Dispersion engineered slot photonic crystal waveguides for slow light operation

We introduce a novel design of wide Slot Photonic Crystal Waveguides (SPCW) by structuring the slot as a comb. This allows performing dispersion engineering in order to achieve very low group velocities over a few nanometers bandwidth. This kind of SPCW offers opportunities to realize devices requiring strong interactions between light and an optically nonlinear low index material by providing an ultrahigh optical density while easing the filling of the slot due to its width. We will present dispersion engineering results by the plane wave expansion method and finite difference time domain analysis, followed by experimental realization.     

Coupling into the slow light mode in slab-type photonic crystal waveguides

Coupling external light signals into a photonic crystal (PhC) waveguide becomes increasingly inefficient as the group velocity of the waveguiding mode slows down. We have systematically studied the efficiency of coupling in the slow light regime for samples with different truncations of the photonic lattice at the coupling interface between a strip waveguide and a PhC waveguide. An inverse power law dependence is found to best fit the experimental scaling of the coupling loss on the group index. Coupling efficiency is significantly improved up to group indices of 100 for a truncation of the lattice that favors the appearance of photonic surface states at the coupling interface in resonance with the slow light mode.     

Enhanced localization of light in slow wave slot photonic crystal waveguides

A flexible design of slot photonic crystal waveguide with a wide comb is investigated. Introduction of a carefully designed comb within the photonic crystal waveguide allows an accurate dispersion engineering in order to achieve slow light and increase the optical confinement within the comb. The strong light confinement results in an extremely small nonlinear effective area around 0.015 μm². We report experimental realization of a comb photonic crystal waveguide with measured group indices higher than 100 in a Mach-Zehnder interferometer configuration and extract losses limited to 3.7?dB for a 100?μm device at n_g=37.     

Nonlinear performance in silicon nitride slow light photonic crystal waveguides with elliptical holes

We propose a slow-light photonic crystal waveguide, which uses a combination of circular and elliptical air holes arranged in a hexagonal lattice with the background material of silicon nitride (refractive index n = 2.06). Large value of normalized delay bandwidth product (NDBP = 0.3708) is obtained. We have analyzed nonlinear performance of the structure. With our proposed geometry strong SPM is observed at moderate peak power levels. Proposed photonic crystal waveguide has slow light applications such as reduction in length and power consumption of all-optical and electro-optic switches at optical frequency.     

Extremely large bandwidth and ultralow-dispersion slow light in photonic crystal waveguides with magnetically controllability

A line-defect waveguide within a two-dimensional magnetic-fluid-based photonic crystal with 45^o-rotated square lattice is presented to have excellent slow light properties. The bandwidth centered at $ \lambda_{0} $  = 1,550 nm of our designed W1 waveguide is around 66 nm, which is very large than that of the conventional W1 waveguide as well as the corresponding optimized structures based on photonic crystal with triangular lattice. The obtained group velocity dispersion $ \beta_{2} $ within the bandwidth is ultralow and varies from ?1,191 $ a/(2\pi c^{2} ) $ to 855 $ a/(2\pi c^{2} ) $ (a and c are the period of the lattice and the light speed in vacuum, respectively). Simultaneously, the normalized delay-bandwidth product is relatively large and almost invariant with magnetic field strength. It is indicated that using magnetic fluid as one of the constitutive materials of the photonic crystal structures can enable the magnetically fine tunability of the slow light in online mode. The concept and results of this work may give a guideline for studying and realizing tunable slow light based on the external-stimulus-responsive materials.     

Enhanced slow light propagation in photonic crystal waveguides using angular properties of scatter elements

Applicability of the angular properties of scatter elements as a tool to achieve improved slow light performance with small group velocity dispersion and large bandwidth in photonic crystal waveguides is investigated. A polyatomic photonic crystal waveguide, including two scatter elements with different geometrical shapes in each primitive cell, is proposed to investigate the feasibility of our method. Numerical results show that a versatile control of the dispersion relation of slow light modes, with large normalized delay-bandwidth products ranging from 0.2085 to 0.3394, can be obtained using a unique geometrical parameter.     

Design and analysis of slow light regime in silicon carbide 2D photonic crystal waveguides

We theoretically demonstrate the slow light capabilities of 2D silicon carbide based photonic crystal W1 waveguides (SiC-PhC-W1Ws) with numerical simulations. The PhC is assumed to be created by devising air-holes with hexagonal lattice in a standard SiC substrate having oscillator type ordinary refractive index. Numerical simulations show that by means of selective optofluidic infiltration and varying the air-holes radii, SiC-PhC-W1Ws are capable of slowing light down by about 473 times while their group velocity dispersions are tailored to near zero values. Our numerical study also suggests the possibility of slow-light guiding with n_g × Δλ/λ_c values as high as 0.42 in SiC-PhC-W1Ws at optical telecommunications wavelengths.     

