Femtosecond Laser-Matter Interactions

Solid-Plasma-Solid Transformations at the Extreme Energy Density

Eugene G. Gamaly

• Publisher: Jenny Stanford Publishing
• Year: 2022
• Print ISBN:
  9789814877404
• eBook ISBN:
  9781000522853
• Topic: Physical Sciences
• Subtopic: Physical & Theoretical Chemistry

This book describes the ultra-short laser–matter interactions from the subtle atomic motion to the generation of extreme pressures inside the bulk of a transparent crystal. It is the successor to Femtosecond Laser–Matter Interactions: Theory, Experiment and Applications (2011). Explanation and experimental verification of the exceptional technique for the phase transformations under high pressure are in the core of the book. The novel phase formation occurs along the unique solid-plasmasolid transformation path: the memory of the initial state is lost after conversion to plasma. New phase forms from chaos during the cooling to the ambient. The pressure-affected material remains detained inside a pristine crystal at the laboratory tabletop. Unique super-dense aluminium and new phases of silicon were created by the confined micro-explosions. The text also describes the recent studies that used the quasi-non-diffracting Bessel beams. The applications comprise the new high-pressure material formation and micromachining. The book is an appealing source for readers interested in the cutting-edge research exploring extreme conditions and creating nanostructures at the laboratory tabletop.

Chapter 1 Basics of Laser–Matter Interactions: Light and Matter

In this chapter, we identify different ultra-short laser–matter interaction regimes. These regimes are separated by the intensity, pulse duration, nature of a target (transparent or opaque solid) beam position on the target (on the surface, inside a bulk, at rear surface) and the focussing conditions. As a first step, we present the description of the major tool, laser beam, and the studied subject—metal and dielectric solids.

1.1 Laser Beam

It is instructive to describe the spatial, temporal and internal structure of the incident laser beam. Let us characterize first the unfocussed beam and approximations used for the characterization of the beam in practice.

1.1.1 Macroscopic Electrodynamics

From the quantum viewpoint, a laser beam is the flow of photons. The photons are massless relativistic particles with the energy ℏω (ℏ is the Planck constant divided by two π, and ω is the cyclic frequency). The energy flow of photons per unit surface per unit time, intensity, expressed through the photons number density, n_ph [cm^–3], moving with the speed of light in vacuum, c, reads

$I=n_{ph}\cdot \hslash \omega \cdot c.$ (1.1)

The photon’s wavelength relates to the frequency by the familiar relation, λ = 2πc/ω. Quantum mechanical description of photons beam is necessary when the number density of photons is less than one photon per cubic wavelength (Landau et al., 1984). One can describe the energy flow expressed through the electric field amplitude in the frames of macroscopic electrodynamics, if the number density of photons is large, n_ph >> λ^–3 (~10¹² photons/cm³ for λ = 1 µm). Expressing the number density through the intensity, n_ph = I/ℏ × c × ω, one can find that even for low intensity of interest for the material modification, ~10⁹ W/cm², the density of photons exceeds 2 × 10¹⁷ photons/ cm³, well over the above limit. Another condition of applicability of macroscopic electrodynamics demands that the field penetration depth into a medium, l_s, should be significantly larger than the inter-atomic distance, l_s>> (n_a)^–1/3, where n_a is the atomic number density. This condition is fulfilled in the all considered situations. Thus, we can use the classical description of the incident photons’ beam in the frame of macroscopic electrodynamics for considered below interaction regimes.

1.1.2 Polarization States

Photon as a quantum particle possesses spin equal to one (in units of the Plank constant), s = 1. In accord with the quantum mechanics, the particle possessing spin, s, can be found in (2s +1) states. In classical electrodynamics, the three spin states of photon correspond to three polarization states. Three quantum states of photon, corresponding to the spin states ±1, 0, are retained in classical description as polarization states corresponding to the left hand and right hand circular polarization (α = ±1) and linear polarization (α = 0).

Slow varying envelope. We will use in this book mainly the slow varying envelope approximation when the major time scale (pulse duration) significantly exceeds laser period (t >> T/2π). The electric field then is expressed as follows: E = E₀(t)(e_x . cosωt ± a · e_y · sin ωt). Here e_x, e_y are the unit vectors in a plane perpendicular to the wave vector of the wave propagating in the z-direction.

The squared laser field averaged over the many laser periods reads

$\langle E^2\rangle =E^2\left(t\right)\cdot \stackrel{t>>2\pi /\omega }{\int }{\left[\cos \omega \tau +\alpha \sin \omega \tau \right]}^{2}d\tau .$ (1.2)

The laser intensity, I (W/cm²), takes the form (brackets indicating averaging are omitted for brevity)

$I=\frac{c{E}^2}{8\pi }\left({\alpha }^2+1\right).$ (1.3)

1.1.3 Spectral Structure

The initial spectral structure of the pulse assumed to be well defined with the central frequency and the band’s width corresponding to the pulse duration. For the transform-limited pulse (Akhmanov et al., 1988), the bandwidth and the pulse duration shall obey the condition, Δω × t_p ≤ 1. For example for the pulse with t_p = 100 fs, λ = 800 nm, one obtains, Δλ ≈ (λΔω/ω) ≈ 3....

Table of contents

• Cover Page
• Half-Title Page
• Title Page
• Copyright Page
• Dedication
• Table of Content
• Preface
• List of frequently used symbols
• 1 Basics of Laser-Matter Interactions: Light and Matter
• 2 Interaction with Metals
• 3 Interaction with Dielectrics
• 4 Non-Destructive Transformations: Formation, Lifetime and Decay of Unconventional States of Matter
• 5 Ablation of Metals and Dielectrics
• 6 Extreme Energy Density Confined inside a Transparent Crystal—Novel Path for New Materials Creation: Solid-Plasma-Solid Transformations
• Conclusions and Future Directions
• Appendices
• Bibliography
• Index