David B. Chang

Restricted Diffusion in Biophysical Systems: Experiment

The pulsed-gradient spin echo nuclear magnetic resonance (PGSENMR) technique was used to measure restricted diffusion of water in three types of animal tissue: human blood plasma and red cells; rat and rabbit heart; rat and rabbit liver. Characteristic lengths (L) for restriction of diffusion are estimated from dependence on the measuring time. Limitations on the range of observable restrictive lengths (1.5-15 mum) are discussed.The decrease in diffusivity due to 1 mum alumina powder (volume fraction = 0.18) in glycerin/water mixtures agrees with the Wang theory assuming spherical particles and no hydration. The characteristic length (L approximately 4 mum) is larger than the particle size (1 mum) or separation (1.8 mum). Comparison of the diffusivities in tissues at short diffusion times with the Wang theory indicates some bound or trapped water.For packed red blood cells, a restriction (L approximately 2.3 mum) was attributed tothe red cell membrane. A permeability p approximately 0.014 cm/s may be estimated from the decrease in diffusivity. Average values of diffusivity ratio in heart were: 0.36 +/- 0.02 for rat; and 0.26 +/- 0.03 for rabbit; and in liver: 0.25 +/- 0.01 for rat; 0.25 +/- .04 for 10-day old rabbit; and 0.195 +/- 0.03 for 2-yr old rabbit. A restriction (L approximately 2.7 mum) in rat liver probably results from the mitochondria.

Upconversion‐pumped infrared erbium laser

A study of upconversion processes in Er3+ ions in CaF2 has led to an unambiguous demonstration of IR lasing produced by upconversion pumping alone. The study has shown: (1) strong upconversion lines originating from the 4I11/2 state; (2) the existence of both two‐photon and three‐photon upconversion processes; (3) an optimum concentration of 5–10 mol % for Er3+; and (4) upconversion efficiencies for the visible bands as high as 10% at 77 K and 1% at 300 K. Measurements of lasing between the 4I11/2 and 4I13/2 levels show that even though the lifetime of the upper (4I11/2) state is only half that of the lower state, self‐quenching does not occur. Upconversion is implicated for this transition both by the nonpersistence of a ‘‘red shift’’ during a lasing pulse, and by the opposite dependence on concentration of laser threshold energy and upconversion‐produced fluorescent intensity. A simple theoretical model shows that a 2.8‐μm laser operating between the 4I11/2 and 4I13/2 levels can be pumped solely by upco...

Threefold upconversion laser at 0.85, 1.23, and 1.73 μm in Er:YLF pumped with a 1.53 μm Er glass laser

Lasing action at 1.73, 1.23, and 0.85 μm was obtained by a threefold upconversion process in Er:YLF at 110 K using a 1.53 μm Er:glass pump laser. The 1.73 μm transition had the lowest threshold and highest pump conversion efficiency.

Continuous wave and Q‐switched infrared erbium laser

Upconversion pumping has been demonstrated both experimentally and theoretically to be important in the 2.8‐μm 4I11/2–4I13/2 lasing transition in a CaF2 (Er3+) system. A minimum pump threshold of only 2.8 J with a 1‐ms Xe lamp pulse has been achieved in CaF2 rods containing 5 mol % of Er3+ ions. A simple theory has been developed to show that upconversion pumping makes possible cw lasing between the 4I11/2 and 4I13/2 states even when the lifetime of the terminal 4I13/2 state is much longer than that of the upper 4I11/2 state. Semi‐cw lasing has been experimentally demonstrated by pumping with six ganged photographic flash bulbs fired in sequence, with each bulb having a duration of 15–20 ms. Lasing at 2.8 μm lasted for 80 ms, well beyond 10 and 20 ms lifetimes of the 4I11/2 and 4I13/2 states. The 2.8‐μm CaF2 (Er3+) laser has also been Q switched, using a 100‐nm thin graphite foil as a shutter.

