Ultraviolet Femtosecond Laser Pulses Research Articles

Ultraviolet laser-induced cross-linking in peptides.

The aim of this study was to demonstrate, and to characterize by high-resolution mass spectrometry that it is possible to preferentially induce covalent cross-links in peptides by using high-energy femtosecond ultraviolet (UV) laser pulses. The cross-link is readily formed only when aromatic amino acids are present in the peptide sequence. Three peptides, xenopsin, angiotensin I, and interleukin, individually or in combination, were exposed to high-energy femtosecond UV laser pulses, either alone or in the presence of spin trapping molecules, the reaction products being characterized by high resolution mass spectrometry. High-resolution mass spectrometry and spin trapping strategies showed that cross-linking occurs readily, proceeds via a radical mechanism, and is the highly dominant reaction, proceeding without causing significant photo-damage in the investigated range of experimental parameters. High-energy femtosecond UV laser pulses can be used to induce covalent cross-links between aromatic amino acids in peptides, overcoming photo-oxidation processes, that predominate as the mean laser pulse intensity approaches illumination conditions achievable with conventional UV light sources.

Multipulse irradiation of silicon by femtosecond laser pulses: Variation of surface morphology

The multipulse interaction of ultraviolet femtosecond laser pulses with silicon and generation of surface structures in a large area spot (≳1mm2) has been studied. The evolution of multiscale structures at the constant fluence strongly depends on the number of pulses, N. For N<200, the “carpet-like” pattern of nano-, and micro-spikes is generated by the bubble explosion in a thin surface foam layer. The accumulation of bubbles and their explosion due to repetition of laser pulses cause damped membrane-like oscillations of the silicon surface. For 200≤N, bifurcation of surface morphology takes place: (i) the surface tension waves of the wavelength ∼200μm appear in the peripheral region of the spot. Generated by the surface thermal gradient in the liquid foam layer, they spread from the hot centerline towards the periphery of the spot. The change of their wavelength with propagation distance indicates onset of the Eckhaus instability caused by the phase modulation in multipulse interaction. (ii) Deep caverns appear in a highly superheated silicon layer in the central region of the spot due to the fast gas–liquid phase separation and the fragmentation process.

Characterization of polarization shaped ultraviolet femtosecond laser pulses

We present full vector-field characterization of amplitude, phase, and polarization controlled femtosecond laser pulses in the ultraviolet (UV) with a method termed DFG-XTURTLE. It combines the recently developed technique of tomographic ultrafast retrieval of transverse light E-fields (TURTLE) with the well-established cross-correlation frequency-resolved optical gating (XFROG) amplitude and phase retrieval algorithm. Three individual linear projections at 0&#xB0;, 45&#xB0;, and 90&#xB0; of the vector-shaped field in the UV are frequency mixed with a well-characterized reference field at 800&#x2009;nm and the generated frequency-resolved difference-frequency cross correlations in the visible are analyzed with a standard XFROG algorithm. The TURTLE search algorithm then establishes the full vector field of the UV pulses by finding the correct phase relationship between the 0&#xB0; and 90&#xB0; projections to reconstruct the projection at 45&#xB0;. To retrieve the interpulse phase of temporally well-separated pulses, additional spectra must be taken. The handedness of shaped pulses can also be assigned with additional measurements.

Generation of High-Density Electrons Based on Plasma Grating Induced Bragg Diffraction in Air

Efficient nonlinear Bragg diffraction was observed as an intense infrared femtosecond pulse was focused on a plasma grating induced by interference between two ultraviolet femtosecond laser pulses in air. The preformed electrons inside the plasma grating were accelerated by subsequent intense infrared laser pulses, inducing further collisional ionization and significantly enhancing the local electron density.

