X-ray diffraction from the ripple structures created by femtosecond laser pulses

In this paper, we present the investigation and characterization of the laser-induced surface structure on an asymmetrically cut InSb crystal. We describe diffraction from the ripple surface and present a theoretical model that can be used to simulate X-ray energy scans. The asymmetrically cut InSb sample was irradiated with short-pulse radiation centred at 800 nm, with fluences ranging from 10 to 80 mJ/cm². The irradiated sample surface profile was investigated using optical and atomic force microscopy. We have investigated how laser-induced ripples influence the possibility of studying repetitive melting of solids using X-ray diffraction. The main effects arise from variations in local asymmetry angles, which reduce the attenuation length and increase the X-ray diffraction efficiency.     

Photoinduced multiple microchannels inside silicon produced by a femtosecond laser

Photoinduced multiple microchannels in the interior of silicon produced by an 800-nm femtosecond laser were observed. The multiple microchannels were aligned spontaneously with a period along the propagation direction of the laser beam, which could be attributed to the interface spherical aberration induced due to refractive-index mismatch. We also observed that the depth of the photoinduced microchannels increased with the increase of the laser power. The power dependence of the depth of photoinduced microchannels in silicon was different from that in transparent materials, which probably arose from the competition between self-focusing due to the nonlinear Kerr effect and self-defocusing related to the thermal accumulation in the process of laser irradiation.     

Wavelength-independent all-optical synchronization of a Q-switched 100-ps microchip laser to a femtosecond laser reference source

We present a Q-switched microchip laser emitting 1064-nm pulses as short as 100 ps synchronized to a cavity dumped femtosecond laser emitting 800-nm pulses as short as 80 fs. The synchronization is achieved by presaturating the saturable absorber of the microchip laser with femtosecond pulses even though both lasers emit at widely separated wavelengths. The mean timing jitter is 40 ps and thus considerably shorter than the pulse duration of the microchip laser.     

Enhancement of resolution and quality of nano-hole structure on GaN substrates using the second-harmonic beam of near-infrared femtosecond laser

By using a second harmonic of near infrared femtosecond (fs) laser (λ=387 nm, 150 fs) with high NA objective lens, fabrication resolution has been greatly improved in nano-fabrication of wide band-gap semiconductor gallium nitride (GaN). We have carried out a wet-chemical-assisted fs laser ablation method, in which the laser beam is focused onto a single-crystal GaN substrate immersed in a concentrated hydrochloric (HCl) acid solution. A two-step processing involving irradiation with a fs laser beam in air followed by wet chemical treatment is also performed for comparison. In the wet-chemical-assisted ablation, theoretical diameters of ablation craters are calculated as a function of pulse energy by assuming that the reaction is based on two-photon absorption. In lower energy, the calculated curve is close to the experimental value, while the actual measured diameters in the region of higher energy are larger than calculated values. In the condition of the highest fabrication resolution, we obtained ablation craters smaller than 200 nm at full width at half maximum. We have also demonstrated the fabrication of two-dimensional (2D) periodic nanostructures on surface of a GaN substrate using the second harmonic single fs-laser pulse. Uniform ablation craters with the size as small as 410 nm in diameter are arranged with a periodicity of 1 μm. Such structures are applicable to 2D photonic crystals which improve the light extraction efficiency for blue LEDs in the near future.     

Birefringent modifications induced by femtosecond filaments in optical glass

We present a study of the refractive index modifications spontaneously induced by infrared femtosecond filaments propagating in the bulk of transparent solids. It was found that extended modified refractive index channels up to several mm in length can be formed under loose focusing conditions in fused silica. The observed birefringence zone at the beginning of these channels is attributed to the anisotropic stress induced via nonlinear losses in the high intensity region of the filament.     

Parallel ripple formation during femtosecond laser grooving of ceramic

The self-organized formation of ripples in the direction parallel to the groove during the femtosecond laser machining of microgrooves on aluminum nitride ceramic at laser fluences much higher than the single-pulse ablation threshold is reported. These parallel ripples are notably different from the commonly observed polarization-perpendicular ripples and are produced in grooves having an appropriate width and depth, irrespective of laser polarization. From subsequent experiments with narrow and wide groove widths, it could be considered that the groove walls play an important role in the formation of these parallel ripples, possibly by confining the laser-induced plasma.     

3 GHz, fundamentally mode-locked, femtosecond Yb-fiber laser

We demonstrate a fundamentally mode-locked Yb-fiber laser with 3?GHz repetition rate and ～206 fs pulse duration. The laser incorporates two enabling technologies: a 1?cm heavily Yb-doped phosphate glass fiber as the gain medium and a high-dispersion (-1300 fs²) output coupler to manage cavity dispersion. The oscillator self-starts and generates up to 53?mW average power.     

