Researchers at the Technion-Israel Institute of Technology have developed coherently controlled spin optical lasers based on single atomic layers

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 coherent spin-dependent interactions between a single atomic layer and a laterally constrained photonic spin lattice, which supports a high-Q spin valley through Rashaba-type spin splitting of photons of bound states in the continuum.

Spin valley optical microcavities are constructed by interfacing photonic spin lattices with inversion asymmetry (yellow core region) and inversion symmetry (cyan cladding region).

The work, published in Nature Materials and highlighted in the journal's research brief, paves the way for the study of coherent spin-related phenomena in classical and quantum regimes, opening new ground for basic research and optoelectronic devices that utilize electron and photon spin.

In the absence of a magnetic field, can we remove the spin degeneracy of a light source at room temperature? According to Dr. Rong, "Spin optical sources combine photon patterns and electron transitions and thus provide a way to study the exchange of spin information between electrons and photons and to develop advanced optoelectronic devices."

In order to construct these sources, one premise is to remove the spin degeneracy between two opposite spin states in the photon or electron part. This is usually achieved by applying magnetic fields under Faraday or Zeeman effects, although these methods usually require strong magnetic fields and cannot produce miniature sources. Another promising approach uses an artificial magnetic field to generate photon spin-split states in momentum space, based on a geometric phase mechanism.

Unfortunately, previous observations of spin split states relied heavily on propagation modes with low mass factors, which imposed undesired limits on the spatial and temporal coherence of the source. This approach is also hampered by the spin-controlled properties of block-laser gain materials, which cannot or cannot easily be used for active control of the source, especially in the absence of a magnetic field at room temperature.

To achieve a high-Q spin-splitting state, the researchers constructed a photonic spin lattice with different symmetries, including a core with an inversion asymmetry and an inversion symmetry envelope integrated with a WS2 single layer to create a laterally restricted spin valley state. The intrinsically reversed asymmetric lattice used by the researchers has two important properties.

A controllable spin-dependent reciprocal lattice vector caused by the space-dependent geometric phase of an inhomogeneous anisotropic nanoppore. This vector splits the spin degradation band into two spin-polarized branches in momentum space, known as the photonic Rashba effect.

A pair of highly Q-symmetric (quasi-) bound states in a continuum, i.e., a ±K(Brillouin Angle) photon spin valley at the edge of a spin-splitting branch, form a coherent superposition of equal amplitude.

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 is widely studied in valley electronics as an alternative information carrier. Specifically, their ±K 'valley excitons (which radiate as planar spin-polarized dipole emitters) can be selectively excited by spin-polarized light according to valley contrast selection rules, allowing active control of spin optical sources in the absence of magnetic fields."

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 room temperature spin exciton laser is realized by strong light feedback. At the same time, the laser mechanism drives the initially phase-free ±K 'valley excitons to find the minimum loss state of the system and re-establish the phase-locking correlation according to the geometric phase opposite the ±K spin valley state.

This laser-driven valley coherence eliminates the need for low temperature suppression of intermittent scattering. In addition, the minimum loss state of the Rashba single-layer laser can be regulated 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 entanglement of quantum information between ±K' valley excitons via qubits."

Our team has long been developing spin optics to use photon spin as an effective tool for controlling the behavior of electromagnetic waves. In 2018, we were intrigued by the valley pseudo-spin in two-dimensional materials and so began a long-term project to investigate the active control of atomic-scale spin optical sources in the absence of magnetic fields. We initially solved the challenge of obtaining coherent geometric phases from a single valley exciton by using a non-local Berry phase defect pattern.

However, due to the lack of a strong synchronization mechanism between excitons, the fundamental coherent superposition of multiple valley excitons of the realized Rashuba single-layer light source remains unsolved. This problem inspires us to think about the high Q photon Rashuba model. After the innovation of new physical methods, we realized the Rashuba monolayer laser described in this paper.

This achievement paves the way for the study of coherent spin-related phenomena in the classical and quantum fields, opening new ground for basic research and optoelectronic devices that utilize electron and photon spin.

Source: Physicists Organization Network