Chemically synthesized biomolecules can self-assemble to bioinspired nanostructures of different morphologies such as dots, tubes, spheres, nanofibers and more. They adopt similar basic ordering as their biological counterparts either α-helical or β-sheet peptide/protein conformations. These two fundamental biomolecular architectures exhibit dissimilar physical properties. One of the most interesting physical properties found in biological and bioinspired structures is a new biophotonic phenomenon of visible fluorescence (FL). It has been observed both in neurodegenerative disease-related amyloid fibrils and in synthetic amyloidogenic biorganic di- and tri-aromatic and aliphatic peptide nanostructures. The FL effect has been also found recently in peptide nanodots and hybrid polymer/peptide thin films. All of them have been assembled to β-sheet secondary structure. In this work we report on a new development of FL optical waveguiding in elongated bioinspired fibrillary structures, self-assembled from ultrashort amylodogenic peptides/proteins and hybrid polymer/peptides biomolecules. We show that FL propagation in these two fiber materials of different origin can be described by two completely different mechanisms. One of them is conventional FL propagation in the region of optical transparency of peptide materials in accordance with optical confinement rules. Another model is FL reabsorption mechanism where anomalous long range FL propagation has been found. We show that this intrinsic FL biophotonic waveguiding effects found in different β- sheet biomaterials is considered as a promising tool for precise biomedicine where new biocompatible visible tunable FL optical waveguides can be applied in advanced nanomedical technologies (local bioimaging, light diagnostics, therapy, optogenetitcs and health monitoring).
Purely organic materials with negative and near-zero dielectric permittivity can be easily fabricated, and propa- gation of surface polaritons at the material/air interface was demonstrated. Here we develop a mean- eld theory of nonlinear light-induced "plasmonic" properties of organic materials. The theory describes both a red shift of the resonance frequency of isolated molecules, according to the Clausius-Mossotti Lorentz-Lorentz mecha- nism, and the wide variations of their spectra related to the aggregation of molecules into J- or H-aggregates. The bistable response of organic materials in the condensed phase has been demonstrated using the electron- vibrational model. We predict the generation of the switching waves, or kinks in the bistable organic thin lms that enable us to observe a bistable behaviour of the surface polaritons at the organic thin lm/dielectric inter- face under the laser irradiation. We present the alternating-sign dependence of the switching wave velocity on pump intensity and discuss a possibility of controlling the polariton propagation by switching waves.
Supramolecular self-assembled bio-inspired peptide nanostructures are favorable to be implemented in diverse nanophotonics applications due to their superior physical properties such as wideband optical transparency, high second-order nonlinear response, waveguiding properties and more. Here, we focus on the optical properties found in di-phenylalanine peptide nano-architectures, with special emphasize on their linear and nonlinear optical waveguiding effects. Using both simulation and experiments, we show their ability to passively guide light at both fundamental and second-harmonic frequencies. In addition, we show that at elevated temperatures, 140-180°C, these native supramolecular structures undergo irreversible thermally induced transformation via re-assembling into completely new thermodynamically stable phase having nanofiber morphology similar to those of amyloid fibrils. In this new phase, the peptide nanofibers lose their second-order nonlinear response, while exhibit profound modification of optoelectronic properties followed by the appearance of visible (blue and green) photoluminescence (PL). Our study propose a new generation of multifunctional optical waveguides with variety of characteristics, which self-assembled into 1D-elongated nanostructures and could be used as building blocks of many integrated photonic devices.
We present a pseudoparticle nonequilibrium Green function formalism as a tool to study the coupling between plasmons and excitons in nonequilibrium molecular junctions. The formalism treats plasmon-exciton couplings and intra-molecular interactions exactly, and is shown to be especially convenient for exploration of plasmonic absorption spectrum of plexitonic systems, where combined electron and energy transfers play an important role. We demonstrate the sensitivity of the molecule-plasmon Fano resonance to junction bias and intra-molecular interactions (Coulomb repulsion and intra-molecular exciton coupling). The electromagnetic theory is used in order to derive self-consistent ¯eld-induced coupling terms between the molecular and the plasmon excitations. Our study opens a way to deal with strongly interacting plasmon-exciton systems in nonequilibrium molecular devices.
