Amitabh Ghoshal - Publications

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Amitabh Ghoshal

Experimental observation of mode-selective anticrossing in surface-plasmon-coupled metal nanoparticle arrays, Amitabh Ghoshal, Ivan Divliansky, and Pieter G. Kik, Applied. Physics Letters 94, 171108 (2009) [download] [link]

Surface plasmon excitation using resonant metal nanoparticles is studied experimentally. Geometry dependent reflection measurements reveal the existence of several optical resonances. Strong coupling of the in-plane nanoparticle plasmon resonance and propagating plasmons is evident from clear anti-crossing behavior. Reflection measurements at high numerical aperture demonstrate the excitation of surface plasmons via out-of plane particle polarization. The thus excited plasmons do not exhibit anti-crossing in the considered frequency range. The results are explained in terms of the known surface plasmon dispersion relation and the anisotropic frequency dependent nanoparticle polarizability. These findings are important for applications utilizing surface coupled nanoparticle plasmon resonances.

Figure from "Influence of particle induced damping on the nanoparticle-mediated excitation of propagating surface plasmons"

Figure from "Influence of particle induced damping on the nanoparticle-mediated excitation of propagating surface plasmons"

Excitation of propagating surface plasmons by a periodic nanoparticle array: trade-off between particle-induced near-field excitation and damping, Amitabh Ghoshal and Pieter G. Kik, Applied Physics Letters 94, 251102 (2009) [download] [link]

We simulate a plasmonic device – an illuminated array of silver nanoparticles in SiO₂ which excites propagating surface plasmons in a nearby silver film. A model to calculate the damping of propagating surface plasmons by nearby nanoparticles is compared to the simulations to demonstrate that excitation of maximum surface plasmon amplitude is due to increased excitation of surface plasmons with increasing nanoparticle volume, counter-acted by an increase in the nanoparticle-induced damping. The nanoparticle resonance frequency does not strongly affect the surface plasmon excitation efficiency in large couplers if the nanoparticle volume can be modified, but affects the nanoparticle volume at which maximum surface plasmon excitation occurs.

Silver nanoparticles on silica.

Single Particle Spectroscopy Study of Metal-Film-Induced Tuning of Silver Nanoparticle Plasmon Resonances, Min Hu, Amitabh Ghoshal, Manuel Marquez, and Pieter G. Kik, under review, Journal of Physical Chemistry

We present a general and simple strategy to tune the optical plasmon resonance of a metal nanoparticle close to a metal thin film by modifying the particle-film distance. A significant tuning of the silver (or gold) particle plasmon resonance into near-IR region can be achieved due to a strong electromagnetic coupling between the nanoparticle and the metal film. Simply altering the thickness of a thin dielectric silica spacer layer incorporated between the metal nanoparticle and the metal film allows accurate controlling of the resonance. The study on single nanoparticles yields rich information from their optical spectra. The horizontal and vertical plasmon modes were observed in the experiment and each mode was assigned based on a dipole-dipole interaction model. Our experimental results are crucial for understanding the optical response of the particle-film hybrid structures and other related optical phenomena at nanometer scales. We believe our investigation also provides a simple and convenient route to a variety of single nanoparticle sensing platforms for novel optical sensor applications in nanotechnology.

Figure from "Theory and simulation of surface plasmon excitation using resonant metal nanoparticle arrays"

Figure from "Theory and simulation of surface plasmon excitation using resonant metal nanoparticle arrays"

Theory and simulation of surface plasmon excitation using resonant metal nanoparticle arrays, Amitabh Ghoshal and Pieter G. Kik, J. Appl. Phys. 103, 113111 (2008) [download] [link]

We discuss a plasmonic coupling device consisting of an array of ellipsoidal silver nanoparticles embedded in SiO₂ and placed near a silver surface. By tuning the shape of the particles in the array, the nanoparticle plasmon resonance is tuned. The resulting resonantly enhanced fields near the nanoparticles in turn excite surface plasmons on the metal film. We have performed Finite Integration Technique simulations of such a plasmon coupler, optimized for operation near a wavelength of 676 nm. Analysis of the frequency dependent electric field at different locations in the simulation volume reveals the separate contributions of the particle and surface resonance to the excitation mechanism. A coupled oscillator model describing the nanoparticle and the metal film as individual resonators is introduced and is shown to reproduce the trends observed in the simulations. Implications of our analysis on the resonantly enhanced excitation of surface plasmons are discussed.

