High-density and high-efficiency small light sources will have a wide range of applications including sensing, spectroscopy, quantum computing, and optical communications. In this regard, the integration of III-V materials on silicon has been pursued for decades to combine the best of both semiconductor families: Silicon for electronics and photonics components such as passive waveguides, wavelength filters, modulators, and photodetectors along with III-V semiconductors as direct bandgap efficient light emitters. Several approaches have been investigated relying overall on either bonding III-Vs to a silicon wafer or direct growth. The main challenge to be overcome in direct growth stems from the significant lattice and thermal mismatch between III-Vs and Si that leads to crystal defects. We have developed a novel growth method named template-assisted selective epitaxy (TASE) to cointegrate seamlessly different materials. The high crystal quality of TASE-grown III-Vs resulted in state-of-the-art electronic devices. TASE-grown materials fill and take the shape of a pre-defined hollow oxide template. Overall benefits of TASE include low defect density, absence of foreign catalyst (Au-free), and precise control of crystal composition, position, shape, and size. For integrated photonics TASE offers several advantages that are not achievable with other integration schemes. As all the fabrication steps involve only Si–no III-V processing–the template can hence be fabricated in a Si photonic foundry on an SOI wafer. This way, based on the mature Si technology, we can create III-V nanostructures featuring arbitrary shapes, e.g., vertical or horizontal nanowires, rings, microdisks, or photonic crystals. It also enables in-plane co-integration with Si-based components. Dense co-integration of different III-Vs was implemented on the same chip by sequential growth runs. We have recently demonstrated microdisk lasers grown by TASE based on GaAs and InP, which compared favorably to similar structures fabricated by the mature wafer bonding approach. However, the true benefit of TASE comes from enabling a seamless integration with silicon passives. In this talk we shall focus on these aspects, and how this enables novel laser architectures including InP-based one-dimensional photonic crystal cavities and whispering gallery mode resonators. This research was supported by the European Union H2020 ERC StG PLASMIC (Grant No. 678567), SiLAS (Grant No. 735008) and Swiss National Science Foundation SPILA (Grant No. CRSK-2_190806). References 1. Schmid, H. et al. Template-assisted selective epitaxy of III–V nanoscale devices for co-planar heterogeneous integration with Si. Appl. Phys. Lett. 106, 233101 (2015). 2. Lee, S. et al. High Performance InGaAs Gate-All-Around Nanosheet FET on Si Using Template Assisted Selective Epitaxy. in 2018 IEEE International Electron Devices Meeting (IEDM) 39.5.1-39.5.4 (2018). doi:10.1109/IEDM.2018.8614684. 3. Borg, M. et al. High-Mobility GaSb Nanostructures Cointegrated with InAs on Si. ACS Nano 11, 2554–2560 (2017). 4. Mauthe, S. et al. InP-on-Si Optically Pumped Microdisk Lasers via Monolithic Growth and Wafer Bonding. IEEE J. Sel. Top. Quantum Electron. 25, 1–7 (2019). 5. Mauthe, S. et al. Hybrid III-V Silicon Photonic Crystal Cavity Emitting at Telecom Wavelengths. Nano Lett. (accepted). 6. Mauthe, S. et al. High-speed III-V nanowire photodetector monolithically integrated on Si. Nat. Commun. 11, 4565 (2020).