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Yin-Pu Huang, Bo-Rui Wu, Soumava Ghosh, Yue-Tong Jheng, [Ya-Lun Ho](https://orcid.org/0000-0001-8274-5978), [Yen-Ju Wu](https://orcid.org/0000-0003-2647-3407), Attaporn Wisessint, [Munho Kim](https://orcid.org/0000-0002-0379-1886), [Guo-En Chang](https://orcid.org/0000-0002-3739-5451)

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[Mid-infrared silicon photonic lasers based on GeSn slab waveguide on silicon](https://mdr.nims.go.jp/datasets/ec208745-3bf8-4fe2-bc94-13832f7d20b6)

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Mid-infrared silicon photonic lasers based on GeSn slab waveguide on siliconResearch Article Vol. 32, No. 22 / 21 Oct 2024 / Optics Express 39560Mid-infrared silicon photonic lasers based onGeSn slab waveguide on siliconYIN-PU HUANG,1,2 BO-RUI WU,1,2 SOUMAVA GHOSH,2YUE-TONG JHENG,2 YA-LUN HO,3 YEN-JU WU,4ATTAPORN WISESSINT,5 MUNHO KIM,6AND GUO-EN CHANG1,2,*1Graduate Institute of Opto-Mechatronics, National Chung Cheng University, Chiayi County 621301,Taiwan2Department of Mechanical Engineering and Advanced Institute of Manufacturing with Hi-TechInnovations (AIM-HI), National Chung Cheng University, Chiayi County 621301, Taiwan3Research Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba305-0044, Ibaraki, Japan4Center for Basic Research on Materials, National Institute for Materials Science, Tsukuba 305-0047,Ibaraki, Japan5Department of Mechanical Engineering, Kasetsart University, Bangkok 10900, Thailand6School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798,Singapore*imgec@ccu.edu.twAbstract: GeSn alloy has emerged as an attractive active material for Si-based mid-infrared(MIR) lasers due to its direct bandgap nature at higher Sn concentrations. Here, we report on anoptically-pumped GeSn MIR lasers based on planar slab waveguide with a top Si ridge structure.The inclusion of 10% Sn transforms the GeSn active layer into a direct bandgap material. The Siridge structure ensures appropriate optical confinements with reduced scattering loss from thewaveguide sidewall. Lasing action was achieved under optical pumping with a low thresholdof 60.85 kW/cm2 and an emission wavelength of 2238 nm at T = 40 K. Lasing action was alsoobserved up to T = 90 K with a threshold of 170 kW/cm2.© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement1. IntroductionSilicon photonics operating in the mid-infrared (MIR) range of 2–20 µm has been increasinglyrecognized for its potential in applications such as chemical analysis, bio-medical diagnosis,free-space communications, and spectroscopy [1,2]. MIR silicon photonic chips necessitate theintegration of various photonic devices on a single Si chip, manufactured using complementarymetal-oxide semiconductor (CMOS) technology. Although various Si-based passive and activephotonic devices capable of operation in the MIR region have been demonstrated [3], the indirectbandgap nature of Si limits the efficiency of Si-based light emitters, hindering the completion ofMIR silicon photonics. Over the past two decades, the development of high-quality group-IVGeSn alloys on Si substrates with suitable buffer layers through low-temperature molecular beamepitaxy (MBE) and chemical vapor deposition (CVD) techniques has opened new avenues forefficient light emitters in MIR silicon photonics [4–6]. With a quasi-direct bandgap nature, Ge hasa direct-band (Γ-valley) conduction band (CB) of only 136.5 meV above the lowest indirect-band(L-valley) CB [7]; the incorporation of α-Sn, another group-IV material, can significantly reducethis energy, transforming the material into a direct-gap material with a sufficiently high Sncontent (> 7%) [8]. This reduction in the direct bandgap also extends the emission wavelengthinto the MIR region [9]. Moreover, a unique momentum (k)-space separation owing to theclose proximity of Γ- and L-valley CB enhances the optical performance of GeSn-based devices#540223 https://doi.org/10.1364/OE.540223Journal © 2024 Received 23 Aug 2024; revised 2 Oct 2024; accepted 2 Oct 2024; published 15 Oct 2024https://orcid.org/0000-0002-0379-1886https://orcid.org/0000-0002-3739-5451https://doi.org/10.1364/OA_License_v2#VOR-OAhttps://crossmark.crossref.org/dialog/?doi=10.1364/OE.540223&amp;domain=pdf&amp;date_stamp=2024-10-16Research Article Vol. 32, No. 22 / 21 Oct 2024 / Optics Express 39561[10,11]. Furthermore, the compatibility with standard Si-CMOS technology, and the potentialfor monolithic integration on the same Si chip have rendered GeSn a promising candidate forlight