## 4 III-V-on-Silicon Devices for Optical Communication Applications
### 1550 nm Resonant Cavity Mirror Lasers  
While microdisk lasers can obtain small footprint, single mode operation and low threshold current, the laser wavelength of the device is determined by the diameter of the III-V microdisk, which is therefore prone to variation on the wafer. Therefore, ideally one would like to use structures defined in the silicon device layer to determine the laser wavelength, while keep the optical confinement in the III-V waveguide layer high, in order to reduce device footprint and power consumption. This can be realized by the resonant cavity mirror design shown in Figure 16a. Two identical passive silicon grating cavities are coupled to the III-V waveguide above to form a linear device. Each silicon cavity is evanescently coupled to the InP waveguide. If the optical mode in the InP waveguide is of the silicon cavity's resonance wavelength, the silicon cavity will be excited. The energy built-up inside the silicon cavity couples back into the InP waveguide both co- and counter-directionally to the incoming light. The co-directional light will interfere destructively with the InP waveguide mode while the counter-directionally coupled light propagates back into the III-V waveguide. Because CMOS manufacturing techniques allow for compact, high-Q silicon cavities, this approach enables high and narrow band reflectivity over a short distance. An optically pumped proof-of-concept device [53] confirmed the potential advantages of using resonant mirrors as resonators in III-V-on-Si lasers. The threshold pump power of the experimentally demonstrated device was mW-level and the laser operated in single mode with a side-mode suppression ratio of up to 39 dB. However, the practical usability of optically pumped lasers is rather limited, as most applications require an electrical pumping mechanism. To achieve electrical pumping, metallic contacts have to be placed on the III-V waveguide structure to allow injection of electrical carriers. These metallic structures should be placed sufficiently far away  
Figure 15: Large signal modulation response of a full optical link based on a microdisk laser, an SOI waveguide and a heterogeneously integrated photodetector for a 10 Gbps 2\({}^{7}\)-1 PRBS pattern. Reproduced from [52]. © 2013 IEEE  
from the optical mode to prevent excessive loss due to absorption in the metal. This leads to relatively thick III-V waveguides (1-2 \(\upmu\)m) and therefore a high modal effective index (\(\sim\)3.3), which prevents phase matching with the underlying silicon cavity. Moreover, such a large waveguide supports multiple transversal modes. Therefore, a novel mesa geometry was adopted, that allows for electrical injection in high vertical index contrast III-V membranes, as shown in Figure 16b. Lasing of resonant cavity mirror structures in pulsed operation was demonstrated, as discussed in [54]. Single mode operation was obtained with a threshold current of 4 mA in a 160 \(\upmu\)m long device. For various applications in the visible-NIR wavelength range, such as short reach optical data communication and spectroscopy, an integrated short-wavelength III-V-on-Si laser is of great interest. GaAs-based VCSELs are very energy-efficient light sources. While silicon photonic integrated circuits are opaque in the visible-NIR, Silicon nitride (SiN) based waveguide circuits can be used. Those circuits can also be fabricated in a CMOS fab, thereby inheriting the same advantages as the silicon-on-insulator waveguide platform. As a first step towards GaAs VCSEL integration on SiN waveguide circuits, we demonstrate a heterogeneously integrated short-wavelength hybrid-cavity VCSEL on Si, by bonding a GaAs-based \"half-VCSEL\" to a dielectric DBR (Ta\({}_{2}\)Os/SiO\({}_{2}\)) on Si using ultra-thin divinylsiloxane-bis-benzocyclobutene (DVS-BCB) adhesive bonding. After bonding, oxide-confined GaAs VCSEL structures were fabricated in Chalmers University of Technology [55].