Recent publications:
RESEARCH INTERESTS
Plasmonics and Metamaterials: Plasmonic properties of semiconductor nanostructures, structured thin films. with metal composites. Fundamental studies and applications of plasmonics in metal and metal/semiconductor nanostructures, for non-linear effects (higher harmonics generation) and plasmon mediated field enhancement organic-inorganic nanostructures of oxides, sulphides and organic-inorganic hybrid systems. Non-linear plasmonics and webmixing.
Optoelectronic devices:
Application of metal-semiconductor plasmonics in opto-electronic devices like light emitting diodes, solar cells, and fluorescence studies of fluorophores etc.
RESEARCH PLAN: Advanced nano-plasmonic systems to understand the light-matter interaction at nanometer length scale through the study of SERS/SEF and non-linear plasmonics
Objective and background of the proposed research work:
Plasmonics is an emerging field which primarily deals with the ‘enhanced electromagnetic near fields’ caused by surface plasmon resonance has attracted much attention from researchers of various fields for potential application to almost all the fields ranging from physics to biomedical science. Currently, surface enhanced Raman scattering (SERS) and surface enhanced fluorescence are of huge interests because of its extreme sensitivity to electromagnetic near-fields. Especially, SERS is a unique process that has been explored tremendously for sensitive molecular detection. Because of nonlinear dependence with enhanced electromagnetic near-fields, a high density of concentrated near-field regions or hot-spots and strong plasmon-photon coupling are expected to play a major role in high output signal. It is also expected that only well designed plasmonic structures having optimized arrays of near-fields regions can serve reliable and reproducible results. Therefore, fabrication of advanced plasmonic patterned structures with desired parameters and functionality is a pre-requisite condition for the realization of ultra sensitive detection of SERS/SEF at single molecular level. Apart from the academic interests, SERS/SEF has also potential for industrial application, especially in biomedical science. For the purpose a high-throughput nanofabrication is needed. Laser Interference Lithography (LIL)/Nano-Imprint Lithography (NIL) techniques seem to offer the solution for the above requirements through which one can efficiently generate reliable and reproducible plasmonic structures of desired functionality at large scale. Based on the academic interests and industrial prospects I set my main objective or goal as follows: to fabricate advanced plasmonic structures using high-throughput LIL/NIL for ultra sensitive detection of SERS/SEF, understanding of optical nonlinearity (non-linear nanoplasmonics). The challenge is to keep required functionality intact at large scale fabrication for reliable and reproducible results. A major set point is to unveil the physics behind light-matter interaction at nanometer length scale for nanoplasmonic systems through above studies. Plan for the proposed research work: The following steps are to be followed to achieve the expected goal. In the first step I will fabricate the advanced plasmonic structures with desired functionality using LIL/NIL followed by topographical characterization to ensure the desired patterns. Subsequently plasmonic properties will also be investigated through SPR measurements. Next, SERS/SEF, optical nonlinear processes will be investigated on these active plasmonic systems for various Raman active probe molecules, especial graphene and graphene composites (infrared plasmonic materials), metal-semiconductor hybrid quantum structures. Finally the output will be analyzed in view of the expected results. The proposed plan is discussed points by points.
A. Experimental approach for plasmonic template fabrication:
Plasmonic nanopatterned templates will be grown using high-throughput Laser Interference Lithography /nano-imprint lithography. These techniques are favoured over other techniques because of less complex processing steps involved but have high output with overall production cost low. As I expect to have high density of hot-spots of intense near-fields, a challenge will be given to fabricate sub 100 nm patterned
Figure-1: Proposed active plasmonic templates of shapes ranging from line pattern to diamond-like structures to chess-board pieces.
