Dingshan Gao

Flat Band Slow Light in Symmetric Line Defect Photonic Crystal Waveguides

Flat band slow light in symmetric line defect photonic crystal waveguides formed by adding dielectric pillars in the air holes nearest to the waveguide core is investigated. By adjusting the radii of the new dielectric pillars, a linear band in the photonic band structure appears which denotes low group velocity dispersion. High average group index of 74.4 with 2.3-nm bandwidth is demonstrated in an optimized waveguide by finite-difference time-domain simulation.

Improvement of delay-bandwidth product in photonic crystal slow-light waveguides.

We report new results about the improvement of delay-bandwidth product in photonic crystal slow light waveguides. Previous studies have obtained large delay-bandwidth product at the price of small average group index. It is pointed out here that the radius and the distance between the two boundary rows of holes have a key contribution for delay-bandwidth product. We show the possibility of improving this factor of merit meanwhile maintaining the same group index. We succeed in improving normal delay-bandwidth product from 0.15 to 0.35, keeping at the same time the group index unchanged at high value of 90. This optimization approach may be applicable for previous flat band slow light devices.

Dispersion Engineering of Wide Slot Photonic Crystal Waveguides by Bragg-Like Corrugation of the Slot

A comb slot photonic crystal waveguide geometry allowing flexible dispersion engineering of wide slot photonic crystal waveguides is proposed. Additionally, a figure of merit combining confinement within the slot and high group index is introduced to quantify the electromagnetic density within slow wave slot waveguides, and is applied to the proposed waveguide geometries. Group indices exceeding 130 and 90 over bandwidths of 1.5 and 2 nm are achieved for slots wider than 300 nm. The figure of merit exhibits sevenfold and fortyfold enhancements in comparison with classical slot waveguides with slot widths around 50 and 100 nm, respectively.

Compact Notch Microwave Photonic Filters Using On-Chip Integrated Microring Resonators

We propose and experimentally demonstrate a compact notch microwave photonic filter (MPF) using two integrated microring resonators (MRRs) on a single silicon-on-insulator (SOI) chip. The free spectral ranges (FSRs) of two cascaded MRRs are 160 GHz and 165 GHz, respectively. Due to the vernier effect, the transmission spectrum of cascaded MRRs is a series of bimodal distribution whose interval is an arithmetic sequence. By locating the laser wavelength at the middle of different bimodal intervals and fine tuning it properly, both central frequency and bandwidth of the notch MPF can be tunable. In the experiment, the tunability of central frequency and 3-dB bandwidth are demonstrated from 2.5 GHz to 17.5 GHz and from 6 GHz to 9.5 GHz, respectively. The best rejection ratio of the notch filter is larger than 40 dB. This approach will allow the implementation of low-cost, very compact, and integrated notch MPFs in a silicon chip.

Wideband slow light in chirped slot photonic-crystal coupled waveguides

Wideband dispersion-free slow light in chirped-slot photonic-crystal coupled waveguides is proposed and theoretically investigated in detail. By systematically analyzing the dependence of band shape on various structure parameters, unique inflection points in the key photonic band with approximate zero group velocity can be obtained in an optimized slot photonic-crystal coupled waveguide. By simply chirping the widths of the photonic-crystal waveguides in the optimized structure, wideband (up to 20 nm) slow-light with optical confinement in the low dielectric slot is demonstrated numerically with relative temporal pulse-width spreading well below 8% as obtained from two-dimensional finite-difference time-domain simulations. The wideband slow-light operation of the proposed structures would offer significant potential for novel compact high-speed optical-signal-processing devices in silicon-based systems.

