GeSn alloys have been regarded as a potential lasing material for a complementary metal-oxide-semiconductor-compatible light source. Despite their remarkable progress, all GeSn lasers reported to date have large device footprints and active areas, which prevent the realization of densely integrated on-chip lasers operating at low power consumption. Here, we present a 1D photonic crystal nanobeam with a very small device footprint of 7 lm 2 and a compact active area of $1.2 lm 2 on a high-quality GeSn-on-insulator substrate. We also report that the improved directness in our strain-free nanobeam lasers leads to a lower threshold density and a higher operating temperature compared to the compressive strained counterparts. The threshold density of the strain-free nanobeam laser is $18.2 kW cm À2 at 4 K, which is significantly lower than that of the unreleased nanobeam laser ($38.4 kW cm À2 at 4 K). Lasing in the strain-free nanobeam device persists up to 90 K, whereas the unreleased nanobeam shows quenching of lasing at a temperature of 70 K. Our demonstration offers an avenue toward developing practical group-IV light sources with high-density integration and low power consumption.

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complementarymetal-oxide-semiconductor (CMOS) processing limits the practical realization of these long-sought-after PICs. [2] Germanium (Ge) has been extensively explored for such a laser owing to its CMOS compatibility and near-direct bandgap configuration. [3,4] Among various approaches to achieve the bandgap directness, strain engineering [5][6][7][8][9][10][11][12][13] and tin (Sn) alloying [14][15][16][17] have been considered as the two most promising paradigms.While lasing action has been observed in strain-engineered Ge at low operating temperatures (<100 K), [18][19][20] the Sn alloying approach has made significant, steady progress toward achieving lasing at practically high temperature over the past few years. [21][22][23][24][25][26][27] Since the first lasing demonstration in GeSn at 90 K, [21] much effort has been focused on increasing the operating temperature. [21][22][23][24][25][26][27] A major route to this end has been to increase the Sn content to further increase the directness of GeSn alloys, [17] which enabled higher operating temperatures reaching 270 K. [27] However, the lasing thresholds at these elevated temperatures are very high (>800 kW cm −2 at 270 K). [27] The exact causes for such high threshold in direct bandgap GeSn lasers have been attributed to the material quality. [17] For instance, it has been suggested that a large content of Sn increases the nonradiative recombination rate, thus leading to the reduction of internal quantum efficiency which influences the lasing threshold significantly. [17] In addition, the Sn alloying is typically accompanied by the compressive strain in the GeSn layer due to the large lattice mismatch between GeSn and Ge buffer layers. [15] Such compressive strain reduces the directness of GeSn, [28,29] thereby hindering the lasing performance. [30] Additionally, the increase in Sn content requires a decrease in growth temperature, which is typically associated with a higher concentration of point defects (vacancies and vacancies complexes) that can also impact the laser performance due to carrier trapping. [31,32] Another route to improve the lasing performance is to simultaneously employ both strain engineering and Sn alloying. [28,33] Recently, a few research groups have made significant progress along this direction by relaxing the compressive strain [22,34] and also by inducing mechanical tensile strain in GeSn. [35,36] Despite the improved directness of strain-engineered GeSn over as-grown compressively strained GeSn, the suspended device configuration, which has thus far been necessary for GeSn alloys are promising candidates for complementary metal-oxidesemiconductor-compatible, tunable lasers. Relaxation of residual compressive strain in epitaxial GeSn has recently shown promise in improving the lasing performance. However, the suspended device configuration that is thus far introduced to relax the strain is destined to limit heat dissipation, thus hindering the device performance. Herein is demonstrated that strain-free GeSn microdisk laser ...

GeSn alloys offer a promising route towards a CMOS compatible light source and the realization of electronic-photonic integrated circuits. One tactic to improve the lasing performance of GeSn lasers is to use a high Sn content, which improves the directness. Another popular approach is to use a low to moderate Sn content with either compressive strain relaxation or tensile strain engineering, but these strain engineering techniques generally require optical cavities to be suspended in air, which leads to poor thermal management. In this work, we develop a novel dual insulator GeSn-on-insulator (GeSnOI) material platform that is used to produce strain-relaxed GeSn microdisks stuck on a substrate. By undercutting only one insulating layer (i.e., Al2O3), we fabricate microdisks sitting on SiO2, which attain three key properties for a high-performance GeSn laser: removal of harmful compressive strain, decent thermal management, and excellent optical confinement. We believe that an increase in the Sn content of GeSn layers on our platform can allow us to achieve improved lasing performance.

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Despite the recent success of GeSn infrared lasers, the high lasing threshold currently limits their integration into practical applications. While structural defects in epitaxial GeSn layers have been identified as one of the major bottlenecks towards low-threshold GeSn lasers, the effect of defects on the lasing threshold has not been well studied yet. Herein, we experimentally demonstrate that the reduced defect density in a GeSn-on-insulator substrate improves the lasing threshold significantly. We first present a method of obtaining high-quality GeSn-on-insulator layers using low-temperature direct bonding and chemical–mechanical polishing. Low-temperature photoluminescence measurements reveal that the reduced defect density in GeSn-on-insulator leads to enhanced spontaneous emission and a reduced lasing threshold by ∼ 10 times and ∼ 6 times, respectively. Our result presents a new path towards pushing the performance of GeSn lasers to the limit.

Two-dimensional (2D) materials-based photodetectors in the infrared range hold the key to enabling a wide range of optoelectronics applications including infrared imaging and optical communications. While there exist 2D materials with a narrow bandgap sensitive to infrared photons, a two-photon absorption (TPA) process can also enable infrared photodetection in well-established 2D materials with large bandgaps such as WSe2 and MoS2. However, most of the TPA photodetectors suffer from low responsivity, preventing this method from being widely adopted for infrared photodetection. Herein, we experimentally demonstrate 2D materials-based TPA avalanche photodiodes achieving an ultrahigh responsivity. The WSe2/MoS2 heterostructure absorbs infrared photons with an energy smaller than the material bandgaps via a low-efficiency TPA process. The significant avalanche effect with a gain of ∼1300 improves the responsivity, resulting in the record-high responsivity of 88 μA/W. We believe that this work paves the way toward building practical and high-efficiency 2D materials-based infrared photodetectors.

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