# Recent Progress in Synthesis and Photonic Applications of Two-Dimensional Bismuthene
## 4 Photonic Applications Based on 2D Bismuth
### Nonlinear Photonic Devices
In addition, the fabricated mode-locked Tm-doped fiber laser successfully generates a 2030 nm pulse with maximum energy of 6.6 nJ, which is higher than that of thegraphene based device [69], suggesting that bismuthene is favorable for an optical fiber communication application. Apart from generating mode-locked pulses, the bismuthene saturable absorber can also be used as a Q-switch in ultrafast lasers. In 2019, bismuthene was reportedly employed in a passively Q-switched fiber laser at the mid-infrared region (MIR) (Figure 4b) [70]. Owing to the broadband nonlinear optical response of bismuthene from the NIR to the MIR region, a 645 fs Er\({}^{3+}\)-doped fiber laser was demonstrated, with a duration of 1.86 us and a signal-to-noise ratio of 37.8 dB at 2789 nm. Recently, [PERSON] et al. grew a few-layer bismuthene nanosheet using the solution based method and demonstrate a passively Q-switched Nd: GYAP laser at 1300 nm [71]. Compared with the center wavelength of the continuous-wave laser, the passively Q-switched laser shows a blueshift center wavelength because of the insertion loss of the bismuthene saturable absorber (Figure 4c). In addition, it was discovered that the peak power and pulse width of the bismuthene-based laser both outperform those of the passively Q-switched laser using MoSe\({}_{2}\)[72] or ReS\({}_{2}\)[73] as saturable absorbers. In 2019, [PERSON] et al. successfully fabricated bismuthene with a modulation depth as high as 22.1% and a saturation fluency of 21.5 \(\upmu\)J/cm\({}^{2}\)[74]. The 880 nm pumped Nd:BGO passively Q-switched laser can produce a stable laser beam with a repetition rate of up to 223.7 kHz and a duration down to 256 ns (Figure 4d). Table 2 summarizes the main figures of merit (FOMs) of the lasers based on bismuthene and other 2D materials. It can be determined that the bismuthene-based lasers exhibit output power and single pulse energy superior to those of other commonly studied 2D candidates. The outstanding performance of the lasers may be attributed to the excellent nonlinear property of bismuthene, which could pave the way for potential applications such as optical communication and laser materials processing. Figure 3: Schematic diagram and key parameters of the bismuthene–based laser. (**a**) Schematic figure of mode–locked fiber device based on the bismuthene saturable absorber. (**b**) Optical spectrum characteristic of the solution pulse at 250 mW. (**c**) Oscilloscope trace of solution pulse. The inset of the picture shows the autocorrelation trace of the pulse [16]. (**d**) The autocorrelation trace of the single conventional soliton. (**e**) The radio frequency spectrum of the generated single conventional solitons. (**f**) The transmission as a function of the average power of bismuthene nanosheets at 1563 nm [23]. Thanks to its layer-dependent bandgaps, strong light-matter interaction ability, and good air-stability, bismuthene can also be used in designing nonlinear optical signal processing systems. Recently, [PERSON] et al. constructed, for the first time, an all-optical signal processing fiber system based on bismuthene-coated fiber [25]. Thanks to the strong light-matter interaction of bismuthene, the optical fiber system can not only be used as an optical Kerr switch, but it also operates as a four-wave-mixing (FWM) wavelength converter (Figure 4e). In 2017, [PERSON] et al. realized all optical switching based on the spatial cross phase modulation of bismuthene owing to its strong nonlinear optical absorption and the refraction effect [26]. The 532 nm switching laser and 633 nm signal laser were injected into bismuthene to obtain a high modulation depth and all optical switching. According to the generated diffraction rings (Figure 4f), the switched light and signal light were generated simultaneously. It can be determined that the variation of rings is strongly dependent on the switching light.