Raz Gvishi

Two-photon absorption based optical limiting and stabilization in organic molecule-doped solid materials☆

Abstract Optical limiting and stabilization via two-photon absorption (TPA) in organic molecule-doped solid materials have been investigated. The nonlinear materials are epoxy rod and a composite glass rod doped with the same organic dopant (2,5-benzothiazole 3,4-didecyloxy thiophene). An ultrashort laser source with 0.5 ps pulsewidth and 602 nm wavelength was employed. The transmissivity of these two materials has been measured as a function of the input beam intensity. The measured results can be well fitted based on the assumption that TPA is the predominant mechanism producing the observed optical limiting behavior. Also, optical stabilization behavior is observed, and, at ∼930 MW/cm 2 input intensity level, the output intensity fluctuation is three times less than the input intensity fluctuation.

Novel, organically doped, sol-gel-derived materials for photonics : Multiphasic nanostructured composite monoliths and optical fibers

Sol-gel-processed organic-inorganic hybrid materials combine the merits of inorganic glass and organic molecules, and are therefore a class of materials with good potential for photonics. In this review, two approaches which have shown promising results for producing useful materials for photonics are described: (i) a novel way to fabricate organically doped, multiphasic nanostructured composite monoliths and (ii) a method of fabrication of organically doped, sol-gel-derived optical fibers. For each approach, the preparation process is presented, together with selected applications such as multidye solid-state tunable laser, multiphasic optical power limiter, a micron-scale chemical-sensing and biosensing fibers and solid-state dye-doped fiber lasers.

New laser medium: dye-doped sol-gel fiber

Abstract Rhodamine-6G, a well-known laser dye, was doped in a sol-gel-derived fiber. For the first time, lasing was demonstrated from a dye-doped sol-gel-derived fiber device. Lasing was demonstrated under excitation with an 8 ns pulsed, frequency-doubled Nd:YAG laser at 532 nm operating at a 1 2 Hz repetition rate. The dye-doped fiber was pumped longitudinally and placed in a cavity consisting of a ∼ 100% reflecting flat mirror and a ∼ 70% reflecting flat output coupler. The lasing emission peak for a 2 × 10 −5 M Rhodamine-6G doped fiber was observed at ∼568 nm with a full width at half maximum of ∼ 7 nm. A slope efficiency of ∼ 2.8% was observed for a 1 × 10 −4 M Rhodamine-6G doped fiber. A much higher slope efficiency is expected by improving the coupling into the fiber, decreasing the intrinsic fiber optical losses, optimizing the concentration-length relationship (preventing aggregation effect) and improving the cavity design.

Sol-gel-derived micron scale optical fibers for chemical sensing

We report for the first time on the preparation of organically-doped room temperature processed sol-gel-derived micron scale optical fibers as platforms for chemical- and bio-sensors. Micron scale optical fibers are drawn from fluorescent dye-doped tetraethoxysilane (TEOS)-derived sol-gel solution processed under ambient conditions. Such a simple methodology to entrap organic and even bioactive species within the optical fiber offers many advantages over more conventional ways of immobilizing organic probes for the development of optical sensors. Specifically, we report on the photophysical properties of fluorescein (a pH sensitive fluorescent dye) and rhodamine 6G (R6G; laser dye) entrapped within sol-gel-derived optical fibers. We present the preliminary results on the viability of such doped optical fibers for chemical sensing. Our results demonstrate that a fluorescein-doped sol-gel-derived optical fiber responds to ammonia and acid vapors with a response time of 1–2 seconds.

Resonance energy transfer in a novel two-component system: two-photon fluorophore and a photo-chromic acceptor molecule

To date, a full-scale solar sail has never flown in space. Furthermore, solar sail technology development represents a field that only recently has enjoyed significant support. The goal of this work is to contribute to the development of a low-mass ODS for solar sails that would include research and development in the areas of photogrammetry and thermography. The focus of this work was on the development of the thermography system. A measurement protocol was designed for obtaining accurate temperature measurements using thermal imaging when heat was applied to the membrane surface. Two main limitations were considered during the experimental process. The first is that conventional infrared detector arrays must be kept cool. To minimize the effect that an imagers operating temperature would have on the ODS, a miniature, un-cooled microbolometer was used to acquire temperature measurements from the membrane surface. A second limitation is that a detector array cannot distinguish between emitted and reflected photons, thus presenting a significant problem if one cannot predict the reflected component or if the reflected component is significantly greater than the emitted. To address this limitation, spectral properties of the membrane, including reflectance and transmission, were analyzed using a Hemispherical Directional Reflectometer (HDR) to predict the effects that optical properties would have on sail membrane temperatures. A thermal modeling strategy was also developed. The results of this investigation are presented.

