Researchers at the Technion Israel Institute of Technology developed an atomic-scale, spin-optical laser. To do so, they incorporated a WS2 monolayer into a heterostructure microcavity that supported high-Q photonic spin-valley resonances. The spin-valley modes were generated from a photonic, Rashba-type spin splitting of a bound state in the continuum. The Rashba monolayer laser has intrinsic spin polarizations, high spatial and temporal coherence, and symmetry-enabled, robust features to enable valley coherence in the WS2 monolayer upon arbitrary pump polarizations at room temperature. It does not require magnetic fields or cryogenic temperatures. Professor Erez Hasman, head of the Atomic-Scale Photonics Laboratory at the Technion Israel Institute of Technology. Courtesy of Technion. The new laser could advance the development of coherent, spin-optical light sources for classical and nonclassical technologies and for optoelectronic devices that use both electron and photon spins. “Spin-optical light sources combine photonic modes and electronic transitions and therefore provide a way to study the exchange of spin information between electrons and photons and to develop advanced optoelectronic devices,” said Kexiu Rong, who led the research. “To construct these sources, a prerequisite is to lift the spin degeneracy between the two opposite spin states either in their photonic or electronic parts.” Professor Erez Hasman, head of the Atomics-Scale Photonics Laboratory, said that his team has been working to harness photonic spin as a tool to control electromagnetic waves for a long time. “In 2018, we were attracted by valley pseudospins in two-dimensional materials, and therefore began a long-term project to study the active control of atomic-scale, spin-optical light sources in the absence of magnetic fields,” he said. The researchers first tried using a nonlocal, Berry-phase defect mode to develop coherent geometric phase pickup from individual valley excitons. However, they could not resolve the coherent addition of multiple valley excitons of the realized Rashba monolayer light sources, due to the lack of a strong synchronizing mechanism between the excitons. “This issue inspired us to think about high-Q photonic Rashba modes,” Hasman said. “Following innovations in new physical approaches, we achieved the Rashba monolayer laser described here.” Some of the researchers involved in the research in the laboratory of professor Erez Hasman. Courtesy of Technion. The new laser is enabled by coherent, spin-dependent interactions between a single atomic layer and a laterally confined photonic spin lattice that supports high-Q spin-valley states. To achieve high-Q spin-split states, the researchers constructed photonic spin lattices with different symmetry properties. They built an inversion-asymmetry core and inversion-symmetry cladding integrated with a WS2 monolayer. The inversion-asymmetry lattice has two critical properties. The first is a controllable, spin-dependent, reciprocal lattice vector. The vector splits a spin-degenerate band into two spin-polarized branches in momentum space, in what is referred to as the photonic Rashba effect. The second property is a pair of high-Q, symmetry-enabled, quasi-bound states in the continuum — that is, ±K photonic spin-valley states at the band edges of the spin-split branches. Together, the two states form a coherent superposition state with equal amplitudes. “We used a WS2 monolayer as the gain material because this direct-bandgap transition metal dichalcogenide possesses unique valley pseudospins, which have been widely investigated as an alternative information carrier in valleytronics,” professor Elad Koren, head of the Laboratory for Nanoscale Electronic Materials and Devices, said. “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 selection rule, thus enabling active control of spin-optical light sources without magnetic fields.” In the monolayer-integrated spin-valley microcavities, ±K' valley excitons couple to ±K spin-valley states due to polarization matching. Spin-optical excitonic lasing is achieved at room temperature through strong optical feedback. Q resonances. Consequently, coherent, controllable, spin-polarized lasing (red and blue beams) is achieved from valley excitons in an incorporated WS2 monolayer (purple region). Courtesy of Scholardesigner co, LTD." style="float: left; margin-top: 7px; margin-right: 10px; margin-bottom: 7px;" /> Illustration of a spin-valley Rashba monolayer laser. The spin-valley optical microcavity is built by interfacing an inversion-asymmetric (yellow core region) and an inversion-symmetric (cyan cladding region) 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. Consequently, coherent, controllable, spin-polarized lasing (red and blue beams) is achieved from valley excitons in an incorporated WS2 monolayer (purple region). Courtesy of Scholardesigner Co. Ltd. Meanwhile, ±K' valley excitons, initially without a phase correlation, are driven by the lasing mechanism to find the minimum-loss state of the system. This leads the ±K' valley excitons to re-establish a phase-locked correlation according to the opposite geometric phases of ±K spin-valley states. Valley coherence, which is driven by the lasing mechanism, eliminates the need for cryogenic temperatures to suppress intervalley scattering. Additionally, the minimum-loss state of the Rashba monolayer laser can be regulated to be satisfied or broken via a linear or circular pump polarization. This provides a way to control the lasing intensity and spatial coherence. “The unveiled photonic spin-valley Rashba effect provides a general mechanism to construct surface-emitting spin-optical light sources,” Hasman said. “The demonstrated valley coherence in the monolayer-integrated, spin-valley microcavity makes a step toward achieving entanglement between ±K' valley excitons for quantum information by means of qubits.” The coherent, controllable spin-optical laser, based on a single atomic layer, could enable the study of coherent, spin-dependent phenomena in the classical and quantum regimes, opening new opportunities in research and for optoelectronic devices exploiting both electron and photon spins. The research was published in Nature Materials (www.doi.org/10.1038/s41563-023-01603-3).
