High-Temperature Continuous-Wave Pumped Lasing from Large-Area

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High Temperature Continuous-Wave Pumped Lasing from Large-Area Monolayer Semiconductors Grown by Chemical Vapor Phase Deposition Liyun Zhao, Qiuyu Shang, Yan Gao, Jia Shi, Zhen Liu, Jie Chen, Yang Mi, Pengfei Yang, Zhepeng Zhang, Wenna Du, Min Hong, Yin Liang, Jingya Xie, Xiaoyong Hu, Bo Peng, Jiancai Leng, Xinfeng Liu, Yue Zhao, Yanfeng Zhang, and Qing Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04511 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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High Temperature Continuous-Wave Pumped Lasing from Large-Area Monolayer Semiconductors Grown by Chemical Vapor Phase Deposition Liyun Zhao,†, ‡ Qiuyu Shang,† Yan Gao,† Jia Shi,§ Zhen Liu,† Jie Chen,†, § Yang Mi,§ Pengfei Yang,†, £ Zhepeng Zhang,†, £ Wenna Du,§ Min Hong,†, £ Yin Liang,† Jingya Xie,ζ Xiaoyong Hu,ζ Bo Peng,& Jiancai Leng,$ Xinfeng Liu,§ Yue Zhao,*, ¥ Yanfeng Zhang,*, †, £ Qing Zhang*, †, ‡ †

Department of Materials Science and Engineering, College of Engineering, Peking University,

Beijing 100871, China ‡

Research Center for Wide Gap Semiconductor, Peking University, Beijing 100871, China

§

Division of Nanophotonics, CAS Key Laboratory of Standardization and Measurement for

Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China £

Center for Nanochemistry (CNC), Academy for Advanced Interdisciplinary Studies, Beijing

National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ζ

State Key Laboratory for Mesoscopic Physics and Department of Physics, Collaborative

Innovation Center of Quantum Matter, Peking University, Beijing 100871, China &

School of Microelectronics and Solid State Electronics, University of Electronic Science and

Technology of China, Chengdu 610054, China $

School of Science, Qilu University of Technology (Shandong Academy of Sciences), Jinan

250353, China ¥

Department of Physics, Southern University of Science and Technology, Shenzhen 518055,

China Email: [email protected]; [email protected]; [email protected]

1 ACS Paragon Plus Environment

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Abstract The realization of low energy-consumption lasers based on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) are crucial for the development of optical communications, flexible displays, and lasers on the chip level. However, among the asdemonstrated TMDC-based lasers so far, the gain materials are mainly achieved by a mechanical exfoliation approach accompanied with poor reproducibility and controllability. In this work, we report a controllable design for generating large scale lasing from chemical vapor deposition (CVD) derived high quality monolayer MoS2 film. Strong continuous-wave (CW) optically driven whispering-gallery-mode (WGM) lasing is achieved in a wide temperature range from 77 to 400 K. The eminent lasing performances result from the strong spatial confinement of carriers and the enhanced efficiency of spontaneous emission owing to the lensing and screening effects of silica microsphere cavities. These findings not only advance the fundamental understanding of 2D lasing effects, but also provide solutions to fabricate low-cost, scalable and integratable TMDC-based lasers.

Keywords：MoS2, small laser, microsphere, continuous-wave lasing, layered materials


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Recently, atomic lasers based on two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted great attention for their potential in advancing low energy-consumption, chip-level and flexible coherent light sources for next-generation optical communications, displays, high-density storages and high-resolution imaging, etc.1-6 The valley-resolved recombination properties and large exciton oscillation strength of TMDCs enable the realization of non-trivial polariton and a variety of valley-resolved lasing device applications.7-12 Benefited from atomic-level quantum confinement effect and their 2D electronic density of states, TMDCbased lasers possess large differential gain, which is quite similar to the quantum well lasers.3, 13 The atomic-level spatial confinement effect also leads to extremely localized carriers, in which the photon-generated carrier concentration can easily reach the amplitude of 1018 cm−3, meeting the requirement of population inversion in lasing process.3 Owing to the advantages, most of the TMDC-based lasers, including MoS2, MoTe2, WS2, WSe2, are operated under continuous-wave (CW) optically pumping conditions, extensively raising the possibility of the realization of electrically driven chip-level lasers.1, 3-5 Since sustainable lasing can only be achieved when the optical losses are overcome by the optical gain, all the approaches reported to date in realizing TMDC-based lasers utilize few layer TMDCs from mechanical exfoliation (ME) as the gain media. However, this route exhibits poor controllability and reproducibility. Moreover, to suppress the optical loss, resonant cavities, such as photonic crystals, free-standing microdisks, microsphere-desks and vertical cavity with distributed Bragg reflectors (DBRs), are adopted. Such devices require complicated microfabrication processes,1-5 which could also hinder the practical applications of 2D TMDCbased lasers. Monolayer MoS2 has a direct band gap of ~ 1.9 eV and is an ideal candidate as gain media for visible lasing devices.14,

