Innovative Application of Subwavelength Periodic Polystyrene Microspheres as Saturable Absorbers in Nonlinear Optics

Abstract

Polystyrene (PS) possesses numerous remarkable properties like high transparency, impressive mechanical strength, and a large specific surface area, making it an excellent mask plate or template for the preparation of anti-opal structures. Moreover, it should be noted that PS also exhibits exceptional nonlinear properties due to the subwavelength periodic configuration. In this paper, a self-assembled PS microsphere photonic crystal saturable absorber (PSM-SA) has been proposed and fabricated. It exhibits impressive properties including high stability, high damage threshold, high refractive index, and large specific surface area. It is suggested that the periodic structure of PS in the film has a significant impact on the photonic band gap, resulting in excellent adjustable optical nonlinear characteristics. By integrating PSM-SA into a self-built ring fiber laser system, a Q-switched laser with a pulse width of approximately 2 μs and a repetition rate of 40 kHz at a wavelength of 1562 nm is obtained. These findings demonstrate its potential for enabling efficient and adjustable nonlinear optical functionalities in various optical devices, contributing to the expanding realm of PS microsphere photonic crystals and their significant impact on advancing nonlinear optics technology.

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. These attributes make them well-suited for the creation of high-performance SAs, such as the PSM. Compared to the inherent properties of PS materials, transforming them into PSM photonic crystals can modulate their absorption characteristics, altering their nonlinear properties and offering a potential approach for the fabrication of organic and inorganic material photonic devices. An assessment of the nonlinear properties of PSM was performed using nonlinear test equipment, leading to the generation of a Q-switched pulse laser with a pulse width of around 2 μs and a repetition rate of 40 kHz at a wavelength of 1562 nm, by incorporating PSM-SA into a homemade ring fiber laser system. These findings not only unveil a novel avenue for exploring photonic crystals, but also extend their scope of application within nonlinear optical devices, thereby significantly advancing many fields such as radar, communication, medical treatment, and scientific research [29].

2. Theoretical Background

The photonic crystal periodic arrangement of PSM-SA plays a crucial role in controlling light propagation, and its saturable absorption effect is the key mechanism for transforming continuous light into pulsed light. Once the light intensity exceeds a specific threshold, the material’s linear absorption capacity becomes saturated, resulting in the saturable absorption effect. This effect is a fundamental factor for achieving efficient pulse generation. By designing the PSM-SA structure, light is confined within specific modes and pathways, effectively preventing its dispersion within the PBG [30], thereby significantly limiting and enhancing light absorption within a particular frequency range. According to Bragg–Snell’s law [31,32], the periodicity or the refractive index contrast of the PSM-SA can lead to a shift in the PBG [33]:

$${\lambda }_{max}=\frac{2}{m}d\sqrt{n_{eff}^2-{sin}^2\theta }$$

(1)

where ${\lambda }_{max}$ is the wavelength of the maximum reflection (photonic band gap) peak, d is the lattice constant, m is the diffraction order, $n_{eff}$ is the effective refractive index and θ is the angle of incidence of the light for the photonic crystals. The effective refractive index can be determined by utilizing the Maxwell Garnett effective medium approximation [34].

$$n_{\mathit{eff}}=n_{\mathit{air}}\frac{2(1-f)n_{\mathit{air}}+(1+2f)n_{\mathit{mat}}}{2(2+f)n_{\mathit{air}}+(1-f)n_{\mathit{mat}}}$$

(2)

where $n_{\mathit{mat}}$ is the dielectric constant of the material, $n_{\mathit{air}}$ is the dielectric constant of air, and $f$ is the filling factor.

The Bragg–Snell’s law states that different sizes of PSM correspond to different wavelengths of light. The ${\lambda }_{max}$ is the maximum reflection peak that forms the photonic band gap, and the optimum working wavelength can be adjusted according to the formula by changing the periodicity or refractive index contrast through any external stimulus. This correlation is further confirmed by the transmission spectrum of polystyrene microspheres, which aligns with the theoretical concept [15]. Notably, microspheres with a size of approximately 500 nm exhibit the highest reflection (or scattering) rate for 1550 nm light, suggesting the presence of a directional band gap.

In the experiments, it is crucial to consider the potential impact of incident light on the refractive index of PS and the high reflective Au mirror [35]. Additionally, the presence of the Au mirror on the surface can induce local plasmon resonance effects [36], which enhance the surface magnetic field [37] and result in a redshift of the wavelength [38,39]. Considering these factors, PSMs with a size of less than 500 nm were chosen to prepare PSM-SA devices for a 1550 nm pulse laser.

