Andriy Shevchenko

Electromagnetic multipole theory for optical nanomaterials

Optical properties of natural or designed materials are determined by the electromagnetic multipole moments that light can excite in the constituent particles. In this paper, we present an approach to calculating the multipole excitations in arbitrary arrays of nanoscatterers in a dielectric host medium. We introduce a simple and illustrative multipole decomposition of the electric currents excited in the scatterers and connect this decomposition to the classical multipole expansion of the scattered field. In particular, we find that completely different multipoles can produce identical scattered fields. The presented multipole theory can be used as a basis for the design and characterization of optical nanomaterials.

Optical Interference Lithography Using Azobenzene‐Functionalized Polymers for Micro‐ and Nanopatterning of Silicon

Azobenzene-containing polymers (azopolymers) have attracted great interest due to their potential use in various technological applications, including holographic recording, photomechanics, diffractive optics, and microand nanopatterning. [ 1–7 ] These applications are brought about by the effi cient and reversible photoisomerization of the azobenzene moieties between a rodlike trans-state and a bent cis-state, which is accompanied by various changes in the properties of the material system both at molecular and macroscopic levels. [ 7 , 8 ] Remarkably, the photoisomerization can give rise to signifi cant surface mass transport phenomena, allowing one-step inscription of high-quality, thermally stable photoinduced surface patterns onto the azopolymer fi lm. Since its fi rst demonstration in 1995, [ 9 , 10 ] the photoinduced surface-relief grating (SRG) formation has been intensively investigated in various types of azobenzene-containing materials. [ 11–15 ] The phenomenon continually keeps fi nding new potential applications. Recently, the SRGs have been combined with organic solar cells and lasers, [ 16 , 17 ] carbon-based nanomaterials, [ 18 ] and block-copolymer nanostructures. [ 19 ] Furthermore, in recent years, azopolymer-based patterns have been increasingly used as templates for fabricating periodic arrays of, e.g., titanium dioxide, [ 20–22 ] indium tin oxide, [ 23 ] and metallic [ 24 , 25 ]

Gas refractometry using a hollow-core photonic bandgap fiber in a Mach-Zehnder-type interferometer

We present the design of a fiber-optic gas refractometer that enables spectrally resolved measurements of both real and imaginary parts of the complex refractive index. The proposed concept is based on a Mach-Zehnder-type interferometer with a hollow-core photonic bandgap fiber in one of the interferometer’s arms. The fiber is used simultaneously as an optical waveguide and an analyte containing cell. We demonstrate the performance of the device by measuring the complete complex refractive index of an air-acetylene gas mixture within the optical C-band. The introduced concept leads towards versatile applications in optics as well as atomic and molecular physics.

Large-area nanostructured substrates for surface enhanced Raman spectroscopy

We demonstrate substantial enhancement of Raman transitions of organic molecules by nanostructured gold-coated substrates at the excitation wavelength of 785 nm and experimentally study the factors that influence the enhancement. The substrates are fabricated by using a robust and cost-effective nanopatterning technique that allows us to create high-density gold- or silver-coated nanopillars simultaneously on the whole surface of a standard silicon wafer.

Characterization of polarization fluctuations in random electromagnetic beams

Two stationary, partially polarized electromagnetic beams with equal degrees of polarization may exhibit completely different time evolutions of the instantaneous polarization state. In this work, we derive a statistical quantity that describes the rate at which the field intensity in the beam, on average, is redistributed between the beams polarization state at any time and the state orthogonal to it. This method allows one to treat the dynamical properties of the polarization fluctuations both theoretically and experimentally. We demonstrate the method by applying it to important special cases, such as fields obeying Gaussian statistics, black-body radiation pencils and depolarized laser beams. We also prove that a geometric approach introduced earlier is closely connected with the present model.

Microscopic derivation of electromagnetic force density in magnetic dielectric media

Macroscopic force density imposed on a linear isotropic magnetic dielectric medium by an arbitrary electromagnetic field is derived by spatially averaging the microscopic Lorentz force density. The obtained expression differs from the commonly used expressions, but the energy-momentum tensor derived from it corresponds to a so-called Helmholtz tensor written for a medium that obeys the Clausius-Mossotti law. Thus, our microscopic derivation unambiguously proves the correctness of the Helmholtz tensor for such media. Also, the expression for the momentum density of the field obtained in our theory is different from the expressions obtained by Minkowski, Abraham, Einstein and Laub, and others. We apply the theory to particular examples of static electric, magnetic and stationary electromagnetic phenomena, and show its agreement with experimental observations. We emphasize that in contrast to a widespread belief the Abraham-Minkowski controversy cannot be resolved experimentally because of incompleteness of the theories introduced by Abraham and Minkowski.

High and stable photoinduced anisotropy in guest-host polymer mediated by chromophore aggregation

We present a novel materials concept for optical inscription of stable birefringent optical elements into guest-host type polymers by making use of chromophore aggregation. The method is based on photoalignment of azobenzene chromophores, the aggregation of which leads to significant enhancement and stabilization of the photoinduced birefringence. The obtained order parameter of the molecular alignment (0.3) in combination with the exceptional thermal stability of the anisotropy renders the material system unique among amorphous azobenzene-containing polymers and provides a route toward designing efficient photoresponsive optical elements through the guest-host type approach.

Theoretical description of bifacial optical nanomaterials

We introduce a formalism that describes the interaction of light with bifacial optical nanomaterials. They are artificial noncentrosymmetric materials in which counter-propagating waves behave differently. We derive electromagnetic material parameters for uniaxial crystalline media in terms of the complex transmission and reflection coefficients of a single layer of the constituent nanoscatterers, which makes the numerical evaluation of these parameters very efficient. In addition, we present generalized Fresnel coefficients for such bifacial nanomaterials and investigate the fundamental role of higher-order electromagnetic multipoles on the bifaciality. We find that two counter-propagating waves in the material must experience the same refractive index, but they can have dramatically different wave impedances. The use of our model in practice is demonstrated with a particular example of a bifacial nanomaterial that exhibits a directional impedance matching to the surrounding medium.

Interferometric description of optical metamaterials

We introduce a simple theoretical model that describes the interaction of light with optical metamaterials in terms of interfering optical plane waves. In this model, a metamaterial is considered to consist of planar arrays of densely packed nanoparticles. In the analysis, each such array reduces to an infinitely thin homogeneous sheet. The transmission and reflection coefficients of this sheet are found to be equal to those of an isolated nanoparticle array and, therefore, they are easy to evaluate numerically for arbitrary shapes and arrangements of the particles. The presented theory enables fast calculation of electromagnetic fields interacting with a metamaterial slab of an arbitrary size, which, for example, can be used to retrieve the effective refractive index and wave impedance in the material. The model is also shown to accurately describe optically anisotropic metamaterials that in addition exhibit strong spatial dispersion, such as bifacial metamaterials.

Trapping atoms on a transparent permanent-magnet atom chip

We describe experiments on the trapping of atoms in microscopic magneto-optical traps on an optically transparent permanent-magnet atom chip. The chip is made of magnetically hard ferrite-garnet material deposited on a dielectric substrate. The confining magnetic fields are produced by miniature magnetized patterns recorded in the film by magneto-optical techniques. We trap Rb atoms on these structures by applying three crossed pairs of counterpropagating laser beams in the conventional magneto-optical trapping geometry. We demonstrate the flexibility of the concept in creation and in situ modification of the trapping geometries through several experiments.