A second-order autocorrelator for single-shot measurement of femtosecond laser pulse durations

A second-order autocorrelator for single-shot measurement of ultrashort laser pulse durations has been set up. It is based on recording the spatial profile of non-collinear phase-matched second harmonic generation in a KDP crystal using a CCD camera-framegrabber combination. Performance of the system is described from measurement of 250 femtosecond transform-limited laser pulses from a passively mode-locked, diode pumped Nd:glass laser. It can also be used for measurement of picosecond laser pulses in the multi-shot scanning mode.     

Femtosecond regenerative amplification in Cr:forsterite

We describe an oscillator-amplifier laser system for the generation of high-power femtosecond pulses near 1.25 mum based on chromium-doped forsterite. Chirped-pulse amplification at a 1-kHz repetition rate raises the pulse energy to >350 muJ. The nearly transform-limited 200-muJ, 135-fs-long recompressed pulses have a peak power of approximately 1.5 GW.     

飞秒激光烧蚀MgAl2O4透明陶瓷的实验研究

实验研究了800nm飞秒激光与MgAl2O4透明陶瓷的相互作用，得到其在单脉冲、多脉冲情况下的损伤阈值和损伤面积，用CCD成像技术和扫描电镜观察了烧蚀点的形貌特征，用显微红外光谱仪测试了烧蚀区域的透过光谱．结果表明；单脉冲烧蚀条件下，烧蚀面积与脉冲能量近似为线性关系，而在多脉冲烧蚀条件下，烧蚀面积随着脉冲数量的增加呈近似波尔兹曼（Boltzmann）增大；当激光功率接近损伤阈值时，烧蚀后的区域在波数为2500-7000cm^-1范围内的红外透过率由82％提高到86％，当激光功率超过损伤阈值后，透过率降低20％左右．     

Trace-gas detection in ambient air with a thermoelectrically cooled, pulsed quantum-cascade distributed feedback laser

A pulsed quantum-cascade distributed feedback laser operating at near room temperature was used for sensitive high-resolution IR absorption spectroscopy of ambient air at a wavelength of ~8 mum. Near-transform-limited laser pulses were obtained owing to short (~5-ns) current pulse excitation and optimized electrical coupling. Fast and slow computer-controlled frequency scanning techniques were implemented and characterized. Fast computer-controlled laser wavelength switching was used to acquire second-derivative absorption spectra. The minimum detectable absorption was found to be 3 x 10(-4) with 10(5) laser pulses (20-kHz repetition rate), and 1.7 x 10(-4) for 5 x 10(5) pulses, based on the standard deviation of the linear regression analysis.     

Femtosecond Traveling-Wave Optical Parametric Amplification in MgO:LiNbO(3)

A mid-infrared femtosecond optical parametric amplifier tunable in the chemically important spectral region between 3.1 and 3.9 mum (at >10-muJ idler pulse energy) has been constructed on the basis of MgO:LiNbO(3) with 7% doping. With femtosecond pumping near 800 nm (Ti:sapphire regenerative amplifier) and narrow-band (long-pulse) seeding, this simple single-stage device provides maximum conversion efficiencies of 40% and exhibits extremely low seed threshold (<10-mW peak seed power for >1-muJ idler output). The generated idler pulses are almost transform limited with <200-fs pulse duration. The pulse-to-pulse fluctuations reproduce the stability of the pump source at 1-kHz repetition rate.     

Spectrum modification of XUV radiation in the presence of a delayed weak control field

We use a delayed weak laser beam to control the spectral features of extreme ultraviolet (XUV) pulses generated by a strong femtosecond laser beam through phase-matched high harmonic generation (HHG) in an atomic medium, e.g. krypton. The variation of the HHG spectrum reveals the influence of the free electrons on the propagation of the XUV field in the medium. In addition, a signature of the autoionization process is visible. Our findings provide a promising technique to study ultrafast dynamics of atomic and molecular gases.     

Phase-matching and femtosecond difference-frequency generation in the quaternary semiconductor AgGaGe5Se12

We present data on the linear (transmission, index of refraction) and nonlinear (second-order susceptibility) optical properties of the quaternary semiconductor AgGaGe5Se12 with orthorhombic symmetry--a solid solution in the AgxGaxGe1-xSe2 system with x = 0.17. The nonlinear coefficients are estimated from phase-matched second-harmonic generation near 3 microm. After numerical analysis of the phase-matching configurations for three-wave nonlinear interactions, the first experimental results on difference-frequency mixing, producing tunable (4-7.5-microm) femtosecond pulses at a 1-kHz repetition rate, are described. The pulses of only five optical cycles (FWHM = 84 fs) are generated near 5 microm with energy of 0.5 microJ. Because of its higher damage threshold, larger birefringence and bandgap, and greater variety of phase-matching schemes, AgGaGe5Se12 could become an alternative to AgGaS2 and AgGaSe2, more widely used in high-power and specific applications.     

