We experimentally demonstrate a robust Si-photonic waveguide architecture that realizes dynamically encircling an exceptional point (EP) in the optical domain and broadband asymmetric modal transmission as an essential consequence. The structure consists of a pair of coupled channel waveguides and an adjacent slab-waveguide patch that enable precise lithographic controls on the phase velocities and radiation rates of the guided photonic modes. Complex modal index and inter-mode coupling constant profiles required for the encircling-an-EP parametric control are precisely coded in the geometry of those elements. The device created on this basis induces the symmetry-exchanging adiabatic state flip for one transmission direction and symmetry-preserving anti-adiabatic state-jump for the transmission in the opposite direction. In fabrication, we use a state-of-the-art electron-beam lithography for creating mm-long devices with nm-scale transversal precision. A comprehensive spectral measurement for the intensity and phase distributions of the transmitted optical states is obtained with a specially designed phase-sensitive infrared microscopy integrated with a tunable diode-laser system and spectrum analyzer. On this basis, we confirm in the experiment the highly asymmetric modal transmission persisting over a broad spectral band exceeding 100 nm in the telecommunications window around 1,550 nm. Hence, we establish a substantive experimental step toward broadband non-reciprocal photonic devices based on the unique non-Hermitian dynamics.
We discuss asymmetric reflectance in surface plasmon Bragg gratings incorporating optical gain, referred to as
active asymmetric surface plasmon Bragg gratings. It is shown that balanced modulation of index and gain/loss with
quarter pitch spatial shift causes unidirectional coupling between contra-propagating modes in long-range surface
plasmon polariton Bragg gratings. Such gratings operate at the breaking threshold of parity-time symmetry
(exceptional point). Two active asymmetric surface plasmon Bragg gratings designs are proposed and their
performance is examined through modal and transfer matrix method computations.