Alexander A. Ezhov

Enhanced Third-Harmonic Generation in Silicon Nanoparticles Driven by Magnetic Response

We observe enhanced third-harmonic generation from silicon nanodisks exhibiting both electric and magnetic dipolar resonances. Experimental characterization of the nonlinear optical response through third-harmonic microscopy and spectroscopy reveals that the third-harmonic generation is significantly enhanced in the vicinity of the magnetic dipole resonances. The field localization at the magnetic resonance results in two orders of magnitude enhancement of the harmonic intensity with respect to unstructured bulk silicon with the conversion efficiency limited only by the two-photon absorption in the substrate.

Surface-plasmon-induced enhancement of magneto-optical Kerr effect in all-nickel subwavelength nanogratings

Enhancement of transversal magneto-optical Kerr effect (TKE) is controlled experimentally in magnetoplasmonic subwavelength nanogratings made of nickel films by resonant excitation of surface plasmon-polaritons (SPPs). Almost one order of magnitude increase of the TKE value is observed in the spectral range of Wood’s anomaly corresponding to the fulfillment of the phase-matching conditions for SPP excitation.

Liquid-crystalline polymer composites with CdS nanorods: structure and optical properties.

We report on the structure, uniaxial orientation, and photoluminescent properties of CdS nanorods that form stable nanocomposites with smectic C hydrogen-bonded polymers from the family of poly(4-(n-acryloyloxyalkoxy)benzoic acids. TEM analysis of microtomed films of nanocomposites reveals that CdS nanorods form small domains that are homogeneously distributed in the LC polymer matrix. They undergo long-range orientation with the formation of one-dimensional aggregates of rods when the composite films are uniaxially deformed. The Stokes photoluminescence was observed from CdS NRs/LC polymer composites with emission peak located almost at the same wavelength as that of NRs solution in heptane. An anti-Stokes photoluminescence (ASPL) in polymer nanocomposites was found under the excitation below the nanoparticles ground state. The mechanism of ASPL was interpreted in terms of thermally populated states that are involved in the excitation process. These nanocomposites represent an unusual material in which the optical properties of anisotropic semiconductor nanostructures can be controlled by mechanical deformation of liquid-crystalline matrix.

Extreme optical activity and circular dichroism of chiral metal hole arrays

We report extremely strong optical activity and circular dichroism exhibited by subwavelength arrays of four-start-screw holes fabricated with single-pass focused ion beam milling of freely suspended silver films. Having the fourth order rotational symmetry, the structures exhibit the polarization rotation up to 90° and peaks of full circular dichroism and operate as circular polarizers within certain ranges of wavelengths in the visible. We discuss the observations on the basis of general principles (symmetry, reciprocity, and reversibility) and conclude that the extreme optical chirality is determined by the chiral localized plasmonic resonances.

Implications of the causality principle for ultra chiral metamaterials

We demonstrate that the fundamental causality principle being applied to strongly chiral artificial materials yields the generalized Kramers-Kronig relations for the observables – circular dichroism and optical activity. The relations include the Blaschke terms determined by material-specific features the zeros of transmission amplitude on the complex frequency plane. By the example of subwavelength arrays of chiral holes in silver films we show that the causality relations can be used not only for a precise verification of experimental data but also for resolving the positions of material anomalies and resonances and quantifying the degree of their chiral splitting.Chiral metamaterials – artificial subwavelength structures with broken mirror symmetry – demonstrate outstanding degree of optical chirality that exhibits sophisticated spectral behavior and can eventually reach extreme values. Based on the fundamental causality principle we show how one can unambiguously relate the metamaterial circular dichroism and optical activity by the generalized Kramers-Kronig relations. Contrary to the conventional relations, the generalized ones provide a unique opportunity of extracting information on material-dependent zeroes of transmission coefficient in the upper half plane of complex frequency. We illustrate the merit of the formulated relations by applying them to the observed ultra chiral optical transmission spectra of subwavelength arrays of chiral holes in silver films. Apart from the possibility of precise verification of experimental data, the relations enable resolving complex eigenfrequencies of metamaterial intrinsic modes and resonances.

Structural properties of silicon nanoparticles formed by pulsed laser ablation in liquid media

Silicon nanoparticles have been formed as a result of the irradiation of single-crystal silicon targets in distilled water and liquid nitrogen, by, respectively, picosecond and femtosecond laser pulses. The main structural properties of these nanoparticles have been investigated by atomic force microscopy, transmission electron microscopy, electron diffraction, Raman scattering, and photoluminescence spectroscopy. These particles are found to be mainly spherical. The presence of crystalline and amorphous silicon phases under picosecond ablation in water is established experimentally. Irradiation by femtosecond pulses in liquid nitrogen can yield nanoparticles smaller than 5 nm in size, which are quantum dots with a characteristic photoluminescence peak near 750 nm.

Ferroelectric properties in KNbO3 thin films probed by optical second harmonic generation

Ferroelectric phase transitions and the structure of KNbO3 nanometer-thick films deposited by Metal-organic chemical-vapor deposition technique on a MgO(100) substrate are studied. Thin KNbO3 films of the average thickness from 20to150nm possess a grainlike structure with a lateral grain size of about 250nm. Such films reveal ferroelectric properties similar to KNbO3 single crystal. Temperature dependencies of optical second harmonic generation intensity show that the Curie temperature TC of the nanometer-thick KNbO3 films is about 30°C lower than the TC of KNbO3 single crystal, which is attributed to size effects in KNbO3 nanograins.

Creation of silicon nanocrystals using the laser ablation in liquid

The method for the formation of silicon nanoparticles by picosecond laser pulses is studied upon the surface irradiation of the single-crystal silicon in various liquids. The ablation products are investigated using the atomic-force microscopy and Raman spectroscopy. The experimental results indicate the crystal-line structure of nanoparticles and the dependence of their size on the ablation medium.

Near-field optical polarimetry of plasmonic nanowires

Local polarization properties of a gold subwavelength grating with plasmon-induced linear dichroism are studied by scanning near-field optical dynamic polarimetry. Periodic modulation of linear dichroism with the period corresponding to the period of the grating and the contrast of 0.3 is found at the distance of λ/20 from the sample.

Lateral 2D-3D phase segregation in fatty acid/fatty amine monolayers induced by langmuir-blodgett deposition.

We describe the formation of lateral 2D-3D patterns in mixed multilayer LB films of stearic acid (SA) and octadecylamine (ODA) deposited from aqueous subphases at a basic pH. The 3D particles of SA constituting the micrometer-scale linear assemblies in the LB film are assumed to segregate at the three-phase contact line in the course of film deposition. This 2D-3D phase separation of the two-component system presumably originates from the substrate-induced lowering of the collapse point of SA that leads to spontaneous 3D condensation of an acid on a solid support. The morphology of SA/ODA LB patterns is sensitively influenced by the deposition speed and surface pressure, while the chemistry of the solid support does not affect the resulting structures. The possible mechanism that controls the specific orthogonal arrangement of the 3D phase of SA in the LB film through wettability oscillations is suggested.