Observation of quincunx-shaped and dipole-like flatband states in photonic rhombic lattices without band-touching

TL;DR

This paper experimentally demonstrates the existence and manipulation of compact localized states in photonic rhombic lattices using external gradients, revealing new flatband phenomena and oscillation behaviors.

Contribution

It introduces a method to control flatband states in photonic lattices through external gradients, showing how to preserve or modify flatband states and induce Bloch-like oscillations.

Findings

Flatband states can be preserved or modified using external gradients.

A quincunx-shaped CLS spans two unit cells under certain conditions.

Superposition of CLS leads to Bloch-like oscillations in momentum space.

Abstract

We demonstrate experimentally the existence of compact localized states (CLSs) in a quasi-one-dimensional photonic rhombic lattice in presence of two distinct refractive-index gradients (i.e., a driven lattice ribbon) acting as external direct current (dc) electric fields. Such a lattice is composed of an array of periodically arranged evanescently coupled waveguides, which hosts a perfect flatband that touches both remaining dispersive bands when it is not driven. The external driving is realized by modulating the relative writing beam intensity of adjacent waveguides. We find that a y-gradient set perpendicularly to the ribbon preserves the flatband while removing the band-touching. The undriven CLS - which occupies two lattices sites over one unit cell - turns into a quincunx-shaped CLS spanned over two unit cells. Instead, an x-gradient acting parallel to the ribbon yields a Stark ladder of CLS whose spatial profile is unchanged with respect to the undriven case. We notably find that their superposition leads to Bloch-like oscillations in momentum space.