Numerical Simulation of Broadband Scattering by Coated and Noncoated Metal Nanostructures Using Discrete Dipole Approximation Method

We suggest numerical method to study the optical response of metal nanostructures. The analysis of optical properties such as scattering and absorption by coated and noncoated nanogeometry has been done using discrete dipole approximation (DDA) method. The core-shell nanogeometry supports surface plasmon resonances, which are highly tunable from 400 to 1100 nm. The tunability of surface plasmon resonance (SPR) highly depends on the structural anisotropy and chosen core-shell material. Further, we have observed that aspect ratio is one of the key parameter to decide the nature and position of the plasmonic peaks and magnitude of optical cross section. We have also shown that coated nanospheroid is a more appropriate geometry as compared to coated nanosphere and noncoated nanospheroid in terms of wide tunability of surface plasmon resonance. The wide tunability in SPR is observed for the effective radii 90 nm core-shell (Au@SiO₂) nanospheroid with aspect ratio 0.1.     

Numerical Simulation of Electromagnetic Wave Interaction with Spheroidal Core-Shell Nanoparticle: Dependence of Surface Plasmon Resonance on Core-Shell Composition

Plasmonics - In the present work, we report our observations drawn from the numerical simulation of absorption and scattering efficiencies of spheroid shape nanostructures using discrete dipole...     

Study of Surface Plasmon Resonances of Core-Shell Nanosphere: A Comparison between Numerical and Analytical Approach

An investigation of the wavelength dependent extinction spectra of coated sphere with different core@shell compositions based on discrete dipole approximation technique has been presented in this paper. We have used combinations of A g, A u, and S i O ₂ for this analysis. Specifically, we study the impact of spherical core-shell thickness on its surface plasmon resonance (SPR) peak positions and corresponding spectral widening in distinct regimes of the spectrum. We observe that SPR peak of core-shell nanoparticle(CSNP) can be tuned over the visible to near-infrared spectrum region by manipulating the core/shell ratio and composition of core and shell. Specifically, for dielectric@metal (core@shell) nanoparticle, SPR peak position shifted towards lower wavelength as we increase the shell thickness, which is opposite to the SPR behavior of metal@dielectric. The extinction spectrum shows linear relation between SPR position and thickness of the shell. Further, we observed two resonant peaks for the case of metal@metal CSNP. The SPR peak of Au@Ag (a _eff 100 nm with shell thickness 8 nm) reveals two resonant peak corresponding to Au (594 nm) in red domain, while the peak in blue domain corresponds to Ag (402 nm). We also observe that optical resonance of CSNP can be tuned across the near-infrared region by changing the surrounding medium of higher refractive index. Further, near field pattern of core@shell geometry at resonance wavelength is also shown in the present study. We have also compared the numerical and analyticalmethod for smaller size CSNP with varying thickness and the results show good agreement.     

In-vitro antimycobacterial drug susceptibility testing of non-tubercular mycobacteria by tetrazolium microplate assay

Background

Signature matching of nucleotide sequences in the V1 and V3 regions 16S rRNA genes using pyrosequencing technology is a powerful tool for typing vaginal Lactobacilli to the species level and has been used for investigating the vaginal microbial niche.

Methods

This study has characterized the normal cultivable vaginal Lactobacillus flora at varying estradiol levels in plasma; the study comprised 17 patients undergoing ovarian stimulation for In Vitro Fertilization (IVF) treatment. The vaginal status of each participant was initially assessed as normal according to Amsel and Nugent criteria.

Results

L. crispatus, L. gasseri and/or L. jensenii were present in 10 of the patients throughout the study period, and little variation among these three species was encountered in individual patients. The flora of three women was dominated by L. delbrüeckii, L. rhamnosus or L. vaginalis. One woman exhibited a dominance of L. iners. The flora of the remaining three women were initially dominated by L. rhamnosus or L. reuteri, but as their estrogen levels rose, their flora composition altered, to become dominated by one of the three species most common in a normal, healthy vagina.

Conclusion

Signature matching of nucleotide sequences in the V1 and V3 regions of 16S rRNA genes is a discriminative tool for the study of vaginal Lactobacilli and can be used to track the Lactobacillus flora under a variety of physiological conditions.     

Numerical Simulation of Extinction Spectra of Plasmonically Coupled Nanospheres Using Discrete Dipole Approximation: Influence of Compositional Asymmetry

We demonstrate plasmon coupling phenomenon between equivalent (homodimer) and non-equivalent (heterodimer) spherical shape noble metal nanoparticle (Ag, Au and Al). A systematic comparison of surface plasmon resonance (SPR) and extinction properties of various configurations (monomer, homodimer and heterodimer) has been investigated to observe the effect of compositional asymmetry. Numerical simulation has been done by using discrete dipole approximation method to study the optical properties of plasmonically coupled metal nanoparticles (MNPs). Plasmon coupling between similar nanoparticles allows only higher wavelength bonding plasmon mode while both the plasmon modes lower wavelength antibonding mode as well as higher wavelength bonding mode in the case of heterodimer. Au monomer of radius 50 nm shows resonance peak at 518 nm while plasmon coupling between Au-Au homodimer results in a spectral red shift around 609 nm. Au-Ag plasmonic heterodimer (radius 50 nm) reveals two resonant modes corresponding to higher energy antibonding mode (422 nm) as well as lower energy bonding mode (533 nm). Further, we have shown that interparticle edge-to-edge separation is the most significant parameter affecting the surface plasmon resonances of MNPs. As the inter particle separation decreases, resonance wavelength shows red spectral shift which is maximum for the touching condition. It is shown that plasmon coupling is a reliable strategy to tune the SPR.