Zinc Oxide (ZnO) is a wide bandgap semiconductor that has been the subject of considerable research due to its potential
applications in the areas of photonics, electronics and sensors. Nano-ZnO offers several advantages over existing biosensing
platforms, most notably a large surface area for greater bio-functionalization and an inherent photoluminescence
(PL) signal consisting of two emission peaks. One peak is in the UV, due to near band edge emission and the other is in
the visible (green) region, due to oxygen vacancies caused by crystalline defects. Real-time detection of surface binding
events may be possible if changes to the PL spectrum of a ZnO-based bio-sensor can be induced. Here we describe the
surface modification of nanocrystalline zinc oxide (nano-ZnO) to introduce chemically reactive functionality for
subsequent bio-functionalization. We have demonstrated through TEM-EDS that nano-ZnO powders have been surface
modified with a heterobifunctional organosilane crosslinking agent that contains an amine-reactive aldehyde group.
Furthermore, we have attached a fluorophore to the reactive aldehyde verifying the modified nano-ZnO surface is
available for subsequent biomolecular covalent attachment. The introduction of a chemically reactive modifier to the
surface of the nano-ZnO presents a template for the design of new, optically responsive bio-sensing platforms.
We report on the physical and optical characteristics of the laser-hardened, solid-state host material polymer-filled nanoporous glass (PFNPG). PFNPG consists of a nanoporous glass structure (average pore size =~7 - 10 nm and matrix porosity =~38 - 40%) filled with a damage-resistant polymer. We have previously used this material as a host matrix for solid-state dye lasers, and in this study have applied it to nonlinear filters. The objectives were twofold: (1) to fabricate PFNPG samples with a high laser damage threshold under f/5 focusing conditions; and (2) to successfully dope a nonlinear absorbing dye into this matrix at millimolar concentrations. Undoped PFNPG plates showed damage thresholds of =~42 J/cm2, a value significantly higher than that observed for a bulk polymer in the same test bed. PFNPG samples doped with the nonlinear dye Zn-TPP showed even greater damage resistance. Samples with dye concentrations ≥1 mM showed good nonlinear filtering.
A numerical parametric investigation using a 2-level factorial design of experiments was performed to determine the effect of the f-number, sample position, sample thickness and the number of passes through the nonlinear optical sample on the beam transmittance through an optical system and on the threshold intensity. Calculated temperature profiles and excited state population dynamics are also reported. These results describe trends that are used in making decisions during the design process.
The optical transmission processes for a nonlinear optical (NLO) material are influenced by the properties of its environment. NLO properties such as intersystem crossing rates may be altered by characteristics of the host such as polarity. The effects of the host material on the optical transmission of the NLO material is investigated using numerical laser beam propagation modeling (LBPM) techniques. Numerical simulations are reported for the optical transmission for zinc meso-tetra(p-methoxyphenyl)tetrabenzporphyrin (ZnTMPTBP) in liquids of differing polarity such as toluene, tetrahydrofuran (THF), dichloromethane (DCM), acetone, and pyridine. In addition to investigating the effect of the solvent on transmission, these calculations explore the effect on transmission of two different singlet lifetimes which have been reported in the literature. Transmission curves are calculated using separately determined parameters obtained from curve fitting using zscan data. Calculated results are compared with experimental data for all cases. Z-scans are also calculated for several cases and the results compared to experimental data. Numerical simulations provide a valuable tool to study the optical transmission behavior of NLO materials such as ZnTMPTBP.
A comparison is made of two, laser-induced-damage assessment techniques. The first technique monitors the sample for changes in linear transmission after high-energy laser illumination. With the second technique, an image is transmitted through the sample, after high-energy laser illumination, at the position of incidence. Both single and multiple shot data are considered. Results show the imaging technique to be an efficient method by which to unambiguously discern the onset of image-degrading laser- damage, regardless of detector noise, shot-to-shot variations and sample inhomogeneities. Practically speaking, the imaging technique is relatively easy to incorporate into a laser-based experimental system and is particularly relevant to the assessment of optical systems for imaging.