Seeded femtosecond optical parametric amplification in the mid-infrared spectral region above 3 mum

Femtosecond optical parametric amplification that results in microjoule mid-infrared pulses at wavelengths exceeding 3 mum is demonstrated. Narrow-band quasi-cw seeding at the signal wavelength is applied to ensure the generation of nearly transform-limited femtosecond pulses at the idler wavelength. The broad bandwidth of the parametric amplification provided by pumping with femtosecond pulses from a Ti:sapphire regenerative amplifier at high intensity results in idler pulse durations shorter than the pump pulse length. The potentials of three nonlinear optical crystals that belong to the potassium titanyl phosphate family are comparatively studied. At 1-kHz repetition rate our all-solid-state system produces highly synchronized ~100-fs pulses in the spectral range between 3 and 4 mum.     

Dispersion measurement of inert gases and gas mixtures at 800 nm

Dispersion of femtosecond laser pulses propagating in Ar, He, Kr, N(2), Ne, Xe, and their mixtures is measured by spectrally and spatially resolved interferometry. By varying the gas pressure in a 4.5 m long tube between 0.05 mbar and ambient pressure, the first, second, and third order phase derivatives of broadband laser pulses are determined at 800 nm under standard conditions. The dispersion of gases and gas mixtures obeys the Lorentz-Lorenz formula with an accuracy of 0.7%. Based on the measured pressure dependent dispersion values in the near infrared and the refractive indices available from the literature for the ultraviolet and visible, a pressure dependent Sellmeier-type formula is fitted for each gas. These common form, two-term dispersion equations provide an accuracy between 4.1x10(-9) (Ne) and 4.3x10(-7) (Xe) for the refractive indices, from UV to near IR.     

具有超快光Kerr效应的线型共轭聚苯基氰的合成

在射频电容耦合的辉光放电装置中进行苯基氰的等离子体聚合。用红外光谱，紫外光谱及原位四级质谱等技术表征聚合过程的聚合物结构。采用超快染料激光器测定聚苯基氰的超快光Ｋｅｒｒ效应。     

Comparative study of femtosecond and nanosecond laser-induced breakdown spectroscopy of depleted uranium

We present spectra of depleted uranium metal from laser plasmas generated by nanosecond Nd:YAG (1064 nm) and femtosecond Ti:sapphire (800 nm) laser pulses. The latter pulses produce short-lived and relatively cool plasmas in comparison to the longer pulses, and the spectra of neutral uranium atoms appear immediately after excitation. Evidence for nonequilibrium excitation with femtosecond pulses is found in the dependence of spectral line intensities on the pulse chirp.     

Plastic bending of sapphire fibers for infrared sensing and power-delivery applications

Sapphire fibers with diameters of 325-850 mum were plastically bent by CO(2) laser beams with typical bending radii as small as 2.8 mm. The additional optical loss caused by a single bend was less than 0.1 dB (at 900 nm), the damage threshold of the bent fibers was higher than 150 MW/cm(2) for Nd:YAG laser pulses, and the high mechanical strength of the bending area was also proved. Several successful applications of bent sapphire fibers have shown that plastically bent sapphire fibers are promising for use in IR sensing and power-delivery applications.     

Kilohertz femtosecond UV-pumped visible beta-barium borate and lithium triborate optical parametric generator and amplifier

We demonstrate optical parametric generation and amplification of femtosecond pulses in the entire visible range using type-I phase-matched beta-barium borate and lithium triborate crystals pumped by the frequency-doubled output of a Ti:sapphire regenerative amplifier at 395 nm. The output is tunable from 470 to 770 nm with a pulse width of ~170 fs at a repetition rate of 1 kHz and a maximum output energy of ~1.1 muJ/pulse. The visible optical parametric amplifier output was then frequency doubled and sum frequency mixed with the fundamental output of Ti:sapphire at 790 nm to produce UV pulses with a conversion efficiency of greater than 25%. The second harmonic generated UV pulses are tunable from 240 to 380 nm with a maximum pulse energy of ~260 nJ/pulse.     

New Optical Fiber Coherence Tomography— Compact and Stabile System for Medical Application

A new idea of the optical fiber coherence tomography system designed for medical application is presented in this paper. Application as a source the femtosecond 400 mW Ti:Al₂O₃ pulse laser operates at wavelength 798 nm with spectral pulse bandwidth about 145 nm provides the longitudinal resolution about 2 mum. The system can be used for scanning the interior of a human body owing to application of a special optical head based on single-mode fiberscope. The structure comprises a set of two fiber-optic interferometers. The first is a Michelson type which enables the doubled pulses generation with spatial separation directly correlated with a deep scanning in the investigated sample. The second one is a partially delayed Fizeau interferometer formed on the end of optical fiber providing a high temperature and polarization system stability. The discussed setup consists also of a new signal processing structure based on synchronous detection scheme and application of the lock-in amplifier allowing investigation of the reflection coefficient of investigated sample via a suitable numerical processing of the first two harmonic amplitudes of the detected signals. The experimental results of OCT reconstruction of a tree leaf in area 150 ×  1750 mum giving the possibility of recognizing various kind of tissues are also presented.     

