Tatiana N. Smirnova

Surface modified ZrO2 and TiO2 nanoparticles embedded in organic photopolymers for highly effective and UV-stable volume holograms

The improved synthesis of large quantities of small (hydrodynamic diameter 6–8 nm) TiO2 and ZrO2 nanoparticles tailored as a high refractive index agent in holographic acrylate photopolymer composites has been developed. The surface of the particles was modified by p-dodecylbenzenesulfonic acid (organic shell weight fraction is about 50–70%) to render them highly compatible with organic monomer mixtures avoiding aggregation. The exposure to the interference pattern provides fast one-step formation of permanent highly efficient volume phase gratings with excellent transparency in the visible range. A high refractive index modulation amplitude n1 of about 0.0165 has been achieved in the composites containing 25 wt% of TiO2 nanoparticles. The level of light scattering in the gratings does not exceed 8–10% compared to the film without nanoparticles. The holographic grating based on acrylates doped with ZrO2 nanoparticles, first employed in this work, exhibited high photostability in contrast to holographic nanocomposites with TiO2 nanoparticles. Possible applications of such photocurable organic–inorganic nanocomposites are holographic diffractive elements for a number of optical and electro-optical applications or devices based on ordered arrays of functional nanoparticles within organic films.

Amplified spontaneous emission in polymer?CdSe/ZnS-nanocrystal DFB structures produced by the holographic method

Amplified spontaneous emission (ASE) is demonstrated in volume-distributed feedback (DFB) structures, formed by colloidal CdSe/ZnS nanocrystals and ZrO2 nanoparticles (NPs) in a polymer matrix. Periodic redistribution of the NPs in an organic matrix was carried out by holographic photopolymerization in a specially developed light-sensitive nanocomposite. The composite consists of two acrylate monomers and two types of inorganic NPs. The NPs provide for the formation of two co-phased gratings-a refractive index grating and an optical gain (losses) grating. The core-shell CdSe/ZnS nanocrystals are used as a gain medium, while ZrO2 NPs create the refractive index grating and enhance the distributed feedback. The period of the volume structure provides the feedback for lasing at the wavelength lambda(las) of about 575 nm in the second diffraction order. In contrast to known laser systems based on volume DFB cavities, in which the different components of the formulation provide optical gain and feedback, in our case the inorganic NPs serve as an emitting material and can provide simultaneously for feedback. By pumping of DFB structures by a titanium-sapphire laser (lambda(pump) = 400 nm, pulse duration of 120 fs) normal to the sample plane, the appearance of a sharp stimulated emission along the grating-vector direction is observed. Output intensity of ASE as a function of the pump energy shows a threshold behavior and full width at half-maximum (FWHM) of the ASE spectral band decreases from 33 to 12 nm.

The fabrication of periodic polymer/silver nanoparticle structures: in?situ reduction of silver nanoparticles from precursor spatially distributed in polymer using holographic exposure

A new approach to producing volume periodic polymer-metal nanoparticle structures is presented. Periodic distribution of Ag nanoparticles in a polymer film can be obtained by applying the holographic patterning in the UV or visible spectral range to the composite material comprising photocurable monomers, photoinitiators and a solution of silver nitrate in acetonitrile. Photopolymerization of the composite in the interference pattern provides formation of a highly efficient volume grating composed of periodic polymer regions and Ag precursor-containing regions. Subsequent homogeneous UV irradiation and/or thermo-treatment of the grating causes reduction of silver ions to Ag nanoparticles in the areas of the film containing the metal precursor. Spectroscopic measurements confirm the formation of the nanoparticles in the gratings. Transmission electron microscopy showed a regular spatial distribution of well-defined Ag nanoparticles in a polymer film with a periodicity governed by the geometry of holographic structuring. The average diameter of nanoparticles can be controlled by the wavelength and intensity of holographic exposure as well as the composite formulation. A possible mechanism of silver nanoparticle formation by free radicals as reducing agents is presented.

Simple and high performance DFB laser based on dye-doped nanocomposite volume gratings

This letter reports on the optimized design and operation performance of a second order distributed feedback (DFB) dye laser based on an active organic waveguide with a volume Bragg grating. The DFB gratings were inscribed holographically in a dye-doped organic nanocomposite containing high refractive index inorganic nanoparticles. In this work we experimentally investigated and theoretically analyzed the influence of waveguide and grating parameters on the spectral and energy characteristics of this kind of DFB laser in order to obtain a narrow-band emission of low divergence. We will show that a tailored improvement of the waveguide and grating parameters provides a low-threshold laser emission in the spectral range of 570–620 nm with a linewidth of less than 0.05 nm and an output beam profile close to a Gaussian distribution.

