Holographic design and band gap evolution of photonic crystals formed with five-beam symmetric umbrella configuration

Fabrication of helical photonic structures with submicrometer axial and spatial periodicities following "inverted umbrella" geometry through phase-controlled interference lithography

In this Letter we report for the first time, to the best of our knowledge, a phase spatial light modulator (SLM)-based interference lithography (IL) approach for the realization of hexagonally packed helical photonic structures with a submicrometer scale spatial, as well as axial, periodicity over a large area. A phase-only SLM is used to electronically generate six phase-controlled plane beams. These six beams from the front side and a direct central backside beam are used together in an "inverted umbrella" geometry setup to realize the desired submicrometer axial periodic chiral photonic structures through IL. The realized structures with 650 nm spatial and 353 nm axial periodicities on negative photoresist can be used as an optical filter and refractive index sensor, as evidenced from the FDTD-based simulation study on its optical properties. Further, the fabricated templates can be transferred to metals such as silver or aluminum for the realization of a metamaterial-based broadband circular polarizer ranging from 1 to 3.5 μm of near-infrared spectra.

Layered nano-gratings by electron beam writing to form 3-level diffractive optical elements for 3D phase-offset holographic lithography

A multi-level nanophotonic structure is a major goal in providing advanced optical functionalities as found in photonic crystals and metamaterials. A three-level nano-grating phase mask has been fabricated in an electron-beam resist (ma-N) to meet the requirement of holographic generation of a diamond-like 3D nanostructure in photoresist by a single exposure step. A 2D mask with 600 nm periodicity is presented for generating first order diffracted beams with a preferred π/2 phase shift on the X- and Y-axes and with sufficient 1(st) order diffraction efficiency of 3.5% at 800 nm wavelength for creating a 3D periodic nanostructure in SU-8 photoresist. The resulting 3D structure is anticipated to provide an 8% complete photonic band gap (PBG) upon silicon inversion. A thin SiO2 layer was used to isolate the grating layers and multiple spin-coating steps served to planarize the final resist layer. A reversible soft coating (aquaSAVE) was introduced to enable SEM inspection and verification of each insulating grating layer. This e-beam lithographic method is extensible to assembling multiple layers of a nanophotonic structure.

Polarization control in flexible interference lithography for nano-patterning of different photonic structures with optimized contrast

Half-wave plates were introduced into an interference-lithography scheme consisting of three fibers that were arranged into a rectangular triangle. Such a flexible and compact geometry allows convenient tuning of the polarizations of both the UV laser source and each branch arm. This not only enables optimization of the contrast of the produced photonic structures with expected square lattices, but also multiplies the nano-patterning functions of a fixed design of fiber-based interference lithography. The patterns of the photonic structures can be thus tuned simply by rotating a half-wave plate.

Large complete bandgaps in a two-dimensional square photonic crystal with isolated single-atom dielectric rods in air

It is well known that the square lattice of isolated single-atom dielectric rods in air does not give rise to complete bandgaps even when asymmetry is introduced to lift some degeneracy. However, in this paper, a new kind of two-dimensional square photonic crystal with isolated single-atom dielectric rods in air formed by holographic lithography is proposed, and the relation between their photonic bandgap properties and their specific holographic design are systematically analyzed. In addition to the large complete relative bandgap, namely 9.68% gap/midgap ratio for the dielectric constant contrast of 13.6:1, this structure has very large tolerance on the system parameters and fabrication conditions. This fact can greatly relax the experimental requirements. This work may demonstrate the unique feature and advantages of photonic crystals made by the holographic method and provide a guideline for their design and fabrication.