Objective With the rapid development of terahertz (THz) technology, requirements for THz radiation sources and detectors have increased. Currently, the high refractive index of photoconductive materials decreases the photoelectric conversion efficiency and the shielding effect of the field leads to the saturation of the radiated power, thereby limiting further improvement of the radiated power of THz sources. Therefore, the study on high-power broadband THz radiation sources is an active field in THz-related technology. Many researchers have conducted several studies on the photoconductive antenna (PCA) optimization design because PCA is one of the most widely used THz radiation sources. The THz radiation performance of PCA can be effectively adjusted by improving the antenna structure, which can further promote the wide application of THz devices.

Methods Herein, the performance of a THz-microstructured PCA based on the dipole PCA with embedded square split-ring resonators is analyzed and investigated. Based on the model of dipole PCA, the dynamic process of the amplitude of the designed microstructure resonance mode under the interaction of incident light and structure is described using the coupling wave theory. The local coupling characteristics between the THz wave and structures are simulated and analyzed by adjusting the structural parameters. Furthermore, the relationships of the time domain pulse waveform and frequency spectrum characteristics of THz radiation between the parameters of the dipole and microstructured PCAs are simulated and compared. Finally, the simulation results of the radiation direction of the dipole and microstructured PCAs are compared.

Results and Discussions In this design, the square split-ring resonator (Fig. 1 (b)) is embedded in the dipole PCA electrode (Fig. 1 (a)) to investigate the interaction between the microstructured PCA and pumped laser, which is incident vertically on the surface gap of the structure. In the simulation, three resonance frequency curves under different effects of local electromagnetic modes can be obtained by adjusting the structural parameters (Fig. 2). To further explain the influence of the resonance frequency values of different electromagnetic localized effects on radiation frequency values and the loss performance of the intrinsic structure, we use three resonance frequency values, namely, ω₁=0.8 THz (solid blue line), 1.0 THz (dotted black line), and 1.2 THz (dotted red line). The results show that the local electromagnetic resonance patterns significantly enhance the electromagnetic energy at the resonant frequencies (Fig. 3). Then, the influence of the curves of the three resonance frequencies and the corresponding coupling coefficients on the radiation waveforms is investigated (Fig. 4). We further evaluate the influence of the microstructure parameters on the position of the resonance peak frequency and the signal intensity of the THz PCA. Results show that the maximum radiation intensity is increased by 340% by adjusting the length, width, and gap between the electrodes. The frequency shift of the resonance peak occurs gradually, and the frequency shift range is 0.2--0.8 THz (Figs. 5, 6, and 7). Finally, the far-field THz radiation modes of the dipole and microstructured PCAs are simulated. The main beam of the microstructured PCAs shows better energy concentration and directivity, providing a reference for fabricating directional THz antennas (Figs. 8 and 9).

Conclusions We design microstructured PCA, which exhibits a higher signal intensity and frequency adjustable degree than the dipole PCA. When the coupling coefficient and strength are increased by optimizing and adjusting the structural parameters, the electromagnetic radiation energy is considerably enhanced at the resonance frequencies. The intensity of the THz wave is increased by 340% with a change in the microstructural parameters. Moreover, the resonance peak gradually shifts in the range of 0.2--0.8 THz and the frequency adjustability is approximately 61%. The far-field THz radiation mode of the microstructured PCA can achieve the strongest THz wave radiation direction, which is crucial for designing directional THz antennas. Finally, the relationship between the geometrical structure parameters of the PCA and the resonance peak frequency and intensity of the THz signal is defined, which provides an important basis for the subsequent optimization design of the THz antenna with a high Q value.