Sublattice Zak phases and passive exceptional points in thin-film lithium niobate waveguide trimers: gauge, symmetry and fabrication-tolerance design rules
Year: 2026
Type: preprint
Abstract: Waveguide arrays on thin-film lithium niobate (TFLN) are a natural platform for non-Hermitian topological photonics. We give quantitative design rules for an inversion-symmetric three-site (trimer) Su-Schrieffer-Heeger chain and a two-waveguide passive parity-time (PT) coupler. (i) The absolute values of sublattice Zak phases depend on the eigenvector gauge and on the unit-cell origin: shifting one sublattice by a cell changes its Zak phase by pi. Only differences within a fixed cell are gauge invariant, and bulk-boundary correspondence holds when the open chain is terminated at the same cell. (ii) In a trimer the central site is invariant under inversion; with a passive absorber on it the biorthogonal Zak phase stays exactly quantized for all tested losses up to 0.8 of the inter-cell coupling, whereas an absorber on an outer site makes it drift. (iii) Edge states tolerate +-6% random coupling disorder, but random on-site (propagation-constant) disorder dominates: it must stay below about 0.1 of the inter-cell coupling, i.e. about 1 nm of random width variation for a 49 cm^-1 coupling and 3 nm for 116 cm^-1. (iv) Finite-element mode solving reproduces a measured coupling length (41.4 vs 40 um) and, after validation, the measured graphene loss (147 vs 132 dB/cm). A graphene monolayer on the lossy arm places the passive-PT coupler at the exceptional point (EP) for a gap of 1.843 um (TE). The EP requires zero detuning and coupling equal to loss, hence two tuning knobs: the wavelength (+-15 nm compensates a +-20 nm gap error) and electro-optic nulling of the arm detuning (4.2 cm^-1 per nm of width mismatch). Code is openly available. Preprint (13 pages, 7 figures, 4 tables). Code and data: doi:10.5281/zenodo.24052931 (Apache-2.0). Computational study; no fabricated devices or measurements by the author.
Author: Babych, Maksym
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7225579835