Fig. 1. (a) Schematic of SHG in monolayer WS₂ enhanced by the microsphere. The incident light and SH signal are coupled through a tapered fiber. (b) Measured spectra of the SH signal (red) as well as the pump laser (blue). (c) Schematic of experimental setup for measurement of SHG and transmission spectra. BS, beam splitter; FPC, fiber polarization controller; SP, 950 nm short pass filter; PD, photon detector; WDM, wavelength division multiplexer.

Fig. 2. (a) The CCD image of the microcavity in the x–y plane. We make Raman spectra measurements at positions A and B, and the results are shown in the insets (left). Inset figure (right) shows the simulation of electric field distribution of the microsphere with a monolayer WS₂ on its surface in the y–z plane. The red arrow represents the polarization direction of the electric field. The x-axis is the direction of the light field propagating. The two-dimensional material depicted in the figure is purely schematic, with an actual thickness of less than 1 nm. (b) Transmission spectrum of a microsphere without a material. (c) Transmission spectrum at 1546.2 nm. The Q factor of the mode reaches 1.2 × 10⁶. (d) SH wavelengths versus the corresponding pump wavelengths. The figure shows the measurement results for resonant modes near wavelengths with equal intervals. As the pump wavelength changes, the mode corresponding to the fundamental frequency also changes, causing the SH intensity to fluctuate. However, the SH wavelength remains half of the pump wavelength. Inset: experimental results at a smaller wavelength range. The color of the data point represents the intensity of the doubled frequency signal.

Fig. 3. (a), (b) Measured SH spectra of the cavity-enhanced monolayer WS₂ under CW laser input and pulse laser input, respectively. (c), (d) Measured SH spectra of the monolayer WS₂ under CW laser input and pulse laser input, respectively. (e) Dependence between SH power and input power for the same pump mode at 1542.83 nm. A–C represent different variation processes of SH power. Inset: schematic of the SHG process. The black arrows represent the pump lights. The red Lorentzian shapes represent the pump cavity modes.

Fig. 4. (a) The effect of material thickness on energy distribution. As the thickness increases, the energy becomes more concentrated near the 2D material while the energy inside the cavity decreases. (b) The relationship between Ω and the coupling efficiency of the tapered fiber to the SH signal. Inset: demonstration of the angle (Ω) between the crystal orientations (x, y) and the coordinates (a, b).