Showing papers on "Pulse duration published in 2005"

Experimental investigation of ablation efficiency and plasma expansion during femtosecond and nanosecond laser ablation of silicon

Abstract: Femtosecond laser (Ti:sapphire, 100 fs pulse duration) ablation of silicon in air was compared with nanosecond laser (Nd:YAG, 3 ns pulse duration) ablation at ultraviolet wavelength (266 nm) Laser ablation efficiency was studied by measuring crater depth as a function of pulse number For the same number of laser pulses, the fs-ablated crater was about two times deeper than the ns-crater The temperature and electron number density of the laser-induced plasma were determined from spectroscopic measurements The electron number density and temperature of fs-induced plasmas decreased faster than ns-induced plasmas due to different energy deposition mechanisms Images of the laser-induced plasma were obtained with femtosecond time-resolved laser shadowgraph imaging Plasma expansion in both the perpendicular and the lateral directions were compared

Pulse duration dependence of femtosecond-laser-fabricated nanogratings in fused silica

Abstract: Femtosecond laser radiation tightly focused in bulk fused silica is used to generate self-ordered nanogratings when the sample is translated under the lens at constant speed. The nanogratings are preserved over a length scale of millimeters. We demonstrate that nanogratings are formed for all pulse durations tested, ranging from 40to500fs, and that the pulse energy threshold for this phenomenon increases with decreasing pulse duration. We use high spatial resolution diagnostics based upon selective chemical etching followed by atomic force microscopy and scanning electron microscopy to reveal the morphology of the nanogratings.

Clocking femtosecond X rays.

Abstract: Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms.

Direct measurement of the critical power of femtosecond Ti:sapphire laser pulse in air

Abstract: We report a simple experiment to directly determine the critical power for self-focusing in air by measuring the focal shift of the focused femtosecond Ti:sapphire laser pulses. The measured critical power is 10 GW for the 42 fs laser pulse; it gradually decreases to 5 GW for (chirped) pulse duration longer than 200 fs.

Production of Doubly Charged Helium Ions by Two-Photon Absorption of an Intense Sub-10-fs Soft X-Ray Pulse at 42 eV Photon Energy

Abstract: We report on the observation of doubly charged helium ions produced by a nonlinear interaction between a helium atom and photons with a photon energy of 42 eV which are generated with the 27th harmonic of a femtosecond pulse from a Ti:sapphire laser. The number of ions is proportional to the square of the intensity of the 27th harmonic pulse, and thus two-photon double ionization should be dominantly induced as compared with other nonlinear processes accompanying sequential ionization via a singly charged ion. This phenomenon is utilized to measure the pulse duration of the 27th harmonic pulse by using an autocorrelation technique, for the first time to our knowledge, and as a result a duration of 8 fs is found.

Pulse self-compression to the single-cycle limit by filamentation in a gas with a pressure gradient.

Abstract: We calculate pulse self-compression of a 30 fs laser pulse traversing gas with different pressure gradients. We show that an appropriate density profile brings significant improvement to the self-compression by filamentation. Under an optimal pressure gradient, the pulse duration is reduced to the single optical cycle limit over a long distance, allowing easy extraction into an interaction chamber.

Double chirped-pulse-amplification laser: a way to clean pulses temporally

Abstract: We demonstrate a double chirped-pulse-amplification (CPA) Ti: sapphire laser system that includes two CPA stages with intermediate nonlinear temporal pulse filtering. The method makes it possible to reduce substantially the background of amplified spontaneous emission (ASE), including prepulses and postpulses. An ASE temporal contrast of 10(10) was demonstrated at 20 mJ of output energy and 50-fs pulse duration. The demonstrated scheme is applicable to petawatt power-level laser systems.

High-power picosecond and femtosecond pulse generation from a two-section mode-locked quantum-dot laser

Abstract: We demonstrate mode locking in a two-section quantum-dot laser that produces output powers up to 45 mW at 1260 nm. The pulse duration could be varied from 2 ps to as short as 400 fs at the 21 GHz pulse repetition rate.

