Sergey M. Novikov

Demonstration of Magnetic Dipole Resonances of Dielectric Nanospheres in the Visible Region

Strong resonant light scattering by individual spherical Si nanoparticles is experimentally demonstrated, revealing pronounced resonances associated with the excitation of magnetic and electric modes in these nanoparticles. It is shown that the low-frequency resonance corresponds to the magnetic dipole excitation. Due to high permittivity, the magnetic dipole resonance is observed in the visible spectral range for Si nanoparticles with diameters of ∼200 nm, thereby opening a way to the realization of isotropic optical metamaterials with strong magnetic responses in the visible region.

Plasmonic black gold by adiabatic nanofocusing and absorption of light in ultra-sharp convex grooves

Excitation of localized and delocalized surface plasmon resonances can be used for turning excellent reflectors of visible light, such as gold and silver, into efficient absorbers, whose wavelength, polarization or angular bandwidths are however necessarily limited owing to the resonant nature of surface plasmon excitations involved. Nonresonant absorption has so far been achieved by using combined nano- and micro-structural surface modifications and with composite materials involving metal nanoparticles embedded in dielectric layers. Here we realize nonresonant light absorption in a well-defined geometry by using ultra-sharp convex metal grooves via adiabatic nanofocusing of gap surface plasmon modes excited by scattering off subwavelength-sized wedges. We demonstrate experimentally that two-dimensional arrays of sharp convex grooves in gold ensure efficient (>87%) broadband (450-850 nm) absorption of unpolarized light, reaching an average level of 96%. Efficient absorption of visible light by nanostructured metal surfaces open new exciting perspectives within plasmonics, especially for thermophotovoltaics.

Sensing using plasmonic nanostructures and nanoparticles.

Nanoparticles are widely used in various fields of science and technology as well as in everyday life. In particular, gold and silver nanoparticles display unique optical properties that render them extremely attractive for various applications. In this review, we focus on the use of noble metal nanoparticles as plasmonic nanosensors with extremely high sensitivity, even reaching single molecule detection. Sensors based on plasmon resonance shifts, as well as the use of surface-enhanced Raman scattering and surface-enhanced fluorescence, will be considered in this work.

Extraordinary Optical Transmission Enhanced by Nanofocusing

We demonstrate that the phenomenon of extraordinary optical transmission (EOT) through perforated metal films can be further boosted up by utilizing nanofocusing of radiation in tapered slits. For one-dimensional arrays of tapered slits in optically thick suspended gold films, we show that the maximum transmission at resonance is achieved for taper angles in the range of 7-10 degrees increasing significantly in comparison with the transmission by straight slits. Transmission spectroscopy of fabricated 500 and 700 nm period tapered slits in a 180 nm thick gold film on a glass substrate demonstrates the enhanced EOT with the resonance transmission being as high as approximately 0.18 for the filling ratio of approximately 0.13 and showing good correspondence with theoretical results. It is also shown that the enhanced transmission can be achieved with either weak (2.5%) or strong (43%) reflection depending on the direction of light (normal) incidence.

Resonant plasmon nanofocusing by closed tapered gaps.

We study radiation nanofocusing by closed tapered gaps, i.e. metal V-grooves, under normal illumination, and discover that the local field inside a groove can be resonantly enhanced due to interference of counter-propagating gap plasmons. Considering V-grooves milled in gold, we analyze this phenomenon theoretically, deriving an analytic expression for the resonance condition and predicting more than 550-fold intensity enhancements at resonance, and observe it experimentally with two-photon photoluminescence microscopy, demonstrating more than 100-fold intensity enhancements.

Surface enhanced Raman imaging: periodic arrays and individual metal nanoparticles

Surface enhanced Raman scattering (SERS) from Rhodamine 6G homogenously adsorbed on both periodic arrays of and individual gold nanoparticles is investigated using high-resolution Raman imaging with polarized excitation. Rectangular 50-nm-high nanoparticles of different sizes chosen to ensure the presence of localized surface plasmon resonances close to the 532-nm excitation wavelength are fabricated with electron-beam lithography on the surface of a smooth gold film and arranged both individually (i.e., placed sufficiently far apart) and in 740-nm-period arrays. Linear reflection spectra and high-resolution Raman images obtained from arrays of nanoparticles are compared revealing good correspondence in the spectral dependences of reflection and local SERS enhancements (measured at the top of nanoparticles). The latter are related to those observed with individual nanoparticles. The results obtained emphasize the importance and quantify the influence of particle dimensions, polarized excitation, collctive resonances and SERS locations.

