Ayan Chakrabarty

Polarization conversion with elliptical patch nanoantennas

In this paper, we demonstrate arrays of optical patch nanoantennas can convert light polarization through reflection. By breaking the azimuthal symmetry, elliptical plasmonic patch nanoantennas exhibit both even and odd cavity modes, which can be expressed by Mathieu functions. It is shown that by properly orienting the incident polarization, a linearly polarized light in resonance with one cavity mode can be converted into an elliptical or circular polarization after reflection. Since the major cavity modes can be excited at all incident angles, the polarization conversion by these elliptical patch nanoantennas can be realized with wide range of incident angles.

Resonant cavity modes of circular plasmonic patch nanoantennas

We present theoretical analysis and numerical studies of cavity modes in circular plasmonic patch nanoantennas. There exist both even and odd cavity modes, while the even cavity modes were often missed in the literature because they can only be excited by oblique illumination. The cavity resonance frequencies are affected by near-field coupling at small periods and by coupling with surface plasmon outside the cavity at large periods. For intermediate periods with non-coupling effects, a simple resonant condition is obtained and validated by numerical simulations to relate the gap plasmon wave number and the effective patch size.

Cavity modes and their excitations in elliptical plasmonic patch nanoantennas.

We present experimental and theoretical studies of two dimensional periodic arrays of elliptical plasmonic patch nanoantennas. Experimental and simulation results demonstrate that the azimuthal symmetry breaking of the metal patches leads to the occurrence of even and odd resonant cavity modes and the excitation geometries dependent on their modal symmetries. We show that the cavity modes can be described by the product of radial and angular Mathieu functions with excellent agreements with both simulations and experiments. The effects of the patch periodicity on the excitation of the surface plasmon and its coupling with the cavity modes are also discussed.

Brownian Motion of Arbitrarily Shaped Particles in Two Dimensions

We implement microfabricated boomerang particles with unequal arm lengths as a model for nonsymmetric particles and study their Brownian motion in a quasi-two-dimensional geometry by using high-precision single-particle motion tracking. We show that because of the coupling between translation and rotation, the mean squared displacements of a single asymmetric boomerang particle exhibit a nonlinear crossover from short-time faster to long-time slower diffusion, and the mean displacements for fixed initial orientation are nonzero and saturate out at long times. The measured anisotropic diffusion coefficients versus the tracking point position indicate that there exists one unique point, i.e., the center of hydrodynamic stress (CoH), at which all coupled diffusion coefficients vanish. This implies that in contrast to motion in three dimensions where the CoH exists only for high-symmetry particles, the CoH always exists for Brownian motion in two dimensions. We develop an analytical model based on Langevin theory to explain the experimental results and show that among the six anisotropic diffusion coefficients only five are independent because the translation-translation coupling originates from the translation-rotation coupling. Finally, we classify the behavior of two-dimensional Brownian motion of arbitrarily shaped particles into four groups based on the particle shape symmetry group and discussed potential applications of the CoH in simplifying understanding of the circular motions of microswimmers.

Numerical Studies of Metal–Dielectric–Metal Nanoantennas

We propose a new design of optical nanoantennas and present numerical studies of their optical properties. The nanoantennas are composed of two metallic parts stacked vertically with a dielectric spacer. The calculated near-field and far-field spectra show cavity resonances, which produce sharp peaks in local field spectra and leave their fingerprints as multiple dips in the far-field scattering spectra. Numerical results also show that the local field enhancements inside the cavity can be tuned by varying the size of the dielectric spacer and that a gap of 5 nm or less promises an extraordinary electromagnetic field enhancement on the order of 102. The uniqueness of this new design lies in the ability to fabricate them with high accuracy in gap size control.

High-Precision Tracking of Brownian Boomerang Colloidal Particles Confined in Quasi Two Dimensions

In this article, we present a high-precision image-processing algorithm for tracking the translational and rotational Brownian motion of boomerang-shaped colloidal particles confined in quasi-two-dimensional geometry. By measuring mean square displacements of an immobilized particle, we demonstrate that the positional and angular precision of our imaging and image-processing system can achieve 13 nm and 0.004 rad, respectively. By analyzing computer-simulated images, we demonstrate that the positional and angular accuracies of our image-processing algorithm can achieve 32 nm and 0.006 rad. Because of zero correlations between the displacements in neighboring time intervals, trajectories of different videos of the same particle can be merged into a very long time trajectory, allowing for long-time averaging of different physical variables. We apply this image-processing algorithm to measure the diffusion coefficients of boomerang particles of three different apex angles and discuss the angle dependence of these diffusion coefficients.

DNA–WT1 protein interaction studied by surface-enhanced Raman spectroscopy

AbstractInteractions of proteins with DNA play an important role in regulating the biological functions of DNA. Here we propose and demonstrate the detection of protein–DNA binding using surface-enhanced Raman scattering (SERS). In this method, double-stranded DNA molecules with potential protein-binding sites are labeled with dye molecules and immobilized on metal nanoparticles. The binding of proteins protects the DNA from complete digestion by exonuclease and can be detected by measuring the SERS signals before and after the exonuclease digestion. As a proof of concept, this SERS-based protein–DNA interaction assay is validated by studying the binding of a zinc finger transcription factor WT1 with DNA sequences derived from the promoter of the human vascular endothelial growth factor. FigureA representative dark-field optical microscopic image of Ag nanoparticles attached with dye labeled DNA molecules and two Raman spectra from AAg nanoparticles attached with FAM (left) and TAMRA (right) labeled DNA molecules respectively.