By virtue of a photonic Rashba-type spin splitting of a bound state in the continuum, this heterostructure enables a selective lateral confinement of the emerging photonic spin-valley states inside the core for high-Q resonances. To build these sources, a prerequisite is to lift the spin degeneracy between the 2 opposite spin states either in their electronic or photonic parts.
To accomplish high-Q spin-split states, the researchers built photonic spin lattices with different symmetry homes, which comprise an inversion-asymmetry core and inversion-symmetry cladding incorporated with a WS2 monolayer to create laterally confined spin-valley states. (2) A set of high-Q symmetry-enabled (quasi-) bound states in the continuum, that is, ± K (corners of the Brillouin zone) photonic spin-valley states, at the band edges of the spin-split branches. The 2 states form a coherent superposition state with equal amplitudes.
The spin-valley optical microcavity is built by interfacing an inversion-asymmetric (yellow core region) and an inversion-symmetric (cyan cladding area) photonic spin lattice. By virtue of a photonic Rashba-type spin splitting of a bound state in the continuum, this heterostructure enables a selective lateral confinement of the emergent photonic spin-valley states inside the core for high-Q resonances.
Researchers at the Technion– Israel Institute of Technology have unveiled a manageable and meaningful spin-optical laser based on a single atomic layer. This advancement is made it possible for by meaningful spin-dependent interactions between a single atomic layer and a laterally restricted photonic spin lattice, the latter of which supports high-Q spin-valley states through the photonic Rashba-type spin splitting of a bound state in the continuum.
Released in the prestigious journal Nature Materials and featured in the journals Research Briefing, the accomplishment paves the method to study coherent spin-dependent phenomena in both classical and quantum regimes. It opens new horizons in fundamental research and optoelectronic devices exploiting both electron and photon spins.
Research Team and Collaborations
The research study was conducted in the research group of Professor Erez Hasman, head of the Atomic-Scale Photonics Laboratory, in partnership with Professor Elad Koren, head of the Laboratory for Nanoscale Electronic Materials and Devices in the Department of Materials Science and Engineering, and Professor Ariel Ismach at Tel Aviv University. The 2 groups at the Technion are in association with the Helen Diller Quantum Center and Russel Berrie Nanotechnology Institute. Dr. Kexiu Rong conducted and led the research, and teamed up with Dr. Xiaoyang Duan, Dr. Bo Wang, Dr. Vladimir Kleiner, Dr. Assael Cohen, Dr. Pranab K. Mohapatra, Dr. Avinash Patsha, Dr. Subhrajit Mukherjee, Dror Reichenberg, Chieh-li Liu, and Vladi Gorovoy.
The Challenge of Spin Degeneracy
Can we raise the spin degeneracy of source of lights in the absence of electromagnetic fields at room temperature level? According to Dr. Rong, “Spin-optical lights combine electronic transitions and photonic modes and therefore supply a way to study the exchange of spin details between photons and electrons and to develop sophisticated optoelectronic devices. To build these sources, a prerequisite is to lift the spin degeneracy between the 2 opposite spin states either in their photonic or electronic parts.
This is normally achieved by using electromagnetic fields under a Faraday or Zeeman impact, although these approaches usually need strong electromagnetic fields and can not produce miniaturized sources. Another appealing way benefits from artificial magnetic fields for photonic spin-split states in momentum space, underpinned by a geometric phase system.
Sadly, previous observations of spin-split states have actually relied heavily on proliferation modes with poor quality aspects, which impose unwanted restrictions on spatial and temporal coherence of the sources. This technique is also impeded by the spin-controllable properties of a bulk laser gain material being not available or nontrivial to gain access to for active control of the sources, specifically in the lack of magnetic fields at space temperature.”
Achieving High-Q Spin-Split States
To achieve high-Q spin-split states, the researchers constructed photonic spin lattices with different symmetry properties, which comprise an inversion-asymmetry core and inversion-symmetry cladding integrated with a WS2 monolayer to create laterally confined spin-valley states. The important inversion-asymmetry lattice the scientists use has 2 crucial residential or commercial properties. (1) A controllable spin-dependent mutual lattice vector due to space-variant geometric stages from its making up inhomogeneous-anisotropic nanoholes.
This vector divides a spin-degenerate band into 2 spin-polarized branches in momentum area, which is referred to as the photonic Rashba result. (2) A pair of high-Q symmetry-enabled (quasi-) bound states in the continuum, that is, ± K (corners of the Brillouin zone) photonic spin-valley states, at the band edges of the spin-split branches. Additionally, the two states form a meaningful superposition state with equivalent amplitudes.
Professor Koren noted that, “We used a WS2 monolayer as the gain material since this direct-bandgap transition metal dichalcogenide has unique valley pseudospins, which have actually been commonly investigated as an alternative details provider in valleytronics. Specifically, their ± K valley excitons (radiated as in-plane spin-polarized dipole emitters) can be selectively excited by spin-polarized light according to a valley-contrasted choice guideline, hence allowing active control of spin-optical lights without magnetic fields.”
In the monolayer-integrated spin-valley microcavities, ± K valley excitons couple to ± K spin-valley states owing to polarization matching, and spin-optical excitonic lasing is accomplished at room temperature levels through strong optical feedback. Meanwhile, ± K valley excitons (at first without a stage connection) are driven by the lasing system to discover the minimum-loss state of the system, which leads them to re-establish a phase-locked connection according to the opposite geometric stages of ± K spin-valley states.
This lasing-mechanism-driven valley coherence removes the requirement for cryogenic temperatures to reduce the intervalley scattering. Furthermore, the minimum-loss state of the Rashba monolayer laser can be managed to be pleased (broken) via a linear (circular) pump polarization, which supplies a way to manage the lasing strength and spatial coherence.
Ramifications and Future Directions
” The unveiled photonic spin valley Rashba effect offers a basic mechanism to construct surface-emitting spin-optical source of lights. The demonstrated valley coherence in the monolayer-integrated spin– valley microcavity makes a step towards attaining entanglement in between ± K valley excitons for quantum information by methods of qubits,” discusses Professor Hasman.
” For a long period of time, our group has been dealing with developing spin optics to harness photonic spin as an efficient tool to control the behavior of electromagnetic waves. In 2018, we were attracted by valley pseudospins in two-dimensional materials, and therefore began a long-term task to study the active control of atomic-scale spin-optical light sources in the absence of electromagnetic fields.
We at first took on the difficulty of meaningful geometric phase pickup from individual valley excitons by utilizing a non-local Berry-phase problem mode.
The underlying meaningful addition of numerous valley excitons of the recognized Rashba monolayer light sources remained unsolved, owing to the lack of a strong integrating system between the excitons.
This issue motivated us to think of high-Q photonic Rashba modes. Following developments in new physical approaches, we attained the Rashba monolayer laser described here.”
Recommendation: “Spin-valley Rashba monolayer laser” by Kexiu Rong, Xiaoyang Duan, Bo Wang, Dror Reichenberg, Assael Cohen, Chieh-li Liu, Pranab K. Mohapatra, Avinash Patsha, Vladi Gorovoy, Subhrajit Mukherjee, Vladimir Kleiner, Ariel Ismach, Elad Koren and Erez Hasman, 6 July 2023, Nature Materials.DOI: 10.1038/ s41563-023-01603-3.
The research was supported by the Israel Science Foundation (ISF), the Helen Diller foundation, and the joint Technion NEVET grant by RBNI. The fabrication was performed at the Micro-Nano Fabrication & & Printing Unit (MNF&PU) of the Technion.