## 4 Experimental Results and Discussion  
### Nonlinear Optical Feature  
To comprehensively characterize the nonlinear optical properties of the PSM-SA, a dual-arm nonlinear testing system was utilized. In Figure 3a, our test configuration featured  
Figure 1: Schematic diagram of the preparation process for the photonic crystals of PSM-SA.  
Figure 2: Characterization of PSM-SA device: (**a**) Scanning electron microscopy of PSM-SA; (**b**) Raman spectra of PSM-SA.  
a meticulously crafted self-made 1550 nm femtosecond laser, finely tuned to ensure the precision of our measurements. This laser setup was pivotal to our research, providing the optical stimulus required for probing the PSM-SA's response. The pump source for the laser is composed of a semiconductor laser with a central wavelength of 976 nm. The generated pump light from the two lasers was expertly coupled using a 2 \(\times\) 1 wavelength division multiplexer (WDM) coupler. This coupled light was then directed into a 10 m long erbium-doped fiber (EDF) acting as the gain medium. Within the EDF, it underwent amplification to the desired level. Subsequently, residual pump light was filtered out using an isolator (ISO) and then equally divided into two channels by a 5:5 optical coupler (OC). In this experimental setup, one channel of light was directed toward the PSM-SA through a circulator (CIR) and then reflected back along the fiber. Meanwhile, the reflected power and the standard power of the other channel were precisely measured using a power meter. The ratio of the reflected power to the standard power provided the essential data for assessing the reflectivity in our nonlinear tests. The optical fibers of all devices were meticulously fused together using an optical fiber welding machine, combining them into a closed nonlinear testing loop.  
The results of our rigorous nonlinear optical reflectivity experiments, and the fitting curve, are presented in Figure 3b. At the target wavelength of 1550 nm, the PSM-SA exhibited a noteworthy modulation depth of 1.6% and an unsaturated loss of 39.8%. Notably, we observed a distinct rising edge in the reflectivity curve. This observation is pivotal, as it signifies the remarkable ability of the PSM thin-film photonic crystals to efficiently absorb and modulate the intensity of the incident light. The photonic crystals themselves exhibit a photonic bandgap effect. When the input light is precisely at the edge of the band gap, due to its absorption rate in that spectral range, it exhibits a distinct nonlinear characteristic. The input light experiences a low-transmittance band gap at low light intensities, also known as the \"OFF\" state. Conversely, the transmittance becomes high at high light intensities, resulting in the \"ON\" state. This stark contrast in the strength of the transmitted signal between these two states holds significant implications for potential applications, such as optical switching and modulation. However, it should be noted that defects introduced during the PSM-SA preparation process act as non-radiative recombination centers, contributing to the relatively high unsaturated loss observed. To optimize the PSM-SA, improvements can be made to the preparation process and other relevant factors, aiming to reduce these losses and enhance the overall performance of the PSM-SA.