Controlling light polarization interaction with matter is crucial in optics, spanning fundamental science to technological applications. Metasurfaces, composed of resonant subwavelength structures, offer nanoscale light control, enabling applications like high-harmonic generation, ultrathin optical elements, and biosensing. Recent advancements include a shift from plasmonic to all-dielectric materials to reduce Ohmic losses and the utilization of photonic quasi-bound states in the continuum (qBICs) for radiative loss control. All-dielectric qBIC metasurfaces exhibit ultrasharp resonances, high Q factors, broad spectral tunability, and enhanced near-fields. Metasurfaces with broken in-plane inversion symmetry offer control over radiative lifetimes through geometric perturbations. However, most qBIC metasurfaces rely on modifying in-plane geometry due to fabrication challenges of varying resonator heights. This limits applications in holography, optical angular momentum (OAM) generation, chirality sensing, and chiral nanophotonics, which require non-planar structures for efficient interaction with complex polarization states. Chiral qBICs with broken in-plane mirror symmetry have been theoretically proposed to achieve maximum optical chirality, meaning selective interaction with one helicity and transparency to the other. While achieving maximum chirality for arbitrary incident directions is complex, focusing on specific incident directions simplifies the design. A key measure is the difference in co-polarized transmittances (ΔT), approaching ±1 for maximum chirality. Experimental implementations have been limited to microwaves or optical realizations with weak resonance modulation and small ΔT. This work experimentally demonstrates out-of-plane symmetry-broken qBIC metasurfaces in the visible spectrum using a novel multi-step fabrication process for arbitrary height control of resonators. The smallest height difference achieved is 10 nm, with potential for Angstrom-level control. This approach is applied to create height-driven qBIC resonances and maximally chiral qBIC metasurfaces that selectively couple to circularly polarized light.

The introduction extensively reviews the existing literature on metasurfaces, highlighting the transition from plasmonic to all-dielectric materials and the importance of photonic quasi-bound states in the continuum (qBICs) for controlling radiative losses. It discusses the advantages of using all-dielectric qBIC metasurfaces for achieving ultrasharp resonances, high Q factors, and enhanced near-fields. The limitations of existing qBIC designs, primarily their reliance on in-plane symmetry breaking, and the need for three-dimensional structures for applications in chiral nanophotonics are also emphasized. The literature review also touches upon theoretical proposals for chiral qBICs and the challenges associated with their experimental realization, particularly in the optical regime. The concept of maximum optical chirality and its quantification using the transmittance difference (ΔT) are explained, along with the limitations of existing experimental implementations.

The optical properties of qBIC metasurfaces are modeled using coupled-mode theory (CMT), describing light scattering as the interference of a direct background channel and a resonant channel. Coupling parameters (me) are calculated, proportional to the integral of the displacement current within the meta-unit volume. Power transmission coefficients are expressed in terms of these parameters and the eigenfrequency. In-plane symmetry breaking, achieved by varying rod length, and out-of-plane symmetry breaking, using rod height difference (Δh), are discussed. The coupling to the far-field is described, showing how Δh affects the dipole moments and their locations. Combining height-driven and in-plane symmetry breaking enables the realization of chiral qBIC metasurfaces. A meta-unit consisting of two rods with equal lengths, rotated in-plane by an angle θ and having different heights, breaks all point symmetries, creating a three-dimensional chiral arrangement. Coupling parameters for left circularly polarized (LCP) and right circularly polarized (RCP) waves are derived, showing how precise geometry tailoring can achieve efficient LCP coupling and RCP isolation. The transmittance difference (ΔT) is introduced as a measure of transmission chirality. Numerical simulations using COMSOL are performed to verify the height-driven qBIC engineering. For linearly polarized qBICs, simulations of transmittance spectra for various Δh values show sharp resonances and high Q factors. The Q factor's inverse quadratic dependence on Δh is observed, confirming the qBIC nature. Near-field simulations show high field enhancements. For chiral metasurfaces, simulations are performed for different opening angles (θ), optimizing the design for maximum chirality. The experimental realization employs a multi-step nanofabrication approach combining electron beam lithography and deposition processes to create metasurfaces with multiple height levels. The process starts with PECVD of an a-Si layer on a SiO2 substrate. Electron beam lithography, metal deposition, and lift-off create the first resonator layer. A second a-Si layer deposition and lithography steps are then performed to define the second, higher resonator, creating the different height levels.

The research demonstrates a novel multi-step nanofabrication process for creating all-dielectric metasurfaces with resonators of varying heights. This approach enables out-of-plane symmetry breaking in quasi-BIC metasurfaces, offering a new degree of freedom for controlling optical properties. Numerical simulations confirm the height-driven qBIC engineering, showing that the quality factor (Q) of the resonances follows an inverse quadratic dependence on the height difference (Δh), a hallmark of qBICs. High near-field enhancements (exceeding 50) are observed. The study further demonstrates the creation of maximally chiral qBIC metasurfaces by introducing both in-plane and out-of-plane symmetry breaking. The resulting metasurfaces exhibit a transmittance difference (ΔT) close to the theoretical maximum (±1), meaning they selectively transmit one circular polarization while blocking the other. The experimental results validate the numerical simulations, demonstrating the successful fabrication and optical characterization of the designed metasurfaces. The smallest experimentally achieved height difference is 10 nm, with the potential for Angstrom-level precision using techniques like atomic layer deposition (ALD).

The findings address the research question by successfully demonstrating a new approach to create three-dimensional all-dielectric metasurfaces exhibiting maximum optical chirality. The results significantly advance the field of chiral nanophotonics by overcoming the limitations of previous methods that relied on complex three-dimensional structures or achieved only weak chirality. The ability to control the resonance linewidth and chirality through out-of-plane symmetry breaking opens up new possibilities for designing advanced nanophotonic devices. The high near-field enhancements observed suggest potential applications in enhancing light-matter interactions for biosensing or catalysis. The successful demonstration of near-maximum chirality represents a substantial step towards developing efficient devices for generating optical angular momentum and realizing holographic metasurfaces. The scalability of the fabrication method suggests broader applications in other areas of nanophotonics.

This research introduces a novel nanofabrication method for creating all-dielectric metasurfaces with precise height control of individual resonators. This unlocks an additional degree of freedom for designing metasurfaces with tailored optical properties. The experimental demonstration of maximally chiral qBIC metasurfaces exhibiting near-maximum transmittance difference (ΔT) validates the theoretical predictions and opens new avenues for advanced nanophotonic devices. Future research could focus on further miniaturization of the structures, exploring even higher Q factors and sharper resonances. Investigating the use of other materials and exploring applications in biosensing and other areas are promising research directions.

The current fabrication method, while successful, may still present challenges for mass production and scalability. The achieved height control, while impressive at 10 nm, might still be a limiting factor in achieving even higher Q factors and more pronounced chiral effects. The study focuses on normal incidence; exploring the performance at oblique angles would be valuable. Further optimization of the fabrication process could be undertaken to minimize imperfections and improve the uniformity of the fabricated structures.