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Particle-Surface Interactions in Near Field Optics: Spin-orbit Effects of Light and Optical/Casimir Forces

Periodic Reporting for period 2 - PSINFONI (Particle-Surface Interactions in Near Field Optics: Spin-orbit Effects of Light and Optical/Casimir Forces)

Reporting period: 2018-09-01 to 2020-02-29

Nanophotonics is the science that studies photonics at the nanoscale and promises solutions to the biggest challenges in information and communication technologies (ICTs): In the past century, long-distance data transmission was revolutionized by photonics through the use of optical fibres. Today, the communication bottleneck lies on the switching and routing of signals in data centres, which still depend on electronic processing in microelectronic-based switches and routers, with low bandwidth and high energy consumption and residual heat. The ICT sector including data centres generates up to 2% of the global CO2 emissions, a number on par to the aviation sector contribution, and data centres are estimated to have the fastest growing carbon footprint from across the whole ICT sector, due to the rapid growth of the use of Internet services. Nanophotonics can provide a platform for all-optical switching and routing in integrated optical interconnects, with a huge boost in bandwidth, and importantly, a greatly reduced power consumption, of great environmental and economic importance. Ultrafast light routing at the nanoscale is thus a big current challenge with high potential impact.

Near-field directionality of light, being researched in the PSINFONI project, can provide a fundamental solution to the ultrafast routing challenge described above. In essence, the polarization of light or polarization of a light source may affect light's propagation direction. Hence, all-optical ultrafast switching on light polarization can be used for light switching and routing, without relying on electronics. The inverse scenario suggests the possibility of integrated nano-photonic light polarimeters based on this robust electromagnetic phenomenon. PSINFONI will theoretically and experimentally explore the basic science and potential for devices using these phenomena.

Nanophotonics has also been an enabling technology for advancements in other sciences via the possibility of optical tweezers and optical forces, which received the 2018 Nobel prize in Physics. Researchers in the biological and healthcare sciences, among others, greatly benefit from the possibility of utilizing optical forces to trap, move and manipulate small molecules or even living cells. However, these tools require complex setups to achieve the focusing of a laser beam into very tight volumes to create the optical trap, and subsequent movement of that focus. This allows for movement of one single optical trap at a time, but a massively-parallel movement of several molecules, particles or cells is not easily scalable. The PSINFONI project aims to explore the possibility of achieving optical forces for the manipulation and movement of particles without using focused illumination, instead relying on the particle-surface interactions that occur in the near field when particles close to a surface are illuminated using a simple plane wave, which can then be controlled via the illuminating light polarization. The use of plane waves for optical forces would allow massively-parallel control, movement and/or sorting of vast numbers of particles simultaneously. We also explore the regime of Casimir forces, when no illumination is involved.

Both the near-field directionality of light, and the repulsive and lateral optical forces on particles under plane wave illumination, rely on the near field interactions of polarized particles with a nearby surface or waveguide. The PSINFONI project relies on our expert in-depth knowledge of these near-field particle-surface interactions, allowing us to efficiently explore the space of achievable possibilities in the ideas described above.
In the area of near-field directionality from localized sources for polarization-based switching, one of the aims of the project was to explore the use of magnetic dipoles: indeed, we studied this in depth, and beyond the expected spin directionality of magnetic dipoles, we uncovered a new paradigm: by combining electric and magnetic sources we can achieve completely new physics of near-field directionality beyond spin-orbit interactions. We revealed the existence of the Janus dipole, which we named as such due to its dual-face nature, enabling on/off switchable coupling to a nearby waveguide (see figure). This remarkable source was proposed theoretically by us, and also experimentally demonstrated in collaboration with other groups in the Max Planck Institute for Light.

This work also inspired us into novel applications. A suitably engineered particle can be placed in a region of optical evanescent waves, extremely easy to do in practice via total internal reflection on a nearby surface, such that we can exploit the electric and magnetic fields to achieve an extremely sensitive position sensor. Tiny subwavelength changes in the position of the particle result in dramatic changes to the directionality of the particle in the far-field. Hence, we proposed how a far-field measurement can be used to detect sub-nanometer displacements of the particle.

In the area of exploiting near-field directionality to achieve integrated nanopolarimeters, we have successfully designed, fabricated, and measured, what is to our knowledge the first subwavelength integrated Stokes nanopolarimeter in a silicon photonic circuit, compatible with CMOS mass-manufacturing facilities. It can be placed in the way of the beam with negligible disturbance. It is therefore a non-invasive, integrated, nanopolarimeter. The fabrication and measurement was performed in collaborations with a group at Universitat Politecnica de Valencia.

In the area of novel optical forces of particles near surfaces, we theoretically showed the first instance of a lateral Casimir force acting on particles near a smooth surface, when the particles are experiencing a rotation. This surprising and counterintuitive physics, which we formally describe as being identical to the mechanism of a wheel but with no contact or friction, ultimately stems from the near-field directionality of circularly polarized dipoles. This work received significant media attention and was picked up by several internet blogs and news websites.

We hav also done considerable theoretical efforts on both lateral and repulsive ptical forces on engineered particles near engineeres surfaces, including repulsive forces near graphene, lateral forces above magneto-optical surfaces, and repulsive forces on magnetic particles near conventional metallic surfaces.
Two of our recent theoretical frameworks (optical forces acting on any particle near any surface based on its angular spectrum and angular spectrum of any point particle with dipolar, quadrupolar and arbitrary higher order multipole polarizabilities) will be combined to obtain novel exciting results for optical forces of engineered particles near surfaces. We are working in the study of how engineered substrates can enhance the optical forces of particles within near-field distance, with promising results of several order of magnitude enhancements. We can also exploit the uncovered near-field directionality from multipolar resonances rather than dipoles to achieve a much more complete control of the near-field directionality in a large number of nearby waveguides, simultaneously, with a single multipolar source, with applications on nano-routing.
Numerical simulation of Janus dipole near field directionality