15

Compared with other monolayer TMDCs, large-scale

monolayer MoS2 with high quality (i. e., crystal domains in the range of tens to hundreds


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micrometers, low defect density) can be grown by chemical vapor deposition (CVD), enabling controllable and scalable device fabrication.16-22 Recently, we have successfully grown monolayer MoS2 nanosheets directly on commercial SiO2 microspheres with high intrinsic quality factor (~108, Supporting Information, SI, Section I), in which exciton emission tuning by the resonant whispering-gallery-modes (WGMs) of microspheres has been demonstrated.23 Unfortunately, the lasing action does not occur, possibly due to the poor surface roughness and resulted scattering loss after the deposition process. Here we report the realization of CW lasing by coupling commercial SiO2 microspheres with CVD-grown hundred-micron-sized monolayer MoS2 thin film. The microspheres can effectively reduce the excitation area owing to lensing effect and therefore enhance coupling between gain area and optical modes. Strong WGM lasing with full width at half maximum (FWHM) of ~ 0.3 – 2.05 nm is achieved by CW optical pumping in a wide temperature range (77 – 400 K). Systematic studies on lasing mechanism from the aspect of gain, loss and spontaneous coupling efficiency are also presented. Our work marks an important step forward towards optoelectronic applications for on-chip optical communication technologies. Results and Discussion Figure 1a shows schematics of the configuration of our MoS2/microsphere cavity lasers. SiO2 microsphere arrays are placed on top of the monolayer MoS2 film with 285 nm-SiO2-Si substrate. The substrate is selected to enhance absorption and emission efficiency because of constructive interference (Figure S1) and the reduction of lattice distortion.24-26 The photoluminescence (PL) intensity of monolayer MoS2 film on the 285 nm-SiO2-Si substrate is ~ 6.6 times higher than that on Si substrate (Figure S2). The microspheres serve as high quality WGM microcavities (intrinsic Q-factor ~ 108) with low absorption coefficient at both excitation (532 nm) and emission spectra range.27 Moreover, the SiO2 microsphere cavity is an optical lens


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and focuses excitation energy locally (c. a. ~ 0.5 × 0.5 μm2) at its interface with MoS2/substrate in the transverse directions, enlarging the spatial overlap between the excitation area and the gain area (Figure 1b). Such configuration is expected to improve the lasing performance in three aspects: 1) leading to a larger spontaneous emission efficiency; 2) enhancing excitation density and efficiency to generate higher local density of carriers; 3) compressing the carrier scattering and thermal fluctuation caused by MoS2 that are not coupled with cavity.3 Furthermore, due to the relative higher screening of the SiO2 microsphere compared with atmosphere, the MoS2 beneath the microspheres has a smaller band gap compared with its surrounding monolayer MoS2, which further increases the carrier localization and therefore enhances effective gain in the MoS2cavity coupling area.28, 29 The optical image of the MoS2/microsphere arrays (see Figure 1c), shows excellent reproducibility and controllability of our MoS2/microsphere cavity lasers.1-5 The scanning tuning microscopy (STM) images of CVD-grown MoS2 on highly oriented pyrolytic graphite (HOPG) substrate are shown in Figure 1d. Due to the lattice mismatch between MoS2 (a1 = 0.312 nm) and graphite (a2 = 0.246 nm), hexagonal Moiré patterns with a period of ~ 1.06 ± 0.05 nm are observed over large area (as marked by a rhombus, the corresponding lattice diagram is shown in Figure S3), suggests that the CVD-grown monolayer MoS2 samples have low density of defect states.30 The microsphere shows clean and smooth surface according to scanning electron microscopy (SEM) image as displayed in Figure 1e. The frequency difference between the Raman peaks 384.7 cm-1 (in-plane E2g1 mode) and 405.2 cm-1 (out-of-plane A1g mode) is 20.5 cm−1 for both MoS2/microsphere and its surrounding bare MoS2 (Figure S4), showing that the MoS2 is indeed monolayer and the strain effect could be ignored.31 The PL intensities of astransferred CVD-grown MoS2 are almost the same as the exfoliated ones (Figure S5); two main emission peaks at ~ 611 and 662 nm are observed, which can be assigned to spontaneous


