The central idea in SPINONICS revolves around developing waveguides with controllable polarization and intensity. To achieve this, a system with strong spin-orbit interaction is chosen, utilizing anisotropic materials where the optical response depends on the input polarization. By rotating the axes of the anisotropic material on a point-wise basis, the spin-orbit interaction is maximized. The spatial rotation introduces an additional phase delay, known as the geometric phase, stemming from the rotation angle's gradient in a gauge field. In the structure made of twisted anisotropic materials, light is confined through the interplay between dynamic and geometric phases. The dynamic phase is associated with the material's refractive index, forming the foundation for standard waveguides. However, in this system, the accumulated phase is controlled through full 3D rotation of the material, allowing the shaping of modes in both polarization and intensity. From a fundamental physics perspective, this system can be modeled as a quantum particle in the presence of effective electric and magnetic fields, tunable through the geometry of the structure.
We then proceeded to investigate how short pulses propagate in waveguides solely based on a gradient in the geometric phase. Short pulses comprise several frequencies locked in phase. As each wavelength has a different velocity, optical pulses experience dispersion in a waveguide, meaning their temporal shape changes while propagating. Waveguides built upon the geometric phase exhibit much stronger dispersion than standard waveguides, offering high tunability in both magnitude and sign through geometric parameters. These unique properties have potential applications in dispersion management, such as guiding ultrashort pulses in waveguides or generating pulses in mode-locked lasers based on chirped pulse amplification.
Our exploration did not stop with single waveguides; we also delved into the response of coupled waveguides, a fundamental setting for creating integrated optical modulators. When two waveguides are positioned close to each other, they exchange energy coherently, enabling energy routing driven by the relative phase. Waveguides based on the geometric phase introduce additional degrees of freedom in designing such structures. By carefully designing the relative rotation angle between adjacent waveguides, we found that the coupling constant can be changed while keeping the waveguide positions unaltered. Remarkably, this enables the coupling to even become infinite, allowing two waveguides to be isolated from each other, despite their overlapping exponential tails.
While waveguides typically trap photons in a finite region on the plane perpendicular to the propagation direction, we explored the possibility of extending this confinement to the longitudinal direction, thus trapping light in a definite region of three-dimensional space. The question is if an optical resonator supporting structured modes can be realized. Our numerical investigations demonstrated that this is indeed possible. Coupled with an active material, such a configuration could lead to a laser directly emitting structured light with a tunable profile.
The results from SPINONICS have been published in several peer-reviewed journals as well as presented at many renowned conferences. The Project constitutes a fundamental science endeavour with its implementation for a possible new technological platform based on spin-orbit photonics. Over the last decade metasurface technology has become mainstream and has been commercialized (lab to consumer). On the same lines, we expect potential applications for PBP-based devices. However, at the current stage both the science and technology needed are in their infancy.
