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1. 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.

    

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2. Generation of extreme-ultraviolet beams with time-varying orbital angular momentum

   https://doi.org/10.1126/science.aaw9486  

   Rego, Laura ; Dorney, Kevin M. ; Brooks, Nathan J. ; Nguyen, Quynh L. ; Liao, Chen-Ting ; San Román, Julio ; Couch, David E. ; Liu, Allison ; Pisanty, Emilio ; Lewenstein, Maciej ; et al ( June 2019 , Science)

   Light fields carrying orbital angular momentum (OAM) provide powerful capabilities for applications in optical communications, microscopy, quantum optics, and microparticle manipulation. We introduce a property of light beams, manifested as a temporal OAM variation along a pulse: the self-torque of light. Although self-torque is found in diverse physical systems (i.e., electrodynamics and general relativity), it was not realized that light could possess such a property. We demonstrate that extreme-ultraviolet self-torqued beams arise in high-harmonic generation driven by time-delayed pulses with different OAM. We monitor the self-torque of extreme-ultraviolet beams through their azimuthal frequency chirp. This class of dynamic-OAM beams provides the ability for controlling magnetic, topological, and quantum excitations and for manipulating molecules and nanostructures on their natural time and length scales. 

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3. Demonstration of using two aperture pairs combined with multiple-mode receivers and MIMO signal processing for enhanced tolerance to turbulence and misalignment in a 10  Gbit/s QPSK FSO link

   https://doi.org/10.1364/OL.391120  

   Song, Haoqian ; Li, Long ; Pang, Kai ; Zhang, Runzhou ; Zou, Kaiheng ; Zhao, Zhe ; Du, Jing ; Song, Hao ; Liu, Cong ; Cao, Yinwen ; et al ( May 2020 , Optics Letters)

   We utilize aperture diversity combined with multiple-mode receivers and multiple-input-multiple-output (MIMO) digital signal processing (DSP) to demonstrate enhanced tolerance to atmospheric turbulence and spatial misalignment in a 10 Gbit/s quadrature-phase-shift-keyed (QPSK) free-space optical (FSO) link. Turbulence and misalignment could cause power coupling from the fundamental Gaussian mode into higher-order modes. Therefore, we detect power from multiple modes and use MIMO DSP to enhance the recovery of the original data. In our approach, (a) each of multiple transmitter apertures transmits a single fundamental Gaussian beam carrying the same data stream, (b) each of multiple receiver apertures detects the signals that are coupled from the fundamental Gaussian beams to multiple orbital angular momentum (OAM) modes, and (c) MIMO DSP is used to recover the data over multiple modes and receivers. Our simulation shows that the outage probability could be reduced from$><\#comment/>0.1$to$<<\#comment/>0.01$. Moreover, we experimentally demonstrate the scheme by transmitting two fundamental Gaussian beams carrying the same data stream and recovering the signals on OAM modes 0 and$+1$at each receiver aperture. We measure an up to$\sim <\#comment/>10\mathrm{d}\mathrm{B}$power-penalty reduction for a bit error rate (BER) at the 7% forward error correction limit for a 10 Gbit/s QPSK signal.

    

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4. Photonic Integrated Circuit for Rapidly Tunable Orbital Angular Momentum Generation Using Sb 2 Se 3 Ultra‐Low‐Loss Phase Change Material

   https://doi.org/10.1002/adom.202200098  

   Alam, Md Shah ; Gnawali, Rudra ; Hendrickson, Joshua R. ; Beamer, Diane ; Payne, Tamara E. ; Volk, Andrew ; Agha, Imad ( July 2022 , Advanced Optical Materials)

   The generation of rapidly tunable optical vortex (OV) beams is one of the most demanding research areas of the present era as they possess orbital angular momentum (OAM) with additional degrees of freedom that can be exploited to enhance signal‐carrying capacity by using mode division multiplexing and information encoding in optical communication. Particularly, rapidly tunable OAM devices at a fixed wavelength in the telecom band stir extensive interest among researchers for both classical and quantum applications. This article demonstrates the realistic design of a Si‐integrated photonic device for rapidly tunable OAM wave generation at a 1550‐nm wavelength by using an ultra‐low‐loss phase change material (PCM) embedded with a Si‐ring resonator with angular gratings. Different OAM modes are achieved by tuning the effective refractive index using rapid electrical switching of Sb₂Se₃ film from amorphous to crystalline states and vice versa. The generation of OAM waves relies on a traveling wave modulation of the refractive index of the micro‐ring, which breaks the degeneracy of oppositely oriented whispering gallery modes. The proposed device is capable of producing rapidly tunable OV beams, carrying different OAM modes by using electrically controllable switching of ultra‐low‐loss PCM Sb₂Se₃.

    

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5. Second-harmonic generation of spatiotemporal optical vortices and conservation of orbital angular momentum

   https://doi.org/10.1364/OPTICA.422743  

   Hancock, S. W. ; Zahedpour, S. ; Milchberg, H. M. ( April 2021 , Optica)

   A spatiotemporal optical vortex (STOV) is an intrinsic optical orbital angular momentum (OAM) structure in which the OAM vector is orthogonal to the propagation direction [Optica6,1547(2019)OPTIC82334-253610.1364/OPTICA.6.001547] and the optical phase circulates in space-time. Here, we experimentally and theoretically demonstrate the generation of the second harmonic of a STOV-carrying pulse along with the conservation of STOV-based OAM. Our experiments verify that photons can have intrinsic orbital angular momentum perpendicular to their propagation direction.

    

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