Ultrahigh intensity lasers can potentially be used in conjunction with conventional fusion lasers to ignite inertial confinement fusion (ICF) capsules with a total energy of a few tens of kilojoules of laser light, and can possibly lead to high gain with as little as 100 kJ. A scheme is proposed with three phases. First, a capsule is imploded as in the conventional approach to inertial fusion to assemble a high‐density fuel configuration. Second, a hole is bored through the capsule corona composed of ablated material, as the critical density is pushed close to the high‐density core of the capsule by the ponderomotive force associated with high‐intensity laser light. Finally, the fuel is ignited by suprathermal electrons, produced in the high‐intensity laser–plasma interactions, which then propagate from critical density to this high‐density core. This new scheme also drastically reduces the difficulty of the implosion, and thereby allows lower quality fabrication and less stringent beam quality and symmet...

/pdf/ignition-and-high-gain-with-ultrapowerful-lasers-3jnwrgathf.pdf

Ignition and high gain with ultrapowerful lasers

The rise time of intense radiation determines the maximum field strength atoms can be exposed to before their polarizability dramatically drops due to the detachment of an outer electron. Recent progress in ultrafast optics has allowed the generation of ultraintense light pulses comprising merely a few field oscillation cycles. The arising intensity gradient allows electrons to survive in their bound atomic state up to external field strengths many times higher than the binding Coulomb field and gives rise to ionization rates comparable to the light frequency, resulting in a significant extension of the frontiers of nonlinear optics and (nonrelativistic) high-field physics. Implications include the generation of coherent harmonic radiation up to kiloelectronvolt photon energies and control of the atomic dipole moment on a subfemtosecond $(1{\mathrm{f}\mathrm{s}=10}^{\mathrm{\ensuremath{-}}15}\mathrm{}\mathrm{s})$ time scale. This review presents the landmarks of the 30-odd-year evolution of ultrashort-pulse laser physics and technology culminating in the generation of intense few-cycle light pulses and discusses the impact of these pulses on high-field physics. Particular emphasis is placed on high-order harmonic emission and single subfemtosecond extreme ultraviolet/x-ray pulse generation. These as well as other strong-field processes are governed directly by the electric-field evolution, and hence their full control requires access to the (absolute) phase of the light carrier. We shall discuss routes to its determination and control, which will, for the first time, allow access to the electromagnetic fields in light waves and control of high-field interactions with never-before-achieved precision.

Intense few-cycle laser fields: Frontiers of nonlinear optics

Femtosecond filamentation in transparent media

We review the field of femtosecond pulse shaping, in which Fourier synthesis methods are used to generate nearly arbitrarily shaped ultrafast optical wave forms according to user specification. An emphasis is placed on programmable pulse shaping methods based on the use of spatial light modulators. After outlining the fundamental principles of pulse shaping, we then present a detailed discussion of pulse shaping using several different types of spatial light modulators. Finally, new research directions in pulse shaping, and applications of pulse shaping to optical communications, biomedical optical imaging, high power laser amplifiers, quantum control, and laser-electron beam interactions are reviewed.

Femtosecond pulse shaping using spatial light modulators

Author(s): Vogel, Alfred; Venugopalan, Vasan | Abstract: The mechanisms of pulsed laser ablation of biological tissues were studied. The transiently empty space created between the fiber tip and the tissue surface improved the optical transmission to the target and thus increased the ablation efficiency. It was found that the structure and morphology also affect the energy transport among tissue constituents.

/pdf/mechanisms-of-pulsed-laser-ablation-of-biological-tissues-58cltfa63z.pdf

Mechanisms of pulsed laser ablation of biological tissues.

The self-channeling of ultrashort laser pulses through 20 m of air was demonstrated. The channeled pulse was measured to have 0.75 mJ of energy, a diameter of 80 microm FWHM, and a modulated spectrum. All these values were measured to be fairly constant during the propagation of the pulse. A preliminary model is shown to explain these results.

Self-channeling of high-peak-power femtosecond laser pulses in air

Experimental evidence is presented demonstrating avalanche ionization as the dominant mechanism for dielectric breakdown in silicon with ultrafast laser pulses at above-gap photon energies. Data are presented for pulses between 80 fs and 9 ns at 786 nm and 1.06 \ensuremath{\mu}m. Associated electric fields range from 0.3 to 40 MV/cm. Avalanche ionization coefficients range from ${10}^{10}$ to ${10}^{14} {\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and are discussed in relation to semiempirical dc ionization theory and recent ac Monte Carlo calculations. Correlation is obtained between electron collision times and associated ionization rates.

Avalanche ionization and dielectric breakdown in silicon with ultrafast laser pulses

The collisional effect due to the multi-photon ionization process in dielectric material has been studied. We found that the breakdown threshold of fused silica is the same for both linearly and circularly polarized light at 55 fs and 100 fs, which we believe is an indication of the suppression of multi-photon ionization in solids. By numerically solving the time-dependent Schrodinger equation with scattering, for the first time, we have observed substantial reduction of the multi-photon ionization rate in dielectrics due to collisions.

/pdf/reduction-of-multi-photon-ionization-in-dielectrics-due-to-27t1bfx7sm.pdf

Reduction of multi-photon ionization in dielectrics due to collisions

BACKGROUND We investigated the role of laser pulse width in determining fluence thresholds and efficiency for corneal photodisruption. METHODS A laser system that delivers a wide range of pulse energies and pulse widths was used to produce ablations at pulse widths from 100 femtoseconds (fs) to 7 nanoseconds (ns). The laser-induced breakdown fluence threshold at each pulse width was determined by monitoring individual plasma emissions. Using multiple shots, the photodisruption threshold and cutting depth at each pulse width were determined histologically. RESULTS Corneal breakdown thresholds decreased at a faster rate from 7 ns to approximately 10 picoseconds (ps), compared to further reductions in pulse width below 10 ps, where little variation was seen. Breakdown for pulse widths below 10 ps showed little intershot variability, resulting in highly reproducible fluence thresholds. Corneal tissue examined histologically showed similar fluence dependency. CONCLUSIONS Corneal tissue photodisruption thresholds demonstrate pulse width dependence. At pulse widths less than 10 ps and with fluences near the breakdown threshold, ablations are maximally precise and efficient. These findings suggest optimal laser parameters for corneal surgery.

Photodisruption in the human cornea as a function of laser pulse width

Interactions of intense 400-fs laser pulses with a solid target are studied with time-integrated and time-resolved measurements. The latter are accomplished by means of a pump and probe experiment, in which the motion of the critical surface is measured with 250-fs resolution. It is found that when the average electron quiver energy (${\mathit{mv}}_{\mathrm{os}}^{2}$/2) becomes comparable to the electron thermal energy (${\mathit{kT}}_{\mathit{e}}$), the ponderomotive force of the high-intensity laser significantly reduces the thermal expansion of a laser plasma. This study is performed in an interesting regime not easily accessible with longer pulse lasers, in which the electron density scale length during the laser pulse was much less than the laser wavelength.

/pdf/competition-between-ponderomotive-and-thermal-forces-in-24xa0twqlz.pdf

X. Liu

Papers

Self-channeling of high-peak-power femtosecond laser pulses in air

Avalanche ionization and dielectric breakdown in silicon with ultrafast laser pulses

Reduction of multi-photon ionization in dielectrics due to collisions

Photodisruption in the human cornea as a function of laser pulse width

Competition between ponderomotive and thermal forces in short-scale-length laser plasmas.