Electromechanically Reconfigurable Optical Vortex Array

An optical vortex (OV), which is a topological texture of the electromagnetic field with the intensity singularity and spiral phase rotation, has been garnering significant interest due to the supported orbital angular momentum [1]. The generation and modulation of OVs are important for applications that utilize the orbital angular momentum of light, such as multiplexing and multi-valued logic systems that aim to increase data capacity [2]. To date, active pixel arrays, including spatial light modulators or digital micromirror devices, have been typically employed to create and control a reconfigurable array of OVs [3]. However, these systems suffer from the modulation speed limited by the intrinsic response time of the materials used and the requirement to control each pixel individually. Moreover, the size of the pixel unit imposes constraints on achieving a compact OV array and increasing the number of OVs. Recently, the gradient thickness optical cavity (GTOC) has been proposed as an innovative platform for creating and controlling an array of reconfigurable OVs, capable of exhibiting quasi-particle-like behaviors [4]. The GTOC platform projects the parametrized space defined by the thicknesses of the employed dielectric spacers onto the real space through thickness gradients. The GTOC enables to spontaneously generate OVs and actively change their positions by engineering the optical thickness of the interlayer separating the dielectric spacers.<br/>In this study, we demonstrate the creation and high-speed manipulation of a reconfigurable OV array using an electromechanical GTOC (EM-GTOC). Employing a piezo-actuator to rapidly control the thickness gradients of the dielectric spacers, the EM-GTOC can control the position and periodicity of the generated OV array with a speed exceeding tens of kHz. The EM-GTOC is a multi-layered structure that includes a glass layer combined with a piezo-actuator, an air spacer, an 8-nm-thick Ni layer, a polymer layer (Norland Optical Adhesive), and an Al bottom mirror. The thickness of the air spacer between the glass and Ni layers changes with position, and the piezo-actuator electromechanically controls the thickness gradient. Phase distributions measured by a holographic interferometer reveal that the period of the OV array varies depending on the voltage applied to the piezo-actuator. Furthermore, the OVs can actively move in one dimension over distances of approximately 1.92 times of the period of the OV array.<br/>The high-speed piezo-actuator allows the rapid reconfiguration of the OV array at the region of interest. We assessed the reconfiguration speed of the EM-GTOC employing the orbital angular momentum dependent diffraction by a fork-grating. We observed that the EM-GTOC encodes the orbital angular momentum into the reflected light at a speed of 10 kHz, equivalent to the typical speed limit of the spatial light modulator, or potentially even faster. We also demonstrated a reconfigurable OV array of which the OVs move in two-dimensional space. Another piezo-actuator combined with a Ni-coated cover glass, which replaces the Ni/polymer/Al structure, provides the second OV driving axis. By applying voltage to the two piezo-actuators connected to the cover glass and the slide glass, we successfully maneuvered the vortex and anti-vortex within a space of 1.33 period 2.19 period. We expect that the EM-GTOC platform, which can rapidly reconfigure an array of OVs, can find applications in orbital angular momentum-based multi-level logic networks. We also anticipate that the use of Micro-Electro-Mechanical Systems (MEMS) techniques could potentially control OV arrays at sub-GHz speeds.<br/><br/>[1] Y. Shen et al, <i>Light Sci. Appl</i>, (2019)<br/>[2] J. Wang et al, <i>Nat. Photon</i>, (2012)<br/>[3] S. Ngcobo et al, <i>Nat. Commun</i>, (2013)<br/>[4] D. Kim et al, <i>Nature</i>, (2022)