Tunable slow light and buffer capability in photonic crystal coupled-cavity waveguides based on electro-optic effect

An enhanced photonic crystal coupled-cavity waveguide (PC-CCW) is proposed to realize compact, ultra-fast modulated and high-performance buffering application. The slow light and buffer performances of PC-CCW are optimized by adjusting the cavity spacing and the hole size around the cavity. In the optimized structure, the group velocity is below 2.713 × 10^? 4c. The corresponding delay time, buffer capacity and Q factor can reach as high as 12.286 ns, 68.5 bit and 3525, respectively. Then the dynamic modulation of slow light transmission and buffer capacity in PC-CCW are systemically studied. The simulation shows that the buffer capacity and the physical size of each bit are unchanged as the applied voltage increases. While the wavelength shift and delay time decrease almost linearly as the applied voltage increases. And the modulation sensitivities are about 3.726 nm/mV and 0.875 ns/mV, respectively. The Q factor also decreases accordingly.     

Multiple slow light bands in photonic crystal coupled resonator optical waveguides constructed with a portion of photonic quasicrystals

Coupled resonator optical waveguides (CROWs) in complex two-dimensional (2D) photonic crystals (PCs) constructed with a portion of 12-fold photonic quasicrystals (PQs) are proposed. We show that enhanced transmission and slow light can be simultaneously achieved in such waveguides as well as general CROWs. Moreover, due to higher degree of flexibility and tunability of PQs for defect mode properties compared to conventional periodic PCs, multiple slow light bands can be flexibly obtained in CROWs constructed with complex 2D PCs. Our results may lead to the development of a variety of novel ultracompact devices for photonic integrated circuits.     

High-performance slow light photonic crystal waveguides with topology optimized or circular-hole based material layouts

Photonic crystal waveguides are optimized for modal confinement and loss related to slow light with high group index. A detailed comparison between optimized circular-hole based waveguides and optimized waveguides with free topology is performed. Design robustness with respect to manufacturing imperfections is enforced by considering different design realizations generated from under-, standard- and over-etching processes in the optimization procedure. A constraint ensures a certain modal confinement, and loss related to slow light with high group index is indirectly treated by penalizing field energy located in air regions. It is demonstrated that slow light with a group index up to n_g = 278 can be achieved by topology optimized waveguides with promising modal confinement and restricted group-velocity-dispersion. All the topology optimized waveguides achieve a normalized group-index bandwidth of 0.48 or above. The comparisons between circular-hole based designs and topology optimized designs illustrate that the former can be efficient for dispersion engineering but that larger improvements are possible if irregular geometries are allowed.     

Low loss propagation in slow light photonic crystal waveguides at group indices up to 60

We have designed slow light photonic crystal waveguides operating in a low loss and constant dispersion window of Δλ = 2 nm around λ = 1565 nm with a group index of n_g = 60. We experimentally demonstrate a relatively low propagation loss, of 130 dB/cm, for waveguides up to 800 μm in length. This result is particularly remarkable given that the waveguides were written on an electron-beam lithography tool with a writefield of 100 μm that exhibits stitching errors of typically 10–50 nm. We reduced the impact of these stitching errors by introducing “slow–fast–slow” mode conversion interfaces and show that these interfaces reduce the loss from 320 dB/cm to 130 dB/cm at n_g = 60. This significant improvement highlights the importance of the slow–fast–slow method and shows that high performance slow light waveguides can be realised with lengths much longer than the writing field of a given e-beam lithography tool.     

Real-space observation of ultraslow light in photonic crystal waveguides

We show the real-space observation of fast and slow pulses propagating inside a photonic crystal waveguide by time-resolved near-field scanning optical microscopy. Local phase and group velocities of modes are measured. For a specific optical frequency we observe a localized pattern associated with a flat band in the dispersion diagram. During at least 3 ps, movement of this field is hardly discernible: its group velocity would be at most c/1000. The huge trapping times without the use of a cavity reveal new perspectives for dispersion and time control within photonic crystals.     

Asymmetric light transport in L-shaped and U-shaped photonic crystal waveguides

We present an asymmetric light transport in a modified U-shaped photonic crystal waveguide (PCW). The compact waveguides consist of coupling two L-waveguides in a PC formed by dielectric rods in air arranged in a square lattice. We show that the transmitted power flow depends on the direction of the incident light, the transmission varies between a high and low value upon switching the position of the source beam from the input to the output of the PCW. Computations are performed using the plane wave expansion and finite-difference time-domain methods.     

