Objective With the rapid development of communication and computer technology, people's requirements for information transmission and processing are also increasing. The rapid development of wireless communication leads to the lack of spectrum resources and the development of new frequency bands has become a consensus to solve the lack of spectrum resources. Several frequency bands of the terahertz (THz) band which have not been developed and used have attracted people's attention recently. High-performance equipment and easily integrated products are required in THz wave communication systems. Due to the unique properties of photonic crystals, such as photonic bandgap and photonic localization, they are often used to design and manufacture THz wave devices. Currently, most research on the 4-2 encoder based on the photonic crystal focuses on the optical band, while the research on the 4-2 encoder in the THz band is less. Therefore, it is crucial to study the THz 4-2 encoder with high contrast and fast response to meet the increasing demands in information transmission and processing using the THz wave communication system. The development of THz 4-2 encoders has correspondingly promoted the improvement and progress of THz communication systems, which is also an indispensable and crucial part of the THz communication system.

Methods Based on the bandgap and local properties of photonic crystals, a novel THz 4-2 encoder is proposed using the photonic crystal. This proposed scheme introduces the line defect waveguide into a complete two-dimensional square lattice silicon photonic crystal. The line defects consisting of four input ports (I₀, I₁, I₂, and I₃) and three output ports (V, O₁, and O₀) are used to achieve the logic function of the THz wave 4-2 encoder. When THz wave is input from input port I₀, all THz waves are output from output port V, and the logic state of O₁ and O₀ is “00”; when THz wave is input from input port I₁, most THz waves are output from output port O₀ because of the bending waveguide, the logic state of O₁ and O₀ is “01”; when THz wave is input from input port I₂, most THz waves are received at the output port O₁, and the logic state of O₁ and O₀ is “10”. When the THz wave is input from the input port I₃, the output ports O₀ and O₁ are equivalent to an equal ratio beam splitter, and the logic state of O₁ and O₀ is “11”.The performance of THz wave 4-2 encoder based on the photonic crystal is studied using the plane wave method (PWM) and finite-difference time-domain (FDTD).

Results and Discussions The proposed THz 4-2 encoder based on photonic crystal has high contrast ratio and short response time. When THz wave is transmitted from input port I₀, the transmittance of output port O₁ is 0, and that of output port O₀ is 0; when THz wave is transmitted from input port I₁, the transmittance of output port O₁ is 1.17%, and that of output port O₀ is 83.51%; when THz wave is input from input port I₂, the transmittance of output port O₁ is 83.55%, and that of output port O₀ is 1.16%; When THz wave is input from input port I₃, the transmittance of output port O₁ and output port O₀ are both 47.06%. By calculating the contrast ratio, it can achieve 16.04 dB (Table 2). In addition, the response time is not less than 167 ps. The THz wave 4-2 encoder is also not the same under different working wavelengths. According to the research, the THz wave 4-2 encoder has high contrast ratio between 251 and 252 μm (Fig. 8). The length L of short cavity waveguide W₇ and W₈ will affect the energy of THz wave coupling from input port I₃ to output ports O₁ and O₀. The contrast of the 4-2 encoder can be improved to a certain extent by choosing the appropriate short cavity waveguide length L=5a (Fig. 9). The existence of dielectric pillars A and B also improves the performance of THz wave 4-2 encoder to a certain degree (Fig. 10).

Conclusions A novel THz wave 4-2 encoder using a silicon photonic crystal is proposed. In this proposed scheme, the property of coupling with line defects waveguide made of four input ports and three output ports are utilized. The performance of the proposed THz wave 4-2 encoder is studied using the PWM and FDTD. The results show that the encoder can achieve THz binary logic coding in a wide wavelength range of 251--252 μm. The lowest and highest contrasts are 13.27 dB and 18.02 dB, respectively. Meanwhile, the response time reaches the order of ps. THz wave 4-2 encoder has the advantages of small volume, simple structure, high contrast, fast response, low loss, and easy integration. It will be widely used in THz communication in the future.