The research results on the implementation of micro active vortex laser using laser nanoprinting technology are published in Nano Letters

Introduction
Vortex beams carrying orbital angular momentum (OAM) are widely used for high-throughput optical information multiplexing, and achieving on chip, small-scale vortex lasers is crucial for promoting the industrial implementation of vortex light reuse technology. Recently, Gu Min, an academician of Shanghai University of Technology, and Fang Xinyuan, an associate professor of Shanghai University of Technology, and Wu Dexin, an academician of the Institute of Microelectronics, Chinese Academy of Sciences, proposed a vertical cavity surface emitting vortex laser based on laser nano three-dimensional (3D) printing integrated OAM phase structure, which realized an on chip vortex laser array of micron level, addressable and expandable topological charges. By integrating OAM phase structure with two-photon aggregation laser 3D printing technology on a vertical cavity surface emitting laser (VCSEL), the Gaussian beam emitted by VCSEL can be modulated into a vortex beam. 3D printing provides higher degrees of structural freedom.

In response to the problem of insufficient spatial bandwidth product of the phase structure integrated with existing vortex lasers, which makes it difficult to generate high-order vortex light, the author designed and printed a multi-layer cascaded spiral phase plate structure to improve the spatial bandwidth product by extending the traditional two-dimensional OAM phase structure to three-dimensional. Finally, a vortex beam with a topological charge number of l=15 and a purity of 83.7% was achieved. The laser printing technology based on galvanometer mode has the advantages of high efficiency and low cost, and this method has no special requirements for the type of VCSEL and will not affect the photoelectric characteristics of VCSEL. The research results were published in Nano Letters under the title of "Nanoprinted Diffractive Layer Integrated Vertical Capacity Surface Emitting Vortex Lasers with Scalable Topological Charge".

Research background
With the rapid development of artificial intelligence and big data, the amount of data generated by humans every day is also increasing exponentially. Achieving high-capacity information reuse is an effective way to cope with future high data throughput applications. Vortex light with spiral phase wavefront carries orbital angular momentum, which has infinite orthogonal properties, making it widely used in various optical information multiplexing technologies, including optical communication, holography, optical encryption, optical storage, optical artificial intelligence, etc. The implementation of on-chip and small-scale vortex lasers is crucial for promoting the industrial implementation of vortex light reuse technology. However, in existing methods, there is a problem of the large volume of the entire vortex light source system caused by the separation of OAM phase structure and laser source. Alternatively, in cases where the laser source is integrated with the OAM phase structure, the limited output area of the laser source results in insufficient spatial bandwidth product of the OAM phase structure, making it difficult to generate high-order vortex light, which limits the increase in the number of OAM information multiplexing channels.

Research Highlights 
In the article, the author proposes a vertical cavity surface emitting vortex laser based on laser nano 3D printing integrated OAM phase structure, achieving an addressable and higher topological charge on-chip vortex laser array. Vertical cavity surface emitting laser (VCSEL) is a semiconductor laser source that has the advantages of small volume, high speed, low threshold, circular light field, vertical output, and arrayability. As shown in Figure 1, the author uses two-photon aggregation femtosecond laser 3D printing technology to integrate OAM forked gratings onto a single-mode front emitting VCSEL device through a printed bracket, thereby transforming the Gaussian beam emitted by VCSEL into a vortex beam after being modulated by the forked grating. In the device, the forked grating does not directly contact the output surface of the VCSEL, which can avoid affecting the characteristics of the VCSEL device. At the same time, the output light of the VCSEL has a certain degree of divergence. This scheme can increase the effective illumination area of the OAM phase structure, thereby obtaining a larger spatial bandwidth product. In galvanometer mode, laser 3D printing has high manufacturing efficiency, and the integration of a single OAM phase structure takes only about 20 minutes.

Figure Vertical cavity surface emitting vortex laser. (a) Schematic diagram of the principle. (b) Schematic diagram of laser 3D printing. (c-e) Scanning electron microscope image of the device.

As shown in Figure 2, the author has implemented an addressable vortex laser array with l=1 to l=5, with a single device size of only about 100 micrometers × 100 microns. Through far-field testing, it can be seen that as l increases, the diameter of the vortex light gradually increases, which is consistent with the simulation results. At the same time, in the article, the author tested the vortex light using the Dawey prism interferometry, further verifying the topological charge of vortex light. The power current voltage test results confirm that the integrated OAM phase structure will not affect the optoelectronic characteristics of VCSEL. In addition, the author also studied the experimental results using single-layer spiral phase plates and multimode VCSEL.

In previous studies, researchers typically integrated OAM phase structures onto the output surface of lasers. Due to the limited output surface of micro lasers, the spatial bandwidth product of OAM phase structures was limited. For higher-order vortex beams, the phase step of their phase structure is larger, and the limited spatial bandwidth product makes it difficult for previous methods to achieve vortex beams with higher topological charges. Currently, the reported results are generally lower than l=5. Laser 3D printing gives OAM a higher degree of structural freedom in its phase structure. In this study, the author extended the traditional two-dimensional OAM phase structure to three-dimensional by designing a cascaded spiral phase plate (SPP) to improve the spatial bandwidth product. As shown in Figure 3, the author printed a double-layer cascaded SPP structure (l=5 and l=10, respectively), thereby achieving the generation of a vortex beam with l=15. Through reverse cancellation experiments, it was proven that the purity of the vortex light reached 83.7%. More layers of SPP cascading have also been proven effective in experiments.

Summary and Outlook
This research has achieved a micro, addressable, and scalable topological charge vortex optical array light source, which is expected to promote the miniaturization and integration development of OAM information multiplexing technology. In the next stage, in order to achieve a larger topological charge, we can try to optimize the device structure of VCSEL to increase its divergence angle, thereby increasing the effective illumination area and improving the spatial bandwidth product. In addition, in order to cascade more layers of OAM phase structures, it is necessary to optimize the laser printing process, improve the mechanical strength of the structure, and print resolution. Additive manufacturing technologies such as laser 3D printing can achieve the manufacturing of three-dimensional complex structures, thereby achieving superior performance in some aspects compared to two-dimensional optical components. The research approach can also be applied to the research of other optoelectronic devices, by endowing traditional devices with new functions or improving their critical performance through laser 3D printing integrated optical structures.

Source: Sohu