A strategy for protecting and improving the performance of a nonlinear optical device exposed to a high-energy beam is numerically investigated. In this strategy, a thermally stimulated defocusing material is used in combination with a RSA material. To test this new approach, the ability of a CS2 cell dyed with a liner absorber material to protect a NLO device is determined using calculated values of beam and aperture transmission and the temperature distribution in the NLO device. The results demonstrate that the strategy provides thermal protection and marginally reduces the aperture transmission. These current calculations suggests that other approaches, such as multi-cell devices, may be more effective at providing thermal protection and reducing beam transmission. However, this current approach needs further investigation at other linear transmissions and in addition might be combined with other approaches, such as multiple layers to provide enhanced protection.
The application of a numerical algorithm to the design procedure of a multi-cell limiter (MCL) is discussed and calculated numerical results are compared to experimental results. This application illustrates the ability to predict optical limiting under circumstances that are comparable to the design process of a MCL. Both the optical limiting and the temperature profile through the MCL are determined for three design cases. Also, the numerical analysis is used to investigate thermal damage using the calculated temperature profile. The results illustrate the potential application of a numerical simulation in designing a MCL and that the accuracy of the numerical model is on the same order as the experimental uncertainties.
We carried out systematic experiments to study the effect of solvents on power limiting in zinc meso-tetra (p- methoxyphenyl) tetrabenzoporphyrin (ZnTBP). We used Toluene, pyridine, acetone, dichloromethane (DCM) and a toluene pyridine mixture to evaluate polarity and complex formation of the solvent and solute molecules. Z-scan data were generated using picosecond and nanosecond laser pulses at 532 nm. These data were then curvefit to a 5-level model to obtain excited state and two photon absorption coefficients, and the intersystem crossing rates. An f/5 optical limiting setup was used to determine limiting thresholds. Results indicate that toluene and DCM give the lowest limiting thresholds and that pyridine ligation can be used as a means of molecular modification for future molecular studies.
The application of a numerical algorithm to the design procedure of a multicell stepped limiter (MCSL) is discussed. This application illustrates the ability to predict thermal damage and the thermal lensing effects that occur within the stepped limiter. Both the optical limiting and the temperature profile through the MCSL are determined for three design cases. The results illustrate the potential application of a numerical simulation in designing a MCSL and of predicting thermal damage.
We report optical power limiting in undoped single crystals of the wide band-gap semiconductor Zinc Oxide (ZnO). The ZnO crystals were grown at the Sensors Directorate of the Air Force Research laboratory using the hydrothermal method. The crystals are of high optical quality and are transparent in the visible part of the spectrum. The contributions of the real and imaginary parts of the third-order nonlinear optical susceptibility were studied using the Z-scan technique. The Z-scan experiments were carried out using picosecond pulses at 532 nm wavelength with the electric field polarization oriented orthogonal to the crystal c- axis. The nonlinear refractive index of the crystals is found to be negative. The ZnO samples exhibit two photon absorption and high damage thresholds. Our results indicate that the ZnO crystals may be good candidates for applications in optical power limiting and pulse stabilization.
A series of azulenic compounds are synthesized and their third-order nonlinear optical properties are determined. The compounds are studied in solution using degenerate four wave mixing and z-scan at 532 nm with 30 ps pulses. Extrapolated values for the third-order nonlinear optical susceptibility (chi) (3) of the compounds are in the range 10-9 to 10-8 esu. Z-scan results indicate reverse saturable absorption. Optical power limiting with a f/5 system is demonstrated for all of the compounds studied.