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emission of B-exciton (BX) and A-exciton (AX) recombination of monolayer MoS2, respectively.32, 33 The PL peak position and intensity are uniform over the whole sample region (Figure S5). The PL peak position of MoS2 exhibit slight variation on different areas of SiO2 substrate which may be owing to the change of local environment, sample defects and strain caused in the growth and transfer processes (Figure S6). Both the STM results and the PL spectroscopy confirm that we can preserve the high crystal quality and stability even after the wet transfer processes. The spontaneous emission of MoS2/microsphere laser devices are investigated by optical excitation with 532 nm CW laser at room temperature. The excitation power density is ~ 980 W/cm2. The spontaneous emission intensity of MoS2/microsphere (red) is greater than that of bare monolayer MoS2 (olive), suggesting the lensing effect and increased pumping efficiency of microsphere (Figure 1f).3 As discussed above, the PL peak of the MoS2 beneath microspheres is red shifted by ~ 30 meV compared with its surrounding bare monolayer MoS2, suggesting a smaller bandgap. Although the PL peaks of monolayer MoS2 on the different area is not the same, for a fixed monolayer MoS2 sample, the PL peak center is lower when the MoS2 locates beneath microsphere with energy red shift of 14 – 30 meV (Figure S6). The redshifts may be attributed to the screening and strain effect caused by the microspheres.28, 29, 34, 35 It could provide additional in-plane carrier localization besides the intrinsic out-of-plane confinement in 2D materials (Figure S7) and therefore optical gain. A series of strong oscillation peaks appear above the AX and BX emission peaks of monolayer MoS2, which can be assigned to the optical feedback effect of the WGM cavity.36, 37


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Figure 1. Design of the MoS2/microsphere WGM cavities. (a) Schematic of the SiO2 microsphere cavity on top of monolayer MoS2. The red light represents the laser emission from MoS2/microsphere. (b) The principle diagram of MoS2/microsphere laser operation that results in the highly localized carriers and the large coupling between the gain medium and the optical cavity modes. (c) The optical image of MoS2/microsphere arrays. Scale bar is 15 μm. (d) The Moiré-scale scanning tunneling microscopy (STM) image of MoS2 transferred on highly oriented pyrolytic graphite (HOPG) substrate (VT = – 0.236 V, IT = 5.90 nA), Scale bar is 4 nm. Inset: magnified view of the STM image, Scale bar is 0.8 nm. The arrows lie along the Moiré pattern (blue) and the S atoms (green) of MoS2 respectively. Rhombus indicates the unit cell for the Moiré pattern with a period of ~ 1.06 ± 0.05 nm. (e) The SEM image of single microsphere in arrays with smooth and clean surface. Scale bar is 2 μm. (f) PL spectra of MoS2/microsphere (red) and CVD-grown monolayer MoS2 (olive) on SiO2-Si substrate at room temperature using the 532 nm as excitation source (excitation power density of 980 W/cm2). To clearly display the oscillation peaks of MoS2/microsphere laser devices, the spontaneous emission background is subtracted as shown in Figure 2a. These WGM resonant peaks could be classified into two groups based on the relative intensity and wavelength, as indicated by black and blue lines.38 Figure 2b displays the polarization dependent PL spectroscopy taken from the MoS2/microsphere in reflective symmetry. The detailed schematic diagram of polarization-


