Two-photon fabrication of bio-inspired microstructures for optical topological applications
Creators: name: Goi, Elena
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 510204
subject: Photonics, optoelectronics and optical communications

description: The control of the flow of light using photonic-band-gap materials has received considerable attention over the past decade, and the technological applications of artificially structured metamaterials, like photonic crystals, have demonstrated the potential of artificially engineered media in photonics. Photonic crystals are periodic structures with spatial period comparable in size to the wavelength of light. Unlike unstructured materials, their dispersive properties can be manipulated through the choice of materials and the geometrical design. The idea of periodic systems with a tailored modulation of the dielectric function was motivated by the well-known physics of electronic Bloch states, because the dielectric scattering of light in periodic media presents the same formal solutions as those for the scattering of electrons in periodic potentials. Analogous to the electronic bandgaps formed in semiconductors, photonic crystals possess photonic bandgaps, frequency bands where light is completely reflected due to interference. This unique control of light has inspired the development of many photonic crystal devices such as integrated optical waveguides, cavities, optical-switches and even super-prisms.<br><br>Recently, however, in the study of electronic systems, it has become apparent that even in the absence of interaction effects, the dispersion relations of the energy bands do not fully characterize the dynamics of wave packets in all symmetry conditions. The additional information, which is not obtainable from the simple knowledge of the energy bands is the topological description of the photonic band structure. Topology, a property related to the global structure of the frequency dispersion of a photonic system, emerged as a new tool for the control of momentum space and an additional degree of freedom for the discovery of fundamentally new states of light.<br><br>Topological ideas in photonics branch from exciting developments in solid-state physics, along with the discovery of new phases of matter called topological insulators, materials which are conventional insulators in the bulk but support dissipationless topologically protected edge states. Topological insulators have been of interest to physicists as much for their unique physics as for their plethora of potential applications, which include the whole range of possibilities from nano-scale electronic circuits to the realization of Majorana fermions and large-scale quantum computers. Recent works propose to transfer the key feature of topologically non trivial electronic models to the realm of photonics. There are many advantages to studying band topologies in photonic systems. First, in contrast to the topological insulator state in conventional materials, where we are limited to select atomic compositions and crystalline arrangements, in photonics we can literally build a topological system through the selection of materials and geometry and we can tune continuously the design to create any of the allowed bulk or edge dispersions. Second, photons have no Fermi levels, therefore the whole photonic band structure can be probed using photons with different energy. Moreover, there is no fundamental length scale in Maxwell’s equations, therefore experimentalists can work at any wavelength. Finally, the exploitation of topological effects could dramatically improve the robustness of photonic devices in the presence of imperfections.<br><br>The field of topological photonics has grown exponentially in recent years. Non-trivial topological effects have been proposed across a variety of photonic systems. Much like the field of topological insulators in electronics, topological photonics promises an enormous variety of breakthroughs in both fundamental physics and technological outcomes. Despite these potentials, advancements in topological photonics research has been hindered by diffculties in fabricating 3D structures that fulfill the requirements to exhibit topologically non-trivial properties. Currently, the main challenge in this field is the realisation of topological optical structures for the development of on-chip optical systems that support states of light that are immune to back scatter, robust against perturbation and feature guaranteed unidirectional transmission.<br><br>The aim of this thesis is to realise and investigate three-dimensional photonic microstructures with topologically non trivial properties and operative wavelength in the optical regime for application as optical signal processing devices. In particular, our goal is to realise photonic crystals that possess frequency isolated linear point degeneracies in their three-dimensional dispersion that define the illusive Weyl points. Weyl points act as monopoles or anti-monopoles Berry flux in momentum space, and carry chirality defined by quantised topological charges. Here we demonstrate a new technique for the experimental realisation of photonic type I Weyl points in a bio-inspired three-dimensional photonic crystal with operative wavelength in the middle-infrared. More importantly, we discover the chiral nature of the photonic Weyl points by coupling with spin-angular momentum carried by circularly polarised light.<br><br>Our photonic structures are based on the biomimetic gyroid networks, structures that naturally occur in several biological nanostructures such as the wing scales of the \textit{Callophrys Rubi} butterfly. The gyroid network is a three-dimensional periodic network with both cubic symmetry and chirality and thus is an excellent platform for the development of chiral photonic crystals and a powerful platform for the study of novel photonic topological states.<br><br>We demonstrate that by using a galvo-dithered direct laser writing technique it is possible to fabricate biomimetic gyroid structures with superior control over size, periodicity and filling fraction compared to the biological counterparts. This method is particularly suitable for fabricating achiral double gyroid micro-structures, which are the first step for the Weyl point realisation. Using this technique, we are able to incorporate defects into the double gyroid design and in this way break the parity symmetry, as required in the Weyl points systems. To obtain frequency isolated Weyl points in the band structure and detect them clearly, a high refractive index structure is required. We propose the idea of coating the polymer templates with high refractive index materials creating a core-cladding structure to increase the effective refractive index of the photonic crystals. The core-cladding structure is practically realised via atomic layer deposition of layered-composite nanometric antimony telluride on polymer templates created via three-dimensional direct laser writing. Finally, we characterise the Weyl point structures with angle-resolved transmission spectroscopy and investigate the chiral character of the opposite charged point degeneracies through the coupling with circularly polarised light. In this way we discover a Weyl point-induced mechanism that leads to reversed circular dichroism along the directions that intersect the oppositely charged topological photonic states.<br><br>The operation of these topological structures at optical wavelengths and efficient fabrication via three-dimensional nano-lithography make them highly desirable in integrated photonic chips and nano-photonic devices. The discovery of the Weyl-point induced reversed circular dichroism provides an entirely new platform for developing topologically protected super-robust photonic devices in angular-momentum-based information processing, circular-dichroism-enabled protein sensing, spintronics and quantum optoelectronics.<br><br>