We studied the third-order nonlinear optical properties of five azulenic donor-acceptor compounds. The compounds were studied in solution in Tetrahydrofuran using backward degenerate four-wave mixing and Z-scan with 30 picosecond pulses at 532 manometer wavelength. Extrapolated values for the third-order nonlinear optical susceptibility ((chi) (3)) of the solute molecules were obtained from concentration dependent measurements. The compounds possess large values of (chi) (3) in the range 3 X 10-10 esu to 3 X 10-8 esu. The two orders of magnitude change in the magnitude of the susceptibility that we observe for this series of donor-acceptor azulenic compounds makes them interesting candidates for the study of structure property relationships in nonlinear optical chromophores. The compounds also exhibit large solvatochromism and may also prove to be useful in second-order nonlinear optical applications.
We constructed an optical system for performing 2D logic operations. We demonstrated the logic operations of OR, NOR, XOR, AND. The photoinduced dichroism of Bacteriorhodopsin (bR) is the physical mechanism exploited in the optical system. In its normal state with no light illumination a bR doped polymer film is isotropic with random distribution of bR molecules. When excited by linearly polarized light, only those bR molecules whose transition dipole moments for absorption lie in or near the direction of the electric field ar bleached at the actinic light wavelength due to the B to M transition. If a probe beam is incident in the regions illuminated by the actinic light it will no longer be interacting with an isotropic film. The film is now anisotropic and dichroic. Due to dichroism the actinic light illumination produces an angular rotation of the plane of polarization of the probe beam. In the experiments the bR film is kept between two crossed polarizers to get zero output from the probe beam at a screen when no actinic light is incident on the bR film. The two polarizers and the screen are kept in the path of the probe beam. There are two actinic light beams that induce dichroism and they are made orthogonally polarized with respect to each other by the use of a polarization rotator. The polarization rotator can be arranged so that their polarizations are made parallel for some of the logic operations. The plane of polarization of these two actinic beams are at 45 degrees to the pane of polarization of the probe beam. The combination of rotation of the analyzer and the polarization rotation of one of the actinic beams allows for many logic operations to be performed. No interference recordings are involved in the experiments and hence vibration isolation systems are not required. A coherent source is not a requirement either since a white light source with an appropriate wavelength filter can induce photoanisotropy in the bR film.
We have fabricated Polycarbonate/ Porphyrin blends. The mixtures were prepared with the
use of solvents suitable to both the Porphyrins and the Polycarbonate. Thin films were
prepared by evaporation of the blends on a Teflon substrate. The samples exhibit reverse
saturable absorption. We present results of optical power limiting experiments for the
composites of films sandwiched between Polycarbonate sheets. The composites have high
impact strength and remarkable mechanical robustness.
We studied the nonlinear absorption properties of tetrabenzporphyrins (TBP). Two dimensional delocalized pi- electron systems such as the macrocyclic dyes tetrabenzporphyrins exhibit nonlinear absorption. Their nonlinear absorption and refraction are due mainly to a mechanism that involves the formation of large populations of excited states. This takes place in the spectral regions between B and Q bands. The relevant time frame in which the nonlinear absorption takes place is determined by the dipole dephasing times in the case of the excited singlet states (which is of order picosecond). TBPs have been proposed as candidates for applications in optical power limiting. We have monitored the nature of the nonlinear absorption of TBP samples with picosecond laser pulses at 532 nm wavelength. Open aperture picosecond Z-scan measurements obtained with solutions of TBP in tetrahydrofuran (THF) exhibit reverse saturable absorption (RSA). Our experiments also reveal RSA for a thin film of TBP in a polymethyl methacrylate (PMMA) matrix. On the other hand we observed that for films deposited using the Langmuir-Blodgett (LB) technique the RSA is decreased. A simple extrapolation of solution measurements to thin film samples might result in erroneous conclusions. We investigated the nonlinear absorption of TBP in THF-water mixture to elucidate the role of molecular aggregates in the nonlinear properties of TBP samples. The RSA observed in the case of THF solutions is lost in the mixture.
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