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Nanowires are promising platforms for realizing ultra-compact light sources for photonic integrated circuits. In contrast to impressive progress on light confinement and stimulated emission in III-V and II-VI semiconductor nanowires, there has been no experimental demonstration showing the potential to achieve strong cavity effects in a bottom-up grown single group-IV nanowire, which is a prerequisite for realizing silicon-compatible infrared nanolasers. Herein, we address this limitation and present the first experimental observation of cavity-enhanced strong photoluminescence from a single Ge/GeSn core/shell nanowire. A sufficiently large Sn content (~ 10 at%) in the GeSn shell leads to a direct bandgap gain medium, allowing a strong reduction in a material loss upon optical pumping. Efficient optical confinement in a single nanowire enables many round trips of emitted photons between two facets of a nanowire, achieving a narrow width of 3.3 nm. Our demonstration opens new possibilities for ultrasmall on-chip light sources towards realizing photonic-integrated circuits in the underexplored range of extended SWIR.

Second-order nonlinear χ2 processes hold the key to realizing various promising classical and quantum applications. Only conventional non-centrosymmetric materials like aluminium nitride (AlN) and lithium niobate (LN) exhibit a strong second-order nonlinearity. While germanium (Ge) has the advantage of mature foundry processing due to its complementary metal-oxide-semiconductor (CMOS) compatibility compared to conventional χ2 materials, it has a weak second-order nonlinearity owning to its inversion symmetry. It is predicted that exploiting micro-scale strain gradients induced by silicon nitride (SiN) stressors can break the inversion symmetry of Ge and enable more efficient second harmonic generation (SHG) in mid-infrared (MIR). We herein demonstrate SHG in visible and telecom S-band ranges by pumping our germanium-on-insulator (GOI) sample with a tunable femtosecond pulse laser. The signature quadratic power dependence curve of SHG has also been experimentally observed. In this report, we also propose a novel approach to breaking the inversion symmetry of Ge, which can amplify its weak second-order nonlinearity. We designed a Ge micro-bridge with a periodically changing nano-scale strain gradient that is orders of magnitude higher than the previously proposed structures utilizing SiN stressors. This results in a 3-orders of magnitude enhancement in χ2 according to both a classic anharmonic model and density potential theory. Our work paves the way toward a CMOS compatible and high χ2 nonlinear material for integrated photonic applications.

With the numerous recent demonstrations of germanium-tin (GeSn) semiconductor lasers, this material has become the strongest candidate for the realization of photonic-integrated circuits (PICs). However, the high defect density of this material often results in high lasing thresholds, making them undesirable for real-world applications. Furthermore, the low-thermal budget of high Sn content (>9 at.%) GeSn makes it exceptionally hard to grow onto other materials without introducing point defects. Herein, we present a novel method of directly combining GeSn with other materials through low-temperature wafer bonding. Through this technique, we can reduce the harmful high defect density and improve the lasing performance. To cater to the low thermal budget, we optimized the bonding processes which is critical in preventing Sn segregation to the surface. Through this method, we provide compelling evidence for the enhancement in photoluminescence in GeSn. Additionally, this technique can be extended to fabricate new material stacks, presenting unforeseen opportunities in other fields such as non-linear photonics.