Effective Coherence Length Research Articles

Broadband, static wave-front generation: Na-Ag ion-exchange phase screens and telescope emulation.

We test the statistical properties of static, atmospherelike wave fronts in glass that allow repeatable testing of astronomical adaptive optics instrumentation. The technology is mask-structured ion exchange (MSI) in glass and has significant advantages over other transmissive technologies. The screens are easy to clean, are insensitive to ambient temperature changes, and have high optical-to-near-infrared transmission. However, the effective coherence length (r0) on each of the fabricated screens is systematically too large or, equivalently, the fabricated aberrations are too weak. Despite this strong caveat, the screens appear to be quite useful: Long-exposure point-spread functions have realistic shapes, and power spectrum indices closely match those of the computer-generated wave fronts. Most significant, stacking screens with similar r0 values reduced r0 by the amount predicted by turbulence theory. The refractivity of MSI screens remains unmeasured. Finally, we present our design of an optical system that emulates the key characteristics of the Very Large Telescope, made to contain glass phase screens and to mimic an array of stars for multiconjugate adaptive optics system testing.

Effects of NaCl on the J-aggregation of two thiacarbocyanine dyes in aqueous solutions

The effects of NaCl on the aggregation of two typical thiacarbocyanine dyes (3,3′-di(3-sulfopropyl)-4,5,4′,5′-dibenzo-9-phenyl-thiacarbocyanine triethyl ammonium salt (Dye 1) and 3,3′-di(3-sulfopropyl)-4,5,4′,5′-dibenzo-9-methyl-thiacarbocyanine triethyl ammonium salt (Dye 2)) in aqueous solution have been studied by using absorption spectroscopy, fluorescence spectroscopy, and 1H- and 23Na-NMR measurements. It is found that the J-aggregation of two dyes can be promoted by the addition of NaCl and that the effective coherence length of the J-aggregate is shorter than that obtained without NaCl. Fluorescence spectra demonstrate that the fluorescence intensities of the J-aggregates of two dyes are quenched by addition of NaCl. This is consistent with the decrease of the effective coherence length of J-aggregates of the two dyes in the presence of NaCl. 1H-NMR spectra of two dyes show that the Na + ions penetrate into the J-aggregates and replace the counterion (triethylammonium ions) in two dyes. The measurements of the chemical shifts of 23Na nuclei provide further information about the interaction between the Na + ions and dye anions in the J-aggregates of the two dyes. Due to this interaction, the electrostatic repulsion between the dye anions in the J-aggregates can be reduced and thus accelerate the aggregation of the two dyes in the presence of NaCl. The apparent association constants between Na + ions and dye molecules in J-aggregates of Dye 1 and Dye 2 estimated from the measured chemical shifts of 23Na nuclei are about 2.38 M −1 and 1.35 M −1, respectively.

Real Space Analysis of Excitonic Interactions and Coherence Length in Helical Aggregates

The effective exciton coherence length in molecular aggregates is calculated using a nonlocal, real-space representation of the linear optical response. Simulations of polarized absorption and linear dichroism demonstrate that the spectra depend on local properties of the molecular aggregate segment where the exciton coherence is delocalized. This allows us to limit the calculations to small segments of a large aggregate, avoiding the complexity of computing long-range coherences.

A Comparative Study of MBE-Grown GaN Films Having Predominantly Ga- or N-Polarity

Wurtzitic GaN epilayers having both Ga and N-polarity were grown by reactive molecular beam epitaxy (MBE) using a plasma-activated nitrogen source on c-plane sapphire. The polarities were verified by convergent beam electron diffraction (CBED). High-resolution X-ray diffraction, atomic force microscopy (AFM) imaging and transmission electron microscopy (TEM) were employed to characterize the structural defects present in the films. The different topographic features of Ga and N-polarity samples and their appearance after wet etching were correlated to the measured X-ray rocking curve peak widths for both symmetric [0002] and asymmetric [1014] diffraction. For Ga-polarity samples, the [0002] diffraction is narrower than the [1014] diffraction, while for N-polarity ones the [0002] peaks are broader than [1014]. The half width of [1014] peaks for both polarity types were in the range of 5-7 arcmin indicative of, among possibly other defects, a high density of pure edge threading dislocations lying parallel to the c-axis. The 1-2 arcmin [0002] linewidths of Ga-polarity samples suggest a low density of screw dislocations, which corresponds with the TEM observations where the screw dislocation density is less than 10 7 cm -2 . In N-polarity samples, however, the [0002] diffraction peak was typically wider than 5 arcmin, suggesting either a higher density of edge dislocations and inversion domains in N-polarity samples, or the columnar structural features in AFM images, where the effective coherence length for X-ray diffraction is drastically reduced.

