Light beams with a helical phase-front possess orbital angular momentum along their direction of propagation in addition to the spin angular momentum that describes their polarisation. Until recently, it was thought that these two 'rotational' motions of light were largely independent and could not be coupled during light-matter interactions. However, it is now known that interactions with carefully designed complex media can result in spin-to-orbit coupling, where a change of the spin angular momentum will modify the orbital angular momentum and vice versa. In this work, we propose and demonstrate that the birefringence of plasmonic nanostructures can be wielded to transform circularly polarised light into light carrying orbital angular momentum. A device operating at visible wavelengths is designed from a space-variant array of subwavelength plasmonic nano-antennas. Experiment confirms that circularly polarised light transmitted through the device is imbued with orbital angular momentum of 62" (with conversion efficiency of at least 1%). This technology paves the way towards ultrathin orbital angular momentum generators that could be integrated into applications for spectroscopy, nanoscale sensing and classical or quantum communications using integrated photonic devices. Keywords: light orbital angular momentum; light spin angular momentum; plasmonic metasurface INTRODUCTION Spin angular momentum (SAM) and orbital angular momentum (OAM) are associated with the polarisation and phase of the optical field, respectively. 1 A striking difference between these momenta is the range of allowed values. SAM can be 6" per photon, expressed as left or right circular polarisation, while OAM has an unbounded value of '" per photon, 2 ' being an integer. In an anisotropic and inhomogeneous medium, these otherwise independent momenta can be made to interact, changing both the polarisation and phase of the beam. 3 This change depends on the incident beam's polarisation and the medium's topology stemming from its inhomogeneity. This relationship can be described by the Pancharatnam-Berry (geometrical) phase, 4 and is what allows a beam to experience different optical paths associated with the trajectory of the polarisation evolution on the Poincaré sphere. 5 Recently, this phenomenon has enabled PancharatnamBerry phase optical elements: 6,7 devices that control the output beam's wavefront according to the polarisation of the input beam. These devices could easily be inserted into the beam path of existing spectroscopic, nano-imaging or communication systems, as they do not rely on diffraction, adding OAM-based functionality that has the potential to distinguish between molecules of different chirality, enhance optical circular dichroism 8 and encode multiple bits of information onto a single photon. 9 Existing Pancharatnam-Berry phase optical elements include qplates, 3 made of liquid crystals, and computer-generated subwavelength gratings, 6,10 made of micron-size dielectric features. These

The size and operating energy of a nonlinear optical device are fundamentally constrained by the weakness of the nonlinear optical response of common materials. Here, we report that a 50-nm-thick optical metasurface made of optical dipole antennas coupled to an epsilon-near-zero material exhibits a broadband (∼ 400 nm bandwidth) and ultrafast (recovery time less than 1 ps) intensitydependent refractive index n 2 as large as −3.73 ± 0.56 cm 2 /GW. Furthermore, the metasurface exhibits a maximum optically induced refractive index change of ±2.5 over a spectral range of ∼ 200 nm. The inclusion of low-Q nanoantennas on an epsilon-near-zero thin film not only allows one to design a metasurface with an unprecedentedly large nonlinear optical response but also offers the flexibility to tailor the sign of the response. Our technique allows one to overcome a longstanding challenge in nonlinear optics, namely that of finding a material for which the nonlinear contribution to the refractive index is of the order of unity. It consequently offers the possibility of designing low-power nonlinear nano-optical devices with orders-of-magnitude smaller footprints.All-optical signal processing and computation are often hailed as breakthrough technologies for the next generation of computation and communication devices. Two important parameters of such devices, energy consumption and size, critically depend on 1 the strength of the nonlinear optical response of the materials from which they are made. However, materials typically exhibit an extremely weak nonlinear optical response. This property makes designing subwavelength all-optical active devices extremely difficult. Thus, all-optical active devices tend to have large footprints, which limits the integration density to many orders of magnitude smaller than what can be achieved in a state-of-the-art electronic integrated circuits [1,2]. Thus, materials with much stronger nonlinear optical responses are needed in order to enable integrated high-density on-chip nonlinear optical devices.Over the years several approaches have been explored to enhance the intrinsic nonlinear optical response of materials, including local field enhancement using composite structures [3,4,5], plasmonic structures [6,7], and metamaterials [8,9,10,11]. However, these techniques offer only limited control over the magnitude (and sign when applicable) of the wavelength-dependent nonlinear response, and typically involve a trade-off between the strength of the nonlinearity and the spectral position of the peak nonlinear response. It has been reported recently that materials with vanishingly small permittivitycommonly known as epsilon-near-zero or ENZ material -exhibit intriguing linear [12,13,14,15,16] and large nonlinear responses [17,18,19,20,21]. However, an ENZ material has a large nonlinear response over only a relatively narrow spectral range. Furthermore, the zero-permittivity wavelength, strength of the nonlinear response, and the losses depend on the optical properties of the ENZ material. In com...

Slow light devices such as photonic crystal waveguides (PhCW) and coupled resonator optical waveguides (CROW) have much promise for optical signal processing applications and a number of successful demonstrations underpinning this promise have already been made. Most of these applications are limited by propagation losses, especially for higher group indices. These losses are caused by technological imperfections ("extrinsic loss") that cause scattering of light from the waveguide mode. The relationship between this loss and the group velocity is complex and until now has not been fully understood. Here, we present a comprehensive explanation of the extrinsic loss mechanisms in PhC waveguides and address some misconceptions surrounding loss and slow light that have arisen in recent years. We develop a theoretical model that accurately describes the loss spectra of PhC waveguides. One of the key insights of the model is that the entire hole contributes coherently to the scattering process, in contrast to previous models that added up the scattering from short sections incoherently. As a result, we have already realised waveguides with significantly lower losses than comparable photonic crystal waveguides as well as achieving propagation losses, in units of loss per unit time (dB/ns) that are even lower than those of state-of-the-art coupled resonator optical waveguides based on silicon photonic wires. The model will enable more advanced designs with further loss reduction within existing technological constraints.

We review the different types of dispersion engineered photonic crystal waveguides that have been developed for slow light applications. We introduce the group index bandwidth product (GBP) and the loss per delay in terms of dB ns − 1 as two key figures of merit to describe such structures and compare the different experimental realizations based on these figures. A key outcome of the comparison is that slow light based on photonic crystals performs as well or better than slow light based on coupled ring resonators.