Chiral media exhibit optical activity, which manifests itself as differential retardation and attenuation of circularly polarized electromagnetic waves of opposite handedness This effect can be described by different refractive indices for left- and right-handed waves and yields a negative index in extreme cases Here we demonstrate active control of chirality, optical activity and refractive index These phenomena are observed in a terahertz (THz) metamaterial based on three-dimensionally (3D) chiral metallic resonators and achiral vanadium dioxide inclusions The chiral structure exhibits pronounced optical activity and a negative refractive index at room temperature when vanadium dioxide is in its insulating phase Upon heating, the insulator-to-metal phase transition of vanadium dioxide effectively renders the structure achiral, resulting in absence of optical activity and a positive refractive index The origin of the structure’s chiral response is traced to magnetic coupling between front and back of the structure, while the temperature-controlled chiral-to-achiral transition is found to correspond to a transition from magnetic to electric dipole excitations The use of a fourfold rotationally symmetric design avoids linear birefringence and dichroism, allowing such a structure to operate as tunable polarization rotator, adjustable linear polarization converter and switchable circular polarizer

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adfm.202010249

Temperature‐Controlled Optical Activity and Negative Refractive Index

Active control of strong chiroptical responses in metasurfaces can offer new opportunities for optical polarization engineering. Plasmonic active chiral metasurfaces have been investigated before, but their tunable chiroptical responses is limited due to inherent loss of plasmonic resonances, thus stimulating research in low loss active dielectric chiral metasurfaces. Among diverse tuning methods, electrically tunable dielectric chiral metasurfaces are promising thanks to their potential for on-chip integration. Here, we experimentally demonstrate nano-electromechanically tunable dielectric chiral metasurfaces with reflective circular dichroism (CD). We show a difference between absolute reflection under circulary polarized incident light with orthogonal polarization of over 0.85 in simulation and over 0.45 experimentally. The devices enable continuous control of CD by induced electrostatic forces from 0.45 to 0.01 with an electrical bias of 3V. This work highlights the potential of nano-electromechanically tunable metasurfaces for scalable optical polarization modulators.

NEMS-Tunable Dielectric Chiral Metasurfaces

Abstract. Diffractive optical elements (DOEs) are intricately designed devices with the purpose of manipulating light fields by precisely modifying their wavefronts. The concept of DOEs has its origins dating back to 1948 when D. Gabor first introduced holography. Subsequently, researchers introduced binary optical elements (BOEs), including computer-generated holograms (CGHs), as a distinct category within the realm of DOEs. This was the first revolution in optical devices. The next major breakthrough in light field manipulation occurred during the early 21st century, marked by the advent of metamaterials and metasurfaces. Metasurfaces are particularly appealing due to their ultra-thin, ultra-compact properties and their capacity to exert precise control over virtually every aspect of light fields, including amplitude, phase, polarization, wavelength/frequency, angular momentum, etc. The advancement of light field manipulation with micro/nano-structures has also enabled various applications in fields such as information acquisition, transmission, storage, processing, and display. In this review, we cover the fundamental science, cutting-edge technologies, and wide-ranging applications associated with micro/nano-scale optical devices for regulating light fields. We also delve into the prevailing challenges in the pursuit of developing viable technology for real-world applications. Furthermore, we offer insights into potential future research trends and directions within the realm of light field manipulation.

Diffractive optical elements 75 years on: from micro-optics to metasurfaces

Nanomechanical photonic metamaterials provide a wealth of active switching, nonlinear, and enhanced light-matter interaction functionalities by coupling optically and mechanically resonant subsystems. Thermal (Brownian) motion of the nanostructural components of such metamaterials leads to fluctuations in optical properties, which may manifest as noise, but which also present opportunity to characterize performance and thereby optimize design at the level of individual nanomechanical elements. We show that nanomechanical motion in an all-dielectric metamaterial ensemble of silicon-on-silicon-nitride nanowires can be controlled by light at sub-μW/μm2 intensities. Induced changes in nanowire temperature of just a few Kelvin and nonthermal optical forces generated within the structure change the few-MHz Eigenfrequencies and/or picometric displacement amplitudes of motion, and thereby metamaterial transmission. The tuning mechanism can provide active control of frequency response in photonic metadevices and may serve as a basis for bolometric, mass, and micro/nanostructural stress sensing.

Optical Control of Nanomechanical Brownian Motion Eigenfrequencies in Metamaterials

The chirality‐dependent forces can offer new possibilities for passive optical separation and identification of chiral particles, which opens great opportunities to develop the technologies of pharmaceutics, chemicals, and biomedicine. Here, a robust enantioselective rotation of subwavelength chiral particles using lateral optical forces induced by a tightly focused azimuthally polarized beam is demonstrated. Although this focused field carries neither optical orbital angular momentum nor optical chirality, the lateral optical force can rotate the particle with opposite chirality around opposite directions, achieving an effective optical enantioseparation. Such a counterintuitive phenomenon is closely related to the transformation of the magnetic spin angular momentum of the focused field into the mechanical orbital angular momentum of the particle. In addition, the particle with different chirality parameters will be trapped in different orbits while with opposite chirality trapped in the same orbit, meanwhile its rotation direction is determined by the sign of the chirality parameter, which can realize an efficient chiral identification of single particles. The investigations may open up a new path toward light‐induced rotation or probing of objects with different chirality parameters.

Enatioselective Rotation of Chiral Particles by Azimuthally Polarized Beams

Dataset supports the paper: Q. Zhang, E. Plum, J. Y. Ou, H. Pi, J. Li, K. F. MacDonald, and N. I. Zheludev (2021), Electrogyration in metamaterials: Chirality and polarization rotatory power that depend on applied electric field. Advanced Optical Materials 9(4), 2001826. 
https://doi.org/10.1002/adom.202001826

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/adom.202001826

Electrogyration in Metamaterials: Chirality and Polarization Rotatory Power that Depend on Applied Electric Field

Electrogyration is electric-field-dependent polarization state rotation in chiral media. We demonstrate a metamaterial with a quadratic electrogyration constant of 7.8 × 10−14 rad m2 V−2, which is a million times stronger than in natural materials.

/pdf/giant-electrogyration-in-a-nanomechanical-metamaterial-8r5r8u0alq.pdf

Giant Electrogyration in a Nanomechanical Metamaterial

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Electrogyration in Metamaterials: Chirality and Polarization Rotatory Power that Depend on Applied Electric Field

Giant Electrogyration in a Nanomechanical Metamaterial