Ti-sapphire Laser Pulses Research Articles

Spectral-angular patterns and energy threshold for linear-to-nonlinear femtosecond laser pulse focusing in air

Focused propagation of a high-power ultrashort laser pulse is strongly influenced by optical medium nonlinearity that can significantly affect pulse parameters in the focusing region as compared to the linear focusing regime. We present the results of our experimental study and numerical simulations of frequency-angular patterns and laser plasma distribution of a gigawatt-power femtosecond Ti-sapphire laser pulse propagating in air under various focusing conditions. For the first time to the best of our knowledge, we found experimentally and confirm theoretically the range of pulse numerical aperture (NA), from approximately NA = 2 ⋅ 10−3 to 5 ⋅ 10−3, when both frequency and angular spectra broadening during supercritical laser pulse focusing in air is minimal. We attribute this NA-range to the transition region between quasilinear and nonlinear pulse focusing regimes. Additionally, the energy threshold for different pulse focusing regimes in the wide range of focusing conditions is obtained for the first time through the numerical simulations. This threshold is demonstrated to decrease with pulse NA increase.

Coulomb-Corrected Strong Field Approximation without Singularities and Branching Points

The domain of validity of the Coulomb-Corrected Strong-Field Approximation (CCSFA) is going to be analyzed in relation to the semi-classical dynamics of electrons during ionization of hydrogen-like targets. Our analysis is limited to ionization driven by Ti-sapphire laser pulses with intensities up to roughly 1014 Wcm−2. For such parameters, the effects related to radiation pressure are small and the laser field can be described in the dipole approximation. By applying the Magnus expansion for the exact retarded electron propagator we obtain an effective action which is free of Coulomb singularities and branch points when the complex-time trajectories are used. Furthermore, we show that the classical action is exactly recovered as the asymptotic limit of its effective counterpart. The applicability of such limit is also discussed.

High harmonics generation at the interaction of an intense fs laser pulse with an overdense plasma layer

Harmonics generated with fs laser pulses are not only among the most intense sources in the wavelength range of a few tens of nanometers but also a far better source than X-ray lasers from the coherence point of view. It is anticipated, that in the near future, the harmonics issued from femtosecond laser-produced plasmas will represent a coherent source much brighter than all other XUV sources apart from the XFEL. These particular aspects, correlated with the fact that high order harmonics produced by reflection of an ultrashort and intense laser pulse on an overdense plasma layer impose virtually no restrictions on the intensity of the laser employed, lead the way to a series of potential applications in plasma diagnostics and nonlinear optics. This work presents a theoretical study on the high harmonics generation mechanism occurring at the reflection of a femtosecond Ti-Sapphire terawatt laser pulse (λ0 = 0.8 μm and pulse durations around 100–150 fs) on an overdense steep Al plasma layer. Insights on how different laser parameters influence this generation mechanism and its efficiency are provided.

Optical Biopsy of Benign and Malignant Tissue by Time Resolved Spectroscopy

Pathological condition of malignant tissue could be analyzed by spectral domain or time domain spectroscopy, the two being the complementary to each other in optical biopsy (OB) of cancer. This paper reports results of time resolved emission spectroscopy (TRS) of 24 excised tissue samples of breast and prostate (normal control = 12; benign = 4; malignant = 8), employing a 390 nm, 100 fs, Ti-Sapphire laser pulses.The fluorescence decay times were measured using streak camera and the resultant data were fitted for single and bi-exponential decays with reliability of 97%. Our results show the distinct difference between normal, benign and malignant tissues mostly due to the emission spectra of Nicotinamide Adenine Dinucleotide (NADH), Flavin Mononucleotide (FAD) and also due to the heterogeneity of micro environments associated with the diseased tissues. In this short report, fit is also shown that TRS of breast tissues are similar to those of prostate tissues.

Effects of initial humidity and temperature on laser-filamentation-induced condensation and snow formation

We investigated the effects of initial humidity and temperature on the formation of water condensation and various snowflake patterns by firing high-repetition filaments of femtosecond Ti-sapphire laser pulses at 9 mJ/50 fs per pulse into a cloud chamber. While adjusting initial temperature and humidity distributions near the filaments, snowflakes and ice particles with various shapes and sizes were observed on the bottom cold plate. In addition, significant differences were found in the weights of the laser-induced snow below the filament center. From those data, initial conditions of low temperature ( (90 ± 11) % and to ice RHi > (120 ± 15) %) were found being important to assist an efficient laser-filamentation-induced condensation and snow formation.

