Effects of Optical Turbulence and Density Gradients on Particle Image Velocimetry.

Particle image velocimetry (PIV) is a well-established tool to collect high-resolution velocity and turbulence data in the laboratory, in both air and water. Laboratory experiments are often performed under conditions of constant temperature or salinity or in flows with only small gradients of these properties. At larger temperature or salinity variations, the changes in the index of refraction of water or air due to turbulent microstructure can lead to so-called optical turbulence. We observed a marked influence of optical turbulence on particle imaging in PIV. The effect of index of refraction variations on PIV has been described in air for high Mach number flows, but in such cases the distortion is directional. No such effect has previously been reported for conditions of isotropic optical turbulence in water. We investigated the effect of optical turbulence on PIV imaging in a large Rayleigh-Bénard tank for various path lengths and turbulence strengths. The results show that optical turbulence can significantly affect PIV measurements. Depending on the strength of the optical turbulence and path length, the impact can be mitigated in post-processing, which may reduce noise and recover the mean velocity signal, but leads to the loss of the high-frequency turbulence signal.

Experimental and numerical study of underwater beam propagation in a Rayleigh-Bénard turbulence tank.

The propagation of a laser beam through Rayleigh-Bénard (RB) turbulence is investigated experimentally and by way of numerical simulation. For the experimental part, a focused laser beam transversed a 5 m×0.5 m×0.5 m water filled tank lengthwise. The tank is heated from the bottom and cooled from the top to produce convective RB turbulence. The effect of the turbulence on the beam is recorded on the exit of the beam from the tank. From the centroid motion of the beam, the index of refraction structure constant Cn2 is determined. For the numerical efforts RB turbulence is simulated for a tank of the same geometry. The simulated temperature fields are converted to the index of refraction distributions, and Cn2 is extracted from the index of refraction structure functions, as well as from the simulated beam wander. To model the effect on beam propagation, the simulated index of refraction fields are converted to discrete index of refraction phase screens. These phase screens are then used in a split-step beam propagation method to investigate the effect of the turbulence on a laser beam. The beam wander as well as the index of refraction structure parameter Cn2 determined from the experiment and simulation are compared and found to be in good agreement.

Experimental study of a compressive line sensing imaging system in a turbulent environment.

Turbulence poses challenges in many atmospheric and underwater surveillance applications. The compressive line sensing (CLS) active imaging scheme has been demonstrated in simulations and test tank experiments to be effective in scattering media such as turbid coastal water, fog, and mist. The CLS sensing model adopts the distributed compressive sensing theoretical framework that exploits both intrasignal sparsity and the highly correlated nature of adjacent areas in a natural scene. During sensing operation, the laser illuminates the spatial light modulator digital micromirror device to generate a series of one-dimensional binary sensing patterns from a codebook to encode the current target line segment. A single element detector photomultiplier tube acquires target reflections as the encoder output. The target can then be recovered using the encoder output and a predicted on-target codebook that reflects the environmental interference of original codebook entries. In this work, we investigated the effectiveness of the CLS imaging system in a turbulent environment. The development of a compact CLS prototype will be discussed, as will a series of experiments using various turbulence intensities at the Naval Research Lab's Simulated Turbulence and Turbidity Environment. The experimental results showed that the time-averaged measurements improved both the signal-to-noise radio and the resolution of the reconstructed image in the extreme turbulence environment. The contributing factors for this intriguing and promising result will be discussed.

Quantification of optical turbulence in the ocean and its effects on beam propagation.

The influence of optically active turbulence on the propagation of laser beams is investigated in clear ocean water over a path length of 8.75 m. The measurement apparatus is described and the effects of optical turbulence on the laser beam are presented. The index of refraction structure constant is extracted from the beam deflection and the results are compared to independently made measures of the turbulence strength (Cn2) by a vertical microstructure profiler. Here we present values of Cn2 taken from aboard the R/V Walton Smith during the Bahamas optical turbulence exercise (BOTEX) in the Tongue of the Ocean between June 30 and July 12, 2011, spanning a range from 10-14 to 10-10 m-2/3. To the best of our knowledge, this is the first time such measurements are reported for the ocean.

Determination of flow orientation of an optically active turbulent field by means of a single beam.

