Moiré meta-device for flexibly controlled Bessel beam generation

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 0^th-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.

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°.

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°.

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.

灵活操控贝塞尔光束：为大容量光通信与高精度无损成像提供新契机

贝塞尔光束是一种特殊的无衍射光束，具有中心光斑小、传播过程中受到干扰后可自愈的特性。此外，高阶贝塞尔光束还具备携带不同轨道角动量（也称拓扑荷数）的能力。太赫兹波介于微波和红外光之间，具有超大带宽频谱和极低光子能量的特点，在高速通信、无损检测等领域有着巨大的应用价值。因此，制造太赫兹贝塞尔光束，并实现对其阶数和无衍射距离等特性的灵活操控，对增大通信容量、提高成像精度具有极其重要的意义。

然而，由于太赫兹波长较长，轴棱锥或环形狭缝等传统贝塞尔光束生成器件尺寸偏大，不利于系统小型化。相比之下，超薄二维形态的超构表面具有全方位调控电磁波特性的能力，是生成及操控贝塞尔光束的有效途径。然而，现有器件大多仅实现了0阶贝塞尔光束的生成，高阶贝塞尔光束的生成及灵活操控鲜有研究。

为解决上述问题并实现此特殊光束的生成及动态操控，首都师范大学张岩团队与北京理工大学胡滨团队组合作，设计了可灵活操控太赫兹贝塞尔光束阶数及无衍射距离的莫尔超构表面。该器件由两层超构表面级联而成，当两层超构表面相对旋转时，可连续产生具有不同阶数的太赫兹贝塞尔光束，实现无衍射距离的大范围调制。相关研究成果发表于Photonics Research 2023年第1期（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）。

该工作将涡旋相位板和轴棱锥的功能同时集成到莫尔结构中，在莫尔结构中的两个超构表面上分别赋予特定的相位调制，实现了太赫兹贝塞尔光束的生成及调控。如图1所示，器件由两层介质超构表面构成，其中不同直径的硅圆柱负责调制入射太赫兹平面波的相位。当两层超构表面相对旋转时，在器件出射端就可以得到具有不同阶数的贝塞尔光束。

利用自行搭建的太赫兹焦平面成像系统对器件功能进行表征，贝塞尔光束阶数调控的表征结果如图2所示。其中，相位分布中的2π周期数目表示贝塞尔光束的阶数。可以看出，随着相对旋转角度从0°增加到80°，生成的贝塞尔光束从0阶增加至4阶。

如图3所示，通过更改两层超构表面的相位分布，得到了对贝塞尔光束无衍射区域长度进行调控的莫尔超构器件。以0阶贝塞尔光束为例，研究了两层超构表面的相对旋转角度变化对贝塞尔光束无衍射区域长度的影响。如图3所示，当相对旋转角度从15°增加到60°时，贝塞尔光束无衍射区域的长度从77 mm减小至10 mm。该器件的提出将进一步推进太赫兹功能器件的发展。

胡滨教授表示：“莫尔超构表面是一种新型的动态超构表面器件，是实现可调及多功能器件设计的新途径。这项工作通过巧妙的设计，不仅实现了0阶及高阶贝塞尔光束的生成，而且还对光束的阶数及无衍射区域等特性进行了连续动态调控，这为增大通信容量，提高成像精度提供了新的契机。”