# On-Demand Waveguide-Integrated Microlaser-on-Silicon
## 1 Introduction  
Optical integration, which was first proposed in 1969, has been extensively studied over the past decades [1; 2; 3; 4; 5]. It is defined as the integration of optical components that enable the transmission, modulation, and detection of light on a single chip [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. Numerous types of integrated devices with various optical functions have been theoretically proposed and experimentally realized using several photonic platform materials including compound semiconductors (InP, GaAs), elementary semiconductors (Si, Ge), silica, and polymers [22; 23; 24; 25; 26; 27]. In particular, the potential of leveraging the state-of-the-art nanoscale complementary metal-oxide-semiconductor (CMOS) manufacturing technology and superior material properties such as high purity, low defect density, high thermal conductivity, and high refractive index have made Si-based photonic systems the most promising optical integration platform [3; 4; 5]. Further, Si exhibits almost negligible optical absorption at telecommunication wavelengths and thus already has become a predominant platform for integration. However, despite the substantial progress being made in Si-based light modulation and detection and their large-scale, cost-effective, monolithic device integration technology, the realization of efficient and reliable light sources, such as high-quality lasers on Si, has remained a challenge because of the fundamental limitations of the Si--the phonon-involved slow recombination rate, the enhanced free-carrier absorption (FCA), and the large Auger recombination for high-carrier-injection environments. Two major goals have been pursued simultaneously: the development of high-performance microlasers and their efficient integration into Si platforms [2]. Some strategies using the flip-chip technique and adhesive-polymer-assisted, direct-contact-based and/or oxide-bond-based die-to-wafer bonding combined with special surface treatments as well as template-assisted monolithic integration have enabled the realization of heterogeneous or hybrid integration of III-V compound active semiconductors with passive Si components [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 28]. Further, they successfully demonstrated the lasing emission and evanescent-type light coupling or hybridized-mode-type laser devices. However, these approaches often involve imprecise manual processes and consequently induce a relatively large initial misalignment, which should later be complementarily compensated with a post-integration III-V device fabrication process, such as photolithography. Thus, to use these approaches, one should initially bond a large volume of III-V dies in an extended area of the coupling region, which inevitably sacrifices the massive III-V materials, high integration density, and compactness [7; 10; 12]. In this regard, a recently emerging integration technique of micro transfer printing using a microstructured adhesive polymer stamp has drawn considerable attention, demonstrating its ability to mechanically manipulate micro- and nano-scale materials and structures in a highly precise and accurate manner [18; 19; 20; 21; 30; 31; 32; 33]. For example, an edge-emitting III-V gain media was transfer-printed using a polymer microtip [17; 18]. A bundle of nanoscale semiconductor wires was selectively picked up using a flat-tip polydimethylsiloxane (PDMS) micro-stamp and transferred to a targeted location on a receiving device substrate, thus enabling simultaneous lasing operations at multiple sites [33]. In addition, prefabricated one-dimensional (1D) photonic crystal (PhC) nanocavities with or without a supporting frame structure were successfully transferred, without any damage, onto silicon-on-insulator (SOI) or SiO\({}_{2}\) wafers [19; 20; 34].