Numerical investigations of optimal synthesis of several low coherence sources for resolution improvement

A clever method is proposed for optimal synthesis of several low coherence sources to improve the resolution of optical coherence tomography. An optimization algorithm is introduced to arrange the parameters of sources for decreasing the effective coherence length and inhibiting the side lobes simultaneously. Computer simulations are presented to show the features of this new method. The influences of source fluctuations are also investigated.

Enhancement of second-harmonic generation in a one-dimensional semiconductor photonic band gap

We demonstrate significant enhancement of second-order nonlinear interactions in a one-dimensional semiconductor Bragg mirror operating as a photonic band gap structure. The enhancement comes from a simultaneous availability of a high density of states, thanks to high field localization, and the improvement of effective coherent length near the photonic band edge.

Lateral proximity effect and long-range energy-gap gradients in Ta/Al and Nb/Al superconducting tunnel junctions

We present the results of current-voltage characterizations of $\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}$ and $\mathrm{T}\mathrm{a}/\mathrm{A}\mathrm{l}$ superconducting tunnel junctions, which reveal a dependence of the measured energy gap on the size of the junction. This implies a geometrical dependence of the energy gap, which suggests that the effective local energy gap has a lateral spatial variation on a scale of several \ensuremath{\mu}m. An extended version of the theory of the proximity effect could explain this phenomenon when lateral coherence lengths are introduced, which are of the order of the bulk coherence length in Al. One of the consequences of this theory is that the coherence length in a thin film cannot be taken to be isotropic. Another is that the effective lateral coherence length, which features in the lateral Usadel equation, actually depends on the quasiparticle energy.

A Kinetic Model for J-Aggregate Dynamics

We developed a model to mimic the exciton dynamics in J-aggregates of cyanine dyes on the basis of Monte Carlo simulations. We consider the aggregates to be linear one-dimensional chains, where the in-phase combination of molecular excitons leads to a stabilization. A random distribution of segment lengths limits the effective coherence length and induces an inhomogeneous band broadening. The model takes into account the incoherent energy hopping between aggregates following the excitation. The results of the simulation could be fitted to the experimentally obtained absorption and fluorescence spectra and to the fluorescence decays of the J-aggregates of 3,3‘-disulfopropyl-5,5‘-dichloro-thiacarbocyanine (THIATS).

Theory of Neutron Diffraction from the Vortex Lattice in UPt3

Neutron scattering experiments have recently been performed in the superconducting state of UPt3 to determine the structure of the vortex lattice. The data show anomalous field dependence of the aspect ratio of the unit cell in the B phase. There is apparently also a change in the effective coherence length on the transition from the B to the C phases. Such observations are not consistent with conventional superconductvity. A theory of these results is constructed based on a picture of two-component superconductivity for UPt3. In this way, these unusual observations can be understood. There is a possible discrepancy between theory and experiment in the detailed field dependence of the aspect ratio.

Crossover to 2D strongly coupled state and temperature dependence of the coupling between layers in SNS multilayers

Two types of dimensional crossover are discussed for SNS multilayers: (1) a 3D-2D (three-to-two) crossover occurs when the transverse coherence length of superconducting layers becomes of the order the layer thickness: ξ s ⊥ ( T) ∼ d/2. (2) A2D-2DSC (two-to-2D strongly coupled) crossover occurs when the effective coherence length of the normal layers becomes of the order of the layer thickness: ξ N (T) ∼ d/2. The 2D-2DSC crossover is inherent only to SNS multilayers and is caused by the proximity effect between S and N layers. Due to the proximity effect the superconducting properties of the N layers increase with decreasing temperature and hence the coupling between the layers increases. Thus at low temperatures the crossover to a 2D strongly coupled state could occur. The proximity effect in dirty SNS multilayers was studied theoretically with microscopic Usadell equations. Boundary conditions for arbitrary thicknesses of layers, transparencies of S/N boundaries and conductivities of layers are derived in an Appendix.

