Extreme Huygens' metasurfaces based on quasi-bound states in the continuum

We introduce the concept and a generic approach to realize Extreme Huygens' Metasurfaces by bridging the concepts of Huygens' conditions and optical bound states in the continuum. This novel paradigm allows creating Huygens' metasurfaces whose quality factors can be tuned over orders of magnitudes, generating extremely dispersive phase modulation. We validate this concept with a proof-of-concept experiment at the near-infrared wavelengths, demonstrating all-dielectric Huygens' metasurfaces with different quality factors. Our study points out a practical route for controlling the radiative decay rate while maintaining the Huygens' condition, complementing existing Huygens' metasurfaces whose bandwidths are relatively broad and complicated to tune. This novel feature can provide new insight for various applications, including optical sensing, dispersion engineering and pulse-shaping, tunable metasurfaces, metadevices with high spectral selectivity, and nonlinear meta-optics

Nonlinear dynamics in chiral torsional metamaterials

The advent and rapid development of metamaterials introduced many revolutionary concepts for manipulating electromagnetic waves. As an important class of metamaterials, chiral metamaterials allow us to control the polarization of electromagnetic waves at the subwavelength scale. While much work has been done on using chiral metamaterials to control electromagnetic waves, the accompanying effects, such as the electromagnetic force and torque acting on the structures, as well as nonlinear optomechanical effects, are still largely unexplored. The exploration of these areas could provide useful insight from both fundamental and practical points of view. In this thesis, we study new properties of chiral metamaterials, in particular the optomechanical properties and nonlinear effects that arise from the coupling between electromagnetic and elastic degrees of freedom. An accurate and efficient model based on the free-space Green’s function under the eigenmode approximation is developed for the study. In Chapter 1, we provide a comprehensive introduction to the basic concepts and history of metamaterials, followed by more focused reviews on chiral metamaterials, different paradigms of tunable metamaterials, the nontrivial electromagnetic force and torque, as well as the nonlinear optomechanical effect in different platforms. Finally, the motivation and the scope of the thesis are summarized. To understand the optical activity in coupled structures, in Chapter 2, we employ the model developed to study the near-field coupling, far-field scattering and optical activity of chiral meta-molecules based on twisted coupled cut-wire pairs. The numerical results from our model agree well quantitatively with full-wave calculation. We also discuss the optimum twist angle of the structure. After exploring the optical activity, in Chapter 3, we study the optomechanical properties of chiral meta-molecules based on a pair of twisted split-ring resonators. This structure can provide a strong and tunable torque, and can support different optomechanical dynamics, making it a good candidate for subwavelength light-driven actuators. To achieve strong coupling between electromagnetic resonance and elastic deformation in metamaterials, in Chapter 4, we introduce chiral torsional meta-molecules based on twisted split-ring pairs. We predict a rich range of nonlinear stationary effects including self-tuning and bistability. Importantly, these nonlinear effects including bistability are successfully observed in experiment. After understanding the nonlinear stationary responses of torsional meta-molecules, in Chapter 5, we study their nontrivial nonlinear dynamic effects. We introduce a simple structure based on three connected split-rings and find that this structure can support novel nonlinear dynamics such as chaos, damping-immune self-oscillations and dynamic nonlinear optical activity. To understand how intermolecular interaction can change system dynamics, in Chapter 6, we study the nonlinear effects of ensembles of enantiomeric torsional meta-molecules. We find that spontaneous chiral symmetry breaking can exist due to intermolecular interaction. For the first time in metamaterials, both spontaneous chiral symmetry breaking and self-oscillations are successfully demonstrated experimentally. Our study provides a new route to achieve artificial phase transitions in metamaterials without using naturally occurring phase change materials. In Chapter 7, we summarize the work and discuss the future possible topics in related to the optomechanical effects in metamaterials

Time-varying Huygens' meta-devices for parametric waves

Huygens' metasurfaces have demonstrated almost arbitrary control over the shape of a scattered beam, however, its spatial profile is typically fixed at fabrication time. Dynamic reconfiguration of this beam profile with tunable elements remains challenging, due to the need to maintain the Huygens' condition across the tuning range. In this work, we experimentally demonstrate that a time-varying metadevice which performs frequency conversion can steer transmitted or reflected beams in an almost arbitrary manner, with fully dynamic control. Our time-varying Huygens' metadevice is made of both electric and magnetic meta-atoms with independently controlled modulation, and the phase of this modulation is imprinted on the scattered parametric waves, controlling their shapes and directions. We develop a theory which shows how the scattering directionality, phase and conversion efficiency of sidebands can be manipulated almost arbitrarily. We demonstrate novel effects including all-angle beam steering and frequency-multiplexed functionalities at microwave frequencies around 4 GHz, using varactor diodes as tunable elements. We believe that the concept can be extended to other frequency bands, enabling metasurfaces with arbitrary phase pattern that can be dynamically tuned over the complete 2\pi range

Chiral meta-atoms rotated by light

We study the opto-mechanical properties of coupled chiral meta-atoms based on a pair of twisted split-ring resonators. By using a simple analytical model in conjunction with the Maxwell stress tensor, we capture insight into the mechanism and find that this structure can be used as a general prototype of subwavelength light-driven actuators over a wide range of frequencies. This coupled structure can provide a strong and tunable torque, and can support different opto-mechanical modes, including uniform rotation, periodically variable rotation and damped oscillations. Our results suggest that chiral meta-atoms are good candidates for creating sub-wavelength motors or wrenches controlled by light.This work is supported by the Australian Research Council

Optical activity and coupling in twisted dimer meta-atoms

We analyse the optical activity in twisted dimers, the meta-atoms of a chiralmetamaterial, by introducing a simple yet accurate model for the coupling between them. The near-field interaction coefficients are derived from a Lagrangian model and include the effects of retardation, whereas the far-field radiation is based on a multipole expansion. We show that the optimum twist angle varies with frequency, and near resonance is substantially lower than 45 degrees, which is the lowest symmetry configuration. Our approach is accurate over a wide frequency range, including the resonant regions with the highest optical activity. In contrast to other models of near-field interaction, it requires no fitted parameters or homogenization procedure and is directly applicable to a wide variety of resonant particles.This work is supported by the Australian Research Council

Self-oscillations in nonlinear torsional metamaterials

We study the nonlinear dynamics of torsional meta-molecules - sub-wavelength resonators with strong coupling between electromagnetic excitation and rotational deformation - and show that such structures may undergo self-oscillations. We develop a semi-an

Electromagnetic tuning of resonant transmission in magnetoelastic metamaterials

We demonstrate an analogue of electromagnetically-induced transparency (EIT) in a magnetoelastic metamaterial system and experimentally realize nonlinear electromagnetic tuning of this EIT-like transmission. We study a single meta-molecule, consisting o