2D-Photonic crystal heterostructures for the realization of compact photonic devices

Highlights

• A novel design of a Polarization beam splitter based on a heterostructure 2D-Photonic crystal.

• The high polarization extinction ratio of around 30.5 dB.

• Steering of TE-polarized light in 180^o bend which allows the formation of compact photonic devices.

• The proposed device is easy to realize with the help of CMOS technology.

Abstract

Herein, we presented a novel design of polarization beam splitter (PBS) based on the integration of heterostructure 2D- Photonic crystals (PhCs). Two PhC structures, namely PhC-type 1 and PhC-type 2, with different transmission characteristics are proposed. The first type of PhC is composed of a 2D-periodic array of circular air holes embedded in silicon substrate which facilitates the propagation of self-collimated TE and TM-polarized light. The second type of PhC consists of a 2D-periodic array of square-shaped air holes embedded in silicon substrate which carry a photonic bandgap (PBG) for TE-polarized light whereas TM-polarized light propagates unaffected by diffraction. When PhC-type 2 structure is arranged at 45° on both sides of the PhC-type 1 structure, this combination leads to the formation of PBS with a polarization extinction ratio (PER) as high as 30.7 dB. Furthermore, the same structure can be used to steer the TE-polarized light at 180° in a small footprint. Additionally, the heterostructure PhC can be used for TM-polarization maintain devices.

Introduction

In 1987, Photonic crystals (PhCs) were suggested by Yablonovitch [1] and John [2] has gathered ample attention due to its light management properties. A new prospect has unwrapped in the last few years. The main objective is to tailor the optical properties of the materials. A comprehensive technical developments will be feasible if the materials could be configured to control the electromagnetic (EM) waves over the anticipated frequency range by seamlessly reflecting them, or letting them to travel only in certain directions or to confine them to a certain volume [3]. EM waves disperse within the PhC, for certain wavelengths due to the destructive interference, creating a photonic bandgap (PBG) making it conceivable to control the propagation of light. PhCs have drawn significant attention to practical applications, largely because their band architectures are critically dependent on three controllable degrees of freedom: the lattice geometry, the arrangement of dielectric materials within the unit cell, and the refractive index contrast between the principal materials. These features allow wide prospects to customize and design PhC structures for a multitude of different applications. Recently, PhCs are employed in numerous remarkable applications such as polarization-pass filters [4], [5], [6], light steering [7], [8], optical diode [9], negative refraction [10] and self-collimation [11], among others.

Polarization beam splitters (PBS) are one of the basic components of optical interconnection and optical communication networks for spatially separate light steering in two polarizations. Photonic devices are typically intended for single-polarization operation [12]. Therefore, the beam can be divided into two orthogonal polarizations if the incident light is un-polarized or partially polarized to ensure that the polarization is suitable for the subsequent optical devices. A lot of attention has been given to investigate compact PBSs offering high polarization extinction ratio (PER) [13]. For free-space optical implementation, PBS based on gratings or metasurfaces is suitable [14], [15]. Besides, these PBSs may integrate metal components leading to substantial transmission loss.

In the photonic integrated circuits (PICs), optical waveguide functions as an essential component [16], [17], [18]. Dielectric waveguides usually have comparatively outsized dimensions as they require a big radius of curvature to avoid significant bending loss. Optical waveguides based on PhC structure are different from the conventional dielectric waveguides as they deliver strong optical confinement and malleable light steering. One of the noteworthy applications of the PhC waveguides is the optical transmission of EM wave through a sharp bend with low bending loss [19], [20]. This encourages the designing and manufacturing of PhC based waveguides [21], [22]. However, because of the reflections at the sharp bending corner, non-negligible bending loss also exists, especially when the bending angle is greater than 90°. Therefore, extensive research has been conducted on the optimization of the bends to diminish the reflection loss [23], [24], [25].

In this work, we proposed a novel idea of integrating two PhC heterostructures on silicon-on-insulator (SOI) platform to realize stimulating integrated optical elements such as transverse magnetic (TM) polarization maintaining device, polarization beam splitter and beam steering in a compact footprint. The proposed PhC structures are capable of delivering a self-collimated TE and TM polarized beams and a low transmission loss. In our previous work [26], we have demonstrated the 2D-PhC heterostructure for light steering applications. The numerical simulations are carried out via a 2D finite element method (2D-FEM) where the PhC structure is customized to obtain the best performance. For the practical realization of the device, the layer thickness of SiO₂ and Si should be maintained at ~3000 nm and 400 nm, respectively [13]. The “electromagnetic waves frequency domain (emw)” is used as the physics interface and the “frequency domain” was added to the study. The sub-domains in the PhC structure are distributed into triangular mesh elements with a “Physics-controlled mesh” which is a built-in function in the software. The meshing of the device structure depends on the processing power of the system used. We designated a “fine mesh” size which provides precise simulation results based on the processing speed of our computer. The open geometry is assessed by assigning a scattering boundary conditions (SBC) at the outer edges of the simulation window.

When the propagating wave encounters the interface between a homogenous media and a PhC structure, the Bloch modes are excited at a reduced energy of E = a/λ, where a and λ are the lattice constant and the wavelength in free space, respectively. Despite the presence of the scatterers, Bloch waves move in a particular direction in a PhC [27], a phenomenon that can be theoretically explored by equal frequency contours (EFCs) [28]. By employing Maxwell’s equations into a problem of eigenvalue and solving the problem via the methods such as the expansion of the plane wave, EFCs are accomplished. In PhCs, the propagation of light is regulated by its dispersion surfaces, providing a representation of spatial variations of spectral properties in k-space. A cross-section of the dispersion surfaces at fixed frequency outcomes in an EFC. While the EFC of an unpatterned slab is circular, depending on the material usage, lattice geometry, pitch, and the fill factor, the EFC of PhCs exhibits various shapes. In PhCs, the direction of light propagation is defined by the group velocity that is usual for EFCs. Therefore, the self-collimation effect originates from the flat sections of EFCs and many studies have been dedicated to modifying the dispersion properties of PhCs to artificially regulate the EM-waves for the desired applications, taking into account different lattice symmetries, material parameters, and geometrical shapes of the scatterers [29].