Dispersionless slow light by photonic crystal slab waveguide with innermost elliptical air holes

We report a low-loss photonic crystal slab waveguide formed by deforming the innermost circle air holes in the conventional photonic crystal slab waveguide into elliptical ones. We obtain the photonic bands and group index of guided modes in this photonic crystal waveguide by guided-mode expansion method and investigate the dependence of photonic bands and group index of guided modes on the parameters of the innermost elliptical air holes. The group velocity and group velocity dispersion of this waveguide strongly depend on the innermost elliptical air holes. Photonic crystal slab waveguide with the optimum innermost elliptical air holes possesses a wider single mode region below the light line, in which light can easily propagate without intrinsic loss. At the same time, the guided mode supported by this waveguide has nearly constant group velocity and vanishing group velocity dispersion in a 3-5 nm bandwidth.     

A systematic analysis of hole size,hole-type and rows shifting on slow light characteristics of photonic crystal waveguides with ring-shaped holes

A photonic crystal waveguide embedded in silica is proposed and the effects of number of defect rows adjacent to the line-defect, number of rows shifted in transverse direction to the light propagation and the types of holes in these rows on slow light properties are investigated without changing the line-width by plane-wave expansion method. We observe that the structure with one row of ring-shaped holes exhibits better slow light properties than the structure with two and three innermost rows of ring-shaped holes when outer and inner radius of the holes are considered as free parameters. Shifting the second innermost rows of holes is found to be preferable than shifting the second innermost rows of rings. Besides, shifting the second and third innermost rows together does not make considerable enhancement on the slow light properties as shifting only the second innermost rows, no matter the shifting holes are ring-shaped holes or only holes.     

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.     

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.     

Group index and dispersion properties of photonic crystal waveguides with circular and square air-holes

The slow light and group velocity dispersion properties of 2D triangular lattice photonic crystal line defect waveguide (PCW) with square and circular air-holes are numerically investigated with the plane-wave expansion method. The simulation results show that the guided mode is impacted slightly by the cross section’s shape of the air-holes of the same filling ratio. Adjusting two rows of the inner-hole adjacent to the waveguide and modifying the waveguide width can bring in low-group velocity and low-dispersion (LVLD) region, in which the group index of the square holes can reach 210 which is far better than the circular-holes. At the same air-hole size and waveguide width, the PCW with the square holes can support higher bit rate of the signal up to 35 Gb/s. These results provide important theoretical basis for realizing of optical buffering and optical logic devices in all-optical network.     

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.     

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.     

Numerical demonstration of slow light tuning in slotted photonic crystal waveguide using microfluidic infiltration

Tuning of the operating wavelength of slow light in the slotted photonic crystal waveguide using microfluidic infiltration has been investigated. Using 2D plane wave expansion method, we numerically demonstrate that the operating wavelength can be shifted from the C to L band, simply by choosing the refractive index of the infiltrated fluid. It is also found that, as the refractive index of the infiltrated fluid changes, the group velocity dispersion has slight variation at different operating wavelength. This design opens the possibility for post-fabrication scheme of tuning the operating wavelength of slow light in slotted photonic crystal waveguide, and allows the device to be optimized for different applications.     

Modal selective tuning in a photonic crystal cavity

We present a way to selectively tune the properties of the degenerated modes confined in a single point defect two-dimensional photonic crystal cavity based on a triangular lattice of air holes. We investigate the dependence of the modal properties of the resonator on the position of the first neighbor holes, showing that it is possible to finely tune the resonant frequency of only one of these two modes and to increase the quality factor of the mode that has no frequency shift. This is achieved by controlling the wavevector components inside the cavity. This approach is a viable strategy for the development and the optimization of several innovative devices based on bi-modal cavity arrays, such as arrays of integrated optical filters and optical read-out sections for biosensing applications.     

