In today’s world, with rapidly growing technologies, advancements are needed in every field of research, especially in electronics and communications devices. In this way, we are interested in the context of this advancement form electronics to world of optics.
The photonic crystal devices are periodic nanostructures of alternating layers of certain materials, with different refractive indices, which enable the manipulation of various forms of electromagnetic radiation.
The waveguides of a photonic crystal can have very sharp curves, with low losses in the structure, contributing to an increase in the density of integration of components, by several orders of magnitude.
Photonic crystals offer a wide range of applications in ultra-compact all-optical integrated circuits with a high reduction in energy consumption. Currently, several logic gates based on photonic crystal platforms have been demonstrated (Lee et al. 2008).
In order to recognize the performance of all-optical logic gates, different structures have also been proposed, such as, Multimode Interference, MZI (Mach-Zhender Interferometer), SOA (Semiconductor Optical Amplifier), among others.
Initially, all-optical logic gates based on SOA properties were reported (Zhou et al. 2005; Houbavlis et al. 1999; Kim et al. 2006). However, there are some limitations to these methods, among which we can mention the high input power with a low power transmission, complex and expensive projects, the latency time, in addition to the speed and size of the structures, causing that are less used, making on chip integration difficult (Saranya and Anbazhagan 2020).
The study of photonic crystals allowed the design of certain structures with interesting optical properties, including the specific frequencies range denoted by photonic band gap (PBG) and that can be calculated using the plane wave expansion (PWE) method. The light in this range of frequencies does not propagate through this structure. (Leung and Liu 1990; Meade et al. 1992; Sakoda 2004). In this way, the electromagnetic waves incident with frequencies located in this range are reflected by the crystal, and therefore the light flux can be controlled (Joannopoulos et al. 2008).
Lately, many researchers have become interested in the design of optical logic gates from photonic crystals, as it is one of the most important optical media to form optical processors and optical communications systems. Some of these authors designed logic gates based on resonators, whether linear or non-linear, where they studied the advantages and disadvantages in each method used.
In this direction, some works have already been developed, among which, refer to that of Chunrong Tang et al. (2013), where several completely optical logic gates of a 2-D photonic crystal were designed, based on multimodal interference (MMI). Some logic gates were obtained: OR, NAND, XNOR and XOR. These structures are analyzed and simulated using the FDTD and PWE methods. The contrast ratio for OR and NAND logic gates is 13dB, for XNOR it was 17dB and for XOR, about 21dB in C-Band (C-Band / Conventional Band) with the spectral region from 1530 nm to 1565 nm. These logic gates are potential candidates for constituting photonic integrated circuits (PICs), which will be used in all-optical signal processing, all-optical networks and in photonic computing.
In another study, Yuanliang Zhang et al. (2007), demonstrated a device used to obtain optical logic gates. The working principle is shown through simulations using the FDTD method. This device is applicable for the frequency range of 0.188-0.199 (a/λ). The contrast ratio obtained within the frequency range s is 17 dB with the maximum being about 21dB. Due to its simple structure and its clear operating principle, this structure can be used for future applications in PICs.
In Junjie Bao et al. (2014), the authors investigated a new approach for the design of all-optical logic gates based on 2D photonic crystals in a square lattice of silicon rods (Si) on silica (SiO2). It consists of two photonic crystal resonator rings (PCRRs) and cross-shaped waveguides without the use of optical amplifiers and materials nonlinear. The layout of the optical logic gate is simulated and analyzed by the FDTD and PWE methods. The results of the numerical simulation demonstrate that the structure acts as a NOR and NAND logic gate. The logical levels, high ‘1’ and low ‘0’ are defined. Since this structure is composed of a linear material, it presents low power consumption compared to structures composed of non-linear materials. It is observed that the construction of new structures causes PCRRs to have new applications in ultra-compact PICs.
Goudarzi et al. (2016) proposed an all-optical structure based on two types of defect, the point defect and the line defect. The defects were created in a square lattice formed of silicon dielectric rods in contrast to air. The device design features two input ports with two output ports. The operating frequency range of the device is from 0 to 0.45 (a/λ), however it was adjusted to 0.419 for low dispersion condition, and the structure is implemented with an operational lambda equal to 1.55 µm. Regarding the performance presented after the simulations, the maximum contrast ratio reached was about 6.767 dB. According to the results, the authors reported that the device can act as an XOR logic gate and an OR logic gate.
More recently, Saranya and Anbazhagan (2020) proposed a trifurcation structure of logic gates based on two-dimensional photonic crystals composed of a square lattice of air holes in silicon. The plane wave expansion method (PWE) and the finite difference time domain method (FDTD) are used to analyze the behavior of the structure. The results obtained prove the functionality of OR, AND, NOR and NAND optical logic gates. Regarding the performance obtained by the device, specifically for AND, NAND and NOR gates, the contrast ratio was 6.15 dB, 5.79 dB and 2.97 dB, respectively. Furthermore, the bit rate and footprint were calculated for the simulated optical logic gates. The footprint for the proposed logic gate was around 424.7 µm. In order to improve the value of the contrast ratio, where certain logic functions imply a better output power, then some phase shifts were introduced in the launch field. Then, with a 180° phase shift, the contrast ratio obtained for the AND and OR logic gates, was 6.52 dB and 10.79 dB, respectively.
The previous works are some examples of studies that are characterized by the use of a numerical methodology in order to analyze the all-optical logic gates obtained for use in various applications in the scope of telecommunications.
The electromagnetic simulation has been performed through 2D finite-difference time-domain (FDTD) method (Taflove and Hagness 2005), which is used to simulate electromagnetic wave propagation in any kind of materials in the time domain. We present a new design of a very compact structure to be used as NAND logic gate, contributing to the density of component integration in optical communications systems.
Numerical simulation has been performed through 2D FDTD method (Johnson and Joannopoulos 2000; Yee 2004), which is used to simulate electromagnetic wave propagation in any kind of materials in the time domain.