Changbao Ma

A three-dimensional wide-angle BPM for optical waveguide structures

Algorithms for effective modeling of optical propagation in three- dimensional waveguide structures are critical for the design of photonic devices. We present a three-dimensional (3-D) wide-angle beam propagation method (WA-BPM) using Hoekstras scheme. A sparse matrix algebraic equation is formed and solved using iterative methods. The applicability, accuracy and effectiveness of our method are demonstrated by applying it to simulations of wide-angle beam propagation, along with a technique for shifting the simulation window to reduce the dimension of the numerical equation and a threshold technique to further ensure its convergence. These techniques can ensure the implementation of iterative methods for waveguide structures by relaxing the convergence problem, which will further enable us to develop higher-order 3-D WA-BPMs based on Padé approximant operators.

Phthalocyanine Nanoparticle Formation in Supersaturated Solutions

Self-organization of molecules in solution is an important natural and synthetic process, in particular for the preparation of nanomaterials. However, the mechanism of growth for solution-based nanoparticle formation is not always well understood. We present results that clarify these mechanisms in solutions of magnesium phthalocyanine in which the self-organization is induced by addition of a miscible nonsolvent. From simultaneous measurements of the sizes of the growing nanoparticles by photon correlation spectroscopy and the molecular concentration by absorption and fluorescence spectroscopy, we have found that the particles do not grow by molecular diffusion to the surfaces. These results suggest the importance of unstable clusters in the growth process. We also observed a strong dependence of the particle size on the initial concentration which we attribute to effects of the curvature of the solubility curve.

A simple three dimensional wide-angle beam propagation method

The development of three dimensional (3-D) waveguide structures for chip scale planar lightwave circuits (PLCs) is hampered by the lack of effective 3-D wide-angle (WA) beam propagation methods (BPMs). We present a simple 3-D wide-angle beam propagation method (WA-BPM) using Hoekstras scheme along with a new 3-D wave equation splitting method. The applicability, accuracy and effectiveness of our method are demonstrated by applying it to simulations of wide-angle beam propagation and comparing them with analytical solutions.

Right-angle slot waveguide bends with high bending efficiency

Two right-angle bends for nanoscale slot waveguides with high bending efficiency based on a corner mirror and different resonant cavities are presented, one with a triangular cavity and the other with a square cavity. Through two-dimensional parametric scanning of the position of the mirror and the dimension of the cavity, a maximum bending efficiency calculated using mode overlap integral (MOI) of 94.3% is achieved for the bend with the triangular cavity and 93.1% is achieved for the bend with the square cavity. Although they both have similar bending performance, the position of the mirror is different between the two cases.

High efficiency right-angle bending structures in continuous slot waveguides

We present two new high efficiency right-angle slot waveguide bends based on a corner mirror and a triangular or square cavity. In contrast to previous devices, the nanophotonic slot regions in these bends are continuous, leading to more robust designs and the ability to create nanofluidic slot regions. The additional layer also leads to higher bending efficiencies, 95.4% and 94.1% for the bends with the triangular and square cavity, respectively.

The Refractive Index Change and Self-Defocusing Effect of Methyl Red Doped Polymer

The absorption spectra of trans and cis states of the methyl red doped poly(methyl methacrylate) film are measured separately. The refractive index change distribution of the sample film is determined using the Kramers–Kronig relation along with the absorption spectrum data. The self-defocusing properties of a methyl red sample at 441.6 and 535 nm are investigated and the results indicate a significant optical limiting property.

On the Gaussian approximation and margin measurements in optical amplifier systems

By comparing with the exact optical communication system performance obtained using a quasi-analytical BER calculation method, we systematically analyze the accuracy of the Gaussian approximation (GA) method and the threshold-based margin measurement method for performance evaluation of on-off keying (OOK) optical amplifier systems. We reveal the inherent connection between the GA and the threshold method from both the one-shot case and systems with low inter-symbol interference (ISI). For systems with strong ISI, we show that GA method may fail but the threshold method still provides accurate system performance estimates, even when large threshold bias is used in the measurement.

Confinement Analysis in Symmetric and Asymmetric Nanoscale Slab Slot Waveguides

Using the recently derived analytical TM modal field expression and the transcendental dispersion relation for a general nanoscale asymmetric slab slot waveguide, the confinement performance in symmetric vs. asymmetric geometries was systematically analyzed and compared.

Simulations of thermo-optic effects in beam propagation

The finite difference beam propagation method (FD-BPM) is an effective model for simulating a wide range of optical waveguide structures. We present results of simulations combining this method with finite element modeling of thermal effects using Comsol FEMLAB software. These simulations were developed and are used to examine propagation of optical signals in polymer waveguides in which inhomogeneous temperature profiles are induced using MEMS microheaters, for example, for use in switching applications. Thermal modeling combined with values of the thermo-optic nonlinearity yields three-dimensional refractive index profiles in the active regions of a variety of waveguide structures. The change in the refractive index profile of cladding induces mode deformations and transmission losses due to leakage at the core/cladding interface, in addition to phase shifts in the propagating beams. These effects are used to design thermo-optic switches in both multimode and single mode waveguides, and study the direct effect on propagation due to changes in the applied heater power. In addition, we demonstrate the utility of applying the method to assessing losses due to thermally induced inhomogeneities in planar lightwave circuits such as optical interconnects.

A multimode thermo-optic beam steering switch

Thermooptic switches are viable options for rapidly and reliably switching and routing optical signals in planar lightwave circuits. We present modeling and fabrication of a thermooptic switch made of the polymer SU-8, an epoxy resin commonly used as a MEMS structural material. SU-8 is a good candidate material for use in planar waveguides due to its high refractive index, good transmission properties in the visible and infrared, and excellent thermal and mechanical stability. Furthermore, it has great advantages in fabrication since it is used as a negative photoresist, and so can be patterned directly using photolithography. Light is guided by a refractive index gradient generated by embedded MEMS microheaters, which activate the thermal nonlinearity of the polymer. The thermooptic change in refractive index imparts an inhomogeneous phase shift to the beam in the waveguide, which guides the input into one of two or more outputs. The switch design and operation parameters have been optimized using simulations of the thermal profile using finite element modeling and of the optical propagation using the beam propagation method.