Generation and amplification of dual-color femtosecond pulses by a noncollinear optical parametric up-conversion process

Noncollinear optical parametric up-conversion generation and amplification are realized in a thick β-barium borate (BBO) crystal, and a couple of visible femtosecond up-conversion laser pulses can be achieved by a femtosecond pulse at 800 nm as the pump sources. The theoretical and experimental results indicate that there exist phase-matching conditions for dual-color noncollinear parametric up-conversion generation and amplification, and their wavelengths can be tuned by rotating the BBO crystal. This parametric up-conversion generation and amplification can be attributed to three and five-wave mixing in a thick BBO crystal, and it shows the potential application on optical parametric chirped pulse amplification (OPCPA) to generate multi-color ultraviolet or visible femtosecond laser pulses pumped directly by femtosecond fundamental laser pulses without frequency-doubling or tripling.

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.

Isotope effect on ultrafast charge-transfer-to-solvent reaction from I− to water in aqueous NaI solution

A charge-transfer-to-solvent (CTTS) reaction from a photoexcited iodine atomic anion, I- (aq), in bulk water (H2O and D2O) was studied by time-resolved photoelectron spectroscopy using a liquid beam (microjet) of aqueous NaI solution. The P-2(3/2) CTTS state of I- (aq) was excited by a 226 nm femtosecond laser pulse and the evolution of the nonstationary electronic state was probed using another ultraviolet femtosecond laser pulse. Global fitting of the observed time-dependent photoelectron kinetic energy distributions provided the time constants of individual reaction steps and the photoelectron spectra from the CTTS state, a contact pair, the solvent-separated state, and a hydrated electron. Most of the elementary reaction steps revealed a strong deuterium isotope effect, indicating coupling of the electron dynamics and the hydrogen atomic motion of solvent water. However, nondiffusive geminate recombination processes from the CTTS state and a contact pair were almost insensitive to deuteration. Consequently, geminate recombination processes from the CTTS state and a contact pair occurs more efficiently in D2O, because the response of water is decelerated in D2O. In contrast, the recombination process from the solvent-separated state in the final step of the CTTS reaction is less efficient in D2O, presumably due to the smaller zero point energy.

Spectral modulation of ultraviolet femtosecond laser pulse by molecular alignment of CO2, O2, and N2

We demonstrate efficient third harmonic generation of a near infrared femtosecond pulse through cascaded frequency doubling and sum-frequency generation processes, where the group velocity mismatching between the involved fundamental and generated second harmonic pulses, before they are sent to frequency mixing, are precompensated with a properly inserted nonlinear crystal. The spectrum of the generated third harmonic pulse with energy of 1.1 mJ is further modulated by using impulsive molecular alignments of CO2, O2, and N2, where significantly broadened spectrum in the ultraviolet spectral region is observed due to the additional cross-focusing effect from the parallel aligned molecules and the consequently enhanced self-phase modulation.

Ultrathin amorphization of single-crystal silicon by ultraviolet femtosecond laser pulse irradiation

The mechanisms of amorphization for crystalline Si (c-Si) induced by ultraviolet femtosecond laser irradiation are described in this paper. The wavelength of the laser pulse was 267 nm, which is the third harmonics of a Ti:sapphire laser. We performed a laser scanning microscopy and a transmission electron microscopy for surface and structural analysis and imaging pump-probe measurements to investigate the dynamics of the process. From the analyses, we confirmed that the thickness of the amorphized layer was quite uniform and there is no lattice defect under the amorphized section. The thickness of the amorphous Si (a-Si) layer was 7 nm and the threshold fluence of the amorphization was 44 mJ/cm2. From the Imaging Pump-Probe measurement it was revealed that the melting time is less than 1 ns and ultra high speed melting and re-solidification process was occurred. The melting depth estimated by the Imaging Pump-Probe measurement was 7 nm. The melted portion completely corresponded to the amorphized section.

