2 Device design and fabrication

2.1 Sample fabrication

Figure 1(a) shows a schematic diagram of the ASW lasers. The designed isosceles triangles with 32 μm long equal sides have been introduced at a period of 42.5 μm. Figure 1(b) shows a scanning electron microscope (SEM) image of the ASW lasers. The wafer was grown on an n-doped GaSb substrate by Veeco solid-source Gen-II molecular beam epitaxy (MBE). The active region presented in this work contained two compressively strained 10 nm thick In_0.18GaSb quantum wells separated by a 20 nm Al_0.25GaAs_0.02Sb barrier layer and two undoped 270 nm thick Al_0.25GaAs_0.02Sb separate confinement layers. The top and bottom were 2 μm thick p-doped and n-doped Al_0.5GaAs_0.04Sb cladding layers, respectively. The total thickness of the epitaxy layers was about 5 μm, including a 250 nm thick p⁺ GaSb layer grown on the P-cladding layer and a 360 nm thick GaSb buffer grown on the substrate. After epitaxial growth, the ASW structure and ridge were formed through standard photolithography and an inductively coupled plasma (ICP) dry etching process from the same batch, with an etching depth of 2 μm. After depositing a 250 nm thick SiO₂ insulating layer via plasma-enhanced chemical vapor deposition (PECVD), electrode windows were created on the center of the ridge. Electrode layers consisting of Ti/Pt/Au and AuGe/Ni/Au were sputtered on the p-type and n-type sides, respectively, subsequent to reducing the wafer thickness to 125 μm. The lasers were coated with anti-reflection and high-reflection coatings. The reflectance of the ends of the lasers was 5% and 95%, respectively. Both ASW lasers and BA lasers were cleaved to a length of 1.5 mm and a width of 500 μm, mounted p-side down on C-mount copper heat sinks for better heat dissipation. All devices have the same emission aperture of 100 μm.

Figure 1 (a) Schematic diagram of the ASW lasers. (b) Scanning electron microscope image of the ASW lasers, top view (left) and enlarged view (right).

2.2 Methods and simulation

The optical field distribution of ASW lasers was simulated using the 2D finite difference time domain (FDTD) method. For the simulation, the input parameters were set to the experimental laser parameters, including the epitaxial structure, etching depth and microstructure size. The specific details of the ASW structure are shown in Figure 2. Figure 3 illustrates the calculated optical field distributions of lateral modes through the ASW structure for mode orders m = 0, 1, 2, 3, 6 and 8. As anticipated from the design of the ASW structure based on the characteristics of the near-field mode distribution^[ Reference Piprek ^{19 ]}, high-order modes experience noticeable increased scattering losses at the edges of the ridge, while the transmission of the fundamental mode remains largely unaffected. Figure 4(a) shows the ratios R _u of the residual energy for different order modes transmitted after four-period microstructures with different size ratios $\left(L-d\right)/L$ , where L is the width of the ridge and d is the width of the unetched central region, as depicted in Figure 1(a). Notably, the fundamental mode and higher-order modes exhibit a notable disparity in loss characteristics. As the microstructure size ratio varies from 0.4 to 0.6, there is a synchronous increase in losses in each order mode. To strike a balance between preserving as much energy as possible in the fundamental mode and ensuring high losses in the higher-order modes, a microstructure size ratio of 0.5 was selected for the experiment. At this ratio, the remaining energy in the fundamental mode is approximately 1.4 times that of the higher-order modes. Figures 4(b)–4(f) illustrate the optical field densities of the high-order modes (m = 0, 1, 3, 6 and 8) before and after microstructure tailoring. The majority of the energy of the fundamental mode is preserved, while the intensity of the higher-order modes significantly decreases on both sides. This indicates the enhanced competitiveness of the fundamental mode, thereby reducing the number of lateral higher-order modes as the operating current increases^[ Reference Wang, Tong, Shu, Tian, Sun, Zhao, Lu, Zhang, Hou and Wang ^{16 ]}. Such a selection mechanism for fundamental and high-order modes holds great promise in achieving both high-power and low lateral divergence simultaneously.

Figure 2 Details of the ASW structure. The inset shows the near-field distribution of different order modes of conventional BA lasers.

Figure 3 The optical field distributions of lateral modes with order numbers m = 0, 1, 2, 3, 6 and 8 through the ASW lasers, respectively.

Figure 4 (a) Energy retained after transmitting different lateral modes through the ASW structure with microstructure size ratios of 0.4, 0.5 and 0.6. (b)–(f) Simulated optical field distribution of modes with order numbers m = 0, 1, 3, 6 and 8 at the facet of conventional BA lasers and ASW lasers with the microstructure size ratio of 0.5.