emitters and enabled a reduction in fabrication costs.With the successful growth of direct-bandgap GeSn alloys, the first optically pumped GeSn laserwith 12.6% Sn was achieved with a threshold of 325 kW/cm2 up to T = 100 K [12]. Subsequentresearch has continued to improve the performance of optically pumped GeSn lasers in terms ofoperating temperatures and thresholds [13–22]. Recent advancements include room-temperatureGeSn optically pumped lasers [23,24] and GeSn-based electrically injected lasers up to 140 K[9,25–28]. These developments have established GeSn lasers as viable light sources for MIRsilicon photonics. However, the performance of GeSn lasers still falls short of the commerciallyavailable III-V-based lasers [29–31] in terms of the threshold and operating temperature, thusnecessitating further improvements to meet practical application demands.Optical loss in semiconductor lasers is crucial for performance, and is attributed mainly tomaterial losses (such as free-carrier absorption) and scattering loss from imperfections in thelasers [32–35]. Surface defects on the sidewalls of optical cavities, introduced during the etchingprocess, lead to unwanted surface recombination and additional optical loss, thereby increasingthe threshold and decreasing the maximum lasing temperature of GeSn lasers [35]. Reducingsidewall roughness to suppress surface recombination is crucial for improving performance interms of the threshold and maximum lasing temperature.In this study, we demonstrate an optically pumped GeSn planar slab waveguide laser on a Sisubstrate with a Si-ridge structure on the GeSn gain medium. The incorporation of approximately10% Sn into the GeSn active layer not only transforms the material into a direct bandgap materialenabling efficient direct band-to-band transitions, but also shifts the emission wavelength tothe MIR region, enhancing its utility for MIR silicon photonics. Our device design features aSi-ridge structure that forms guided modes with excellent optical confinement without etchingthe GeSn active layer, thereby minimizing the optical loss caused by surface defects. Lasingaction was observed with a low threshold of 60.85 kW/cm2 and an emission wavelength of 2238nm at T = 40 K, and up to an operating temperature of 90 K with a threshold of 170 kW/cm2.2. Results and discussion2.1. Device design and simulationFigure 1(a) presents a 3D schematic of the proposed optically-pumped GeSn planar slab waveguidelaser on a Si substrate. The GeSn sample comprises a 450 nm-thick strain-relaxed Ge virtualsubstrate (VS) and a 550 nm-thick GeSn active region containing 10% Sn on a Si (001) substrate.A 100 nm-thick amorphous Si ridge structure was engineered to form guiding modes withoutetching the GeSn active layer. As a result, surface non-radiative recombination caused by surfacedefects can be reduced, thereby suppressing optical losses and enhancing the laser’s performance.To assess the optical confinement of the layers, finite element method (FEM) simulations wereconducted using the refractive indices (RIs) of the materials by referring to previous studies[36,37] to determine the field distribution and the optical confinement factors (OCFs) for thevarious layers with different widths of the Si ridge layer. Figure 1(b) illustrates the simulatedOCF as a function of the width of the amorphous Si layer (w) and the energy distribution for thequasi-transverse electric (TE) mode with w= 20 µm at λ= 2200 nm. The energy distributionreveals that the Si ridge structure and the Si substrate provide significant optical confinementfor the GeSn active layer, which is attributable to the substantial contrast in RIs between GeSn(n= 4.3) and Si (n= 3.45). As shown in Fig. 1(b), the OCF for the GeSn active layer increases withthe widening of the Si ridge, following a saturation trend. Consequently, a high OCF of 66.80%was achieved for the GeSn active layer, indicating robust optical confinement. Additionally, theguided mode is positioned far from the sidewall of the Si-ridge structure, suggesting minimizedimpact from sidewall defect-related recombination and reduced optical loss on the lasing action.Research Article Vol. 32, No. 22 / 21 Oct 2024 / Optics Express 39562(a) Pump0 50 100 150 20066.466.566.666.766.8OCF (%)Width w (m)w(b)Fig. 1. Device design and simulation. (a) Schematic of the GeSn slab waveguide laser onSi substrate. (b) Simulated optical confinement factor (OCF) as a function of the width ofamorphous Si top layer. The inset shows the simulated field distribution at λ= 2200 nm forthe