There exists a growing need for fast spatial optical phase modulators in various applications including laser
communication for both terrestrial and ground-to-space communications, ultrafast laser pulse shaping as well as in
medical imaging. The two principal phase spatial light modulator technologies currently available namely, liquid crystal
and digital micro-mirror are limited to frame rates of a few kHz. A need therefore exists for faster MHz-range spatial
phase modulating devices. Existing solid state electro-optical modulators such as based on LiNbO3 crystal, although
capable of GHz rate modulation rates, cannot be used for 2-D spatial light modulation. This is due to their relatively
small electro-optical coefficient which requires the use of a relatively thick layer and its associated large, (100's of Volt)
modulating signal, thereby barring their practical use as spatial light or phase modulators. Surface plasmon polariton
resonances which can be excited at the metal-dielectric interfaces have been shown to significantly affect both the
amplitude and the phase of the traversing optical beam. In this work we present a preliminary study of metallic nanoparticles
embedded in a solid state electro-optical modulator (EOM), as potential spatial phase modulating device. Here,
the spatial refractive index modulation of the EOM, allows, the modulation of either amplitude of phase modulation,
with the added advantage of potentially ultra-fast frame rates. The results of computer simulations, based on finite
difference time domain (FDTD) method, with various nano-particle geometries are reported, describing the achievable
phase modulation along with the associated absorption losses.
KEYWORDS: Eye, Image processing, Near infrared, LCDs, Imaging systems, Electrodes, Head-mounted displays, Liquid crystal on silicon, Retina, Control systems
We propose an Eye Tracker/Display system, based on a novel, dual function device termed ETD, which allows sharing
the optical paths of the Eye tracker and the display and on-chip processing. The proposed ETD design is based on a
CMOS chip combining a Liquid-Crystal-on-Silicon (LCoS) micro-display technology with near infrared (NIR) Active
Pixel Sensor imager. The ET operation allows capturing the Near IR (NIR) light, back-reflected from the eye's retina.
The retinal image is then used for the detection of the current direction of eye's gaze.
The design of the eye tracking imager is based on the "deep p-well" pixel technology, providing low crosstalk while
shielding the active pixel circuitry, which serves the imaging and the display drivers, from the photo charges generated
in the substrate. The use of the ETD in the HMD Design enables a very compact design suitable for Smart Goggle
applications. A preliminary optical, electronic and digital design of the goggle and its associated ETD chip and digital
control, are presented.
The Localized Surface Plasmon Resonance (LSPR) phenomenon exhibited in nano-particles, embedded in a dielectric
medium has recently been shown to enhance the absorption as well as the photo-generation effect in several lightsensitive
structures including solar cells and photo-diodes. The origin of this enhancement has not yet been sufficiently
clarified as there appear to be several mechanisms at play, depending on the particular device structure and
configuration. We have conducted computer simulation studies on the absorption enhancement in a silicon substrate by
nano-shell-related LSPR, based on a Finite Difference, Time-Domain (FDTD) Analysis.
Preliminary results of this study show significant enhancement of up to 10X in the near band gap spectral region of Si,
using 40-100nm diameter nano-shells. The enhancement was studied as a function of the metallic Shell thickness, the
thickness of an externally coating layer of SiO2, as well as of various nanoshell shapes. The results suggest that the main
enhancement mechanism in this case of tubular nanoshells embedded in Si substrate, is that of field-enhanced absorption
caused by the strongly LSPR-enhanced electric field extending into the Silicon substrate.
Spatial resolution is an important performance characteristic of spatial light modulators (SLM). This parameter depends
on the physical properties of the electro-optical material, as well as on the design features of the SLM. One of the key
factors affecting the spatial resolution of liquid crystal (LC)-based SLM is the fringing field effect. This effect can be
reduced in thin LC cells with corresponding reduction in the electro-optical response. A strong electro-optic response in
thin LC layer can be attained using the Surface Plasmon Resonance (SPR) phenomenon. While SPR-based LC SLMs
were already demonstrated about 15 years ago, their development has been hampered by the fact that these devices are
expected to have a relatively low resolution, due to the finite propagation length (several tens of micrometers) of the
surface plasmons (SP).