3 Results and discussion

Figure 5(a) shows the CW light power versus current (L–I) characteristics of the conventional BA lasers and ASW lasers at 288 K, with different periods and positions of microstructures. To validate the superiority of the waveguide design and derive further optimization strategies, a total of four sets of devices were fabricated. Specifically, lasers with eight-period microstructures distributed on both sides or gathered in the middle of the cavity are denoted as ASW-side and ASW-mid, respectively. In addition, lasers with microstructure covering the entire longitudinal direction and conventional BA lasers without microstructures are expressed as ASW and BA, respectively. The ASW-side, ASW-mid and BA structure have similar peak powers of about 1.1 W and slope efficiencies of 0.29 W/A. The maintenance of the output power may be explained by reduced mode competition and better matching of the lateral gain profile to the mode profile^[ Reference Lang, Larsson and Cody ^{20 –} Reference Shi, Yang, Wang, Chen, Yu, Zhang, Wu, Xu, Ni and Niu ^{23 ]}. The ASW structure exhibits two regimes in slope efficiency of 0.27 and 0.16 W/A below and above 2.6 A, respectively, with an output power of 0.98 W at 4.6 A. This observation can be attributed to the substantial increase in losses to higher-order modes compared to other devices, along with the sustained strong modulation effect of the proposed structure at high current levels. As the number of microstructure periods increases, there is a sequential decrease in the threshold current. Compared to conventional BA lasers, the threshold current of the ASW structure decreases from 0.35 to 0.20 A. This favorable outcome can be attributed to the dual suppression of lateral carrier accumulation effects^[ Reference Rieprich, Winterfeldt, Kernke, Tomm and Crump ^{24 ,} Reference Piprek and Li ^{25 ]}, leading to an increase in effective injection efficiency. The power conversion efficiencies versus current (PCE–I) characteristics for conventional BA lasers and ASW lasers are shown in Figure 5(b). Clearly, in comparison to the conventional BA laser, both the ASW-side structure and ASW-mid structure exhibit higher PCE across the whole power range. The maximum PCE for the ASW structure is 25.5%, surpassing the 23.6% of the conventional BA laser. At elevated current levels, these two values tend to converge, reaching 14% at 4.6 A. The inset depicts the spectra recorded at 4.5 A for the BA laser and the ASW laser. The central emission wavelength for both devices is in the 1900 nm region.

Figure 5 (a) L–I curves and (b) PCE–I curves of conventional BA lasers and ASW lasers with different positions and periods. The inset displays the spectra at 4.5 A of the BA laser and the ASW laser, respectively.

Our design, on the other hand, shows a much improved far-field performance. The lateral far-field profiles of the above devices under CW operation at 1 and 4.5 A at a heat sink temperature of 288 K were measured and are shown in Figures 6(a) and 6(b), respectively. The conventional BA laser exhibits a typical double-lobe feature, whereas all the ASW lasers demonstrate narrower and more concentrated far-field profiles. The far-field profiles of the ASW-side structure and ASW-mid structure closely resemble each other. The lateral far-field angle of the ASW-mid structure increases from 4.91° to 11.39° full width at half maximum (FWHM) with increasing drive current from 1 to 4.5 A. This represents a reduction of 56% and 42% compared to the conventional BA laser, which has angles of 11.23° and 19.61° at these respective currents. The broadening is a consequence of the enhanced index guiding in the lateral dimension resulting from self-heating^[ Reference Zediker, An, Xiong, Jiang, Schmidt and Treusch ^{26 ]}. The ASW structure has slightly larger lateral divergence compared to the ASW-side structure and ASW-mid structure, producing 7.67° and 12.73° divergence at a drive current of 1 and 4.5 A, respectively. However, it is worth noting that the advantage of the ASW structure lies in significantly reducing certain minor side lobes, as indicated by the red dashed circles. This further confirms the effective suppression of higher-order modes by the proposed structure, which is consistent with near-field simulation results. Figure 6(c) displays the lateral far-field divergence angle as a function of injection current, ranging from 1 to 4.5 A. Notably, all the ASW lasers exhibit smaller lateral divergence compared to the conventional BA laser in the whole dynamic current range. The divergence angles of the ASW-side structure and ASW-mid structure gradually increase with current-dependent values of 1.68 and 1.82 deg/A, respectively. The ASW structure has the lowest current dependence at 1.35 deg/A, achieving a 38% improvement compared to the 2.19 deg/A observed in BA lasers. This indicates a significant enhancement in the lateral far-field blooming effect^[ Reference Wang, Wang, Shu, Tian, Zhao, Tong and Wang ^{27 ]}. Furthermore, it is evident that for this device, the variations in the position of the microstructure along the longitudinal direction of the cavity have a minor impact on output power and lateral far-field performance. To enhance device performance, simulation and experimental projects will be further improved for better alignment between mode profiles and carrier density profiles^[ Reference Piprek ^{19 ,} Reference Crump, Leisher, Matson, Anderson, Schulte, Bell, Farmer, DeVito, Martinsen, Kim, Choquette, Erbert and Traenkle ^{21 ]}.

Figure 6 The lateral far-field profiles at (a) 1 A and (b) 4.5 A of conventional BA lasers and ASW lasers, respectively. (c) The lateral far-field angles depending on the injection current at 288 K of conventional BA lasers and ASW lasers.