Periodic Reporting for period 4 - iCOMM (New Frontiers in Nanophotonics: Integrating Complex Beams and Active Metasurface Devices)
Reporting period: 2023-03-01 to 2024-08-31
Photonics is one of the most penetrating technologies of the 21st century with impact in such diverse areas as information processing and communications, sensing and security, healthcare and medicine, and quantum technology among others. Its role in the future is only expected to rise, providing novel disruptive solutions needed for various industries. Recent advances in photonics were enabled by our ability to engineer controlled light-matter interactions either by shaping materials (nanoparticles, resonators, waveguides, photonic crystals, metamaterials) or controlling light beam properties (subwavelength focusing, polarisation and optical angular momentum, ultrashort pulses). This approach has allowed to open completely new applications for photonics where the impact in modern technologies is highest. In ICT, photonic integrated circuitry, all-optical routing in transparent networks, all-optical switching and modulation at higher speeds and lower power consumption, new approaches for information encoding, such as OAM, are the driving force. In healthcare and security, advanced sensing based on nanophotonic structures provides label-free sensitivities down to molecular level, real-time sensing and used for cancer diagnostic, drug development, and DNA sequencing to name but a few. Complex, structured optical beams have unique properties offering new degrees of freedom for achieving unusual functions demanded in microscopy, optical trapping and manipulation of nano-objects, information encoding in optical communications, holography, quantum technologies and laser micromachining. Metasurfaces, a subwavelength-thin nanostructured films, which were initially developed for controlling the phase of light and its reflection and transmission beyond the Snell’s law, provide a rich playground for generation and manipulation of structured beams. iCOMM project established a metasurface platform for generating and controlling complex vector beams in space and time and developed its applications in sensing and identification of chiral objects and nonlinear optical trapping. Using unique optical properties of designed metasurfaces capable of controlling both phase and amplitude of light, nonlinear interactions of pulsed vector beams were optimised and explored. The results are important for development of applications of complex optical beams and metasurfaces in optical communications, displays, security and bio- and chemical sensing. The realisation of active metasurface chips for nonlinear generation, transformation and manipulation of pulsed vector beams is also important for their other application in classical and quantum optical information communications and processing, metrology and high-resolution microscopy.
In this project, the work was significantly focused on the understanding of basic physics of ultrashort cylindrical vector beams and their interaction with anisotropic metasurfaces, on designing nonlinear optical intreractions between such beams and metasurfaces, the demonstration of the enhanced chiral interactions and optical trapping with complex beams. In the first strand, all optical phase control of the incident light using a nonlinear Kerr-effects in metasurface was demonstrated, which in turn allowed us to achieve polarisation control of the transmitted and reflected light. Dynamic tailoring and transformation of complex vector beams is required for applications in optical telecoms and quantum communications, photolithography, and optical data storage. Careful engineering of pulsed vector beams allowed generation, control and exploitation of three-dimensional polarization components characteristic to the complex vector beams (conventional Gaussian beams and plane waves support only transverse polarisation in the plane perpendicular to the propagation direction). These new polarisation configurations, which is of particular importance for optical trapping and manipulation, can also result in the nontrivial polarisation states of the light, such as skyrmion, merons and their lattices. Optical spin skyrmions and merons were demonstrated both theoretically and experimentally. We have studied both experimentally and theoretically the polarisation and temporal properties of ultrafast cylindrical vector beams and compared them to their continuous wave counterparts. The role of optical angular momentum cross-talk through the nonlinear optical interactions were studied. Plasmonic metasurfaces capable to enhance the chiral interactions of light have been demonstrated. Some of the other most important achievements include observation of the optical spin of unpolarised light, providing new understanding of polarisation description of vector beams beyond paraxial approximation, discovery of photonic skyrmions and their generalised description allowing understanding of their behaviour in the presence of disorder, the topological textures of the light fields with specific polarisation arising due to the spin-orbit coupling in the evanescent waves, and demonstration of the three-dimensional full-colour image projection based on reflective metasurfaces under incoherent illumination, which is important for practical use of metasurfaces, including in anti-counterfeiting applications. In addition, unidirectional chiral scatterers were designed and fabricated and ultrafast control of polarisation states through nonlinear optical effects in metamaterials and epsilon near-zero media.
The project has achieved a significant progress beyond the state of the art. We have shown that traditional optical theories (such as the optical theorem) valid for conventional optical beams should be corrected for cylindrical vector beams. The use of femtosecond cylindrical vector beams results in additional optical forces in a nonlinear regime due to a peculiar polarisation structure, allowing additional control of nanoparticle trapping. Using anisotropic nonlinear metasurfaces, we have shown that due to the nonlinear phase control, the polarisation of the transmistted and reflected light can be manipulated all-optically. Moreover, with the geometric anisotropy, the time response can also be controlled by selectively exciting the nonlinearity at a given location in the metasurface. Non-interleaved geometric-phase metasurfaces were designed working in a broad spectral range throughout visible spectral range. The combination of magnetic and electric dipoles of the nanoparticles, allowed achieving total phase and amplitude control over the scattered field, important for designing Huygens' type metasurfaces. Finally, the interplay between evanescent waves carrying optical angular momentum was shown to result in photonic skyrmions, a new optical topological structure, with a peculiar spin distributions suitable for high-resolution optical metrology and imaging. The enhanced spin-orbit coupling was demonstrated in the interaction complex vector beams with anisotropic plamonic metamaterials. Based on these achievements in the state of art of complex vector beams and understanding their interaction with metasurfaces and chiral objects, complex optical beams, metasurfaces and their linear and nonlinear interactions can be useful for development of novel applications in optical communications, displays, security and bio- and chemical sensing and metrology.