# On-Demand Waveguide-Integrated Microlaser-on-Silicon
## 2 Results and Discussion
### Device Fabrication  
Figure 2 depicts the device fabrication process. First, we used an SOI wafer with a 260 nm thick Si device layer and a 3.0 \(\upmu\)m thick SiO\({}_{2}\) layer to fabricate the arrays of Si-posts and strip-type Si WGs with different widths, lengths, and gap distances. These arrays were readily patterned in a deterministic manner using either electron beam lithography (EBL) or photolithography. The conventional dry-etching technique (CCP-RIE) and a few rounds of O\({}_{2}\)-plasma treatment completed the passive device fabrication. As per the requirements, another round of dry etching was briefly introduced to the Si WG to slightly reduce the height of the WG with respect to that of the Si-post. Importantly, when the microdisk was transferred onto the post, this height difference induced a small air gap between the bottom of the microdisk and the top of the WG, which allowed the evanescent coupling of the light. Thus, the light transmission through the waveguide could be controlled by systematically controlling the air gap. Figure 2a depicts a representative SEM image of the fabricated Si-post and WG. The width and length of the Si WG were approximately 500 nm and approximately 105.8 \(\upmu\)m, and the size of Si-post was approximately 1.6 \(\upmu\)m in diameter, respectively. The small post was designed to firmly support the entire microdisk structure but minimally perturb the resonant modes. The gap between the post and the WG was approximately 1.1 \(\upmu\)m (inset, Figure 2a). In this device, we designed the Si-post to be located in the middle of the longitudinal Si WG to examine how accurately we could integrate the laser device and achieve equal light couplings toward the right and left directions. Figure 1: On-demand WG-integrated microlaser-on-silicon. (**a**) Schematic of the device. A high-index microdisk cavity (dark blue color) with an optical gain medium (red color) is transferred onto a target post and coupled with a strip-type WG using the microtip-assisted transfer-printing technique. (**b**) Tilted-view SEM image of the fully fabricated WG-integrated laser-on-silicon device. A MQW InGaAsP microdisk with a diameter of approximately 4.5 \(\upmu\)m was on-demand transferred on top of a Si-post and coupled to a Si WG. Scale bar: 1 \(\upmu\)m.  
The microdisks were fabricated using a 260 nm thick InGaAsP slab containing six InGaAs/InGaAsP quantum-wells (QWs). Underneath the slab, a 900 nm thick InP sacrificial layer was introduced for the selective chemical etching, which readily enabled the microdisks to be air-suspended at the boundaries and post-supported at the center ((i), Figure 2b). Thereafter, we used the EBL to define arrays of microdisks of different sizes, which were mechanically dry-etched using chemically assisted ion beam etching (CAIBE). After removing the residual resist using O\({}_{2}\)-plasma treatments, time-sensitive anisotropic chemical etching (HCI:DI = 3:1) was applied to produce a submicron-sized post structure ((ii), Figure 2b). In particular, an effort was made to keep the post size smaller than approximately 400 nm to facilitate the pick-up and separation process. Panel (iii) of Figure 2b depicts the fabricated single InGaAsP microdisk with a supporting InP-post of size of approximately 280 nm. The schematics in Figure 2c illustrate the key steps involved in the on-demand laser-on-Si integration. We used a previously developed PDMS microtip-assisted transfer printing technique to pick up the fabricated single microdisk on the InP substrate ((i)), move it to a receiving Si device substrate ((iii)), and align-transfer it on a target Si-post ((iii)) [21; 30; 31]. Figure 2: Fabrication processes for the on-demand WG-integrated microlaser-on-silicon. **(a)** SEM images of fabricated strip-type Si WG and Si-post.