# Recent Progress in Synthesis and Photonic Applications of Two-Dimensional Bismuthene
## 4 Photonic Applications Based on 2D Bismuth
### Nonlinear Photonic Devices  
Recently, bismuthene has received significant attention for its potential use in saturable absorbers thanks to its unique electron mobility properties, spin-orbit coupling, and incredible stability in ambient conditions [20; 23; 64]. In 2018, [PERSON] et al. first demonstrated a sub-200 fs soliton mode-locked fiber laser using bismuthene as the saturable absorber (Figure 3a) [16]. The self-started mode-locking laser process only occurred when the bismuthene-saturable absorber was located in the ring laser cavity. The 1561 nm stable soliton pulses with width as low as 193 fs were generated when the pump power increased from 100 mW to 350 mW. As shown in Figure 3b, symmetrically distributed Kelly sidebands can be observed in the output optical spectrum, which reveals the soliton state of the pulse. According to the 2 us-long soliton pulse trace on the oscilloscope (Figure 3c), the soliton pulse with a period of 113.4 ns and a frequency of 8.85 MHz is propagated in the laser cavity. Furthermore, the autocorrelation trace of the soliton pulse was also measured in the picosecond time scale. The extracted bandwidth is 193 fs, and the corresponding time-bandwidth product (TBP) is 0.342. Importantly, this soliton is almost perfect because the TBP of the device is similar to the theoretical limit (0.315). In addition, according to the radio frequency (RF) spectrum of the device, the peak of the soliton pulse is located at the repetition rate of 8.85 MHz with a signal-to-noise ratio of \(\sim\)55 dB, which demonstrates the pulse with high stability. It significant that both the pulse duration time (193 fs) and the output power (5.6 mW) of the device outperformed those of the antimonene microfiber-based laser, attributable to the better nonlinear ability of bismuthene [65]. Additionally, a passively mode-locked ytterbium-doped mode-locked fiber laser was constructed by using bismuthene as the saturable absorber, with modulation depth and saturation intensity of 2.4% and 0.3 MW/cm\({}^{2}\), respectively [23]. When the pump power was set at 122.1 mW, a single-pulse soliton molecule was generated, with a 621.5 fs pulse duration at a central wavelength of 1557.5 nm (Figure 3d), a spectral width of 10.35 nm, and a fundamental cavity repetition of 22.74 MHz (Figure 3e). In addition, by adjusting the pump power, the generated single molecules can react with each other. Owing to the outstanding nonlinear optical properties and semimetal characteristics of bismuthene, two-pulse, eight-pulse, and fourteen-pulse soliton molecules, with both tightly and loosely temporal separation, were observed for the first time. Over the last 5 years, [PERSON] et al. have adopted a bismuthene nanosheet-saturable absorber to achieve harmonic dual-wavelength mode-locked picosecond pulses [66]. As shown in Figure 3f, the modulation depth (7.7%) and saturation intensity (16 MW/cm\({}^{2}\)) of bismuthene nanosheets at 1563 nm are obtained due to the enhanced light modulation ability [67]. The proposed nonlinear fiber laser can generate stable harmonic mode-locked pulses or harmonic dual-wavelength mode-locking pulses by adjusting the pump power. In contrast to the previous research, [PERSON] et al. not only characterized the modulation depth of bismuthene, but also studied the underlying mechanism of nonlinear saturable absorption, which is related to the excited-state dynamics [68]. As depicted in Figure 4a, the relaxation of photogenerated carriers is distributed through three channels, including rapid intraband (-3 ps) carrier-carrier or carrier-phonon scattering, slower interband relaxation (-420 ps), and the slowest trap state-mediated electro-hole recombination. This mechanism indicates the important role of intermediate states and the origin of saturable absorption.