Photonic Crystal Slab Waveguides Research Articles

Ultra-compact dual-purpose photonic crystal slab waveguide for both generating and isolating optical frequency comb

In this article, we present a novel approach to achieve a highly compact Si3N4 photonic crystal waveguide by incorporating air holes. This structure is developed in two functional applications that simultaneously perform the functions of generating optical frequency combs and demultiplexing them. The presence of air holes enhances control and precision in the dispersion engineering process. At the same time, the light conduction, based on the refractive index contrast between the core and cladding, along with the conduction occurring in the photonic bandgap region of the structure, leads to enhanced trapping of light within the core of the structure. As a result, a nonlinear coefficient of 4.2 m-1W−1 is obtained, which originates from the small effective mode area of 0.67 µm2. By employing four-wave mixing, a total of 40 comb lines are generated with a frequency spacing range of 10 GHz, 50 GHz, 60 GHz, and 100 GHz over a 6 mm length waveguide. The same waveguide structure is also developed for demultiplexing the comb lines, with four filters at the end which, individually extract the optical combs possessing wavelengths of 1317.8 nm, 1318.5 nm, 1319.2 nm, and 1319.9 nm. This demultiplexer has a Q-factor, average transmission efficiency, channel spacing, and spectral linewidth of 7209.35, 99.2 %, 0.7 nm, and 0.185 nm, respectively. The suggested design can significantly advance integrated photonic circuits, optical communications, and remote sensing applications requiring simultaneous information delivery, such as remote surgical operations and sensing, because of its substantial compactness.

Numerical modeling of an integrated non-volatile reflector switch and mode converter switch based on a low loss phase change material (Sb2Se3) in SiN platforms

Programmable integrated photonics is an emerging research field due to its range of applications, from data processing to computing. Phase change materials (PCMs) on waveguides provide enormous flexibility for programmable integrated photonics. These materials show a large contrast in the optical properties (such as refractive index and optical loss) between the two stable states (i.e., amorphous to crystalline) of the PCM. These states are reversible and reproducible with an external stimuli which could be optical, thermal or electrical. Their non-volatile behavior allows PCMs to serve as an active layer for programmable photonics. In this work, we investigate hybrid device architectures utilizing the non-volatile properties of PCMs for integrated programmable photonics in a Si3N4 platform. FDTD modeling was carried out to design two configurations of non-volatile reconfigurable switches: (1) A non-volatile reconfigurable reflector switch consisting of a photonic crystal (PhC) slab waveguide and having a thin layer of Sb2Se3 on top of the waveguide and (2) 1×2 mode converter switch. In the case of a non-volatile reconfigurable reflector switch, the reflectivity of the waveguides is controlled by shifting the mode-gap of the photonic crystal slab waveguide by changing the phase of the PCM. It has applications as a reconfigurable reflective filter in the optical communication system and on-chip smart Bragg mirror. Non-volatile broadband directional switches provide a new paradigm for designing programmable multifunctional nanophotonics, which works in the same way as electronic field-programmable gate arrays. A 1×2 mode converter switch — based on Sb2Se3 as a clad layer on a coupling waveguide of a directional coupler — is proposed. The optical switches proposed in this paper offer very low optical insertion loss (∼ 0.5 dB), low coupling length (∼ 12 µm), broadband operation (∼ 80 nm), small cross talk (∼ 16 dB), and zero static power consumption.

Optimizing the chiral Purcell factor for unidirectional single-photon emitters in topological photonic crystal waveguides using inverse design

We present an inverse design approach to significantly improve the figures-of-merit for chiral photon elements and quantum emitters in topological photonic crystal slab waveguides. Beginning with a topological waveguide mode with a group index of approximately 10 and a maximum forwards or backwards Purcell factor at a chiral point of less than 0.5, we perform optimizations of the directional Purcell factor. We use a fully three dimensional guided-mode expansion method to efficiently calculate waveguide band dispersion properties and modes, while automatic differentiation is employed to calculate the gradient of objective functions. We present two example improved designs: (i) a topological mode with an accessible group index of approximately 30 and a maximum unidirectional Purcell factor at a chiral point greater then 4.5 representing a nearly 10-fold improvement to the Purcell factor, and (ii) a slow light mode, well away from the Brillouin zone edge with a group index greater then 350 and a maximum unidirectional Purcell factor at a chiral point greater than 45.