The influence of structure and environment on spectroscopic and lasing properties of dye-doped glasses

Abstract A new class of hemicyanine dyes was studied as laser dyes. From the influence of structure and solvent effects on the spectroscopic and lasing properties, a basic understanding of the involved processes was obtained. The studied dyes were found to have two distinguishable mesomeric forms, one predominant in the ground-state, and the other in the excited state, leading to a large Stokes shift. The dyes exhibited low fluorescence quantum yields, which were attributed to the presence of a counter iodide ion, which increases singlet-to-triplet intersystem crossing, and to a twisted intramolecular charge-transfer (TICT) of the amino moiety. However, significant lasing efficiencies were observed under pulsed pump conditions, possibly because the stimulated emission competes with the nonradiative processes. The laser losses are mainly due to the cavity. Solvent effect studies showed that the chromophore is very sensitive to hydrogen bonding donor (HBD) solvents. The dye-doped sol-gel composite glass exhibits a behavior close to that of water, suggesting that the dye is attached to the silica skeleton of the composite glass through a hydrogen bonding. Energy transfer between two dyes copoding a multiphasic composite glass was found to be significant; therefore, this composite exhibited simultaneous lasing from both dyes. Codoped tunability was achieved through the range of both dyes, from 560 to 610, nm with an average efficiency of 7%. The lasing properties of this lasing medium was studied and compared to reference dye solutions.

New two-photon fluorescent probe for multiphoton microscopy in biological media

An important ingredient in improving Multi Photon Laser Scanning Microscopy, MPLSM, is the development of efficient two-photon fluorescent (TPF) probes. We previously reported on a new class of TPF probes, specifically designed in order to maximize their efficiency in potential MPLSM applications. The fluorophores are based on a tetraketo derivative (TK) with a symmetric structure Donor-Acceptor-Donor (D-A-D). Those fluorophores have the following properties: a) Very large two-photon absorption coefficients (δ ~ 1000GM); b) Two-photon excitation (TPE) peak wavelength strongly shifted to the red (λ ~ 1µm); c) High fluorescence quantum efficiency; d) Large Stokes shifts of the fluorescence bands. We extended our work to a new fluorophore from this class that is more suitable for biological settings. This new fluorophore has a structure of crown-TK-crown that incorporates the ability to trap metal ions such as calcium. The TPE wavelength dependence of the TK-crown derivative is very similar to its analogous linear derivative with enhancement in the value of the cross-section, due to the stronger donor moieties. The TPE cross-section for the TK-crown derivative was about δ = 950 GM at λmax = 980 nm.

New long-wave and highly efficient two-photon fluorophores for multiphoton microscopy

An important ingredient in improving Multi Photon Laser Scanning Microscopy, MPLSM, is the development of efficient exogenous two-photon fluorescent (TPF) probes. Here we report on a new class of two-photon fluorophores, specifically designed in order to maximize their efficiency in potential MPLSM applications. The fluorophores possess a symmetric Donor-Acceptor-Donor (D-n-A-n-D) structure with varying conjugation length and have strong donors and acceptors. We have studied the two-photon excitation (TPE) properties of these fluorophores and found the following properties: (1) Very large two-photon absorption coefficients (6 > 1000 GM); (2) Peak TP excitation wavelength which are strongly shifted to the red ((lambda) 1 micrometer); (3) Large fluorescence quantum efficiency; (4) Large Stokes shifts of the fluorescence bands. These properties make them particularly suitable for imaging thicker samples, relying on the large improvement in TPE cross-sections and the reduced attenuation at both the excitation and emission wavelengths. We also describe TPE fluorescence anisotropy experiments revealing the tensorial shape of the fluorophores.

Novel C60 multiphasic nanocomposites for photonics

Novel multiphasic nanostructured inorganic : polymer composites of C60 have been prepared by sol-gel processing. These composites offer the prospect of introducing multifunctionality by optimizing each phase for a particular response. Since the phase separation is at a nanometer size scale, the composites are optically transparent. Results are presented form the studies of spectroscopic characterization, optical power limiter behavior, and third-order nonlinear optical response.

Studies of new two-photon fluorescent probes suitable for multiphoton microscopy in biological settings

Multi-Photon Laser Scanning Microscopy (MPLSM) requires efficient two-photon absorbing fluorescent (TPF) probes. In particular, probes exhibiting bio-functionality are very attractive for MPLSM studies of biological samples. We have synthesized and studied a new class of TPF probes capable of caging metal ions, such as Ca+2 and Na+, which play an important role in neuronal mechanisms. The TPF probes are based on a tetraketo derivative with a symmetric Donor-Acceptor-Donor (D-A-D) structure. The donor is an azacrown moiety, which also serves as a metal ion-caging unit. We studied the linear and the non-linear spectroscopic properties of these TPF probes as a function of conjugation length and the size of the crown ring. We find that this new class of TPF probes possesses very large two-photon excitation cross-section coefficients (~1000GM) at near IR wavelengths as well as affinity to metal ions. In the presence of changing sodium ion concentration the dye spectra reveals four distinguishable forms and the TPF efficiency changes strongly. We therefore conclude that the dye can perform as a sensitive metal ion TPF probe.