Traditional toplogical insulators (TIs) are a special phase of matter in which a material acts as an insulator in its bulk yet as a conductor on its boundary. More importantly, the conductive edge state is characterized by being unidirectional and immune to reflections from any surface imperfections or local disorders. These materials were originally discovered in the field of condensed-matter physics from studies of the Quantum Hall Effect (QHE) and have been attracting significant interest from the scientific community ever since. In recent years, several works have devised new approaches to extend TI concepts from fermionic systems to classical photonics and acoustics. The underlying principle in all these works is to break the symmetry at a degeneracy point in the momentum space of a finite array by either parametrically modulating the Hermitian potential of the constituent elements or by exploiting the protected phases arising from certain spatial symmetries. Parametric modulation has been shown to be a more effective approach since it guarantees the absence of reflected modes for all kinds of disorders while parity-protected topological states are only immune against certain imperfections.
Unlike QHE-based TIs, Floquet TIs do not require an external magnetic field nor do they rely on rare-earth materials which makes them very attractive for numerous applications. For example, multi-port microwave Floquet TIs can be used in wireless communications as a multi-element antenna interface in order to route the forward (transmitted) and reverse (received) signals into different directions thus enabling simultaneous transmission and reception on the same carrier frequency and doubling the spectral efficiency across multiple antennas. Floquet TIs can also be used in integrated photonics to miniaturize the size of waveguide bends and transitions while mitigating transmission loss that could otherwise arise from unwanted reflections. Despite all these benefits, so far Floquet TIs have been proposed across narrow bandwidths and are also very challenging to implement in practice since they require the accurate generation and distribution of phase-synchronized modulation signals across a large array. As a result, none of the Floquet TIs presented to-date have been experimentally validated nor has a realistic approach to do so been even proposed.
In collaboration with Prof. Alu’s group, we addressed these crucial problems and developed a new Floquet TI based on infinitesimal, reconfigurable meta-molecules which are implemented using ultra-broadband circulator circuit. We demonstrated a 4×4 lattice consisting of 16 inductively-coupled meta-molecules and on-chip multi-phase clock generation in a 45nm SOI CMOS process. This Floquet TI has a bandwidth from DC to GHz frequencies, hence for the first time thus opening the door for practical applications of TIs in classical microwave and photonic systems. Check our the related publications:
- Aravind Nagulu*, Xiang Ni*, Ahmed Kord*, Mykhailo Tymchenko, Andrea Alu and Harish Krishnaswamy, “Dispersion-Free Quasi-Electrostatic Chip-Scale Floquet Topological Insulator,” in review with Nature Electronics. * -equal contributors.
- Aravind Nagulu*, Ahmed Kord*, Xiang Ni*, Andrea Alu, and Harish Krishnaswamy, “Dispersion-Free Floquet Topological Insulator on a Chip Based on Magnetless, Ultra-Compact Quasi-Electrostatic Meta-Molecules,” International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials) , Sep. 2020. * – equal contributors