Formation of Void Array Inside Transparent and Absorptive Glasses by Femtosecond Laser Irradiation

We demonstrate self-fabrication of void arrays in a fused silica transparent in the visible and a color-filter borosilicate glass strongly absorptive at 800 nm using tightly focused Ti-sapphire femtosecond laser pulses at 1 kHz without scanning. The period, the size, the number of voids, and the length of the aligned void structure were controlled by changing the laser pulse energy, and the position of the focal point inside two materials. The void arrays were observed by an optical microscope and also estimated by an optical diffraction experiment. The void size and period were smaller in the absorptive glass than in the transparent glass. The submicrometer-sized void was observed by a scanning electron microscope. The smaller and clearer void arrays were formed in the color filter than the fused silica glass. With increasing the laser focal depth, the void-array length increased in the fused silica and decreased in the color filter.

Femtosecond x-ray pulse length characterization at the Linac Coherent Light Source free-electron laser

Two-color, single-shot time-of-flight electron spectroscopy of atomic neon was employed at the Linac Coherent Light Source (LCLS) to measure laser-assisted Auger decay in the x-ray regime. This x-ray-optical cross-correlation technique provides a straightforward, non-invasive and on-line means of determining the duration of femtosecond (>40 fs) x-ray pulses. In combination with a theoretical model of the process based on the soft-photon approximation, we were able to obtain the LCLS pulse duration and to extract a mean value of the temporal jitter between the optical pulses from a synchronized Ti-sapphire laser and x-ray pulses from the LCLS. We find that the experimentally determined values are systematically smaller than the length of the electron bunches. Nominal electron pulse durations of 175 and 75 fs, as provided by the LCLS control system, yield x-ray pulse shapes of 120±20 fs full-width at half-maximum (FWHM) and an upper limit of 40±20 fs FWHM, respectively. Simulations of the free-electron laser agree well with the experimental results.

Geometry-dependent terahertz emission of silicon nanowires

THz emission was observed from the vertically aligned silicon nanowire (Si NW) arrays, upon the excitation using a fs Ti-sapphire laser pulse (800 nm). The Si NWs (length = 0.3 approximately 9 microm) were synthesized by the chemical etching of n-type silicon substrates. The THz emission exhibits significant length dependence; the intensity increases sharply up to a length of 3 mum and then almost saturates. Their efficient THz emission is attributed to strong local field enhancement by coherent surface plasmons, with distinctive geometry dependence.

Terahertz Emission from Vertically-aligned Silicon Nanowires

AbstractLarge-area vertically aligned silicon nanowire (Si NW) arrays were synthesized with a controlled length (0.3 ˜ 9 μm) by the chemical etching of n-type silicon substrates. Upon their excitation using a fs Ti-sapphire laser pulse (800 nm), their THz emission intensity exhibits strong dependence on their length; the intensity increases sharply up to a length of 3 μm and then decreases slightly, due to the complete absorption of the optical pump power. The Raman scattering spectrum exhibits the same behavior as that of the THz emission. We suggest that the field enhancement by localized surface plasmons induces more efficient THz emission or Raman scattering for the longer Si NWs. The photocurrent measured in a photoelectrochemical cell showed consistently the length dependence with a maximum value at the length of 5 μm.

Laser-induced breakdown spectroscopy of liquid media with femtosecond laser excitation

Optical breakdown spectra on seawater and distilled water faces excited by Ti-sapphire laser pulses with durations of 50 fs and 650 fs were studied experimentally in normal atmosphere. The time dependence of the intensity of the Na I (588.9 nm) and Mg II (279.5 nm) emission lines was studied, and the limit of detection of Na in an aqueous NaCl solution was established at 10−6 g/l.