Dielectric function dynamics during femtosecond laser excitation of bulk ZnO

Using a broad band dual-angle pump-probe reflectometry technique, we obtained the ultrafast dielectric function dynamics of bulk ZnO under femtosecond laser excitation. We determined that multiphoton absorption of the 800-nm femtosecond laser excitation creates a large population of excited carriers with excess energy. Screening of the Coulomb interaction by the excited free carriers causes damping of the exciton resonance and renormalization of the band gap causing broadband (2.3–3.5 eV) changes in the dielectric function of ZnO. From the dielectric function, many transient material properties, such as the index of refraction of ZnO under excitation, can be determined to optimize ZnO-based devices.     

197 nm femtosecond laser-pulse duration: comparison of autocorrelation measurements

An easy and reliable way is presented to measure the duration of UV femtosecond laser pulses of λ < 200 nm. The used autocorrelation techniques are based on two-photon absorption (TPA) in different TPA media, especially calcium fluoride (CaF₂). For 197 nm, the laser-pulse energy transmission and the laser-induced fluorescence of self-trapped excitons at 278 nm are applied. Both methods yield nearly the same second-order autocorrelation functions allowing to analyze the investigated laser pulse and obtain its duration of $(350\pm10)$  fs.     

Self-organized periodic surface structures on ZnO induced by femtosecond laser

Self-organized periodic surface structures on ZnO have been observed after multiple linearly polarized femtosecond laser pulse irradiation. The observed self-organized structures are attributed to the second harmonics in the sample surface excited by the incident laser. The grating orientation could be adjusted by the laser polarization direction. We also find that fluences play an important role in the formation of self-organized nanostructures.     

Comparison of plume expansion in femtosecond laser ablation on oxidized and non-oxidized Sm surfaces

Time development of Sm⁺ and Sm ablation plume produced by the femtosecond laser irradiation has been investigated. The two-dimensional spatial profiles of Sm and Sm⁺ emitted from oxidized and non-oxidized Sm surface were visualized using a planar laser-induced fluorescence method. It was observed that the flow velocity of Sm⁺ is much faster than that of Sm plume in both surfaces. The plumes from the oxidized Sm surface show higher velocity than that from non-oxidized surface, which is originated by the small electric conductivity at the surface. Expansion property observed for Sm⁺ and Sm plume in the oxidized Sm surface ablation implies the formation of the Knundsen layer nearby the surface. Meanwhile, continuous emission of Sm indicates the large contribution of heating effect to emission process at the non-oxidized surface. We conclude that the fsLA process strongly depends on the electric property of the ablated surface and the heating effect contributes to the particle emission process on the conductive material surface.     

On the evaluation of gluon condensate effects in the holographic approach to QCD

In holographic QCD the effects of gluonic condensate can be encoded in a suitable deformation of the 5D metric. We develop two different methods for the evaluation of first order perturbative corrections to masses and decay constants of vector resonances in 5D Hard-Wall models of QCD due to small deformations of the metric. They are extracted either from a novel compact form for the first order correction to the vector two-point function, or from perturbation theory for vector bound-state eigenfunctions: the equivalence of the two methods is shown. Our procedures are then applied to flat and to AdS 5D Hard-Wall models; we complement results of existing literature evaluating the corrections to vector decay constant and to two-pion–one-vector couplings: this is particularly relevant to satisfy the sum rules. We concentrate our attention on the effects for the Gasser–Leutwyler coefficients; we show that as in the Chiral Quark model, the addition of the gluonic condensate improves the consistency, the understanding and the agreement with phenomenology of the holographic model.     

Nonlinear refraction of silver nanowires from nanosecond to femtosecond laser excitation

In order to investigate the effect of pulse width and solvent on the nonlinear properties of metal nanostructures, silver nanowires were fabricated in a direct current electric field (DCEF) using a solid-state ionic method and characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The nonlinear refractive index (γ) of silver nanowires suspended in ethanol was measured using the Z-scan technique and laser radiation of various (femto-, pico-, and nanosecond) pulse durations. Experimental results indicated that silver nanowires have obvious positive refractive nonlinearities and γ (the Kerr-induced self-focusing) increases as the pulse duration increases from 7.4×10⁻⁸ cm²/GW at 110 fs to 1.6×10⁻⁴ cm²/GW at 8 ns, due to the additional influence of the atomic reorientational Kerr effect in the case of longer pulses. Due to the solvent dependence of the nonlinear behavior of the silver nanowires, the nonlinear absorption and refraction of silver nanowires suspended in de-ionized water are smaller than those of silver samples suspended in ethanol. The thermal nonlinearities are insignificant in our experimental conditions.     

Polarons in magnesium-doped lithium niobate crystals induced by femtosecond light pulses

Strong light-induced absorption has been observed in lithium niobate crystals doped with magnesium after application of femtosecond illumination. In this material there are no Nb-on-Li-site defects and hence no antisite polarons occur, but small free polarons close to the conduction band can be generated. The light-induced absorption observed is attributed to these polarons. For LiNbO₃:Mg, their decay times are about two orders of magnitude smaller than those of the Nb-on-Li-site polarons in undoped material. The results are relevant for a better understanding of the suppression of the so-called optical damage in these crystals and for their use in femtosecond applications.     

