Absorption of subpicosecond UV laser pulses during interaction with solid targets.

Femtosecond laser pulses are capable of producing very high light intensities at moderate pulse energies. We study the laser light absorption at flux densities of 1017 W/cm2 and higher during the plasma formation and the plasma heating process. In the first stage multiphoton and collisional ionization dominate. Modification of the inverse bremsstrahlung absorption occur owing to ionization dephasing. In the second stage beam energy conversion is mainly by collisional absorption at normal incidence, and by resonance absorption at oblique illumination. Simultaneously strong electron heat conduction provides for plasma formation and heating in deeper layers of the solid not accessible to the laser light. The penetration depth of the laser beam is modified by the anomalous skin effect. The electron-ion collision frequency at the high laser intensities under consideration is determined by the oscillation energy of the electrons rather than by their thermal motion. Although it reaches high values (1014-1016 S-1), all kinds of solid targets become strongly reflecting (>60%) owing to the formation of electron densities largely exceeding those of solid-state plasmas. Effects modifying the degree of absorption are briefly discussed.

The absorption of 250-fs KrF laser pulses incident on solid targets of aluminum and gold has been measured as a function of polarization and angle of incidence for the intensity range of ${10}^{14}$--2.5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. Maximum absorption of over 60% occurs for p-polarized radiation at angles of incidence in the range of 48\ifmmode^\circ\else\textdegree\fi{}--57\ifmmode^\circ\else\textdegree\fi{}. The measured results are in agreement with absorption on a steep density gradient.

The absorption of 250 fs KrF laser pulses incident on solid targets of aluminum, copper and gold has been measured for normal incidence as a function of laser intensity in the range of 1012–1014 W cm−2 and as a function of polarization and angle of incidence for the intensity range of 1014−2.5×1015 W cm−2. As the intensity increases from 1012 W cm−2 the reflectivity at normal incidence changes from the low-intensity mirror reflectivity value to values in the range of 0.5–0.61 at 1014 W cm−2. For this intensity maximum absorption of 63–80% has been observed for p-polarized radiation at angles of incidence in the range of 54°–57°, increasing with atomic number. The results are compared with the expected Fresnel reflectivity from a sharp vacuum-plasma interface with the refractive index given by the Drude model and also to numerical calculations of reflectivity for various scale length density profiles. Qualitative agreement is found with the Fresnel/Drude model and quantitative agreement is noticed with the numerical calculations of absorption on a steep density profile with normalized collision frequencies, v/ω, in the range of 0.13–0.15 at critical density and normalized density gradient scale lengths, L/λ0, in the range of 0.018–0.053 for a laser intensity of 1014 W cm−2.

The effect of light intensity on resonance absorption of p-polarization laser pulses in plasma with steep density gradients is studied by particle-in-cell simulations. For given incident angle and density scale length, the absorption rate decreases with an increase of the laser intensity. This can be attributed to a relativistic reduction of the local plasma frequency and thus detuning of the driving laser field from the excited electron plasma waves near the critical surface. With a further increase of the laser intensity, the absorption rate begins to increase since the laser field can now further penetrate into the plasma up to higher densities and drive density cavities and peaks there. As a result, multiple-resonant absorption is found at different locations.

Mathematical expressions for the shape and amplitude of a photoacoustic signal generated due to the multiphoton absorption of Gaussian laser pulses are derived. It was found that the sensitivity of a photoacoustic spectrometer increases in proportion to n 3/4 when changing from one-photon linear absorption to nonlinear multiphoton absorption, where n is the nonlinearity exponent. Calibration of the photoacoustic spectrometer used in the measurements of multiphoton absorption cross sections is discussed.

It has been well recognized that the presence of a skull imposes harsh restrictions on the use of ultrasound and optoacoustic techniques in the study, treatment and modulation of the brain function. We propose a rigorous modeling and experimental methodology for estimating the insertion loss and the elastic constants of the skull over a wide range of frequencies and incidence angles. A point-source-like excitation of ultrawideband acoustic radiation was induced via the absorption of nanosecond duration laser pulses by a 20 μm diameter microsphere. The acoustic waves transmitted through the skull are recorded by a broadband, spherically focused ultrasound transducer. A coregistered pulse-echo ultrasound scan is subsequently performed to provide accurate skull geometry to be fed into an acoustic transmission model represented in an angular spectrum domain. The modeling predictions were validated by measurements taken from a glass cover-slip and ex vivo adult mouse skulls. The flexible semi-analytical formulation of the model allows for seamless extension to other transducer geometries and diverse experimental scenarios involving broadband acoustic transmission through locally flat solid structures. It is anticipated that accurate quantification and modeling of the skull transmission effects would ultimately allow for skull aberration correction in a broad variety of applications employing transcranial detection or transmission of high frequency ultrasound.

Resonant absorption is a process that electromagnetic (EM) energy is converted to plasma energy with a mode conversion on the resonant layer where the incident EM wave frequency equals to the local frequency of a plasma normal modes. With a finite collision between charged and background neutral particles in a plasma, the plasma oscillation is dissipated to widen the resonance layer and heat the plasma. In this work a modified scattering matrix methods (SMM) are applied to study the effects of the collision frequency, incident angle, and plasma thickness on resonant absorption. We analyze the energy absorption caused by resonance in comparison with collisional absorption for different parameters. It is found that the resonant absorption dissipates about nearly half of the incident EM energy in an overdense inhomogeneous plasma when the collision is weak, and the rest half portion is reflected. If the collision is strong, however, the collisional absorption is then more significant than the resonant and affects the entire wave propagation process.

