L. B. Da Silva

Birefringence characterization of biological tissue by use of optical coherence tomography.

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.

Short wavelength x-ray laser research at the Lawrence Livermore National Laboratory*

Laboratory x‐ray lasers are currently being studied by researchers worldwide. This paper reviews some of the recent work carried out at Lawrence Livermore National Laboratory. Laser action has been demonstrated at wavelengths as short as 35.6 A while saturation of the small signal gain has been observed with longer wavelength schemes. Some of the most successful schemes to date have been collisionally pumped x‐ray lasers that use the thermal electron distribution within a laser‐produced plasma to excite electrons from closed shells in neon‐ and nickel‐like ions to metastable levels in the next shell. Attempts to quantify and improve the longitudinal and transverse coherence of collisionally pumped x‐ray lasers are motivated by the desire to produce sources for specific applications. Toward this goal there is a large effort underway to enhance the power output of the Ni‐like Ta x‐ray laser at 44.83 A as a source for x‐ray imaging of live cells. Improving the efficiency of x‐ray lasers in order to produce s...

High-Energy X-ray Microscopy Techniques for Laser-Fusion Plasma Research at the National Ignition Facility.

Multi-kilo-electron-volt x-ray microscopy will be an important laser-produced plasma diagnostic at future megajoule facilities such as the National Ignition Facility (NIF). However, laser energies and plasma characteristics imply that x-ray microscopy will be more challenging at NIF than at existing facilities. We use analytical estimates and numerical ray tracing to investigate several instrumentation options in detail, and we conclude that near-normal-incidence single spherical or toroidal crystals may offer the best general solution for high-energy x-ray microscopy at NIF and similar large facilities. Apertured Kirkpatrick-Baez microscopes using multilayer mirrors may also be good options, particularly for applications requiring one-dimensional imaging over narrow fields of view.

Accurate measurement of laser-driven shock trajectories with velocity interferometry

We describe a velocity interferometer used to measure the velocity and trajectory of laser driven shocks in liquid deuterium accurately and continuously. This demonstration of velocity interferometry to measure shock velocity and shock trajectory in condensed matter shows strong potential for future studies of laser-driven shocks in transparent media. Accuracy of this technique can be better than 1% in velocity and ±0.2 μm in position during a 10 ns interval.

Power measurements of a saturated yttrium x-ray laser

We report on measurements of the output power from the J = 2–1 (λ = 15.5 nm) neonlike yttrium soft-x-ray laser. The results show peak powers of 32 MW in an output pulse width of 200 ps (FWHM) and an integrated energy of 7 mJ. The output is consistent with the expected saturation intensity of this laser and makes this system one of the brightest XUV sources available.

Electronic conduction in shock-compressed water

The optical reflectance of a strong shock front in water increases continuously with pressure above 100 GPa and saturates at ∼45% reflectance above 250 GPa. This is the first evidence of electronic conduction in high pressure water. In addition, the water Hugoniot equation of state up to 790 GPa (7.9 Mbar) is determined from shock velocity measurements made by detecting the Doppler shift of reflected light. From a fit to the reflectance data we find that an electronic mobility gap ∼2.5 eV controls thermal activation of electronic carriers at pressures in the range of 100–150 GPa. This suggests that electronic conduction contributes significantly to the total conductivity along the Neptune isentrope above 150 GPa.

Physical characterization of ultrashort laser pulse drilling of biological tissue

Abstract Ultrashort laser pulse ablation removes material with low-energy fluence required and minimal collateral damage. The ultimate usefulness of this technology for biomedical application depends, in part, on characterization of the physical conditions attained, and determination of the zone of shockwave and heat-affected material in particular tissues. Detailed numerical modeling of the relevant physics (deposition, plasma formation, shockwave generation and propagation, thermal conduction) are providing this information. A wide range of time scales is involved, ranging from picosecond for energy deposition and peak pressure and temperature, to nanosecond for development of shockwave, to microsecond for macroscopic thermophysical response.

Absolute measurements of the equations of state of low-Z materials in the multi-Mbar regime using laser-driven shocks

Although high intensity lasers offer the opportunity to explore the equations of state (EOSs) of materials under high energy density conditions, experimental difficulties have limited the application of laser-driven shocks to EOS measurements. However, we have recently performed absolute EOS measurements on the principal Hugoniot of liquid deuterium near one Mbar and of polystyrene from 10 to 40 Mbar. The D2 measurements were made with direct drive; the polystyrene experiments were indirectly driven. The data were sufficiently accurate to differentiate between existing EOS models and were surprising, particularly for D2. The results demonstrate that laser driven shocks can be used effectively to investigate high pressure EOSs.

Optimization of x‐ray sources for proximity lithography produced by a high average power Nd:glass laser

We measured the conversion efficiency of laser pulse energy into keV x rays from a variety of solid planar targets and a Xe gas puff target irradiated using a high average power Nd:glass slab laser capable of delivering 13 ns full width at half‐maximum pulses at up to 20 J at 1.053 μm and 12 J at 0.53 μm. Targets were chosen to optimize emission in the 10–15 A wavelength band, including L‐shell emission from materials with atomic numbers in the range Z=24–30 and M‐shell emission from Xe (Z=54). With 1.053 μm a maximum conversion of 11% into 2π sr was measured from solid Xe targets. At 0.527 μm efficiencies of 12%–18%/(2π sr) were measured for all of the solid targets in the same wavelength band. The x‐ray conversion efficiency from the Xe gas puff target was considerably lower, at about 3%/(2π sr) when irradiated with 1.053 μm.

Fringe formation and coherence of a soft-x-ray laser beam illuminating a Mach--Zehnder interferometer

We investigated the fringe visibility produced by a Mach-Zehnder interferometer illuminated by a collisionally pumped yttrium x-ray laser operating at 15.5 nm. Fringe visibility varied as a function both of relative path delay and of relative spatial overlap of the beams. This visibility information was extracted quantitatively from several interferograms and analyzed to produce a characterization of the temporal coherence, yielding a gain-narrowed linewidth of 1.3 pm for the 15.5-nm laser transition and spatial coherence consistent with an effective source size of approximately 220 microm +/- 50% at the x-ray laser output.