PHYSICAL REVIEW APPLIED 12, 064007 (2019) Editors’ Suggestion Featured in Physics Manipulation of Orbital-Angular-Momentum Spectrum Using Pinhole Plates Yuanjie Yang , 1,2, * Qi Zhao, 1 Linli Liu, 1 Yidong Liu, 1 Carmelo Rosales-Guzmán, 3 and Cheng-wei Qiu 2 1 School of Physics, University of Electronic Science and Technology of China, Chengdu 610054, China 2 Department of Electrical and Computer Engineering, National University of Singapore, 117576 Singapore 3 Wang Da-Heng Collaborative Innovation Center, Harbin University of Science and Technology, Harbin 150080, China (Received 7 June 2019; revised manuscript received 20 July 2019; published 4 December 2019) The orbital angular momentum (OAM) of light has attracted a great amount of interest in recent times, due to the wide variety of applications it has made possible. Generally, the OAM spectrum is produced by coaxial superposition of several vortex beams, which might increase the complexity of the system. Here, we propose and experimentally demonstrate a simple way to produce a state of light with a controllable OAM spectrum using a binary array of pinholes. More specifically, we show that a spiral structure can convert a plane wave into a beam with a wide OAM spectrum, which can be easily tuned to pure or multiple OAM modes by adapting the structure of the pinhole plate. Furthermore, we show that a simple pinhole plate can produce structured beams with particular OAM states, such as photonic gears (superposition of OAM modes with opposite topological charges ±ℓ) and OAM combs (an optical mode formed by a series of discrete and equally spaced OAM modes, akin to an optical frequency comb). It is worth noting that we demonstrate the OAM comb experimentally. This study provides an avenue for the flexible generation of OAM spectra and the simplicity of the setup could make this approach convenient for many applications, such as optical communications and quantum information. DOI: 10.1103/PhysRevApplied.12.064007 I. INTRODUCTION Vortices are inherent to any wave phenomena and denote singular (zero intensity) points in the plane perpen- dicular to the propagation axis where the phase becomes indeterminate; these occur in multiples of 2π [1,2]. In gen- eral, phase singularities can be expressed mathematically as an exponential function of the form exp(iℓθ ), where θ is the azimuthal index of the cylindrical coordinates and ℓ, known as the topological charge, is associated with an orbital angular momentum (OAM) of ℓ - h per photon, with - h denoting the reduced Plank constant [3,4]. The on-demand generation of optical fields embedded with OAM [5,6] has triggered a wide variety of applications [7,8], spanning fields as diverse as optical manipulation [9], classical and quantum communications [10–14], optical metrology [15– 18], and imaging [19], to mention but a few. Besides opti- cal vortices, recent studies have shown that electron [20], neutron [21], atom [22], plasmonic [23], and radio [24–27] vortices can carry OAM, leading to a wide range of appli- cations in fields such as the manipulation of nanoparticles, memory devices, or imaging [28,29]. * dr.yang2003@uestc.edu.cn Over the past 20 years, a wide variety of techniques to produce vortex beam have been elucidated. Besides classic methods, based on spiral phase plates [30] or computer-generated holograms [31], the recent introduc- tion alternative methods, namely, photon sieves [32,33] and Vogel spiral arrays [34,35], have paved the way towards the generation of OAM-carrying matter waves with electrons, atoms, or neutrons. However, most of these aforementioned methods are only able to produce modes with a unique OAM value or simple superposi- tions of the same. Crucially, OAM-based applications will greatly benefit from beams carrying a high-dimensional OAM spectrum [27,28], especially if this spectrum can be tailored. Recently, the applications of tailored OAM spec- tra have drawn much attention, for instance, to measure object parameters [36] or to perform geometric operations on images, such as rotations and reflections [37]. More recently, a quadratic phase plate has been proposed theo- retically and may expand the initial single OAM mode into a comblike OAM spectrum [38]. Here, we propose a technique based on the use of a struc- tured pinhole plate for generating structured beams with a particular OAM spectrum. We first simulate these struc- tures and then experimentally corroborate the quality of the generated beams. More specifically, we measure the 2331-7019/19/12(6)/064007(9) 064007-1 © 2019 American Physical Society