Zexuan Qiang

Optical Add-Drop Filters Based on Photonic Crystal Ring Resonators

We present here an optical add-drop filter (ADF) design based on ultra-compact photonic crystal ring resonators (PCRRs). The normalized transmission spectra for single-ring and dual-ring configurations have been investigated by using the two-dimensional finite-difference time-domain (FDTD) technique in a square lattice dielectric-rod photonic-crystal structure. With the introduction of four scatterers at the corners of quasisquare- ring PCRR, high wavelength selectivity and close to 100% drop efficiency can be obtained. Both backward- and forward-dropping were achieved by controlling the coupling efficiency between two PCRR rings for resonant modes with different symmetry. The resonant-mode quality factor Q and the wavelength tunability were also analyzed, opening opportunities for PCRRs as ultra-compact filters, optical add-drop multiplexers, electrooptical N x N switches and electrooptical modulators.

Flexible photonic-crystal Fano filters based on transferred semiconductor nanomembranes

Crystalline semiconductor nanomembranes (NMs), which are transferable, stackable, bondable and manufacturable, offer unprecedented opportunities for unique and novel device applications. We report and review here nanophotonic devices based on stacked semiconductor NMs that were built on Si, glass and flexible PET substrates. Photonic-crystal Fano resonance based surface-normal optical filters and broadband reflectors have been demonstrated with unique angle and polarization properties. Such a low temperature NM stacking process can lead to a paradigm shift on silicon photonic integration and inorganic flexible photonics.

Polarization-independent drop filters based on photonic crystal self-collimation ring resonators.

We report here a polarization-independent drop filter (PIDF) based on a photonic crystal self-collimation ring resonator (SCRR). Despite of the large birefringence associated with the polarization-dependent dispersion properties, we demonstrate a PIDF based on multiple-beam interference theory and polarization peak matching (PPM) technique. The PIDF performance was also investigated based on finite-difference time-domain (FDTD) technique, with excellent agreement between the theory and the simulation. For the designed drop wavelength of 1550 nm, the polarization-independent free spectral range is about 36.1 nm, which covers the whole optical communication C-band window. The proposed PIDFs are highly desirable for applications in photonic integrated circuits (PICs).

Polarization and angular dependent transmissions on transferred nanomembrane Fano filters

We report angular and polarization dependent transmission properties of Fano resonance optical filters with transferred silicon nanomembrane on glass substrate. The transmission spectra of the filters can have either weak or strong polarization and angular dependence, depending on properties of individual Fano resonance modal dispersion. Measurement results agree very well with simulations based on a rigorous coupled-wave analysis for the transmission spectra, on planewave expansion wave-vector technique for the dispersion property analysis, and on a three-dimensional finite-difference time-domain technique for the propagating modal study. These results will provide importance guidance for the design of a new class of ultra-compact surface-normal frequency selective components with preferred polarization and angular properties. These components are highly desirable for silicon photonic integration.

Photonic NOT and NOR gates based on a single compact photonic crystal ring resonator

New all-optical NOT and NOR logic gates based on a single ultracompact photonic crystal ring resonator (PCRR) have been proposed. The PCRR was formed by removing the line defect along the GammaM direction instead of the conventional GammaX direction in a square-pattern cylindrical silicon-rod photonic crystal structure. The behavior of the proposed logic gates is qualitatively analyzed with the theory of beam interference and then numerically investigated by use of the two-dimensional finite-difference time-domain method. No nonlinear material is required with less than a 2.2 microm effective ring radius. The wavelengths of the input signal and the probe signal are the same. This new device can potentially be used in on-chip photonic logic-integrated circuits.

Design of Fano Broadband Reflectors on SOI

We report here the design of Fano resonance broadband reflectors based on two-dimensional photonic crystals on silicon-on-insulator. The impact of vertical confinements and buffer configurations was investigated with a three-dimensional finite-difference time-domain technique. Spectral red- and blue-shifts were obtained by controlling the effective buffer layer indices, with very good agreement between experimental and simulation results.

Resonance control of membrane reflectors with effective index engineering

We report here broadband membrane reflectors based on Fano resonances in patterned silicon nanomembranes. Resonance control of the reflectors was realized either by partially removing buried oxide layer underneath the device layer or by controlled SiO2 film deposition on top of the devices. Both blueshifts and redshifts were demonstrated with a turning range of 50 nm for a center wavelength of 1550 nm. These results demonstrate practical postprocess means for Fano resonance engineering for both narrowband filters and ultracompact broadband reflectors.

Photonic crystal defect mode cavity modelling: a phenomenological dimensional reduction approach

A phenomenological dimensional reduction approach (PDRA) for the cavity characteristics in defect mode based photonic crystal (PC) lasers is presented. Based on the fully vectorial three-dimensional finite-difference time-domain (3D FDTD) technique, simultaneous enhancement and suppression in spontaneous emission and absorption were obtained in an absorptive photonic crystal slab (PCS) cavity. Effective index perturbation (EIP) was proposed for fast and accurate determination of the effective index and the dominant resonant cavity frequency in a 3D PCS structure, with two-dimensional (2D) FDTD simulation. Further dimensional reduction from 2D to one-dimensional planar cavity enables phenomenological modelling of lasing characteristics via the effective reflectivity calculation and rate equation analysis. Very fast and accurate results have been achieved with this PDRA approach. A high spontaneous emission factor and cavity quality factor Q were obtained in a single defect cavity, which led to over an order reduction in lasing gain threshold. The model offers a fast and accurate tool for the design and modelling of PC defect mode cavity based devices and aids the research in the proposed novel defect mode based devices such as ultra-compact light sources on Si and spectrally resolved PC infrared photodetectors.

Fast Calculation of Cavity-Mode Characteristics of Photonic Crystal Cavities

A fast approach based on effective index perturbation method is proposed to evaluate the intrinsic characteristics of photonic-crystal-slab-based microcavity with two-dimensional finite-difference time-domain (2-D FDTD) technique. For two widely used single defect structures, less than 2% computational error was obtained in calculating the defect mode frequencies. Accurate prediction of cavity modal properties and resonant peak frequencies is feasible based on 2-D FDTD simulation by adjusting the effective index to match the dielectric band edge for donor-like defect mode. The correlation between the modified effective index and the cavity (lasing) mode with the highest quality factor Q offers an efficient tool in the design of defect mode based photonic crystal microcavities

Colloidal quantum dot absorption enhancement in flexible Fano filters

We report here modified absorption property of colloidal quantum dots (CQDs) inside flexible Fano filters—made of patterned single crystalline silicon nanomembrane transferred onto flexible plastic substrates. Enhanced optical absorption was obtained both experimentally and theoretically, when the CQD absorption peak spectrally overlaps with Fano resonance peak. On the other hand, suppressed absorption was observed when the Fano resonance has no spectral overlap with the CQD absorption bands.