L. B. Da Silva

Bio: L. B. Da Silva is an academic researcher from Lawrence Livermore National Laboratory. The author has contributed to research in topics: Laser & X-ray laser. The author has an hindex of 35, co-authored 104 publications receiving 4418 citations. Previous affiliations of L. B. Da Silva include University of British Columbia & Lawrence Berkeley National Laboratory.

Birefringence characterization of biological tissue by use of optical coherence tomography

Abstract: An improved polarization-sensitive optical coherence tomography (OCT) system is developed and used to measure birefringence in porcine myocardium tissue and produce two-dimensional birefringence mapping of the tissue. Signal-to-noise issues that cause systematic measurement errors are analyzed to determine the regime in which such measurements are accurate. The advantage of polarization-sensitive OCT systems over standard OCT systems in avoiding image artifacts caused by birefringence is also demonstrated.

Measurements of the Equation of State of Deuterium at the Fluid Insulator-Metal Transition

Abstract: A high-intensity laser was used to shock-compress liquid deuterium to pressures from 22 to 340 gigapascals. In this regime deuterium is predicted to transform from an insulating molecular fluid to an atomic metallic fluid. Shock densities and pressures, determined by radiography, revealed an increase in compressibility near 100 gigapascals indicative of such a transition. Velocity interferometry measurements, obtained by reflecting a laser probe directly off the shock front in flight, demonstrated that deuterium shocked above 55 gigapascals has an electrical conductivity characteristic of a liquid metal and independently confirmed the radiography.

Absolute equation of state measurements on shocked liquid deuterium up to 200 gpa (2 mbar)

Abstract: We present results of the first measurements of density, shock speed, and particle speed in liquid deuterium compressed by laser-generated shock waves to pressures from 25 to 210Gpa (0.25 to 2.1Mbar). The data show a significant increase in D{sub 2} compressibility above 50Gpa compared to a widely used equation of state model. The data strongly suggest a thermal molecular dissociation transition of the diatomic fluid into a monatomic phase. {copyright} {ital 1997} {ital The American Physical Society}

Mapping of birefringence and thermal damage in tissue by use of polarization-sensitive optical coherence tomography

Abstract: We demonstrate cross-sectional birefringence- and polarization-independent backscatter imaging of laser-induced thermal damage in porcine myocardium in vitro, using a polarization-sensitive optical coherence tomography system. We compare the generated images with histological sections of the tissue and demonstrate that birefringence is a more sensitive indicator of thermal damage than is backscattered light. Loss of birefringence in thermally damaged regions is quantified and shown to have significant contrast with undamaged sections of the tissue. A detailed theoretical analysis of the birefringence measurements is provided, including a calculation of the systematic errors associated with background noise, system imperfections, and tissue dichroism.

Plasma mediated ablation of biological tissues with nanosecond-to-femtosecond laser pulses: relative role of linear and nonlinear absorption

Abstract: Plasma mediated ablation of collagen gels and porcine cornea was studied at various laser pulse durations in the range of 1 ns-300 fs at 1053-nm wavelength. It was found that pulsed laser ablation of transparent and weakly absorbing gels is always mediated by plasma. On the other hand, ablation of strongly absorbing tissues is mediated by plasma in the ultrashort-pulse range only. Ablation threshold along with plasma optical breakdown threshold decreases with increasing tissue absorbance for subnanosecond pulses. In contrast, the ablation threshold was found to be practically independent of tissue linear absorption for femtosecond laser pulses. The mechanism of optical breakdown at the tissue surface was theoretically investigated. In the nanosecond range of laser pulse duration, optical breakdown proceeds via avalanche ionization initiated by heating of electrons contributed by strongly absorbing impurities at the tissue surface. In the ultrashortpulse range, optical breakdown is initiated by multiphoton ionization of the irradiated medium (six photons in case of tissue irradiated at 1053-nm wavelength), and is less sensitive to linear absorption. High-quality ablation craters with no thermal or mechanical damage to surrounding material were obtained with subpicosecond laser pulses. Experimental results suggest that subpicosecond plasma mediated ablation can be employed as a tool for precise laser microsurgery of various tissues.

Intense few-cycle laser fields: Frontiers of nonlinear optics

Abstract: 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.

Optical coherence tomography - principles and applications

Abstract: There have been three basic approaches to optical tomography since the early 1980s: diffraction tomography, diffuse optical tomography and optical coherence tomography (OCT). Optical techniques are of particular importance in the medical field, because these techniques promise to be safe and cheap and, in addition, offer a therapeutic potential. Advances in OCT technology have made it possible to apply OCT in a wide variety of applications but medical applications are still dominating. Specific advantages of OCT are its high depth and transversal resolution, the fact, that its depth resolution is decoupled from transverse resolution, high probing depth in scattering media, contact-free and non-invasive operation, and the possibility to create various function dependent image contrasting methods. This report presents the principles of OCT and the state of important OCT applications. OCT synthesises cross-sectional images from a series of laterally adjacent depth-scans. At present OCT is used in three different fields of optical imaging, in macroscopic imaging of structures which can be seen by the naked eye or using weak magnifications, in microscopic imaging using magnifications up to the classical limit of microscopic resolution and in endoscopic imaging, using low and medium magnification. First, OCT techniques, like the reflectometry technique and the dual beam technique were based on time-domain low coherence interferometry depth-scans. Later, Fourier-domain techniques have been developed and led to new imaging schemes. Recently developed parallel OCT schemes eliminate the need for lateral scanning and, therefore, dramatically increase the imaging rate. These schemes use CCD cameras and CMOS detector arrays as photodetectors. Video-rate three-dimensional OCT pictures have been obtained. Modifying interference microscopy techniques has led to high-resolution optical coherence microscopy that achieved sub-micrometre resolution. This report is concluded with a short presentation of important OCT applications. Ophthalmology is, due to the transparent ocular structures, still the main field of OCT application. The first commercial instrument too has been introduced for ophthalmic diagnostics (Carl Zeiss Meditec AG). Advances in using near-infrared light, however, opened the path for OCT imaging in strongly scattering tissues. Today, optical in vivo biopsy is one of the most challenging fields of OCT application. High resolution, high penetration depth, and its potential for functional imaging attribute to OCT an optical biopsy quality, which can be used to assess tissue and cell function and morphology in situ. OCT can already clarify the relevant architectural tissue morphology. For many diseases, however, including cancer in its early stages, higher resolution is necessary. New broad-bandwidth light sources, like photonic crystal fibres and superfluorescent fibre sources, and new contrasting techniques, give access to new sample properties and unmatched sensitivity and resolution.

Mechanisms of pulsed laser ablation of biological tissues.

Abstract: 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.

A compact X-ray free-electron laser emitting in the sub-ångström region

Abstract: Researchers report sub-angstrom fundamental-wavelength lasing at the SPring-8 Angstrom Compact Free-Electron Laser in Japan. The output has a maximum power of more than 10 GW, a pulse duration of 10−14 s and a lasing wavelength of 0.634 A.

Optical coherence tomography (OCT): a review

Abstract: This paper reviews the state of the art of optical coherence tomography (OCT), an interferometric imaging technique that provides cross-sectional views of the subsurface microstructure of biological tissue. Following a discussion of the basic theory of OCT, an overview of the issues involved in the design of the main components of OCT systems is presented. The review concludes by introducing new imaging modes being developed to extract additional diagnostic information.