Broadband Nonlinear Photonics in Zeolite‐Based Selenium Chains

As one of the 1D nanomaterials, Se nanochains have been widely used in photoelectric devices, such as photodetectors, battery devices, and optical sensors due to their excellent photoelectric properties. Among them, ultrafast photonics has attracted much attention due to its wide applications; for instance, environmental monitoring, material processing, biomedical imaging, nonlinear optical generation, and free‐space optical communication. Herein, Se nanochains are fabricated by trapping Se species in the circular 1D channel of AlPO4‐5 (AFI) crystal, and the characteristics of Se@AFI are systemically investigated. In addition, the AFI single crystal loaded with Se nanochains in the channel is successfully prepared as a saturable absorber in the fiber laser. The mode‐locking pulse at 1.5 and 1 μm with the repetition rate of 13.16 and 14.74 MHz can be obtained by integrating the Se@AFI composite material in the cavity. The signal‐to‐noise ratios are both over 70 dB, suggesting the high operational stability of the pulses. These results indicate that Se@AFI has good potential as saturable absorber material with a broad wavelength range for ultrafast pulsed fiber lasers. Our results will also provide a new idea for designing air‐stable ultrafast photonic devices and other composite materials in various fields.

absorption-wavelength tunable range than SESAM. However, the development of original nanomaterials is also hindered by their inherent properties, such as the complex preparation process [34] and the instability of materials. [35] Thus, investigating the saturated absorption characteristics of Se nanochains in the fiber laser is very interesting since the confined space effect of zeolites makes Se chains stable in the air and shows unique optical properties.
In this article, Se@AFI composite material was fabricated by using the thermal diffusion method and applied in the ultrafast fiber laser as a new saturable absorber (SA). By inserting the Se@AFI into the erbium-doped fiber laser (EDFL) cavity and ytterbiumdoped fiber laser (YDFL), ultrafast pulses can be realized in EDFL and YDFL. The Se@AFI SA has many advantages, such as a relatively high optical damage threshold, cost-effectiveness, broadband absorption, and operation stability, indicating the great potential application of Se@AFI SA in advanced photonics. Meanwhile, we believe that the fabrication method of Se@AFI will provide readers with a new idea to prepare composite materials in various fields. In the future, we will further control the proportion of Se with different structures in the channels by adjusting the synthesis temperature and studying the nonlinear optical properties of Se with different structures.

Synthesis of Se@AFI
The AFI single crystals were synthesized through the hydrothermal method in our experiments, which has been repeated several times to insure the consistency of the AFI crystal. The synthesis steps were as follows: 1) dissolved the aluminum triisopropoxide in deionized water and stirred for 12 h; 2) added the diluted orthophosphoric acid dropwise into the Teflon beaker and stirred for 3 h; 3) slowly added the TPA to the gel and stirred for 3 h; and 4) slowly added the diluted HF solution to the homogeneous gel. Finally, the composition of the beginning gel was 1.6Al 2 O 3 :1.2P 2 O 5 :1.2TPA:1.0HF:600H 2 O. After 1 h, the gel was sealed into the autoclaves with Teflon-lined stainless steel and heated to 170°C for 20 h. Finally, the products were dried at under 80°C for 12 h after washing with deionized water. Figure 1a shows the synthesis process of the Se@AFI composite materials. Among all zeolite structures, the AFI crystal is known for its circular 1D channel structure with 7.3 Å Â 7.3 Å of pore diameter along the c-axis (see Figure 1c). To remove the TPA molecules in the channels, the AFI crystals were calcined at 600°C under an air atmosphere for 72 h, emptying the channels. Then, pure Se powder and the calcined AFI crystals were packaged into a quartz glass tube together using the vacuum tube sealing machine at a vacuum of 10 À1 Pa. Se species were introduced into the channels by physical diffusion at 400°C for 10 h. Hundreds of the as-synthesized samples show the same color and dopant density after thermal diffusion.
X-ray powder diffraction (XRD, PANalytical Empyrean) was used to identify the structure of synthesized materials at room temperature. Optical microscopy and scanning electron microscopy (SEM, FlexSEM1000) were used to observe the topographical properties of the samples. www.advancedsciencenews.com www.adpr-journal.com

