2D-Photonic crystal heterostructures for the realization of compact photonic 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].
Section snippets
PhC-type 1 structure
In this section, the PhC-type 1 structure is designed where self-collimated TE and TM polarized light can propagate free of beam divergence due to periodic structure. The lattice constant (a) and circular air hole radius (R) is intended in such a way that there is no photonic bandgap (PBG) for both TE and TM-polarization in the operational wavelength range [26]. 2D PhC lattices are composed of a periodic plane and are extruded or non-periodic in their third dimension; periodically arranged
PhC-type 2 structure
Here PhC-type 2 structure is proposed which is composed of square air holes periodically arranged on a silicon substrate. The side length of the square air hole and lattice constant is denoted as R1 and a1, respectively. The PBG is a fascinating feature of PhCs with the help of which certain polarization of light can be blocked. We have used this phenomenon to design a PhC structure which is capable of blocking TE-polarized light while allowing TM-polarized light to propagate unaffected. The
TM-polarization maintaining 2D-PhC
Numerous integrated photonics devices operating with TM-polarized light delivers outstanding efficiency. Moreover, for sensing applications, TM-polarized light is realistic, since its evanescent field penetrates deeper into the top and bottom cladding relative to that of the TE-polarized light. Also, to assist most of the channel capacity, devices used for polarization multiplexing require TM-polarized light. This is the reason, TM-polarization maintaining devices are important for realizing
Heterostructure 2D-PhC 180° bend reflector and polarizer
In this section, two major applications of PhC heterostructure are discussed. The unique arrangement of PhC-type 1 and PhC-type 2 structures allow the steering of self-collimated TE-polarized light in a small footprint of 180° bend. Moreover, it can also assist in the formation of the polarization beam splitter (PBS). In [37], a beam power splitter is realized based on heterostructure 2D-PhC. In [38], a rapid preparation of TiO2-SiO2 heterostructure PhC is realized via a modified
Conclusion
In conclusion, the disposal of heterostructures in PhC delivers a wide prospect for the expansion of optical elements. In this work, we have numerically investigated some interesting applications of heterostructure 2D-PhC structure formed by the amalgamation of PhC-type 1 and PhC-type 2 structure via 2D-finite element method. The lattice parameters of both the structures are optimized in such a way that first structure can allow the transmission of both TE and TM-polarized light to transmit
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was financially supported by the Ministry of Science and Higher Education within the State assignment FSRC "Crystallography and Photonics" RAS (No. 007-GZ/Ch3363/26) for numerical calculations and Russian Science Foundation (No. 20-69-47110) for theoretical results.
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