Punnag Padhy

On the Field Enhancement and Performance of an Ultra-Stable SPR Biosensor Based on Graphene

The effect of graphene on the electric field enhancement and performance of SPR-based sensor has been proposed and compared with Ag-Au bimetallic configuration. We found that a monolayer of graphene on Ag not only addresses the oxidation problem of Ag, but it also shows field enhancement as compared with the widely reported Ag-Au bimetallic combination. Detailed calculations and simulations show that the proposed graphene-based sensor has higher sensitivity and narrower full-width at half-maximum than bimetallic. In addition, the better biomolecules adhesion due to graphene because of π-stacking interaction may open a new window for ultra-stable high performance biosensors for real time bimolecular interactions.

Adjoint method for estimating Jiles-Atherton hysteresis model parameters

A computationally efficient method for identifying the parameters of the Jiles-Atherton hysteresis model is presented. Adjoint analysis is used in conjecture with an accelerated gradient descent optimization algorithm. The proposed method is used to estimate the Jiles-Atherton model parameters of two different materials. The obtained results are found to be in good agreement with the reported values. By comparing with existing methods of model parameter estimation, the proposed method is found to be computationally efficient and fast converging.

On the substrate contribution to the back action trapping of plasmonic nanoparticles on resonant near-field traps in plasmonic films

Nanoparticles trapped on resonant near-field apertures/engravings carved in plasmonic films experience optical forces due to the steep intensity gradient field of the aperture/engraving as well as the image like interaction with the substrate. For non-resonant nanoparticles the contribution of the substrate interaction to the trapping force in the vicinity of the trap (aperture/engraving) mode is negligible. But, in the case of plasmonic nanoparticles, the contribution of the substrate interaction to the low frequency stable trapping mode of the coupled particle-trap system increases as their resonance is tuned to the trap resonance. The strength of the substrate interaction depends on the height of the nanoparticle above the substrate. As a result, a difference in back action mechanism arises for nanoparticle displacements perpendicular to the substrate and along it. For nanoparticle displacements perpendicular to the substrate, the self induced back action component of the trap force arises due to changing interaction with the substrate as well as the trap. On the other hand, for displacements along the substrate, it arises solely due to the changing interaction with the trap. This additional contribution of the substrate leads to more pronounced back action. Numerical simulation results are presented to illustrate these effects using a bowtie engraving as the near-field trap and a nanorod as the trapped plasmonic nanoparticle. The substrates role may be important in manipulation of plasmonic nanoparticles between successive traps of on-chip optical conveyor belts, because they have to traverse over regions of bare substrate while being handed off between these traps.

A semi-analytical model of a near-field optical trapping potential well

A semi-analytical model is proposed to describe the force generated by a near-field optical trap. The model contains fitting parameters that can be adjusted to resemble a reference force-field. The model parameters for a plasmonic near-field trap consisting of a C-shaped engraving are determined using least squares regression. The reference values required for the regression analysis are calculated using the Maxwell stress tensor method. The speed and accuracy of the proposed model are compared with the conventional method. The model is found to be significantly faster with an acceptable level of accuracy.

Extracting the potential-well of a near-field optical trap using the Helmholtz-Hodge decomposition

The non-conservative nature of the force field generated by a near-field optical trap is analyzed. A plasmonic C-shaped engraving on a gold film is considered as the trap. The force field is calculated using the Maxwell stress tensor method. The Helmholtz-Hodge decomposition is used to extract the conservative and the non-conservative component of the force. Due to the non-negligible non-conservative component, it is found that the conventional approach of extracting the potential by direct integration of the force is not accurate. Despite the non-conservative nature of the force field, it is found that the statistical properties of a trapped nanoparticle can be estimated from the conservative component of the force field alone. Experimental and numerical results are presented to support the claims.

Effect of substrate in optical trapping of metallic nanoparticle on nano apertures and engravings

The localized plasmon mode of the trapped metal nanoparticle shifts as it draws closer to the metallic substrate. This dynamically modifies its interaction energy with the trap and the trap force at the operating wavelength.