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Abstract
Surface waves with their ability to pass through nanostructures opened the way to convert the electronic circuits to optical circuits. The 4 × 2 optical encoder have been designed, analyzed, and simulated optically using plasmonic structure using the COMSOL version 5.5 software package with finite element method (FEM). 4 × 2 optical encoder using silver as metal and SiO2 and Al2O3 as a dielectric with a Tthreshold of 40%, and operating wavelength of 1550 nm. The results were delivered in graphs containing the Transmission values (T) versus a wavelength range between 800 nm and 2000nm, electric field distribution (EF), contrast ratio (CR), modulation depth (MD), and insertion loss (IL). The maximum T was 50%, the MD was 87.2%, and the CR 8.84 dB.
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1. Introduction
The surface plasmon polariton (SPP) phenomenon generates waves at the interface region between the metal and dielectric material due to the interaction of light waves and free electrons of the metal, which represents the principal operations of the optical system devices [1–3]. The SPP can break the limitation of inherent delay and heat generation of integrated electronic circuits (IECs) and the diffraction limits of integrated photonic circuits (IPCs) [4,5] The SPP opens the way for the wave to travel in waveguides at a specific wavelength, this guide is known as plasmonic waveguides (PW) [6,7]. The SPPs transfer the electromagnetic waves within the boundary between the dielectric and the metal due to the interaction between light waves and free electrons of the metal. The SPPs with high bandwidth open the way for optical signal to travel within the waveguides at a specific wavelength. There are two structures used to design the PW for guiding the light in a specific wavelength due to high confinement: metal insulator metal (MIM) [8], and insulator metal insulator (IMI) [9]. A comparison between the two structures is shown by Table 1. Typically, the metal layer is selected to be noble materials such as silver (Ag), gold (Au), etc [10,11]The IMI was fabricated using two dielectric layers (low-index medium) separated by a metal layer (conducting medium) [12]. The IMI represents a good choice for designing the plasmonic structure due to the long propagation length and low loss [13,14]. The principles of constructive and destructive interferences between the light waves in the PW lead to the mechanism of the plasmonic devices [15–17]. Transverse electric (TE) and transverse magnetic (TM) modes control the propagation in the PWs, but the surface plasmon polariton phenomena typically deal with the TM mode [18–20].
Table 1. The difference between IMI and MIM [29].
The PWs are proposed to design several passive and active devices such as logic gates, multiplexer /de-multiplexer [21,22]flip-flop circuits [23], Bragg reflectors, combiners [24], splitters [25], switches, modulators [26], resonators [27], filters, and decoders/encoders [28].
In this article, an optical OR gate and 4 × 2 optical encoder based on the PW have been simulated, designed, and analyzed. Ag noble material is used as metal. SiO2 and Al2O3 are used as low-index dielectric material. The suggested design operates at 1550 nm wavelength and is designed using four square shapes and three straight strips of Ag metal. This article was laid out as follows: section 2 contains the mathematical concept and the system design. The result and discussion comes in section 3, and section 4 includes the conclusion.
2. Mathematical concept and system design
The OR logic gates with two input ports (IPs) one control port (CP) and one output port (OP) were designed by this paper as shown in Fig. 1 using IMI plasmonic structure at 1550 nm operating wavelength with Ag as metal and 99.14 SiO2%, and 0.59% Al2O3 as dielectric. In Fig. 1, the OR gate is designed using four square shapes and three straight lines of Ag, and the remaining part is designed using dielectric material. Two OR logic gates with one straight line of Ag are connected to design the 4 × 2 encoder plasmonic circuit as shown in Fig. 2. The two sub-structures of OR logic gates are separated using perfect electronic conductor (PEC) material. The PCE material represents a perfect mirror that is used to isolate the substructures from each other’s [10,11]. The 4 × 2 encoder is designed using four IPs, two CPs, and two OPs as shown in Fig. 2.
A PW is a multi-layer system consisting of Ag material with a dielectric constant of metal (ɛm) as a conducting medium and SiO2, and Al2O3 with a dielectric constant of insulator (ɛd,) as a dielectric medium. Equation (1) proposed to understand the nature of the transverse magnetic (TM) of plasmonic structure with respect to the dielectric constant of the medium as [24]:
3. Results and discussions
The 4 × 2 encoder is an electronic circuit that receives input bits and creates an equivalent binary code. It is generated by combining two OR logic gates with two IPs and one OP as shown in Fig. 3. The trial-and-error method was proposed by this paper to decide the suitable IPs and CPs. The truth table for the 4 × 2 encoder is listed in Table 2. It consists of four inputs, namely U0, U1, U2, and U3, and two outputs which are W0 and W1. These IPs and OPs can be projected on the proposed configuration depicted in Fig. 2, where U1 is assigned to IP 9, U2 is pointed to IP 7, U3 is common for IPs 8 and 10, respectively, and S0 is directed to IP 3. The outputs are W0 which is on OP 5, and W1 for OP 4. Ports 2 and 1 are CPs.
Table 2. Truth table of 4 × 2 encoder [34].
The design of the OR logic gate is shown in Fig. 1. To design OR gate ports 1, and 2 are selected to be IPs, port 3 is CP, and port 4 is OP. Table 3 shows the results of the simulation OR logic gate. When the IPs have an OFF-OFF condition the OP has an OFF condition due to that the T level is equal to 0.05 and this value is lower than the Tthreshold. (0.4).
