Photonic Crystal Defect Waveguides Research Articles

Eigenfunction and eigenmode-spacing statistics in chaotic photonic crystal graphs.

The statistical properties of wave chaotic systems of varying dimensionalities and realizations have been studied extensively. These systems are commonly characterized by the statistics of the eigenmode spacings and the statistics of the eigenfunctions. Here, we propose photonic crystal (PC) defect waveguide graphs as a physical setting for chaotic graph studies. Photonic crystal waveguides possess a dispersion relation for the propagating modes, which is engineerable. Graphs constructed by joining these waveguides possess junctions and bends with distinct scattering properties. We present numerically determined statistical properties of an ensemble of such PC graphs including both eigenfunction amplitude and eigenmode-spacing studies. Our proposed system is compatible with silicon nanophotonic technology and opens chaotic graph studies to a new community of researchers.

Study of a hexagonal lattice defect photonic crystal waveguide slow wave structure for G-band traveling wave tube

A defect photonic crystal waveguide (DPCW) slow wave structure (SWS) for G-band traveling wave tube (TWT) is proposed. This DPCW SWS is arrayed in hexagonal lattice with the center defect cells. The TWT based on DPCW SWS operates at 16 kV beam voltage and 120 mA beam current with a 0.6 T focusing magnetic field. A saturated output power of 22.3W, corresponding to a gain of 36.5 dB, is achieved with an input power of 5 mW. The output power is over 10W in a 30 GHz bandwidth. Stable output signal can be achieved without attenuator or sever which shows that the proposed DPCW SWS can depress the oscillation and harmonic in nature. A DPCW SWS sample was fabricated by computer control (CNC) milling, and the measured results show great agreement with the simulation results.

Intelligent Forward-Wave Amplifier Design With Deep Learning and Genetic Algorithm

In this work, exploitation of artificial intelligence algorithms for the design of forward-wave amplifier is proposed and discussed. The multilayer perceptron (MLP), a quintessential example of deep learning, is employed to get the cost function, i.e., the relationship between the geometrical parameter set and dispersion characterization from only a small proportion of the parameter sets. Due to the structural complexity, there did not exist a closed-form solution and the neural network shows its superiority compared to the time-consuming numerical calculation. Subsequently, the genetic algorithm (GA) is used to find the parameter sets best matching the goals that broadband beam-wave interaction at specific voltages and even specific phases or frequencies, from the huge 4-D parameter-set space. The intuitive goals can be easily accessible, as long as the goals are within the ability of the parameter-set space and converting the goals into the goal functions for GA with some attempts and adjustments. As a practical example, the recently proposed defect photonic crystal waveguide is utilized to demonstrate the design pattern. Through almost full-automatic design with little judgment and analysis, a response of 14-GHz 3-dB bandwidth and 25-dB gain at the 168.5-GHz central frequency has been realized, indicating the effectiveness of the design pattern. This design pattern will ease researchers from numerous simulation work and help to get deeper insight of the structure.

Manipulation of acoustic localizations based on defect mode coupling in a corrugated waveguide

Defects in periodic structures can always result in localization of various waves, such as Anderson localization, line defect photonic crystal waveguides, and locally resonant sonic metamaterials. Due to the unique physical properties of localization, it has found applications in many functional devices including optical/acoustical tweezers, laser cavities, spatial modulators, and detectors. Here, we introduce two defects into an acoustic waveguide with perfect periodicities and experimentally observe the related two defect modes and their coupling effects. The double defects can cause the two extraordinary transmissions in the forbidden band of periodic structures, and the induced acoustic localization is quite different from that in a single defect that the measured sound pressure along the axis of the waveguide shows the maximum in each defect. The measured sound pressure also indicates that the localization of defect modes has in-phase or out-of-phase characteristics, which change alternately with increasing number of periods between the defects. When the distance between the two defects increases, the defect coupling turns to be weaker and the transmitted peaks fuse together as one. Finally, the defect mode disappears for a larger distance. The experiments also reveal that the localization of double defects is very sensitive to the defect geometry. The maximum sound pressure of defect modes with a longer wavelength always appears in the longer defect. Based on our findings, the acoustic localization can be efficiently manipulated by the regulation of structures with defects, which may pave the way for various functional devices in wave control engineering.

