Metafiber transforming arbitrarily structured light

Structured light has proven useful for numerous photonic applications. However, the current use of structured light in optical fiber science and technology is severely limited by mode mixing or by the lack of optical elements that can be integrated onto fiber end-faces for complex wavefront control, and hence generation of structured light is still handled outside the fiber via bulky optics in free space. We report a metafiber platform capable of creating arbitrarily structured light on the hybrid-order Poincar\'e sphere. Polymeric metasurfaces, with unleashed height degree of freedom and a greatly expanded 3D meta-atom library, were laser nanoprinted and interfaced with polarization-maintaining single-mode fibers. Multiple metasurfaces were interfaced on the fiber end-faces, transforming the fiber output into different structured-light fields, including cylindrical vector beams, circularly polarized vortex beams, and an arbitrary vector field. Our work provides a new paradigm for advancing optical fiber science and technology towards fiber-integrated light shaping, which may find important applications in fiber communications, fiber lasers and sensors, endoscopic imaging, fiber lithography, and lab-on-fiber technology

Permittivity-asymmetric quasi-bound states in the continuum

Broken symmetries lie at the heart of nontrivial physical phenomena. Breaking the in-plane geometrical symmetry of optical systems allows to access a set of electromagnetic states termed symmetry-protected quasi-bound states in the continuum (qBICs). Here we demonstrate, theoretically, numerically and experimentally, that such optical states can also be accessed in metasurfaces by breaking the in-plane symmetry in the permittivity of the comprising materials, showing a remarkable equivalence to their geometrically-asymmetric counterparts. However, while the physical size of atoms imposes a limit on the lowest achievable geometrical asymmetry, weak permittivity modulations due to carrier doping and electro-optical Pockels and Kerr effects, usually considered insignificant, open up the possibility of infinitesimal permittivity asymmetries for on-demand, and dynamically tuneable optical resonances of extremely high quality factors. We probe the excitation of permittivity-asymmetric qBICs (${\varepsilon}$-qBICs) using a prototype Si/TiO$_{2}$ metasurface, in which the asymmetry in the unit cell is provided by the refractive index contrast of the dissimilar materials, surpassing any unwanted asymmetries from nanofabrication defects or angular deviations of light from normal incidence. ${\varepsilon}$-qBICs can also be excited in 1D gratings, where quality-factor enhancement and tailored interference phenomena via the interplay of geometrical and permittivity asymmetries are numerically demonstrated. The emergence of ${\varepsilon}$-qBICs in systems with broken symmetries in their permittivity may enable to test time-energy uncertainties in quantum mechanics, and lead to a whole new class of low-footprint optical and optoelectronic devices, from arbitrarily narrow filters and topological sources, biosensing and ultrastrong light-matter interaction platforms, to tuneable optical switches.Comment: Manuscript and Supplementary Information, 27 pages, 4 Figures manuscript + 4 Supplementary Figure

Optically addressable spin defects coupled to bound states in the continuum metasurfaces

Van der Waals (vdW) materials, including hexagonal boron nitride (hBN), are layered crystalline solids with appealing properties for investigating light-matter interactions at the nanoscale. hBN has emerged as a versatile building block for nanophotonic structures, and the recent identification of native optically addressable spin defects has opened up exciting possibilities in quantum technologies. However, these defects exhibit relatively low quantum efficiencies and a broad emission spectrum, limiting potential applications. Optical metasurfaces present a novel approach to boost light emission efficiency, offering remarkable control over light-matter coupling at the sub-wavelength regime. Here, we propose and realise a monolithic scalable integration between intrinsic spin defects in hBN metasurfaces and high quality (Q) factor resonances leveraging quasi-bound states in the continuum (qBICs). Coupling between spin defect ensembles and qBIC resonances delivers a 25-fold increase in photoluminescence intensity, accompanied by spectral narrowing to below 4 nm linewidth facilitated by Q factors exceeding $10^2$. Our findings demonstrate a new class of spin based metasurfaces and pave the way towards vdW-based nanophotonic devices with enhanced efficiency and sensitivity for quantum applications in imaging, sensing, and light emission.Comment: 13 pages, 4 Figures + 4 Supplementary Figure

Confined Etching within 2D and 3D Colloidal Crystals for Tunable Nanostructured Templates: Local Environment Matters

We report the isotropic etching of 2D and 3D polystyrene (PS) nanosphere <i><i>hcp</i></i> arrays using a benchtop O₂ radio frequency plasma cleaner. Unexpectedly, this slow isotropic etching allows tuning of both particle diameter and shape. Due to a suppressed etching rate at the point of contact between the PS particles originating from their arrangement in 2D and 3D crystals, the spherical PS templates are converted into polyhedral structures with well-defined hexagonal cross sections in directions parallel and normal to the crystal <i>c</i>-axis. Additionally, we found that particles located at the edge (surface) of the <i><i>hcp</i></i> 2D (3D) crystals showed increased etch rates compared to those of the particles within the crystals. This indicates that 2D and 3D order affect how nanostructures chemically interact with their surroundings. This work also shows that the morphology of nanostructures periodically arranged in 2D and 3D supercrystals can be modified via gas-phase etching and programmed by the superlattice symmetry. To show the potential applications of this approach, we demonstrate the lithographic transfer of the PS template hexagonal cross section into Si substrates to generate Si nanowires with well-defined hexagonal cross sections using a combination of nanosphere lithography and metal-assisted chemical etching

