Terahertz (THz) waves are electromagnetic ones with frequencies ranging from 0.1 THz to 10 THz. Due to their high penetration, low photon energy, and high communication capacity, terahertz waves are widely employed and have important applications in broadband communication, medical imaging, nondestructive testing, security, and other fields. However, as how to generate high-quality terahertz waves becomes a major technical problem, the THz frequency band is once called the THz gap. With the rapid development of ultrafast optoelectronics, photoconductive antenna (PCA), a THz source involved in electronics and photonics, can be applied at room temperature, with high frequency of THz wave generation and low requirements for laser pump power, which makes it stand out among other THz sources. However, due to the high refractive index of photoconductive materials, the photoconversion efficiency of THz PCA is low. Meanwhile, due to the shielding effect of electric fields, the THz radiation power is easily saturated and difficult to improve.

The radiation power of THz PCA is affected by many parameters, such as current density, bias voltage, selected laser power, and repetition frequency. However, the micro-nano structures can effectively improve carrier mobility to form more obvious local enhancement of electric fields at the interface with LT-GaAs substrate. Therefore, based on the surface plasmas theory, our paper adopts the finite difference time domain (FDTD) method. With the purpose to study the efficiency enhancement of THz PCA, the period and structural parameters of micro-nano structures are calculated and simulated by FDTD software. In simulating THz wave radiation of PCA, the laser irradiates at the gap between the PCA electrodes, which stimulates the transient photocurrent in the substrate. The transient photocurrent is incorporated into the FDTD calculation as the current source, which makes the FDTD algorithm can be employed in semiconductor calculation.

Adding a dielectric anti-reflection layer on the substrate surface can increase the absorption rate of incident light of micro-nano structures. Therefore, the Si₃N₄ anti-reflection layer can be added during simulation to improve efficiency. After the THz PCA model is built, micro-nano columnar structures are added on the substrate surface to study the enhancement of electric fields and the reduction of reflection after-wave. The changes in transmittance and reflectance monitors before and after adding micro and nano structures are observed and recorded. It should be noted that if the micro-nano structures between the electrodes are too small, it is easy to melt under a strong photocurrent, which results in a short PCA circuit. If the micro-nano structures among the electrodes are too thick, the absorption rate will also be greatly reduced despite significantly reduced reflectivity, causing decreased overall efficiency of the THz PCA. Therefore, a balanced structure should be selected during the simulation to reduce the reflectivity with a high absorption rate. Additionally, the distribution period of the columnar micro-nano structures also affects the generation of THz waves. The micro-nano structures with different arrangement distribution forms and densities are simulated respectively, and the results of single-layer structures are selected (Fig. 4).

To explore the relationship between the period and the transmittance of double-layer micro-nano structures, we expand the simulation range to try a variety of different period combinations, which can achieve maximum efficiency and ensure feasibility. During the simulation, other parameters of the upper micro-nano structures are not changed to ensure that the selected structure has the same upper layer transmittance, except for the increasing number after expanding the simulation range. In addition, the lower micro-nano structures should be interspersed below the gap of the upper micro-nano structures at an appropriate spacing. On the contrary, the generation efficiency of THz waves may be reduced if the lower micro-nano structures are too dense.

Under the 1550 nm 100 fs laser pulse light source, the cylindrical micro-nano structure with a diameter of 0.1225 μm and a height of 0.75 μm arranged by 3×4 triangles with a period of 1 μm has a small transmittance to generate THz waves, which means a high absorption rate. Meanwhile, the electric field intensity (Fig. 5) and the transmittance (Fig. 6) are shown in the figures. The maximum intensity of the central electric field is 0.55 and the transmittance is about 0.16. The double-layer heterogeneous micro-nano structure (Fig. 9), electric field intensity (Fig. 10), and transmittance obtained through calculation and scanning (Fig. 11) are shown. The transmittance is about 0.09, and the electric field intensity of the central part reaches the highest value of 1.02, which is 185.45% of the single-layer micro-nano structure. It can be concluded that under the 1550 nm 100 fs laser pulse light source, the addition period is 1 μm and the diameter of the 4×5 equilateral triangle arrangement is 0.1225 μm, with the height of 0.75 μm and the period of 0.5 μm. The cylindrical micro-nano structure with a diameter of 0.17 μm and a height of 0.5 μm in the 8×8 square arrangement has a higher absorption rate.

Single-layer equilateral triangular cylindrical nanocrystals are proven to be better than other single-layer structures in other conditions. The double-layer heterogeneous micro-nano structures are superior to the single-layer micro-nano structures and other double-layer structures. This double-layer heterogeneous micro-nano structure has structural innovation, which can make the THz PCA generate high-quality THz waves. Additionally, the depth-to-width ratio and frequency of the proposed micro-nano structure can be processed by the existing plasma etching technology.