Figure-2: Proposed active plasmonic templates through assembling of metal nanostructures into the dielectric/ metal grooves.
structures. Apart from this, in the present approach I will utilize LIL/NIL extensively to generate other promising structures as shown in Figure-1 (note: these are primarily dielectric patterns). The primary dielectric structures will be made plasmonically active through: (i) Top metal coating onto the structured dielectric patterns (can be derived from Figure-1) (ii) Selective ion etching followed by removal of top dielectric layer will be performed if the initial dielectric patterns are on the top of the flat metal layer (can be derived from Figure-1). (iii)Assembling of pre-grown metal nanostructures of appropriate shape and size onto structured templates’ groove (Figure-2) to manipulate interaction between metal nano-objects, hence near-fields.
B. Structural and topological characterization: The fabricated plasmonic structures will be characterized using SEM and AFM for detailed topological information which would be helpful in obtaining various parameters such as duty cycle, periods, shape and size, and depth. These parameters will be important for computation.
C. Plasmonic properties (SPR) and near-fields characterization: Plasmonic guided modes, localized modes etc will be investigated using SPR measurements (angle resolved surface plasmon resonance spectrometer). For direct near-field mapping of the patterned plasmonic templates near-field scanning optical microscope (NSOM) will be employed. The near-field patterns generated through computer simulation of the desired structures (square arrays of square pillars as shown in Figure-3 for an example) with parameters values obtained from SEM studies will be analyzed in view of direct near-field distribution obtained by NSOM.
D. SERS/SEF measurements aiming at ultra sensitive detection limit: The fabricated advanced plasmonic nano-patterned templates will be used extensively for SERS/SEF aiming at ultra-sensitive detection or single molecular detection. Various Raman probe molecules will be used to detect SERS with a special care on novel graphene and graphene composite. Probe molecules will be coated onto plasmonic templates in a variety of ways such as chemical bonding, spin coating, drop casting, thermal/ebeam evaporation. Proper excitation wavelength will be selected to achieve highly enhanced SERS signal. Once an optimization is achieved the particular structure possibly be set for the feasibility of large scale fabrication in view of application.
E. Optical non-linearity/optical gain: The concentrated near-fields in a well organized manner are a key issue to study non-linear phenomena. Nonlinear materials will be put into these near-fields regions and under intense laser optical non linearity will be studies. We will be mainly interested in finding the higher harmonics generation.
Metal nanostructures (arrays) as template
Figure-3: A typical square array of square pillars and its un-normalized electric field distribution (along growth direction) around a representative pillar. High near-field is present around the corners. of the pillar.Proposed active plasmonic templates of shapes ranging from line pattern to diamond-like structures to chess-board pieces.
Expected results and overall view: I expect a large scale nanofabrication of novel plasmonic structures using high-throughput Laser Interference Lithography/nano-imprint lithography for ultra-sensitive detection of SERS/SEF which has huge biomedical application. I hope a methodology can be developed to achieve promising optimized plasmonic structure for the use of various surface sensitive optical phenomena with academic interests and technological prospects. With the availability and accessibility of lab and Institute facilities (or setting up) such as laser interference lithography (can be home built)/ advanced nano-imprint lithography, SEM/AFM, NSOM, Raman spectrometer I can dream of the feasibility of the proposed research plan which will also help me in developing my further research carrier and understanding the basic physics. Therefore it would be a great pleasure for me to have an opportunity to carry out advanced research excellence in the projected direction.
List of instrumental facilities for the proposed research
Appeal points from a view point of the originality:
The proposed research will focus on the basic understanding and unveiling the underlying physics/mechanism of various surface sensitive optical phenomena at nanometer length scale by using advanced plasmonic systems (structures/templates). High-throughput laser interference lithography/Nano-Imprint Lithography will be employed to generate the nanoplasmonic structures. Engineering the hot-spots or strong near-field regions with the desired functionality (wavelength specific for efficient coupling of photon to plasmon at nanometer length scale) intact is an important aspect. Computer simulation and experimental investigation are the keys for the unique generation of expected plasmonic patterns. The study will help in realizing the futuristic plasmonic devices.