Distortionless Pulse Transmission in Valley Photonic Crystal Slab Waveguide

A valley photonic crystal is one type of photonic topological insulator, the realization of which needs only P-symmetry breaking. The domain wall between two valley-contrasting photonic crystals supports robust edge states, which can wrap around sharp corners without backscattering. Using the robust edge states, one can achieve pulse transmission. Here, using time-domain measurements in the microwave regime, we show distortionless pulse transmission in a sharply bended waveguide. An \ensuremath{\Omega}-shaped waveguide with four 120\ifmmode^\circ\else\textdegree\fi{} bends is constructed with the domain wall between two valley photonic crystal slabs. Experimental results show the progress of Gaussian pulse transmission without distortion, and the full width at half maximum of the output signal changes slightly in the \ensuremath{\Omega}-shaped waveguide. By measuring the steady-state electric field distribution, we also confirm the confined edge states without out-of-plane radiation, which benefits from dispersion below the light line. Our work provides a way for high-fidelity optical pulse signal transmission and the development of high-performance optical elements, such as photonic circuits or optical delay lines.

Theory of intrinsic propagation losses in topological edge states of planar photonic crystals

Using a semi-analytic guided-mode expansion technique, we present theory and analysis of intrinsic propagation losses for topological photonic crystal slab waveguide structures with modified honeycomb lattices of circular or triangular holes. Although conventional photonic crystal waveguide structures, such as the W1 waveguide, have been designed to have lossless propagation modes, they are prone to disorder-induced losses and backscattering. Topological structures have been proposed to help mitigate this effect as their photonic edge states may allow for topological protection. However, the intrinsic propagation losses of these structures are not well understood and the concept of the light line can become blurred. For four example topological edge state structures, photonic band diagrams, loss parameters, and electromagnetic fields of the guided modes are computed. Two of these structures, based on armchair edge states, are found to have significant intrinsic losses for modes inside the photonic band gap, more than 100 dB/cm, which is comparable to or larger than typical disorder-induced losses using slow-light modes in conventional photonic crystal waveguides, while the other two structures, using the valley Hall effect and inversion symmetry, are found to have a good bandwidth for exploiting lossless propagation modes below the light line.

Numerical modeling of integrated electro-optic modulators based on mode-gap shifting in photonic crystal slab waveguides containing a phase change material

This paper presents novel designs and simulation results of electro-optic modulators in integrated silicon photonics platforms. The electro-optic modulators described in this paper are based on the modulation of the photonic bandgap in a silicon photonic crystal slab waveguide, consisting of a phase change material (germanium selenide) sandwiched in between silicon layers. Three-dimensional finite difference time domain (FDTD) modeling is employed to simulate two configurations of electro-optic modulators based on mode-gap shifting in photonic crystal slab waveguides—one that is designed for operation with the transverse electric (TE) polarization of light and the other for the transverse magnetic (TM) polarization. When the electric field is applied across the germanium selenide layer surrounded by doped silicon layers, there is a change of phase of the germanium selenide layer. A large refractive index contrast between the two phases of the germanium selenide layer—i.e., between the amorphous phase and the crystalline phase—on application of the electric field is responsible for shifting of the mode gap in the dispersion characteristics of the PhC slab waveguide. Our results show excellent performance of the electro-optic modulators in terms of the high On/Off extinction ratio ( > --> 37 d B in the TE configuration and > --> 28 d B in the TM configuration), broadband switching capability ( ∼ 100 n m ), low insertion loss in the On state ( < 2 d B ), high range of tunability (by changing the lattice constant and radius of air holes), and low operating voltage ( < 4 V ). The proposed devices can be used as electro-optic modulators in future nanoscale photonic integrated circuits.