Twin-crystal walk-off-compensated type-II second-harmonic generation: single-pass and cavity-enhanced experiments in KTiOPO_4

A twin-crystal device consisting of a pair of two identically cut KTP crystals of length l = 6 mm mounted with their optical axes symmetrically crossed is used to circumvent conversion-efficiency limitations that are due to the aperture effect in type-II (oeo) critically phase-matched second-harmonic generation (SHG) at λ = 1.3 and λ = 2.532 μm. A single-pass increase of as great as 3.2–3.5 times the conversion efficiency of a single crystal is obtained with this angle-tuned device at these wavelengths. This enhancement results from an increased effective coherence length of the interaction compared with that of a single bulk crystal of length 2l. We show that cavity-enhanced critically phase-matched type-II SHG can be easily performed with the twin device, allowing for walk-off as well as phase compensation for the fundamental-resonating ordinary and extraordinary waves at λ = 1.30 μm. Potential implementations of this device in other single-cavity-enhanced parametric interactions, such as doubly resonant optical parametric oscillation and degenerate sum-or difference-frequency mixing, are discussed.

Nonlocal theory of the size and temperature dependence of the radiative decay rate of excitons in semiconductor quantum dots.

A nonlocal theory of the radiative decay rate of excitons in semiconductor quantum dots is developed and the observed size and temperature dependence for CuCl microcrystallites is explained successfully. The retardation effect due to the finite ratio of the quantum dot radius to the wavelength of light is found to be irrelevant for explaining the experiments. Instead, the homogeneous broadening effect is shown to be essential in saturating the mesoscopic enhancement of the excitonic radiative decay rate or, in other words, in determining the effective exciton coherence length.

Long-range proximity effect in aluminum thin films with regions of nearly identical transition temperatures.

We have investigated the resistive transition of dirty aluminum thin films whose transition temperature is spatially modulated by \ensuremath{\sim}2--4 % along the current path. At current densities as high as 600 A/${\mathrm{cm}}^{2}$, the system exhibits a single homogeneous transition for modulation lengths up to 50 \ensuremath{\mu}m, an order of magnitude longer than predicted by current theories. In the limit that the modulation length is 50 \ensuremath{\mu}m, the ${\mathit{T}}_{\mathit{c}}$ depends sensitively and monotonically on the ratio of the lengths of the etched and the unetched regions (${\mathit{d}}_{1}$/${\mathit{d}}_{2}$), but only weakly on their absolute magnitudes. This behavior is reproduced for ${\mathit{T}}_{\mathit{c}}$ modulation parallel or perpendicular to the current flow. For samples with a controlled aperiodic (Fibonacci) modulation, the ${\mathit{T}}_{\mathit{c}}$ is compatible with that of periodically modulated films. The dependence of the ${\mathit{T}}_{\mathit{c}}$ on ${\mathit{d}}_{1}$/${\mathit{d}}_{2}$ can be fitted to a simple Ginzburg-Landau theory which assumes that the effective pair coherence length is long in comparison with the modulation period. Despite the good fit, data for the full range of experimentally accessible modulation length scales are not consistent with a more detailed microscopic theory. We believe this long-range proximity effect is not the consequence of pair tunneling, but is a manifestation of the long distance over which quasiparticle phase coherence is maintained in a two-dimensional metal film.

All high T c edge-geometry weak links utilizing Y-Ba-Cu-O barrier layers

High quality YBa2Cu3O7−x/normal-metal/YBa2Cu3O7−x edge-geometry weak links have been fabricated using nonsuperconducting Y-Ba-Cu-O barrier layers deposited by laser ablation at reduced growth temperatures. Devices incorporating 25–100 Å thick barrier layers exhibit current-voltage characteristics consistent with the resistively shunted junction model, with strong microwave and magnetic field response at temperatures up to 85 K. The critical currents vary exponentially with barrier thickness, and the resistances scale linearly with Y-Ba-Cu-O interlayer thickness and device area, indicating good barrier uniformity, with an effective normal metal coherence length of 20 Å.