3. Materials and Experimental Setup

The soap-free method was utilized to prepare the PSM, resulting in microspheres with excellent homogeneity and stable properties. The synthesis process is outlined as follows: first, a sealed glass reactor connected to an agitator was prepared. A total of 500 mL of deionized water was added to the reactor, and the system was treated with nitrogen for 2 h. The agitator speed was set to 500 rpm, and the constant temperature tank was set to 90 °C. Next, the styrene was cleaned using a 5% NaOH solution. This cleaning process was repeated 2–3 times to remove the inhibitor from the styrene. Subsequently, the styrene was washed with deionized water 3–4 times, and the pH value of the cleaning solution was monitored until it reached 7. Afterward, 35 mL of the washed styrene was measured and quickly added to the reactor. Following a 15 min interval, 30 mL of KPS (Potassium Persulfate) initiator was measured and swiftly added to the reactor. The reaction was allowed to proceed for 4 h. The agitator and the constant temperature tank were turned off, and the reactor was cooled to room temperature. The PSM obtained through these steps can be further utilized in the research and fabrication of PS photonic crystals.

To obtain the PSM-SA, the treated PSM suspension was transferred to a gold mirror using the liquid-level transfer method. The preparation process is illustrated in Figure 1. Initially, the PSM suspension and anhydrous ethanol were first transferred in a 1:1 ratio, using a pipette gun, to a centrifuge tube, with 50 μL of each being transferred. The resulting solution is enough to cover the slide completely, allowing just enough for a complete study. To ensure thorough mixing of the PS solution and ethanol, the mixture was subjected to 10 min of ultrasonication. Subsequently, deionized water was placed on a glass slide. Utilizing the surface tension and capillary forces between the fine particles, the uniformly mixed PS solution was gently floated on the surface of the liquid droplet. It was allowed to stand for 30 min. Next, to save experiment time, filter paper was used to absorb excess water from the slide and the slide placed into a beaker filled with water, allowing the self-assembled PSM film to float on the water surface. The PSM film on the water surface was carefully extracted using tweezers while holding a gold mirror. The film was then allowed to stand for 20 min to ensure a secure and completely dry adhesion. This process resulted in the production of the PSM-SA.

The morphological analysis and characterization of the PSM are crucial steps in our investigation. Field-emission scanning electron microscopy (SEM) is employed to explore the morphology and size distribution of the PSM, as depicted in Figure 2a. The SEM images provide valuable insights into the structural characteristics of these microspheres. It can be seen that the microspheres are uniform in shape and size and uniform in arrangement. In addition to morphological analysis, we conducted Raman spectroscopy to assess the material quality and composition of the PSM-SA samples, as presented in Figure 2b. Raman spectroscopy is a powerful tool for identifying and quantifying molecular vibrations in materials. In our spectra, specific peaks corresponding to the PS material [40] are clearly observed, providing a deeper understanding of the chemical characteristics of our samples. For example, the Raman spectra reveal distinctive peaks indicative of PS. The peak at 1002 cm⁻¹ corresponds to aromatic respiration, while the peak at 620 cm⁻¹ indicates the deformation of the benzene ring. Further insights are gained from peaks at 902 cm⁻¹ and 1023 cm⁻¹, which are associated with the out-of-plane bending vibrations of C–H and C–O bonds, respectively. The C–H bending vibration occurs at 1030 cm⁻¹. Additionally, peaks at 1490 cm⁻¹, 1447 cm⁻¹, and 1602 cm⁻¹ correspond to the stretching vibrations of the C=C bond on the benzene ring. The peaks at 2846 cm⁻¹ and 2924 cm⁻¹ arise from the symmetric and asymmetric stretching vibrations of –CH₂ bonds, while the region of 3100–3000 cm⁻¹ corresponds to the stretching of the =C–H bond on the benzene ring. This comprehensive Raman spectroscopic analysis provides evidence of the PS material and offers valuable insights into its molecular structure and chemical composition. Such characterization is fundamental to our understanding of the PSM-SA samples and their suitability for the intended applications in our study.

4. Experimental Results and Discussion

4.1. 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 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 × 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.

