Curtis W. Neff

High-filling-fraction inverted ZnS opals fabricated by atomic layer deposition

The infiltration of three-dimensional opal structures has been investigated by atomic layer deposition. Demonstrations using ZnS:Mn show that filling fractions >95% can be achieved and that the infiltrated material is of high-quality crystalline material as assessed by photoluminescence measurements. These results demonstrate a flexible and practical pathway to attaining high-performance photonic crystal structures and optical microcavities.

ZnS‐Based Photonic Crystals

Recent studies of the application of II-VI materials to the fabrication of photonic crystals are reported. Modeling studies show the potential of ZnS photonic crystals to provide a new method of controlling the emission characteristics of this material system so as to enhance color, intensity and decay time. The same structures are also shown to possess giant refraction and dispersion properties that can be used to control (collect, focus and steer) light. The fabrication of these period structures is addressed. Two approaches are being considered: the fabrication of two-dimensional ZnS photonic crystals by conventional electron beam lithography and the formation of three-dimensional ZnS photonic crystals by cost-effective self-assembly methods. ZnS and related II-VI compounds are very attractive for applications in photonic crystal devices operating in the visible and near IR region due to their high indices of refraction and large bandgaps that make them highly transparent in the visible. We report recent studies on the self-assembly of nanoparticles and subsequent ZnS infiltration techniques that can be used to control the emission and out-coupling properties of phosphors embedded in a photonic crystal.

Photonic band tuning in two-dimensional photonic crystal slab waveguides by atomic layer deposition

The photonic bands of two-dimensional (2D) triangular lattice photonic crystal Si slab waveguides were statically tuned using low temperature atomic layer deposition (ALD) of TiO2. Angular dependent reflectance measurements of bare and coated devices were well fitted by three-dimensional finite-difference time-domain calculations. The technique not only allows the physics of photonic band effects in 2D photonic crystals to be systematically studied but also demonstrates large static tuning and precise fine-scale control over band frequency and dispersion, with a frequency tuning range of 12% and precision of 0.005% per ALD cycle. Band tuning to achieve zero group velocity is demonstrated.

ACTIVE PHOTONIC CRYSTAL NANO-ARCHITECTURES

The development of nano-scaled photonic crystal structures has resulted in many new devices exhibiting non-classical optical behavior. Typically, in these structures a photonic band gap and associated defect mode are used to create waveguides, resonators, couplers and lters. In this paper we propose that the functionality of these structures can be signican tly enhanced by the inltration of the photonic crystal with other classes of materials, particularly highly nonlinear liquid crystals and electro-optical materials. The properties of conventional 2D PC slab waveguides were simulated by the nite difference time domain method and shown to exhibit very large refraction and dispersion, and signican t tunable eects under bias when inltrated with liquid crystal. In particular, a new superlattice photonic crystal concept is proposed and shown to exhibit up to 50 tunability in the angle of refraction when alternate liquid crystal inltrated pixel rows were modulated from their aligned to unaligned state. This modulation corresponds to index changes from 1.5 to 2.1; it is assumed that a refractive index change of up to approximately n = 0:6 can be achieved. The superlattice eect was also demonstrated to induce new switching and out-coupling eects that were strongly dependent on the direction of propagation and index modulation. These simulations demonstrate the potential of a new class of optically-active photonic crystal architectures to tune giant refraction and dispersion characteristics and to enable new switching phenomena.

Observation of Brillouin zone folding in photonic crystal slab waveguides possessing a superlattice pattern

The optical properties of superlattice photonic crystal (PC) patterns in two-dimensional slab waveguides were investigated using coupled-resonance angular dependent reflectivity measurements. Additional features were found in the superlattice PC spectra in comparison to that of the triangular lattice, indicating Brillouin zone folding (BZF) due to the larger unit cell and reduced symmetry of the superlattice. Calculated band structures corroborate these measurements and confirm the BZF effect which was shown to extract portions of the triangular lattice guided bands into the waveguide radiating regime, making the dielectric bands excitable by an out-of-plane source.

A photonic crystal superlattice based on triangular lattice.

A two-dimensional superlattice photonic crystal structure is investigated in which the holes in adjacent rows of a triangular lattice alternate between two different radii. The superimposition of a superlattice on a triangular lattice is shown to reduce the photonic bandgap, introduce band splitting, and change the dispersion contours so that dramatic effects are seen in the propagation, refraction, and dispersion properties of the structure. For single mode propagation, the superlattice shows regions of both positive and negative refraction as well as refraction at normal incidence. The physical mechanisms responsible for these effects are directly related to Brillouin Zone folding effects on the triangular lattice that lowers the lattice symmetry and introduces anisotropy in the lattice.

TUNING OF PHOTONIC CRYSTAL BAND PROPERTIES BY ATOMIC LAYER DEPOSITION

We report the application of atomic layer deposition to manipulate the dielectric architecture of conventional and superlattice two-dimensional photonic crystal waveguides fabricated in silicon. Conformal deposition of a second dielectric layer is shown to have a dramatic influence on the photonic band structure and produces unique effects that cannot be emulated in a single dielectric slab photonic crystal material. With additional dielectric coatings, a strong decrease in photonic band frequencies and change in band slope are observed, which for the lowest photonic states produces strong degeneracies. The capability, in principle, to tune the position of bands to within 0.005% accuracy, is demonstrated. Additionally, new features are observed when differential band shifts result in band-crossing and for which like polarizations activate perturbation mechanisms that result in local and strong band curvatures. The extremely strong band bending resulting from band-band interactions could have applications, in slow light devices, and provide a way to introduce non-linear effects into tunable photonic crystal structures.

Optical and Crystallographic Properties of Inverse Opal Photonic Crystals Grown by Atomic Layer Deposition

We report a technique for the formation of infiltrated and inverse opal structures that produces high quality, low porosity conformal material structures. ZnS:Mn and TiO 2 were deposited within the void space of an opal lattice by atomic layer deposition. The resulting structures were etched with HF to remove the silica opal template. Infiltrated and inverse opals were characterized by SEM, XRD, and transmission/reflection spectroscopy. The reflectance spectra exhibited features corresponding to strong low and high order photonic band gaps in the (111) direction (γ-L). In addition, deliberate partial infiltrations and multi-layered inverse opals have been formed. The effectiveness of a post-deposition heat treatment for converting TiO 2 films to rutile was also studied.

Manipulation of Dispersion Properties of Two-dimensional Photonic Crystal Slab Waveguides by Atomic Layer Deposition

In addition to the growing interest within the semiconductor industry, atomic layer deposition (ALD) has also become an attractive tool to engineer and manipulate with extreme precision the complex dielectric architecture of three-dimensional photonic crystals (PCs). Very recently, we proposed to apply this technique to 2D PC slab waveguides.

Photonic band tuning in 2D photonic crystals by atomic layer deposition

In this paper, we have demonstrated a technique in which the photonic bands of two-dimensional triangular lattice photonic crystal slab waveguides can be statically tuned by nanoscale modifications, thereby enabling unprecedented adjustment to the dispersion properties of any 2D photonic crystal. Additionally, the technique facilitates the formation of layered and composite 2D PC waveguides thus opening new fabrication routes to control dispersion, propagation and dielectric contrast