Narrowband optical parametric gain in slow mode engineered GaInP photonic crystal waveguides

We predict narrowband parametric amplification in dispersion-tailored photonic crystal waveguides made of gallium indium phosphide. We use a full-vectorial model including the dispersive nature both of the nonlinear response and of the propagation losses. An analytical formula for the gain is also derived.     

Transmission of slow light through photonic crystal waveguide bends

The spectral dependence of the bending loss of cascaded 60 degrees bends in photonic crystal (PhC) waveguides is explored in a slab-type silicon-on-insulator system. An ultralow bending loss of (0.05 +/- 0.03) dB/bend is measured at wavelengths corresponding to the nearly dispersionless transmission regime. In contrast, the PhC bend is found to become completely opaque for wavelengths corresponding to the slow-light regime. A general strategy is presented and experimentally verified to optimize the bend design for improved slow-light transmission.     

Semi-analytic method for slow light photonic crystal waveguide design

We present a semi-analytic method to calculate the dispersion curves and the group velocity of photonic crystal waveguide modes in two-dimensional geometries. We model the waveguide as a homogenous strip, surrounded by photonic crystal acting as diffracting mirrors. Following conventional guided-wave optics, the properties of the photonic crystal waveguide may be calculated from the phase upon propagation over the strip and the phase upon reflection. The cases of interest require a theory including the specular order and one other diffracted reflected order. The computational advantages let us scan a large parameter space, allowing us to find novel types of solutions.     

Wideband and low dispersion slow light in slotted photonic crystal waveguide

We present a procedure to generate wideband and low dispersion slow light in slotted photonic crystal waveguide. By shifting the first and second rows of air holes of slotted photonic crystal waveguide, the bandwidth of slow light can be increased, with small group velocity dispersion. Using 2D plane wave expansion method, we numerically demonstrate slow light with the nearly constant group indices of 23, 42, and 54 over 17.6 nm, 6.7 nm and 3.3 nm bandwidth, respectively. The maximal normalized delay-bandwidth product is 0.26. From the fabrication's point of review, shifting the position of holes is easier to be controlled technically than changing the diameters of air holes. In addition, our simulations suggest this design is tolerant to deviation for positions of the first two rows of air holes. Therefore, the proposed approach decreases the dependence on the fabrication accuracy.     

Coupling loss minimization of slow light slotted photonic crystal waveguides using mode matching with continuous group index perturbation

We experimentally demonstrate highly efficient coupling into a slow light slotted photonic crystal waveguide. With optical mode converters and group index tapers that provide good optical mode matching and impedance matching, a nearly flat transmission over the entire guided mode spectrum of 68.8 nm range with 2.4 dB minimum insertion loss is demonstrated. Measurements also show up to 20 dB baseline enhancement and 30 dB enhancement in the slow light region, indicating that it is possible to design highly efficient and compact devices that benefit from the slow light enhancement without increasing the coupling loss.     

Influences of supercell termination and lateral row number on the determination of slow light properties of photonic crystal waveguides

We systematically analyze the effects of the use of an inaccurate supercell termination and an insufficient supercell size of plane-wave expansion method on the dispersion and the slow light properties of the photonic crystal waveguides. The inattentive use of supercells of photonic crystal waveguides appeared in the literature is found to be yielding errors in the dispersion and slow light characteristics of the fundamental guided mode of photonic crystal waveguides. In addition, extra modes appear in the photonic band gap of the photonic crystal waveguide due to inaccurate supercell termination. By examining the field distribution of the modes, the extra modes can be determined and removed from the band diagram. The dispersion, group index and bandwidth characteristics are observed to be less affecting from inaccurate supercell termination as the number of rows adjacent to the waveguide increases. Moreover, the dispersion and the group index-frequency curves of the fundamental guided mode of correctly terminated supercells are found to be converging as the lateral row number along the line-defect is increased.     

一种新型无色散慢光光子晶体薄板波导

利用椭圆形孔替代传统光子晶体薄板波导中邻接波导的最内层两排圆孔构成一种新型低损耗光子晶体薄板光波导.该波导的群速度和群速度色散特性强烈依赖于波导中这两排邻接波导的椭圆孔的特性.借助波导导模展开方法,计算得到波导的能带结构和群指数,并分析了它们与椭圆孔的参数关系.通过优化这些椭圆孔的参数,可以增加光子晶体光波导导模在光锥以下的无固有传输损耗带宽,在2—45 nm 的带宽上实现无色散的常数群速度.这些理论结果将为低损耗低色散慢光波导的设计制造提供理论基础. 关键词： 光子晶体薄板波导 群速度 群指数 群速度色散