Three-Dimensional Electrochemical Axial Lithography on Si Micro- and Nanowire Arrays

A templated electrochemical technique for patterning macroscopic arrays of single-crystalline Si micro- and nanowires with feature dimensions down to 5 nm is reported. This technique, termed three-dimensional electrochemical axial lithography (3DEAL), allows the design and parallel fabrication of hybrid silicon nanowire arrays decorated with complex metal nano-ring architectures in a flexible and modular approach. While conventional templated approaches are based on the direct replication of a template, our method can be used to perform high-resolution lithography on pre-existing nanostructures. This is made possible by the synthesis of a porous template with tunable dimensions that guides the deposition of well-defined metallic shells around the Si wires. The synthesis of a variety of ring architectures composed of different metals (Au, Ag, Fe, and Ni) with controlled sequence, height, and position along the wire is demonstrated for both straight and kinked wires. We observe a strong enhancement of the Raman signal for arrays of Si nanowires decorated with multiple gold rings due to the plasmonic hot spots created in these tailored architectures. The uniformity of the fabrication method is evidenced by a homogeneous increase in the Raman signal throughout the macroscopic sample. This demonstrates the reliability of the method for engineering plasmonic fields in three dimensions within Si wire arrays.P-28797(VLID)354731

Unlocking the out-of-plane dimension for photonic bound states in the continuum to achieve maximum optical chirality

Abstract The realization of lossless metasurfaces with true chirality crucially requires the fabrication of three-dimensional structures, constraining experimental feasibility and hampering practical implementations. Even though the three-dimensional assembly of metallic nanostructures has been demonstrated previously, the resulting plasmonic resonances suffer from high intrinsic and radiative losses. The concept of photonic bound states in the continuum (BICs) is instrumental for tailoring radiative losses in diverse geometries, especially when implemented using lossless dielectrics, but applications have so far been limited to planar structures. Here, we introduce a novel nanofabrication approach to unlock the height of individual resonators within all-dielectric metasurfaces as an accessible parameter for the efficient control of resonance features and nanophotonic functionalities. In particular, we realize out-of-plane symmetry breaking in quasi-BIC metasurfaces and leverage this design degree of freedom to demonstrate an optical all-dielectric quasi-BIC metasurface with maximum intrinsic chirality that responds selectively to light of a particular circular polarization depending on the structural handedness. Our experimental results not only open a new paradigm for all-dielectric BICs and chiral nanophotonics, but also promise advances in the realization of efficient generation of optical angular momentum, holographic metasurfaces, and parity-time symmetry-broken optical systems

Optically addressable spin defects coupled to bound states in the continuum metasurfaces

Abstract Van der Waals (vdW) materials, including hexagonal boron nitride (hBN), are layered crystalline solids with appealing properties for investigating light-matter interactions at the nanoscale. hBN has emerged as a versatile building block for nanophotonic structures, and the recent identification of native optically addressable spin defects has opened up exciting possibilities in quantum technologies. However, these defects exhibit relatively low quantum efficiencies and a broad emission spectrum, limiting potential applications. Optical metasurfaces present a novel approach to boost light emission efficiency, offering remarkable control over light-matter coupling at the sub-wavelength regime. Here, we propose and realise a monolithic scalable integration between intrinsic spin defects in hBN metasurfaces and high quality (Q) factor resonances, exceeding 102, leveraging quasi-bound states in the continuum (qBICs). Coupling between defect ensembles and qBIC resonances delivers a 25-fold increase in photoluminescence intensity, accompanied by spectral narrowing to below 4 nm linewidth and increased narrowband spin-readout efficiency. Our findings demonstrate a new class of metasurfaces for spin-defect-based technologies and pave the way towards vdW-based nanophotonic devices with enhanced efficiency and sensitivity for quantum applications in imaging, sensing, and light emission

Unraveling the Chirality Transfer from Circularly Polarized Light to Single Plasmonic Nanoparticles

Due to their broken symmetry, chiral plasmonic nanostructures have unique optical properties and numerous applications. However, there is still a lack of comprehension regarding how chirality transfer occurs between circularly polarized light (CPL) and these structures. Here, we thoroughly investigate the plasmon-assisted growth of chiral nanoparticles from achiral Au nanocubes (AuNCs) via CPL without the involvement of any chiral molecule stimulators. We identify the structural chirality of our synthesized chiral plasmonic nanostructures using circular differential scattering (CDS) spectroscopy, which is correlated with scanning electron microscopy imaging at both the single-particle and ensemble levels. Theoretical simulations, including hot-electron surface maps, reveal that the plasmon-induced chirality transfer is mediated by the asymmetric distribution of hot electrons on achiral AuNCs under CPL excitation. Furthermore, we shed light on how this plasmon-induced chirality transfer can also be utilized for chiral growth in bimetallic systems, such as Ag or Pd on AuNCs. The results presented here uncover fundamental aspects of chiral light-matter interaction and have implications for the future design and optimization of chiral sensors and chiral catalysis, among others