Scott R. Greenfield

Cooling to 208K by optical refrigeration

We report cooling to record low temperatures by optical refrigeration with ytterbium-doped zirconium–barium–lanthanum–aluminum–sodium-fluoride glass. The glass cooling element was mounted in vacuum in a low-thermal-emissivity chamber and pumped with the light from a diode-pumped solid state Yb:YAG (ytterbium-doped yttrium-aluminum-garnet) laser. Starting from room temperature, the glass cooling element reached a minimum temperature of ∼208K when pumped with ∼10W of 1026-nm light. The heat load at minimum temperature was ∼29mW.

Advances in laser cooling of thulium-doped glass

Recent developments in cooling thulium-doped heavy-metal fluoride glass are presented. Thulium-doped fluorozirconate (ZBLANP) is cooled to 19 K below ambient with a multiple-pass pump scheme. This represents over an order of magnitude increase from the previously reported single-pass geometry. The results agree with a simple model for anti-Stokes fluorescence cooling that includes considerations of quantum efficiency and parasitic heating mechanisms. Issues relating to a practical optical refrigerator are examined, including a general model for the effects of multiple pump passes.

Near-transform-limited visible and near-IR femtosecond pulses from optical parametric amplification using Type II β-barium borate

Type II phase-matched beta-barium borate is used in the f irst stage of amplif ication of a white-light continuum in a two-stage optical parametric amplifier pumped by the second harmonic of a regeneratively amplified Ti:sapphire laser system operating at 824 nm. Near-transform-limited sub-190-fs pulses with microjoule energies are achieved in the signal branch, which is tunable from 475 nm to degeneracy. This system effectively bridges the wavelength gap between the fundamental and the second harmonic of amplified Ti:sapphire laser systems.

Laser-launched flyer plate and confined laser ablation for shock wave loading: Validation and applications

We present validation and some applications of two laser-driven shock wave loading techniques: laser-launched flyer plate and confined laser ablation. We characterize the flyer plate during flight and the dynamically loaded target with temporally and spatially resolved diagnostics. With transient imaging displacement interferometry, we demonstrate that the planarity (bow and tilt) of the loading induced by a spatially shaped laser pulse is within 2-7 mrad (with an average of 4+/-1 mrad), similar to that in conventional techniques including gas gun loading. Plasma heating of target is negligible, in particular, when a plasma shield is adopted. For flyer plate loading, supported shock waves can be achieved. Temporal shaping of the drive pulse in confined laser ablation allows for flexible loading, e.g., quasi-isentropic, Taylor-wave, and off-Hugoniot loading. These techniques can be utilized to investigate such dynamic responses of materials as Hugoniot elastic limit, plasticity, spall, shock roughness, equation of state, phase transition, and metallurgical characteristics of shock-recovered samples.

TRIDENT high-energy-density facility experimental capabilities and diagnostics

The newly upgraded TRIDENT high-energy-density (HED) facility provides high-energy short-pulse laser-matter interactions with powers in excess of 200 TW and energies greater than 120 J. In addition, TRIDENT retains two long-pulse (nanoseconds to microseconds) beams that are available for simultaneous use in either the same experiment or a separate one. The facilitys flexibility is enhanced by the presence of two separate target chambers with a third undergoing commissioning. This capability allows the experimental configuration to be optimized by choosing the chamber with the most advantageous geometry and features. The TRIDENT facility also provides a wide range of standard instruments including optical, x-ray, and particle diagnostics. In addition, one chamber has a 10 in. manipulator allowing OMEGA and National Ignition Facility (NIF) diagnostics to be prototyped and calibrated.

