Kundan Chaudhary

Colloidal ribbons and rings from Janus magnetic rods

Dipolar particles are fundamental building blocks in nature and technology, yet the effect of particle anisotropy is seldom explored. Here, we fabricate colloidal silica rods coated with a hemicylindrical magnetic layer to satisfy multiple criteria: nearly monodisperse, easily imaged and magnetic interaction that dominates over gravity. We confirm long-predicted features of dipolar assembly and stress the microstructural variety brought about by shape and constituent anisotropy, especially by extrapolating knowledge learned from literal molecules. In this colloidal system, we describe analogies to liquid crystalline deformations with bend, splay and twist; an analogy to cis/trans isomerism in organic molecules, which in our system can be controllably and reversibly switched; and a field-switching methodology to direct single ribbons into not only single but also multiple rings that can subsequently undergo hierarchical self-assembly. We highlight subtle material issues of control and design rules for reconfigurable dipolar materials with building blocks of complex shape.

Anisotropic Colloidal Templating of 3D Ceramic, Semiconducting, Metallic, and Polymeric Architectures

3D-porous anisotropic solids are fabricated by using horizontally and vertically aligned assemblies of silica rods with a length of ca. 2 μm and a diameter of 500 nm as templates. Templated materials include examples from metals, semiconductors, ceramics, and polymers, Ni, Si, HfO2, and PMMA, respectively. By varying the infilling conditions, the detailed mesoscale structure and degree of anisotropy can be controlled.

High‐Operating‐Temperature Direct Ink Writing of Mesoscale Eutectic Architectures

High-operating-temperature direct ink writing (HOT-DIW) of mesoscale architectures that are composed of eutectic silver chloride-potassium chloride. The molten ink undergoes directional solidification upon printing on a cold substrate. The lamellar spacing of the printed features can be varied between approximately 100 nm and 2 µm, enabling the manipulation of light in the visible and infrared range.

Selective excitation and imaging of ultraslow phonon polaritons in thin hexagonal boron nitride crystals

We selectively excite and study two new types of phonon-polariton guided modes that are found in hexagonal boron nitride thin flakes on a gold substrate. Such modes show substantially improved confinement and a group velocity that is hundreds of times slower than the speed of light, thereby providing a new way to create slow light in the mid-infrared range with a simple structure that does not require nano-patterning. One mode is the fundamental mode in the first Restrahlen band of hexagonal boron nitride thin crystals on a gold substrate; the other mode is equivalent to the second mode of the second Restrahlen band of hexagonal boron nitride flakes that are suspended in vacuum.The new modes also couple efficiently with incident light at the hexagonal boron nitride edges, as we demonstrate experimentally using photo-induced force microscopy and scanning near-field optical microscopy. The high confinement of these modes allows for Purcell factors that are on the order of tens of thousands directly above boron nitride and a wide band, with new perspectives for enhanced light-matter interaction. Our findings demonstrate a new approach to engineering the dispersion of polaritons in 2D materials to improve confinement and light-matter interaction, thereby paving the way for new applications in mid-infrared nano-optics.2D materials: Ultraslow phonon polaritonsApplying flakes of hexagonal boron nitride (h-BN) to a gold-coated substrate provides a platform that can slow light in the mid-infrared range, according to US researchers. Antonio Ambrosio and coworkers from Harvard University in the USA transferred a 121 nm-thick flake of the 2D material h-BN to a silicon substrate coated with a 100 nm layer of gold. They discovered that the h-BN-gold material system supports two types of phonon polariton guided modes, one of which has a very large group refractive index in the mid-infrared (191 at a wavenumber of 1500 cm−1) indicating slow light propagation. Scanning Near-field Optical Microscopy (s-SNOM) and Photo-Induced Force Microscopy (PiFM) were used to characterize the modes. PiFM allowed unprecedented high-resolution imaging of these modes in the 795 and 1900 cm−1 spectral range.

Metasurfaces with wavelength-controlled functions

We demonstrate single-layer metasurfaces with controllable multi-wavelength functions. A multiwavelength achromatic metalens for red, yellow, green and blue light, and metasurfaces generating focused beams with different orbital angular momentum states are designed and fabricated.

Ultra-confined mid-infrared resonant phonon polaritons in van der Waals nanostructures

Resonant polaritonic nanoantennas allow manipulation of light and optical forces at the nanoscale for sensing applications. Hexagonal boron nitride has been proposed as an excellent candidate to achieve subwavelength infrared light manipulation owing to its polar lattice structure, enabling excitation of low-loss phonon polaritons with hyperbolic dispersion. We show that strongly subwavelength hexagonal boron nitride planar nanostructures can exhibit ultra-confined resonances and local field enhancement. We investigate strong light-matter interaction in these nanoscale structures via photo-induced force microscopy, scattering-type scanning near-field optical microscopy, and Fourier transform infrared spectroscopy, with excellent agreement with numerical simulations. We design optical nano-dipole antennas and directly image the fields when bright- or dark-mode resonances are excited. These modes are deep subwavelength, and strikingly, they can be supported by arbitrarily small structures. We believe that phonon polaritons in hexagonal boron nitride can play for infrared light a role similar to that of plasmons in noble metals at visible frequency, paving the way for a new class of efficient and highly miniaturized nanophotonic devices.