Photonic crystals, materials with periodically varying refractive indices, show exciting optical properties that enable many technological applications. Conventional photonic crystals have optical properties that are determined at the time of fabrication and the ability to tune them is quite limited, particularly at visible frequencies. We investigate theoretically the possibility to use nanowires or nanotubes as the building block for tunable two-dimensional photonic crystals. Tunability is achieved by fabricating flexible nanowires in a periodic pattern and actuating them electrostatically. This changes the lattice basis, which in turn modifies the optical properties of the photonic crystal. We use a finite-difference time-domain method to model photonic crystals with changeable bases. We show that the optical transmission through a two-dimensional photonic crystal with only a few rows of nanowires in the light propagating direction can be electrostatically tuned from over 90% transmission to less than 10%. We demonstrate that tunability is maintained in realistic three-dimensional experimental geometries. Finally, we analyse the performance of the photonic crystals in terms of actuation voltages and tuning speeds, and conclude that the response time of a tunable carbon-nanofibre-based photonic crystal lies in the microsecond range.
A novel type of hybrid measurement facility comprising a chamber antenna array (CAA) inside an overmoded waveguide (WG) is proposed and analyzed numerically. The reflecting walls of the metal rectangular WG are used in conjunction with a CAA to synthesize obliquely incident plane-wave (PW) fields at the device under test (DUT). This enables increased flexibility in emulating almost any PW multipath testing conditions in the WG chamber without the high cost and complexity of classical anechoic measurement systems employing relatively large phase-steered PW generators (PWGs).A modeling framework is proposed that has been used to devise first-order design rules (e.g., the number of independent propagating modes, dimensions of the WG, CAA, and DUT). Afterward, an optimally beamformed CAA example is presented to numerically validate the quality of the on-and off-axis PW fields in the test zone (TZ). This study shows design tradeoffs between the amplitude ripple in the DUT region, the total power focused in this region, the DUT size, and the angle of incidence.
Carbon nanofibers (CNF) are used as components of planar photonic crystals. Square and rectangular lattices and random patterns of vertically aligned CNF were fabricated and their properties studied using ellipsometry. We show that detailed information such as symmetry directions and the band structure of these novel materials can be extracted from considerations of the polarization state in the specular beam. The refractive index of the individual nanofibers was found to be n CN F = 4.1.
A square planar photonic crystal composed of carbon nanofibers was fabricated using e-beam lithography and chemical vapor deposition. The diffraction properties of the system were characterized experimentally and compared with theory and numerical simulations. The intensities of the (-1,0) and (-1,-1) diffraction beams were measured as functions of the angles of incidence for both s and p-polarization. The obtained radiation patterns can be explained using a simple ray interference model, but finite-difference time-domain (FDTD) calculations are necessary to reproduce the observed dependence of the scattered radiation intensity on incident laser polarization. We explain this in terms of the aspect ratio of the nanofibers and the excitation of surface plasmon polaritons at the substrate interface.
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