Boosted Nonlinear Optical Properties of Polypyrrole Nanoplates Covered with Graphene Layers

Abstract

The combination of polypyrrole (PPy) with graphene has attracted extensive attention as a nonlinear optical material with various optoelectronic applications. Here, we describe the development of PPy nanoplates prepared using a simple spin-coating method. The appropriate volume of the dropped PPy solution was determined to be 50 drops by comparing the surface morphologies, chain structures, elementary compositions, and optical properties of PPy saturable absorbers (SAs). The hybrid PPy/graphene heterostructure SA was obtained using the wet transfer process of a graphene layer. This approach led to significant improvements in optical properties, including a ~7.2% increase in linear optical absorption, a 2.5-fold increase in modulation depth, and a third decrease in saturable intensity at 1550 nm due to the additional optical absorption and the π-π interaction between PPy nanoplates and the graphene layer. By inserting the PPy/graphene heterostructure SA into the passively mode-locked fiber laser cavity, 1559 nm ultrashort laser pulses were generated, with an average output power of 1.24 mW, a 815 fs pulse width, and a repetition frequency of 3.26 MHz. Our experimental results demonstrate that the prepared PPy SA has excellent nonlinear optical characteristics, providing a new opportunity for the generation of ultrashort laser pulses.

1. Introduction

Ultrafast fiber lasers emitting femtosecond laser pulses in the near-infrared (NIR) region have been the focus of considerable research interest due to the advantages of high peak power, compact design, good stability, low cost, and easy operation. They have been widely used in many applications such as medicine, surgery, optical coherence tomography, optical frequency comb, material processing, and so on [1,2,3,4]. For ultrafast fiber lasers based on the passive mode-locking method, key performance parameters such as pulse duration, output power, and long-term stability are highly dependent on the saturable absorber (SA)’s nonlinear optical absorption properties, including modulation depth (ΔT) and saturable intensity (I_sat) [5,6]. Recently, a variety of nonlinear optical materials, such as graphene, carbon nanotubes, black phosphorus, transition metal chalcogenides, and topological insulators, have exhibited optical nonlinearities at various wavelengths in the near-infrared range and have been used as SAs to generate ultrashort laser pulses [7,8]. However, it has been found that SAs based on a single material usually possess some inherent shortcomings, which hampers their practical application in ultrafast fiber lasers. Fortunately, the issue can be effectively solved by constructing hybrid heterostructures to boost both the linear and nonlinear optical properties of SAs.

Conjugated organic polymers with extended π-electron delocalization along the molecular chain have attracted extensive research attention in a wide variety of applications such as solar batteries, supercapacitors, electrochemical sensors, and nonlinear optics [9,10,11,12]. Among these organic conducting polymers, polypyrrole (PPy), with its advantages of good stability, cost effectiveness, and ease of synthesis, has been considered a promising nonlinear optical material for optoelectronic applications. The nonlinear optical characteristics of PPy materials have been demonstrated to originate mainly from the ultrafast and lossless excitation of highly charge-correlated π-electron states [13,14]. Therefore, PPy employed as a SA enables the direct generation of ultrafast laser pulses. Recently, significant progress has been made in the nonlinear optical characteristics of PPy-based materials for optoelectronic device applications [15,16]. Saturable absorption and reverse saturable absorption characteristics have been demonstrated in PPy-based materials, and it is also found that the two kinds of absorption properties can transform each other by constructing PPy heterostructures [17,18]. Therefore, it is highly necessary to explore an effective strategy to obtain PPy SAs with optimized saturable absorption for ultrafast photonics applications.

In this study, we successfully obtained a series of pure PPy SAs using a simple spin-coating method, and the tunable linear and nonlinear optical characteristics were obtained by adjusting the volume of the dropped PPy solution. To further improve the optical properties of the PPy SA, we constructed a new type of PPy/graphene hybrid heterostructure by the wet transfer method of graphene, and noticeable improvements, including ~7.2% linear optical absorption and 2.5 times ΔT at 1550 nm, were achieved due to the increased photon harvest process and the strong π-π interaction in the PPy/graphene hybrid heterostructure. In addition, a 1550 nm passively mode-locked Er-doped fiber (EDF) laser was experimentally used based on the PPy/graphene heterostructure SA, which exhibits stable mode-locked pulse trains with a pulse width of 815 fs and a repetition frequency of 3.26 MHz.

