Multiplication of the Orbital Angular Momentum

Schematics of multiplication and division of the orbital angular momentum of light with diffractive transformation optics. (Source: G. Ruffato et al.)

Optical beams carrying orbital angular momentum (OAM) have attracted a growing attention during the last decades, exhibiting disruptive appli­cations in a wide range of fields: particle trapping and tweezing, high-resolution micro­scopy, astro­nomical corona­graphy, high-capacity telecommunication and security. Light beams carrying OAM are endowed with peculiar twisted wavefronts, and modes with different OAM are orthogonal to each other and can carry inde­pendent information channels at the same frequency without any inter­ference. Then, in the telecom field, the potentially unbounded state space provided by this even-unex­ploited degree of freedom offers a promising solution to increase the infor­mation capacity of optical networks and solve in a sustainable way the impelling problem of frequency saturation, otherwise called as the ‘optical crunch’, this approach being valid both for free-space and optical fibre propa­gation.

Currently, it is urgent to further develop novel devices that can reconfigure and switch between distinct OAM modes to fully exploit the extra degree of freedom provided by the OAM both for classical and quantum communications. So far, conven­tional methods are useful for imple­menting only shift operations on the OAM, i.e., addition or sub­traction. For the first time, novel optical elements have been designed and fabri­cated to perform the multi­plication and division of the orbital angular momentum of light in a compact and efficient way. The study has been conducted by Gianluca Ruffato, Michele Massari, and Filippo Romanato at the Depart­ment of Physics and Astronomy of Padova University, in Italy.

The key element of these optics is represented by an optical trans­formation mapping the azimuthal phase gradient of the input OAM beam onto a circular sector. By combining multiple circular-sector transformations into a single optical element, it is possible to multiply the value of the input OAM state by splitting and mapping the phase onto comple­mentary circular sectors. Conversely, by combining multiple inverse trans­formations, the division of the initial OAM value is achievable by mapping distinct comple­mentary circular sectors of the input beam into an equal number of circular phase gradients.

The designed optical elements have been fabri­cated in the form of minia­turized and compact phase-only diffractive optics with high-resolution electron-beam litho­graphy, and optically charac­terized in the visible range to demonstrate the expected capability to either multiply or divide the OAM of the input beam. This study can find promising appli­cations for the multi­plicative generation of higher-order OAM modes, optical infor­mation processing based on OAM-beam trans­mission, and optical routing/­switching in telecom, both in the classical and single-photon regimes. (Source: CAS)

Reference: G. Ruffato et al.: Multiplication and division of the orbital angular momentum of light with diffractive transformation optics, Light: Sci. & Appl. 8, 113 (2019); DOI: 10.1038/s41377-019-0222-2

Link: LaNN – Laboratory for Nanofabrication of Nanodevices, EcamRicert, Padova, Italy

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