quasi-transverse electric fundamental mode.2.2. Material growth and characterizationThe sample used in this study was grown by reduced-pressure chemical vapor deposition (RPCVD)on a Si (001) substrate with Ge2H6 and SnCl4 precursors for Ge and Sn, respectively. The epitaxialprocess included the growth of a strain-relaxed Ge VS using a two-step growth technique, and thegrowth of a GeSn layer at 320 °C to reduced Sn segregation. The material characterization wasconducted using cross-sectional transmission electron microscopy (XTEM), secondary ion massspectrometry (SIMS), selected area electron diffraction (SAED), X-ray reciprocal space mapping(RSM) and photoluminescence (PL) techniques. Figure 2(a) depicts the XTEM image alongsidethe SIMS depth profile showing the atomic distribution of Ge and Sn atoms. Misfit dislocationswere evident in the Ge VS near the Si substrate, indicating that the Ge VS was strain-relaxed.Additionally, the XTEM image revealed sharp and flat interfaces between the Ge VS and GeSnactive layers. The SIMS profile indicates a uniform distribution of Sn atoms within the GeSnlayer. The inset in Fig. 2(a) shows the SAED image for the active GeSn layer, demonstrating itssingle-crystalline nature. The high-resolution XTEM image at the Ge/GeSn interface as shownin the inset of Fig. 2(a) provides clear evidence of high material quality. Figure 2(b) displaysthe X-ray RSM of the (224) plane. Three distinct peaks were identified, corresponding to the Sisubstrate, Ge VS, and GeSn active layer. The diagonally passing strain-relaxed line confirmsthat the Ge VS was tensile strained while the GeSn layer was compressively-strained. From theposition of the diffraction peaks, the in-plane and out-of-plane lattice constants of the layerswere extracted to determine the strain and composition. The analysis indicates that the Ge VS issubject to an in-plane biaxial tensile strain of 0.17% due to the difference in thermal expansioncoefficients between Ge and Si during epitaxial growth [19]. In contrast, the GeSn layer exhibitsan Sn concentration and in-plane biaxial compressive strain of 10% and 0.264%, respectively.Temperature-dependent PL experiments were conducted using a 1064 nm continuous-wavelaser as the pumping source and a Fourier transform infrared (FTIR) spectroscope equipped withan LN2-cooled InSb photodetector operating in a 1-5 µm spectral range. Figure 2(c) illustratesthe temperature-dependent PL spectrum of our sample. A strong emission signal peak at 2222nm was observed at T = 77 K (corresponding to a photon energy of 0.558 eV). Compared withthe direct-gap emission wavelength of approximately 1550 nm for pure Ge, the extension ofemission wavelength was attributed to the incorporation of Sn in the GeSn layer. From thetemperature-dependent PL spectra, the PL peak position as a function of temperature was depictedin Fig. 2(d). To clearly understand the effect of temperature on the direct bandgap of GeSn, wefitted the experimental data using the Varshni equation [38,39],EΓg (T) = EΓg (T = 0) −α × T2T + β(1)Research Article Vol. 32, No. 22 / 21 Oct 2024 / Optics Express 39563 4  Fig. 2. Material characterization and analysis. (a) Cross-sectional transmission electronmicroscopy (XTEM) image with secondary ion mass spectrometry (SIMS) diagram ofthe atomic distribution of Ge and Sn atoms. The inset shows the selected area electrondiffraction (SAED) pattern of the GeSn active layer and high-resolution XTEM image at theGeSn/Ge interface. (b) X-ray (224) reciprocal space mapping (RSM) of the sample used inthis study. The dark red line represents full strain relaxation. (c) Temperature-dependentphotoluminescence (PL) spectra of the GeSn sample. (d) PL peak position as a function oftemperature with the Varshni fitting. (e) Calculated band structure of the GeSn active layerat T = 77 K.where α and β are the Varshni parameters, EΓg (T = 0) is the direct bandgap energy at T = 0K. From the fitting shown in Fig. 2(d), we obtained α = 7.05 × 10−4 eV/K, β=559.73 K, andEΓg (T = 0 K) = 0.569 eV, which are useful for estimating the direct bandgap energy of theGeSn alloy at various temperatures. To gain a deeper understanding of the emission peak, wetheoretically calculated the electronic band structure of the GeSn active layer considering thestrain effect via the multi-band k p method [38]. The calculated band structure of the GeSnactive layer at T = 77 K is illustrated in Fig. 2(e). The results suggest that the introduction of10% Sn in the GeSn active layer shifts the Γ-valley CB to be lower than the L-valley CB