2. Materials and Methods

To obtain PPy-based SAs, PPy powder was purchased from Aladdin Co., Ltd., Shanghai, China. Subsequently, 0.1 g of PPy powder (C₄H₅N, >99%) was dissolved in 10 mL of anhydrous ethanol solution. The solution was then stirred for 2 h at room temperature to obtain a polymeric precursor solution. Before the spin-coating process, reflectors of SAs were prepared by depositing 100 nm Au films onto the Si substrates with the magnetron sputtering method. This kind of reflective structure has been effectively used for measuring nonlinear optical absorption properties of SAs and realizing passively mode-locked operations in our previous work [19]. Then, the prepared PPy polymeric precursor solutions were spin-coated onto Au reflectors at a rotation speed of 100 revolutions per minute, and the spin-coating processes were set to be 10 min to ensure adequate coverage of the PPy materials. During the spin-coating process, 10, 50, and 100 drops of PPy polymeric precursor solutions were dropped onto Au reflectors, with different thicknesses of PPy films, marked as Py10, Py50, and Py100 SAs, respectively. The PPy/graphene hybrid SA (named as Py50/Gr. SA below) was prepared by the wet transfer process of the graphene onto Py50 SA. All the preparation processes for PPy-based SAs were carried out at room temperature in an atmospheric environment.

Scanning electron microscopy (SEM) images were acquired by using the Hitachi SU-70 instrument to obtain the surface morphologies of the SAs. Raman spectra were carried out on a scanning near-field optical microscope system (Ntegra Spectra, NT-MDT, Moscow, Russia) with a 532 nm solid-state laser source. Ultraviolet–visible–NIR (UV-vis-NIR) reflection spectra were obtained on a UV visible absorption spectrometer. X-ray photoelectron spectroscopy (XPS) was recorded on a Thermo ESCALAB 250Xi X-ray photoelectron spectrometer with a monochromatic Al Kα radiation as the excitation source (Thermo Fisher Scientific, Waltham, MA, USA).

3. Results

3.1. Material Characterizations

SEM was used to study the surface morphologies of the PPy and PPy/graphene hybrid SAs. SEM images were directly obtained on the SAs without further treatment. Figure 1a,b, and c present SEM images of various PPy SAs corresponding to Py10, Py50, and Py100 SAs, respectively. As shown in Figure 1a–c, PPy nanostructures can be observed. The coverage rate of the PPy material on the Au/Si reflector surface gradually increases with the PPy solution volume increasing from 10 to 50 drops, indicating that the coverage rate of PPy materials can be effectively controlled by adjusting the dropped PPy solution volume. Upon further increasing the PPy solution volume to 100 drops, no noticeable increase in coverage rate is observed. To have a clearer view of the PPy nanostructures, a high-magnification SEM image of the Py50 SA is shown in Figure 1d. A 3D architecture with a smooth surface is observed for the PPy materials, suggesting the formation of PPy nanoplates. To optimize the nonlinear optical properties of the PPy SAs, the graphene layer was transferred onto the PPy SAs, and Figure 1e,f show low- and high-magnification SEM images of the Py50/Gr. SA, respectively. The transferred graphene layer is covered on the PPy nanoplates, and an obvious edge of the graphene layer can be observed, as shown in Figure 1e, indicating that the PPy/graphene hybrid SA was successfully obtained. To obtain clearer observations of the PPy/graphene hybrid SA, an SEM image measured at a 1 μm scale for the hybrid SA edge is presented in Figure 1f. Some wrinkles and ripples of the graphene layer are induced by the van der Waals forces between PPy nanoparticles and the graphene layer, implying the presence of an π-π interaction at the interface of the Py50/Gr. heterostructure. More importantly, PPy nanoplates that are covered below the three-dimensional network structure of the graphene layer can provide multi-dimensional channels for photon-generated carriers, which is beneficial for improving the nonlinear optical characteristic of PPy SAs.

Raman spectroscopy, a nondestructive, no-special-sample-preparation, and contactless measurement technology [20,21], was employed to confirm the formation of PPy chains and the interaction in the PPy/graphene heterostructure [22]. Therefore, to further describe the effect of the transferred graphene layer on the structural properties of PPy nanoplates, Raman spectra measurements for PPy and PPy/graphene heterostructure SAs were carried out, as shown in Figure 2. Py10, Py50, and Py100 SAs exhibit five prominent Raman peaks at 934, 976, 1051, 1396, and 1586 cm⁻¹, corresponding to C-C ring deformation (bipolarons), C-C ring deformation (polarons), C-H in-plane deformation (polarons), C-C in the ring, and C=C backbone stretching mode, respectively [23,24]. For the Py50/Gr. SA, besides these five Raman peaks of the PPy nanoplates, 3D and 2D bands of the characteristic peaks of the graphene layer can be obtained at 1330 and 2699 cm⁻¹, respectively. In comparison with C=C backbone stretching mode of Py10, Py50, and Py100 SAs, the Raman peak for the Py50/Gr. SA shifts to 1571 cm⁻¹ due to the overlap with the G band of the graphene layer. The observed peaks prove the formation of PPy chains and the successful transfer of the graphene layer. In comparison with the Py50 SA, the Py50/Gr. SA exhibits much stronger peak intensity for C-C ring deformation (bipolarons), C-C ring deformation (polarons), and C-H in-plane deformation (polarons) in Raman modes due to the induced orderly manner of PPy nanoplates, further demonstrating the π-π interaction between PPy nanoplates and the graphene layer.

