated capillary fibers

We demonstrate that hollow core fibers with aluminum-coated bores of ϕ=0.7mm and ϕ=1.0mm are well suited for guiding high-power ultraviolet femtosecond pulses. We consider 1-m-long fibers in two geometries: straight and bent with a 30-cm radius of curvature. The straight fibers transmit approximately 60% of the power at 200nm and 85% at 266nm, while the corresponding numbers for the bent fibers are 25% and 35%. The duration of the femtosecond pulses increases by 10% and 50% per meter at 200 and 266nm, respectively. The broadening increases to a factor of two when the fiber is bent. The maximum transmitted pulse energy at 266nm is 100μJ corresponding to 0.5GW or an intensity of 1011W∕cm2. However, this value is limited only by the 266nm pulse generation and is expected to go even higher. The applicability of the powerful femtosecond pulses from the fiber is demonstrated by an experiment in which water is ionized by two-photon absorption. This experiment indicates the potential of using aluminized hollow core fibers in medical therapy with ultraviolet femtosecond pulses.

Optical resolution of oriented enantiomers via photodissociation: quantum model simulations for H2POSD

We demonstrate quantum mechanically how to resolve enantiomers from an oriented racemic mixture taking advantage of photodissociation. Our approach employs a femtosecond ultraviolet (UV) laser pulse with specific linear polarization achieving selective photodissociation of one enantiomer from a mixture of L and R enantiomers. As a result, the selected enantiomer is destroyed in the electronically excited state while the opposite enantiomer is left intact in the ground state. As an example we use H2POSD which presents axial chirality. A UV pulse excites the lowest singlet excited state which has nsigma* character and is, therefore, strongly repulsive along the P-S bond. The model simulations are performed using wavepackets which propagate on two dimensional potential energy surfaces, calculated along the chirality and dissociation reaction coordinates using the CASSCF level of theory.

Sub-micron patterning of solid materials with ultraviolet femtosecond pulses

The treatment of solid materials using subpicosecond laser pulses at 248 nm is studied. The potential of the combination of femtosecond pulse durations with ultraviolet wavelengths for precise material processing is demonstrated. Imaging techniques combined with diffractive optical elements are exploited for the fabrication of nanometer-scale features on practically all classes of materials including metals, semiconductors, and dielectrics. Two-dimensional periodic surface structures with excellent quality and reproducibility over large areas with feature sizes down to the 100 nm range are reached .

Conductivity of solvated electrons in hexane investigated with terahertz time-domain spectroscopy.

We present investigations of the transient photoconductivity and recombination dynamics of quasifree electrons in liquid n-hexane and cyclohexane performed using terahertz time-domain spectroscopy (THz-TDS). Quasifree electrons are generated by two-photon photoionization of the liquid using a femtosecond ultraviolet pulse, and the resulting changes in the complex conductivity are probed by a THz electromagnetic pulse at a variable delay. The detection of time-domain wave forms of the THz electric field permits the direct determination of both the real and the imaginary part of the conductivity of the electrons over a wide frequency range. The change in conductivity can be described by the Drude model, thus yielding the quasifree electron density and scattering time. The electron density is found to decay on a time scale of a few hundred picoseconds, which becomes shorter with increasing excitation density. The dynamics can be described by a model that assumes nongeminate recombination between electrons and positive ions. In addition, a strong dependence of the quasifree electron density on temperature is observed, in agreement with a two-state model in which the electron may exist in either a quasifree or a bound state.

Femtosecond measurements of two-photon absorption coefficients at lambda = 264 nm in glasses, crystals, and liquids.

Using ultraviolet femtosecond pulses with high irradiance stability, we measured the two-photon absorption (TPA) coefficients in a number of substances with a total accuracy of approximately 10%. Six commercial fused-silica samples (KU-1, Coming 7940, SQ, Suprasil, Herasil, and Infrasil) possess TPA coefficients (beta values) of approximately 2 x 10(-11) cm/W. For crystalline quartz and sapphire, the following beta values were obtained: (1.2 +/- 0.2) x 10(-11) and (9.4 +/- 1.2) x 10(-11) cm/W, respectively. In beta-barium borate crystal the TPA coefficient depends on crystal cut, beam polarization, or both and varies from (47 +/- 5) x 10(-11) to (68 +/- 6) x 10(-11) cm/W. For eight liquids that were studied (water, heavy water, ethanol, methanol, hexane, cyclohexane, 1,2-dichloroethane, and chloroform) the beta value lies from (34 +/- 3) x 10(-11) to (95 +/- 11) x 10(-11) cm/W.

Chirped-pulse amplification of ultraviolet femtosecond pulses by use of Ce^3+:LiCaAlF_6 as a broadband, solid-state gain medium

Chirped-pulse amplification in the ultraviolet region is demonstrated by use of a broadband Ce(3+): LiCaAlF(6) laser medium. A modified bow-tie-style four-pass amplifier pumped by 100-mJ, 266-nm pulses from a Q -switched Nd:YAG laser has a gain factor of 370 and delivers 6-mJ, 290-nm pulses. After dispersion compensation, the output pulses can be compressed to 115 fs.

Ce/sup 3+/:LiCaAlF/sub 6/ crystal for high-gain or high-peak-power amplification of ultraviolet femtosecond pulses and new potential ultraviolet gain medium: Ce/sup 3+/:LiSr/sub 0.8/Ca/sub 0.2/AlF/sub 6/

To develop high-peak-power ultrashort pulse laser systems in the ultraviolet region, a large Ce/sup 3+/:LiCaAlF/sub 6/ (Ce:LiCAF) crystal, a tunable ultraviolet laser medium with large saturation fluence and broad gain spectrum width, was grown successfully with a diameter of more than 70 mm. To demonstrate high small signal gain, a four-pass confocal amplifier with 60 dB gain and 54 /spl mu/J output energy was constructed. Chirped pulse amplification (CPA) in the ultraviolet region was demonstrated using Ce:LiCAF for higher energy extraction. A modified bow-tie-style four-pass amplifier pumped by 100-mJ 266-nm 10-Hz pulses from a Q-switched Nd:YAG laser had 370-times gain and delivered 6-mJ 290-nm pulses. After dispersion compensation, the output pulses can be compressed down to 115 fs. This is the first ultraviolet, all-solid-state high-peak-power CPA laser system using ultraviolet gain media, and this demonstration shows further scalability of the Ce:LiCAF CPA system. Additionally, a new gain medium, Ce/sup 3+/:LiSr/sub 0.8/Ca/sub 0.2/AlF/sub 6/, with longer fluorescence lifetime and sufficient gain spectrum width over 18 nm was grown to upgrade this system as a candidate for a final power amplifier gain module.

Femtosecond ultraviolet pulses generated using noncollinear optical parametric amplification and sum frequency mixing

We demonstrate the generation of ultraviolet 33-fs pulses with a shot-to-shot energy fluctuation of less than 3% using sum frequency mixing of visible pulses of a noncollinear optical parametric amplifier with sub-40-fs pulses of a 1-kHz Ti:sapphire-amplified system. The pulses are transform-limited (ΔνΔτ=0.36) and tunable in the range from 315 nm to 355 nm with energy above 1 μJ (2.6 μJ at 330 nm).