We have recently achieved a non-dispersive, low-order pseudogap in the blue region near the L-point in electrodeposited ZnO inverse opal. This behavior is a consequence of the change in the interconnecting network of interstices from a low dielectric (air) in the opal to a high dielectric one in the inverse opal. To verify this, we present the calculated photonic bands along the LW direction, which corresponds to the angular range explored experimentally by means of angle-resolved spectroscopy. We also studied the dispersion as a function of refractive index contrast (RIC). Further increase of the RIC above the threshold necessary to open up a complete photonic bandgap between the 8th and 9th bands does not have significant effects on improving the non-dispersive characteristic in the pseudogap. The results could be extended to make other inverse photonic structures of different symmetry with non-dispersive bands suitable for the study of optical processes involving low group velocity.

To reach the full theoretical potential advantages of ideal Quantum Dots (QDs) for diode lasers and photodetectors, elimination of the wetting layer, which is inherent to self-assembled QDs of Stranski-Krastnow (SK) growth mode, and achieving a uniform mono-modal QD size distribution is needed. The SK QD approach is complicated by the randomness of the QD size distribution and inherent presence of the wetting layer. These factors have been experimentally identified as the underlying cause for low optical gain and high temperature sensitivity in diode lasers which result from carrier leakage out of the QDs into the wetting layer.

An alternate approach to QD formation is the use of nanopatterning with diblock copolymers combined with selective MOCVD growth. We utilize cylinder-forming PS-b-PMMA which have the ability of preserving the hole size through the pattern transfer procedures. The combination of diblock copolymer lithography with selective MOCVD growth of the QDs could lead to a higher degree of control over QD shape, size uniformity, and composition over the self-assembly process. Since the SK self-assembly process is not employed, the problematic wetting layer states are eliminated and improved optical gain can be expected. Control over the QD height, shape, and strain, also allows for the design of increased energy spacing between ground and excited QD states and hence a wider control or selection of the emission wavelengths. Since the QD strain is decoupled from the size, the process also has potential for achieving longer wavelength emission compared with SK QDs. On a GaAs substrate, hexagonally arranged uniform size QD arrays have been fabricated. The QD patterning is prepared by dense nanoscale diblock copolymer lithography, which consists of perpendicularly ordered cylindrical domains of polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA) matrix. To transfer polymer patterns to QD arrays, two mask materials have been taken under consideration. Firstly, to characterize the profile and distribution of the QDs, a dielectric template mask was utilized and the polymer patterning is transferred on it. After the pattern transfer to the dielectric and subsequent removal of the polymer, single crystal GaAs QDs are selectively grown by MOCVD. SEM images indicate that the QD density is larger than 5×10^10/cm2, comparable to SK growth mode and the size distribution peaks at a 12nm diameter. Finally, to grow QDs and subsequently cover the QDs in situ, an AlxGa1-xAs template mask is being investigated. After the pattern transfer to a 15nm thick AlGaAs template layer using RIE, the native oxide on the patterned AlGaAs surface acts as a selective growth mask for InAs QDs. After deposition of the QDs, capping layers can be grown on QDs without removing the sample from the reactor. We are currently investigating optimal growth conditions for covering the QDs and characterizing the photoluminescence. University of Wisconsin -Madison gratefully acknowledges support from the ARO MURI W911NF-05-1-0262 (Dr. John Prater) and NSF NSEC DMR-0425880.

We suggest a photonic structure with energy selective and diffractive properties to be incorporated in thin-film tandem solar cells. Our device enhances the pathway of incident light within a amorphous silicon photovoltaic (PV) top cell in its spectral region of low optical absorption. This leads to an increase in the short-circuit current of the top cell. For a conductive inverted opal structure as intermiediate layer, we numerically determine an current increase of 1.44mA/cm2 for an a-Si:H / c-Si thin-film tandem cell corresponding to an increase in the absolute efficiency from 11.1% to 12.4%.

Photonic crystal type nanoarchitectures (built of chitin and air) occurring in the scales of two male butterflies, with dorsal blue and ventral green coloration were investigated by optical microscopy, SEM and TEM. Reflectance data were collected by normal incidence measurements and by integrated reflectance measurements, too. Reflectance peaks with close positions in the blue range of the spectrum were found both for the single crystalline scales and for the most disordered photonic band gap material. Polycrystalline structures with grain size on the micron scale produce matt reflectors, while moderately ordered “pepper-pot” type structures can yield surfaces with shiny aspect. It was found that the cover scales on each sides of the wing are primarily responsible for the observed colors. The character of the observed colors (shiny, or matt) correlates with the habitat of the investigated butterflies.

