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1. Angle- and polarization-resolved luminescence from suspended and hexagonal boron nitride encapsulated MoSe ₂ monolayers

   https://doi.org/10.1364/OPTICA.464533  

   Han, Bo ; Stephan, Sven ; Thompson, Joshua J. P. ; Esmann, Martin ; Antón-Solanas, Carlos ; Shan, Hangyong ; Kunte, Nils ; Brem, Samuel ; Tongay, Sefaattin ; Lienau, Christoph ; et al ( October 2022 , Optica)

   The polarized photoluminescence from atomically thin transition metal dichalcogenides is a frequently applied tool to scrutinize optical selection rules and valley physics, yet it is known to sensibly depend on a variety of internal and external material and sample properties. In this work, we apply combined angle- and polarization-resolved spectroscopy to explore the interplay of excitonic physics and phenomena arising from the commonly utilized encapsulation procedure on the optical properties of atomically thin${\mathrm{M}\mathrm{o}\mathrm{S}\mathrm{e}}_2$. We probe monolayers prepared in both suspended and encapsulated manners. We show that the hBN encapsulation significantly enhances the linear polarization of exciton photoluminescence emission at large emission angles. This degree of linear polarization of excitons can increase up to$\sim <\#comment/>17\mathrm{\%<\#comment/>}$in the hBN encapsulated samples. As we confirm by finite-difference time-domain simulations, it can be directly connected to the optical anisotropy of the hBN layers. In comparison, the linear polarization at finite exciton momenta is significantly reduced in a suspended${\mathrm{M}\mathrm{o}\mathrm{S}\mathrm{e}}_2$monolayer, and becomes notable only in cryogenic conditions. This phenomenon strongly suggests that the effect is rooted in the k-dependent anisotropic exchange coupling inherent in 2D excitons. Our results have strongmore »implications on further studies on valley contrasting selection rules and valley coherence phenomena using standard suspended and encapsulated samples.

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2. On-chip spin-orbit locking of quantum emitters in 2D materials for chiral emission

   https://doi.org/10.1364/OPTICA.463481  

   Ma, Yichen ; Zhao, Haoqi ; Liu, Na ; Gao, Zihe ; Mohajerani, Seyed Sepehr ; Xiao, Licheng ; Hone, James ; Feng, Liang ; Strauf, Stefan ( August 2022 , Optica)

   Light carries both spin angular momentum (SAM) and orbital angular momentum (OAM), which can be used as potential degrees of freedom for quantum information processing. Quantum emitters are ideal candidates towards on-chip control and manipulation of the full SAM–OAM state space. Here, we show coupling of a spin-polarized quantum emitter in a monolayer${\mathrm{W}\mathrm{S}\mathrm{e}}_2$with the whispering gallery mode of a${\mathrm{S}\mathrm{i}}_3{\mathrm{N}}_4$ring resonator. The cavity mode carries a transverse SAM of$\mathrm{\sigma <\#comment/>}=\pm <\#comment/>1$in the evanescent regions, with the sign depending on the orbital power flow direction of the light. By tailoring the cavity–emitter interaction, we couple the intrinsic spin state of the quantum emitter to the SAM and propagation direction of the cavity mode, which leads to spin–orbit locking and subsequent chiral single-photon emission. Furthermore, by engineering how light is scattered from the WGM, we create a high-order Bessel beam which opens up the possibility to generate optical vortex carrying OAM states.
3. Optical signatures of Dirac nodal lines in NbAs ₂

   https://doi.org/10.1073/pnas.1809631115  

   Shao, Yinming ; Sun, Zhiyuan ; Wang, Ying ; Xu, Chenchao ; Sankar, Raman ; Breindel, Alexander J. ; Cao, Chao ; Fogler, Michael M. ; Millis, Andrew J. ; Chou, Fangcheng ; et al ( January 2019 , Proceedings of the National Academy of Sciences)

   Using polarized optical and magneto-optical spectroscopy, we have demonstrated universal aspects of electrodynamics associated with Dirac nodal lines that are found in several classes of unconventional intermetallic compounds. We investigated anisotropic electrodynamics of${\mathrm{N}\mathrm{b}\mathrm{A}\mathrm{s}}_2$where the spin-orbit coupling (SOC) triggers energy gaps along the nodal lines. These gaps manifest as sharp steps in the optical conductivity spectra${\sigma }_1\left(\omega \right)$. This behavior is followed by the linear power-law scaling of${\sigma }_1\left(\omega \right)$at higher frequencies, consistent with our theoretical analysis for dispersive Dirac nodal lines. Magneto-optics data affirm the dominant role of nodal lines in the electrodynamics of${\mathrm{N}\mathrm{b}\mathrm{A}\mathrm{s}}_2$.
4. Optical analog of valley Hall effect of 2D excitons in hyperbolic metamaterial

   https://doi.org/10.1364/OPTICA.404063  

   Guddala, Sriram ; Khatoniar, Mandeep ; Yama, Nicholas ; Liu, Wenxiao ; Agarwal, Girish S. ; Menon, Vinod M. ( January 2021 , Optica)

   The robust spin and momentum valley locking of electrons in two-dimensional semiconductors makes the valley degree of freedom of great utility for functional optoelectronic devices. Owing to the difference in optical selection rules for the different valleys, these valley electrons can be addressed optically. The electrons and excitons in these materials exhibit the valley Hall effect, where the carriers from specific valleys are directed to different directions under electrical or thermal bias. Here we report the optical analog of valley Hall effect, where the light emission from the valley-polarized excitons in a monolayer${\mathrm{W}\mathrm{S}}_2$propagates in different directions owing to the preferential coupling of excitonic emission to the high momentum states of the hyperbolic metamaterial. The experimentally observed effects are corroborated with theoretical modeling of excitonic emission in the near field of hyperbolic media. The demonstration of the optical valley Hall effect using a bulk artificial photonic media without the need for nanostructuring opens the possibility of realizing valley-based excitonic circuits operating at room temperature.
5. Observation of Chiral Surface Excitons in a Topological Insulator Bi_2Se_3

   Kung, H ; Goyal, A ; Maslov, D ; Wang, X ; Lee, A ; Kemper, A ; Cheong, S ; Blumberg, G ( January 2019 , Proceedings of the National Academy of Sciences of the United States of America)

   We observe novel composite particles -- chiral excitons -- residing on the surface of a topological insulator (TI), Bi_2Se_3. Unlike other known excitons composed of massive quasiparticles, chiral excitons are the bound states of surface massless electrons and surface massive holes, both subject to strong spin-orbit coupling which locks their spins and momenta into chiral textures. Due to this unusual feature, chiral excitons emit circularly polarized secondary light (photoluminescence) that conserves the polarization of incident light. This means that the out-of-plane angular momentum of a chiral excitonis preserved against scattering events during thermalization, thus enabling optical orientation of carriers even at room temperature. The discovery of chiral excitons adds to the potential of TIs as a platform for photonics and optoelectronics devices.