Biopolymer photonics: from nature to nanotechnology

Biopolymers offer vast potential for renewable and sustainable devices. While nature mastered the use of biopolymers to create highly complex 3D structures and optimized their photonic response, artificially created structures still lack nature's diversity. To bridge this gap between natural and engineered biophotonic structures, fundamental questions such as the natural formation process and the interplay of structural order and disorder must be answered. Herein, biological photonic structures and their characterization techniques are reviewed, focusing on those structures not yet artificially manufactured. Then, employed and potential nanofabrication strategies for biomimetic, bio-templated, and artificially created biopolymeric photonic structures are discussed. The discussion is extended to responsive biopolymer photonic structures and hybrid structures. Last, future fundamental physics, chemistry, and nanotechnology challenges related to biopolymer photonics are foreseen.Peer ReviewedPostprint (published version

Sol-gel Barium Titanate Nanohole Array as a Nonlinear Metasurface and a Photonic Crystal

The quest of a nonlinear optical material that can be easily nanostructured over a large surface area is still ongoing. Here, we demonstrate a nanoimprinted nonlinear barium titanate 2D nanohole array that shows optical properties of a 2D photonic crystal and metasurface, depending on the direction of the optical axis. The challenge of nanostructuring the inert metal-oxide is resolved by direct soft nanoimprint lithography with sol-gel derived barium titanate enabling critical dimensions of 120 nm with aspect ratios of 5. The nanohole array exhibits a photonic bandgap in the infrared range when probed along the slab axis while lattice resonant states are observed in out-of-plane transmission configuration. The enhanced light-matter interaction from the resonant structure enables to increase the second-harmonic generation in the near-UV by a factor of 18 illustrating the potential in the flexible fabrication technique for barium titanate photonic devices

Nonlinear Photonic Crystals and Metasurfaces in Soft-Nanoimprinted Barium Titanate Nanomaterials

Nonlinear optics was made accessible through the high intensities achievable with lasers. It enabled fiber optic communication as well as ultra-fine resolution spectroscopy and extended the applications of lasers. In recent years, nonlinear optical processes have helped to reveal interesting effects in the quantum realm including entangled photons, quantum interference effects in free space and integrated devices. Second-harmonic generation is the lowest order nonlinearity i.e., the process with the highest conversion efficiency, occurring however only in non-centrosymmetric materials. Two materials that exhibit strong second-harmonic responses in the visible are lithium niobate and barium titanate. Additionally, these materials have very large electro-optic coefficients and transparencies ranging from the near-ultraviolet to mid-infrared. Barium titanate, a metal oxide, is a material with a very high electro-optic coefficient. Lithium niobate, another metal oxide, is established since a long-time as electrooptic modulator. The chemical inertness of these oxides prohibits any top-down chemistry based etching fabrication such that only physical etching methods can be applied. This makes the fabrication as well as the process development extremely time-consuming and energy intensive. In addition, barium titanate thin films are not commercially available. Therefore, in this work, we present the development of a solution-based bottom-up soft-nanoimprint lithography as a method for producing barium titanate based devices. By this method, we achieve the successful fabrication and optical characterization of 3D nonlinear photonic crystals with up to 1 cm2 large surface areas. These structures show a cubic crystal structure and a relative linear transmission of 48.4% at 757 nm. We also numerically investigate the influence of geometric parameters on the photonic band gap position of woodpile photonic crystals. Due to our advances in process optimization, critical dimensions of 60 nm became feasible with a sol-gel chemistry approach. This allowed to fabricate 2D hexagonal photonic crystal with L3 cavities with aspect-ratios of 0.4. These devices can either be used to enhance optical nonlinearities as second-harmonic generation or spontaneous parametric down-conversion as well as to fabricate tunable biosensors. In order to assign defined optical functionalities to the structures, simulation models were developed. These structures had then to be tested at specifically built optical setups to investigate the consistency between simulation and fabrication. To extend the application range of the present photonic crystal structures, active tuning of the optical responses is necessary. The electro-optic effect is one of the fastest active tuning mechanisms with modulation speeds up to the GHz-regime. We successfully fabricated a metasurface in lithium niobate thin films and detected, for the first time, a dispersive electro-optic effect over a metasurface resonance. At the turning point of the metasurface resonance, we could enhance the electrooptic modulation intensity by 80 times compared to an unstructured thin film. We showed modulation speeds up to MHz speeds by applying AC voltages starting from less than 1Vpp. To achieve these measurements, we developed an optical setup in free-space configuration to measure the electro-optic modulation from these metasurfaces based on a lock-in technique. In addition, we also succeed in fabricating barium titanate metasurfaces from barium titanate nanoparticle thin films. However, the quality factors of these resonances were too low to allow for electro-optic modulation. This was improved by applying the developed sol-gel chemistry based soft-nanoimprint lithography to the field of metasurfaces. We could recently demonstrate the first barium titanate metalenses with aspect ratios as high as 8 and demonstration of electro-optic modulation of these devices is in active progress. This thesis also outlines future research directions and analyses how barium titanate 3D nonlinear photonic crystals can contribute to the field of quantum optics, photonic time crystals and disordered photonic crystals

