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Register a new userEvolution of vortices created by conical diffraction in biaxial crystals versus orbital angular momentum
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A Brenier
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Light states evolution versus their fractional orbital angular momentum (OAM) has been analyzed in the conical diffraction process occurring through biaxial crystals. Experimental results are provided by a non-degenerate cascade of KGd(WO$_2$)$_4$ and Bi$_2$ZnOB$_2$O$_6$ biaxial crystals. The continuous $0to 1to 2$ $hbar$/photon increasing of the fractional OAM in passing through integer values was operated with the help of the spin-orbit coupling in the Bi$_2$ZnOB$_2$O$_6$ crystal. The phase of the state light and its vortices were visualized by interference patterns with a reference beam. The evolution of the fractional OAM value is accompanied by a continuous evolution of pairs of vortices with opposite signs and linked by a $-pi/+pi$ discontinuous phase line. The phase pattern evolution around half-integer OAM is observed to be continuous. In other cases, the evolution can be interrupted by the breaking of a $-pi/+pi$ discontinuous phase line and a new pair of vortices with opposite charges is born.
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We reveal for the first time a direct relationship between the diffraction of optical beams and their carrying orbital angular momentum (OAM). We experimentally demonstrate a novel phenomenon that the anisotropic diffraction can be induced by the OAM, predicted by us [Opt. Express, textbf{26}, 8084 (2018)], via the propagations of the elliptic beams with the OAM in linearly and both-linearly-and-nonlinearly isotropic media, respectively. In the former case, when its carrying OAM equals the so-called critical OAM, the spiraling elliptic Gaussian beam (fundamental eigenmode) is observed in the free space, where only the eigenmode with cylindrical-symmetry is supposed to exist for the beam without the OAM. In the latter case, the spiraling elliptic soliton, predicted by Desyatnikov et al. [Phys. Rev. Lett, textbf{104}, 053902 (2010)], is observed to stably propagate in a cylindrical lead glass. The power-controllable rotation of such an elliptic beam is also experimentally demonstrated.
As an attractive degree of freedom in electromagnetic (EM) waves, the orbital angular momentum (OAM) enables infinite communication channels for both classical and quantum communications. The exploration of OAM generation inspires various designs involving spiral phase plates, antenna arrays, metasurfaces, and computer-generated holograms. In this work, we theoretically and experimentally demonstrate an approach to producing OAM carrying EM waves by a point defect in three-dimensional (3D) photonic crystals (PCs). Simultaneous excitation of two vibrational-defect states with an elaborately engineered phase retardation generates a rotational state carrying OAM. Through converting guided waves in a line defect to localized waves in a point defect and then to radiated vortex waves in free space, the lowest four OAM-mode emitters, i.e., OAM indices of 1, -1, 2, and -2, are successfully realized. This work offers a physical mechanism to generate OAM by PCs, especially when the OAM generation is to be integrated with other designs.
Single photons with orbital angular momentum (OAM) have attracted substantial attention from researchers. A single photon can carry infinite OAM values theoretically. Thus, OAM photon states have been widely used in quantum information and fundamental quantum mechanics. Although there have been many methods for sorting quantum states with different OAM values, the nondestructive and efficient sorter of high-dimensional OAM remains a fundamental challenge. Here, we propose a scalable OAM sorter which can categorize different OAM states simultaneously, meanwhile, preserving both OAM and spin angular momentum. Fundamental elements of the sorter are composed of symmetric multiport beam splitters (BSs) and Dove prisms with cascading structure, which in principle can be flexibly and effectively combined to sort arbitrarily high-dimensional OAM photons. The scalable structures proposed here greatly reduce the number of BSs required for sorting high-dimensional OAMstates. In view of the nondestructive and extensible features, the sorters can be used as fundamental devices not only for high-dimensional quantum information processing, but also for traditional optics.
In this work, an explicit formula is deduced for identifying the orbital angular moment (OAM) of vectorial vortex with space-variant state of polarization (SOP). Different to scalar vortex, the OAM of vectorial vortex can be attributed to two parts: the azimuthal gradient of Pancharatnam phase and the product of the azimuthal gradient of orientation angle of SOP and relevant solid angle on the Poincar{e} sphere. With our formula, a geometrical description for OAM of light beams can be achieved under the framework of the traditional Poincar{e} sphere. Numerical simulations for two types of vectorial vortices have been carried on to confirm our presented formula and demonstrate the geometrical description of OAM. Furthermore, the finding will pave the way for precise characterization of OAM charge of vectorial vortices.
Recently, photons have been observed to possess transverse orbital angular momentum (OAM); however, it is unclear as whether they can hold a transverse OAM higher than 1. Here, we theoretically and experimentally demonstrate that high-order spatiotemporal Bessel optical vortices (STBOVs) can stably carry transverse OAM even beyond $10^2$. Through the inverse design of the spiral phase, an STBOV of any order can be controllably generated using a 4f pulse shaper. In contrast to conventional longitudinal OAM, the vector direction of the transverse OAM can be distinguished by the unique time-symmetrical evolution of STBOVs. More interestingly, the stability of STBOVs improves with their increasing orders owing to enhanced space-time coupling, making these beams particularly suitable for the generation of ultra-high transverse OAM. Our work paves the way for further research and application of this unique OAM of photons.
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