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).
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.
Quasi-phase matching in periodic nonlinear structures provides an efficient method to phase match a single nonlinear process. However, periodic modulation is not suitable for multiple-wavelength interactions, such as dual wavelength second harmonic generation, frequency tripling and frequency quadrupling. Multiple processes may be phase-matched by using aperiodic, rather than periodic, modulation of the nonlinear coefficient. We have developed a method to phase match any two arbitrary processes by quasi-periodic modulation of the nonlinear coefficient. This method was experimentally verified by performing dual wavelength second harmonic generation, as well as frequency tripling (based on simultaneous phase matching of frequency doubling, followed by sum frequency generation of the first and second harmonic waves) in a single quasi-periodically-poled KTP crystal. Additional degrees of freedom are obtained by using two dimensional periodic or quasi-periodic modulation of the nonlinear coefficient. We discuss frequency tripling by a 2D periodic structure, as well as the application of 2D quasi-periodic structures for nonlinear frequency conversion.
Reversal of the spontaneous polarization direction under an applied electric field is a basic property of ferroelectrics. However the traditional techniques used for fabrication of domain gratings have been able to produce domains not smaller then 2 micrometers. Sub-micron and nanometer scale domains may be fabricated using atomic force microscopy based techniques; however, to date there was no success in fabricating stable domains that elongate without widening throughout thick ferroelectrics. A breakthrough in the field emerged with the recent development of the high voltage atomic force microscope that enabled to obtain sub-micrometer stable domain configurations in bulk ferroelectrics. Diverse stable domain configurations were fabricated in several ferroelectric crystals like LiNbO3 and RbTiOPO4. Studying the influence of the applied high voltage, and the tip velocity on the domain strips has allowed fabricating domain gratings (with a domain width of 590 micron) useful for backward propagating quasi-phase-matched frequency conversion. It is found that string-like domains are formed due to the super-high electric field of the high voltage atomic force microscope tip. The domains, which resemble channels of an electrical breakdown, nucleate under an electric field of around 10 in a power of seven Volts per centimeter at the ferroelectric surface, and grow throughout the crystal bulk where the external electric field is practically zero. A theory explaining the shape of the formed domains shows that the driving force for the domain breakdown is the decrease of the total free energy of the system with increasing domain length.
Periodically poled KTiOPO4 (KTP) wafers with short period length are required for generation of green and blue coherent light. Electric field poling processes developed for producing inverted micrometer scale domain structures in other ferro-electric materials cannot be directly applied to flux-grown KTP due to its relatively high (super ionic) conductivity at room temperature. In this paper we describe the low temperature method developed by us for poling flux- grown KTP crystals without modifying their composition. High voltage switching pulses were applied to KTP samples at a temperature below the superionic insulating transition and the switching charge was continuously monitored. This way, high quality domain gratings of 3.8 - 10 micrometers periods were fabricated in 0.5 - 1.0 mm thick flux-grown KTP plates. Second harmonic generation in the range of 400 - 530 nm light by these samples were tested with different types of IR lasers including diode, diode pumped solid state and fiber lasers. The results demonstrate that the low temperature poling technique can provide high quality, short period periodically poled KTP for blue and green coherent light generation.
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