Study On Tungsten Based Two-Dimensional Transition Metal Dichalcogenides Whispering Gallery Mode Exciton Laser

The whispering gallery mode optical microcavity light field forms a closed and stable propagation mode around the inner wall of the curved cavity under the condition of surface total reflection,which can provide resonance feedback for stimulated emission of the gain medium,and has a large quality factor and mode volume ratio.The coupling of cavity photons with semiconductor exciton states can not only achieve exciton lasers with small size,low threshold,narrow linewidth,and low power consumption,but also expand the research of traditional light matter coupling systems,bringing some novel freedom degrees of regulation and physical properties.It is a frontier topic in the field of micro and nano photonics devices.Compared with exciton states in bulk materials,the exciton binding energy in quantized low dimensional systems is much larger and the exciton effect is stronger.Monoatomic layer transition metal dichalcogenides,as a quasi two-dimensional system,have exciton binding energy of up to several hundreds of me V,about an order of magnitude higher than traditional quantum well structures.When combined with whispering gallery mode microcavities,they can achieve coupling of cavity photons and excitons at higher temperatures.In this paper,tungsten based transition metal dichalcogenides（monolayer WS₂,monolayer WSe₂）were used as gain medium,combined with SiO₂ microsphere cavities,to achieve low threshold exciton laser output in different wavelength ranges.On this basis,the effects of microsphere cavity diameter,substrate thickness,and other factors on exciton laser mode were further investigated.The main research works and achievements of this paper are as follows:1.Study on optically pumped monolayer WS₂-SiO₂ microsphere cavity exciton laser at room temperature.The monolayer WS₂ has a very large exciton binding energy（380 me V）,which can be combined with a WGM microcavity to achieve a low threshold WGM exciton laser in the visible light band.In the experiment,the simulation software COMSOL was used to simulate a spherical WGM microcavity based on the finite element method to determine the distribution of the electric field mode in the microcavity of a certain microsphere diameter.Based on theory and simulation,the monolayer WS₂-SiO₂ microsphere exciton laser was built,and a SiO₂ sphere with 9.2μm diameter was used to realize monolayer WS₂-SiO₂ WGM exciton laser in the range of 590 nm to 670 nm at room temperature,with laser threshold～32μW.The laser linedwidth is 1.5 nm.Compared with the monolayer WSe₂-SiO₂ microsphere cavity laser with similar diameter in the lasing band of 720 nm～800 nm,the monolayer WS₂-SiO₂ microsphere cavity exciton laser have higher quality factor and threshold.2.Study on mode tuning of optically pumped monolayer WSe₂-SiO₂ microsphere exciton lasers.Changes of the microsphere cavity diameter can lead to changes of the intrinsic radiation loss and mode distribution in the cavity,thereby adjusting the exciton laser mode characteristics.In the experiment,the diameter of 10μm,11.3μm,12.4μm was selected.The room temperature exciton laser emission at 720 nm～800 nm was realized in a SiO₂ microsphere cavity under continuous wave pumping at 532 nm.The linewidths of the main lasing modes of three diameter lasers are 3 nm,2.3 nm,and 1.4nm,respectively.The free spectral ranges are 12.2 nm,11 nm,and 9.5 nm,respectively.The thresholds of three diameter lasers are 10μW,3μW and 2μW,respectively.It can be seen that with the increase of the diameter of the microsphere cavity,the free spectral range and the laser linewidth of the laser mode show a decreasing trend,and the quality factor shows a gradually increasing trend.3.Study the effect of SiO₂ layer substrate thickness variation on the mode of the monolayer WSe₂-SiO₂ microsphere cavity exciton laser.Changes of the silicon dioxide dielectric layer thickness will affect the degree of optical mode leakage at the contact points between the microcavity and the substrate.We used the SiO₂ microspheres with a diameter of 12μm,combined with the monolayer of WSe₂ on the thickness of 285nm,300 nm,and 500 nm SiO₂ layer,to explore the effect of substrate thickness on the limitation of optical mode and the performance of the WSe₂-SiO₂ laser.The results show the quality factor of the microcavity of the 300 nm substrate is the highest,and the laser threshold and mode linewidth are lower.As the substrate thickness increases to 500 nm,the quality factor of the microcavity decreases and ultimately no laser is produced.