Optically reconfigurable ferroelectric metasurfaces

We use ferroelectric-plasmonic metasurfaces to demonstrate volatile and non-volatile optical switching of near-infrared light. Plasmonic metasurfaces on lithium niobate enable high-contrast optical switching with ratios up to 2.37:1 (3.7 dB) due to photogalvanic and photorefractive effects, therefore rendering a compact platform for photonic computing

Photonic assemblies of randomly oriented nanocrystals for engineered nonlinear and electro-optic effects

Nonlinear crystals that have a noncentrosymmetric crystalline structure, such as lithium niobate (LiNbO3) and barium titanate (BaTiO3) exhibit nonzero second-order tensor susceptibilities (χ(2)) and linear electro-optic coefficients (rij). The constraints associated with top-down nanofabrication methods have led to bottom up approaches to harness the strong nonlinearities and electro-optical properties. Here, we present an overview of photonic assemblies made of randomly oriented noncentrosymmetric nanocrystals via bottom-up fabrication methods. In this configuration, nanocrystals can form objects with tunable dimensions, increased complexity, and a great span of symmetry level, ranging from thin layers to spheres. At the same time, according to their shape, photonic assemblies may support optical modes, that is, Mie or guided, which can tailor linear optical properties and enhance nonlinear and electro-optic responses. As a result, assemblies of noncentrosymmetric nanocrystals can form a disruptive platform to realize photonic integrated devices free of etching process and over large surface areas. Last, we foresee potential applications of noncentrosymmetric nanocrystals in various fields of nano-optics and sensing.ISSN:2330-402

Electro‐Optic Metasurfaces Based on Barium Titanate Nanoparticle Films

Metal‐oxides are promising candidates to substitute silicon in intra‐chip optical interconnects, as they exhibit great electric field tuning capabilities. The development of crystal ion slicing of thin films from bulk crystals and the advances over epitaxial growth have allowed the integration of metal‐oxides on a single chip. In terms of performance, they possess strong electro‐optic response over broad bandwidths across near‐infrared. However, lattice and thermal expansion coefficient mismatch limits the compatibility with available substrates and other materials, while physical hardness makes high quality nanostructures difficult to implement. Here, a novel concept of electro‐optic (EO) switching is introduced: an adjacent BaTiO3 nanoparticle film to a plasmonic metasurface provides reflection changes up to 0.15% under 4 V of control signal for modulation frequencies up to 20 MHz, in the near‐infrared. The nanoparticle films show EO coefficients (37.04 ± 25.6 pm V−1) comparable to lithium niobate crystals, are deposited uniformly over large scale and on any type of substrate, while retain optical nonlinear properties (e.g. second‐harmonic generation). Photonic nanostructures such as metasurfaces incorporated with nanoparticle films can harness the multifunctional properties of metal‐oxides such as BaTiO3 to form a new family of switchable nano‐devices across the entire visible to near‐infrared part of the spectrum.ISSN:2195-107

Towards active electro-optic lithium niobate metasurfaces

We present the design and fabrication advances on active lithium niobate metasurfaces. We determine by numerical calculations a metasurface design with electro-magnetic resonances in the visible and near-infrared, by taking into account the constraints for fabrication on thin films of lithium niobate. We suggest that the optical properties of the metasurface can be switched using the electro-optical properties of lithium niobate