Vortex beams are characterized by a helical wavefront and a phase singularity point on the propagation axis that results in a doughnut-like intensity profile. These beams carry orbital angular momentum proportional to the number of intertwined helices constituting the wavefront. Vortex beams have many applications in optics, such as optical trapping, quantum optics and microscopy. Although beams with such characteristics can be generated holographically, spin-to-orbital angular momentum conversion has attracted considerable interest as a tool to create vortex beams. In this process, the geometrical phase is exploited to create helical beams whose handedness is determined by the circular polarization (left/right) of the incident light, that is by its spin. Here we demonstrate high-efficiency Spin-to-Orbital angular momentum-Converters (SOCs) at visible wavelengths based on dielectric metasurfaces. With these SOCs we generate vortex beams with high and fractional topological charge and show for the first time the simultaneous generation of collinear helical beams with different and arbitrary orbital angular momentum. This versatile method of creating vortex beams, which circumvents the limitations of liquid crystal SOCs and adds new functionalities, should significantly expand the applications of these beams.
In the Cherenkov effect a charged particle moving with a velocity faster than the phase velocity of light in the medium radiates light that forms a cone with a half angle determined by the ratio of the two speeds. Here, we show that by creating a running wave of polarization along a one-dimensional metallic nanostructure consisting of subwavelength-spaced rotated apertures that propagates faster than the surface plasmon polariton phase velocity, we can generate surface plasmon wakes, a two-dimensional analogue of Cherenkov radiation. The running wave of polarization travels with a speed determined by the angle of incidence and the photon spin angular momentum of the incident radiation. By changing either one of these properties we demonstrate controlled steering of the Cherenkov surface plasmon wakes.
Surface plasmons polaritons (SPPs) are light-like waves confined to the interface between a metal and a dielectric. Excitation and control of these modes requires components such as couplers and lenses. We present the design of a new lens based on holographic principles. The key feature is the ability to switchably control SPP focusing by changing either the incident wavelength or polarization. Using phase-sensitive near-field imaging of the surface plasmon wavefronts, we have observed their switchable focusing and steering as the wavelength or polarization is changed.
V-shaped nanoantennas are among the popular choices for the unit element of a metasurface, a nanostructured surface used for its ability to mold and control the wavefront of light. In general, the motivation for choosing the V-antenna as the unit element comes from its bimodal nature, where the introduction of the second mode offers extra control over the scattered wavefronts. Here, through near-field scanning optical microscopy, we study a 1D metastructure comprised of V-antennas in the context of generating asymmetric surface plasmon polariton (SPP) wavefronts. The key point is that the use of the V-antenna allows for the creation of a two-dimensional phase gradient with a single line of antennas, where the extra phase dimension offers additional control and allows for asymmetric features. Two different asymmetries are created: (1) SPP wavefronts that have different propagation directions on either side of the metastructure, and (2) SPP wavefront asymmetry through focusing: one side of the metastructure focuses SPP wavefronts, while the other side has diverging SPP wavefronts.
In the Cherenkov effect a charged particle moving with a velocity faster than the phase velocity of light in the medium radiates light that forms a cone with a half angle determined by the ratio of the two speeds. In this paper, we show that by creating a running wave of polarization along a one dimensional metallic nanostructure consisting of subwavelength spaced rotated apertures that propagates faster than the surface plasmon polariton phase velocity, we can generate surface plasmon wakes, which are the two-dimensional analogue of Cherenkov radiation. The running wave of polarization travels with a speed determined by the angle of incidence and the photon spin angular momentum. We utilize this running wave of polarization to demonstrate controlled steering of the wakes by changing both the angle of incidence and the polarization of light, which we measure through near-field scanning optical microscopy.Whenever a disturbance travels in a medium faster than the phase velocity of the waves it creates, a so-called 'wake' is formed. In fact, this is a general wave phenomenon that accounts for things as diverse as boat wakes, sonic booms, and Cherenkov radiation [1,2]. Surface plasmon polaritons (SPPs) are a light-like mode confined to the interface between a metal and a dielectric. In principle, nothing prohibits the existence of SPP wakes; however, creating a disturbance that both excites SPPs and travels faster than their phase velocity is difficult owing to wavevector matching conditions and their light-like nature. In this paper, we excite a running wave of polarization (RWP) that travels faster than the phase velocity of the SPPs, and thus are able to create SPP wakes. Furthermore, by employing a metasurface [3-5], we are able to change the propagation direction of the wakes and steer them in a controllable way.The running wave of polarization (RWP) is created by having light incident at an oblique angle onto a slit etched in a metal film. Light impinging on different sections of the slit will have different phase because of the different propagation distances (Fig. 1a,b). This wave, polarized perpendicularly to the slit, excites SPPs and travels with a phase velocity: = / sin > . As can be seen in Fig. 1c, SPP wakes propagate away from the slit at an angle given by the equation: sin = sin / , where = / . In order to controllably steer the wakes, we replace the slit with a series of rotated nanoslits. (Fig.1 d,e).
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