by51 meV, highlighting the directness of the band structure. Furthermore, the compressive strainof approximately 0.264% breaks the degeneracy of the heavy hole (HH) and light hole (LH)bands in the valance band (VB), shifting the HH band above the LH band by approximately26 meV. As a result, the lowest direct-band transition occurs when electrons jump from theΓ-valley CB to the HH band by releasing energy of ∼562 meV which gives a good agreementof our experimental findings. This analysis proves that the observed emission was caused bythe interband transition from the Γ-valley CB to the HH band (Γ→HH). When the temperaturedecreased, the PL emission peak redshits owing to the decreased bandgap energy. The PLResearch Article Vol. 32, No. 22 / 21 Oct 2024 / Optics Express 39564intensity also decreased with increasing temperature, which also indicates that the GeSn activelayer is a direct bandgap material.2.3. Optical characterizationThe sample was fabricated into devices using CMOS-compatible processes. Mesas with a widthof w= 200 µm were patterned by standard optical lithography followed by SF4-based reactive ionetching (RIE) to form ridge-shaped waveguide structures. The sample was subsequently cleavedinto devices of various lengths; the device used in this study measured a length of L= 1.78 mm.Scanning electron microscopy (SEM) image of the fabricated GeSn device is shown in Fig. 3(a).0 50 100 150 200Integrated PL intensity (a.u.)Power density (kW/cm2)1000 2000 3000 4000Wavelength (nm)Intensity (a.u.)(a)(b)10 µmSi (001)SiGeSn/Ge(b)(c) T=40KPth=60.85 kW/cm2T=40K0.8 Pth1.0 Pth1.2 Pth1.4 Pth40 50 60 70 80 900255075100FWHM (nm)Power density (kW/cm2)Thresholdx10ThresholdFig. 3. Optical characterization of our GeSn laser. (a) Scanning electron microscope (SEM)image of the fabricated device. (b) Light-in-light-out (L-L) curve of the GeSn device atT = 40 K. The threshold is denoted by the grey dashed line. (c) Power-dependent emissionspectra of the GeSn device at T = 40 K. The inset shows the full-width-at-half-maximum ofthe emission peak at various power densities.PL experiments were conducted using a 1064 nm pulsed laser with a pulse width of 12 ns anda repetition rate of 12.5 kHz as the pumping light source, which was modulated using an opticalchopper at a frequency of 200 Hz. The pumping laser was incident normally on the GeSn devicesmounted in a cryogenic system through a cylindrical lens. The light emitted from one facet of theGeSn device was collected and analyzed using a FTIR. A special step-scan technique was adoptedto enhance the signal-to-noise ratio and eliminate background thermal radiation. Figure 3(b)presents the light-in-light-out (L-L) curve of the GeSn device at T = 40 K. The PL intensity waslow at a low pumping power intensity, where spontaneous emission dominated. Above a certainpumping power density, the PL intensity increased sharply, indicating that stimulated emissionbegan to dominate the light emission. The onset of the L-L curve defines the threshold of theGeSn laser as Pth = 60.85 kW/cm2. The obtained threshold of our GeSn lasers is lower thanthe typical values of 100–600 kW/cm2 reported for optically pumped GeSn lasers on Si withSn contents of 12–16% [12,13,19–21]. The lower threshold of our GeSn lasers is attributed toreduced loss from surface defects, highlighting the importance of our planar slab waveguideResearch Article Vol. 32, No. 22 / 21 Oct 2024 / Optics Express 39565design. Figure 3(c) exhibits the measured PL spectra at T = 40 K above and below the threshold.Fitting the emission peaks using the Gaussian function yielded the full-width-at-half-maximum(FWHM). The inset in Fig. 3(c) shows the FWHM extracted from the emission spectra as afunction of the pumping density. Below the threshold, a broad and weak emission peak wasobserved at λ= 2239 nm with an FWHM of 87 nm (21.5 meV). Contrarily, when the pumpingpower density reaches the threshold, the intensity of the emission peak increased sharply witha narrowed FWHM of 35 nm (8.6 meV), comparable to the results reported for other GeSnoptically pumped lasers [18,19]. As the pumping power intensity increased further, the intensityof the emission peak increased more significantly, and the FWHM sharply decreased, confirminglinewidth narrowing. These results verify the achievement of lasing action from the developedGeSn lasers.Next, we evaluated the temperature-dependent characteristics of the GeSn laser. Figure 4(a)illustrates the temperature-dependent PL spectra measured from our fabricated GeSn slabwaveguide laser at different temperatures under an optical pumping density of 500 kW/cm2. Asthe temperature increased from T = 40 K, robust lasing peaks were clearly observed up to T = 90K. Beyond this temperature, the broadening of the emission spectra and the absence of a distinctlasing peak indicate that spontaneous emission predominated at higher operating temperatures.The corresponding L-L curves, shown in Fig. 4(b), also demonstrated the “turn-on” behavior upto T = 90 K. Therefore, the highest lasing temperature of our GeSn laser was determined to be 90K. The L-L curves were used to determine the thresholds at different temperatures, as shown inFig. 