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.     

Low-group-velocity and low-dispersion slow light in photonic crystal waveguides

Photonic crystal slab line defect waveguides with slightly small innermost holes are theoretically expected to show light transmission with low-group-velocity and low-dispersion (LVLD) characteristics owing to a linear and almost flat photonic band. In this study, the LVLD characteristics of such waveguides were experimentally confirmed by using modulation phase shift measurement and transmission of ultrashort optical pulses. These results will be applicable to buffering and nonlinearity enhancement of optical signals.     

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.     

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.     

Anomalous propagation loss in photonic crystal waveguides

Propagation loss can occur in photonic crystal waveguides without complete optical confinement. We employ a highly efficient transfer-matrix method which allows for accurate and reliable extraction of the propagation loss even at an extremely low level. The results for a two-dimensional photonic crystal waveguide shows that the loss exponentially decays via the waveguide wall thickness. An anomalous phenomenon is found where the loss for guided modes near the upper band gap edge can be several orders of magnitude smaller than that for modes in the middle of the band gap. This anomaly can be well explained by the localization degree of guided modes at different frequency domains.     

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.     

Tunable properties of light propagation in photonic liquid crystal fibers

Tunable properties of light propagation in photonic crystal fibers filled with liquid crystals, called photonic liquid crystal fibers (PLCFs) are presented. The propagation properties of PLCFs strongly depend on contrast between refractive indices of the solid core (pure silica glass) and liquid crystals (LCs) filing the holes of the fiber. Due to relatively strong thermo-optical effect, we can change the refractive index of the LC by changing its temperature. Numerical analysis of light propagation in PLCF, based on two simulation methods, such as finite difference (FD) and multipole method (MM) is presented. The numerical results obtained are in good agreement with our earlier experimental results presented elsewhere [1].     

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.     

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.     

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.     

Semianalytic treatment for propagation in finite photonic crystal waveguides

We present a semianalytic theory for the properties of two-dimensional photonic crystal waveguides of finite length. For single-mode guides, the transmission spectrum and field intensity can be accurately described by a simple two-parameter model. Analogies are drawn with Fabry-Perot interferometers, and generalized Fresnel coefficients for the interfaces are calculated.     

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.     

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.     

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.     

Enhanced stimulated Raman scattering in slow-light photonic crystal waveguides

We investigate for the first time, to our knowledge, the enhancement of the stimulated Raman scattering in slow-light silicon-on-insulator (SOI) photonic crystal line defect waveguides. By applying the Bloch-Floquet formalism to the guided modes in a planar photonic crystal, we develop a formalism that relates the intensity of the downshifted Stokes signal to the pump intensity and the modal group velocities. The formalism is then applied to two prospective schemes for enhanced stimulated Raman generation in slow-light photonic crystal waveguides. The results demonstrate a maximum factor of 104(66,000) enhancement with respect to SOI channel waveguides.     

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.     

Evidence of Bloch wave propagation within photonic crystal waveguides

We report in this paper results obtained from characterizations of photonic band gap waveguide, using near field optical microscopy. We will show evidence of a Bloch wave propagating within our W1 photonic crystal waveguides (PCWs) structure, using a 2D Fourier transform approach. The processed image will then be compared to simulations obtained from plane wave method. This comparison exhibits that near-field measurements, using tapered optical fibers as probes, are mainly sensitive to the electric field propagating within the structure. PACS 42.25.-p; 42.30.Va; 42.770.-Qs; 42.82.-Et     

Enhanced all-optical tuning of leaky eigenmodes in photonic crystal waveguides

We demonstrate all-optical switching in an active two-dimensional photonic crystal waveguide, observing as large as 16 nm blueshifts of a leaky eigenmode at 839 nm and switching ratios of almost 70%. These results are larger than those previously observed in similar experiments performed on passive photonic crystal waveguides; the enhancement is due to resonant photogeneration of carriers by In(0.12)Al(0.2)Ga(0.68)As quantum wells in the core of the waveguide. The effective change in the refractive index of the structure is approximately10(-2), with a rise time of approximately1 ps and a decay time of approximately10 ps, potentially allowing high-speed switching and fast modulation rates.