Numerical optimization design for waveguide bends with low-loss and wide-bandwidth in two-dimensional photonic crystal slabs

In this paper, we demonstrate some optimization designs for waveguide bends in two-dimensional photonic crystal slabs based on the effective index approximation. Four different types of waveguide bends with low-loss and wide-bandwidth are designed, which involve 60°, Z-shaped, Y-shaped and 120° waveguide bends in the two-dimensional triangular lattice photonic crystals. In all numerical experiments, the electromagnetic fields in the photonic crystal slab waveguides are solved by the finite element method in COMSOL software. Several strategies are applied to design slab waveguide bends with low-loss and wide-bandwidth, for example, adding some elliptical air-holes near the corner or adjusting the radii of some air-holes. For four optimized structures, the transmission efficiencies at 1550 nm are all more than 92%. As we show, this is achieved over a −0.915 dB bandwidth of more than 65 nm in all waveguide bends. The numerical results indicate that the design scheme we applied to optimize waveguide bends in photonic crystal slabs is simple and efficient, which can provide some useful guidance for further three-dimensional design.

Enhancement of magneto-optical properties in magnetic photonic crystal slab waveguide based on yttrium iron garnet

In this work, a polarization-independent waveguide based on magnetic photonic crystal (MPC) with a triangular lattice of air holes in Yttrium Iron Garnet (YIG) slab grown on alumina (Al2O3) substrate is proposed, where both TE-like and TM-like periodic band gaps overlap. YIG is well known for its attracting magneto-optical (MO) properties and used to produce a coupling between the TE and TM modes. Thus, a nonreciprocal effect can be obtained by applying an external magnetic field parallel to the direction of propagation. At 1550 nm, the complete photonic band gap is simulated and optimized using the three dimensional plane-wave expansion method. The aim of this study is to enhance Faraday rotation (FR) while maintaining a low modal birefringence. A numerical analysis in function of magnetic gyration (g) has been reported, using the BeamProp software. The results reveal a proportional relation between FR, Δn and g, such for g = 0.5, a large FR of 26.11×104 °/cm with Δn = 7×10−6. The results show a real improvement of this MPC structure based on YIG with larger FR, lower modal birefringence and minimal losses. The notable enhancement in the MO behaviour could improve the performance of optical isolators, and makes it suitable for nonreciprocal devices.

Simulation of magneto-optical properties of magnetic photonic crystal waveguides

We propose a kind of magnetic photonic crystal (MPC) slab waveguide made from magneto-optical material, namely bismuth iron garnet. The finite difference beam propagation method is employed for numerical calculations, which is the most widely used propagation technique for modeling integrated and photonic devices. The mode conversion and the influence of corresponding parameters are studied theoretically. As a numerical example, a TE-TM mode conversion waveguide-type optical isolator based on this designed MPC is described. An effective away to enhance the Faraday rotation at the 1.55-μm telecommunication wavelength is shown.

Polarization-independent waveguides based on the complete band gap of the two-dimensional photonic crystal slabs

Two-dimensional photonic crystal (PC) slabs exhibiting photonic band gaps (PBGs) in both TE and TM polarizations are of particular interest for a better control of light propagation and easy experimental fabrication. Through reducing the symmetry of the PC lattice, simultaneous realization of PBGs in both TE and TM polarizations within a certain frequency region can be achieved. Based on the air holes immersed in the silicon background, two kinds of two-dimensional PC slab structures are proposed. These are a honeycomb lattice of circular air holes with different sizes and larger circles, and smaller triangles alternately arranged, respectively. The light transmittances show that these two structures offer large complete band gaps. Different kinds of straight waveguides are designed, and their abilities of localizing the incident beam are studied by analysing the light transmittances and spatial field distributions, where the light energy is localized perfectly in the optimized waveguide structure. Our proposed PC slab waveguides are convenient for fabrication, and could be applied in future all-optical integrated circuits.

Photonic crystal waveguides as sources of counterpropagating factorizable biphoton states.

