## 1 Introduction  
Photonic crystals [1] consist of materials that exhibit periodic variations in the dielectric constant or the refractive index. The periodic structural arrangement significantly impacts the properties of photon modes within these materials, and hence the photonic band gap (PBG) [2]. Analyzing the periodic variation in material dielectric constants in one, two, and three spatial directions, photonic crystals can be classified into one-dimensional, two-dimensional, and three-dimensional structures. Additionally, based on the number of components constituting the photonic crystal material, they can be categorized as binary, ternary, or even higher-order compositions. The manipulation of photonic properties and selective control of light reflection or propagation within specific wavelength ranges can be achieved through the design of material structures. Based on applications, photonic crystals can be classified into two major categories: photonic crystal slab and photonic crystal fiber [3; 4; 5; 6]. Multilayer dielectric mirrors [7] that have been widely used in optical devices, such as one-quarter wavelength [8] multilayer reflective films composed of alternating layers of materials with different permittivity, belong to the photonic crystal slab. The simple structure of one-dimensional periodicity can confine three-dimensional light beams through bandgaps and refractive indices. While most photonic crystal slab exhibits two-dimensional periodicity with finite thickness, defects within photonic crystal slab can create waveguides and resonant cavities, which can achieve impressive performance but require meticulous design to minimize losses resulting from resonant cavities and disruption of periodicity. This phenomenon, that a specific wavelength of light shining on the surface of this material will be fully reflected, is also the basis for many applications, including dielectric mirrors, dielectric F-P filters [9], and distributed feedback lasers [10]. Moreover, photonic crystals are widely used in optical fields such as optical waveguides [11], sensors [12] and optical filters [13]. With the development of photonic crystals, the materials that constitute photonic crystals also tend to be diversified. Some of the most commonly utilized materials include silicon dioxide [14], polymer polystyrene [15], and silicon [16], as well as metals like gold and silver [17]. Among them, polystyrene (PS) has attracted much attention because of its many remarkable properties, such as excellent optical nonlinear characteristics [18], high transparency [19], impressive mechanical strength [20], large specific surface area [21], high chemical stability [22], and easy preparation feature. These properties have made PS a focus of application and research, particularly in its use as a mask plate or template for the fabrication of anti-opal structures. The densely packed arrangement of the multi-layered PS microsphere forms a three-dimensional photonic crystal [23], offering a promising avenue for optical switches by exploiting its third-order nonlinear magnetic susceptibility [24; 25; 26; 27]. However, the potential use of its nonlinear properties in devices has yet to be thoroughly investigated. To extend the utility of the nonlinear properties inherent in the PS microsphere (PSM), this study introduces a novel application involving thin-film photonic crystals comprised of PSM. The utilization within the nonlinear optical device's saturable absorbers (SA) marks a new application for PSM photonic crystals. The PSM-SA is prepared using a 430 nm polystyrene microsphere photonic crystal monolayer, showcasing excellent third-order optical nonlinearity. Photonic crystals manifest a conspicuous photonic band gap effect [28]. To take advantage of its high damage threshold and robust system stability, we designed an SA with a PSM structure, thereby endowing it with an impressive photonic crystal-like performance, to a significant extent.