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resolved PL spectroscopy can be found in Figure 2c, the z-direction represents the incidence direction of the laser (Figure S8). The PL intensities of the resonant modes with verticallypolarization (x-direction) are much larger than that of horizontally-polarization (y-direction), which suggests that the optical resonant modes possess the same polarization attributed to transverse magnetic modes (TM). According to the mode simulation (Figure 2d-e), the strong and weak modes are attributed to the first-order TM modes (TM1) and second-order TM modes (TM2), respectively.39 The relative intensity ratios of TM1 and TM2 in unpolarized and vertically polarized condition are different owing to the distinct polarization distribution of the two resonant modes. The FWHM of the higher order TM2 modes are larger than the adjacent TM1 modes due to increasing propagation losses.40 In WGM cavities, the free spectral range (FSR) of the modes, or the spacing between two adjacent resonant modes, can be written as FSR=λ2/(πDneff), where λ is the resonant wavelength, neff is the effective refractive index of the microsphere.41 For a fixed microsphere, FSR is proportional to the inverse of wavelength. Using D = 9.7 μm, neff = 1.455 (λ = 658 nm) and 1.463 (λ = 664 nm), FSRs are calculated to be 9.8 and 9.9 nm, respectively, which is in good agreement with the experimental values (10.0 and 10.1 nm, Figure S9a). The FSR of a typical resonant mode, i. e. TM161 modes, as a α/D (where α is a constant) function of diameters of microsphere cavities (olive, Figure S9b), supporting that the oscillation modes are attributed to WGMs.42 In the microsphere cavities, radiation, absorption and scattering loss are the main sources of the optical losses (see SI Section-I for details). Since both the scattering and radiation losses are inversely proportional to the diameter, Q-factor is higher for larger microspheres, which agrees well with the experimental result (red, Figure S9b).27 Meanwhile, the FWHMs of lasing modes significantly decrease from 1.66 (2.05) to 0.30 (0.45) nm for TM161 (TM256 ) when the resonant modes decrease from 750 to 560 nm. This trend can be associated with the reduction of radiation and absorption losses at shorter wavelength. It


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also suggests that the scattering loss is greatly suppressed in our 2D lasers, as this loss tends to increase and thus broaden the resonant modes at shorter wavelength.

Figure 2. Resonant modes analysis of the MoS2/microsphere WGM cavities. (a) PL spectra of MoS2/microsphere after subtracting the background spontaneous emission of MoS2. WGM peaks are classified into two groups by the calculated WGM positions, as indicated by TM1 (black) and TM2 (blue) lines. (b) Plot of the unpolarized, vertically (along x-direction) and horizontally (along y-direction) polarized lasing spectra of MoS2/microsphere cavity. (c) Schematic diagram of polarization resolved PL spectroscopy for MoS2/microsphere. (d-e) Mode simulation of the electric field distribution patterns of TM256 mode at 658 nm (angular mode number of 56) and TM161 at 664 nm (angular mode number of 61). Our lasing devices exhibit an excitation-intensity-dependent PL emission and a linewidth narrowing effect at room temperature. Figure 3a shows the PL spectra from MoS2/microsphere as a function of power density in the range from 70 to 1080 W/cm2 under 532 nm CW laser excitation. A 150 g/mm grating with relatively higher collection efficiency is adopted to reduce data acquisition time and experimental uncertainty. PL spectroscopy is dominated by spontaneous emission with large FWHMs far below the lasing threshold. When the power


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intensity approaches lasing threshold, the intensities of WGMs increase quickly. As shown in Figure 3b, the integrated PL intensities of TM161 (~ 664 nm, red) mode undergo linear and superlinear increase processes with power density increasing, while in contrast, the PL intensities of bare monolayer MoS2 (olive) is always linear to the excitation intensity. The experimental data of TM161 mode are fitted by lasing rate equations (SI, Section-III) with the spontaneous emission factor (β) as a fitting parameter. The best fitting to the experimental data of TM161 modes is obtained when β = 0.22, which is quite close to the values reported previously (i.e., 0.19 – 0.77).15

