J. P. Balthasar Mueller

Polarization-Controlled Tunable Directional Coupling of Surface Plasmon Polaritons

Controlling Light Propagation Surface plasmons are light-induced collective electronic excitations in a metal that offer the possibility of manufacturing optoelectronic devices at nanometer scale. Before such shrinking can be achieved, the propagation direction and lifetime of the plasmonic excitations have to be controlled (see the Perspective by Miroshnichenko and Kivshar). Rodríguez-Fortuño et al. (p. 328) show how this is done using polarized light. Alternatively, using an array of metallic nanoantennae (in this case, slits) patterned into a thin gold film, Lin et al. (p. 331) present a further improvement on current plasmonic coupling schemes that has the potential to encode information contained in both the intensity and polarization of light. Control over the generation and propagation direction of light-induced surface plasmons in a thin metal film is demonstrated. [Also see Perspective by Miroshnichenko and Kivshar] Light can be coupled into propagating electromagnetic surface waves at a metal-dielectric interface known as surface plasmon polaritons (SPPs). This process has traditionally faced challenges in the polarization sensitivity of the coupling efficiency and in controlling the directionality of the SPPs. We designed and demonstrated plasmonic couplers that overcome these limits using polarization-sensitive apertures in a gold film. Our devices enable polarization-controlled tunable directional coupling with polarization-invariant total conversion efficiency and preserve the incident polarization information. Both bidirectional and unidirectional launching of SPPs are demonstrated. The design is further applied to circular structures that create radially convergent and divergent SPPs, illustrating that this concept can be extended to a broad range of applications.

Metasurface Polarization Optics: Independent Phase Control of Arbitrary Orthogonal States of Polarization

We present a method allowing for the imposition of two independent and arbitrary phase profiles on any pair of orthogonal states of polarization-linear, circular, or elliptical-relying only on simple, linearly birefringent wave plate elements arranged into metasurfaces. This stands in contrast to previous designs which could only address orthogonal linear, and to a limited extent, circular polarizations. Using this approach, we demonstrate chiral holograms characterized by fully independent far fields for each circular polarization and elliptical polarization beam splitters, both in the visible. This approach significantly expands the scope of metasurface polarization optics.

Ultracompact metasurface in-line polarimeter

In-line polarimeters perform nondestructive polarization measurements of optical signals, and play a critical role in monitoring and controlling the polarization environment in, for example, optical networks. While current in-line polarimeters are constructed with multiple optical components, either fabricated into an optical fiber or using free-space optics, we present here a novel architecture conducive to monolithic on-chip integration. This enables the scalable fabrication of high-performance polarization sensors with exceptional stability, compactness, and speed. The method relies on the detection of the highly polarization-dependent scattered field of a subwavelength antenna array known as a metasurface, and is shown here to provide polarization state measurements matching those of a state-of-the-art commercial polarimeter.

Lateral chirality-sorting optical forces

Significance In light of the difficulty often associated with sorting and characterizing materials by chirality, new research aimed toward the development of passive optical methods has stirred considerable excitement at the interface of analytical chemistry and physical optics. We describe here a mechanism through which chirality-sorting optical forces emerge through the interaction with the spin-angular momentum of light, a property that the community has recently learned to control with great sophistication using modern nanophotonics. In particular, the forces may be oriented perpendicularly to the propagation direction of evanescent waves, leading to a helicity-dependent lateral deflection of chiral particles in opposite directions. The highly unusual transverse optical force described herein is the strongest of its kind discovered so far to our knowledge. The transverse component of the spin angular momentum of evanescent waves gives rise to lateral optical forces on chiral particles, which have the unusual property of acting in a direction in which there is neither a field gradient nor wave propagation. Because their direction and strength depends on the chiral polarizability of the particle, they act as chirality-sorting and may offer a mechanism for passive chirality spectroscopy. The absolute strength of the forces also substantially exceeds that of other recently predicted sideways optical forces.

Arbitrary spin-to–orbital angular momentum conversion of light

From spins to spirals The polarization state of light can be used in imaging applications and optical communications. Light can also be structured into vortices that carry optical angular momentum, which can be used for micromanipulation and enhancing the capacity of optical communication channels. Devlin et al. present a metasurface converter for optical states that transforms polarization states into optical angular momentum states. The coupling between arbitrary spin and optical angular momentum states of light in a compact planar structure may find applications in producing complex structured light fields. Science, this issue p. 896 A designed metasurface can transform polarization states into optical angular momentum states. Optical elements that convert the spin angular momentum (SAM) of light into vortex beams have found applications in classical and quantum optics. These elements—SAM-to–orbital angular momentum (OAM) converters—are based on the geometric phase and only permit the conversion of left- and right-circular polarizations (spin states) into states with opposite OAM. We present a method for converting arbitrary SAM states into total angular momentum states characterized by a superposition of independent OAM. We designed a metasurface that converts left- and right-circular polarizations into states with independent values of OAM and designed another device that performs this operation for elliptically polarized states. These results illustrate a general material-mediated connection between SAM and OAM of light and may find applications in producing complex structured light and in optical communication.

Polarization-Selective Coupling to Long-Range Surface Plasmon Polariton Waveguides

We use plasmonic antenna arrays to unidirectionally couple incident light in two different polarization states to long-range surface plasmon polariton waveguide modes propagating in opposite directions. The structures enable polarization-sorting with extinction rates in excess of 30dB.

Performance characteristics of 4-port in-plane and out-of-plane in-line metasurface polarimeters

In-line polarimeters perform nonterminating measurements of the polarization state of light by sampling only a small part of the total light intensity. In-line polarimeters are used in applications such as polarization state generators and in optical communications. Current polarimeters use multiple optical components in sequence for polarization analysis and therefore often become bulky and expensive. Here, we experimentally demonstrate the operation of compact fiber-coupled polarimeters with high sampling rates, operating at telecom wavelengths, each polarimeter comprising a single ultra-thin metasurface aligned to four photodetectors. We compare two configurations of such metasurface polarimeters, with in-plane and out-of-plane detection, respectively. The metasurface polarimeters reported here show excellent agreement with commercial polarimeters and cover a bandwidth of at least 100 nm.

High-efficiency chiral meta-lens

We present here a compact metasurface lens element that enables simultaneous and spatially separated imaging of light of opposite circular polarization states. The design overcomes a limitation of previous chiral lenses reliant on the traditional geometric phase approach by allowing for independent focusing of both circular polarizations without a 50% efficiency trade-off. We demonstrate circular polarization-dependent imaging at visible wavelengths with polarization contrast greater than 20dB and efficiencies as high as 70%.

Packaged inline metasurface polarimeters with in-plane and out-of-plane detection

In-line Polarimeters perform nonterminating measurements of a light beams state and degree of polarization by sampling only a small part of the total light intensity. In-line polarimeters are used in applications such as polarization state generators and in optical communications. A new class of engineered optical structures, the so-called metasurfaces, offers a way to tailor the amplitude, phase or polarization of light waves using subwavelength structures. Metasurfaces provide a promising platform for simplifying and miniaturizing existing optical components [1]. We present a new in-line, polarization preserving polarimeter design based on our previously reported metasurface formed by arrays of nanoantennas [2]. Our design promises excellent accuracy, compact design, potential for low-cost mass production, and wavelength scalability well beyond the wavelength range presently investigated.