Spatial coherence properties of planar antennas

Lateral spatial coherence of an antenna is modeled by random phase front tilts. An effective source coherence length is introduced, and a generalized Fresnel coefficient which takes into account the coherence of the source is abstracted. Generalized reciprocity relationships for beamwidth and source coherence as well as for observed coherence and source power distribution valid in the near and far field are developed. >

Time division multiplexing of coherence tuned optical fibre sensors based upon a multimode laser diode

We describe a scheme in which a multimode laser diode, with effective coherence length less than 250 μm, is used to illuminate sensors multiplexed in time and with their status read by coherence sensing. Data is presented which shows that the spectral characteristics of the laser light remain essentially unchanged upon direct modulation of the laser injection current or whilst subject to laser feedback. This has allowed the multiplexing scheme network to be implemented without recourse to either a Bragg cell modular or an optical isolator. The technique was applied to measure displacement, strain and temperature.

Excess magnetization due to interfilamentary proximity coupling in NbTi multifilamentary wires

It is shown theoretically that the excess magnetization of a multifilamentary superconducting wire caused by interfilamentary proximity coupling, ΔM p, is proportional to the twist pitch of the wire, I. It is also shown that this theoretical prediction can be confirmed experimentally for Nb - Ti multifilamentary wires. The critical current density of the superconducting shielding current flowing through the matrix, which becomes a weak superconductor due to interfilamentary proximity coupling, is given by J cp = J 0( B B c2p ) −1.4 × [1 − ( B B c2p )] 1.4 with J 0 ∝ [ cosh( d N 2ξ N )] −2 , where B is the magnetic flux density, and B c2p, ξ N and d N are the upper critical flux density, the effective coherence length and the width of the weakly superconducting matrix, respectively. It is concluded that ΔM p can be estimated quantitatively by measuring the I dependence of total magnetization, and also that the most effective method of decreasing ΔM p is to decrease ξ N to the dirty limit.

Josephson Junctions with Links of Super-Normal Bilayer

Josephson junctions with links of super-normal bilayer are proposed as reproducible Josephson weak links. By an analysis using Ginzburg-Landau equations, the effective coherence length of the super-normal bilayer is estimated to become longer than that of the superconductor. Junctions are fabricated using Nb/Cu bilayers, and exhibit good electrical properties. Critical current dependence on Cu thickness agrees qualitatively with the result of analysis.

Dispersion and noise of 1.3 mu m multimode lasers in microwave digital systems

The intensity noise and the performance in a 1.7 Gb/s digital system of 1.3 mu m InGaAsP multilongitudinal mode lasers is discussed. The total intensity noise, mode partitioning, and the impact of dispersion on optical noise are measured. It is found that under CW conditions the total simulated emission from unpackaged lasers is inherently quiet, with an integrated optical signal-to-noise ratio (SNR) of 26.8+or-1.5 dB over a bandwidth of 1.5 GHz and 5 mW/facet. The optical SNR decreased as a function of increasing reflection. Intense mode partitioning decreased the SNR of the main mode by approximately 20 dB and reduced the effective coherence length to approximately 2 cm in glass fiber. At 1.7 Gb/s, the power penalities associated with laser bias and fiber dispersion are reported. The best receiver sensitivity is obtained when the laser is biased 1.3 mA below threshold. In general, it is found that as the bit rate increases, the optimum transmitter design becomes progressively more restrictive.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Preparation of long-coherence-length second-harmonic-generating optical fibers by using mode-locked pulses

We have prepared an optical fiber that produces efficient (0.24%) second-harmonic conversion by using modelocked 1.064-microm laser pulses along with a harmonic seeding beam. The effective coherence length increased to 35 cm as compared with only a few centimeters in fibers prepared with mode-locked Q-switched pulses. We show that the input bandwidth of the pump pulse imposes a fundamental limit on the effective coherence length and that this length can be severely reduced by competing nonlinear effects.