Fabrication Techniques:
Laser Interference Lithography tools/Basic Nano Imprint Lithography
Thin film growth/ coating techniques
Physical vapour deposition system such as thermal/e-beam evaporation, spin coating, sputtering unit
Characterization techniques
Angle resolved Surface plasmon resonance spectrometer
Raman scattering measurement system
Near-Field Scanning Optical Microscope
SEM/AFM/UV-VIS-IR
Various Laser sources (248nm, 403 nm, 447 nm, 473 nm, 532 nm, 633 nm, 940 nm)/optical components for polarization anisotropy measurements
Computation facilities
Photonics, Plasmonics and nanostructures simulation using commercial softwares, such as OPTIWAVE
Previous research activity:
Understanding the light-matter interaction at nanometer length scale:
During Postdoctoral Studies: Development of plasmonic/photonic systems My postdoctoral studies mainly focus on the plasmonic aspects of patterned metallo-dielectric photonic systems through experimental and numerical simulation approach. Fundamental investigations of these plasmonic and photonic structures have technological importance towards the realization of plasmonic/photonics devices that have attracted considerable interests in recent past. Apart from the fundamental studies on plasmonics I am also involved in investigating plasmonic aspects of surface enhanced Raman scattering and surface enhanced fluorescence with special emphasis on near field engineering. Our findings show that SERS (Surface Enhanced Raman Scattering) signal highly depends on the template periodicity as if there is a periodicity dependent modulation of SERS signal. We showed that this is caused by the combined effect of enhanced near fields arisen due to the resonance of localized plasmon as well as that of propagating plasmons. The results infer that the smaller structures not necessarily always offer better results as per SERS and SEF (Surface Enhanced Fluorescence) concerned. Here I write SEF as we have also investigated the similar periodic effect of SEF provided fluorescence quenching is being reduced. There is a contribution to SERS signal due to the surface lattice type. In all the cases it is necessary that the coupling of incident photons to the plasmonic system is strong. In our recent study (soon to be communicated) we have investigated the polarization controlled light transmission through ladder structures fabricated using laser interference lithography. The plasmonic ladder structures are seen to be highly anisotropic in nature and more interestingly one can control the anisotropic properties through the control of structuring the materials during the nanofabrication. Other research involved is to design and fabricate and study of the possibility of polarization control switch using special shape metamaterials. These plasmonic metamaterial structures have very particular shape and dimension and need the involvement of advanced fabrication technique such as E-BEAM, FIB. Colour tunability is an interesting aspect of these plasmonic metamaterials. The plasmonic photonic system I am working on has strong fundamental and technological prospects. SERS and SEF have wide biomedical application. The special plasmonic and photonic structures are important in optical devices and the latter systems are also promising for nonlinear plasmonics study where nonlinear optical activity such as higher harmonics generation is primarily involved. For the fabrication of these plasmonic photonic structures I used simple two-beam laser interference lithography (LIL) which is very simple and cost effective technique. Large area fabrication is possible in short period of time. I have the complete experience to develop the set up. For the optical characterization of these plasmonic systems I used home build set ups (developed at IIT Kanpur) such as SPR spectrometer, polarization measurement, fluorescence and fluorescence imaging microscope.
During PhD studies: Development of nanophosphors I have following research work experience: During PhD program I worked on semiconductor nanostructures, thin films of sulphides and oxides of inorganic materials systems. I developed methodology to fabricate various semiconductor nanophosphors or quantum dots of CdS, ZnS and alloys embedded in fatty acid thin film matrix. Understanding the optical properties of these low dimensional structures or nanostructures through the study of absorption and emission, Raman, and IR measurements was a major set point. The nanostructured semiconductor quantum dots I developed have special class of optical properties such as quantum size effects and tunability of the spectral absorption and emission and we found that these materials can be considered as excellent phosphors for potential optoelectronic application. The phosphors are developed thorough chemical route and post deposition chemical treatments.
samplepost — Oct 14, 2009 2:31:02 PM