Pulse compression with supercontinuum generation in microstructure fibers

Abstract: We demonstrate the generation of 5.5-fs pulses by dispersive compression of a supercontinuum generated with 15-fs pulses from a Ti:sapphire laser in a 5-mm-long microstructure fiber. The generated continuum is characterized with a setup for cross-correlation spectral-phase interferometry for direct electric-field reconstruction (SPIDER). The reconstructed spectral phase is used as on input for an iterative algorithm optimizing the compressor phase profile. We also discuss the limitations of this technique concerning the achievable pulse duration, including aspects like limited spectral coherence of the supercontinuum, limited compressor resolution, and a limitation of the SPIDER technique. The coherence is limited by power fluctuations and the instability of the seed laser.

Pulse width effect in ultrafast laser processing of materials

Abstract: A systematic study of the influence of laser pulse width in ultrafast laser micromachining of silicon and stainless steel was carried out. Experiments were performed using a Ti:sapphire laser with 800 nm and 400 nm (frequency doubling) wavelengths in an air environment with a pulse duration ranging from 110 fs to 10 ps. Hole drilling and line cutting of silicon and stainless steel were studied. Structural details of the machined area were characterized using a number of techniques such as optical microscopy, atomic force microscopy and stylus profilometry. The effect of pulse width on melting was observed at ∼2.5 ps for stainless steel and at ∼5 ps for silicon. Also, the polarization effect on the laser machining is reported. The machined quality using femtosecond laser pulses with high energy fluence was found to be limited by the laser intensity contrast.

Direct measurement of transient pulses induced by laser and heavy ion irradiation in deca-nanometer devices

Abstract: This paper investigates the transient response of 50-nm gate length fully and partially depleted SOI and bulk devices to pulsed laser and heavy ion microbeam irradiations. The measured transient signals on 50-nm fully depleted devices are very short, and the collected charge is small compared to older 0.25-/spl mu/m generation SOI and bulk devices. We analyze in detail the influence of the SOI architecture (fully or partially depleted) on the pulse duration and the amount of bipolar amplification. For bulk devices, the doping engineering is shown to have large effects on the duration of the transient signals and on the charge collection efficiency.

Three-dimensional carrier-dynamics simulation of terahertz emission from photoconductive switches

Abstract: A semi-classical Monte Carlo model for studying three-dimensional carrier dynamics in photoconductive switches is presented. The model was used to simulate the process of photoexcitation in GaAs-based photoconductive antennas illuminated with pulses typical of mode-locked Ti:Sapphire lasers. We analyzed the power and frequency bandwidth of THz radiation emitted from these devices as a function of bias voltage, pump pulse duration and pump pulse location. We show that the mechanisms limiting the THz power emitted from photoconductive switches fall into two regimes: when illuminated with short duration ( 40 fs) pulses, screening is the primary power-limiting mechanism. A discussion of the dynamics of bias field screening in the gap region is presented. The emitted terahertz power was found to be enhanced when the exciting laser pulse was in close proximity to the anode of the photoconductive emitter, in agreement with experimental results. We show that this enhancement arises from the electric field distribution within the emitter combined with a difference in the mobilities of electrons and holes.

Surface modification of steels and magnesium alloy by high current pulsed electron beam

Abstract: High current pulsed electron beam (HCPEB) is now developing as a useful tool for surface modification of materials. When concentrated electron flux transferring its energy into a very thin surface layer within a short pulse time, superfast processes such as heating, melting, evaporation and consequent solidification, as well as dynamic stress induced may impart the surface layer with improved physico-chemical and mechanical properties. This paper presents our research work on surface modification of steels and magnesium alloy with HCPEB of working parameters as electron energy 27 keV, pulse duration ∼1 μs and energy density ∼2.2 J/cm2 per pulse. Investigations performed on carbon steel T8, mold steel D2 and magnesium alloy AZ91HP have shown that the most pronounced changes of phase–structure state and properties occurring in the near-surface layers, while the thickness of the modified layer with improved microhardness (several hundreds of micrometers) is significantly greater than that of the heat-affected zone. The formation mechanisms of surface cratering and non-stationary hardening effect in depth are discussed based on the elucidation of non-equilibrium temperature filed and different kinds of stresses formed during pulsed electron beam melting treatment. After the pulsed electron beam treatments, samples show significant improvements in measurements of wear and corrosion resistance.