Metallo-dielectric photonic crystals are sharp thermal emitters at infrared wavelengths, and are being employed in sensors. We describe the theory of thermal emission and enhanced absorption in these photonic crystals using a scattering matrix approach, where Maxwell's equations are solved in Fourier space. A sub-wavelength hole array in a metal layer is coupled to a two-dimensional photonic crystal of the same periodicity in these metallo-dielectric photonic crystals. The sub-wavelength hole array has an enhanced transmission mode that couples to a weakly guided mode of the photonic crystal having similar modal character. The transmissive mode of the hole array is absorbed by the photonic crystal to create a sharp absorption and reflective minimum. The enhanced absorption is investigated in different lattice symmetries.

We report the nanofabrication of Bragg reflector waveguide structure based on barium titanate (BTO) epitaxial thin film grown on MgO substrate and characterized its optical transmission. A low pressure (~1 Torr) nano-lithography was used for the formation of Bragg reflector waveguide structure. We demonstrate that there is sufficient refractive index contrast to form a well-defined stop band by forming gratings in a top strip layer of Si3N4 that was grown on BTO thin film. Simulation of the optical transmission and stop band of TE mode, and electro-optic (EO) modulator tunability are discussed as well. The non-linear photonic crystal waveguides are potentially suitable as tunable optical filters and ultra-wide bandwidth modulators.

Photonic crystal (PhC) can potentially result in enhanced optical properties around critical points in the photonic band gap. Of interest here are enhanced nonlinear optical effects. In this study, one and two dimensional nonlinear PhC were designed and fabricated using barium titanium oxide (BTO) thin films as the active medium. Nonlinear PhC made with barium titanate thin films potentially provide integrated devices with the advantages of wide tunability and high stability. Films 500 nm thick deposited on MgO substrates were utilized. Two dimensional PhC structures were defined by focused ion beams (FIB). Before patterning, a thin metal layer was deposited on the barium titanate layers in order to improve the conductivity of the samples. After writing the patterns, cylindrical air holes were generated in the thin film layers. The PhC lattice constant and the hole radius were selected in sub-micron region in order to satisfy the requirement of wave resonance. The PhCs with sub-micron features were characterized by the atomic force microscopy (AFM), scanning electron microscopy (SEM), and near field optical microscopy (NSOM). The transmission spectra of the PhC waveguides were calculated with a continuous wide band source that covered 1 to 2 micron wavelength. Simulations of the transmission characteristics were performed using the two dimensional finite difference time domain method (FDTD).

Photonic crystals currently hold exciting potential in waveguiding applications with the ability to greatly reduce waveguide bend radii, allowing for the realization of photonic networks with increased complexity and device density. Square spiral photonic crystals are comprised of periodic arrays of slanted cylindrical columns with periodic abrupt 90° changes in the column growth direction. The abrupt changes produce elbows which correspond to points in the three-dimensional diamond lattice, which are commonly referred to as diamond:n photonic crystal structures based on the nth nearest neighbor lattice point between column arms. The diamond:n structures are characterized by four parameters: column separation, spiral pitch, column diameter, and column growth angle. These structures have been shown theoretically to have a three-dimensional relative bandgap of approximately 15% for a direct silicon diamond:1 structure and approximately 24% for an inverse silicon diamond:5 structure with circular column cross sections. Silicon square spiral photonic crystals can be grown by a single-step thin film deposition process known as glancing angle deposition (GLAD), which utilizes rotation of lithographically patterned substrates and advanced control algorithms. While using GLAD to grow square spiral photonic crystals in a single-step process is desirable, it does have inherent limitations. Physical vapor deposited films deposited at oblique incidence angles form columns with angle ≤ 60° relative to the substrate normal. This deviates from the optimal diamond:1 and diamond:5 structures which require film growth angles of approximately 64° and 74°, respectively. Here we present SEM images of periodic silicon square spiral films grown with a vapor source incidence angle of 85° that indicate an average column tilt angle of 57˚ for films grown with the basic GLAD process and an average column tilt angle between 61˚ and 65˚ using an ion-assisted GLAD process. Photonic simulations indicating that square spiral films with complete three-dimensional photonic bandgaps are only achievable via ion-assisted GLAD are also presented.

A novel technique for fabricating highly ordered colloidal photonic crystals has been developed. In this method, a droplet of water containing polystyrene microspheres was added to the surface of a fluorinated solvent bath. Consequently, the two liquids remained separated and the colloidal particles self-assembled into close-packed structures at the interface between them. By transferring the droplet onto a glass slide, a highly ordered crystal was obtained. This technique offers a new, potentially easier, and more effective approach than currently used. We believe that it will open new ways for fabricating materials based on colloidal crystals as well as applying the colloidal photonic crystals to optical devices.

It is commonly assumed that the long-wavelength limit of a metamaterial can always be described in terms of effective permeability and permittivity tensors. Here we report that this assumption is not necessary–there exists a new class of metamaterial consisting of several interlocking disconnected metal networks, for which the effective long-wavelength theory is local, but the effective field is non-Maxwellian, and possesses much more internal degrees of freedom than effective Maxwellian fields in a local homogeneous medium.