Israel Builds Coherent, Controllable Atomic-scale Spin-optic Laser

Recently, researchers at the Technion-Israel Institute of Technology (TI) are pushing the limits of the field of spin-optic lasers by creating a coherent and controllable spin-optic laser based on a single atomic layer of tungsten disulfide (WS2).
Their advances in spin-optical lasers are opening up new horizons for fundamental research and optoelectronic devices by utilizing electron and photon spins.
Kexiu Rong, a postdoctoral scholar who led the research, said, "In 2018, we were fascinated by the valley pseudo-spins in two-dimensional (2D) materials, which are considered to be well known alternative carriers of information such as electron charge and spin, and they are being widely explored for the next-generation of valley electronic devices."
Collaboration was reached with a large group of colleagues from the Laboratory of Atomic Scale Photonics, the Laboratory of Nanoelectronic Materials and Devices at the Technion-Israel Institute of Technology (TI) and Tel Aviv University.
Valleytronics, in which electrons flow through the lattice of a two-dimensional semiconductor in the form of waves with two distinct electronic valleys, is a promising field for quantum computing. Excited electrons accumulate in one valley and acquire a valley index (-K′ or K′), which can be used to represent 1 or 0 to encode information.
Spin-optic laser
Spin-optical lasers combine photon modes (intrinsic modes of a cavity) and electron leaps (emission from a gain material), providing a way to explore the exchange of spin information between electrons and photons and to develop advanced optoelectronic devices.
"To obtain high-quality photonic spin-splitting modes, we constructed photonic spin lattices with different symmetry properties, which compromise an anti-asymmetric core and an antisymmetric cladding integrated with a WS2 monolayer to create transversely confined spin valley states," said Rong, adding that "both lattices are both photonic analogs of ordered electronic spin lattices modeled by anisotropic and non-uniform nanostructures."
Note: A spin-valley optical microcavity is a combination of an anti-asymmetric (yellow core region) and an anti-symmetric (green-blue cladding region) photonic spin lattice. Rashba-type spin splitting of acoustic states in a continuous medium can selectively and laterally confine the emergent photonic spin valley states of high-q resonances within the core. Coherently controllable spin-polarized lasers (red and blue beams) are produced by valley excitation in tungsten disulfide.
The basic inverse asymmetric lattice has two important properties: first, it has a controllable spin-dependent inverse easy lattice vector due to its composition of nonuniform anisotropic nanopores with spatially varying geometrical phases. This vector splits a spin-simple band into two spin-polarized branches in momentum space, i.e., the photonic Rashba effect.
Second, a pair of high-quality symmetric (quasi-) bound states, or ±K (Brillouin zone angle) photonic spin valley states, are obtained at the band edges of the spin-split branches. These two spin-split states are "bound states" in the continuum because they are highly confined in space due to the symmetry mismatch between their near and outward propagating fields. These two states form a coherent superposition of states of equal amplitude.
Prof. Elad Koren, head of the Nanoelectronic Materials and Development Laboratory, said, "To obtain spin-controllable electron leaps, we used a WS2 monolayer as a gain material, as this direct bandgap transition metal disulfide has a unique valley pseudo-spin, which has been widely investigated as an alternative information carrier for valley electrons."
A key takeaway from the team's work is that by focusing on the design of the photonic modes and the choice of the monolayer gain material, they have solved the challenge of removing spin simplicity from coherent light sources in the absence of a magnetic field and at room temperature.
Perhaps the most surprising aspect of the team's work is that it goes well beyond the fabrication of monolayer lasers, Hasman said, "Our lasing mechanism leads to the long-sought valley coherence within the WS2 monolayer, without cryogenics, and with the laser intensity and spatial coherence controlled by different pump polarizations."
The team's lasing mechanism drove the ±K valley excitons to find the system's minimum loss state, which allowed them to re-establish locking correlations based on the opposite geometrical phases of the ±K spin valley states.
"This laser mechanism-driven valley coherence eliminates the need for low-temperature suppression of inter-valley scattering," Hasman added, "In addition, the minimum loss state of the Rashba monolayer can be tuned to satisfy (disrupt) by linear (circular) pump polarization, which provides a means of controlling the laser intensity and spatial coherence provides a way to control the laser intensity and spatial coherence."