Holographic patterning of organic-inorganic photopolymerizable nanocomposites

We present here novel easily processible organic-inorganic nanocomposites suitable for holographic fabrication of diffraction optical elements (DOE). The nanocomposites are based on photocurable acrylate monomers and inorganic nanoparticles (NP). The compatibility of inorganic NP with monomers was achieved by capping the NP surface with proper organic shells. Surface modification allows to introduce up to 50wt.% of inorganic NP in organic media. Depending on the NP nature (metal oxides, phosphates, semiconductors, noble metals) and their properties, the materials for both efficient DOE and multifunctional elements can be designed. Organic-inorganic composites prepared have been successfully used for the effective inscription of periodic volume refractive index structures using the holographic photopolymerization method. The nanocomposite preparation procedure, their properties and optical performance of holographic gratings are reported. The use of functional NP makes it possible to obtain effective holographic gratings having additional physical properties such as light-emission or NLO. Some examples of such functional polymer-NP structures and their possible application fields are presented. The combination of easy photo-patterning of soft organic compounds with physical properties of inorganic materials in new nanocomposites and the flexibility of the holographic patterning method allow the fabrication of mono- and multifunctional one- and multi-dimensional passive or active optical and photonic elements.

PPC: self-developing photopolymers for holographic recording

The performance of the developed family of highly efficient self-processing photopolymer compositions (PPC) for holographic recording in real time and use of these materials as the recording media for some applications is investigated. We consider the mechanism of holographic recording, peculiarities of recording process in self- developing photopolymers, dependence of holographic characteristics on the thermodynamic properties of compositions. ON the basis of obtained data we offer the method of PPC modification that results in increasing of the materials light-sensitivity, the efficiency of recording and the threshold of the gratings optical damage. We also cite the instances of use of PPC for production of different holographic optical elements.

Relief structures in the self-developing photopolymer materials

Summary Surface-relief structures are optically induced by visible laser exposure in the self-developing photopolymers. We describe the processes caused a relief formation, fabricating technologies, analyze the influence of recording process parameters (recording intensity, grating period, the value of adhesion between polymer film and substrate) and present the diffusive-polymerization theory to modeling a surface relief formation in photopolymers. With a holographic method the fan-out element that generates 18 and 25 beams of almost equal intensity is experimentally realized.

Analysis of light wave diffraction and amplification by reflection grating operating in the second-order Bragg regime. 1. Approximate theory

A new approximate theory was developed and applied to analysis of the second-order Bragg diffraction by a thick reflection grating formed in a medium with and without optical gain. To derive the general system of equations describing the optical wave interaction with a grating, the method of variation of constants was used, which allowed us to obtain the analytical formulas for the electric-field strength of transmitted and reflected waves. The proposed approach was extended to the case of grating formed in a material with nonlinear response to the recording field when dielectric permittivity modulation of a medium includes higher spatial harmonics.

Synthesized phase objects used instead of real ones for optical-digital recognition systems: experiment

The method of pattern recognition based on replacement of object images incoming to the correlator input by object-dependent synthesized phase objects calculated using the iterative Fourier-transform algorithm was developed by us earlier. In this work, we performed experimental testing the above method by using an opticaldigital 4F-correlator. Synthesized phase objects were introduced into the correlator through the spatial light modulator LC 2002. Holographic matched filters were recorded using self-developing photopolymers PPC-488. For two test objects, we obtained unified (δ-like) correlation signals with the signal-to-noise ratio reaching 24 dB, while the diffraction efficiency of these filters was up to 30%.

Control over the sensitivity of a correlator in the method of synthesized phase objects

As a result of optical experiments, it is established that the controlled variation of a recognition sensitivity at the comparison of the input and reference objects can be realized, while solving the recognition task within the method of synthesized phase objects,1 as distinct from the standard approach. We have obtained and analyzed the results of the recognition for a collection of objects with the use a joint Fourier-transform correlator. For each of the objects, we studied the character of the signal amplitude decrease depending on the growth of distortions in the object structure. We show that it is possible to obtain a family of cross-correlation curves that decrease differently within the method of synthesized phase objects, rather than a single curve. This allows one to vary the sensitivity of the correlator in the process of recognition.