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New Frontiers in Nanophotonics: Integrating Complex Beams and Active Metasurface Devices

Periodic Reporting for period 3 - iCOMM (New Frontiers in Nanophotonics: Integrating Complex Beams and Active Metasurface Devices)

Reporting period: 2021-09-01 to 2023-02-28

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 aims to establish a metasurface platform for generating and controlling complex vector beams in space and time and develop its applications in sensing and identification of chiral objects and nonlinear optical trapping. Using unique optical properties of designer-metasurfaces capable of controlling both phase and amplitude of light, nonlinear interactions of pulsed vector beams are optimised and explored. This will be a transformative development for the applications of complex optical beams and metasurfaces in optical communications, displays, security and bio- and chemical sensing and metrology. The realisation of active metasurface chips for nonlinear generation, transformation and manipulation of pulsed vector beams will also be a transformative development for their other application in classical and quantum optical information communications and processing, metrology and high-resolution microscopy. The success of the project will transform the areas of both complex optical beams and metasurfaces by introducing real-time active control and consolidate and enhance the European leadership in this field.
In the reporting period, 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 and the demonstration of the enhanced chiral interactions with such 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 already required for applications in optical telecoms and quantum communications, photolithography, and optical data storage. Careful engineering of pulsed vector beams will allow exploring three-dimensional polarization components and not just transverse ones (in the plane perpendicular to the propagation direction), which is of particular importance for optical trapping and manipulation, where longitudinal components (the field components along the propagation direction, are not allowed or weak in a plane wave or Gaussian beam) are important. In order to address these, 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. We also worked on the development of metasurfaces to enhance the chiral interactions of light and have demonstrated the enhancement of circular dichroism for both circular polarised beams and complex vector beams. Some of the most important achievements include observation of the optical spin of unpolarised light, providing new understanding of polarisation description beyond paraxial approximation, discovery of photonic skyrmions, 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.
The project has already 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 the photonic skyrmion, a new optical topological structure, with a peculiar spin distributions suitable for high-resolution optical metrology and imaging. Based on this achievements in the state of art of complex vector beams, we will continue development of the planned research programme to advance the applications of complex optical beams, metasurfaces and their nonlinear interactions in optical communications, displays, security and bio- and chemical sensing and metrology.
A metasurface emulating a 3D cube in RGB colours.