Upconversion‐pumped 2.8–2.9‐μm lasing of Er3+ ion in garnets

Upconversion‐pumped lasing characteristics near 3 μm on the 4I11/2‐4I13/2 transition of the Er3+ ion in Er:YSGG (erbium‐yttrium‐scandium‐gallium garnet) and Er:YAG (erbium‐yttrium‐aluminum‐garnet), laser state spectroscopy, and population kinetics, are the main subjects discussed. The wide difference in lasing patterns of both garnets can be attributed to the difference in population inversion kinetics which was studied by analyzing the rise and decay of fluorescence starting from the laser states. It is shown that the effective decay times, τ1 and τ2 of the lower and upper laser states, are changed during lasing by the nonlinear energy‐transfer processes such that the resulting ratio becomes τ1/τ2<1. In Er:YSGG this condition is established during the declining phase of the 4I11/2 population, whereas in Er:YAG it occurs only for a short time during the rising phase. Consequently Er:YAG lases in a self‐terminating pulsed mode. The role of various energy‐transfer processes in the upconversion‐pumped popula...

Van der Waals attraction between two conducting chains

Abstract The Van der Waals force between two conducting chains is shown from the zero point energies of the strongly spatially dispersive plasmon modes to vary at small separations L as L −3 instead of L −6 as for nonconducting chains.

Restricted diffusion in biophysical systems: Theory☆

Abstract Theoretical results are presented on measurements of restricted diffusion in biophysical systems by the pulsed gradient spin echo nuclear magnetic resonance (PGSENMR) technique. A Fokker-Planck equation is developed to describe restricted diffusion, and it is shown that only two basic types of penetrable diffusion barriers exist, those in which the diffusing particles are partially excluded from the barrier region because of an increased free energy, and those in which the diffusing particles are not excluded but experience increased viscosity in the region. The Fokker-Planck equation is used to obtain expressions for the spin echo amplitude in PGSENMR experiments, and it is shown that for restricted diffusion the average diffusion coefficient measured in these experiments over short intervals is larger than that measured over long intervals. The possibility of distinguishing between the two types of barriers is considered. The experimental parameters required for intracellular restricted diffusion measurements are discussed, and it is shown that the interpretation of PGSENMR results in animal tissues should include the possibility of penetrable barriers rather than just the impenetrable barriers of previous PGSENMR calculations.

A theoretical discussion of diffusion and convection in the lung

Abstract A differential equation is derived to describe the effects of diffusion and convection on gas concentrations in the lung. The terms in this equation are discussed. Attention is drawn to the effect of the changing cross-sectional area of airways. This leads to an effect called “pseudoconvection”, which causes a peak in gas concentration to move toward the mouth. A Greens-function solution is presented for conditions of zero flow and end exponential growth of cross-sectional area. A similar differential equation is derived for total gas as a function of distance and time. For the static exponential lung model, a Greens function solution is derived. The effects of convective dispersion are discussed and found to be relatively minor over most of the respiratory tree at physiological flows. Finally, the effect of protrusions into a tube, such as alveolar walls in an alveolar duct down which a gas diffuses, is discussed, and an analytical expression is derived to describe their effect on the rate of diffusion.

PLASMA EFFECTS IN ANNULENE MOLECULES

Some plasma effects of the pi electrons in annulene molecules are studied. The dispersion relation for plasma oscillations, obtained by using Huckel molecular orbitals in the second quantization formalism, is found to describe soundlike waves. The resulting polarization response of the pi electrons causes a spatial modulation of the interaction potential between two pi electrons. The polarization response defines the natural correlation units in annulenes to be quartets in all but the angular momentum l = ± 2n states of 4n‐annulenes and zero angular momentum states, where the correlation unit is a pair of electrons. Correlation is discussed by using the modified interaction potential with a Wannier hamiltonian in pair and quartet equations. The effect of band alternation is considered in connection with the variation of induced currents in unsaturated annulenes with temperature.

Calculation of van der Waals forces in systems with nonlocal conductivity

A method is presented for calculating van der Waals forces in systems with nonlocal conductivity. The fluctuating fields in the electromagnetic stress tensor are obtained by applying the fluctuation‐dissipation theorem to the response of the system to an external polarization. As an example, the method is used to calculate the contribution of conduction electrons to the interaction of two blocks of metal. When the electrons are described in a simple hydrodynamic approximation keeping only the lowest order nonlocal terms, the interaction force is the same as that obtained by Lifshitz assuming local conductivity. At separations large compared to the London penetration depth c /ωp the interaction is due to electromagnetic waveguide modes, while at distances small compared to c /ωp, the force is due to surface plasmons. As a second example, the method is applied to the interaction of two graphitelike blocks. With a quantum mechanically derived description of the electron motion, the resulting force differs fr...