# In-Plane Monolithic Integration of Scaled III-V Photonic Devices
## 3 Hybrid III-V/Si Photonic Crystal Cavity Lasers
### Characterization of Hybrid III-V/Si PhC Cavities
This peak corresponds to the fundamental cavity mode. Additionally, higher order modes are faintly visible in the spectra taken with high pump power. The emission spectra for hybrid devices with InP as the gain material are plotted in Figure 4c for increasing lattice constant. Resonant modes are visible in each of these spectra as well, shifting to longer wavelengths with increasing lattice constant in good agreement with simulations. Here, the different cavities also cover the entire gain bandwidth between 800 nm and 900 nm. While the combination of InGaAs with an emission wavelength in the telecom range together with the low Si absorption is desirable, we can still learn from hybrid devices with InP as gain material, especially because of the additional absorption from Si rods outside of the central cavity. InP provides material gain centered around a wavelength of 900 nm, which corresponds to an energy greater than the Si bandgap. The Si part of the PhC cavity  
Figure 4: (**a**) Emission spectra for hybrid InGaAs/Si PhC cavities with different lattices measured at room temperature. The cavity design is displayed in the inset. Simulated emission wavelengths for each PhC cavity are marked by a shaded area due to an uncertainty on the refractive index of III-Vs. Reprinted and adapted with permission from [33] © 2020 American Chemical Society. (**b**) PL intensity for a hybrid InP/Si PhC cavity under increased optical pumping at 130 K in logarithmic scale, showing the emergence of a resonant mode at higher pump powers. (**c**) Emission wavelength dependency of InP-based cavities with a slightly different cavity design and for regularly increasing lattice constant, measured at 100 K.  
then not only does not provide material gain but also absorbs light as it is reflected in the PhC mirror. A quantitative comparison of light absorption can be found in Figure 5a, where the wavelength dependence of the absorption coefficient is displayed (data taken from [44]). Considering an emission wavelength of 900 nm, the absorption coefficient of Si is two orders of magnitude lower than for InP. The emission of InGaAs predominantly lies above the Si absorption edge, i.e., in the wavelength range where Si is transparent. In our setup, only the central part of the PhC cavity is pumped, and the spot-size of the pump source is about 1 \(\upmu\)m in width. The pump-light might also be reflected within the PhC cavity, but for regions increasingly removed from the center of the structure, the III-V material is expected to contribute more loss than gain. We investigate whether there is a correlation between device performance, in terms of emission peak intensity versus input power, and the number of III-V rods in the cavity center. For the same number of total rods (\(n=41\)) in the cavity, an increase in the number of III-V rods relates to a larger proportion of III-V compared to Si rods. This ratio between III-V and Si rods will be used as metric in the following. While the cavity resonance wavelength and \(Q\) factor can be recovered for a different III-V/Si ratio by adjusting the rod width, the device properties directly depend on the amount of light absorption. This allows us to compare different devices with a varying number of III-V rods in the cavity center. In Figure 5b, hybrid InP/Si devices which possess an emission wavelength of 840 nm are optically excited with increasing power and the integrated PL intensity is displayed. These light in-light out (LL) curves are measured at different temperatures and with different InP/Si rods ratio. The kink visible in the LL curve corresponds to the onset of stimulated emission in the cavity mode. However, we note that this alone is not sufficient proof of lasing, but we use it here as a basis for comparing different devices. As expected, the efficiency decreases with a higher temperature, as the slopes of the curves indicate. Additionally, a trend is visible for the three different InP/Si ratios, where the curve is shifted to lower pump power for the cavity with fewer InP rods.