Nanophotonics Lab @ IITH

Gallery

Thu, 01 Jan 2026 00:00:00 +0000

PhD Graduations

Jinal Tapar — PhD Graduation, 2021

Vinod Sharma — PhD Graduation, 2026

Lab Infrastructure

Group Activities

Dielectric Nanophotonics

Wed, 26 Aug 2020 00:00:00 +0000

A major challenge in plasmonics, which has been actively studied for over a decade, is the large optical loss which degrades the performance of most plasmonic devices. High index dielectric materials provide a promising alternative route to realize many of the functionalities achievable in plasmonics. Distinctive advantages of high index dielectric resonant nanostructures over their metallic counterparts are low dissipative losses and the existence of both electric and magnetic resonances of comparable strengths. Moreover, semiconductor nanostructures like III-V materials with optical gain provide a route to design active nanophotonic devices. In our research we investigate methods to improve optical gain in nanostructures. We are interested in design and implementation of high Q cavities for light emission from Semiconductor Nanostructures. We also aim to seek a judicious engineering of the interplay between gain, loss and coupling in (parity-time) PT-symmetric nanostructures in order to design directional light-emitting devices with non-trivial characteristics. Some of our recent work is highlighted below:

Investigation of the carrier surface recombination in III-V semiconductors for on-chip nanophotonic applications

Our numerical investigation of carrier dynamics in GaAs nanocylinder on optical pumping lead us to estimate the qualitative and quantitative impact of surface recombination on the optical gain. The optically induced carrier density is lower in nanocylinder due to the dangling bonds at the surface that give rise to recombination of electron hole pairs. With reduced dimensions of the nanostructures, surface area-to-volume (A/V) ratio increases, and therefore the lasing threshold also increases due to higher non-radiative surface recombination process. The optical gain and bandwidth both reduce with increasing surface recombination velocity (SRV). The study gives an insight that p-doping of active material and strain incorporation as a viable approach for enhancing luminescence quantum yield of III-V semiconductor nanostructures. This can complements the surface passivation techniques widely adopted in the III-V research community.

Figure 1: Impact of Surface Recombination and nanocylinder size on optical gain. Further details can be found in reference [1]

PT-symmetric semiconductor nanoantenna metasurface

The concept of (parity-time) PT symmetry has opened a fundamentally new chapter in photonics and expanded the toolbox of photonics by broadening the design parameter space. In the context of photonics, PT symmetry can be realized by requiring an even distribution in the real part of the refractive index, and an antisymmetric gain/loss profile, associated with the imaginary part of the refractive index. The interplay of gain-loss contrast and coupling in PT-symmetric systems has manifested as a number of intriguing breakthrough effects like single-mode lasing, unidirectional invisibility, (coherent perfect absorber) CPA-laser, etc. In this project, we investigated vertically stacked GaInP PT-symmetric nanodisk resonators arranged in two-dimensional periodic lattice using full-wave numerical simulations and scattering matrix theory. The proposed dielectric metasurface supports spectral singularities (SS) with asymmetric reflection and highly anisotropic far-field scattering patterns. At SS, the scattering coefficients tend to infinity, corresponding to lasing threshold. This lasing spectral singularities can occur only in the presence of gain and being poles of the scattering matrix at real frequencies, imply zero-width resonances. Our analysis suggests that a proper combination of geometry, gain, and subwavelength resonances provides flexibility for designing tunable and directional nanophotonic light sources employing non-Hermitian physics.

Figure 2: PT symmetric nanostructures can be designed to exhibit extremely narrow-width resonances known as Spectral Singularities. Further details can be found in reference [2].

Jinal presented her research at the prestegious Frontiers in Optics conference held virtually in September 2020. Hope you will find the video of her talk informative.

References

Jinal Kiran Tapar, Saurabh Kishen, Kumar Prashant, Kaushik Nayak and Naresh Kumar Emani. Enhancement of the optical gain in GaAs nanocylinders for nanophotonic applications. Journal of Applied Physics, 127, 153102 (2020). [ doi | pdf ]
Jinal Kiran Tapar, Saurabh Kishen and Naresh Kumar Emani. Spectral singularities and asymmetric light scattering in PT-symmetric 2D nanoantenna arrays. Optics Letters, 45, 18, pp. 5185-5188 (2020). [ doi ]

Electrical Excitation of Plasmons in Metallic Nanostructures

Sun, 23 Aug 2020 00:00:00 +0000

Surface plasmons (SP), the collective oscillations of electrons on a metal-dielectric interface, have enabled squeezing of light into nanoscale dimensions leading to unparalleled control and manipulation of information at length scales much smaller than the wavelength of light. These electromagnetic waves propagate along metal-dielectric interfaces and can be guided by metallic nanostructures beyond the diffraction limit. Plasmonic circuits, therefore, possess both the capacity of photonics and miniaturization of electronics, paving the way for the development of nanoscale optoelectronic circuitry for computing and sensing applications. Electrical excitation of surface plasmons through the process of inelastic electron tunneling (IET) has gained immense popularity in recent years due to its unique potential to create ultra-compact photon and SP sources. In our group, we are investigating ways to improve the efficiencies of surface plasmon generation from electrically excited metal-insulator-metal tunnel junctions for the realization of ultra-compact on-chip optical sources which are readily integrable with existing CMOS technologies for practical on-chip applications. Our recent work in this direction is highlighted below –

Enhanced light emission from electrically excited nano-strip MIM tunnel junctions

We explored the light generation through inelastic electron tunneling in a periodic Ag-SiO2-Ag tunnel junction. We investigated the complex interplay between the optical modes of the tunnel junction, the antenna modes, and the lattice resonance arising from the periodicity of the structure. Our analysis revealed an enhanced radiative efficiency from hybridization of the optical gap plasmon mode and the lattice resonance, with an enhancement of ~30% as compared to the structure without the lattice resonance. This improvement is due to the direct out-coupling of the propagating SPs to free-space photons mediated by the grating. The emitted spectrum can be tuned to the desired wavelength range, from visible to near IR, by designing the geometrical and material parameters of the device accordingly for on-chip photonic and plasmonic applications.

Figure 1: Optimization of MIM tunnel junction geometry to improve the radiative emission of electrically excited plasmons. Further details can be found reference [1]..

Reference

Saurabh Kishen, Jinal Kiran Tapar and Naresh Kumar Emani. Enhanced light emission from gap plasmons in nano-strip MIM tunnel junctions. Journal of Optics, 22, 095006 (2020). [ doi | pdf ]