Bessel beam is a special non-diffractive beam with the unique natures of small central spot and superior self-healing capabilities. In addition, higher-order Bessel beams have the ability of carrying different orbital angular momentum (also known as topological charge). Terahertz wave, which is an electromagnetic wave between microwave and infrared light, have attracted intensive attention in applications of high-speed communication and high-resolution non-destructive imaging due to their ultra-large bandwidth spectrum and extremely low photon energy. Therefore, generating terahertz Bessel beams and flexibly manipulating their characteristics, including order and non-diffraction length, is of great importance to increase communication capacity and improve imaging accuracy.
However, due to the long wavelength of terahertz, the traditional devices for generating Bessel beams, such as axicons or annular apertures, suffer from bulky size, which will hinder miniaturization of the system. In contrast, the metasurface with ultra-thin two-dimensional form has the ability to fully control the electromagnetic waves, which is an effective way to manipulate Bessel beams. Unfortunately, the implemented devices mostly focused on the generation of 0th-order Bessel beams, the generation and flexible control of high-order Bessel beams are yet to be thoroughly unexplored.
To address this problem, the research group led by Prof. Yan Zhang from the Capital Normal University, cooperating with Prof. Bin Hu from the Beijing Institute of Technology, proposed a novelty Moiré metasurface with flexible manipulation of terahertz Bessel beam order and non-diffraction length. The device consists of two metasurfaces. Not only the order, but also the non-diffraction length of the generated Bessel beam can be continuously tuned with mutual rotation between two metasurface. The relevant research results have been published in Photonics Research, Volume. 11, Issue 1, 2023 (Guocui Wang, Tian Zhou, Jianzhou Huang, Xinke Wang, Bin Hu and Yan Zhang. Moiré meta-device for flexibly controlled Bessel beam generation[J]. Photonics Research, 2023, 11(1):100)
Here, an all-dielectric Moiré meta-device integrated with functions of an axicon and a spiral phase plate was designed. As shown in Fig. 1, the device consists of two layers of metasurfaces in which silicon cylinders of different diameters modulate the phase of the incident terahertz plane wave. A specific phase modulation is assigned to the two metasurfaces in the Moiré meta-device to enable the generation of terahertz Bessel beams. One can flexibly manipulates the order of Bessel beams when the two metasurfaces rotate mutually with each other.
Fig. 1. Schematic of the proposed Moiré meta-device for order-variable Bessel beams generation.
A THz focal-plane imaging system was utilized to characterize the function of the proposed device. Fig. 2 shows experimental and simulated results for order-variable Bessel beam generation based on the Moiré meta-device, where the number of periods of 2π going around the circle in the phase distribution indicates the order of the Bessel beam. It can be seen that the orders of the Bessel beam are changed from 0 to 4 by rotating one metasurface relative to another metasurface from 20° to 80° with a step of 20°.
Fig. 2. Experimental and simulated results for order-variable Bessel beam (a, b) Normalized amplitude and phase profiles measured in experiment. (c, d) Normalized amplitude and phase profiles obtained in simulation.
To demonstrate the ability of this flexibly control, we designed another Moiré meta-device consisted of two other metasurfaces with different phase profiles, as shown in Fig. 3. For the convenience of quantitative characterization, the non-diffraction regions of zero-order Bessel Beams under differents mutal rotation angles are characterized As show in Fig. 3, the length of non-diffraction region decreases from 77 mm to 10 mm as the angle increases from 15° to 60°.
Fig. 3. Simulated results for non-diffaraction region variable Bessel beam generation based on the Moiré meta-device.
The Moiré meta-device platform is powerful in dynamically manipulating the wavefront of electromagnetic waves and provides an effective strategy for continuously controlling the properties of the Bessel beam, which may find applications in stable long-range optical quantum communication, particle manipulation, and high accuracy imaging.
Future work can focus on the meta-atom design of metasurfaces to enlarge the modulation freedom degree of the device. It may also be possible to achieve the active meta-devices with various functions to promote the development of terahertz functional devices.