Generation of optical orbital angular momentum light by an azimuthally-graded refractive index plate
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THE EUROPEAN PHYSICAL JOURNAL D
Regular Article
Generation of optical orbital angular momentum light by an azimuthally-graded refractive index plate Rafif Hamama and Ali J. Sabbah Department of Mathematics and Physics, Lebanese International University, Beirut, Lebanon Received 23 January 2020 / Received in final form 1 August 2020 / Accepted 2 September 2020 Published online 6 October 2020 c EDP Sciences / Societ`
a Italiana di Fisica / Springer-Verlag GmbH Germany, part of Springer Nature, 2020 Abstract. We propose and numerically demonstrate a method to generate orbital angular momentum (OAM) light by using an azimuthally graded index (AGI) plate of uniform subwavelength thickness. We illustrate, theoretically, through numerical simulations, how a plane polarized fundamental gaussian beam incident onto such an AGI plate acquires a nonzero orbital angular momentum, and exhibits an annular far-field intensity profile with a phase singularity line along its axis. We further confirm the OAM property of the output beam by calculations of the overlap integral between its far-field and the paraxial Laguerre Gaussian modes of nonzero OAM. We also highlight the advantages of the proposed AGI plate over existing methods to generate OAM light.
1 Introduction Given the promising potential of orbital angular momentum (OAM) light [1] to advance technology on many fronts, ranging from optical communication [2–5], to optical tweezers [6,7] and traps, to imaging [8,9] and sensing [10,11], etc, a significant effort has been put to develop simple, efficient, and cost-effective sources of OAM light. Since the cylindrically symmetrical Laguerre Gaussian (LG) beams Epl (ρ, ϕ, z), with nonzero positive integer p and any integer l value, carry an orbital angular momentum of l~ per photon, laser systems [12] directly emitting pure Laguerre-Gaussian beams have been initially developed as sources of OAM light (also called helical light). The rotor filter [13] was then introduced, and cylindrical lenses [14–16] were also used to convert high-order Hermite Gaussian (HG) modes into LG ones. Forked holograms [17,18] and spiral phase plates [19–22] (SPPs) were also introduced and demonstrated to produce optical vortices. More recently, new metamaterial approaches [23–25] have emerged in spite of the fabrication complexity involved in them. However, the most widely used OAM light generation method, that also proved to be the most flexible, is based on devices called spatial light modulators [26,27] (SLMs), which can be addressed by computer to modulate, via reflection or transmission, the amplitude and phase of the input light. While the direct generation of OAM light through lasers results in a pure OAM mode, and while cylindrical lenses and SLMs suffer from some astigmatism [28,29] in the output beam, SPPs were proposed as an astigmatism-free source of paraxial OAM light [30], introducing a phase of a
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(∆l)ϕ in the output beam, where ϕ is the azimuthal angle around the SPP’s axis, and ∆l is an integer given
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