Research Supervisor: Dr. Tapajyoti Das Gupta
Title: Compact and Scalable Metasurface Designs for Optical Needle Generation
Date :12th August 2026 (Wednesday)
Time: 3 pm
Venue: SVN Auditorium, Department of IAP
Thesis Abstract:
Optical needles are highly elongated focal beams that combine subwavelength transverse confinement with extended depth of focus, making them attractive for applications such as optical manipulation, laser nanolithography, high-resolution microscopy, and optical coherence tomography (OCT). Conventional approaches for generating optical needles, including annular phase masks and axicon-based Bessel beams, often require complex theoretical design and computationally intensive optimization.
This colloquium presents two alternative metasurface-based approaches for generating optical needles using comparatively simple and scalable optical architectures. Metasurfaces, consisting of quasi-periodic arrays of subwavelength dielectric meta-atoms, provide an efficient platform for tailoring the phase, amplitude, and polarization of light with high spatial precision.
Building upon a previously developed meta-atom library, two distinct metasurface designs are investigated: (i) a compact circular metalens (~40 μm diameter) composed of concentric rings, and (ii) a larger square metalens (~300 μm side length) constructed from smaller sub-units. The circular design is developed and analyzed using Finite-Difference Time-Domain (FDTD) simulations, while the square metalens is designed using a computationally efficient implementation of the Fresnel approximation, followed by FDTD validation to confirm its optical performance.
The circular metasurface generates an optical needle approximately 17 μm long with sharp intensity rise and decay, whereas the square metasurface produces a significantly extended optical needle of nearly 10 mm, exhibiting a sharp onset and a more gradual intensity decay. The advantages, trade-offs, and potential applications of these two design strategies will be discussed, along with future directions toward compact, high-performance metasurface-based beam shaping for advanced photonic systems.