Ultrafast electron dynamics in metals: Real-time analysis of a reflected light field using photoelectrons

We propose an approach to address ultrafast charge-carrier dynamics of metals by analyzing the momentum change in photoelectrons interacting with a transient optical grating at a metal surface. Photoelectrons are excited by an ultraviolet femtosecond laser pulse which precedes an infrared pulse setting up the transient grating. We measure the kinetic energy of the photoelectrons which are accelerated by the grating's ponderomotive potential and thus sample the respective electric field. The method is capable to access phase and amplitude differences between the incoming and the reflected light fields. The latter is determined by the response of the conduction-band electrons to the light field. We report on a demonstration of such an experimental scheme using a Gd(0001) surface and calculate the reflected field by a simplified transport equation. We derive a method to determine the average electron-scattering rate and study the time-dependent evolution of amplitude and phase of the reflected electric field including memory effects in the optically induced polarization dynamics. Finally, we discuss the required steps of this approach to probe ultrafast dynamics in metals experimentally.

Frequency doubling and Fourier domain shaping the output of a femtosecond optical parametric amplifier: easy access to tuneable femtosecond pulse shapes in the deep ultraviolet

Tuneable, shaped, ultraviolet (UV) femtosecond laser pulses are produced by shaping and frequency doubling the output of a commercial optical parametric amplifier (OPA). A reflective mode, folded, pulse shaping assembly employing a spatial light modulator (SLM) shapes femtosecond pulses in the visible region of the spectrum. The shaped visible light pulses are frequency doubled to generate phase- and amplitude-shaped, ultrashort light pulses in the deep ultraviolet. This approach benefits from a simple experimental setup and the potential for tuning the central frequency of the shaped ultraviolet waveform. A number of pulse shapes have been synthesised and characterised using cross-correlation frequency resolved optical gating (XFROG). This pulse shaping method can be employed for coherent control experiments in the ultraviolet region of the spectrum where many organic molecules have strong absorption bands.

Nonlinear absorption of ultraviolet femtosecond laser pulses in argon

The optogalvanic and optoacoustic methods are used to determine the mechanisms of the nonlinear absorption of intense ultraviolet femtosecond laser pulses in gaseous argon. An increase in the nonlinearity of the absorption process from 3 to 4 with an increase in the laser radiation intensity in the range 0.03–12 TW/cm2 is revealed. The possible physical mechanisms of this phenomenon are discussed.

Ultraviolet Femtosecond Laser Creation of Corneal Flap

To study parameters for ocular femtosecond laser surgery in terms of process efficiency and safety aspects using ultraviolet (UV) femtosecond laser pulses. Studies on corneal surgery and flap processing on enucleated porcine eyes were performed using a newly developed ytterbium-doped gain media laser source. Ultraviolet femtosecond laser pulses centered at a wavelength of 345 nm and working at a repetition rate of 100 kHz were generated by the third harmonics of the 1035-nm fundamental wavelength. Flaps with a diameter of 6 mm and a thickness of 100 microm were created in less than 2 minutes with low energy pulses. Transmissions and spectral measurements were performed during flap processing. Less than 2% UV radiation reaches the retina during corneal flap processing. A detectable transmittance towards the retina of visible light centered on 440 nm was found for UV pulses. Ultraviolet corneal refractive surgery is a novel procedure and has the potential to be an alternative to infrared refractive surgery considering safety aspects.

Multistep Ionization of Argon Clusters in Intense Femtosecond Extreme Ultraviolet Pulses

The interaction of intense extreme ultraviolet femtosecond laser pulses (lambda = 32.8 nm) from the FLASH free electron laser (FEL) with clusters has been investigated by means of photoelectron spectroscopy and modeled by Monte Carlo simulations. For laser intensities up to 5x10(13) W/cm(2), we find that the cluster ionization process is a sequence of direct electron emission events in a developing Coulomb field. A nanoplasma is formed only at the highest investigated power densities where ionization is frustrated due to the deep cluster potential. In contrast with earlier studies in the IR and vacuum ultraviolet spectral regime, we find no evidence for electron emission from plasma heating processes.

