Figure 1 (a) Schematic diagram of 6-junction VCSEL; (b) Schematic diagram of p-DBR of single-junction VCSEL; (c) Schematic diagram of p-DBR of multi-junction VCSEL; (d) Output mirror reflectivity and surface phase layer under different logarithms of DBR Relationship between Si ₃ N ₄ thickness

Next, based on the previously constructed multi-junction VCSEL model that achieved ultra-high efficiency breakthrough ( Light Sci Appl 13, 60, 2024 ), the work analyzed the threshold gain changes of single-junction and multi-junction VCSEL based on the thickness of the surface phase layer, and Multi-junction VCSEL efficiency expansion characteristics based on the current level of single-junction single-mode VCSEL. As shown in Figure 2, the results show that as the surface Si ₃ N ₄ thickness changes, the difference in reflectivity change amplitude will lead to significant differences in the maximum threshold gains of the two types of VCSELs. The maximum threshold gain of multi-junction VCSELs is approximately junction VCSEL 2 times. The core idea of ​​realizing single-mode VCSEL is to increase the threshold gain difference between high-order modes and single-mode, thereby maintaining single-mode operation within a certain range of operating conditions. However, most current methods are based on various surface microstructures, which makes the mode field distribution concentrated. Higher-order dies outside the light-emitting aperture have greater losses. Therefore, this work reveals for the first time that multi-junction VCSEL can enhance the modulation ability of the surface microstructure to high-order modes while maintaining low-threshold operation. The proposed method can significantly increase the threshold gain difference between high-order modes and the fundamental mode. Thus, high-order modes can be realized in a wider range. Simulation experiments on the extended characteristics of multi-junction VCSELs show that the efficiency of high-power single-mode VCSELs is expected to exceed 60%.

Figure 2 19 pairs of p-DBR single junction and 9 pairs of p-DBR 6-junction VCSEL, (a) relationship between surface Si ₃ N ₄ optical thickness and threshold gain; (b) multi-junction VCSEL power conversion efficiency expansion characteristics

Subsequently, the team prepared 6-junction VCSEL samples with surface relief structures of different sizes and characterized the photoelectric properties. The results are shown in Figure 3. The 6-junction VCSEL achieved a laser output power of 20.2 mW under continuous current driving, with a side-mode suppression ratio greater than 35 dB, a corresponding electro-optical conversion efficiency of 42%, and a divergence angle of 9.8°. The near-field spot indicates that it is a single fundamental mode laser operating mode. To the best of the team's knowledge, this is the highest single-mode power of a single VCSEL to date under continuous operation at room temperature , and the power value is almost twice the known record. In addition, this method only requires simple micron-level silicon nitride etching on the surface of the VCSEL, without the need for special photolithography processes and complex process flows, ensuring the high reliability and low-cost advantages of single-mode VCSELs.

Figure 3 (a) LIV curve; (b) Spectrogram under different currents when SR=1 μm; (c) Near field diagram at 20.5 mW; (d) Far field spot diagram at 20.5 mW; (e) 20.5 Far field divergence angle at mW

Summary and Outlook