Alina Karabchevsky

Surface-enhanced fluorescence from metal sculptured thin films with application to biosensing in water

Surface-enhanced fluorescence from porous, metallic sculptured thin films (STFs) was demonstrated for sensing of bacteria in water. Enhancement factors larger than 15 were observed using STFs made of silver, aluminum, gold, and copper with respect to their dense film counterparts. The STFs used are assemblies of tilted, shaped, parallel nanowires prepared with several variants of the oblique-angle-deposition technique. Comparison between the different films indicates that the enhancement factor is higher when the tilt is either small ( 80 deg); thus, the enhancement is higher when only a single resonance in the nanowires is excited.

Tuning the chemiluminescence of a luminol flow using plasmonic nanoparticles

We have discovered a strong increase in the intensity of the chemiluminescence of a luminol flow and a dramatic modification of its spectral shape in the presence of metallic nanoparticles. We observed that pumping gold and silver nanoparticles into a microfluidic device fabricated in polydimethylsiloxane prolongs the glow time of luminol. We have demonstrated that the intensity of chemiluminescence in the presence of nanospheres depends on the position along the microfluidic serpentine channel. We show that the enhancement factors can be controlled by the nanoparticle size and material. Spectrally, the emission peak of luminol overlaps with the absorption band of the nanospheres, which maximizes the effect of confined plasmons on the optical density of states in the vicinity of the luminol emission peak. These observations, interpreted in terms of the Purcell effect mediated by nano-plasmons, form an essential step toward the development of microfluidic chips with gain media. Practical implementation of the discovered effect will include improving the detection limits of chemiluminescence for forensic science, research in biology and chemistry, and a number of commercial applications.

Nanoprecision algorithm for surface plasmon resonance determination from images with low contrast for improved sensor resolution

A forward-projection algorithm based on Radon transform for two-dimensional surface plasmon imaging was devised to achieve nanoscale precision in determining the surface plasmon signal. A diverging laser beam at the chosen frequency was used to overcome the angular scanning in the well-known Kretschmann configuration. Multichannel sensing with improved resolution was realized. The technique was also used to find the lateral resolution of the sensor using a patterned layer of 40-nm thick SiO 2 layer on top of the metallic surface. As a surface plasmon resonance signal detector, the use of the proposed Radon transform algorithm shows nanoprecision accuracy in cases of single and multichannel sensing. The method also provides the filtered output of the signal without any extra modification and therefore, it is nonsensitive to noise.

Microspot sensing based on surface-enhanced fluorescence from nanosculptured thin films

Nanosculptured thin films (STF) are prepared by the oblique angle deposition technique and take different forms of nano columnar structures. Varieties of STFs were investigated to find the optimum structure for biosensing based on the surface enhanced fluorescence. A comparative study was carried out with STFs containing the nanocolumnar structures that differ in their shape, height (h), and tilt angle with respect to the surface (α), thickness (d), and arrangement. The greatest enhancement of the fluorescent signal was found for Ag-based STFs on Si(100), giving an enhancement factor of ×71, where h = 400  nm, d = 75  nm, and α = 23° relative to Ag closed film using fluorescent dye Rhodamine 123. We immobilized the fluorescent receptor to the thiol self-assembly monolayer on Ag-based STF and Ag dense film to demonstrate the applications of STFs for specific biosensing. Upon excitation of the fluorophore by an Hg light source, a CCD camera with controlled exposure time would detect the pattern of fluorescent receptor Anti-Rabbit IgG on the surfaces. A specially designed optical fiber housing attached to the microscope allowed quantitative measurement of the fluorescence spectrum on a microspot parallel to the image grab.

Dual-surface plasmon excitation with thin metallic nanoslits

Based on the experimental results and comparison between analytical and rigorous calculations, we have found that dual-surface plasmon (SP) waves are excited at both interfaces of a periodic array of thin metallic nanoslits: one at the grating-substrate interface and one at the grating-superstrate interface. Dual plasmons are excited for each diffraction order at two different wavelengths when the substrate differs from the superstrate. The splitting of the plasmons was investigated as a function of the refractive index difference between the substrate and superstrate. Verification of the extended nature of the double SPs is presented by comparing the rigorous calculation and analytic dispersion relation of extended SPs.