4.2. Output Characteristics of Q-Switched Laser Based on PSM-SA

In the experiment described in this paper, the prepared PSM-SA was integrated into a ring cavity system [41], providing insights into its behavior within the laser setup as depicted in Figure 4. In the experimental test system, the 976 nm pump laser is coupled into the ring cavity through a WDM. The 4 m long EDF gain medium, a key component, played a pivotal role. It enabled the excitation of erbium-doped ions to a higher energy state upon interaction with the pump source. Subsequently, these excited ions rapidly relaxed to a metastable state, initiating the emission of C-band laser radiation through a well-established process [42]. To ensure the stability and efficiency of our laser system, we incorporated a 1550 nm ISO for unidirectional transmission. This essential component ensured that the optical signal traveled in a single direction within the setup, preventing damage to the pump source. Once the continuous wave light from the gain medium passes through the CIR, it irradiates the PSM-SA. This interaction led to the modulation of light into a pulsed form, a pivotal step in our laser operation. Subsequently, these modulated pulsed light signals were recoupled back into the ring cavity. To further manipulate the characteristics of the output pulses, we utilized a polarization controller (PC) to control the polarization state of the pulsed laser. This precise control allowed us to tailor the output pulses to meet our specific requirements. After passing through the OC, an ingeniously designed optical component, 20% of the optical signal was extracted for monitoring and meticulous measurement purposes. This extracted signal allowed us to continuously monitor the laser’s performance characteristics, including output power, pulse duration, and spectral properties. The remaining 80% of the optical signal was cleverly recoupled back into the ring cavity using the WDM, thus ensuring that the laser reaches the gain threshold. The output characteristics of the fiber laser are monitored and analyzed using a photodetector, a digital oscilloscope (Keysight DSOS054A, Keysight Technologies, Santa Rosa, CA, USA), and a spectral analyzer (Anritsu MS9740A, Anritsu Corporation, Atsugi, Japan).

These instruments provide detailed measurements, such as output power, pulse duration, and spectral properties, allowing for a thorough understanding and analysis of the performance. Through this designed experimental setup and precise measurement apparatus, we gained a profound understanding of the capabilities and performance of laser, further advancing our research and insights into photonic crystal applications in nonlinear optical devices and the high performance of PSM-SA.

Within the confines of our experimental setup, we made a remarkable observation regarding the PSM-SA’s behavior. It has been observed that the PSM-SA in this setup exhibits the ability to self-initiate Q-switched output characteristics when the output power is increased. In Figure 5a, we present the compelling time–domain pulse sequence captured from the laser system’s output via the OC. This pulse sequence illustrates the dynamic evolution of the Q-switched pulses as the pumping power gradually escalates. On the horizontal axis we traverse through time, while the vertical axis portrays the pulse intensity. As the pumping power progressively rises and reaches the threshold of 80 mW, the laser system seamlessly transitions into a Q-switched regime. In this regime, the laser generates distinct short, high-energy pulses with precisely defined time intervals between them. The phenomenon is indicative of the laser’s newfound capability to produce pulsed output with remarkable temporal precision. When the pumping power approaches around 340 mW, the fiber laser system operates at its most stable state, consistently producing Q-switched pulses. The corresponding spectrum is shown in Figure 5b.

However, as the pumping power further escalates and reaches approximately 390 mW, a special transformation occurs, as seen in Figure 5a. The increase in power corresponds to a rise in temperature, which, in turn, exerts an influence on the PS polymer chain. This thermal effect triggers shear vibrations that affect the relative positions and structures of the molecules within the PSM-SA. Consequently, the molecular polarity and electric dipole moment decrease, resulting in a reduced refractive index. As the pumping power continues to rise beyond this critical point, the Q-switched behavior begins to diminish. Under the sampler power, the amplitude of the spectral waveform has begun to decrease and deviate from the Q-switched state, the spectrum has redshifted, and the shape has begun to become irregular. This distinctive behavior is observed when the laser system goes beyond the ultimate limit of the Q-switched state, which is intrinsically linked to characteristics of the 430 nm PSMs and their interaction with the laser system. This observation provides a profound insight into the intricate dynamics within our experimental setup, illuminating the delicate balance between pumping power, temperature, and the material properties of the PSM-SA. Interestingly, a stable Q-switched pulse can be obtained again when the pumping power is decreased. This compelling phenomenon underscores the unique attributes of the PSMs employed in our research, specifically their substantial volume and expansive specific surface area. These characteristics endow them with remarkable resilience to thermal stress and enable the restoration of their stable Q-switched behavior. These observations demonstrate the influence of pumping power and heat on the Q-switched dynamics of the laser system based on PSM-SA and the resilience and recovery capabilities of the PSM used in the experiment. PSM-SA was compared with samples containing only PS film on the gold mirror inserted into the ring cavity, in which the PS-only samples did not show any Q-switched behavior, and the nonlinear properties of PS did not seem to be activated, as shown in Figure 5c,d. The spectral characteristics were distinct, with the PS-only sample’s spectral profile closely resembling that of the CIR output spectrum. This comparison highlights the fact that incorporating a microsphere structure into the design of photonic crystals can significantly enhance the intrinsic nonlinear characteristics of the material, potentially leading to the generation of nonlinear effects. Despite PS not displaying Q-switched signals, the introduction of PSM successfully enabled the Q-switched laser operation.