4(c). The threshold increased from 60.85 kW/cm2 at T = 40 K to 170 kW/cm2 at T = 90 K.This increase in temperature thermally excited the optically generated electrons from the Γ-valleyto the L-valley in the CB, necessitating higher optical pumping to achieve lasing action as thetemperature increased. From the temperature-dependent threshold, the characteristic temperature(T0) of the laser, which is crucial for understanding the thermal stability, can be determined using[38]Pth(T) = Pth(Ta) × exp(︃T − TaT0)︃(2)where Pth(Ta) is the threshold at T = Ta. Fitting the data using Eq. (1) yields a characteristictemperature of T0 =51.6 K for our GeSn laser. Ideally, a higher T0 is desirable for stable laserapplications. A further increase in Sn content in the GeSn layer could enhance the directness ofband structure and improves laser passivation, thereby enhancing laser performance.Figure 5(a) presents a high-resolution scan of the emission spectrum at T= 77 K with aspectral resolution of 2 cm−1 (∼0.5 nm) and a pumping power density of 146.85 kW/cm2. Asshown in the inset of Fig. 5(a), periodic peaks, as indicated by the arrows, were clearly observed,corresponding to the Fabry–Pérot longitudinal modes of the cavity, which suggests multiple-modelasing. The peak wavelength positions of the cavity modes extracted from the emission spectrumare depicted in Fig. 5(b). A linear fit of the peak positions yielded a longitudinal mode spacingof 5.47 ± 0.22 nm. Additionally, fitting the emission peak with a Gaussian function yields aFWHM of approximately 6.2 nm, revealing narrow linewidth of the lasing modes.We then examine the polarization behavior of the optically-pumped GeSn slab laser. TheGeSn laser was optically-pumped at T= 77 K above the threshold (146.85 kW/cm2). The lasingemission was filtered using a linear polarizer mounted on a motorized rotation stage to preciselycontrol the polarization angle (θ). Figure 6(a) shows the emission spectra at various polarizationangles. The emission intensity was strongest at θ=0°, and decreased with increasing polarizationangle. Figure 6(b) shows the corresponding polar plot of the emission intensity. The emissionintensity reaches a maximum (minimum) at θ=0° (θ=90°), indicating the TE component wasdominant over the transverse magnetic (TM) component. This observation aligns with thecalculated band structure of the GeSn active layer (Fig. 2(e)), where the Γ→HH transition is thelowest interband transition, leading to the generation of TE-polarized light [38]. The polarizationResearch Article Vol. 32, No. 22 / 21 Oct 2024 / Optics Express 39566 6   = 40 K above and below the threshold. Fitting the emission peaks using Gaussian function yields the full-width-at-half-maximum (FWHM). The inset in Fig. 3(c) shows the FWHM extracted from the emission spectra as a function of the pumping density. Below threshold, a broad and weak emission peak was observed at λ = 2239 nm with an FWHM of 87 nm (21.5 meV). In contrast, when the pumping power density reaches the threshold, the intensity of the emission peak increases sharply with a narrowed FWHM of 35 nm (8.6 meV), comparable to the results reported for other GeSn optically pumped lasers [18,19]. As the pumping power intensity increases further, the intensity of the emission peak increases more significantly, and the FWHM sharply decreases, confirming linewidth narrowing. These results verify the achievement of lasing action from the developed GeSn lasers.    1000 2000 3000 4000Wavelength (nm)Intensity (a.u.)40 60 80 10050100150Temperature (K)Threshold Pth (kW/cm2 ) Exp. data FitT=40 K T=77 K T=90 K T=100 K T=110 K (a) (b)(c)0 150 300 450 600Intensity (a.u.)Power density (kW/cm2) 40k 77k 90k 100kFig. 4. Temperature-dependent optical characteristics. (a) Temperature-dependent PLspectra from our GeSn slab laser measured at various temperatures. (b) Measured L-L curvesat various temperatures. (c) Extracted threshold power density as a function of temperature. 