Experimental Setup of Fiber Lasers
The laser schematic diagram of EDFL and YDFL is shown in Figure 4a. The pump source was a 976 nm laser diode and was combined into the cavity by using a wavelength division multiplexer coupler (980/1550 nm in EDFL and 980/1060 nm in YDFL). The unidirectional operation in the cavity was forced by a polarization-independent optical isolator (ISO) and the polarization state was changed through the polarization controller (PC). By using a thin copper wire under an optical microscope, one single Se@AFI crystal was put on a fiber optic jumper and then integrated into the laser cavity. The optical coupler (OC) could extract 10% of the final pulses. The gain medium adopted a piece of 1.5 m long erbium-doped fiber (EDF, Nufern SM-ESF-HI-HP, cutoff wavelength: 1400 AE 60 nm, 55 AE 5 dB m À1 absorption at 1.53 μm) and 1.4 m long ytterbium-doped fiber (YDF, Coherent SM-YSF-HI-HP, cutoff wavelength: 860 AE 70 nm, 250 dB m À1 absorption at 976 nm), respectively. In the EDFL and YDFL, the length of other standard single-mode fibers was about 19 and 12 m, respectively. And the whole laser cavity lengths of EDFL and YDFL were about 20 and 14 m, respectively. An optical spectrum analyzer (YOKOGAWA, AQ6370D) was used to analyze the spectrum of the output lasers, and an autocorrelator (APE, Pulsecheck SM1200) was used to analyze the pulse width. The pulse envelope and RF spectrum were recorded by a 12.5 GHz photodetector (Newport, 818-BB-35F) with a 4 GHz bandwidth oscilloscope (WaveRunner 9000, Teledyne Lecroy) and a 26.5 GHz RF spectrum analyzer (N9020B, Keysight), respectively. Figure 2a illustrates an optical micrograph of Se@AFI SA, and the optical micrograph (Figure 2b) shows the high optical transparency of the AFI crystal. Figure 2c shows the corresponding SEM image of Se@AFI.  Figure 2d. All peaks match well with the characteristic peaks of the AFI crystal, [36] confirming that the synthesis process will not destroy the structure of the AFI crystal. The Se@AFI single crystal has a broadband absorption from 800 to 1600 nm, as shown in Figure 2f. Figure 2e shows the Raman spectrum (Horiba Jobin Yvon LabRan HR800) of Se@AFI composite materials under a 514.5 nm laser excitation source at room temperature. There are three distinct peaks in the Raman spectrum, located at 236, 258, and 267 cm À1 . The peak at 236 cm À1 is caused by the symmetric bond-stretching mode of trigonal crystalline-like Se (t-Se), and the remaining two peaks are caused by the symmetric bond-stretching mode of the Se single helix (258 cm À1 ) and isolated Se 8 rings (267 cm À1 ), respectively. [37] The t-Se is formed inside the mesopores due to the absence of interchannel walls of AlPO 4 -5 single crystals. [37] The main structure of Se molecules in the channel of AFI is Se single helix and Se 8 rings. According to the Ramman mapping of Se@AFI in Figure 3, the Se molecules are uniformly distributed in the channel of  the AFI crystal. The concentration of different elements in Se@AFI has been detected by energy dispersive spectrometer, as shown in Table 1.

Material Characterizations
The saturated absorption characteristics of the Se@AFI composite materials were investigated by using a balanced twin detector measurement system, as shown in Figure 4b. The homemade fiber lasers were used as the pump sources in the 1.56 and 1.03 μm measurement systems, respectively. Through a 50:50 OC, the pump source was spilled into two fibers, one fiber detects the transmission power of the SA and another fiber serves as a reference. By increasing the pump power gradually, the data curve of the transmission ratio was obtained. According to a simple two-level SA model with the formula, [38][39][40][41] α(I) = α 0 Â (1 þ I/ I sat ) þ α ns , [α(I) is the intensity-dependent absorption coefficient, α 0 is the saturable absorption, I is the input intensity, I sat is the saturation power intensity, α ns is the nonsaturable absorption]. The data of saturated absorption characteristics could be wellfitted. [42] The results show that the modulation depth is 7.7% and the saturable intensity is 0.17 MW cm À2 at 1560 nm ( Figure 4c). Figure 4d shows that the modulation depth is 3.5% and the saturable intensity is 0.86 MW cm À2 at 1030 nm. The normalized nonabsorption loss is larger, compared with other low-dimensional materials. [43][44][45] In fiber lasers, there is a relatively large single roundtrip gain coefficient. [43,45] Thus, the large nonabsorption can be tolerable in the cavity. In our case, the large nonsaturable loss is mainly caused by the scattering and refraction of the AFI crystal. Thus, crystals with smooth surfaces and high optical transparency are significant for experiments. These results clearly illustrate that the Se@AFI SA has great potential for short pulse generation in a broad wavelength band.

