Tsinghua University and Shenzhen University of Technology use laser processing boron nitride to achieve large-scale high-performance quantum light source

Single photon source is an important quantum light source and one of the cores of quantum information technology. In quantum key distribution of quantum secret communication, single photon source is very important for the safe transmission of information using quantum key distribution protocol; In quantum computing, in order to meet the requirements of all optical quantum repeaters and other applications, a single photon source with extremely high purity is required. However, the current single photon light source preparation technology is far from meeting the needs of various quantum technology applications, especially in the controllable large-scale preparation of high-purity single photon light sources, facing various difficulties and challenges.

In view of this, Ning Cunzheng's team combined the controllable large-scale production capability of laser processing with the excellent properties of two-dimensional wide band gap semiconductor material boron nitride (hBN), solved several key problems existing in the current single photon light source, and realized the space controllable large-scale production of high purity, high brightness single photon light source.

Hexagonal boron nitride (hBN) is a new broadband semiconductor. Due to its layered structure, it is easy to peel into thin layers or even monolayer thickness, excellent material properties and easy integration with other two-dimensional materials, it has received great attention recently. In particular, it is found that a single photon source at room temperature can be prepared in hexagonal boron nitride, and it has the characteristics of good stability and easy integration, making it the most promising single photon source for practical applications.

At present, researchers usually generate single photon source by high temperature annealing of hBN, chemical etching, electron beam/ion beam/neutron irradiation, stress induction and other methods. Although these methods can produce single photons in hBN, they all have some shortcomings, such as low brightness or purity of single photon source, low yield of single photon source, or high requirements for processing equipment and technology. Therefore, it is necessary to explore a simple and efficient method to prepare high-quality single photon sources.

Laser processing single photon source uses the defects produced by ultra short intense pulse irradiation as the light source. This method has the advantages of space controllability and mass production. However, the previous method has not been solved well due to thermal effects, and there are efficiency and purity problems. The main method of this research is to optimize the single pulse parameters, and only single pulse irradiation is carried out at the given space point, which effectively avoids the thermal effect and low purity problems. The experimental team realized the efficient production of single photon source on the thin layer of hBN through the method of single pulse femtosecond laser irradiation. Every 100 single pulse femtosecond laser irradiation positions can produce 43 single photon sources, which is the highest yield in the top-down production method. And the purity and brightness of the single photon source produced are very high. The minimum value of the second-order correlation function g2 (0) measuring the single photon index is 0.06 ± 0.03, and the maximum single photon emission intensity is 8.69 Mcps. It is one of the brightest single photon sources produced at present. In addition, the processing of materials by femtosecond laser direct writing technology is based on nonlinear processes such as multiphoton absorption, which can break through the diffraction limit and induce the generation of artificially controllable micro nano structures with high spatial resolution. It does not require expensive micro nano processing equipment and process conditions, and can be used for the production of large-scale models.

Fig. 1 The defect structure arrays (a) with different sizes prepared and the corresponding photoluminescence images (b), (c) and (d) are the single photon source yields with different sizes.

Figures 1a to c show four defect patterns of different sizes and corresponding photoluminescence images. Figure 1d shows the single photon (g2) produced in the total number of defects（ τ） ＜ 0.5) Percentage of defects, i.e. single photon yield. As shown in Figure 1d. The yield of single photon source increases with the increase of defect pattern size, at 3.0 μ The highest yield of 42.9% is achieved in defect patterns of m size, which is the highest in all top-down fabrication methods. Then with the further increase of the size, the single photon source yield gradually decreased. The black curve in Figure 2a shows the relationship between the photon emission rate of the single photon source and the pump power. After fitting, it can be obtained that the saturated photon emission rate is 8.69 Mcps, which is the highest brightness among the single photon sources made by top-down processing technology.

Fig. 2 (a) The relationship between the photon emission rate and the pump power of the single photon source with the highest brightness is obtained. (b) The emission peak of a typical single photon source and its second-order correlation function (illustration).

The relevant achievements were recently published in the Nanometer Journal of the American Chemical Society under the title of "Large scale and high output laser manufacturing bright and pure single photon emitters in hexagonal boron nitride at room temperature". Gan Lin and Zhang Danyang were the co first authors, and Ning Cunzheng was the corresponding author. Ning Cunzheng was once a professor in the Department of Electronic Engineering of Tsinghua University and is now a chair professor of Shenzhen University of Technology. Gan Lin is an assistant researcher in the Department of Electronic Engineering of Tsinghua University, and Zhang Danyang is a doctoral student. This work was completed by the Department of Electronic Engineering of Tsinghua University and the Institute of Integrated Circuits and Optoelectronic Chips of Shenzhen University of Technology. This research work has been supported by the National Natural Science Foundation of China and the Beijing Natural Science Foundation.

Source: Department of Electronic Engineering, Tsinghua University