Novel design for all-semiconductor PCSEL with very high power in a narrow beam
Fig. 1: Schematic depiction of a) the final PCSEL device structure, and b) the optimised photonic crystal unit.
Fig. 2: Varying the cavity length, <i>L</i>, of the optimised device and plotting a) the mode loss of the fundamental mode and mode discrimination to higher order modes, and b) the efficiency that power is coupled out of the substrate, ηsubstrate, and the lateral edges of the device, ηedge.
Fig. 3: For a cavity length of <i>L</i>=2 mm plotting a) the mode spectrum of the device, mode loss as a function of wavelength, and b) the near- and far-field distributions of lowest loss fundamental mode.
High-power diode lasers are crucial components for use in material processing, medicine, automotive applications, and more, due to their ability to supply optical power at high conversion efficiency. However, these lasers often suffer from low beam quality at high power levels, operating with low brightness compared to fiber and solid-state lasers. Recent advancements in GaAs-based photonic crystal surface emitting lasers (PCSELs) have addressed this challenge by using a two-dimensional photonic crystal in the laser structure. This enables scaling of output power while maintaining emission in a narrow, circular beam.
Until now, all reported high-power PCSELs have utilised a high-index-contrast photonic crystal (PC) structure, where embedded air-voids are realised in the semiconductor structure through two-step epitaxial growth. Such PCs provide very strong two-dimensional optical feedback and successfully pin these large-area structures in a single coherent optical mode, for vertical emission via the substrate. However, this approach comes at the cost of reduced power conversion efficiency.
In cooperation with the Weierstrass Institute in Berlin, we have recently proposed an alternative scheme for realising such large-area, high-power PCSELs: a so-called all-semiconductor design [1]. Instead of using air-voids, this approach realizes contrast in the photonic crystal by combining two different semiconductor materials. The method is similar to the one used in semiconductor laser gratings in edge emitters (e.g., in distributed feedback lasers), where such gratings enable devices to be realized with very low loss and high conversion efficiency. However, until now it has been suggested that PCs based on such low-index-contrast gratings are not suitable for use in PCSELs due to the need for strong coupling in the photonic crystal to realise 2D coherence and surface emission.
The proposed design emits at a target wavelength of 1070 nm and utilises an optimised stretched isosceles triangle (SIT) photonic crystal unit cell (Fig. 1) with index contrast in the PC supplied by InGaP/GaAs materials. The photonic crystal has been optimised to stabilize a single large area mode (high 2D coupling), that is strongly out-coupled from the device via the substrate, while suppressing unwanted optical modes (low 1D coupling) in the plane of the laser. Using a newly developed modelling tool [2], we demonstrated that even for large cavity lengths of L=3 mm, for a LxL square cavity PCSEL, this design can enable high external efficiencies and large mode discrimination (Fig. 2) with characteristics comparable to current best-in-class air-void designs. Furthermore, it was shown through inspection of the individual modes in the device that this can be achieved while maintaining a very narrow circular far-field (Fig. 3). The design approach is especially suited for large-area, very high-power PCSELs, and has potential to be realized with very low optical losses. Diode laser devices based on such SIT designs provide a new and exciting route toward realising highly efficient, high-power, and easily manufacturable PCSELs , as summarized in [1] and recently presented at Photonics West and the PCSEL workshop [3,4].
This work was performed in the frame of the project PCSELence (K487/2022) funded by the German Leibniz Association.
Publications
[1] B. King, H. Wenzel, E. Kuhn, M. Radziunas, and P. Crump, "Design of very-large area photonic crystal surface emitting lasers with an all-semiconductor photonic crystal," Opt. Express 32, 44945-44957 (2024).
[2] M. Radziunas, E. Kuhn, H. Wenzel, B. King, and P. Crump, “Optical Mode Calculation in Large-Area Photonic Crystal Surface-Emitting 344 Lasers,” IEEE Photonics J. 16, 1–9 (2024).
[3] B. King (Invited), “Design of High-Power GaAs PCSELs with an All-Semiconductor Photonic Crystal”, Proc. International Workshop on PCSELs, Aston, UK (2024).
[4] B. King, H. Wenzel, E. Kuhn, M. Radziunas, and P. Crump, “Design study on large-area all-semiconductor PCSELs”, Proceedings SPIE Conference 13385 “Novel In-Plane Semiconductor Lasers XXIV”, San Francisco, CA, USA, Paper 13385-51 (2025).