Figure 6 unveils a comprehensive depiction of the output characteristics, providing a nuanced insight into the dynamic evolution of the Q-switched pulse waveform as previously illustrated in Figure 5a. First and foremost, the average output power of the fiber laser demonstrates an intriguingly linear increase in correspondence with the ascending pump power. At its zenith, the system achieves an impressive output power of 1.17 mW, mirroring the ascending waveform height illustrated in Figure 6a. This linear correlation is a testament to the system’s capacity to efficiently harness and convert pump power into useful laser output. As the pump power intensifies, the duration of pulse energy accumulation within the PSM notably diminishes. This reduction in pulse duration translates into an elevation in the repetition frequency, denoted as f. When the pump power reaches its maximum value, the frequency increases to 42.1 kHz, as depicted in Figure 6b. This surge in repetition frequency coincides with an increase in peak power, reaching a maximum of about 12.5 mW. The continuous decrease in pulse width is a notable observation, primarily attributed to the enhanced bleaching effect of the SA [43,44]. However, it is important to acknowledge that there are inherent limitations to further reducing the pulse width. This limitation arises from the saturation effect inherent to the gain fiber within the system and the constraints imposed by the damage threshold and conversion efficiency of the PSM-SA itself. The minimum value of the pulse width is approximately 22.3 μs. In terms of pulse energy, it exhibits a pronounced upward trend in tandem with the pump power’s ascent, gradually stabilizing as it approaches its maximum value of approximately 28 nJ. The relationship between these parameters illustrates the dynamic interaction of output power, repetition rate, peak power, pulse width and pulse energy in the laser system with the change in pump power, reflecting the characteristics and limitations of the process of PSM-SA, providing a comprehensive understanding of its operational dynamics. Their relationship is:

E_p = E_ave/f = Δt × P_peak

(3)

where E_p is single-pulse energy, P_ave is average output power, f is pulse repetition frequency, Δt is pulse width, and P_peak is output peak power.

A novel PSM-SA operating at a wavelength of 1562 nm is successfully prepared, enabling the achievement of stable Q-switched output in an ultrafast fiber–laser system. After repeated experiments, our experimental phenomenon is stable and the results are reproducible. Furtherly, there exists a compelling avenue for further refinement and optimization of the PSM-SA. One pivotal objective is the reduction of its unsaturated loss, a factor that can significantly enhance the potential of the laser system. By addressing this aspect, we can open doors to increased mutual oscillation mode and mode selection within the laser cavity [45]. This optimization endeavor holds the promise of improving the Q value of the laser system, potentially even leading to the achievement of mode locking. Therefore, our current achievement with the PSM-SA marks just the beginning of a journey filled with possibilities. It has illuminated the innovative potential of photonic crystal technology and its application in advanced laser systems, setting the stage for future breakthroughs and advancements in this dynamic field.

5. Conclusions

In summary, PS microsphere thin-film photonic crystals were first utilized as an SA in fiber lasers for generating Q-switched pulse output. While PS typically cannot achieve Q-switched pulse lasers, converting PS into PS microsphere photonic crystals unveils new possibilities. By harnessing the nonlinear characteristics of PSM-SA and integrating the PSM-SA into a self-constructed cavity system, it has successfully produced a Q-switched pulse laser with a duration of about 2 μs and a repetition rate of 40 kHz at a wavelength of 1562 nm. These results emphasize the substantial potential of PS micro-sphere photonic crystals as effective and versatile nonlinear optical components in a range of optical devices. This innovative approach showcases the viability of utilizing organic and inorganic materials for Q-switched pulse lasers, furthering the field of photonic crystals and promoting advancements in nonlinear optics technology for innovative optical applications.