High power subterahertz electromagnetic wave radiation from GaN photoconductive switch

The authors present apparently the observation of subterahertz electromagnetic wave emission from GaN based large aperture photoconductive switches (LA-PCSWs) excited by ultraviolet femtosecond laser pulses. The photoconductive layer of the GaN LA-PCSW is doped with carbon giving rise to a resistivity as high as 60MΩcm. The absolute energy of the emitted radiation pulses is measured to be 93.3pJ/pulse under 500V bias voltage. The Fourier spectrum of the measured pulse by the time-domain-spectroscopy shows the main components lie in 0.1–0.2THz regime.

Generation of shaped ultraviolet pulses at the third harmonic of titanium-sapphire femtosecond laser radiation

We experimentally demonstrate a method to generate shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 267 nm, the third harmonic of conventional titanium-sapphire femtosecond laser systems. Employing a 128-pixel liquid-crystal spatial light modulator, we impose variable spectral phase modulations upon the near-infrared laser pulses. By this, complex laser pulses can be shaped whose overall spectrum is still conserved. Our experiments show that it is possible to easily transfer these pulses into the ultraviolet at 267 nm via sum-frequency mixing in nonlinear crystals and to predictably generate multistructured ultraviolet femtosecond laser pulses. We analyze the temporal and spectral composition of these pulses after frequency conversion into the ultraviolet using difference-frequency cross-correlation and XFROG (cross-correlation frequency-resolved optical gating) techniques with an unmodulated fundamental laser pulse. The method can be employed to facilitate adaptive quantum control experiments in the ultraviolet wavelength regime, where the major absorption bands of many organic molecular systems are located.

Femtosecond laser microprinting of biomaterials

This Letter demonstrates a laser rapid prototyping method that can be used for fabricating high-density resolution patterns of biomaterials. Ultraviolet femtosecond laser pulses have been used for directly printing a wide range of biomaterials in complicated patterns and structures. The ultrashort laser pulses reduce the thermal effects, thus allowing the effective deposition of sensitive biomaterials at high spatial resolution for microfabricating patterns. We present the microprinting of different biomaterial patterns, such as DNA (deoxyribonucleic acid) and proteins, with spatial resolution down to 50μm and we demonstrate that they maintain their properties and biological functions and, thus, can be practically used as biosensors.

Enhanced harmonic conversion efficiency in the self-guided propagation of femtosecond ultraviolet laser pulses in argon

Harmonic generation during the self-guided propagation of femtosecond ultraviolet (UV) laser pulses (248-nm, 450-fs) in argon is investigated. The third (82.7-nm) and fifth (49.6-nm) harmonics are generated in the UV filament. The energy-conversion efficiencies for the harmonics are found to be at least two orders of magnitude higher than those reported in the literature for similar gas pressures. The enhancement is attributed to the quasi-phase matching of the harmonics due to the self-guiding of the driving pulse.

Physical chemistry of the femtosecond and nanosecond laser–material interaction with SiC and a SiC–TiC–TiB 2 composite ceramic compound

The interaction of nanosecond laser pulses in the ultraviolet wavelength range and femtosecond laser pulses in the near-infrared region with the semiconductor SiC and the composite compound SiC–TiC–TiB 2 was investigated. Surface analytical techniques, such as XPS, depth profile (DP), and micro-Raman spectroscopy (μ-RS) were used to identify the chemical changes between untreated and laser-treated areas. Single-pulse irradiation led to material modifications in the condensed state in most instances. Multi-pulse results differed depending on the pulse duration. Crystal structure changes were observed as a consequence of laser-induced melting and resolidification. In air contact all components underwent oxidation reactions according to thermodynamic expectations. Exceptions were observed under exclusion of oxygen, SiC was reduced to elemental Si.

Demonstration of high gain amplification of femtosecond ultraviolet laser pulses

Femtosecond pulses at 400 nm were amplified using a noncollinear optical parametric amplifier pumped by picosecond pulses at 267 nm. A flat spectral gain exceeding 3500 was achieved in single pass within the available 17 nm bandwidth of the signal pulse. The effect of pump depletion, group delay difference, and the geometry of the interacting pulses on the spectral gain are also investigated.