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Open Access Publications from the University of California

Engineering of highly efficient metasurfaces for flat optics

  • Author(s): Kim, Sung Woon
  • Advisor(s): Fainman, Shaya Y
  • et al.

Conventional optical components, such as lenses, mirrors, waveplates and polarizers, have been widely developed and used in many electronic and optical devices. Because these components are bulky, they are not suitable for miniaturization and integration. In recent years, metasurfaces have emerged as a platform to realize the transformation of the field of optical devices as they have the potential to revolutionize the way light is controlled on a chip.

Metallic nanostructures are intrinsically lossy in the optical spectral region due to the absorption in metals. In addition, the design parameters of metasurfaces have limitations for controlling the optical phase-front in the full range of 0 to 2π. These restrictions lead to the introduction of several undesirable losses, including reflection, diffraction, and polarization conversion. Compared to metallic nanostructures, dielectric metasurfaces have several significant advantages such as high transmission efficiency because they do not suffer from the intrinsic nonradiative losses in metals. All-dielectric metasurfaces can allow a diverse range of practical efficient wave-shaping applications of novel materials.

In this dissertation, we report on the experimental study of the anomalous transmission effect in ultrathin metallic gratings, where the metal thickness is much thinner than the skin depth. In particular, incident TM polarized waves are reflected while incident TE polarized waves are transmitted. The anomalous transmission strongly depends on the metal width, thickness and refractive indices of the surrounding dielectric material. We systematically investigate and demonstrate the anomalous effect and determine the optimized nanostrip thickness and width by introducing a shadow-mask fabrication approach. The combined effect of thickness and width is experimentally investigated, and shown to match well with theoretical analysis. The main advantage of our ultrathin metal gratings lies in insertion loss reduction by utilizing the ultrathin metallic film fabrication. This advantage makes our structure readily suitable for a variety of applications including high efficiency metasurfaces, polarization steering, and polarization dependent spectral filter applications.

Also, we explore the design, fabrication, and characterization of dielectric metasurface lens created by varying the density of subwavelength low refractive index nanoholes in a high refractive index substrate, resulting in a locally variable effective refraction index. It is demonstrated that constructed graded index lenses can overcome diffraction effects when the aperture to wavelength ratio (D/λ) is smaller than 40. Our design parameters for engineering the effective refractive index of a composite dielectric are created by controlling the density of deeply subwavelength low index nanoholes in a high index dielectric layer (e.g., Si). The phase of the optical wavefront incident on such a composite dielectric is modulated by the local effective index of the layer. We have demonstrated that the microlenses can be made polarization dependent by asymmetric design as well as polarization independent by symmetric design operating with radiation from a broad spectral range. The main advantages of our dielectric nanosurface lenses include further reduction of insertion loss by adding antireflection (AR) coating of element size and weight via submicron thickness fabrication and miniaturization. Such advantages make our structure readily suitable for a variety of applications, such as microlens arrays, high resolution CCD sensors, and other miniature imaging systems. The experimental results demonstrate the practical potential of polarization and position dependent graded index components by asymmetric designs. We envision using Cartesian and polar coordinate designs for future nanohole region realizations, such as space variant circular nanohole patterns or space invariant elliptical nanohole patterns.

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