Showing papers by "Gerard Mourou published in 2003"

A study of the deterministic character of optical damage by femtosecond laser pulses and applications to nanomachining

Abstract: A remarkable feature of material damage induced by short-pulsed lasers is that the energy threshold becomes deterministic for sub-picosecond pulses. This effect, coupled with the advent of kHz and higher repetition rate chirped pulse amplification systems, has opened the field of femtosecond machining. Yet the mechanism of optical breakdown remains unclear. By examining the damage threshold as a function of polarization, we find that, contrary to established belief, multiphoton ionization plays an insignificant role in optical breakdown. The polarization independence, combined with the observed precise and uniform dielectric breakdown threshold even for nanometer-scale features, leads us to conclude that the fundamental mechanism is ‘self-terminated’ Zener-impact ionization, and that the deterministic and uniform damage threshold throughout the sample threshold stems from the uniform valence-electron density found in good-quality optical materials. By systematically exploring optical breakdown near threshold, we find that we can consistently machine features as small as 20 nm, demonstrating great promise for applications ranging from Micro ElectroMechanical Systems (MEMS) construction and microelectronics, to targeted disruption of cellular structures and genetic material.

Electron acceleration by few-cycle laser pulses with single-wavelength spot size.

Abstract: Generation of relativistic electrons from the interaction of a laser pulse with a high density plasma foil, accompanied by an underdense preplasma in front of it, has been studied with two-dimensional particle-in-cell (PIC) simulations for pulse durations comparable to a single cycle and for single-wavelength spot size. The electrons are accelerated predominantly in forward direction for a preplasma longer than the pulse length. Otherwise, both forward and backward electron accelerations occur. The primary mechanism responsible for electron acceleration is identified. Simulations show that the energy of the accelerated electrons has a maximum versus the pulse duration for relativistic laser intensities. The most effective electron acceleration takes place when the preplasma scale length is comparable to the pulse duration. Electron distribution functions have been found from PIC simulations. Their tails are well approximated by Maxwellian distributions with a hot temperature in the MeV range.

High resolution hard x-ray spectroscopy of femtosecond laser-produced plasmas with a CZT detector

Abstract: We present measurement of characteristic Kα emission from Mo, Ag, and La targets irradiated by a 60 fs, 600 mJ, 10 Hz Ti:sapphire laser pulse at 1017–1019 W/cm2. These x-ray emissions can potentially be used in applications from laser-based hard x-ray sources to x-ray mammography so detailed knowledge of the spectra is required to assess imaging of the figure of merit. We show here that high resolving hard x-ray spectroscopy can be achieved, with resolving powers (E/ΔE) of 60 at 18 keV, with cadmium–zinc–telluride detection system. The Kα conversion efficiency from the laser light to the Kα photon was optimized thanks to this diagnostic and values as high as 2×10−5 were obtained.

Laser-based intense hard x-ray source for mammography

Abstract: Characteristic Kα emissions from Mo, Ag and La targets irradiated by 60 fs, 600 mJ, 10 Hz Ti: Sapphire laser pulse at 1017 W/cm2 - 1019 W/cm2 can be potentially used in x-ray mammography. We have investigated x-ray spectra created by this novel x-ray source in this context. All the obtained spectra exhibited a dominating narrow emission lines with only a small portion of x-ray emission in Bremsstrahlung. Such spectra might be very usful in mammography and might improve contrast and dose utilizaion, as compared to a conventional mammographic x-ray tube. The effective focal spot size was of the order of 50 μm, i.e. significantly smaller than in conventional mammography. In contradiction to conventional mammography the effective x-ray focal spot size and the effective dose remained constant across the field of view. Kα conversion efficiency, from laser light to x-rays, was optimized and values as high as 2 x 10-5 have been obtained.

Suppression of parasitic lasing in multi-pass Ti-sapphire amplifiers

Abstract: Large aspect ratio (ratio of diameter to length) Ti- sapphire crystals, used in amplifiers of high-power lasers [1], suffer from amplified spontaneous emission and parasitic generation inside the crystal [2,3] that causes decay of population inversion and limits achievable gain. Parasitics can be suppressed by cladding the edge surface of a crystal [2,3], but only below certain transverse gain. And any way, it is difficult to use this method for high average power and/or cryogenically cooled laser systems [4] because cladding generally reduces thermal conductivity. In some cases, however, it is possible to avoid parasitics and reduce ASE by changing method of pumping and amplification in Ti-sapphire amplifiers. In the multi-pass amplifiers the high gain causing ASE and parasitics could be reduced by partial-extraction of the stored energy by the amplified pulse during pumping process.

Development of Petawatt scale Ti:sapphire laser at 0.05 Hz repetition rate

Abstract: In our design of Petawatt scale Ti:sapphire laser ("Hercules") a pump laser based on high average power glass (QX-ND, Kigre, Inc) is used at the 100 TW stage. This pump laser generates 14 J of a frequency doubled output at 0.1 Hz repetition rate in 80 ns pulse. In this paper we present a design of a relatively simple and inexpensive extension of this laser allowing to generate pump energy 60-80 J, necessary to reach Petawatt powers. The pump laser consists of a Nd:YLF oscillator, a preamplifier and two 2-pass glass amplifiers with 16-mm diameter phosphate glass rods.

Development of novel ultrafast-laser-based micro-CT system for small-animal imaging

Abstract: We investigated the performance of an ultrafast-laser-based X-ray source as a possible replacement of a microfocal X-ray tube in a micro-CT system for small-animal imaging. Using a number of solid targets (Ge, Mo, Ag, Sn, BaF/sub 2/, La, and Nd) with matching filters, we optimized conditions for X-ray generation and measured X-ray spectra, conversion efficiency, X-ray fluence, and X-ray focal-spot size. We obtained images of small animals. X-ray spectra created by ultrafast laser are advantageous for micro-CT imaging because most of the emission is in narrow characteristic lines. The spectra could be rapidly changed and matched to the imaging task (e.g. animal thickness and density). This novel X-ray source can be also easily applied in dual-energy micro-CT for small-animal imaging with suitable contrast agent (e.g. I-, Ba-, or Gd-based) and matching targets and filters for low- and high-energy beams. We have established that the effective X-ray focal-spot size can be smaller than 5 /spl mu/ and that the average power can surpass the power delivered by a microfocal X-ray tube with 5 /spl mu/m focal-spot size.