Philip Muñoz

Bioinspired micrograting arrays mimicking the reverse color diffraction elements evolved by the butterfly Pierella luna

Significance In the course of evolution, many organisms have developed unique light manipulation strategies that rely on intriguing combinations of a broad range of optical effects generated by materials with sophisticated multiscale hierarchical structural arrangements. By exploiting the optical principles underlying natural structural color, we can generate new photonic materials. Researchers have only just begun to match nature’s morphological and compositional complexity in man-made materials using nanofabrication. We present a bioinspired photonic material that mimics the reverse color-order diffraction found in the butterfly Pierella luna. Exploiting and improving the butterfly’s strategy, we create photonic materials that increase our basic understanding of the optical interplay of hierarchical structures and provide a platform for the development of novel photonic devices. Recently, diffraction elements that reverse the color sequence normally observed in planar diffraction gratings have been found in the wing scales of the butterfly Pierella luna. Here, we describe the creation of an artificial photonic material mimicking this reverse color-order diffraction effect. The bioinspired system consists of ordered arrays of vertically oriented microdiffraction gratings. We present a detailed analysis and modeling of the coupling of diffraction resulting from individual structural components and demonstrate its strong dependence on the orientation of the individual miniature gratings. This photonic material could provide a basis for novel developments in biosensing, anticounterfeiting, and efficient light management in photovoltaic systems and light-emitting diodes.

Integrated Super-Couplers Based on Zero-Index Metamaterials

There has been strong interest in the confinement of electromagnetic energy in sub-diffraction limit waveguide configurations. Such an achievement would offer applications in in telecommunications, subwavelength imaging, optical memory storage, and on-chip photonic processes. Materials with a refractive index of zero have been considered as strong contenders for such “super-coupling” applications (Silveirinha MG, Engheta N, Phys Rev B 76:245109, 2007; Engheta N, Science 340:286–287, 2013). Though e-near-zero and μ-near-zero metamaterials, where zero index is obtained by tuning the effective electric permittivity or permeability to zero, have been proposed as a possible candidate, their infinite or zero impedance causes large reflections which pose a challenge for coupling applications.

Direct laser writing of 3D gratings and diffraction optics

We fabricate 3D gratings and diffraction optics using direct laser writing. Diffraction patterns of gratings agree with Laue theory. We demonstrate zone plates for visible wavelengths. Direct laser writing is promising for integrated diffraction optics.

On-chip super-robust all-dielectric zero-index material

The robustness of the modal degeneracy for photonic Dirac-cone can be engineered by designing all-dielectric pillar arrays giving on-chip platform of zero index material for any wavelength regime. We demonstrate this concept for telecom regime.

Microscroll Invisibility Cloak

We present a design for a self-assembled cylindrical invisibility cloak, based on strained-induced rolling. Here, a 2D slab metamaterial is patterned by electron beam lithography on a compressively strained InGaAs thin film. The metamaterial is composed of an array of periodically spaced silver nanorods embedded in a polymer background. The anisotropic effective permittivity of the slab is defined by on the aspect ratio and pitch of the nanorods, and is modeled by Effective Medium Theory and the Finite Difference Time Domain method (FDTD). The stained film is released from the substrate by wet etching, and the film strain relaxes, causing the combined metamaterial/thin-film to curl into a tight roll. The rolling radius is modeled by continuum strain theory and confirmed by experiment. Resulting microscroll has an anisotropic and radially-dependent effective permittivity, which is inherited from the original slab metamaterial. Depending on the nanorod spacing in the metamaterial layer, we can tune the radial dependence of the permittivity. This design can be used to realize a variety of transformation optical devices with cylindrical symmetry. In particular, we analyze the case of a TE cylindrical invisibility cloak with reduced parameters using FDTD.