The transition kink between the spontaneous emission regime and the stimulated emission

smears leads to a lasing threshold of ~ 380 W/cm2. The superlinear response in the light-light curve is a strong signature of lasing behavior for TM161 resonant modes.43 The second-order (i.e.TM256 ) modes also show the superlinear response and lasing behaviors as shown in Figure S10, leading to a lasing threshold of ~ 765 W/cm2. At the threshold, the FWHMs of the TM161 and TM256 modes exhibit an immediate decrease from 1.41 to 1.14 nm and 2.32 to 1.85 nm, respectively (Figure 3c), which is the so-called linewidth narrowing effect.44 Measurements of the resonant modes using a different grating of 1800 g/mm lead to similar results (Figure 3d, Figure S11), where FWHMs decrease from 0.90 to 0.71 nm for TM161 mode (2.12 to 1.76 nm for TM256 mode). These behaviors not only confirm the occurrence of lasing effect in our devices, but also suggest the feasibility of stable and low threshold lasing. We also investigated the statistics of lasing from 17 MoS2/microspheres devices (Figure 3e). All the devices have shown lasing effects with thresholds around 400 – 500 W/cm2, suggesting good reproducibility.


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b 2

TM56

290K

c TM

2.4

1 61

@150 g/mm

FWHM (nm)

TM161

PL Intensity (a.u.)

a

2.1

MoS2

β=1

2

TM56

1.8 1.5 1

TM61

1.2

β = 0.22 ± 0.02 100

1080 W/cm 2

810 W/cm 2

TM161

PL Intensity (a.u.)

d

200 500 1000 22000 Power Density (W/cm )

0.71 nm 640 W/cm2

380 W/cm 2 70 W/cm2 620 640 660 680 700 Wavelength (nm)

@1800 g/mm

0.9

0

300 600 900 2 Power Density (W/cm )

e

Counts (a.u.)

PL Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.90 nm 2 380 W/cm 662

663 664 665 Wavelength (nm)

300 450 600 750 900 2 Power Density (W/cm )

Figure 3. Room temperature lasing of MoS2/microsphere WGM cavities. (a) PL spectra as a function of excitation power density at room temperature using a grating of 150 g/mm with a spectral resolution of 0.9 nm. (b) Plot of the integrated intensity as a function of excitation power for TM161 mode of MoS2/microsphere (red) and its surrounding monolayer MoS2 background (blue) in log-scale. The emission intensity of MoS2 background keeps linear to the excitation power density. Red lines are the simulated curves using the laser rate equation with different βfactors. β = 0.22 is the best fit to the experimental data and gives a threshold pump intensity of ∼ 380W/cm2. (c) FWHMs of typical TM161 (red) and TM256 (blue) modes versus excitation power density. The groove density of used grating is 150 g/mm. Linewidth narrowing of the lasing mode is observed while the excitation power exceeds the lasing threshold. (d) The Lorentz fit for a typical TM161 mode at threshold (red) and above lasing threshold (blue) using a grating of 1800 g/mm. (e) The statistics distribution of lasing threshold for 17 MoS2/microsphere cavities. High-temperature performance up to 400 K is also investigated for potential device application. Figure 4a shows the 2D pseudo-color image of the laser emission spectra of MoS2/microsphere cavity under different power density at 400 K. A series of narrow peaks appear with power density increasing. The corresponding PL spectra of MoS2/microsphere cavity are shown in Figure S12. When the pumping density is higher than 580 W/cm2, TM161 lasing is


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realized on the basis of super-linearly-increasing emission intensity and linewidth narrowing effect (Figure 4b). Using the rate equation, the β = 0.40 gives the best fitting to the experimental data of TM161 modes. Figure 4c shows the lasing threshold and FWHM of TM161 resonant mode as a function of temperature. As the temperature increases from 77 K (Figure S13) to 400 K, the lasing threshold (FWHM) increases from 32 to 580 W/cm2 (from 0.9 to 1.62 nm), respectively. b

c

400K

600 500 400

TM161

β=1

β = 0.40 ± 0.01 630

660 690 720 Wavelength (nm)

750

50

1.5 1.4 1.3

300 200

FWHM (nm)

700

1.6

1.8 PL Intensity (a.u.)

PL Intensity (a.u.)

