Inverse lattice design and its application to bent waveguides

TL;DR

This paper explores inverse spectral problems for tight-binding Hamiltonians, characterizes their solution space, and applies these findings to design bent waveguides with specific spectral properties, including Dirac and oscillator spectra.

Contribution

It introduces a new formula for characteristic polynomials, characterizes the solution space of inverse problems, and demonstrates practical applications in waveguide design.

Findings

Multiple solutions exist for inverse spectral problems.

A new method for calculating coupling constants is developed.

Designed waveguides reproduce specific spectra like Dirac and oscillator.

Abstract

This paper is divided in two parts. In the first part, the inverse spectral problem for tight-binding hamiltonians is studied. This problem is shown to have an infinite number of solutions for properly chosen energies. The space of such solutions is characterized by a hypersurface in the space of hopping amplitudes (i.e. couplings), whose dimension is half the number of sites in the array. Low dimensional examples for short chains are carefully studied and a table of exactly solvable inverse problems is provided in terms of Lie algebraic structures. With the aim of providing a method to generate lattice configurations, a set of equations for coupling constants in terms of energies is obtained; this is done by means of a new formula for the calculation of characteristic polynomials. Two examples with randomly generated spectra are studied numerically, leading to peaked distributions of couplings. In the second part of the paper, our results are applied to the design of bent waveguides, reproducing specific spectra below propagation threshold. As a demonstration, the Dirac and the finite oscillator are realized in this way. A few partially isospectral configurations are also presented.