Nikita A. Toropov

Mutual modification of silver-nanoparticle plasmon resonances and the absorptive properties of polymethine-dye molecular layers on a sapphire surface

Extinction spectra of a hybrid material comprised of molecular layers of polymethine dyes and granulated silver films preliminarily deposited onto sapphire substrates are studied experimentally. Specific features of the dye layer composition and broadening of plasmon absorption bands are discussed. In hybrid samples, both an increase and a decrease in the silver-nanoparticle plasmon resonance frequency upon spreading of dyes have been observed, as well as a considerable increase in the dye molecule absorption. Possible mechanisms of the phenomena are discussed.

Granular metal films on the surfaces of transparent dielectric materials studied and modified via optical means

Granular films of alkali and coinage metals are the most popular objects for exploring plasmonic effects. They are easy to obtain via physical vapor deposition and to study via optical means. In this contribution we show several ways not only to record but also to modify the granular metal films using thermal and nonthermal optical effects.

Thin films of organic dyes with silver nanoparticles: enhancement and spectral shifting of fluorescence due to excitation of localised surface plasmons

Localised surface plasmon resonances of silver nanoparticles were used for modification of spectral response of organic dye thin films. Silver nanoparticles were deposited in the form of an island film on the quartz substrate. Three kinds of organic thin films with different absorption spectra were used: coumarin, rhodamine, and malachite green. A prominent increase of the thin films absorption was observed. Along with the enhancement of the dye absorption, shifts of the plasmon frequencies were observed. In the presence of silver nanoparticles either fluorescence quenching or enhancement was observed depending on the thickness of the dye layers. In the presence of silver nanoparticles maximum of the dye fluorescence shifts by 30 nm. To enhance the fluorescence of organic thin films we used poly(methyl metacrylate) as a matrix isolating molecules from the metallic surface. In this case, five-fold enhancement of the dye fluorescence was registered.

Boosting Terahertz Photoconductive Antenna Performance with Optimised Plasmonic Nanostructures

Advanced nanophotonics penetrates into other areas of science and technology, ranging from applied physics to biology, which results in many fascinating cross-disciplinary applications. It has been recently demonstrated that suitably engineered light-matter interactions at the nanoscale can overcome the limitations of today’s terahertz (THz) photoconductive antennas, making them one step closer to many practical implications. Here, we push forward this concept by comprehensive numerical optimization and experimental investigation of a log-periodic THz photoconductive antenna coupled to a silver nanoantenna array. We shed light on the operation principles of the resulting hybrid THz antenna, providing an approach to boost its performance. By tailoring the size of silver nanoantennas and their arrangement, we obtain an enhancement of optical-to-THz conversion efficiency 2-fold larger compared with previously reported results for similar structures, and the strongest enhancement is around 1 THz, a frequency range barely achievable by other compact THz sources. We also propose a cost-effective fabrication procedure to realize such hybrid THz antennas with optimized plasmonic nanostructures via thermal dewetting process, which does not require any post processing and makes the proposed solution very attractive for applications.

Optical properties of semiconductor CdSe/ZnS quantum dots in the near field of silver nanoparticles

We experimentally investigate the influence of the near field of silver nanoparticles on the optical and photophysical properties of semiconductor CdSe/ZnS quantum dots. The samples are grown via deposition of metal onto dielectric substrates in a high vacuum chamber and are covered by a layer of quantum dots by means of centrifugation. We show that there is an increase in the quantum dot absorption in the near field of silver nanoparticles upon excitation in the plasmon band domain.

The optical properties of thin films of cyan dyes with silver nanoparticles and how they change when they are photostimulated

Hybrid materials consisting of silver nanoparticles and thin films of cyan dyes have been investigated by means of scanning electron microscopy and absorption and fluorescence spectroscopy. It is detected that the molecular absorption and fluorescence of the dye become stronger in the presence of Ag nanoparticles, and that the plasmon absorption band in the silver nanoparticles shifts. The mutual modification of the optical properties of the components of the hybrid materials is explained by the dependence of the plasmon frequency on the permittivity of the surrounding substance and by the influence of the near fields of the silver nanoparticles on the dye molecules. Photoinduced transformations have been studied in organic thin films in the presence of Ag nanoparticles. It is established that the change of the optical density of a dye layer with nanoparticles during irradiation exceeds the analogous changes in a dye layer without nanoparticles by a factor of 10.

Optical properties of silver nanoparticles coated by cyanine dyes molecular overlayers

Optical properties of supported silver nanoparticles with cyanine dyes overlayers were investigated. The spectrum of the hybrid material was treated as a result of mutual interactions between the plasmon oscillations in the metal nanoparticles and resonance absorption and refraction of dye molecules. The spectral positions of the plasmon resonances are shifted due to the anomalous refraction of dye molecules while the absorption of dye molecules is enhanced due to the incident field amplification in the near field of metal nanoparticles. The photoinduced transformations of hybrid material were also observed.

Chemically Synthesized Gold and Silver Particles Absorbing in the Near-IR Spectral Range

We propose methods for creating spherical gold particles of submicron size and silver rod-like particles with transverse dimensions of ~10 nm and an aspect ratio of 1: 10. Factors determining the frequency of plasmon resonances are considered, reagents are selected, and their ratios for obtaining prolate silver particles are determined. An optimal concentration of the surfactant is determined for creating most elongated silver particles. A shift of the plasmon absorption toward the near-IR range of the spectrum is obtained.

Chemically synthesized silver nanorods intended for near IR applications

A method for preparation silver nanoparticles supporting plasmon oscillations is proposed. Using a seed-mediated growth approach in a rod-like micellar media, silver nanorods of varied aspect ratio were prepared from nearly spherical silver nanoparticles. The concentrations of synthesis reagents were determined to obtain nanorods whose plasmon resonances are shifted toward larger wavelengths relative to resonances of spherical particles.

Optimization of nanoantenna-enhanced terahertz emission from photoconductive antennas

We present the results of hybrid photoconductive antenna THz emission enhanced by silver nanoantenna arrays. By varying the size of nanoantennas and the distance between them, we obtain the greatest value of optical-to-THz conversion efficiency reached so far. The results of experimental investigations are in a good agreement with numerical simulations. The conversion efficiency reveals over 5-fold improvement at certain frequencies, if compared with similar photoconductive antenna without silver nanoparticles, while previous results for this type of antenna barely exceeded 2-fold conversion efficiency gain.