Nanooptics

The physical basis of laser nanostructuring of solid surfaces and bulk are studied and the corresponding methods are developed in the laboratory headed by N. M. Bityurin.

Currently, laser nanopolymerization is the main method of producing solid nanostructures of arbitrary three-dimensional configurations. Structures are formed by a focused laser beam that initiates photopolymerization in the appropriate medium. This technology is in great demand in photonics, microfluidics, in problems of three-dimensional data storage, as well as in biomedical applications.

An important issue is the resolution of this method of three-dimensional nanostructuring. The resolution is limited by fluctuation inhomogeneities of polymer gel that is a network of entangled macromolecules. A theoretical model was developed which allows assessing a minimum size of structures. Possible ways of increasing the resolution were proposed.

Implementations of polymer voxels simulated by the Monte Carlo method (gradient percolation model). The blue “glass” spheres denote mathematical expectations of the simulated voxels. Top – voxels with diameter smaller than the critical one are not reproducible because of fluctuations; bottom – voxels with diameter larger than the critical one are reproducible

It was shown theoretically that the diffusion of a specially introduced inhibitor may improve resolution of the nanostructures produced by laser polymerization.

Theory of inhibited polymerization: (a) without inhibitor diffusion the polymerization region size and maximum conversion are interrelated, both increasing with an increase in the irradiation dose; (b) inhibitor diffusion leads to localization of polymerization region. A decrease in irradiation intensity leads to a smaller size of inhibitor-free region where polymerization occurs, while maximum conversion is determined by irradiation time; (c, d) spatial distribution of inhibitor concentration. Laser intensity is less in the case (c) than in the case (d). The beam intensity profiles are plotted with black lines. It is seen that the region of polymerization localization may be substantially less than the radius of the laser beam

This effect was verified experimentally in collaboration with the Laser Center IESL FORTH (Heraklion, Greece). By introducing the inhibitor into a polymerization medium and choosing appropriate irradiation regime ensuring significant inhibitor diffusion, samples of pho tonic crystals of hybrid organic-inorganic polymer with a grating period of 400 nm were obtained. It is the best result obtained by the traditional multiphoton laser polymerization.

Organic glass with imbedded gold precursors developed and synthesized at IAP RAS: left – nonirradiated sample, right – sample after irradiation by XeCl-laser (308 nm) through a special mask and thermal treatment. Red color of the material indicates the presence of nanoparticles
Laser nanosphere lithography utilizes layers of dielectric microspheres that act as near-field lenses for focusing the laser beam on the surface of the material. This method was used to produce structures with dimensions of 100 nm on the surface of organic and inorganic glasses by means of laser ablation and swel-ling. When producing nanostructures by this method, it is necessary to take into consideration the effects of electrodynamic interaction of the spherical lenses.

Another way to produce nanostructured materials is laser treatment which leads to formation of nanoclusters or nanoinhomogeneities in the initially homogeneous material due to developing instabilities. The inhomogeneities change the optical properties of these materials dramatically, which is of great practical importance. Of particular significance is creation of materials capable of nanostructuring in this fashion. IAP RAS researchers developed a method of UV-induced production of gold nanoparticles directly in a solid polymer matrix doped with gold precursor, without introducing special reducing agents and stabilizers.

A technology of producing bulk samples with record high concentration of gold precursors (~1019 cm-3) by means of polymerization was developed. UV laser irradiation of a sample may be used to form two- and three-dimensional structures in the polymer matrix.

Using an updated method of measuring fast optical nonlinearity by spectrally resolved two-wave mixing with the aid of a femtosecond erbium-doped fiber laser it was shown that the formation of gold nanoparticles leads to a significant increase in the nonlinear refractive index of the medium at wavelengths of 1530–1600 nm (infrared region) in UV irradiated areas.

Laser formation of nanostructures on solid surfaces by means of focusing microobjects. Left – micrograph of a hexagonal structure of spheres 1 µm in diameter. The surface of solid is covered by a monolayer of spheres. Right – result of the action of powerful bichromatic (radiation with a wavelength of 800 nm and its second harmonic) femtosecond laser pulse. Convex submicron structures (laser swelling) on glass surface and sharp 100 nm craters (laser ablation) on polymer surface. Images of the structures were obtained with atomic force microscope