The cross-flow orientation of an optically active turbulent field was determined by Fourier transforming the wander of a laser beam propagating in the ocean. A simple physical model for the measured effect is offered, and numerical simulations are performed. The simulations are in good agreement with measurements, validating the assumptions made in the model.

Optimization of band structure and quantum-size-effect tuning for two-photon absorption enhancement in quantum dots.

The two-photon absorption, 2PA, cross sections of PbS quantum dots, QDs, are theoretically and experimentally investigated and are shown to be enhanced with increasing quantum confinement. This is in contrast to our previous results for CdSe and CdTe QDs where the reduced density of states dominated and resulted in a decrease in 2PA with a decrease in QD size. Qualitatively this trend can be understood by the highly symmetric distribution of conduction and valence band states in PbS that results in an accumulation of allowed 2PA transitions in certain spectral regions. We also measure the frequency nondegenerate 2PA cross sections that are up to five times larger than for the degenerate case. We use a k·p four-band envelope function formalism to model the increasing trend of the two-photon cross sections due to quantum confinement and also due to resonance enhancement in the nondegenerate case.

Role of symmetry breaking on the optical transitions in lead-salt quantum dots.

The influence of quantum confinement on the one- and two-photon absorption spectra (1PA and 2PA) of PbS and PbSe semiconductor quantum dots (QDs) is investigated. The results show 2PA peaks at energies where only 1PA transitions are predicted and 1PA peaks where only 2PA transitions are predicted by the often used isotropic k x p four-band envelope function formalism. The first experimentally identified two-photon absorption peak coincides with the energy of the first one photon allowed transition. This first two-photon peak cannot be explained by band anisotropy, verifying that the inversion symmetry of the wave functions is broken and relaxation of the parity selection rules has to be taken into account to explain optical transitions in lead-salt QDs. Thus, while the band anisotropy of the bulk semiconductor plays a role in the absorption spectra, especially for the more anisotropic PbSe QDs, a complete model of the absorption spectra, for both 1PA and 2PA, must also include symmetry breaking of the quantum confined wave functions. These studies clarify the controversy of the origin of spectral features in lead-salt QDs.

Linear and nonlinear spectroscopy of a porphyrin-squaraine-porphyrin conjugated system.

The linear and nonlinear absorption properties of a squaraine-bridged porphyrin dimer (POR-SQU-POR) are investigated using femto-, pico-, and nanosecond pulses to understand intramolecular processes, obtain molecular optical parameters, and perform modeling of the excited-state dynamics. The optical behavior of POR-SQU-POR is compared with its separate porphyrin and squaraine constituent moieties. Linear spectroscopic studies include absorption, fluorescence, excitation and emission anisotropy, and quantum yield measurements. Nonlinear spectroscopic studies are performed across a wide range (approximately 150 fs, approximately 25 ps, and approximately 5 ns) of pulsewidths and include two-photon absorption (2PA), single and double pump-probe, and Z-scan measurements with detailed analysis of excited-state absorption induced by both one- and two-photon absorption processes. The 2PA from the constituent moieties shows relatively small 2PA cross sections; below 10 GM (1 GM = 1 x 10(-50) cm4 s/photon) for the porphyrin constituent and below 100 GM for the squaraine constituent except near their one-photon resonances. In stark contrast, the composite POR-SQU-POR molecule shows 2PA cross sections greater than 10(3) GM over most of the spectral range from 850 to 1600 nm (the minimum value being 780 GM at 1600 nm). The maximum value is approximately 11,000 GM near the Nd:YAG laser wavelength of 1064 nm. This broad spectral range of large 2PA cross sections is unprecedented in any other molecular system and can be explained by intramolecular charge transfer. A theoretical quantum-chemical analysis in combination with different experimental techniques allows insight into the energy-level structure and origin of the nonlinear absorption behavior of POR-SQU-POR.

Synthesis and two-photon spectrum of a bis(porphyrin)-substituted squaraine.

A chromophore in which zinc porphyrin donors are linked through their meso positions by ethynyl bridges to a bis(indolinylidenemethyl) squaraine core has been synthesized using Sonogashira coupling. The chromophore exhibits a two-photon absorption spectrum characterized by a peak cross section of 11,000 GM and, more unusually, also exhibits a large cross section of >780 GM over a photon-wavelength window 750 nm in width.