Metasurfaces are planar structures that locally modify the polarization, phase and amplitude of light in reflection or transmission, thus enabling lithographically patterned flat optical components with functionalities controlled by design. Transmissive metasurfaces are especially important, as most optical systems used in practice operate in transmission. Several types of transmissive metasurface have been realized, but with either low transmission efficiencies or limited control over polarization and phase. Here, we show a metasurface platform based on high-contrast dielectric elliptical nanoposts that provides complete control of polarization and phase with subwavelength spatial resolution and an experimentally measured efficiency ranging from 72% to 97%, depending on the exact design. Such complete control enables the realization of most free-space transmissive optical elements such as lenses, phase plates, wave plates, polarizers, beamsplitters, as well as polarization-switchable phase holograms and arbitrary vector beam generators using the same metamaterial platform.

/pdf/dielectric-metasurfaces-for-complete-control-of-phase-and-3pa0oy7av0.pdf

Dielectric metasurfaces for complete control of phase and polarization with subwavelength spatial resolution and high transmission

We propose and demonstrate an efficient coupler for compact mode conversion between a fiber and a submicrometer waveguide. The coupler is composed of high-index-contrast materials and is based on a short taper with a nanometer-sized tip. We show that the micrometer-long silicon-on-insulator-based nanotaper coupler is able to efficiently convert both the mode field profile and the effective index, with a total length as short as 40 microm. We measure an enhancement of the coupling efficiency between an optical fiber and a waveguide by 1 order of magnitude due to the coupler.

Nanotaper for compact mode conversion.

Flat optical devices thinner than a wavelength promise to replace conventional free-space components for wavefront and polarization control. Transmissive flat lenses are particularly interesting for applications in imaging and on-chip optoelectronic integration. Several designs based on plasmonic metasurfaces, high-contrast transmitarrays and gratings have been recently implemented but have not provided a performance comparable to conventional curved lenses. Here we report polarization-insensitive, micron-thick, high-contrast transmitarray micro-lenses with focal spots as small as 0.57 λ. The measured focusing efficiency is up to 82%. A rigorous method for ultrathin lens design, and the trade-off between high efficiency and small spot size (or large numerical aperture) are discussed. The micro-lenses, composed of silicon nano-posts on glass, are fabricated in one lithographic step that could be performed with high-throughput photo or nanoimprint lithography, thus enabling widespread adoption.

Subwavelength-thick Lenses with High Numerical Apertures and Large Efficiency Based on High Contrast Transmitarrays

All optical fabrication of Surface Relief Grating (SRG) on azobenzene functionalized polymer films is reviewed in this article. The uniqueness of the single step erasable photofabrication of large amplitude surface relief structures on azo polymer films is mainly due to the photoisomerization and photoanisotropic behavior of the azobenzene group. The design and synthesis of several classes of suitable azobenzene functionalized polymers for the SRG formation is presented. Experimental studies are carried out in a systematic manner to understand the chemical and physical intricacies involved in the SRG formation process. Structural requirements and their limitations for the efficient fabrication of SRGs on azo functionalized polymer films are also discussed. Understanding the fundamental mechanism of SRG formation on the azo functionalized polymer films enables us to make use of these structures in a variety of applications.

Surface relief structures on azo polymer films

Accurate modeling of photonic devices is essential for the development of new, higher performance optical components required by current and future high-bandwidth communications systems. This paper reviews several key techniques for such modeling, many of which are used in commercial design tools. These include several mode-solving techniques, the beam propagation method, the method of lines, and the finite-difference time-domain technique.

Numerical techniques for modeling guided-wave photonic devices

We describe a finite difference solution technique for the full-vector waveguide equation based upon the alternating-direction-implicit (ADI) iterative method. Our technique accurately treats dielectric boundaries (including corners), requires minimal computer resources, and executes faster (by factors of 3-10) than other iterative approaches. In addition, we employ a transparent boundary condition that effectively removes the sensitivity of the calculated results to the size of the computational domain. This feature greatly facilitates the examination of modes near cutoff. >

Full-vector waveguide modeling using an iterative finite-difference method with transparent boundary conditions

A monolithic tapered rib waveguide for transformation of the spot size of light between a semiconductor optical device and an optical fiber or from the fiber into the optical device. The tapered rib waveguide is integrated into the guiding rib atop a cutoff mesa type semiconductor device such as an expanded mode optical modulator or and expanded mode laser. The tapered rib acts to force the guided light down into the mesa structure of the semiconductor optical device instead of being bound to the interface between the bottom of the guiding rib and the top of the cutoff mesa. The single mode light leaving or entering the output face of the mesa structure then can couple to the optical fiber at coupling losses of 1.0 dB or less.

Tapered rib fiber coupler for semiconductor optical devices

We present the results of subwavelength antireflection surfaces etched into GaAs for use at 975 nm. These surfaces comprise linear gratings with periods less than the wavelength of light in GaAs. The structure appears as a homogeneous birefringent film. For one of the two polarizations, the film is directly analogous to the well-known quarter-wavelength antireflection coating. For the other polarization there is little effect on the surface reflectivity.

Polarization-sensitive subwavelength antireflection surfaces on a semiconductor for 975 nm

We have fabricated subwavelength diffractive optical elements with binary phase profiles for operation at 975 nm. The individual surface-relief features of the elements are smaller than the wavelength of light in the material. By modulating the size and spacing of the features we form artificial, gradient, effective index-of refraction surfaces. The blazed transmission gratings were designed with rigorous coupled-wave analysis and fabricated by direct-write electron-beam lithography and reactive ion-beam etching in GaAs. The gratings have minimum features 63 nm wide. Transmission measurements show 85% diffraction efficiency into the first order.

High-efficiency subwavelength diffractive optical element in GaAs for 975 nm

We present the design and experimental demonstration of a tapered-rib adiabatic-following fiber coupler (TRAFFIC). This device is an adaptation of a Shani-Henry mode converter fabricated in (Al)GaAs and designed to increase the coupling efficiency of conventional optical fibers to tightly-confined semiconductor rib waveguide devices. This approach offers the potential of significantly reducing fiber butt coupling losses from the typical values of 7 to 10 dB to values of <1 dB. This long-standing packaging problem is one of the major impediments to the widespread acceptance of semiconductor-based optoelectronics. Moreover, the design can be implemented with minimal increase in fabrication complexity since it uses only a straightforward modification to epitaxial growth, and one additional lithography and etching step.

R.E. Smith

Papers

Full-vector waveguide modeling using an iterative finite-difference method with transparent boundary conditions

Tapered rib fiber coupler for semiconductor optical devices

Polarization-sensitive subwavelength antireflection surfaces on a semiconductor for 975 nm

High-efficiency subwavelength diffractive optical element in GaAs for 975 nm

Reduced coupling loss using a tapered-rib adiabatic-following fiber coupler