[Paper Review] Spin and valley polarized one-way Klein tunneling in photonic topological insulators

Xiang Ni, David Purtseladze+4 more|arXiv (Cornell University)|Jul 18, 2017

Topological Materials and Phenomena53 references3 citations

TL;DR

This paper proposes a photonic topological insulator platform with engineered synthetic gauge fields that induce spin- and valley-polarized one-way Klein tunneling. By creating competing gauge fields in pseudo-spin, chirality, and valley subspaces, the system opens a band gap at one valley while preserving Dirac-like dispersion at the other, enabling topologically protected, polarization-selective unidirectional photon transport with potential for on-chip optical isolation and multiplexing.

ABSTRACT

Advances of condensed matter physics in exploiting the spin degree of freedom of electrons led to the emergence of the field of spintronics, which envisions new and more efficient approaches to data transfer, computing, and storage [1-3]. These ideas have been inspiring analogous approaches in photonics, where the manipulation of an artificially engineered pseudo-spin degrees of freedom is enabled by synthetic gauge fields acting on light [4,5,6]. The ability to control these additional degrees of freedom can significantly expand the landscape of available optical responses, which may revolutionize optical computing and the basic means of controlling light in photonic devices across the entire electromagnetic spectrum. Here we demonstrate a new class of photonic systems, described by effective Hamiltonians in which competing synthetic gauge fields engineered in pseudo-spin, chirality/sublattice and valley subspaces result in band gap opening at one of the valleys, while the other valley exhibits Dirac-like conical dispersion. It is shown that such effective response has dramatic implications on photon transport, among which: (i) spin-polarized and valley-polarized one-way Klein tunneling and (ii) topological edge states that coexist within the Dirac continuum for opposite valley and spin polarizations. These phenomena offer new ways to control light in photonics, in particular for on-chip optical isolation, filtering and wave-division multiplexing by selective action on their pseudo-spin and valley degrees of freedom.

Motivation & Objective

• To develop a photonic system that mimics spintronic and valleytronic effects using synthetic gauge fields for light.
• To engineer competing gauge fields in pseudo-spin, chirality/sublattice, and valley subspaces to break time-reversal symmetry selectively.
• To achieve band gap opening at one valley while preserving Dirac-like conical dispersion at the other, enabling topological transport.
• To demonstrate polarization-selective one-way tunneling and coexisting topological edge states for opposite spin and valley states.
• To enable new functionalities in integrated photonics, such as optical isolation and wave-division multiplexing, via control of pseudo-spin and valley degrees of freedom.

Proposed method

• Engineer a photonic lattice with subwavelength unit cells to realize synthetic gauge fields in pseudo-spin and valley subspaces.
• Apply spatial modulation to break time-reversal symmetry and induce effective magnetic fluxes in the lattice, mimicking spin-orbit coupling.
• Design the system such that competing gauge fields lead to band gap opening at one valley (e.g., K) while preserving Dirac cones at the other (e.g., K’).
• Utilize effective Hamiltonians derived from tight-binding models to describe the system's band structure and topological properties.
• Analyze photon transport through the system using numerical simulations and analytical solutions to demonstrate unidirectional tunneling and edge states.
• Confirm topological robustness by examining transmission spectra under varying incident angles and polarization states.

Experimental results

Research questions

• RQ1 Can synthetic gauge fields in photonic lattices induce spin- and valley-polarized one-way tunneling analogous to electronic systems?
• RQ2 How do competing gauge fields in pseudo-spin, chirality, and valley subspaces affect band structure and topological protection?
• RQ3 Can topological edge states coexist with Dirac-like continua for opposite spin and valley polarizations in a single photonic system?
• RQ4 What is the role of band gap opening at one valley in enabling unidirectional transport with polarization selectivity?
• RQ5 To what extent can this platform support on-chip optical isolation and multiplexing through control of pseudo-spin and valley degrees of freedom?

Key findings

• The system exhibits a band gap at one valley (e.g., K) while maintaining Dirac-like conical dispersion at the other (e.g., K’), enabling topological protection.
• Spin-polarized and valley-polarized one-way Klein tunneling is achieved, with high transmission only for specific pseudo-spin and valley states.
• Topological edge states coexist with the Dirac continuum for opposite spin and valley polarizations, enabling simultaneous unidirectional transport channels.
• The system supports unidirectional photon transport with minimal backscattering, even under disorder, due to topological protection.
• Numerical simulations confirm selective transmission based on pseudo-spin and valley polarization, validating the design for optical isolation and multiplexing.
• The effective Hamiltonian model accurately predicts the observed band structure and transport phenomena, confirming the theoretical framework.