Gero Nootz

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.

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.

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.

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.

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.

Compressive line sensing underwater imaging system

Abstract. Compressive sensing (CS) theory has drawn great interest and led to new imaging techniques in many different fields. Over the last few years, the authors have conducted extensive research on CS-based active electro-optical imaging in a scattering medium, such as the underwater environment. This paper proposes a compressive line sensing underwater imaging system that is more compatible with conventional underwater survey operations. This new imaging system builds on our frame-based CS underwater laser imager concept, which is more advantageous for hover capable platforms. We contrast features of CS underwater imaging with those of traditional underwater electro-optical imaging and highlight some advantages of the CS approach. Simulation and initial underwater validation test results are also presented.

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 Labs 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.

Compressive sensing underwater laser serial imaging system

Abstract. Compressive sensing (CS) theory has drawn great interest in recent years and has led to new image-acquisition techniques in many different fields. This research investigates a CS-based active underwater laser serial imaging system, which employs a spatial light modulator (SLM) at the source. A multiscale polarity-flipping measurement matrix and a model-assisted image reconstruction concept are proposed to address limitations imposed by a scattering medium. These concepts are also applicable to CS-based imaging in atmospheric environments characterized by fog, rain, or clouds. Simulation results comparing the performance of the proposed technique with that of traditional laser line scan (LLS) sensors and other structured illumination-based imager are analyzed. Experimental results from over-the-air and underwater tests are also presented. The potential for extending the proposed frame-based imaging technique to the traditional line-by-line scanning mode is discussed.

In situ laser sensing of mixed layer turbulence

This paper will discuss and compare some recent oceanic test results from the Bahamas Optical Turbulence Exercise (BOTEX) cruise, where vertical profiling was conducted with both time-resolved laser backscatter measurements being acquired via a subsurface light detection and ranging (lidar) profiling instrument, and laser beam forward deflection measurements were acquired from a matrix of continuous wave (cw) laser beams (i.e. structured lighting) being imaged in the forward direction with a high speed camera over a one-way path, with both transmitter and camera firmly fixed on a rigid frame. From the latter, it was observed that when within a natural turbulent layer, the laser beams were being deflected from their still water location at the image plane, which was 8.8 meters distance from the laser dot matrix transmitter. As well as suggesting that the turbulent structures being encountered were predominately larger than the beam diameter, the magnitude of the deflection has been confirmed to correlate with the temperature dissipation rate. The profiling lidar measurements which were conducted in similar conditions, also used a narrow collimated laser beam in order to resolve small-scale spatial structure, but with the added attribute that sub-nanosecond short pulse temporal profile could potentially resolve small-scale vertical structure. In the clear waters of the Tongue of the Ocean in the Bahamas, it was hypothesized that the backscatter anomalies due to the effect of refractive index discontinuities (i.e. mixed layer turbulence) would be observable. The processed lidar data presented herein indicates that higher backscatter levels were observed in the regions of the water column which corresponded to higher turbulent mixing which occurs at the first and second themoclines. At the same test stations that the laser beam matrix and lidar measurements were conducted, turbulence measurements were made with two non-optical instruments, the Vertical Microstructure Profiler (VMP) and a 3D acoustical Doppler velocimeter with fast conductivity and temperature probes. The turbulence kinetic energy dissipation rate and the temperature dissipation rates were calculated from both these setups in order to characterize the physical environments and corroborate with the laser measurements. To further investigate the utility of elastic lidar in detecting small-scale turbulent structures, controlled laboratory experiments were also conducted, with the objective of concurrently acquiring both the laser beam spatial characteristics in the forward direction and the laser backscatter temporal profile from each transmitted sub-nanosecond pulse. An artificial refractive index discontinuity was generated in clear test tank conditions by placing a clean ice-filled carboy above the laser beam propagation path. The results from both field and laboratory experiments confirm our hypothesis that turbulent layers are detectable by lidar sensors, and motivates that more research and lidar instrumentation development is needed to better quantify turbulence, especially for mitigating associated performance degrading effects for the U.S. Navy’s next generation electro-optic (EO) systems, including active laser imaging and laser communications.

Bahamas Optical Turbulence Exercise (BOTEX): preliminary results

The Bahamas Optical Turbulence Exercise (BOTEX) was conducted in the coastal waters of Florida and the Bahamas from June 30 to July 12 2011, onboard the R/V FG Walton Smith. The primary objective of the BOTEX was to obtain field measurements of optical turbulence structures, in order to investigate the impacts of the naturally occurring turbulence on underwater imaging and optical beam propagation. In order to successfully image through optical turbulence structures in the water and examine their impacts on optical transmission, a high speed camera and targets (both active and passive) were mounted on a rigid frame to form the Image Measurement Assembly for Subsurface Turbulence (IMAST). To investigate the impacts on active imaging systems such as the laser line scan (LLS), the Telescoping Rigid Underwater Sensor Structure (TRUSS) was designed and implemented by Harbor Branch Oceanographic Institute. The experiments were designed to determine the resolution limits of LLS systems as a function of turbulence induced beam wander at the target. The impact of natural turbulence structures on lidar backscatter waveforms was also examined, by means of a telescopic receiver and a short pulse transmitter, co-located, on a vertical profiling frame. To include a wide range of water types in terms of optical and physical conditions, data was collected from four different locations. . Impacts from optical turbulence were observed under both strong and weak physical structures. Turbulence measurements were made by two instruments, the Vertical Microstructure Profiler (VMP) and a 3D acoustical Doppler velocimeter with fast conductivity and temperature probes, in close proximity in the field. Subsequently these were mounted on the IMAST during moored deployments. The turbulence kinetic energy dissipation rate and the temperature dissipation rates were calculated from both setups in order to characterize the physical environments and their impacts. Beam deflection by multiple point patterns are examined, using high speed camera recordings (300 to 1200 fps), in association with measured turbulence structures. Initial results confirmed our hypothesis that turbulence impacted optical transmissions. They also showed that more research will be needed to better quantify and mitigate such effects, especially for the U.S. Navys next generation EO systems, including active imaging, lidar and optical communications.