Non-Hermitian optics

Non-Hermitian and topological physics

Loss is usually regarded as a undesirable effect to make many wave propagation phenomena diminish, such as negative refraction. However, the reality is far from this simple picture. A system with loss contrast or balanced gain-loss supports exceptional points in a Hamiltonian picture. It allows phase transition of optical properties. This is expected as a basis for a generation of sensitive optical components to be designed. We are interested in the case of metamaterials where we have magnetic responses as well as electric ones.

Non-Hermitian topological coupler
While a topological coupler confines the direction of edge states on a 4-port device based on toplogical argument. Gain and los add more control on the branching ratio between the ports: Science China Physcs, Mechanics & Astronomy 65, 224611 (2022)

Non-adiabatic transition near exceptional point
A cold-atom system with loss can be described by non-Hermitian Hamiltonian with the spin-orbit-coupled gap closs by loss. A loop in two dimensional phase space traveling near an exceptional point triggers non-adiabatic transition: Nature Phys. 18, 385 (2022)

Topological corner mode
Based on a phononic crystal, a Dirac vortice approach is used to generate topological corner mode at corners with arbitrary shape: Phys. Rev. Lett. 126, 226802 (2021)

Exceptional point at plasmonic frequencies
A Q-plate fabricated by putting geometric-phase elements with exceptional point at the Jones matrix of each element. Exceptional point effect is revealed in the diffraction far-field pattern.
Exceptional point-based plasmonic metasurfaces for vortex beam generation: Opt. Exp. 28, 503 (2020)

Exceptional point in nonlinear optics
Laser and nonlinear optical process can be viewed as non-Hermitian systems. Exceptional points take parts in the nonlinear dynamics, including
Pulsed lasing of Fano laser: Phys. Rev. A 100, 053808 (2019)
Four-wave mixing: Phys. Rev. Lett. 123, 193604 (2019)

Unidirectional zero reflection as gauged Parity-time symmetry
PT-symmetry can be gauged by a Unitary transformation. It corresponds to a familary of equivalent PT-symmetric systems from a given seed system with conventional PT-symmetry.
New J. Phys. 19, 123041 (2017)
A particular gauge can be taken so that eps(x) = mu(-x)* is regarded as PT-symmetric.
Phys. Rev. A 91, 033825 (2015)
Laser-absorber can be designed by detailed matching of permittivity and permeability.
New J. Phys. 18, 085004 (2016)
Acoustic analogy
Phys. Rev. B 98, 201102 (2018)

PT-phase transition
Hamiltonian of a pair of bright and dark atom in the intermediate coupling regime is found to have an effective PT-symmetric Hamiltonian. Phase transition of coherent perfect absorption is observed.
Theory: Phys. Rev. A 87, 053824 (2013)
Experiment: Phys. Rev. Lett. 112, 143903 (2014)

Controlling scattering
A gain/loss pair of metasurfaces can be used to yield perfect transmission across an impurity in an epsilon-near-zero medium, as an exceptional-point phenomenon.
Phys. Rev. X 8, 031035 (2018)

Multiple one-way edge states from homogeneous medium
High gap Chern number is usually generated at photonic band gaps at high frequency. Can we geneat lower frequency where effective medium is valid? We show examples in constructing a maximum of 4 one-way edge states in the effective medium regime.
Phys. Rev. Appl. 12, 064014 (2020).