7   is desirable for stable laser applications. A further increase in Sn content in the GeSn layer enhances the directness of band structure and improves laser passivation, thereby enhancing laser performance.   Fig. 5. (a) High-resolution scan of the emission spectrum at T= 77 K. The inset shows theenlarged part of the emission spectrum, with the emission peaks indicated by arrows. (b)Peak wavelength position versus cavity mode.anisotropy can be characterized by ρ = (Imax − Imin)/(Imax + Imin), where Imax and Imin are themaximum and minimum emission intensities, respectively. A high polarization anisotropy ofρ = 97.5% was obtained, indicating the lasing emission was almost linearly polarized.Figure 7 shows a comparison between the threshold of our GeSn laser and previous optically-pumped GeSn lasers with Fabry–Pérot cavities [12,13,40–43] in terms of Sn composition andoperating temperature. Among these optically-pumped GeSn lasers, our GeSn slab waveguidelaser features a low threshold and/or a higher lasing temperature than the reported GeSn-basedwaveguide lasers with higher Sn contents. This can be attributed to the good material quality,good optical confinement, and reduced surface and sidewall recombination losses, highlightingResearch Article Vol. 32, No. 22 / 21 Oct 2024 / Optics Express 39567 8    Fig. 6. Laser emission polarization characteristics. (a) Emission spectra at variouspolarization angles measured at T= 77 K. (b) The corresponding polar plot of the integratedemission intensity.both the importance of our slab waveguide laser structure design and the competitiveness ofour device. It is anticipated that increasing the Sn content in the GeSn layer could enhance thedirectness of the band structure and improve the threshold and/or increase the lasing temperature[29]. Additionally, the emission wavelength can be tuned in the MIR region by adjusting the Sncontent, making it suitable for MIR silicon photonics. 8   was obtained, indicating the lasing emission is nearly linear polarized.     10 15 20 25102103104  [12] S. Wriths et. al. (2015) [13] Y. Zhou et. al. (2019) [40] S. A. Kabi et. al. (2016) [41] D. Buca et. al. (2016) [42] W. Dou et. al. (2018) [43] J. Margetis et. al. (2017) This workThreshold (kW/cm2)Sn composition (%)1060110160210260Temperature (K)Fig. 7. Comparison of lasing threshold and temperature of optically pumped GeSn laserswith Fabry–Pérot cavities [12,13,40–43].3. ConclusionIn conclusion, we have successfully demonstrated an optically pumped GeSn slab waveguidelaser on a Si platform with a Si ridge structure. This planar GeSn laser structure, combinedwith the Si ridge, demonstrates excellent optical confinement within the active layer whileminimizing overlap with the ridge structure’s sidewall, thereby reducing optical loss and enablingefficient lasing action. With the incorporation of 10% Sn, the emission wavelength extends to2254 nm. Lasing action was confirmed by the clear threshold behavior, linewidth narrowing,and TE dominant polarized emission. Additionally, this fabricated GeSn slab laser achievesa low threshold of 60.85 kW/cm2 at T = 40 K and maintains effective lasing up to 90 K witha threshold of 170 kW/cm2. The characteristic temperature of 51.6 K indicates the need forResearch Article Vol. 32, No. 22 / 21 Oct 2024 / Optics Express 39568further improvements to enhance thermal stability. These findings suggest that a highly credibleGeSn laser with a lower threshold is achievable. By carefully designing and optimizing thelaser structure to reduce surface recombination, decreasing the threshold further and achievingefficient lasing even at higher temperatures is feasible. These results demonstrate immensepotential for efficient CMOS-compatible lasers on Si for mid-infrared photonics, paving the wayfor broader applications in areas such as environmental sensing, healthcare diagnostics, andtelecommunications.Funding. National Science and Technology Council of Taiwan (NSTC 112-2636-E-194-001, NSTC 113-2636-E-194-001); Taiwan Semiconductor Research Institute (JDP112-Y1-028, JDP113-Y1-044).Disclosures. The authors declare that there are no conflicts of interest related to this article.Data availability. Data underlying the results presented in this paper may be available from the corresponding authorupon reasonable request.References1. R. Soref, “The past, present, and future of silicon photonics,” IEEE J. Select. Topics Quantum Electron. 12(6),1678–1687 (2006).2. R. 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