A new world of optoelectronic devices

Researchers at the Technion-Israel Institute of Technology have developed a coherently controlled spin optical laser based on a single atomic layer. This discovery was made possible by a coherent spin-dependent interaction between a single atomic layer and a horizontally constrained photonic spin lattice, which supports a high-Q spin valley through Rashaba-type spin splitting of photons of bound states in the continuum.

The result, published in Nature Materials and highlighted in its research brief, paves the way for the study of coherent spin-related phenomena in classical and quantum systems, and opens new avenues for fundamental research and applications of electron and photon spin in optoelectronic devices. The spin optical source combines the photon mode with the electron transition, which provides a method for studying the spin information exchange between electrons and photons and developing advanced optoelectronic devices.

Spin valley optical microcavities are constructed by interfacing photonic spin lattices with inversion asymmetry (yellow core region) and inversion symmetry (cyan cladding region).
In order to build these sources, a prerequisite is to eliminate the spin degeneracy between two opposite spin states in the photon or electron part. This is usually achieved by applying a magnetic field under a Faraday or Zeeman effect, although these methods usually require a strong magnetic field and cannot produce a microsource. Another promising approach is based on a geometric camera system that uses an artificial magnetic field to generate spin-split states of photons in momentum space.
Unfortunately, previous observations of spin split states have relied heavily on low-mass factor propagation modes, which impose adverse constraints on the spatial and temporal coherence of sources. This approach is also hampered by the spin-controlled nature of blocky laser-gain materials, which cannot or cannot easily be used to actively control light sources, especially in the absence of magnetic fields at room temperature.
To achieve high-Q spin-splitting states, the researchers constructed photonic spin lattices with different symmetries, including a core with inversion asymmetry and an inversion symmetric envelope integrated with a WS2 single layer, to produce laterally constrained spin valleys. The basic inverse asymmetric lattice used by the researchers has two important properties.
The controllable spin-dependent reciprocal lattice vector caused by the geometric phase space variation of the heterogeneous anisotropic nanoporous composed of them. This vector splits the spin degradation band into two spin-polarized branches in momentum space, known as the photonic Rushberg effect.
A pair of high Q symmetric (quasi) bound states in the continuum, namely ±K(Brillouin band Angle) photon spin valleys at the edge of spin splitting branches, form a coherent superposition of equal amplitudes.
Professor Koren noted: "We used the WS2 monolides as the gain material because this direct band-gap transition metal disulfide has a unique valley pseudo-spin and has been extensively studied as an alternative information carrier in valley electrons. Specifically, their ±K 'valley excitons (which radiate in the form of planar spin-polarized dipole emitters) can be selectively excited by spin-polarized light according to valley comparison selection rules, thus actively controlling a magnetically free spin optical source.
In a single-layer integrated spin valley microcavity, the ±K 'valley excitons are coupled to the ±K spin valley state by polarization matching, and the spin exciton laser at room temperature is realized by strong light feedback. At the same time, the laser mechanism drives the initially phase-independent ±K 'valley excitons to find the minimum loss state of the system and re-establish the lock-in correlation based on the geometric phase opposite the ±K spin valley.
Valley coherence driven by this laser mechanism eliminates the need for low temperature suppression of intermittent scattering. In addition, the minimum loss state of the Rashba monolayer laser can be modulated by linear (circular) pump polarization, which provides a way to control laser intensity and spatial coherence."
Professor Hasman explains: "The revealed photonic spin valley Rashba effect provides a general mechanism for constructing surface-emitting spin optical sources. The valley coherence demonstrated in a single-layer integrated spin valley microcavity brings us one step closer to achieving quantum information entanglement between ±K 'valley excitons via qubits.
For a long time, our team has been developing spin optics, using photon spin as an effective tool for controlling the behavior of electromagnetic waves. In 2018, intrigued by the valley pseudo-spin in two-dimensional materials, we began a long-term project to investigate the active control of atomic-scale spin optical sources in the absence of magnetic fields. We use the non-local Berry phase defect model to solve the problem of obtaining coherent geometric phase from a single valley exciton.
However, due to the lack of a strong synchronization mechanism between excitons, the fundamental coherent superposition of multiple valley excitons in the Rashuba single-layer light source that has been achieved remains unsolved. This problem inspires us to think about the Rashuba model of high Q photons. After innovating new physical methods, we have implemented the Rashuba single-layer laser described in this paper."
This achievement paves the way for the study of coherent spin correlation phenomena in classical and quantum fields, and opens a new way for the basic research and use of spintronic and photonic optoelectronic devices.