Cyclohexadiene ring opening observed with 13 fs resolution: coherent oscillations confirm the reaction path

The third harmonic (270 nm, 11 fs), produced in a short argon cell from Ti-sapphire laser pulses (810 nm, 12 fs), was used to excite 1,3-cyclohexadiene to its lowest pipi* state (1B). Probing was done by transient ionization by the 810 nm pulses, measuring the yields of the parent and a fragment ion. As previously found with 10 times longer pulses, the molecule leaves in two steps (time constants tau(1), tau(2)) from the spectroscopic (1B) to a dark (2A) state and from there (within tau(3)) to the ground-state surface. In addition to slightly improved values for tau(1)-tau(3), we found in all three locations (L(1)-L(3)) on the potentials coherent oscillations, which can be assigned to vibrations. They are stimulated by slopes (driving forces) of the potentials, and the vibrational coordinates indicate the slope directions. From them we can infer the path following the initial excitation: the molecule is first not only accelerated towards CC stretching in the pi system but also along a symmetric C[double bond, length as m-dash]C twist. The latter motion-after some excursion-also erects and stretches the CH(2)-CH(2) bond, so that Woodward-Hoffmann interactions are activated after this delay (in L(2)). On leaving L(2) (the 1B minimum) around the lower cone of the 1B/2A conical intersection, the wave packet is rapidly accelerated along an antisymmetric coordinate, which breaks the C(2) symmetry of the molecule and eventually leads in a ballistic path to (and through) the last (2A/1A) conical intersection. The ring opening begins already on the 1B surface; near the 2A minimum it is already far advanced, but is only completed on the ground-state surface.

Ultrafast Dynamics and Coherent Oscillations in Ethylene and Ethylene-d4Excited at 162 nm

The fifth harmonic (162 nm, 11 fs), generated in a short argon cell from 12 fs Ti-sapphire laser pulses, was used to excite C2H4 and C2D4 in the maximum of the first pi pi* transition. Around 10% of the molecules were excited to the pi3s Rydberg state instead. The subsequent motion of the wave packet, moving over the potentials from the Franck-Condon region down to the ground state, was monitored by nonresonant ionization at 810 nm with mass-selective detection of the ion yield. Five time constants (from approximately 20 fs in excited states to 0.6-11 ps in the hot ground state) and four coherent oscillations (CC stretch and torsion vibrations or hindered free rotation) were determined for each isotopomer. The initial relaxation follows a superposition of CC twist and stretch coordinates; this explains a surprisingly small deuterium isotope effect of the initial time constant (21 versus 24 fs). Also the vibrations in the Franck-Condon region have such a mixed character and a correspondingly small isotope shift. From the perpendicular minimum the wave packet reaches (within 17 or 21 fs for the two isotopomers) a conical intersection via a direction that also involves partial hydrogen migration. This is concluded from the detection of ethylidene (CH3CH), formed simultaneously with ground-state ethylene. This carbene isomerizes in the ground state within 0.6 ps (1.6 ps for CD3CD) to ethylene. Two time constants for dissociation (4.5 and 11 ps) in the hot ground state were also identified. The small yields of bimolecular reactions (photodimerization, addition reactions involving a "suddenly polarized" excited state, carbene reactions) are interpreted in terms of the short lifetimes. It is pointed out that the relaxation path starting from the Rydberg state merges into that from the pi pi* state; nevertheless, there is a wavelength dependence in the photochemistry of olefins, because due to a momentum effect the wave packet remembers from which state it came.

포토폴리머와 희토류이온이 첨가된 유리에서의 이광자흡수를 이용한 광정보저장

We studied feasibility of three-dimensional optical memory by utilizing femtosecond laser-induced changes of transmission in photopolymers and photoluminescence in Eu and Sm ion doped sodium borate glasses. We produced transmission change by two photon absorption and obtained sub-Um size spots in photopolymers using 780 nm modelocked Ti-sapphire laser pulses. We also changed valence state of Eu and Sm ions by multi-photon absorption and achieved sized spot formation in Sm-doped glasses.

Waveguide writing in silica glass with femtosecond pulses from an optical parametric amplifier at 1.5 μm

Optical waveguides with a circular small-size core were inscribed in the bulk of fused silica glass by use of focused 1.5μm femtosecond infrared pulses at 1kHz repetition rate generated by an optical parametric amplifier. A refractive index difference between core and cladding in excess of 6×10−3 was achieved which is as large as that induced by Ti-sapphire laser pulses at 800nm. The observed higher resistance of the silica against structural damage could allow micro-machining of waveguide structures under a broader range of writing parameters. The great stability of the white light pattern suggests that the filamentation of femtosecond pulses at 1.5μm plays a major role in the formation of the waveguides. The mechanisms of the induced refractive index change were discussed based on the obtained experimental evidences.