High-accuracy measurement of 240-m distance in an optical tunnel by use of a compact femtosecond laser

A high-accuracy optical distance meter with a mode-locked femtosecond laser is proposed for distance measurements in a 310-m-long optical tunnel. We measured the phase shift of the optical beat component between longitudinal modes of a mode-locked laser. A high resolution of 50 mum at 240-m distance was obtained without cyclic error correction. The group refractive index of air is automatically extracted to an accuracy of 6 parts per million (ppm) by two-color measurement with the pulses of fundamental and second-harmonic wavelengths. Finally, an absolute mechanical distance of 240 m was obtained to within 8-ppm accuracy by use of a series of beat frequencies with the advantage of a wide range of intermode frequency, together with the results of the two-color measurement.     

Long-range detection and length estimation of light filaments using extra-attenuation of terawatt femtosecond laser pulses propagating in air

High-energy femtosecond laser pulses propagating in the atmosphere undergo self-focusing resulting in the appearance of the phenomenon of filamentation. We observed an extra-attenuation of such (terawatt) femtosecond laser pulses propagating in the atmosphere when compared with long pulses (200 ps) with the same energy. This is because, in contrast to the linear propagation of the long pulse, the input femtosecond laser pulse is attenuated owing to either absorption through multiphoton ionization/tunnel ionization or to scattering on the laser-induced plasma; self-phase-modulation and self-steepening further convert partially the energy initially contained in the fundamental bandwidth into the broad side bands of the laser, becoming eventually a white-light laser pulse (supercontinuum). The experimental data allow us to extract an effective extra-attenuation coefficient for an exponential decay of the input pulse energy with the propagation distance. Such a coefficient allows us to estimate an upper bound of the filament length under the experimental conditions used. More generally, our observation leads to a new technique to remotely detect light filaments in the atmosphere.     

Inducing and probing non-thermal transitions in semiconductors using femtosecond laser pulses

Soon after it was discovered that intense laser pulses of nanosecond duration from a ruby laser could anneal the lattice of silicon, it was established that this so-called pulsed laser annealing is a thermal process. Although the radiation energy is transferred to the electrons, the electrons transfer their energy to the lattice on the timescale of the excitation. The electrons and the lattice remain in equilibrium and the laser simply 'heats' the solid to the melting temperature within the duration of the laser pulse. For ultrashort laser pulses in the femtosecond regime, however, thermal processes (which take several picoseconds) and equilibrium thermodynamics cannot account for the experimental data. On excitation with femtosecond laser pulses, the electrons and the lattice are driven far out of equilibrium and disordering of the lattice can occur because the interatomic forces are modified due to the excitation of a large (10% or more) fraction of the valence electrons to the conduction band. This review focuses on the nature of the non-thermal transitions in semiconductors under femtosecond laser excitation.     

Clustered gases as a medium for efficient plasma waveguide generation

Clustered gas jets are shown to be an efficient means for plasma waveguide generation, for both femtosecond and picosecond generation pulses. These waveguides enable significantly lower on-axis plasma density (less than 10(18) cm(-3)) than in conventional hydrodynamic plasma waveguides generated in unclustered gases. Using femtosecond pump pulses, self-guided propagation and strong absorption (more than 70%) are used to produce long centimetre scale channels in an argon cluster jet, and a subsequent intense pulse is coupled into the guide with 50% efficiency and guided at above 10(17)W cm(-2) intensity over 40 Rayleigh lengths. We also demonstrate efficient generation of waveguides using 100 ps axicon-generated Bessel-beam pump pulses. Despite the expected sub-picosecond cluster disassembly time, we observe long pulse absorption efficiencies up to a maximum of 35%. Simulations show that in the far leading edge of the long laser pulse, the volume of heated clusters evolves to a locally uniform and cool plasma already near ionization saturation, which is then efficiently heated by the remainder of the pulse.     

Phase, amplitude, and polarization shaping with a pulse shaper in a Mach-Zehnder interferometer

We present a shaper scheme that fully controls the spectral phase, amplitude, and polarization of femtosecond laser pulses. In particular, it enables independent manipulation over the major axis orientation and the axis ratio of the polarization ellipse. This is accomplished by integrating a 4f-shaper setup in both arms of a Mach-Zehnder interferometer and rotating the polarization by 90 degrees in one of the arms before overlaying the beams. The generated pulses are resolved in a simple and intuitive detection scheme.     

Plasmonic near-electric field enhancement effects in ultrafast photoelectron emission: correlated spatial and laser polarization microscopy studies of individual ag nanocubes

Electron emission from single, supported Ag nanocubes excited with ultrafast laser pulses (λ = 800 nm) is studied via spatial and polarization correlated (i) dark field scattering microscopy (DFM), (ii) scanning photoionization microscopy (SPIM), and (iii) high-resolution transmission electron microscopy (HRTEM). Laser-induced electron emission is found to peak for laser polarization aligned with cube diagonals, suggesting the critical influence of plasmonic near-field enhancement of the incident electric field on the overall electron yield. For laser pulses with photon energy below the metal work function, coherent multiphoton photoelectron emission (MPPE) is identified as the most probable mechanism responsible for electron emission from Ag nanocubes and likely metal nanoparticles/surfaces in general.