arXiv subjects

Prajit Dhara

Publications and source records attributed to Prajit Dhara.

21 records · Page 2Linked to original sources

arXiv2019

Role of polarized tip-enhanced Raman spectroscopy in the enhancement of interface optical phonon modes in AlGaN multi-quantum wells

Group III nitride based two-dimensional multi-quantum well (MQW) nanostructures find remarkable applications in the visible to ultraviolet light sources. The interface optical (IFO) phonon modes in a c-axis oriented superlattice of [Al0.35Ga0.65N (~1.75 nm)/Al0.55Ga0.45N (~2nm)]20 MQWs are observed using tip-enhanced Raman spectroscopic (TERS) studies. The near-field studies using TERS probe with an Au spherical nanoparticle of ~ 200 nm diameter were carried out at ambient conditions showing approximately two to three orders of enhancement in the Raman intensities. The interface phonon mode belonging to E1 symmetry [IFO(E1)] vibrating normal to the c-axis of MQWs appeared to be more prominent in the case of TERS measurement compared to that for the other interface phonon mode of A1 symmetry. The confined electric field of the polarized electro-magnetic excitation using TERS probe, parallel to the plane of the interface of MQW, is made responsible for the plasmonic enhancement of IFO(E1) phonon mode. The confinement was verified using finite-difference time-domain simulation.

cond-mat.mtrl-sci↗

arXiv2017

Near Field Scanning Optical Imaging of Gold Nanoparticles in the Sub-Wavelength Limit

The near-field scanning optical microscopic (NSOM) imaging of Au nanoparticles with size in the sub-wavelength limit (<wavelength/2N.A.) is reported. The NSOM imaging technique can resolve the objects which is beyond the scope of optical microscope using visible light (wavelength=500 nm) with objectives having a numerical aperture (N.A.) close to unity. The role of evanescent waves which is an exponentially decaying field with higher momenta i.e., lower wavelengths compared to that of normal light, in the metal dielectric interface is realized for imaging of noble metal nanostructures with sub-wavelength dimension in the near field. However, the confined light with components of evanescent waves, emanating from the NSOM probe, interacts with the oscillating dipoles present in the sub-diffraction limited nanostructures and produce propagating waves, which can be recorded by the far field detector. The light-matter interactions of Au nanoparticles of diameters in the range of 10-150 nm probed by the NSOM technique with a visible excitation of 532 nm are reported. The strong surface plasmon resonance (SPR) related absorption of Au nanoparticles is envisaged for explaining the contrast variations in the recorded NSOM images.

cond-mat.mtrl-sci↗

arXiv2017

Optical imaging of metallic and semiconductor nanostructures at sub wavelength regime

The near field scanning optical microscopy (NSOM) is not only a tool for imaging of objects in the sub wavelength limit but also a prominent characteristic tool for understanding the intrinsic properties of the nanostructures. The effect of strong localized surface plasmon resonance absorption of excitation laser in the NSOM images for Au nanoparticles is observed. The role of electronic transitions from different native defect related energy states of AlGaN are also discussed in understanding the NSOM images for the semiconductor nanowire.

cond-mat.mtrl-sci↗