This study is aimed at improving the spatial resolution of the SPR-SLM by optimizing the metal-dielectric structure of
the device. In particular, a small-scale patterning of the metal layer supporting the propagation of SPs is considered a
possible solution for reducing the spatial blurring associated with long propagation length of SPs.
Detailed computer simulations of the spatial resolution of the SPR-based LC SLM structure have been carried out using
both the rigorous coupled wave analysis (RCWA) and the finite difference time domain (FDTD) method. These
simulations were performed for an SLM structure based on the well-known prism-type, Kretschmann excitation
configuration. The SLM performance for various spatial resolutions was simulated by introducing a dielectric layer with
periodically modulated refractive index. The RCWA technique was used for an initial estimate of the SP excitation angle
and the optimal thickness of the silver layer supporting the SP propagation. The FDTD method was used for detailed
analysis of near and far field spatial distribution of the modulated light. The results of this study showing improved
resolution LC-SP-SLM are presented here.r
One of the key factors affecting the performance of liquid crystal devices is the fringing field effect. This effect is the principal cause for the current resolution limitations of LCDs as well as the reduction in both the maximum deflection angle and the diffraction efficiency of beam steering devices. Recent studies in the reduction of the fringing field effects will be presented with applications in the development of ultra-small pixel sizes in LCD's and high performance LC-beam steering devices. A particular implementation using Gires-Tournois structure will be discussed. Another area of research to be discussed is a study of the fundamental limits of LCs and other electro-optic materials, with respect to their electro-optic coefficient. Fundamental physical limitations based on material stability considerations will be presented.
Low Vision (LV) due to Age Related Macular Degeneration (AMD), Glaucoma or Retinitis Pigmentosa (RP) is a growing problem, which will affect more than 15 million people in the U.S alone in 2010. Low Vision Aid Goggles (LVG) have been under development at Ben-Gurion University and the Holon Institute of Technology. The device is based on a unique Image Transceiver Device (ITD), combining both functions of imaging and Display in a single chip. Using the ITD-based goggles, specifically designed for the visually impaired, our aim is to develop a head-mounted device that will allow the capture of the ambient scenery, perform the necessary image enhancement and processing, and re-direct it to the healthy part of the patient's retina. This design methodology will allow the Goggles to be mobile, multi-task and environmental-adaptive. In this paper we present the optical design considerations of the Goggles, including a preliminary performance analysis. Common vision deficiencies of LV patients are usually divided into two main categories: peripheral vision loss (PVL) and central vision loss (CVL), each requiring different Goggles design. A set of design principles had been defined for each category. Four main optical designs are presented and compared according to the design principles. Each of the designs is presented in two main optical configurations: See-through system and Video imaging system. The use of a full-color ITD-Based Goggles is also discussed.
A 3-electrode LC beam deflector, consisting of a glass substrate with two transparent lateral electrodes and a third transparent grounded electrode, placed at the opposite glass substrate was designed and fabricated. A differential voltage, corresponding to the required phase step change, was applied to the lateral electrodes of this beam deflector. The gap between the glass substrates was filled with a nematic liquid crystal mixture. It was shown, that the large refractive index gradient generated by the field between the two lateral electrodes, resulted in the deflection of the incident optical beam with deflection angles as large as 25o, at a high deflection efficiency of approximately 98%. A theoretical analysis based on both ray tracing and far-field diffraction calculations in such a 3-electrode LC cell configuration, confirmed the above experimental results. The advantages of this type of LC deflector are the large deflection angle attainable as well as the continuous deflection operation. The main drawbacks are the difficulties in focusing the beam into the narrow region of the refractive index gradient at the electrode gap, as well as the significant divergence of the deflected optical beam.
Liquid crystal (LC) devices including displays, beam-steering devices, electrically- and optically-controlled spatial light modulators, are widely used in a variety of applications. Some important operational properties of these devices, such as spatial resolution and diffraction efficiency, are severely limited by the influence of fringing electrical fields, generated between adjacent pixel electrodes. This work combines the results of three recent studies encompassing computer simulation, the development of an approximate analytical model and its experimental verification. The approximate analytical model ties the fringing-field-dependent broadening kernel, to the physical LC Cell properties. In particular, it is shown that, the broadening of the phase profile due to the fringing field is proportional to the LC cell thickness. These results are found to be in an excellent agreement both with high-precision computer simulations and experimental results. Finally, the phase broadening kernel is found to be independent of the particular shape of the phase profile, allowing the model to be used for other LC device architectures such as LCDs.
