| PhD Seminar

Name of the Speaker: Mr. Anil Babarao Ringne (EE20D018)
Guide: Dr. Ananth Krishnan
Venue: ESB-244 (Seminar Hall)
Date/Time: 30th July 2025(Wednesday), 3 pm
Title: On-axis composite vortex beams generation via moiré and Mie resonant metallo-dielectric moiré gratings

Abstract :

Composite vortex beams, carrying structured phase and intensity profiles, have conventionally been generated using spiral phase plates, fork gratings, and metasurfaces. However, spiral phase plates are inherently non-planar, fork gratings produce composite beams only in higher diffraction orders and off-axis, and metasurfaces demand intricate subwavelength unit cell designs with tight fabrication tolerances [1].

In this work, we demonstrate a method for generating composite vortex beams at the zeroth diffraction order of moiré-patterned binary gratings [1]. These gratings are compatible with planar fabrication, require no subwavelength unit cell design, and are implementable using spatial light modulators, demonstrating their scalability. By the logical superposition of two binary forked gratings with distinct topological charges, we engineer moiré patterns that enable the generation of composite vortex beams in the zeroth order of the binary gratings [2]. The resulting beam shape is determined by the individual topological charges of the involved binary forked gratings. An intense non-vortex residual is typically present in the zeroth order of these moiré binary gratings. This intense non-vortex residual is mitigated by incorporating Mie resonances into the moiré binary gratings [1]. This is achieved through metallo-dielectric structuring and the use of crossed-polarization microscopy techniques. This approach also enables color-selective generation of composite vortex beams. The color-selective characteristic of metallo-dielectric moiré gratings enhances temporal coherence and optical contrast (low chromatic dispersion) under broadband illumination. The fabricated gratings are compact and thus help to maintain spatial coherence in the experimental setup. Together, these factors result in the generation of composite vortex beams suited for on-axis broadband use.

References: 1 A. Ringne et al., Sci. Rep., (2025). 2 A. Ringne et al., J. Opt. Soc. Am. B, (2024).