Nonlinear equation method for band structure calculations of photonic crystal slabs

In this letter a nonlinear equation method based on effective-medium approximation is presented to calculate the band structure of photonic crystal slabs. At a definite wave vector, the normalized frequency is inversely proportional to the effective refractive index of the photonic crystals. Effective-medium approximation is used to calculate the effective refractive indices of the two dimensional photonic crystals and photonic crystal slab. The corresponding relation between the band structure of the photonic crystal slab and that of two dimensional photonic crystals is given by a nonlinear equation. Through the nonlinear equation, the three dimensional photonic crystal slabs problems are simplified as two dimensional simulations. The results by this method agree very well with those of three dimensional finite-difference time-domain method and the calculation is much faster.

High-order photonic differentiator employing on-chip cascaded microring resonators.

We propose and experimentally demonstrate a high-order photonic differentiator using on-chip complementary metal oxide semiconductor-compatible cascaded microring resonators, including first-, second-, and third-order differentiators. All the microring resonator units have a radius of 150 μm and a free spectral range of 80 GHz. The microring resonator can implement the first-order derivative of the optical field near its critical coupling region. Hence higher-order differentiation can be obtained by cascading more microring units on a single chip. For the periodical Gaussian optical pulse injection, the average deviations of all differentiators are less than 6.2%. The differentiation of pseudo-random bit sequence signals at 5 Gbit/s is also demonstrated. Our scheme is a compact and low-power-consumption solution since the cascaded microring units are fabricated with compact size on the silicon-on-insulator substrate.

Compact, flexible and versatile photonic differentiator using silicon Mach-Zehnder interferometers

We propose and experimentally demonstrate the flexibility and versatility of photonic differentiators using a silicon-based Mach-Zehnder Interferometer (MZI) structure. Two differentiation schemes are investigated. In the first scheme, we demonstrate high-order photonic field differentiators using on-chip cascaded MZIs, including first-, second-, and third-order differentiators. For single Gaussian optical pulse injection, the average deviations of all differentiators are less than 6.5%. In the second scheme, we demonstrate multifunctional differentiators, including intensity differentiator and field differentiator, using an on-chip single MZI structure. These different differentiator forms rely on the relative shift between the probe wavelength and the MZI resonant notch. Our schemes show the advantages of compact footprint, flexible functions and versatile differentiation forms. For example, high order field differentiators can be used to generate complex temporal waveforms, such as high order Hermite-Gaussian waveforms. And intensity differentiators are useful for ultra-wideband pulse generation.

Highly efficient phase-matched second harmonic generation using an asymmetric plasmonic slot waveguide configuration in hybrid polymer-silicon photonics.

We theoretically investigate the possible increase of the second harmonic generation (SHG) efficiency in silicon compatible waveguides by considering an asymmetrical plasmonic slot waveguide geometry and a χ((2)) nonlinear polymer infiltrating the slot. The needed phase matching condition is satisfied between the fundamental waveguide mode at the fundamental frequency (FF) and second-order waveguide mode at the second harmonic frequency (SHF) by an appropriate design of the waveguide opto-geometrical parameters. The SHG signal generated in our starting waveguide is three orders of magnitude higher than those previously reported for a FF corresponding to λ = 1550 nm. Then, the SHG performance was further improved by increasing the asymmetry of the structure where nonlinear coupling coefficients as large as 292 psm(-1)W(-1/2) are predicted. The device length is shorter than 20 µm and the normalized SHG conversion efficiency comes up to more than 1 × 10(5) W(-1)cm(-2).

Self-collimated waveguide bends and partial bandgap reflection of photonic crystals with parallelogram lattice

The photonic crystal structure with parallelogram lattice, capable of bending a self-collimated wave with free angles and partial bandgap reflection, is presented. The equifrequency contours show that the direction of the collimation wave can be turned by tuning the angle between the two basic vectors of the lattice. Acute, right, and obtuse angles of collimating waveguide bends have been realized by arc lattices of parallelogram photonic crystals. Moreover, partial bandgap reflection of the parallelogram lattice photonic crystals is validated from the equifrequency contours and the projected band structures. A waveguide taper based on this partial bandgap reflection is also designed and proved to have above 85% transmittance over a very wide operating bandwidth of 180 nm.