Dynamics of laser-induced molecular alignment in the impulsive and adiabatic regimes : A direct comparison

Abstract: Quantum-mechanical calculations are performed of the dynamic alignment of linear molecules induced by a strong nonresonant laser field. Within this framework we have treated in a unified fashion the alignment with laser pulses of varying duration from the short pulse impulsive limit $({\ensuremath{\tau}}_{\text{pulse}}⪡{T}_{\mathrm{rot}})$ to the long pulse adiabatic limit $({\ensuremath{\tau}}_{\text{pulse}}g{T}_{\mathrm{rot}})$. The temporal behavior of the alignment in both these limits, and in the intermediate pulse duration regime, have been analyzed. For the impulsive limit the dependence of the degree of maximum alignment upon the laser pulse duration was examined and the intensity-dependent optimum pulse duration explained. A comparison between the degree of alignment under the same conditions of pulse intensity and rotational temperature was performed between the impulsive and adiabatic cases. The adiabatic case was found to always provide a better degree of alignment for a given intensity which we show is due to the zero relative phasing between the component states of the superposition that form the pendular states. We have explicitly calculated the angular distribution of an ensemble of linear molecules as it evolves through a rotational revival; a rich structure is found that may be useful in guiding future experiments that utilize the field free alignment in a revival.

Coherent probe-pump-based Brillouin sensor for centimeter-crack detection

Abstract: We provide a theoretical explanation for a coherent probe-pump-based Brillouin sensor system that achieves centimeter spatial resolution with high-frequency resolution. It was recently discovered that, when a combination of cw and pulsed light (the probe beam) interacts with a cw laser (the pump beam), centimeter spatial resolution with high-frequency resolution can be achieved even though the probe-pulse duration is 1.5 ns [Opt. Lett.29, 1485 (2004)]. Our study reveals that the coherent portion inside the pulse length of these two interactions caused by the same phase is responsible for this behavior. It allows us to detect 1.5-cm outer-layer cracks on an optical ground-wire cable.

Laser based synchrotron radiation

Abstract: Beams of x rays in the kiloelectronvolt energy range have been produced from laser-matter interaction. Here, energetic electrons are accelerated by a laser wakefield, and experience betatron oscillations in an ion channel formed in the wake of the intense femtosecond laser pulse. Experiments using a 50 TW laser (30 fs duration) are described, as well as comparisons with numerical simulations. These results pave the way of a new generation of radiation in the x-ray spectral range, with a high collimation and an ultrafast pulse duration, produced by the use of compact laser system.

Phase change memory bits reset through a series of pulses of increasing amplitude

Abstract: A set bit in a phase change memory may be programmed to a reset bit using a series of pulses of increasing amplitude. An initial start pulse is applied. After the start pulse is applied, a check determines whether the bit has been reset. If not, a higher amplitude pulse is applied. Each time the pulse amplitude is to be incremented, a check determines whether a maximum safe pulse amplitude has been exceeded. The pulse amplitude is continually incremented until either the maximum is reached or all the bits to be programmed have been programmed into the correct reset state.

Dependence on laser intensity and pulse duration in proton acceleration by irradiation of ultrashort laser pulses on a Cu foil target

Abstract: The dependence on laser intensity and pulse duration in energetic proton acceleration by irradiation of ultrashort laser pulses on a 5μm thick copper tape target was measured. The laser intensity was varied from 8.5×1017W∕cm2 to 1.1×1019W∕cm2, and the pulse duration from 55 fs to 400 fs. The maximum proton energy increased as the pulse duration was increased while the laser intensity was kept constant. The dependence of the maximum proton energy on laser intensity and pulse duration was in good agreement with an analytical plasma-expanding model.