Field enhancement and extraordinary optical transmission by tapered periodic slits in gold films

We investigate field enhancements by one-dimensional periodic arrays of tapered slits fabricated to a high quality (nm precision) using focused ion beam milling in a 180 nm-thick gold film. Tapering of periodic slits in metal was recently shown to boost the extraordinary optical transmission (EOT) exhibited by similar, but non-tapered, plasmonic structures. Here, both simulated and experimental reflection spectra, along with high-resolution two-photon luminescence (TPL) scanning optical images and simulated electric field plots of the metal slits, are compared, revealing good correspondence between spectral dependences and field intensity enhancements (FEs) estimated via the local TPL. Experimentally investigated structures had a fixed taper angle α=20.5° for two different widths, w=80 and 130 nm, having gaps g=25 and 65 nm, respectively, both fabricated at two different periods, Λ=500  and 700 nm. We attributed the obtained FE reaching ~110 to nanofocusing and resonant interference of counter-propagating plasmons by the periodic tapered gaps. As both simulated and experimentally achieved FEs depend on taper angle, gold film thickness, period and gap of the slit arrays, the resonances can actually be tuned in the wavelength range from visible to infrared, making this configuration promising for a wide range of practical applications, e.g. within surface-enhanced spectroscopies.

Probing cytochrome c in living mitochondria with surface-enhanced Raman spectroscopy

Selective study of the electron transport chain components in living mitochondria is essential for fundamental biophysical research and for the development of new medical diagnostic methods. However, many important details of inter- and intramembrane mitochondrial processes have remained in shadow due to the lack of non-invasive techniques. Here we suggest a novel label-free approach based on the surface-enhanced Raman spectroscopy (SERS) to monitor the redox state and conformation of cytochrome c in the electron transport chain in living mitochondria. We demonstrate that SERS spectra of living mitochondria placed on hierarchically structured silver-ring substrates provide exclusive information about cytochrome c behavior under modulation of inner mitochondrial membrane potential, proton gradient and the activity of ATP-synthetase. Mathematical simulation explains the observed enhancement of Raman scattering due to high concentration of electric near-field and large contact area between mitochondria and nanostructured surfaces.

Collective Plasmonic Properties in Few-Layer Gold Nanorod Supercrystals

Gold nanorod supercrystals have been widely employed for the detection of relevant bioanalytes with detection limits ranging from nano- to picomolar levels, confirming the promising nature of these structures for biosensing. Even though a relationship between the height of the supercrystal (i.e., the number of stacked nanorod layers) and the enhancement factor has been proposed, no systematic study has been reported. In order to tackle this problem, we prepared gold nanorod supercrystals with varying numbers of stacked layers and analyzed them extensively by atomic force microscopy, electron microscopy and surface enhanced Raman scattering. The experimental results were compared to numerical simulations performed on real-size supercrystals composed of thousands of nanorod building blocks. Analysis of the hot spot distribution in the simulated supercrystals showed the presence of standing waves that were distributed at different depths, depending on the number of layers in each supercrystal. On the basis of these theoretical results, we interpreted the experimental data in terms of analyte penetration into the topmost layer only, which indicates that diffusion to the interior of the supercrystals would be crucial if the complete field enhancement produced by the stacked nanorods is to be exploited. We propose that our conclusions will be of high relevance in the design of next generation plasmonic devices.

Surface enhanced Raman microscopy with metal nanoparticle arrays

We present high-resolution images of surface enhanced Raman scattering (SERS) from Rhodamine 6G homogeneously adsorbed on periodic arrays of square gold nanostructures, fabricated with electron-beam lithography on the top of a smooth gold film and having surface plasmon resonances close to the 532 nm excitation wavelength. We use reflection spectroscopy to map the resonances and present the first (to our knowledge) point-by-point comparison of SERS spectra from locations separated by submicron distances within nanoparticle arrays, establishing good correspondence of the spectral resonance shape with the spectral dependence of local SERS enhancements and qualitatively comparing SERS images for different spectral frequency ranges. These results illustrate that, in addition to considering only the well established approximation of |E| 4 -dependence of the surface enhanced Raman signal on the local electric field E, one should also take into account the spectral shape of the resonances as well as the spatial position of SERS spectrum acquisition.