We demonstrate numerically that photonic crystal slab waveguides can generate spectrally unentangled biphoton states, highly desired for heralding of single photons. We achieve this by modally phase matching a counterpropagating spontaneous parametric down-conversion process, in a fully integrated scheme and without the need for periodic poling. Such a configuration is an ideal integrated source of heralded single photons, as it spatially separates the photons of a pair at the source without any extra components, while allowing for generation of spectrally narrow photons on a very short length scale.

Giant enhancement of the effective Raman susceptibility in metasurfaces made of silicon photonic crystal nanocavities.

We demonstrate that stimulated Raman amplification can be enhanced by more than four orders of magnitude in a silicon metasurface consisting of a periodic distribution of specially engineered photonic crystal (PhC) cavities in a silicon PhC slab waveguide. In particular, by designing the PhC cavities so as they possess two optical modes separated by the Raman frequency of silicon, one can achieve large optical field enhancement at both the pump and Stokes frequencies. As a consequence, the effective Raman susceptibility of the nonlinear metasurface, calculated using a novel homogenization technique, is significantly larger than the intrinsic Raman susceptibility of silicon. Implications to technological applications of our theoretical study are discussed, too.

Circularly polarized thermal emission from chiral metasurface in the absence of magnetic field

We report on the thermal radiation source which emits circularly polarized thermal radiation in the absence of an external magnetic field. The geometry of thermal source includes a photonic crystal slab waveguide with chiral morphology. We show that due to the absence of a mirror symmetry of such metasurface, the thermally generated electromagnetic waves are circularly polarized with the circular polarization degree of 0.87. Using the Fourier modal method we study the structure eigenmodes and analyze the field distributions in them.

Optical coupling characteristics between a circular defect resonator and a waveguide in a two-dimensional photonic crystal slab

To develop a wavelength division multiplexing (WDM) device using a photonic crystal (PC), we investigate coupling strength between a circular cavity resonator and a waveguide with various structural parameters. In the WDM device, light signals with various wavelengths generated in different cavity resonators travel through a single waveguide. Careful tuning of the cavity-waveguide coupling is necessary for practical operations, since the light signals must have the same intensities in the waveguide. Carrying out three-dimensional finite-differential time-domain (FDTD) calculations along with photonic bands, we search for the optimum structure for the WDM.

Radiation losses in photonic crystal slab waveguide to enhance LEDs efficiency

Two-dimensional photonic crystal slabs with different hole shapes in which air holes etched in slabs as a key factor of losses, is analyzed. As far as higher efficiency in Light Emitting Diodes (LED) structure is concerned, the research undertook to analyze these losses by investigating the role of the etched-holes shape in cladding through modifying the refractive index of holes and as a result changing the effective refractive index of cladding, entire space. Then, validating such results, photonic crystal slab waveguide is simulated in three ways namely regular three dimension, two-dimension cylindrical and cylindroconical, which demonstrates a good agreement with experimental data. Finally, it is shown that interaction of the guided light improves extraction of both lower order modes as well as higher order modes, which enhance LED efficiency.

Fine tuning of the center frequency of the slow light regime in an infiltrated W09 photonic crystal waveguide

We report the tuning process of center frequency in a dispersion engineered slow light regime in W09 photonic crystal slab waveguide (W09-PhCSW). The W09-PhCSW consists of hexagonal lattice of air holes of radii r = 0.3ɑ in a Si substrate with a reduced waveguide width. This end is achieved by means of simultaneous infiltration of magneto and opto-fluidics in first and second rows adjacent to the waveguide. In particular, change in infiltration of opto-fluidics (1.5 ≤ nf ≤ 2.1) is used for selecting the optimized group index with low group velocity dispersion. Concurrently, the center frequency in the constant group index regime (Δng = ±5%) get tuned with an external magnetic field. This course of external controllability originated from the field dependent refractive index of magneto-fluidic. After all, this design leads to a considerable low group velocity dispersion of |β2| ≤ 2000 (ɑ/2πc2), the group index values in the range of 23 ≤ ng ≤ 103 and reasonable group index-bandwidth product values (0.114 ≤ ng × Δω/ωc ≤ 0.26).

