# On-Demand Waveguide-Integrated Microlaser-on-Silicon
## 2 Results and Discussion
### Optical Characterizations
Scale bar: 5 \(\upmu\)m. **(b)** Laser characterization. (i) Light-in versus light-out curve. The threshold power, \(P_{\text{th}}\), was estimated to be approximately 378.0 \(\upmu\)W. Laser peak linewidth (ii) and wavelength (iii) plotted as a function of \(P_{\text{peak}}\). **(c)** Captured IR camera images showing the scattered light intensities at WG ends (left, right) and the laser emission (middle) at (i) \(P_{\text{peak}}\) = 398.8 \(\upmu\)W, (ii) \(P_{\text{peak}}\) = 497.6 \(\upmu\)W. The intensities at the ends of the WG were increased by three-fold for comparison. Scale bar: 10 \(\upmu\)m. **(d)** Measured spectra from the left (i) and right ends of WG (iii) and the microdisk center (ii) at \(P_{\text{peak}}\) = 497.6 \(\upmu\)W, respectively. The right insets of all panels were high resolution spectra. The left insets of (i) and (iii) were the angle-resolved intensity distributions. We believe that these unusual observations can be partly explained by the steady-state stimulated light coupling to the WG, which is discussed in a later section. Next, to examine how the light from the laser efficiently couples to the Si WG, we repeatedly imaged the light scattered at both ends of the WG while increasing \(P_{\text{peak}}\) from below- to above-threshold. Figure 3c exhibits captured IR camera images of the scattered light at both terminations of WG (left and right panels) and the light emission from the laser device (middle panel) at different \(P_{\text{peak}}\) of 398.8 \(\mu\)W (i) (near-threshold) and 497.6 \(\mu\)W (ii) (above-threshold), respectively. At below-threshold, no noticeable light scattering is observed. However, evident light spots were observed at both terminations for \(P_{\text{peak}}\) > \(P_{\text{th}}\), indicating successful light coupling and transmission toward both directions. For the qualitative analysis, we first compared the recorded maximum light intensities from single pixels at the WG end (\(I_{\text{left}}\) or \(I_{\text{right}}\)) with that of the laser (\(I_{\text{disk}}\)) at \(P_{\text{peak}}\) = 398.8 \(\mu\)W, and the ratios of WG left end (\(I_{\text{left}}/I_{\text{disk}}\)) and WG right end (\(I_{\text{right}}/I_{\text{disk}}\)) to the laser were approximately 0.21 and 0.22, respectively ((i), Figure 3c). This discrepancy in the intensity ratios can be attributed to unequal light coupling from the clockwise (CW) and counterclockwise (CCW) WGMs to the propagating WG modes, which originated from the uneven and off-center-transferred microdisk. In addition, this ratio difference became more evident with an increasing \(P_{\text{peak}}\). For example, the measured \(I_{\text{left}}/I_{\text{disk}}\) and \(I_{\text{right}}/I_{\text{disk}}\) ratios are approximately 0.35 and 0.31 at \(P_{\text{peak}}\) = 497.6 \(\mu\)W, respectively ((ii), Figure 3c). Moreover, the ratios of the same WG end at different \(P_{\text{peak}}\) values exhibit noticeable variations. Based on these observations, we hypothesized that besides the light coupling from the laser mode to the fundamental WG mode (\(k_{\text{WG}}\)), there should be a small portion of spontaneous emission that coupled to several higher-order WG modes. Significantly, the transmitted power carried by the fundamental WG mode thus varies in response to laser power at \(P_{\text{peak}}\) > \(P_{\text{th}}\), whereas the transmitted powers carried by other WG modes might remain constant. This can partly explain the total intensity ratio that eventually converges to the ratio associated with the light coupling of the stimulated emission alone at high \(P_{\text{peak}}\). For the spectroscopic study, we measured the spectra from the selected area, including each WG end ((i) left, (iii) right, Figure 3c) and the laser center ((ii), Figure 3c). The intensities of the spectra were normalized to the peak values shown in panel (ii).