A theoretical and experimental investigation was made of the thermal generation of sound by the absorption of laser pulses whose duration was short compared with the transit time of sound across the absorption region. An approximate theoretical model was used in calculations of the acoustic field in the near and far zones of a bulk antenna formed by the absorption zone. Experiments were carried out using a YAG laser emitting pulses of 30 nsec duration and 0.06 J energy. The experimental results were in good agreement with the theory.

Acoustic transients resulting from the absorption of normal mode and Q-switched laser pulses in the mammalian eye have been implicated in damage to retinal tissue. A theoretical analysis indicated that acoustic transients with durations of the same order as the laser pulse (e.g., 100 msec for a Q-switched pulse) and amplitudes of greater than 100 atm could be produced in model systems that simulate the retina. Piezoelectric transducers and a schlieren optical system were used to verify experimentally the theoretical predictions. By the use of these devices, studies were made of the production of laser-induced acoustic transients in the rabbit eye. An 18-mJ Q-switched ruby-laser pulse produced acoustic transients with amplitudes of 1000 atm in the immediate vicinity of the site of absorption, a 750-μ-diam spot on the retina. Normal-mode laser pulses of the same energy but with durations of 0.5 msec produced a train of pulses with amplitudes 500–1000 times less than the Q-switched pulse. The biological significance of these findings is discussed.

This paper presents the precipitation of copper (Cu) from a surfactant-added glyoxylic acid copper (GACu) complex ink, in femtosecond laser multiple pulse-induced thermochemical processes to achieve high-resolution Cu direct writing. The study specifically examines the impact of the surfactant, n-decanoylsarcosine sodium, (NDSS), on the nonlinear optical absorption properties of GACu complex ink. Findings reveal that the addition of NDSS did not alter the molecule binding and single-photon absorption properties of the ink. However, nonlinear optical absorptions evaluated through the open aperture z-scan method displayed differences between the NDSS-added and non-added GACu complex inks when subjected to femtosecond laser pulse irradiation. These results suggest that Cu nanoparticles were generated from the GACu complex ink via nucleation and surfactant-mediated growth, leading to the saturable absorption of the subsequent irradiated laser pulses. By employing a pulse repetition frequency of 5 MHz and a pulse number of 2 × 104, a minimum dot diameter of 1.6 ± 0.1 μm with stable reproductivity was attained with NDSS-added ink, corresponding to the laser spot diameter, without excessive precipitation due to thermal diffusion. This comprehensive understanding of the surfactant NDSS and pulse number effects on the Cu precipitation process holds potential for the direct writing of various materials. Furthermore, this insight offers a novel approach for affordable and scalable manufacturing with minimized environmental impact.

We demonstrate intense pulse guiding in efficient femtosecond end-pumped waveguides generated in clustered gases. This novel scheme provides a route to significantly lower on-axis plasma density (< 10(18) cm(-3)) more than is feasible in conventional hydrodynamic plasma waveguides. Self-focused propagation and strong absorption of intense femtosecond laser pulses are used to produce long centimeter scale channels in an argon cluster jet, and a subsequent pulse is guided with 3 x 10(17) W cm(-2) intensity and approximately 50% coupling efficiency. Preliminary results with hydrogen clusters also show guiding.

We investigated the optical properties of pure copper irradiated by a femtosecond laser pulse. Self-absorption of 50-fs laser pulses at 800 nm and 400 nm wavelengths (below and above the interband absorption threshold, respectively) is studied for peak laser intensities up to 1015 W/cm2. Theoretical description of laser interaction with copper target is developed, solving numerically the energy balance equations for electron and ion subsystems together with Maxwell equations for laser radiation field inside the target. The theory accounts for both intraband and interband absorption mechanisms. We treated in detail the changes in electron structure and distribution function with an increase in electron temperature, as well as the ensuing changes in thermodynamic properties, collision frequencies, optical and transport coefficients. Experimental work on self-absorption of femtosecond laser pulses in copper targets at 800 nm and 400 nm wavelengths is ongoing. Results for 800 nm wavelength are reported. Theory and experiment are in good agreement.

On the law of the partial polarization of light by reflexion

In this work, the feasibility of optoacoustic tomography to act as diagnostic tool in laser cyclophotocoagulation was investigated. The experiments were performed with enucleated porcine and rabbit-eyes. The absorption of short laser pulses in the wavelength range between 750 and 1000 nm generated thermoelastic stress waves in the ciliary muscle and the underlying pigmented epithelial layers. These stress transients were detected at the surface of the sclera with piezoelectric transducers in order to analyze the structure. Depths of the ciliary body could be determined at different distances from the corneoscleral limbus. Furthermore, changes in the optoacoustic signal due to coagulation with a diode laser using typical therapeutical parameters could be observed.

Low photon energy of mid-infrared radiation supports generation of non-equilibrium free-carrier plasma in transparent optical materials via nonlinear absorption and associated inter-band electron excitation. The collision rate of the plasma may be low so that the free carriers experience from zero to very few collisions per electron per pulse for pulses shorter than 200 fs. Those features of the free-carrier plasma support specific mechanisms of light absorption. We consider solid-state analogs of the Brunel absorption at interfaces, Landau damping, and nonlinear absorption by free-carrier stimulated inverse bremsstrahlung. Parametric scaling of those mechanisms is compared against that of the traditional models.