A controllable, refractive, electro-optical liquid crystal (LC) beam deflector is studied both theoretically and experimentally. The principle of operation of this device is based on a beam deflection from a large gradient of refractive index, generated in an LC layer by a transversal electric field, formed between two lateral electrodes, in a simple three-electrode LC cell. A continuous, controllable deflection of a Gaussian laser beam, focused into the narrow region at the electrode gap is demonstrated. Maximum deflection angles of ~18 to 25 deg, depending on the LC birefringence, as well as a high deflection efficiency of up to 98% are demonstrated experimentally, backed by a theoretical analysis.
A simple method of applying mechanical pressure to vary the cell thickness of a liquid crystal cell in a controlled, repeatable manner is presented. Experimental evaluation of several cells subjected to a controlled, mechanical point pressure show the capability of attaining a liquid crystal cell thickness range of 3 to 12 µm, at an accuracy of 5%, within a 2- to 6-mm radius around the pressure point, for the 22×25-mm cells studied. Temporal stability tests show no measurable drift in the cell thickness for more than 120 h. The method offers the advantages of simplicity (avoiding the need of multiple liquid crystal cells), accuracy, repeatability, and temporal stability, as well as speedy change of the cell thickness.
Two novel concepts of liquid crystal (LC) diffractive structures are introduced and analyzed. Both structures are aimed at overcoming the fringing field effect in thin LC cells while allowing sufficiently large phase dynamic range to be attained. The first structure is a combination of a sub-wavelength metal grating configuration, combined with a built-in reflective, blazed grating structure and a uniform thickness LC cell. The reflective blazed grating provides a periodic, linear phase modulation, while the metal-strip sub-wavelength grating acts as a polarization-sensitive transparent multi-electrode element. The thin liquid crystal layer provides the spatially-varying dynamic phase profile. It is shown that this structure allows a triple-beam deflection operation. A diffraction analysis based on the LC director simulation shows a diffraction efficiency of over 66% in all three diffraction angles. A detailed high-spatial resolution analysis of the fringing field effect on the LC alignment for this structure is described. The second configuration is based on a built-in blazed diffractive grating, composed of two optical substrates with different refractive indices and a uniform-thickness LC layer, enclosed in a Fabry-Perot cavity. It is shown that this structure which overcomes the difficulties of LC alignment and fringing field effects in hybrid, blazed LC-glass structures, allows a dynamic switching of a laser beam with a diffraction efficiency exceeding 75%.
A theoretical analysis of liquid crystal (LC) beam steering structures based on Sub-Wavelength Diffractive Optical Elements was performed. Rigorous Diffraction Analysis shows a significant diffraction efficiency gain of up to 40% in using Sub-Wavelength LC structures, compared to regular DOE structures. However, due to Fringe-Field Effects, the formation of a high Diffraction Efficiency Index Grating Structure inside a liquid crystal layer is possible, only if the grating pixel's aspect ratio (height to width) is much smaller than unity (approx. 0.1). This in turn, requires the use of a small aspect ratio LC cell. However, simulations of LC director behavior show, that such ultra-thin LC Cells will fall short of providing the 2(pi) phase modulation necessary for an effective beam steering. One possible solution for this issue is the generation of a Blazed Phase Grating inside a Non-Symmetrical Reflective Fabry-Perot Resonator. Such configuration essentially allows an increased phase modulation magnitude at the expense of a very high sensitivity of such structure to liquid crystal thickness variations--as was verified by computer simulations. A second possible solution is the formation of a Cascaded Diffractive Gratings Stack based on ultra-thin LC layers. These gratings are identically reproduced in each of due to the Talbot Effect. We have studied a Cascaded system, consisting of four Ultra-thin liquid crystal layers separated by glass plates. The results show, that the thickness of each layer in the cascade decreases proportionally to number of layers, followed by an increased absorption. An interesting feature of the cascaded structure is the discrete set of possible beam steering directions, which are determined by constraints of the Talbot Effect.
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