A Terahertz Traveling-Wave Tube Based on Defect Photonic Crystal Waveguide

A defect photonic crystal waveguide (DPCW) is presented as a slow-wave structure (SWS) for terahertz (THz) traveling-wave tube (TWT). Due to the characteristics of DPCW, the mode competition and backward wave are greatly depressed, improving the stability of TWT. The DPCW SWS has flat dispersion characterization and high interaction impedance at 220 GHz. Simulation results show that saturated output power of 12.1 W, corresponding to a gain of 24.3 dB, and the 3-dB gain ripple within 20 GHz can be achieved at the center frequency of 220 GHz. The DPCW-TWT has a low working voltage at 9.52 kV compared to that of folded waveguide (FWG) SWS in this frequency band. The proposed DPCW for the THz regime is easy to fabricate and assemble using the modern fabrication processing technology.

Optimization of PBG-Waveguides for Terahertz-Driven Electron Acceleration

The properties of 2-D photonic bandgap dielectric structures, also called photonic crystals, are numerically investigated to assist the design of waveguides for terahertz (THz)-driven linear electron acceleration. Given the broadband nature of the driving pulses in THz acceleration regimes, one design aim is to maximize the photonic bandgap width to allow propagation of the relevant frequencies within the photonic crystal linear defect waveguide. The proposed design is optimized to provide the best compromise between effective acceleration bandwidth and strong beam–wave interaction at the synchronism central frequency. Considerations on achieved acceleration bandwidth, accelerating voltage, and surface magnetic field are given to compare the proposed geometry to one of the main counterparts in the literature—the dielectric-lined waveguide.

Symmetry-engineered waveguide dispersion in $ {\cal P}{\cal T} $PT symmetric photonic crystal waveguides

This work demonstrates how the crystal symmetry of photonic crystal defect waveguides interacts with simple, experimentally realizable parity-time ( P T ) symmetric regions of chip-scale absorption and amplification to control the existence and location of exceptional points in the first Brillouin zone. Our analysis is based on Heesh–Shubnikov group theory and is generalizable to a large class of devices for which the symmetry groups can be identified. Transverse, longitudinal, and transverse–longitudinal hybrid P T symmetries are considered, and for each, a triangular lattice photonic crystal waveguide with lattice-aligned and lattice-shifted cladding orientations is analyzed. We find that various symmetry combinations produce either strictly real-valued or strictly complex-valued eigenfrequencies at the Brillouin zone boundary. We also show how symmetry can be used to predict P T transitions at accidental degeneracies in the waveguide bands. It is shown how symmetry can be used to design single-mode waveguides, and we discovered exceptional points whose propagation constants are highly sensitive to the non-Hermiticity factor.

Graphene Photodetector Integrated on a Photonic Crystal Defect Waveguide

We present a graphene photodetector for telecom applications based on a silicon photonic crystal defect waveguide. The photonic structure is used to confine the propagating light in a narrow region in the graphene layer to enhance light-matter interaction. Additionally, it is utilized as split-gate electrode to create a pn-junction in the vicinity of the optical absorption region. The photonic crystal defect waveguide allows for optimal photo-thermoelectric conversion of the occurring temperature profile in graphene into a photovoltage due to additional silicon slabs on both sides of the waveguide, enhancing the device response as compared to a conventional slot waveguide design. A photoresponsivity of 4.7 V/W and a (setup-limited) electrical bandwidth of 18 GHz are achieved. Under a moderate bias of 0.4 V we obtain a photoconductive responsivity of 0.17 A/W.

Photonic-crystal-based all-optical NOT logic gate.

In the present paper, we have utilized the concept of photonic crystals for the implementation of an optical NOT gate inverter. The designed structure has a hexagonal arrangement of silicon rods in air substrate. The logic function is based on the phenomenon of the existence of the photonic bandgap and resulting guided modes in defect photonic crystal waveguides. We have plotted the transmission, extinction ratio, and tolerance analysis graphs for the structure, and it has been observed that the maximum output is obtained for a telecom wavelength of 1.554μm. Dispersion curves are obtained using the plane wave expansion method, and the transmission is simulated using the finite element method. The proposed structure is applicable for photonic integrated circuits due to its simple structure and clear operating principle.