800

1.6 1.4 1.2 1.0

1.2

100 200 400 900 2 1800 Power Density (W/cm )

FWHM (nm)

a Power Density (W/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8 100

200 300 400 Temperature (K)

Figure 4. Lasing of MoS2/microsphere WGM cavities at 400 K. (a) 2D pseudo-color plot of the laser emission spectra under different power density showing a series of narrow peaks. (b) Plot of the integrated intensity (red) and FWHM (blue) of TM161 mode as a function of excitation power density in log-scale. The “S” response and linewidth narrowing effect of the lasing mode is observed with a threshold of 580 W/cm2. Red lines are the simulated curves using the laser rate equation with different β-factors. β = 0.40 is the best fit to the lasing data. (c) The thresholds (red) and FWHMs (blue) of TM161 mode as a function of temperature (77 – 400 K). The characteristic temperature is 203 K extracted from an exponential fit (red solid line). This temperature dependent lasing threshold can be well fitted by an exponential function with a characteristic temperature of 203 K, which is higher than that of other semiconductor materials such as ZnO (67 – 138 K), GaAs (133 K) and InGaN/GaN (162 K).45, 46 The lasing characteristic temperature is primarily determined by the thermal stability of spontaneous emission yield and carrier lifetime. The high characteristic temperature of our MoS2 lasing devices could be attributed to the temperature-insensitive radiative lifetime of monolayer MoS2.47 Table 1 presents types of existing lasers based on TMDCs. So far, lasing effects have been realized in four types of TMDCs prepared by mechanical exfoliation: WS2, MoS2, WSe2, and


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MoTe2. Despite of the WS2 laser that is driven by femto-second pulsed laser at 10 K with a power density of 5 – 8 MW/cm2, the other TMDC-based lasers are operated under CW excitation and room temperature with threshold of ~ 0.44 – 6.66 W/cm2 (corresponding to 5 nW – 97 μW) respectively. The as-demonstrated MoS2/microsphere laser is driven by CW optically pump with threshold of 32 – 580 W/cm2 from 77 to 400K (corresponding to 1 – 18 μW), which is comparable with previous work (~ 5 μW).3 The geometry of microsphere cavity leads to 1) the large spatial overlap between gain and excitation area, and 2) the decrease of optical band gap of the gain materials beneath the cavities, which consequently enhance the optical gain coefficient and spontaneous emission factors. With the high quality and special design with microspheres, we are able to study the lasing mechanism from loss and gain aspects. Our results provide detailed insight to understand whether such ultimately thin material is capable to provide sufficient optical gain to overcome the losses. The intrinsic optical losses, including scattering, radiation and absorption losses, are related to the roughness and diameter of cavity, resonant wavelength and dielectric environments, etc. The optical gain and amplification are accomplished by stimulated emission process between two energy levels and enlarged by the increasing of carrier density. The modal gain threshold Γgth (Γ is modal confinement factor, gth is bulk gain) is the gain value which equals total optical losses, δ c = 2π neff ΓQλ (SI, section IV).3 The Q-factors for the first (second) order TM modes are ~ 740 @ 665 nm and ~ 1900 @ 620 nm (411 to 1370) respectively. The mode confinement factor in as-proposed cavity is Γ ≈ 0.0015, which is in the same order of magnitude as that in previous work.3 After adopting the estimated values of Γ and gth, optical gain threshold of the MoS2/microsphere laser device at room temperature is 1.2×105 cm−1 and 2.3×105 cm−1 for TM161 and TM256 modes, respectively. Eventually, the carrier density Nth can be estimated as 1.19×1020 cm−3. The excitation power required to create such a high carrier concentration is 3593 W/cm2 (SI, Section-V), which is much larger than the


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experimentally determined power density of 380 W/cm2, indicating an enhanced optical gain coefficient by the designed use of microsphere cavities. Table 1. Types of existed lasers based on TMDCs and their lasing performance. Materials

Cavity

Temperature

Threshold 2

Linewidth

Q

Excitation source

Reference [2]

1L WS2

microdisk cavity

10 K

5-8 MW/cm

0.24 nm

2604

473 nm, pulse

1L WSe2

photonic crystal cavity

130 K 80 K

27 nW (1 W/cm2) –

– 0.3 nm

– 2465

632 nm, CW

1L MoTe2

silicon nanobeam cavity

RT

97 μW (6.6 W/cm2)

0.202 nm

5603

633 nm, CW

[4]

4L MoS2

coupled microsphere and microdisk cavity

RT

5 μW

High-Temperature Continuous-Wave Pumped Lasing from Large-Area

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