An all-solid-state laser system for generation of 100 μJ femtosecond pulses near 625 nm at 1 kHz

Frequency doubling the output of a high-power femtosecond Cr:forsterite regenerative amplifier with >50% conversion efficiency in a temperature-tuned noncritically phase-matched LBO crystal produces femtosecond pulses of >100 μJ energy in the visible range near 625 nm at a pulse duration of about 200 fs or >65 μJ at <170 fs. Received: 29 March 1999 / Revised version: 27 April 1999 / Published online: 24 June 1999     

Review of searches for Higgs bosons and beyond the Standard Model Physics at the Tevatron

The energy frontier is currently at the Fermilab Tevatron accelerator, which collides protons and antiprotons at a center-of-mass energy of 1.96 TeV. The luminosity delivered to the CDF and DØ experiments has now surpassed the 4 fb^?1. This paper reviews the most recent direct searches for Higgs bosons and beyond-the-standard-model (BSM) physics at the Tevatron. The results reported correspond to an integrated luminosity of up to 2.5 fb^?1 of Run II data collected by the two Collaborations. Searches covered include the standard model (SM) Higgs boson (including sensitivity projections), the neutral Higgs bosons in the minimal supersymmetric extension of the standard model (MSSM), charged Higgs bosons and extended Higgs models, supersymmetric decays that conserve or violate R-parity, gauge-mediated supersymmetric breaking models, long-lived particles, leptoquarks, compositeness, extra gauge bosons, extra dimensions, and finally signature-based searches. Given the excellent performance of the collider and the continued productivity of the experiments, the Tevatron physics potential looks promising for discovery with the coming larger data sets. In particular, evidence for the SM Higgs boson could be obtained if its mass is light or near 160 GeV. The observed (expected) upper limits are currently a factor of 3.7 (3.3) higher than the expected SM Higgs boson cross section at m _H =115 GeV and 1.1 (1.6) at m _H =160 GeV at 95% C.L.     

Multifilamentation of high-power femtosecond laser pulse in turbulent atmosphere with aerosol

The filamentation of a femtosecond laser pulse in atmosphere under the joint influence of turbulence and aerosol has been numerically studied for the first time. A result of these studies is that the presence of aerosol in a turbulent atmosphere increases the distance to the onset of multifilamentation. We have studied the competition between two factors of aerosol coherent scattering in the process of multiple filamentation of a femtosecond pulse in the atmosphere: generation of light field perturbations and energy losses. Similarity parameters have been determined for different conditions in the problem of multifilamentation in aerosol.     

The Effect of the Relative Motion of Atoms on the Frequency of the Emitted Light and the Reinterpretation of the Ives-Stilwell Experiment

We examine the process of the emission of light from an atom that is in a relative translational motion with respect to the medium at rest in which the electromagnetic excitations propagate. The effect of Lorentz contraction of the of electron orbits on the emitted frequency is incorporated in the Rydberg formula, as well as the emitter’s Doppler effect is acknowledged. The result is that the frequency of the emitted light is modified by a factor that is identical with what is called the ‘relativistic Doppler effect’. The new emission formula is applied for reinterpretation of the Ives-Stilwell experiment and shown that within the second order of approximation with respect to the speeds of the atom and the ‘absolute speed’ (Earth’s speed relative to the medium), the absolute motion does not affect the interference. The expression for the modification of the frequency involves both a first and a second-order term with respect to the speed of the atoms in the cathode tube. The latter turns out to be quantitatively the same as if the time would have changed its rate in the frame moving with the atoms. Thus, a new interpretation of the results of this famous experiment is provided without stipulating time dilation.     

Structural changes induced on strontium barium niobate glass by femtosecond laser irradiation

Localized modification of the optical properties of erbium doped strontium barium niobate (SBN) glass has been performed using femtosecond laser irradiation. The samples, with composition SrO–BaO–Nb₂O₅–B₂O₅ and doped with 5%mol of Er³⁺, were fabricated using a melt-quenching method. The samples were irradiated with different number of pulses per spot (1–50 pulses) at two different laser fluences (2.6 and 5.6 J/cm²) by using an fs laser amplifier operating at 800 nm and generating pulses with a duration of 120 fs. Micro-luminescent microscopy, using an Ar⁺ laser as excitation source, has been used to analyze the modifications of the luminescent properties of the sample upon fs laser exposure. The emissions of the Er³⁺: ⁴I_11/2→⁴I_15/2 and ⁴I_13/2→⁴I_15/2 transitions allow appreciating the structural modifications caused by femtosecond laser exposure. The lifetimes of the levels involved in these transitions were measured inside and outside the laser irradiated region. These measurements have been compared with those obtained in bulk glass ceramic sample, which is obtained from the glass precursor by a thermal treatment in order to estimate the optimal conditions to produce nanocrystals in a localized region by ultrafast laser irradiation.