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Nanostructured composite materials

Final Report Summary - NANOCOM (Nanostructured composite materials)

In order to design new materials with enhanced functionality for applications in photonics, glass-metal nanocomposites (GMN) comprising of metal nanoparticles in glass matrix are of special interest. The reason is three-fold. First, in visual and near-infrared spectral range, GMN show pronounced resonance associated with collective oscillations of conduction electrons in the nanoparticles, so-called surface plasmon resonance (SPR). Second, these composites demonstrate fast and very strong optical nonlinearity that make them prospective for photonic and optoelectronic applications. Finally, GMN are technologically attractive because they are compatible with photonic and electronic circuits, and can be manufactured via self-arrangement in the course of the decomposition of solid solution of metals in glasses. In the framework of the NANOCOM project we employ nanostructuring of glassy composites to develop innovative novel materials for photonics. This includes modeling, fabrication and characterization of sub-micron patterned nanocomposites, fabrication of bi-metallic GMN, and modification of GMN with static electric field and femtosecond laser pulses.
In accordance with the objectives, we report the following main scientific results:
The model describing physical and chemical processes occurring in the formation of glass-metal nanocomposites have been developed. This model enables quantitative description of the reactive diffusion of reducible metal in glasses containing reducing agent (e.g. trivalent cerium ions) and accounts for the diffusion of metal ions and their reducing, depletion of the glass with the metal ions, as well as formation, growth and Oswald repining of metal nanoparticles. Glasses suitable for the formation of the nanoparticles have been designed. We have shown that by choosing the concentration of cerium ions, glass composition and parameters of the ion-exchange process one can produce GMN with prescribed spatial distribution of nanoparticles.
Quantitative model of the reactive diffusion of reducing agent (hydrogen) in glasses with account for all species including sodium has been developed. This model predicts two possible scenarios of nanoparticles formation depending on the relations between diffusion coefficients of the involved species. In the first scenario, the concentration of nanoparticles in the GMN monotonously decreases when we increase the distance from the glass surface. In the second scenario, the concentration of nanoparticles is a non-monotonous function of the depth, i.e. the layers of nanoparticles are created in the glass slab. Both scenarios have been demonstrated in experiments. By using double - silver and copper - ion exchange in glass matrix, we fabricated bi-metallic glass-metal nanocomposites. These GMN possess two surface plasmon resonances, which strength can be independently tuned depending on the fabrication conditions, glass composition and metal concentration. In the bi-metallic GMN, the copper nanoparticles are formed closer to the surface of the glass slab than silver nanoparticles do. This enables designing a quasi-layered structure with submicron period that is of interest for the all-optical controlled filters and reflectors.
The theory of the electric field assisted dissolution (EFAD) of metal nanoparticles in glass matrix has been developed. This phenomenon enables modification of GMN by applied static electric field. The theory has been verified experimentally via characterizing the modified GMN using various techniques including optical absorption spectroscopy, Raman spectroscopy and near field optical microscopy. It was shown that the ratio of the mobilities of hydrogen and alkaline-earth ions can crucially influence both the dynamic of the electric filed induced processes and redistribution of the glass constituents in the modified region. We have demonstrated that the EFAD process can be employed for nanopattering of the GMN. The developed numerical model has allowed us to perform quantitative modelling of the GMN modification/nanopatterning and to evaluate the spatial resolution that can be achieved. These findings open avenues towards application of E-field processing of glassy materials for fabrication of photonic and optoelectronic components.
The model describing anisotropy of the GMN modified under irradiation with femtosecond laser pulses has been developed. Being combined with the experimental result this model allows one to control the anisotropy of the modified GMN with sub-micron lateral resolution. Such nanostructures can be used for fabrication of 2D GMN-based devices that enable spatial control of the polarization state of light beams. The performed experiments have allowed us to establish correlation between parameters of the femtosecond laser pulses and the shape of the modified nanoparticles. This experimental finding opens an opportunity to choose the regime of the femtosecond micromachining of GMN that provides anisotropic planar structure with required parameters.