A study on photonic crystal slab waveguide with absolute photonic band gap

Most of the conventional photonic crystal (PhC) slab waveguides have a photonic band gap (PBG) only for one polarization state of two orthogonal polarization states. In this paper, we study on an absolute PBG that can realize PBG for both polarizations in the same frequency range and demonstrate that an absolute PBG can be realized in PhC structures proposed here. In the numerical analysis and design of PhC structures, we employ the two-dimensional finite element method (FEM) based on the effective index method (EIM). First, we propose two-types of PhC structures with an absolute PBG and show that a steering type PhC is superior to an air-ring type PhC to obtain a wideband absolute PBG. It is also shown that the optimized steering type PhC has the absolute PBG whose bandwidth of 164 nm at the center wavelength of 1.55 µm. Furthermore, we design PhC waveguides based on the obtained PhC structure having an absolute PBG in order to obtain guided modes for both polarization states within the same wavelength range. The transmission properties of the designed PhC waveguides are also investigated and 60 degree bends which are required in compact photonic circuits are designed. From these results, the possibility to realize compact polarization multiplexing photonic devices is shown.

Flat-Band Slow Light in a Photonic Crystal Slab Waveguide by Vertical Geometry Adjustment and Selective Infiltration of Optofluidics

In this paper, a photonic crystal slab waveguide (PhCSW) for slow light applications is presented. To obtain widest possible flat-bands of slow light regions—regions with large group index (n g), and very low group velocity dispersion (GVD)—two core parameters of PhCSW structure are investigated. The design procedure is based on vertical shifting of the first row of the air holes adjacent to the waveguide center and concurrent selective optofluidic infiltration of the second row. The criteria of $${{\langle n_g \rangle}}$$ ± 10% variations is used for ease of definition and comparison of flat-band regions. By applying various geometry optimizations for the first row, our results suggest that a waveguide core of W 1.09 would provide a reasonable wide flat-band. Furthermore, infiltration of optofluidics in the second row alongside with geometry adjustments of the first row result in flexible control of 10 < n g < 32 and provide flat-band regions with large bandwidth (10 nm < Δλ < 21.5 nm). Also, negligible GVD as low as β 2 = 10−24 (s2/m) is achieved. Numerical simulations are calculated by means of the three-dimensional plane wave expansion method.

Statistics of Anderson-localized modes in disordered photonic crystal slab waveguides

We present a fully three-dimensional Bloch mode expansion technique and photon Green function formalism to compute the quality factor, mode volume, and Purcell enhancement distributions of a disordered W1 photonic crystal slab waveguide in the slow-light Anderson localization regime. By considering fabrication (intrinsic) and intentional (extrinsic) disorder we find that the quality factor and Purcell enhancement statistics are well described by log-normal distributions without any fitting parameters. We also compare directly the effects of hole size fluctuations as well as fluctuations in the hole position. The functional dependence of the mean and standard deviation of the quality factor and Purcell enhancement distributions is found to decrease exponentially on the square root of the extrinsic disorder parameter. The strong coupling probability between a single quantum dot and an Anderson-localized mode is numerically computed and found to exponential decrease with the squared extrinsic disorder parameter, where low disordered systems give rise to larger probabilities when state-of-art quantum dots are considered. The optimal regions to position quantum dots in the W1 waveguide are also discussed. These theoretical results are fundamental interesting and connect to recent experimental works on photonic crystal slab waveguides in the slow-light regime.

Generation of Counterpropagating Path-Entangled Photon Pairs in a Single Periodic Waveguide.

We propose the use of nonlinear periodic waveguides for direct and fully integrated generation of counterpropagating photon pairs by spontaneous parametric down-conversion. Using the unique properties of Bloch modes in such periodic structures, we furthermore show that two counterpropagating phase-matching conditions can be fulfilled simultaneously, allowing for the generation of path-entangled Bell states in a single periodic waveguide. To demonstrate the feasibility of our proposal, we design a photonic crystal slab waveguide made of lithium niobate and numerically demonstrate Bell-state generation.