Slow light in photonic crystal with combinations of gradient dielectric constant

A line defect waveguide formed by a new kind of two-dimensional photonic crystal with combinations of gradient varied dielectric was designed and the properties of slow light will been studied by using the Plane Wave expansion Method (PWM). The result shows that the group velocity is slower when the corresponding combination of gradient dielectric constants are with higher dielectric constant and lower interval. In our new structure of photonic crystal, the corresponding group velocity can reach as low as 0.06c (c: light velocity in vacuum), two orders magnitude lower than light velocity. Meanwhile, the group velocity dispersion effect is relatively small.     

Slow light performance enhancement of graphene-based photonic crystal waveguide

We propose a graphene-based photonic crystal (PC) slow light waveguide, which is realized by creating periodical air holes in a silicon layer to achieve spatially varying chemical potentials of graphene. The structure is optimized around 30 THz, and a large group index of 166.6 is obtained, with a very low propagation loss of ?2.1 dB/um. The corresponding normalized delay-bandwidth product reaches as high as 4.00. Furthermore, the slow light performance can be dynamically tuned by changing a bias voltage. The center frequency of the slow light waveguide can be tuned between 19.1 THz and 27.4 THz. Our results suggest that graphene-based PC structures are very promising for slow light devices.     

Characteristics analysis of extruded elliptical hole photonic crystal fibers with square air-core

An extruded elliptical hole photonic crystal fibers PCF with square air-core is proposed. By using a full vector finite-element method FV-FEM and anisotropic perfectly matched layers APML, the structure and optical properties of the proposed PCF are analyzed. Simulation results show that the birefringence of the proposed photonic crystal fiber can be up to the order of 10⁻², and has a flattened dispersion from 1.20 μm to 1.80 μm. The proposed PCF may have important application in super-continuum SC generation, dispersion compensation, fiber-optic sensing systems and other aspects.     

Dispersion parameters and nonlinear optical properties of silicon nitride rib waveguides

Silicon nitride rib waveguides are numerically studied by use of a full-vectorial mode solver program based on finite difference method. Dispersion parameters, up to the third-order, are computed for waveguides of heights 0.8 μm, 0.9 μm, and 1 μm. Like silicon-on-insulator waveguides, silicon nitride rib waveguides allow us to tailor dispersion parameters at telecom wavelengths. Deeply-etched silicon nitride rib waveguides of height up to 1.5 μm are investigated for correct geometries to achieve polarization independence. The computations lead to a minimum etch depth which can be written as linear function of the waveguide height. Etch-depth and waveguide height dependencies of the nonlinear refractive index coefficient of the silicon nitride are studied. It is shown that third-order optical nonlinearities in silicon nitride rib waveguides can be enhanced by suitable choices of waveguide parameters.     

Slow light engineering in polyatomic photonic crystal waveguides based on square lattice

In this paper, the slow light properties of the polyatomic Photonic Crystal (PhC) which has multiple different air holes in each primitive cell are investigated. A slow light waveguide with “U-type” group index-frequency curve, which results in nearly constant group index over large bandwidth, is achieved using this new photonic crystal geometry based on the square lattice. Also, the radius and position of the innermost rows of small air holes have been modified to investigate the feasibility of controlling the dispersion relation by subtle structural modification. Numerical results demonstrate that decreasing the group velocity effectively and meanwhile maintaining a large Normalized Delay-Bandwidth Product (NDBP) can be achieved by only modifying the radius of the innermost rows of small air holes. Shifting the innermost rows of small air holes toward the waveguide core is highly beneficial to enlarge the slow light bandwidth, but it contributes nothing to the promotion of NDBP. Our results provide important theoretical basis for the potential application offered by the polyatomic photonic crystal in future optical networks.     

