Current research overview:
The main focus of my current research is engineering photonic modes in nanostructures of atomically thin materials such as graphene and monolayer dichalcogenides (eg. MoS2). Such materials display many unconventional optical properties which are not found in usual three dimensional materials. I am interested in applications of my work in energy harvesting, sub-wavelength imaging and optical circuitry.
For the theoretical part of my work, I use a combination of 1) first principle calculations via density functional theory and 2) classical and quantum electrodynamic calculations via numerous open source and some in house developed semi-analytical codes. For the experimental aspects, I mostly use Raman, FTIR and AFM and sometimes SEM and FIB at various MIT facilities including ISN, CMSE and MTL.
Other than that, I am also involved in two acoustics projects: 1) Rayleigh waves interacting with discrete contact resonances, for which I was mostly involved in modeling and some initial fabrication; 2) non-local effective medium theory for acoustic metamaterials, which is at a theory + computation stage right now. More about my current work can be found in my publications.
- Controlling light-matter interaction using plasmon-phonon coupling
Graphene has been shown to be good candidate for tunable plasmonics in the mid-IR and terahertz range, owing to the possibility of electrostatic doping and its ability to produce higher confinement and lower losses compared to metals. Substantial modification of emission spectrum of quantum emitters placed near graphene has been theoretically predicted. While recently, hBN is now being used as a substrate of choice due to the preservation of high carrier mobility, as opposed to conventional SiO2 substrates, the unique optical properties of hBN arising from its phonon modes, has largely been ignored in the context of emission problems. The phonon modes in hBN have been shown to give rise to natural hyperbolicity, a property which has so far only been restricted to the realm of artifical materials. Both graphene plasmons and hBN phonons reside in the mid-IR and we have shown (Nano Letters, 15:3172, 2015) how the optical properties of graphene-hBN heterostructures would allow one to marry the advantage of their constituents, electrical tunability in the former and high quality factor of the latter, through their hybrid plasmon-phonon polaritons. Through extensive electrodynamical computation via analytical and numerical methods, we have shown distinctively different interaction between an external quantum emitter and the plasmon-phonon modes in the two hyperbolic bands (arising from in-plane and out-of-plane phonons in hBN), leading to substantial modification of its spectrum. The coupling to graphene plasmons allows for additional gate tunability in the Purcell factor, and narrow dips in its spontaneous emission enhancement spectra. In an earlier collaborative work with a postdoc, we also addressed the question of plasmon-phonon coupling in a novel system comprising of graphene-lithium niobate. Interestingly, lithium niobate being a ferroelectric material, provides an excellent route to doping graphene(Applied Physics Letters, 102:-, 2013). Both these works were widely recognized and featured in several news articles.
- Emission control using valley induced chirality in gapped Dirac systems
In gapped graphene or transition metal dichalcogenides, electrons in the two valleys have opposite Berry curvature, ensured by time-reversal symmetry (TRS) of their chiral Hamiltonians. Hence, far field light scattering coefficients of these system does not differentiate between circularly polarized light, i.e. zero circular dichroism in the classical sense. Optical pumping with circularly polarized light naturally breaks TRS, and a net chirality ensues. However, under typical experimental conditions, the transverse conductivity due to Berry curvature is less than the quantized conductivity minimum , and the associated optical dichroism effect is not prominent. These effects, however, can be amplified through enhanced light-matter interaction with plasmons. To this end, we have proposed (arXiv:1509.00790, under review) the emergence of new chiral electromagnetic plasmonic modes which can lead to experimentally observable optical dichroism effect. I showed how a generic gapped Dirac system under continuous pumping with positive cicularly polarized light can lead to the appearance of edge chiral plasmons, and how they can allow launching of one-way propagating edge plasmons in a semi-infinite geometry. Through extensive analytical and numerical calculation, the hybridization of these chiral edge modes in a nanoribbons geometry was shown to lead to an all optical valley induced giant Faraday rotation. Since unlike a magnetic field, the field profile of the optical pump can be easily manipulated on the subwavelength scale by the use of nanostructures, our work is expected to pave the way for chip scale nonreciprocal photonics.
- Emission control using bright and dark plasmon modes in graphene nanostructures
Plasmon modes in a single graphene disc are known to provide high spontaneous emission enhancement for nearby quantum dots. We were interested in the question of whether we can address two regimes of far field spectra: 1) both the emitter as well as the plasmon radiate and 2) only the emitter radiates but not the plasmon. To tackle this question, we formulated a semi-analytical solution for a pair of graphene discs, whereby emission and absorption properties of dark and bright plasmonic modes were studied, as a function of graphene doping. Furthermore, we employed an open quantum systems formalism to show that under certain conditions, both the dark and bright dipolar modes in this system can support vacuum Rabi splittings for the plasmon-emitter coupling (Optics Express, 22:6400, 2014). This work has potential applications in cloaking and superabsorption.
- Efficient surface waveguiding via transformation optics and nonreciprocity
I got interested in this problem during the first semester of his PhD at MIT, when we wrote a conference paper (A. Kumar, etal, Frontiers in Optics 2011/Laser Science XXVII, OSA Technical Digest (Optical Society of America, 2011), paper FMG7.) to propose a method to improve the performance of defect tolerant surface waveguides. This paper was presented at the Frontiers in Optics conference, San Jose in 2011. A natural extension of this work was to look at how by tuning surface impedances, one can prevent scattering of surface waves from sharp corners and protrusions: Transformation optics has proved to be a powerful technique to control propagation of electromagnetic waves. Numerous applications of this technique have been proposed and demonstrated. Some examples are invisility cloaks, waveguides with sharp bends, subwavelength image manipulation, etc. With the recent discovery of two dimensional materials such as graphene, there is an enormous interest in studying their optical properties. Such materials are usually characterized by a surface conductivity instead of the volume conductivity which describes the usual three dimensional materials. As such it is expected that the techniques of transformation optics, as is usually employed, will have to be modified to take into account the change in the dimensionality of the material parameter. To this end, for the first time, we formulated a fully general transformation optics scheme involving surface conductivity (Optics Letters, 39:2113, 2014).