Results and Discussion
We constructed EDFL and YDFL to demonstrate that the Se@AFI can be applied in fiber laser to generate an ultrafast pulse, as shown in Figure 4a. Before we integrated the Se@AFI into the cavity, the phenomenon of laser modulation could not be     observed by adjusting the PC in a wide range of pump power. Therefore, the phase self-modulation in the laser cavity cannot cause an ultrafast pulse. When we integrated Se@AFI into the EDFL system and carefully adjusted the PC, the mode-locking pulse was obtained at the pump power of 106 mW. Figure 5a shows the output pulse trains of the mode-locking EDFL and the repetition rate is 13.16 MHz (period τ = 76 ns). Figure 5b shows a typical spectrum of the dissipative soliton mode-locking and the central wavelength is 1564 nm with a 3 dB spectrum bandwidth of 3.62 nm. The pulse width is about 1.427 ps, assuming a sech 2 pulse profile, as shown in Figure 5c. The time-bandwidth product is about 0.634, suggesting a large chirp in the cavity. Therefore, the measured pulse width after magnification is broadened to 1.427 ps. The radiofrequency (RF) spectrum of the EDFL is shown in Figure 5d. The signal-to-noise ratio is about 70 dB, indicating excellent pulse stability which is higher than other mode-locking pulses based on nanomaterials SA. [46][47][48] When we integrated a bare AFI crystal into the EDFL system, no pulse modulation phenomenon could be observed by adjusting the pump power and PC. It confirmed that bare AFI crystal in the cavity cannot cause the mode-locking pulse, and the emission spectrum is shown in Figure 5e. Figure 6a shows the output spectrum over 24 h under the same experimental conditions to indicate the long-term stability of the EDFL. The 3 dB bandwidth and the central wavelength of the spectrum approximately remain unchanged, suggesting the mode-locked pulse has good long-term stability. The output power of the EDFL is shown in Figure 6b. When the pump power is increased from 95 to 266 mW, the output power increases from 0.45 to 2.5 mW. The results show that the slope efficiency is 1.3%. To evaluate the damage threshold of the Se@AFI composite material, the pump power was further increased to 600 mW and maintained for 5 h. Then, the stable ultrafast pulse can be generated again when we decreased the pump power from 600 to 0 mW, indicating the high optical damage threshold of Se@AFI. Table 2 shows a detailed comparison between Se@AFI and other material-based SAs.
Similarly, when we inserted the Se@AFI SA into the YDFL cavity, the noise-like mode-locked pulse (NLPs) of YDFL could be obtained over 250 mW of the pump power. The NLPs can be maintained from 250 to 750 mW pump power (limited by the pump source), suggesting the high damage threshold of Se@AFI at 1043 nm. Figure 7a shows the output pulse trains of the NLPs, and the repetition rate is 14.74 MHz (period τ = 67 ns). As shown in Figure 7b, the central wavelength is 1043 nm with a 3 dB bandwidth of 13.89 nm at a certain pump power of 500 mW. Figure 7c shows a wide base with a narrow coherent spike, which confirms that the soliton state is a typical NLP regime. The pulse width of the noise-like spike is 188 fs. Figure 7d shows the RF spectrum of the NLPs in YDFL. The signal-to-noise ratio is about 73 dB which is higher than other NLPs based on nanomaterials SA, indicating excellent stability of NLPs. Similarly, the mode-locked pulse could not be obtained when we used a bare AFI crystal instead of Se@AFI under the same experiment condition, and the emission spectrum is shown in Figure 7e. Figure 8a shows the output spectrum over 24 h under 500 mW of pump power to show the long-term stability of the NLPs in YDFL. The 3 dB bandwidth and the central wavelength of the spectrum approximately remain unchanged, indicating the NLPs in YDFL have good long-term stability. However, the slope efficiency shown in Figure 8b is only 1.1% due to a large normalized nonabsorption loss caused by the scattering and refraction of AFI crystal. A detailed comparison between Se@AFI SA and material-based SAs applied in NLPs of YDFL is summarized in Table 3.

Conclusion
In summary, Se@AFI SAs were fabricated by incorporating Se species into the channel of the AFI crystal at the temperature of 400°C. The nonlinear optical property and applications for ultrafast fiber lasers of the Se@AFI composite were investigated. The results show that Se@AFI as a new broadband SA has great potential in the generation of the ultrafast pulse. The synthesis process has good repeatability, which is beneficial to the wide application of Se@AFI. In EDFL, the central wavelength of the mode-locking pulse is 1563 nm with a repetition rate of 13.16 MHz and a pulse width of 1.427 ps. In YDFL, the central wavelength of   Meanwhile, the damage threshold of Se@AFI SA is much higher than other low-dimensional materials film. Therefore, we believe that Se@AFI has great potential applications in the field of advanced photonics research. In addition, the fabrication process of Se@AFI SA could provide a new idea for readers to synthesize other composite materials in various fields.