Design and optimization of a low loss tapered photonic crystal waveguide structure

We present a two-dimensional study of low-loss photonic crystal defect waveguides built from a III–V type semiconductor-based system surrounded by cylindrical air pillars in a prototypical lattice. Varying the parameters of the lattice, including several taper geometries and lengths in the typical wavelengths usually employed in telecommunications, we perform an extensive statistical survey that determines a parameter map that can be taken as a guide by transmission hardware designers to choose the best lattice geometry in terms of optimum transmittance and minimization of the impedance mismatch.

Photonic crystal thin-film micro-pressure sensors

We have engineered both the optical and mechanical behaviour of a suspended line defect photonic crystal waveguide to demonstrate an improved sensor response. Utilizing thinner than the conventional photonic crystal membrane increases the mechanical sensitivity and the dynamic range of the sensor. The mechanical sensitivity is also increased by optimizing the membrane pad connected to the core photonic crystal waveguide. The pad optimization also ensures maximum pad flatness under uniform pressure. The overall sensitivity obtained is as high as 6% optical transmission change per Pa of pressure.

Terahertz wave polarization splitter using full band-gap photonic crystals

Manipulation of terahertz wave polarization state exhibits tremendous potential in developing application of terahertz science and technology. A terahertz polarization beam splitter based on three linear defect photonic crystal waveguides is designed and numerically demonstrated. Both plane wave expansion method and finite-difference time-domain method are used to calculate and analyze the characteristics of the proposed device. The simulation results show that the designed polarization beam splitter can split TE and TM polarized mode into different propagation directions with high efficiency in the frequency range from 1.225THz to 1.260THz. Especially, the extinction ratios for both TE and TM modes are larger than 20dB.

Slow Light with High Normalized Delay Bandwidth Product in Photonic Crystal Line Defect Waveguide

In this paper, to generate slow light with large group index, wideband, and low dispersion in suggested line defect photonic crystal waveguide, a systematic optimization procedure is performed. Two structural parameters in the adjacent rows of the waveguide are carefully adjusted. Dispersion engineering in the proposed structure is done by using plane wave expansion method. By tuning the structural parameters, normalized delay-bandwidth product (NDBP) with nearly constant group index is significantly improved. In the optimum case, a high NDBP of 0.373 with a group index of 38.6 and a bandwidth of 14.97 nm is obtained, while restricting the group-index variation within a 10% range. Also,a negligible group velocity dispersion over a broad wavelength range is obtained that can cause a small expansion of the pulse through the waveguide.

Integrated Grating-Nanoslot Probe Tip for Near-Field Subwavelength Light Confinement and Fluorescent Sensing

We demonstrate a near-field sub-wavelength light confinement probe tip comprised of compact embedded metallic focus grating (CEMFG) coupler and photonic crystal (PhC) based λ/4 nano-slot tip, in terms of its far-field radiation directivity and near-field sub-wavelength light enhancement. The embedded metallic grating coupler increases the free space coupling at tilted coupling angle of 25° with over 280 times light intensity enhancement for 10 μm coupler size. Further, 20 nm air slot embedded in single line defect PhC waveguide are designed, using the impedance matching concept of the λ/4 “air rod”, to form the TE mode light wave resonance right at the probe tip aperture opening. This leads to the light beam spot size reduction down to λ/20. The near-field center peak intensity is enhanced by 4.2 times from that of the rectangular waveguide input, with the total enhancement factor of 1185 from free space laser source intensity. The near-field fluorescence excitation and detection also demonstrate its single molecular enhanced fluorescence measurement capability.

Unit Cell Topology Optimization of Line Defect Photonic Crystal Waveguide

In this paper we utilize a systematic method to optimize the topology of a line defect Photonic Crystal Wave guide (PCW), a very popular and effective optical device for realizing optical buffer in room temperate operation. In order to do this, we divide the unit cell PCW to 25 squares and utilize a Binary Particle Swarm Optimization (BPSO) algorithm to find the optimum topology. During the optimization process, we apply some constraints to avoid band mixing as well. Calculation results show that Normalized Delay-Bandwidth Product (NDBP) for the structure optimized is 0.27. This structure optimized possesses a high bandwidth (20.9 nm) with group index of 21.7 as well.