It has been found in the experiments that at initial stages of GMN formation, metal island film (MIF) forms on the glass surface. At these stages, the MIF dominates the light absorption, i.e. the SPR associated with metal islands is red-shifted with respect to the SPR of bulk GMN. The developed theory of the optical properties of the MIF describes the electric field distribution in the vicinity of nanoislands and the position of SPR as a function of the refractive index of substrate and dispersion parameters of the metal. The theory has allowed us to propose a method that enables monitoring the kinetics of the nanoislands formation on the glass surface by optical means and to explain optical properties of the samples at the initial stage of the thermal processing.
The MIF has been employed for the formation of 1D and 2D gratings comprised of silver nanoislands on the glass surface. This has been achieved via poling the silver ion-exchanged glass with nanostructured glassy carbon electrode. When electric filed is applied, the silver ions move from the glass surface everywhere except nano-sized apertures etched in the electrode. As a result the reduction of metal via thermal processing in hydrogen (i.e. formation of the metal islands) takes place only beneath the nanoapertures. We have demonstrated 1D and 2D MIF gratings with submicron periodicity, which are of interest for sensing and plasmonic devices.
The theory describing nonlinear optical properties of the GMN has been developed using anharmonic oscillator model. In the pump-probe configuration, we have shown theoretically that the nonlinear absorption coefficient has a broader resonance as a function of the pump wavelength than as a function of the probe wavelength. We have also demonstrated that the spectral features in the nonlinear response of glass-metal nanocomposites can be engineered by varying the size of nanoparticles. The pronounced dependence of the magnitude and the sign of the third-order nonlinearity on the particles size explains the diversity of experimental data related to optical nonlinearity of GMNs obtained in different experiments. The developed theory shows that the strong frequency dependence of the Lorentz factor for silver nanoparticles plays decisive role for the magnitude and spectra behavior of the optical nonlinearity.
Three approaches for the fabrication of GMN nanophotonic components have been developed. These are EFAD nanostructuring, electron beam lithography (EBL) followed by reactive ion etching (RIE) nanostructuring, and laser nanostructuring. In EFAD nanostructuring, the metal nanoparticles beneath the sample surface are dissolved when the electric filed applied. We have demonstrated that the in silver based GMN, this technique can be used to fabricate elements as small as 150 nm. In order to implement the EFAD technique, we have developed the advanced glassy carbon electrodes by using experimentally optimized parameters of the ion etching process. In the GMN nanostructuring using EBL and RIE, we have found the optimal processing modes that provide the best spatial resolution. We discovered that in the RIE processing, the presence of the nanoparticles within the glass matrix may result in a considerable roughness of the etched surface. However EBL and RIE allow one to create nanostructures with the element size much smaller than that available with the EFAD. In the laser nanostructuring of the GMN, we have shown that it allows modification of the shape of initially spherical nanoparticles to spheroidal one. Spatial resolution of the laser nanostructuring is limited by the focused laser beam waist, however the laser nanostructuring enables fabrication GMNs that possess optical anisotropy. We have demonstrated experimentally structures with periodic variation of the linear birefringence and dichroism.
Developed nanostructuring techniques were employed to fabricate GMN-based 1D and 2D diffraction gratings by using EFAD and EBL/RIE techniques. Using Fourier Modal Method and effective media approach we have established theory of the GMN-based sub-wavelength diffraction gratings. It was shown that the ion-etched gratings formed in copper-based GMN demonstrate the most attractive feature that is the capability of controlling the position of the spectral region corresponding to the maximal optical nonlinearity of the structure. This makes GMN-based gratings good candidates for future photonic and nonlinear optical devices that rely on the saturable absorption and reverse saturable absorption in a given spectral range. We have demonstrated experimentally 10% saturable absorption change in the pump-probe experiment with copper-based GMN grating.
As a whole, the project contributes essentially into development of glass metal nanocomposites for photonics —new materials providing broaden functionality of photonic components. Joint efforts of scientists working on glassy materials, experimental photonics and theory of the light-matter interaction produced synergetic effect and led to the development advanced photonic devices. The results of the project can be used across the European Research Area. The project results were disseminated through 36 publications in peer-reviewed journals and monographs, 50 presentations at the International conferences on discipline. The the course of the project implementation 3 MSci and 1 PhD dissertations have been defended. The UEF and SPU teams have extended research cooperation via ERA-NET.RUS project “Thermal-electric-field imprinting in nanocomposite materials for novel nanophotonic applications” and via Academy of Finland project “Advanced sensing with self-assembled nanostructured silver island films”.