Resonant forward scattering of light by high-refractive-index dielectric nanoparticles with toroidal dipole contribution

In this Letter, we demonstrate and investigate the Kerker-type effect in high-index dielectric nanoparticles for which the third-order multipoles give a considerable contribution to the light scattering process. It is shown that the Kerker-type effect (strong suppression of the backward light scattering and, simultaneously, resonant forward light scattering) can be associated with the resonant excitation of a toroidal dipole moment in the system. This effect is realized due to the interference of the scattered waves generated by electric, magnetic, and toroidal dipole moments of high-index nanoparticles.

Transmittance and surface intensity in 3D composite plasmonic waveguides.

A detailed theoretical study of composite plasmonic waveguide structures is reported. Expressions for modal expansion coefficients, optical transmittance and surface intensity are presented and used to describe the behavior of dielectric channel waveguides containing a short gold-coated section. The superstrate refractive index is shown to control modal beating and modal attenuation in the gold-coated region leading to distinctive features in the surface intensity and device transmittance. The model presented allows detailed prediction of device performance, enabling improved design of highly sensitive miniature devices for evanescent refractometry and vibrational spectroscopy, and can be extended to the design and optimization of composite waveguides structures with nano-patterned overlayers.

Optical immunosensor for endocrine disruptor nanolayer detection by surface plasmon resonance imaging

Endocrine disrupting compounds (EDCs) such as bisphenol A (BPA) and female hormone Estrone are especially prevalent in surface and waste-waters in nano-molar concentrations and therefore, there is a need for sensitive analytical device for their monitoring. We have designed a miniature, low cost and fast surface plasmon resonance (SPR) imaging liquid sensor based on the angular interrogation using Kretschmann configuration with diverged incident monochromatic light. During this paper we present a surface plasmon resonance imaging (SPRI) biosensor to detect EDCs such as BPA and estrone. A pattern of SPR line which is dark intensity line on bright area was reflected at angles range depending on the dielectric constant of the analye: Rabbit Anti-Estrone polyclonal IgG + Estrone 11-MUA attached to the silver or non-specific sensing of BPA in water with nanoprecision. For analyzing the SPR signals we used an efficient detection algorithm based on Radon Transform with less sensitivity to laser speckle noise and nonuniformity of the illumination.

‘Photonic Hook’ based optomechanical nanoparticle manipulator

Specialized electromagnetic fields can be used for nanoparticle manipulation along a specific path, allowing enhanced transport and control over the particle’s motion. In this paper, we investigate the optical forces produced by a curved photonic jet, otherwise known as the “photonic hook”, created using an asymmetric cuboid. In our case, this cuboid is formed by appending a triangular prism to one side of a cube. A gold nanoparticle immersed in the cuboid’s transmitted field moves in a curved trajectory. This result could be used for moving nanoparticles around obstacles; hence we also consider the changes in the photonic hook’s forces when relatively large glass and gold obstacles are introduced at the region where the curved photonic jet is created. We show, that despite the obstacles, perturbing the field distribution, a particle can move around glass obstacles of a certain thickness. For larger glass slabs, the particle will be trapped stably near it. Moreover, we noticed that a partial obstruction of the photonic jet’s field using the gold obstacle results in a complete disruption of the particle’s trajectory.

Figure of Merit of All-Dielectric Waveguide Structures for Absorption Overtone Spectroscopy

The figure of merit is proposed for all-dielectric waveguides for absorption overtone spectroscopy as the measure of probing efficiency of molecular overtones. It is defined as the power in the evanescent tail over the total power carried by the guided mode. The figure of merit was calculated for the proposed waveguide structures and then compared. We address each waveguide structure in probing overtones. We show that the figure of merit can be substantially increased due to the downscaling of the physical dimensions of the waveguide and microfiber. Such a configuration enables integration possibilities for ultrasensitive devices harnessing evanescent excitation of molecular overtones on miniature and portable chips.