Aleksey M. Komashko

An overview of LLNL high-energy short-pulse technology for advanced radiography of laser fusion experiments

The technical challenges and motivations for high-energy, short-pulse generation with the National Ignition Facility (NIF) and possibly other large-scale Nd : glass lasers are reviewed. High-energy short-pulse generation (multi-kilojoule, picosecond pulses) will be possible via the adaptation of chirped pulse amplification laser techniques on NIF. Development of metre-scale, high-efficiency, high-damage-threshold final optics is a key technical challenge. In addition, deployment of high energy petawatt (HEPW) pulses on NIF is constrained by existing laser infrastructure and requires new, compact compressor designs and short-pulse, fibre-based, seed-laser systems. The key motivations for HEPW pulses on NIF is briefly outlined and includes high-energy, x-ray radiography, proton beam radiography, proton isochoric heating and tests of the fast ignitor concept for inertial confinement fusion.

Multilayer dielectric gratings for petawatt-class laser systems

Existing Petawatt class lasers today based on Nd:glass architectures operating at nominally 500 J, 0.5 ps use meter-scale aperture, gold-overcoated master photoresist gratings to compress the amplified chirped pulse. Many lasers operating in the >1kJ, >1ps regime are in the planning stages around the world. These will require multilayer dielectric diffraction gratings to handle larger pulse energy than can be accommodated with gold gratings. Models of the electric field distribution in the solid material of these gratings suggest that high aspect-ratio structures used at high incidence angles will have better laser damage resistance. New tooling for transfer etching these submicron-grating patterns and for nondestructive critical-dimension measurement of these features on meter-scale substrates will be described.

Relativistic self-focusing in underdense plasma

We present the description of powerful beam self-focusing in underdense plasma. The importance of electron cavitation, i.e. total electron evacuation under the effect of ponderomotive forces, is emphasized. Cavitation results in suppression of filamentation and the possibility to channel power well above the nominal critical power of self-focusing for a distance of many Rayleigh lengths.

Femtosecond laser materials processing

Femtosecond lasers enable materials processing of most any material with extremely high precision and negligible shock or thermal loading to the surrounding area. Applications ranging from drilling teeth to cutting explosives to making high-aspect ratio cuts in metals with no heat-affected zone are made possible by this technology. For material removal at reasonable rates, we developed a fully computer-controlled 15-Watt average power, 100-fs laser machining system.

Interferometric analysis of ultrashort pulse laser-induced pressure waves in water

Pressure waves generated by the surface ablation of water with ultrashort laser pulses in the 140 fs–10 ps duration range were studied with a Mach–Zehnder interferometry. Formation and propagation of spherical and, in ablation with longer pulses, combination of spherical and cylindrical pressure waves were observed. Measurements of the amplitude and dynamics of the pressure waves show that the conversion efficiency of laser energy to mechanical energy of the pressure waves is less than 1%, thus justifying the claim that ultrashort laser pulse ablation induces low-collateral damage and has a large potential in biomedical applications.

Modeling of long term behavior of ablation plumes produced with ultrashort laser pulses

Expansion of ablation plumes created by intense ultrashort lasers is determined by various complicated physical processes which have very different spatial and time scales. Since complete simulation by one model is practically impossible, we suggest using two models describing initial and final stages that can be matched at an intermediate time. The proposed modeling procedure connects laser parameters to plume properties far away from the ablation spot. Laser material interaction and beginning of the expansion are simulated with a one-dimensional hydrodynamics code and the final stage is modeled using an analytical solution for an expanding three- dimensional ellipsoidal gas cloud.

Metal processing with ultrashort laser pulses

Femtosecond laser ablation has been shown to produce well-defined cuts and holes in metals with minimal heat effect to the remaining material. Ultrashort laser pulse processing shows promise as an important technique for materials processing. We will discuss the physical effects associated with processing based experimental and modeling results. Intense ultra-short laser pulse (USLP) generates high pressures and temperatures in a subsurface layer during the pulse, which can strongly modify the absorption. We carried out simulations of USLP absorption versus material and pulse parameters. The ablation rate as function of the laser parameters has been estimated. Since every laser pulse removes only a small amount of material, a practical laser processing system must have high repetition rate. We will demonstrate that planar ablation is unstable and the initially smooth crater bottom develops a corrugated pattern after many tens of shots. The corrugation growth rate, angle of incidence and the polarization of laser electric field dependence will be discussed. In the nonlinear stage, the formation of coherent structures with scales much larger than the laser wavelength was observed. Also, there appears to be a threshold fluence above which a narrow, nearly perfectly circular channel forms after a few hundred shots. Subsequent shots deepen this channel without significantly increasing its diameter. The role of light absorption in the hole walls will be discussed.

Analysis of Stress Waves Generated in Water Using Ultrashort Laser Pulses

A Mach-Zehnder interferometer was used for analysis of pressure waves generated by ultrashort laser pulse ablation of water. It was found that the shock wave generated by plasma formation rapidly decays to an acoustic wave. Both experimental and theoretical studies demonstrated that the energy transfer to the mechanical shock was less than 1%.

Experimental observation of resonance effects in intensely irradiated atomic clusters

We have resolved the expansion of intensely irradiated atomic clusters on a femtosecond time scale. These data show evidence for resonant heating, similar to resonance absorption, in spherical cluster plasmas.