# On-Demand Waveguide-Integrated Microlaser-on-Silicon
## 2 Results and Discussion
### Optical Characterizations  
We systematically performed micro-photoluminescence (PL) spectroscopy to investigate the optical properties of the integrated microdisk laser and the light coupling and propagation through the WG. The fabricated device, depicted in Figure 2d, was mounted on an XYZ stage and optically pumped at room temperature using a pulsed laser diode with a wavelength of 976 nm. Incident pulses with a width of 100 ns at a repetition rate of 1 MHz were used. Light emitted from the device was collected using a 50\(\times\) microscope objective with a numerical aperture of 0.42 and sent to an infrared (IR) camera for imaging or a monochromator for spectroscopic analysis. We note that the collection of maximum light intensities can be further optimized by using an objective lens with high numerical aperture and different magnification. First, we gradually increased the peak pump power (\(P_{\text{peak}}\)) and imaged the light emission from the laser device. The captured IR camera images depicted in Figure 3a exhibit the spontaneous (i), amplified spontaneous (ii), and lasing (iii) emission patterns, respectively. Light intensities in all panels were normalized to the maximum value of panel (iii). In particular, we observed nearly uniform light emissions from the entire body of microdisk at \(P_{\text{peak}}\) = 301.8 \(\upmu\)W but stronger light intensities from the boundary than those from the center at \(P_{\text{peak}}\) = 398.8 \(\upmu\)W. This clearly indicates that the excitation of resonant WGMs and the corresponding amplification of spontaneous emission developed at a \(P_{\text{peak}}\) between the two pump powers. At further increased \(P_{\text{peak}}\) = 497.6 \(\upmu\)W, a clearer donut-shaped intensity distribution with a strong light interference pattern, which is a typical profile of resonant WGMs, was observed. This indicated the successful lasing operation in the device. We also noted that the slight misalignment in the micro-transfer process might contribute little to the lasing oscillations; thus, the result demonstrated the transfer tolerance in our device design. For the quantitative characterization, we measured the lasing spectra with increasing \(P_{\text{peak}}\). The characteristic light-in versus light-out curve exhibited a superlinear increase in intensity with a threshold peak pump power (\(P_{\text{th}}\)) of approximately 378.0 \(\upmu\)W ((i), Figure 3b). The peak power per unit area of approximately 30.1 \(\upmu\)W/\(\upmu\)m\({}^{2}\) falls within the range of typically accepted incident power for micro-resonators, which often allows some intriguing nonlinear phenomena such as optical bistability [38]. As anticipated, the measured threshold was larger than the \(P_{\text{peak}}\) of panel (i) but smaller than that of panel (ii) of Figure 3a, thus strongly supporting the observations and analysis depicted in Figure 3a. In addition, we plotted the linewidth (\(\Delta\lambda_{1/2}\)) ((ii), Figure 3b) and peak wavelength (\(\lambda_{\text{Laser}}\)) of lasing spectra as a function of \(P_{\text{peak}}\) ((iii), Figure 3b). From these data, the experimental \(Q\)-factors (\(\lambda/\Delta\lambda_{1/2}\)) were estimated as \(>\) 2300 at near threshold, which is comparable to those reported in previous study [30]. A typical linewidth reduction and blue shift of the peak wavelength were observed from below- to near-threshold regimes. Interestingly, an abrupt linewidth broadening from 0.64 nm to 0.67 nm near \(P_{\text{peak}}\) = 497.6 \(\upmu\)W was observed and its plateau clamped at 0.68 nm afterward. In addition, the lasing peak was noticeably red shifted by 0.07 nm near \(P_{\text{peak}}\) = 497.6 \(\upmu\)W and remained a constant for the increased \(P_{\text{peak}}\)  
Figure 3: Optical characterization. **(a)** The emission patterns of the laser captured by an IR camera at different \(P_{\text{peak}}\) values of (i) 301.8 \(\upmu\)W, (ii) 398.8 \(\upmu\)W, and (iii) 497.6 \(\upmu\)W. Intensities were normalized with the maximum value of panel (iii).