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.

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 ...

The creation of pseudo-magnetic fields in strained graphene has emerged as a promising route to investigate intriguing physical phenomena that would be unattainable with laboratory superconducting magnets. The giant pseudo-magnetic fields observed in highly deformed graphene can substantially alter the optical properties of graphene beyond a level that can be feasible with an external magnetic field, but the experimental signatures of the influence of such pseudo-magnetic fields have yet to be unveiled. Here, using time-resolved infrared pump-probe spectroscopy, we provide unambiguous evidence for slow carrier dynamics enabled by the pseudo-magnetic fields in periodically strained graphene. Strong pseudo-magnetic fields of ~100 T created by non-uniform strain in  graphene on nanopillars are found to significantly decelerate the relaxation processes of hot carriers by more than an order of magnitude. Our findings offer alternative opportunities to harness the properties of graphene enabled by pseudo-magnetic fields for optoelectronics and condensed matter physics.

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.

The creation of CMOS compatible light sources is an important step for the realization of electronic-photonic integrated circuits. An efficient CMOS-compatible light source is considered the final missing component towards achieving this goal. In this work, we present a novel crossbeam structure with an embedded optical cavity that allows both a relatively high and fairly uniform biaxial strain of ∼0.9% in addition to a high-quality factor of >4,000 simultaneously. The induced biaxial strain in the crossbeam structure can be conveniently tuned by varying geometrical factors that can be defined by conventional lithography. Comprehensive photoluminescence measurements and analyses confirmed that optical gain can be significantly improved via the combined effect of low temperature and high strain, which is supported by a three-fold reduction of the full width at half maximum of a cavity resonance at ∼1,940 nm. Our demonstration opens up the possibility of further improving the performance of germanium lasers by harnessing geometrically amplified biaxial strain.