Figure from "Optimization of surface plasmon excitation using resonant nanoparticle arrays above a silver film"

Figure from "Optimization of surface plasmon excitation using resonant nanoparticle arrays above a silver film"

Optimization of surface plasmon excitation using resonant nanoparticle arrays above a silver film, Amitabh Ghoshal and Pieter G. Kik, Proc. SPIE 6641, 664119 (2007) [download]

A plasmonic coupling device consisting of an array of ellipsoidal silver nanoparticles embedded in silica in close proximity to a silver surface is studied. By tuning the inter-particle spacing, the shape of the particles in the array, and the height of the array above the silver film, the array-mediated surface plasmon excitation is studied. Finite Integration Technique simulations of such a plasmon coupler optimized for operation at a free space wavelength of 676 nm are presented. Plane wave normal incidence excitation of the system results is seen to result in resonantly enhanced fields near the nanoparticles, which in turn excite surface plasmons on the metal film. The existence of an optimum particle-surface separation for maximum surface plasmon excitation efficiency is demonstrated. Analysis of the frequency dependent electric field in the simulation volume as a function of particle aspect ratio reveals the influence of the particle resonance and the surface plasmon resonance on the excitation efficiency.

Figure from "In-situ experimental study of a near-field lens at visible frequencies"

Figure from "In-situ experimental study of a near-field lens at visible frequencies"

In-situ experimental study of a near-field lens at visible frequencies, Grady Webb-Wood, Amitabh Ghoshal, and Pieter G. Kik, Appl. Phys. Lett. 89, 193110 (2006) [download]

We present frequency-dependent near-field scanning optical microscopy (NSOM) measurements of plasmon mediated near-field focusing using a 50 nm Au film. In these studies the tip aperture of an NSOM probe acts as a localized light source, while the near-field image formed by the metal lens is detected in-situ using nanoscale scatterers placed in the image plane. By scanning the relative position of object and probe we resolve the near-field image generated by the lens. NSOM scans performed at different illumination frequencies reveal an optimum near-field image quality at frequencies close to the localized surface plasmon frequency.

Figure from "Coherent far-field excitation of surface plasmons using resonantly tuned metal nanoparticle arrays"

Coherent far-field excitation of surface plasmons using resonantly tuned metal nanoparticle arrays, Amitabh Ghoshal, Grady Webb-Wood, Clarisse Mazuir, and Pieter G. Kik, Proc. SPIE 5927, 592714 (2005) [download]

Recent work in plasmon nanophotonics has shown the successful fabrication of surface plasmon (SP) based optical elements such as waveguides, splitters, and multimode interference devices. These elements enable the development of plasmonic integrated circuits. An important challenge lies in the coupling of conventional far-field optics to such nanoscale optical circuits. To address this coupling issue, we have designed structures that employ local resonances for far-field excitation of SPs. The proposed coupler structure consists of an array of ellipsoidal silver nanoparticles embedded in SiO₂ and placed close to a silver surface. To study the performance of the coupler we have performed simulations using the Finite Integration Technique. Our simulations show that normal incidence illumination at a freespace wavelength of 676 nm leads to the resonant excitation of SP oscillations in the Ag nanoparticles, accompanied by coherent near-field excitation of propagating SPs on the Ag film. The excitation efficiency can by maximized by tuning the aspect ratio of the nanoparticles, showing optimum coupling at an aspect ratio of 3.0 with the long axis (75 nm) along the polarization of the excitation signal. We discuss the origin of these observations.

amitabh.ghoshal@gmail.com