A tri-objective Particle Swarm Optimizer for designing line defect Photonic Crystal Waveguides

This paper proposes a novel tri-objective approach for optimizing the structure of line defect Photonic Crystal Waveguides (PCW). A nature-inspired algorithm called Multi-Objective Particle Swarm Optimization (MOPSO) is employed as the optimizer. The three objectives considered are maximization of group index, maximization of bandwidth, and minimization of Group Velocity Dispersion (GVD). In addition, the optimization process is subject to five constraints in order to guarantee the feasibility of the structures obtained and prevent bad mixing in the final optimized structures. The results show that the tri-objective MOPSO is able to find 20 optimized structures for line defect PCW. The comparative study verifies the significant improvement of the optimized structures compared to current structures. Moreover, post analysis of the results reveals the importance of holes and their possible physical behaviours in line defect PCW. Finally, the implementation considerations and investigations show that the optimized structures are feasible for manufacturing with a resolution of 1nm.

Analysis of spatial hole burning and temperature effects in vertical-cavity surface-emitting lasers with internal photonic crystal waveguide

We investigate theoretically the effect of two-dimensional photonic crystal (PC) defect waveguide parameters embedded into vertical-cavity surface-emitting lasers (VCSELs) on static operation of PC-VCSEL, including spatial hole burning (SHB) and temperature in the active regions. In structures with larger pitch of PC holes, SHB occurs dramatically and temperature increases in the active region. In large-hole diameter to pitch ratio, SHB has little effect and temperature is decreased in the active regions. We also show that with higher input current, temperature rises and SHB occurs.

The study of electro-optical sensor based on slotted photonic crystal waveguide

A compact and sensitive electro-optical sensor based on slotted photonic crystal waveguide (S-PhCW) is demonstrated. The electro-optical sensor can be realized in photonic crystal (PhC) slabs of silicon in Silicon-on-Insulator (SOI). Nonlinear optical polymer is used as infiltration. By applying three-dimensional finite difference time domain (3D-FDTD), the sensitivity and quality factor of electro-optical sensor with different slotted waveguide width are calculated. In addition, sensitivity and the optical properties such as transmission spectrum and field distributions are compared between electro-optical sensor based on line defect photonic crystal waveguide (W1-PhCW) and that based on slotted photonic crystal waveguide (S-PhCW). Simulation results demonstrate that, compared with electro-optical sensor based on line defect photonic crystal waveguide, the sensitivity and quality factor is improved by 30 times and 6.6 times respectively in sensor based on slotted photonic crystal waveguide. Besides, the proposed PhC sensor devices have the advantage of a compact structure with the potential for monolithic integration with optical-to-electrical on-chip conversion and detection.

Low-Loss Photonic Crystal Defect Waveguides and Taper Designs in InP/InGaAsP for Transverse Magnetic Polarized Light

We present the simulation, fabrication and characterization of novel low-loss photonic crystal (PhC) defect waveguides for transverse magnetic (TM) polarized light in the InP/InGaAsP material system. Minimal propagation losses for the proposed W4 PhC waveguides as low as ∼300 dB/cm at λ= 1550 nm for lattice constant a = 350 nm, and ∼500 dB/cm at λ= 1532 nm for lattice constant a = 590 nm are obtained from the cut-back analysis. In addition, an associated application of proposed wide PhC waveguides as intermediate short tapers between deeply etched access waveguides and narrower quasi-single-mode PhC waveguides is investigated. Predicted improved coupling efficiency has been validated experimentally.

Hybrid InP-based photonic crystal lasers on silicon on insulator wires

We report on InP-based photonic crystal lasers operating at 1.585 μm at room temperature, integrated with and evanescently coupled to silicon on insulator (SOI) waveguides. By optically pumping at 1.18 μm through the SOI wires, pulsed laser emission from line defect photonic crystal waveguides accurately aligned (<30 nm) to the silicon circuitry is demonstrated.