Slow light transmission in a photonic crystal power splitter with parallel outputs

In this paper, we investigate coupling of light to slow modes in a photonic crystal power splitter composed of a Y-junction and two 60° bends. First, a combination of two cascaded bends which is commonly used in integrated photonic crystal circuits is studied in slow light frequency regime. We propose a structure that its transmission spectrum covers the high group-index frequencies near the band edge. Also, by structural modifications, high transmission near to 95% is achieved in slow light bandwidth. Next, we study the complete structure of a photonic crystal power splitter with parallel outputs based on a Y-junction integrated with two 60° bends. Using modified bends and reducing sharpness of Y-junction, the efficiency of splitting increases in both high and low group-index frequency bands. The optimized structure has an average efficiency of 82% in slow mode regime. This structure can be used in photonic crystal based slow light devices, such as Mach-Zehnder interferometers.     

Microcavity filters based on hexagonal lattice 2-D photonic crystal structures embedded in ridge waveguides

Two-dimensionally periodic photonic crystal microcavity filters in a ridge waveguide format have been designed and fabricated. Transition mode-matching features were added to increase the optical throughput by more than a factor of two. An increase of Q-factor (more than 100%) was achieved by the addition of two further rows of photonic crystal holes to the microcavity filters. Attempts have also been made to tailor the filter response by applying design concepts used in other Bragg-grating optical filter technologies.     

Influence of defect mode displacement in photonic crystal band gap over light group velocity

The light group velocity of defect mode in photonic crystal is active controlled in order to gain controllable light delay by using thermo-optic effect to change the reflective index of material or changing the geometry structure of photonic crystal. From former research we know that the position of defect mode in band gap is changed with light group velocity. Our research further finds that the position of defect mode can be kept by changing more than one parameter correspondingly, and meanwhile the group velocity is not changed too. It comes to the relationship between position of defect mode in photonic band gap and light group velocity, and it is proved theoretically using the principle of plane wave expand method.     

Advanced impedance matching in photonic crystal waveguides

We demonstrate that the dispersion of guided propagating modes in certain Photonic Crystal Waveguides (PCWGs) can be kept constant when the waveguide’s structure changes along the propagation direction. This suggests that the principle of constant group velocity matching may be utilized to improve impedance matching between different types of PCWGs while at the same time providing significant design flexibility. We illustrate this principle through the design of several efficient coupling structures between two different PCWGs via a local density of states and Fourier transform analysis of the associate electromagnetic fields. The couplers consist of heterostructures whose individual sections exhibit rather distinct structural parameters. Furthermore, we compare these structures to an adiabatic coupler.     

Enhancement of room temperature sub-bandgap light emission from silicon photonic crystal nanocavity by Purcell effect

We report the enhancement of sub-bandgap photoluminescence from silicon via the Purcell effect. We couple the defect emission from silicon, which is believed to be due to hydrogen incorporation into the lattice, to a photonic crystal (PhC) nanocavity. We observe an up to 300-fold enhancement of the emission at room temperature at 1550 nm, as compared to an unpatterned sample, which is then comparable to the silicon band-edge emission. We discuss the possibility of enhancing this emission even further by introducing additional defects by ion implantation, or by treating the silicon PhC nanocavity with hydrogen plasma.     

Abnormal behaviour of one-dimensional photonic crystal with defect

The effect of introduction a defect in a conventional one-dimensional photonic band gap (PBG) structure in respect of the dispersion relation, reflectivity, group velocity and effective group index of such a structure has been studied. In particular, the dependence of various properties of such a structure on the angle of incidence of the electromagnetic waves has been given more importance in the present study. The study shows that inside the conventional PBG structure as well as defect PBG structure, the group velocity and effective group index become negative for certain ranges of normalized frequency. In a defect PBG structure, it is possible to achieve desired values (negative or positive) of group velocity and effective group index by choosing appropriate angle of incidence, whereas in conventional PBG structures, the maximum (negative and positive) values of group velocities and effective group index are found to be almost independent of the angle of incidence. This is a unique property of defect PBG structure which is different from the properties of conventional PBG structures. Because of this unique property, defect PBG structure may be widely used for construction super lenses, lasing without inversion and other optical systems in photonics.