Table 3. Transmission values the OR logic gate plasmonic structure.
If the IPs have an OFF-ON condition the OP is selected to be ON state due to the T level (47%) being higher than the Tthreshold (40%).
If the IPs have an ON-OFF status, the OP has 1 logic number due to that the T level is equal to 0.52 higher than the Tthreshold and the phase shift between IPs and CP is 0°.
Finally, if the IPs has 11 logic number the T level is equal to 1.35 higher than the Tthreshold (40%), and the OP is selected to be 1 logic number.
Three different factors are proposed to describe the performance of the OR logic circuit as shown in Table 4. If the minimum value of the ON condition is equal to 0.47 and the maximum value of the OFF condition is equal to 0.05. The CR offers a good performance with 9.73 dB when the CR describes the ratio of the minimum power in the ON condition to the maximum power in the OFF condition. The ideal dimension of the plasmonic structure is described by the MD. The MD of the suggested structure is 96.29% and the IL is equal to −3.27 dB.
Table 4. Characteristic of OR logic gate plasmonic structure.
Figure 4 shows the optical spectrum analyzer of the OR logic gate over a range from 800 nm to 2000nm operating wavelength at 00, 01, 10, and 11 logic numbers and found that the best performance at 1550 nm wavelength. The electric field distribution of PW is shown in Fig. 5. The high constriction of electric field distribution was illuminated by red about 1.8 × 106 V/m, and the lower electric field distribution was given by blue about 0.2 × 106 V/m.
Two sub-structures of OR logic gates were connected to design 4 × 2 encoder plasmonic circuit as shown in Fig. 2. When the IPs have 1000 logic number as shown in Table 4, the OPs have an OFF status for W1, and W2 and the T level (0.063 for W0, and 0.064 for W1) lower than the Tthreshold (0.4).
If the four IPs have 0100 logic number, the OPs select to be ON condition (1 logic) for W0 and OFF condition (0 logic) for W1, when the T level of W0 is equal to 0.49 and this value higher than the Tthreshold (0.4) and the T level of W1 is equal to 0.064 lower than the Tthreshold (0.4).
When the IPs have 0010 logic number, the OPs select to be OFF condition (0 logic) for W0 and ON condition (1 logic) for W1, due to that the T level of W0 equal to 0.063 is lower than the Tthreshold (0.4) and the T level of W1 (0.5) larger than the Tthreshold (0.4) and 0° phase shift between the IPs and CPs are listed in Table 5.
Table 5. Transmission values the 4 × 2 optical encoder structure.
Finally, Fig. 4 shows that the IPs have 0001 condition, the OPs have ON-ON condition (11 logic) for W0, and W1 and the T level equal to 0.48, and 0.49 higher than the Tthreshold.
Several factors proposed to describe the performance of 4 × 2 encoder plasmonic circuit as shown in Table 6. When the minimum value of ON condition equal to 0.48, and 0.49 and maximum value of OFF condition equal to 0.064, and 0.063, the CR offer a good performance with 8.81 dB, and 8.84 dB. The ideal dimension of plasmonic structure describe by the MD. The MD of suggested structure has 87.1%, and, 87.2%. The IL describe the loss in power when the signal guide from the IPs to the OPs as shown in Eq. (13). The IL of our structure has −3.18 dB, and −3.09 dB.
Table 6. Characteristic of 4 × 2 optical encoder structure.
Figure 6 shows the optical spectrum analyzer of 4 × 2 encoder plasmonic circuit over a range from 800 nm to 2000nm operating wavelength at 1000, 0100, 0010, and 0001 logic number and found that the best performance at 1550 nm wavelength. The constriction of electric field distribution for 01, and 11 OPs PW shown by Fig. 7. The electric field distribution varied between 0.2 × 106 V/m in blue to 1.2 × 106 V/m in red.
Fig. 6. The T values versus wavelength for the 4 × 2 plasmonic encoder circuit at (a) 1000, (b) 0100, (c) 0010 and (d) 0001.
A comparison between our work and recent works is demonstrated in Table 7. Our work evaluates the performance of the nano-structure plasmonic IMI waveguides using CR, MD, and IL. The plasmonic encoder circuit has a special design with square resonator of silver and the remaining part has 99.14 of SiO2%, and 0.59% of Al2O3.
Table 7. The offered 4 × 2 encoder plasmonic circuit versus previous designs.
4. Conclusion
A new structure made by the PW suggested by this article to design an optical 4 × 2 encoder using two OR logic gates. The nature of the material employed the procedure of establishing the operating wavelength, which in this research elected to be 1550 nm, that essential window utilized in the design of optical system applications. The transmission level at the OPs can be increased or decreased with respect the phase shift between the incidence optical waves and because of the constructive and destructive interferences among the optical signals, and also depending on the port's position of the suggested structure. The ON condition and the OFF condition can be acknowledged based on the transmission level. How much is the optical power ratio at the OPs away or close from this value which is in this article 40%?.
Acknowledgment
The final manuscript was read and approved by all writers.
Disclosures
The writers declare no competing interests.
Data availability
The corresponding author will provide the datasets and curves produced during the current investigation upon reasonable request.
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