Tim LaFave

Physical phaseplate for the generation of a millimeter-wave hermite-Gaussian beam

This paper demonstrates the design, fabrication and characterization of a physical phaseplate, which transforms plane waves to Hermite-Gaussian mode beams. The phaseplate is designed for a HG11 mode, working at E-band from 71 to 76 GHz. Simulations were run in ANSYS HFSS, a full-wave electromagnetic simulator, and compared to measured results for phaseplates fabricated with laminate substrates and polymer sheets. The simulations and measurements are in good agreement and demonstrate the designed Hermite-Gaussian beam.

Active Integrated Filters for RF-Photonic Channelizers

A theoretical study of RF-photonic channelizers using four architectures formed by active integrated filters with tunable gains is presented. The integrated filters are enabled by two- and four-port nano-photonic couplers (NPCs). Lossless and three individual manufacturing cases with high transmission, high reflection, and symmetric couplers are assumed in the work. NPCs behavior is dependent upon the phenomenon of frustrated total internal reflection. Experimentally, photonic channelizers are fabricated in one single semiconductor chip on multi-quantum well epitaxial InP wafers using conventional microelectronics processing techniques. A state space modeling approach is used to derive the transfer functions and analyze the stability of these filters. The ability of adapting using the gains is demonstrated. Our simulation results indicate that the characteristic bandpass and notch filter responses of each structure are the basis of channelizer architectures, and optical gain may be used to adjust filter parameters to obtain a desired frequency magnitude response, especially in the range of 1–5 GHz for the chip with a coupler separation of ∼9 mm. Preliminarily, the measurement of spectral response shows enhancement of quality factor by using higher optical gains. The present compact active filters on an InP-based integrated photonic circuit hold the potential for a variety of channelizer applications. Compared to a pure RF channelizer, photonic channelizers may perform both channelization and down-conversion in an optical domain.

Square dielectric THz waveguides.

A holey cladding dielectric waveguide with square cross section is designed, simulated, fabricated and characterized. The TOPAS waveguide is designed to be single mode across the broad frequency range of 180 GHz to 360 GHz as shown by finite-difference time domain simulation and to robustly support simultaneous TE and TM mode propagation. The square fiber geometry is realized by pulling through a heat distribution made square by appropriate furnace design. The transmitted mode profile is imaged using a vector network analyzer with a pinhole at the receiver module. Good agreement between the measured mode distribution and the calculated mode distribution is demonstrated.

Discrete charge dielectric model of electrostatic energy

Abstract Studies on nanoscale materials merit careful development of an electrostatics model concerning discrete point charges within dielectrics. The discrete charge dielectric model treats three unique interaction types derived from an external source: Coulomb repulsion among point charges, direct polarization between point charges and their associated surface charge elements, and indirect polarization between point charges and surface charge elements formed by other point charges. The model yields the potential energy, U ( N ), stored in a general N point charge system differing from conventional integral formulations, 1 / 2 ∫ E · D ⅆ V and 1 / 2 ∫ ρ Φ ⅆ V , in a manner significant to the treatment of few-electron systems.

Experiment and Theory of an Active Optical Filter

The role of gain in an optical filter is advanced by good agreement between theory and experiment presented herein. The particular integrated photonic filter is composed of four semiconductor optical amplifiers and one four-port coupler located at the intersection of the amplifiers. The four-port coupler is realized using frustrated total internal reflection off a very thin slab of alumina embedded in the substrate. The delta function response of the filter is measured using an ultra-fast laser and cross-correlator, and the measured transfer functions agree well with a z-transform-based description of the device.

The value of monophasic capacitance of few-electron systems

Due to the discreteness of electronic charges in a nanoscale system, capacitance is defined in terms of the total interaction energy of N-electrons confined in a dielectric sphere. Specifically, the distribution of N-electrons is obtained from minimization of the total Coulomb and polarization interaction energy and the formation energy, the work done on the system. Our discrete charge dielectric (DCD) model gives rise to an electrostatic capacitance agreeing with the N=1 and ~ cases. For nanometer-size devices, the Schrodinger equation should be used; however, for size greater than 10nm, the Poisson equation accounts for spatial symmetry properties resulting from the discrete nature of interacting electrons. Without metallic components, the equal potential landscape does not coincide with our spherical boundary except for the N=1 case. There is a special configuration associated with each N. Hence, the capacitance defined is monophasic, representing a single electrostatic phase. The most important application of this work may lie in optoelectronics and biological systems.

Two dimensional optical lattice filters with gain: Fabrication and experimental results

A scalable adjustable photonic integrated circuit is developed. A key enabling component is a nanophotonic coupler requiring very high aspect ratio InP etching. The measured filter response of the device agrees well with theory.

Active Lattice Filter with nanophotonic FTIR-couplers for integrated photonic channelizer

A 2D Active Lattice Filter ALF combining a Manhattan grid of waveguide gain segments with nanophotonic couplers at the waveguide intersections is presented. The particular application reported herein is a basic channelizer. The resulting structure is readily described, designed, and analyzed in Z-transform space. Critical to the implementation of the 2D ALF circuit is a highly efficient small footprint coupler based on frustrated total internal reflection (FTIR).