- Other experimental projects
I have been actively involved in two collaborative experimental projects within the general context of surface waves. The first one involved study of plasmonic and excitonic devices based on graphene and MoS2 . For this purpose, I obtained extensive first hand experience in nanofabrication, including thin film deposition, focused ion beam milling, electron beam lithography and spin coating. I characterized numerous batches of these samples using Raman, FTIR, AFM, SEM and TEM. In addition, with the help of postdocs in the group, I built various optical setups such as ATR, Nd:YAG laser and z-scan saturation intensity measurement system. The second project involved the study of coherent coupling between surface acoustic waves and local contact resonances of microspheres. I was attracted to this project because this mechanics problem is analogous to the problem of plasmon-exciton coupling, which I was studying simultaneously. For this project, together with a postdoc, I carried out several monolayer deposition and subsequent laser vibrometer measurement of contact resonance frequency. Subsequently, we built a grating interferometer to resolve surface acoustic waves optically. Part of this work was published in Physical Review Letters, 111:036103, 2013.
Moreover, my master’s thesis was purely experimental, where I built a vapor-liquid-solid (VLS) deposition system for fabricating zinc oxide nanowires. I fabricated organic thin film transistors and performed current-voltage and capacitance voltage measurements. This work was published in International Journal of Nanoscience, 10:761, 2011. A number of supplemental experimental techniques were learnt by me to accomplish this project: for eg. gold nanoparticle synthesis and UV-Visible spectroscopic characterization, wet etching of Hafnium oxide films using hydroflouric acid treatment, surface modification using self assembled monolayers (OTS and HMDS treatment), etc.
Previous research overview:
In the following paragraphs, I will present some of my earlier works which are not directly related to my current thesis. Details will only be described for items which have not been published so far in peer-reviewed journals.
During my undergraduate years at IITB, I worked on two main topics:
- Performance enhancement of p-type organic thin film transistors using Zinc Oxide nanostructures (thesis project)
One of the causes of low mobility in organic transistors is the scattering across grain boundaries. We tried to enhance the mobility by using zinc oxide nanostructures to form conducting bridges between the organic crystals. We were able to experimentally demonstrate a mobility enhancement by over ten times.
The ZnO nanostructures considered here are nanorods (300500 nm), that were deposited in the high temperature zone during vapor phase deposition involving carbothermal reduction of solid zinc precursor. Organic thin film transistors (OTFTs) based on the dispersion of these ZnO nanostructures in the p-type organic semiconductor, P3HT, show a mobility enhancement by 10 times for the organic-inorganic composite, without any significant deterioration in threshold voltage or on-off current ratio. More details can be found in the publication.
- A comparative Study of AlGaN/GaN and ZnMnO/ZnO high electron mobility transistors (pre-final project)
In this work, we investigated the mobility versus temperature dependence of two HEMT structures, one based on ZnMnO/ZnO and the other, AlGaN/GaN.
Instead of going through the rigorous Schrodinger Poisson approach for the potential profile, we have a used a very simplistic procedure for determining the band profile, using a recursion equation relating the sheet carrier density with the surface polarization. The ground state of the triangular potential well was related to the sheet density by employing the well-known Fang-Howard technique. We modeled transport in the 2DEG by considering the scattering due to remote donors, background impurities, acoustic phonons, polar optical phonons and piezoelectric field. The mobility was calculated by solving the Boltzmann Transport Equation using an iterative technique. A comparison of the mobilities for the two structures was then attempted. We found that AlGaN/GaN outperforms ZnMnO/ZnO in terms of mobility. Schematic of a HEMT structure shown on the left (image credit: this webpage from MTL).
Other than these, I got a chance to work with some very smart people for several internships. One of the more relevant ones is listed below.
- Spatial Distribution of Emission in Unidentified Infrared Bands in Galactic Star Forming Regions
In short, I analyzed spectral data from two satellites (Midcourse Space Experiment(MSX) and SPITZER-IRAC) and proposed a new model which successfully accounted for the emission from one of the galactic regions in the Milky Way.
The Midcourse Space Experiment or MSX (full survey) and SPITZER/IRAC (limited region surveyed) have surveyed the Galactic Plane in a total of eight infrared bands between 3 and 25 microns. Five of these bands cover several Unidentiﬁed Infrared Bands (UIB). With the aim of extracting the contribution and the spatial distribution of the UIB emission, a scheme has been developed to model the data with emission in UIBs alongwith underlying thermal continuum from two kinds of dust (at diﬀerent temperatures). The scheme used in a previous paper by SK Ghosh etal. (using only 3 parameters) was also implemented by developing a fresh code. But the ﬁt at lower wave-lengths (namely, of IRAC) was poor, due to which the aforesaid scheme was applied. The second temperature (of the ’hotter dust’) was forced by the IRAC data.The ratio of continua ﬂuxes (in 3.6 and 4.5 µm bands of IRAC) was used to determine this temperature. In order to test this scheme, one of the Galactic compact H-II regions, namely Sh-138 has been studied. The results of this comparative study are as follows: (i) an estimate of the relative abundances of the two kinds of dust has been obtained, and (ii) a UIB emission feature has been predicted in the E-band of MSX (this was forced by MSX data alone). Shown on the left is a picture of Sh 138 region which we analyzed (taken from galaxymap.org).