2-ps passively mode-locked Nd:YVO4 laser using an output-coupling-type semiconductor saturable absorber mirror

Abstract: We have demonstrated stable self-starting passive mode-locking in a diode-end-pumped Nd: YVO4 laser using a semiconductor saturable absorber mirror (SESAM). An ln(0.25)Ga(0.75)As single quantum-well SESAM, which was grown by the metalorganic chemical-vapor deposition technique at low temperature, acts as a passive mode-locking device and an output coupler at the same time. Continuous-wave mode-locked transform-limited pulses were obtained at 1064 nm with a pulse duration of 2.1 ps and an average output power of 1.28 W at a repetition rate of 96.5 MHz. (c) 2005 American Institute of Physics.

Active retina implant with a multiplicity of pixel elements

Abstract: An active retina implant has a multiplicity of pixel elements that convert incident light into electric stimulation signals for cells of the retina with which stimulation electrodes are to make contact. Each pixel element is provided with at least one image cell that converts incident light into electric signals, there being provided at least one amplifier whose input is connected to the image cell and whose output is connected to at least one stimulation electrode to which it supplies a stimulation signal. Also provided is an energy supply which provides externally coupled external energy as supply voltage for the image cells and the amplifiers. The image cell has a logarithmic characteristic according to which incident light of specific intensity is converted into electric signals of specific amplitude. The stimulation signal is supplied in the form of analog voltage pulses of specific pulse length and pulse spacings, the pulse amplitude being a function of the intensity of the incident light.

High-power subnanosecond beams of runaway electrons and volume discharge formation in gases at atmospheric pressure

Abstract: A non-local criterion for runaway electrons generation is proposed, which is a universal two-valued dependence of the critical voltage U cr on pd for a certain gas (where p is the pressure and d is the interelectrode distance). This dependence subdivides the (U, pd) plane into an area of efficient electron multiplication and an area in which electrons leave the gas gap without multiplication. Electron beams of subnanosecond pulse duration and an amplitude of hundreds of amperes are generated at atmospheric pressure in various gases. A volume nanosecond discharge with a high specific excitation power was realized without the pre-ionization of the discharge gap by using an additional external source. The role of the electron avalanches propagating from the cathode to the anode in a dense gas was considered.

Nd:YAG/V:YAG microchip laser operating at 1338 nm

Abstract: Q-switched microchip laser emitting radiation at wavelength 1338 nm was designed and constructed. This laser was based on a monolith crystal which combines in one piece a cooling undoped part (undoped YAG crystal), an active laser part (Nd3+:YAG), and a saturable absorber (V3+:YAG, T0=85%). The microchip resonator consists of dielectric mirrors directly deposited on the monolith surfaces. The output coupler with reflection 90% was placed on the V3+-doped part. Q-switched microchip laser was tested under pulsed, and CW diode pumping. The pulse length was the same for all regimes and it equals to 6.2 ns. The wavelength of linearly polarized laser emission was 1338 nm. For pulsed pumping the output pulse energy was stable up to mean pump power 1 W and it was equal to 131 μJ, which corresponds to peak power 21 kW. In CW regime for pumping up to 14 W the pulse energy was 37 μJ.

Alignment of symmetric top molecules by short laser pulses

Abstract: Nonadiabatic alignment of symmetric top molecules induced by a linearly polarized, moderately intense picosecond laser pulse is studied theoretically and experimentally. Our studies are based on the combination of a nonperturbative solution of the Schr\"odinger equation with femtosecond time-resolved photofragment imaging. Using methyliodide and tert-butyliodide as examples, we calculate and measure the alignment dynamics, focusing on the temporal structure and intensity of the revival patterns, including their dependence on the pulse duration, and their behavior at long times, where centrifugal distortion effects become important. Very good agreement is found between the experimental and numerical results. This allows us to use our theory and numerical results to provide additional insight into the origin of the experimental findings.