Ultrafast photoinduced charge-shift reactions in electron donor-acceptor 9-arylacridinium ions

Abstract Charge-shift species arising from intramolecular electron transfer have been identified for 9-aryl derivatives of the 10-methyl-acridinium ion through observation of red-shifted fluorescence emission bands and the appearance of radical (ion) transients (CH 3 CN and CH 2 Cl 2 solvents) using laser flash photolysis. Rates of forward and return charge-shift processes have been determined, the former occurring in the 1 ps time domain with small effects associated with changes in solvent and the electron donor substituent. Rates for return electron transfer were a strong function of the donor oxidation potential and showed acceleration with decreased driving force (inverted behavior). The lifetimes for charge-shift intermediates could be ‘tuned’ over three orders of magnitude as the result of changes in acridinium structure and solvent.

Three-wavelength electronic speckle pattern interferometry with the Fourier-transform method for simultaneous measurement of microstructure-scale deformations in three dimensions

We present the simultaneous measurement of three-dimensional deformations by electronic speckle pattern interferometry using five object beams and three colors. Each color, corresponding to an orthogonal direction of displacement, is separated through dichroic filtering before being recorded by a separate CCD camera. Carrier fringes are introduced by tilting the beam path in one arm of each of the three interferometers. The measured deformation modulates these carrier fringes and is extracted using the Fourier-transform method to achieve high displacement sensitivity. The field of view is on the order of a millimeter, making the system suitable for study of microstructural deformations. We compare experimental results with calculated values to validate out-of-plane and in-plane deformation measurements and demonstrate sensitivity on the order of 10 nm.

EXCITATION ENERGY TRANSFER AND CHARGE SEPARATION IN THE ISOLATED PHOTOSYSTEM II REACTION CENTER

The nature of excitation energy transfer and charge separation in isolated Photosystem II reaction centers is an area of considerable interest and controversy. Excitation energy transfer from accessory chlorophyll a to the primary electron donor P680 takes place in tens of picoseconds, although there is some evidence that thermal equilibration of the excitation between P680 and a subset of the accessory chlorophyll a occurs on a 100-fs timescale. The intrinsic rate for charge separation at low temperature is accepted to be ca. (2 ps)−1, and is based on several measurements using different experimental techniques. This rate is in good agreement with estimates based on larger sized particles, and is similar to the rate observed with bacterial reaction centers. However, near room temperature there is considerable disagreement as to the observed rate for charge separation, with several experiments pointing to a ca. (3 ps)−1 rate, and others to a ca. (20 ps)-1 rate. These processes and the experiments used to measure them will be reviewed.

Optical parametric amplification of femtosecond pulses tunable from the blue to the infrared with microjoule energies

A white-light continuum is used to seed a two-stage optical parametric amplifier pumped by the second harmonic of a regeneratively amplified Ti:sapphire laser system operating at 824 nm. Microjoule energies are achieved in the signal branch, which is tunable from 472 to 785 nm. Near-transform-limited sub-200-fs pulses are attainable over the vast majority of the tuning range.

Wavelength and intensity dependent primary photochemistry of isolated Photosystem II reaction centers at 5°C

Abstract The long wavelength absorption band of the isolated Photosystem II reaction center was directly excited at five wavelengths between 655 and 689 nm to study the effects of excitation wavelength and intensity on both excitation energy transfer and charge separation processes. Subpicosecond transient absorption measurements were made monitoring principally the bleach of the pheophytin a Q x band at 544 nm. At all pump wavelengths, the kinetics require three exponentials (1–3, 10–25 and 50–100 ps) to be fit properly. The pump energy was varied by a factor of twenty-five (40–1000 nJ), with no apparent effect on either the rates or the amplitude ratios of the three components, although clear evidence of nonlinear behavior was observed at the higher excitation energies. The dependence of both the rates and amplitude ratios of the three components upon pump wavelength will be discussed in terms of excitation energy transfer occurring on a 30 ps timescale. Selective excitation into the short and long-wavelength sides of the composite Q y band give identical transient spectra at 500 ps, indicating near-unity efficiency of excitation energy transfer. At 1 ps, the spectra are quite different, calling into question the extent of ultrafast (∼ 100 fs) excitation energy transfer. The time after the excitation pulse at which the transient crosses ΔA = 0 was found to be a highly sensitive measure of both the excitation energy and the identity of the pigment pool that had been excited.