Helical resonant transport and purified amplification at an exceptional point

We propose an application of a parity-time symmetric non-Hermitian Su-Schrieffer-Heeger (SSH) model by embedding it in a two-dimensional square lattice tube. The coalescence state at the exceptional point of the non-Hermitian SSH model is chiral and selectively controls helical transport and amplification. Two typical helicity-dependent scattering dynamics are observed. If the incidence has an identical helicity with the embedded non-Hermitian SSH model, we observe a perfect transmission without reflection. However, if the incidence has an opposite helicity with the embedded non-Hermitian SSH model, except for a full transmission, we observe an amplified transmission with different helicity from the incidence; but the amplified reflection has identical helicity with the incidence. These intriguing features are completely unexpected in a Hermitian system. Moreover, the helical amplification at high efficiency can be triggered by an arbitrary excitation. The different dynamics between incidences with opposite helicities are the results of unidirectional tunneling, which is revealed to be capable of realizing without introducing magnetic field. We give a discussion about the helical dynamics under system imperfections. Our findings open a direction in an all-optical device and provide perspectives in non-Hermitian transport.