Creation of high energy electronic excitations in inorganicinsulators by intense femtosecond laser pulses

We measured photoemission spectra for a number of insulators (CsI, Diamond, SiO2, CeF3) excited by femtosecond Ti-Sapphire laser pulses at peak intensities, from 0.5 to 6 TW/cm2, which are at least one order of magnitude below the optical breakdown threshold. An intense and pronounced plateau of high energy electrons appears in the photoelectron spectra in this intensity range, which extends up to 30-40 eV at the highest intensities, which we used. The excitation of electrons at high energies is treated in terms of direct interbranch transitions in the conduction band of insulator. These processes are described using calculations based on numerical solution of time dependant Schrodinger equation (TDSE) for Bloch electrons in electric fields. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of collisionally pumped optical-field-ionization soft X-ray lasers

We give an overview of recent experimental results on optical-field-ionization collisional soft X-ray lasers developed at LOA. By focusing a 30-fs, circularly polarized Ti-sapphire laser pulse at an intensity of up to 8×1017 Wcm-2 into a low-density gas cell containing Xe or Kr, we produced a few mm long plasma column for soft X-ray amplifier. Saturated amplification has been achieved on the 4d95d(1 S 0)–4d95p(1 P 1) transition at 41.8 nm in Xe8+, and on the 3d94d(1 S 0)–3d94p(1 P 1) transition in Kr8+ at 32.8 nm. Under optimum pumping conditions the Xe8+ laser provides about (5±2)×109 photons per pulse whilst the Kr8+ laser delivers up to (2.5±1)×109 photons per shot. The repetition rate of these soft X-ray lasers is 10 Hz. The beam wavefront of the Xe8+ laser has been measured by a Shack–Hartmann soft X-ray wavefront sensor, and the pulse duration by a cross-correlation technique.

Observation of high energy photoelectrons from solids at moderate laser intensity

We investigate the photoemission for a set of wide band-gap crystals irradiated by femtosecond Ti-Sapphire laser pulses at intensities varying from 0.5 to 6 TW/cm2 (below the optical breakdown threshold). The measured total electron yield increases linearly with the laser intensity in this intensity range. An intense and wide plateau of high energy electrons appears in the photoelectron spectra at excitation intensities larger than 1 TW/cm2. The exponential cut-off of this plateau reaches 40 eV at maximal applied intensities. In order to explain such a behavior, we propose a mechanism where the heating is due to a sequence of direct interbranch one- and multi-photon transitions in the conduction band.

Visualization of the evolution of multiple filaments in methanol

The full propagation dynamics of a powerful ∼50 fs Ti-sapphire laser pulse is visualized for the first time in a dilute solution of Rhodamine B dissolved in methanol through two-photon fluorescence. These processes include self-focusing, group velocity dispersion, intensity clamping, self-steepening, anti-Stokes spectral broadening and multiple filaments competition.

Superexciplex of Coumarin Molecules using Tunable Ti-Sapphire Laser

Certain highly polar dye molecules exhibit an additional optical gain band under pulsed laser excitation, while there is no such band under steady-state continuous wave (CW) lamp excitation. This new band is not due to an excimer, an exciplex or a two-photon fluorescence band but stems from the formation of a new molecular complex in which two excited molecules remain associated with a solvent molecule acting as a bridge. In this paper, the characteristics of superexciplexes of four related coumarin molecules are presented. All molecules were excited using a tunable Ti-sapphire laser pulse 10 ns in width. The distinct difference between the amplified spontaneous emission (ASE) spectra obtained with tunable laser and tunable lamp excitation demonstrated that twisted intramolecular charge transfer (TICT) conformations might also assist in the formation of these superexciplexes.

Lasing action in air induced by ultra-fast laser filamentation

The backscattered fluorescence of nitrogen from long filaments generated by intense ultrafast Ti-sapphire laser pulses propagating in air is studied. It shows an exponential increase with increasing filament length, indicating amplified spontaneous emission (ASE).