The nonlinear optical characteristics of PPy materials were mainly induced by the ultrafast and lossless excitation of highly charge-correlated π-electron states [13,14], which are highly dependent on the molecular structures of the PPy-based materials. Therefore, XPS measurements are usually performed to reveal the elemental compositions and molecular structures of PPy and PPy/graphene hybrid heterostructures [25,26,27]. As presented in Figure 3, all the SAs exhibit three prominent peaks at around 284.8, 399.8, and 532.0 eV in the total XPS spectra, corresponding to the C 1s, N 1s, and O 1s, respectively, which is in agreement with the chemical compositions of the PPy materials. Similar element contents can be found in the Py10, Py50, and Py100 SAs. After the wet transfer process of the graphene to fabricate the Py50/Gr. SA, due to the additional C element provided by the graphene layer, the C element content ratio increases from 67.93 to 77.41% and the N and O element content ratios decrease from 11.67 to 7.37% and 20.4 to 15.22%, respectively.

High-resolution XPS spectra of C 1s, N 1s, and O 1s were obtained to study the difference in chemical states between the Py50 and Py50/Gr. SAs. As shown in Figure 4a, the C 1s peak of the Py50 SA is composed of four components, including C-C bonds (284.6 eV), =C-N bonds (285.6 eV), -C=N bonds (287.7 eV), and C=O (288.9 eV). Peaks from these four chemical bonds are also observed in the XPS spectra of Py50/Gr. SA, as depicted in Figure 4b. An increase in the peak intensity of C-C bonds for the Py50/Gr. SA is observed due to the C-C bonds provided by the graphene layer. The C-C bonds at 284.9 eV for Py50/Gr. SA are shifted to the higher binding energy by 0.2 eV compared with that of the Py50 SA, which is mainly attributed to the increased electron density due to the π-π interaction between PPy nanoplates and the graphene layer, indicating that the charge-correlated π-electron states of PPy are extended by graphene via the π-π interaction. The N 1s peak of the Py50 SA is fitted into three peaks at 398.5, 399.9, and 400.8 eV, corresponding to the neutral-like structure (-C=N-), the neutral secondary amine structure (-NH-), and positively charged nitrogen (-N⁺-), respectively (Figure 4c). Three fitting peaks for the N 1s peak of Py50/Gr. SA are located at 398.5, 400.1, and 401.1 eV, respectively, as shown in Figure 4d. The peaks of the neutral secondary amine structure (-NH-) and positively charged nitrogen (-N⁺-) for the Py50/Gr. SA is shifted towards a higher binding energy, implying a strong π-π interaction and a noticeable increase in the electronic state, which is consistent with that of the C 1s peak. In comparison with the N1s peak of the Py50 SA, no apparent intensity variation in these three fitted peaks is found due to no additional N element being introduced in the transfer process of the graphene layer. Figure 4e and f present O 1s spectra of the Py50 and Py50/Gr. SAs. O 1s spectra can be divided into three peaks at 532.3 and 534.0 eV, originating from -C-O-C- bonds and C-O bonds. The peak intensity of C-O bonds becomes smaller after the transfer process of the graphene layer. One possible explanation is that the C elements provided by the graphene layer combine with the C-O bonds, inducing the carrier transport channel, increasing the amount of -C-O-C- bonds, and decreasing the amount of C-O bonds.

Reflection spectra of all SAs in the NIR range from 900 to 1800 nm were measured to analyze the effect of the PPy solution volume and the transferred graphene layer on the PPy SAs, as shown in Figure 5. Increasing the solution volume of PPy nanoplates from 10 to 50 drops reduces the reflectivity at 1550 nm by a factor of 3. Further increasing the solution volume to 100 drops results in a slight increase in reflectivity. The variation trend of the reflectivity can be attributed to the increase in the coverage rate of PPy nanoplates. When the solution volume of PPy nanoplates increases from 10 to 50 drops, the coverage rate of PPy nanoplates increases. Simultaneously, the amount of PPy absorbers increases, which contributes to the decrease in the reflectivity. After the graphene layer was transferred onto the Py50 SA, a decrease in the reflectivity for the Py50/Gr. SA was observed, corresponding to a ~7.2% increase in the linear optical absorption. The main reason for the decrease in reflectivity is due to the additional absorption of the graphene layer. Moreover, the increased electron density due to the π-π interaction between PPy nanoplates and the graphene layer plays an important role in the decrease in the reflectivity.

3.2. Nonlinear Absorption Characteristics

Nonlinear optical absorption characteristics, including ΔT and I_sat, determine the performance of ultrafast fiber lasers, which is usually investigated by the balanced twin-detector measurement system, as shown in Figure 6a [19]. A home-made femtosecond pulse fiber laser based on semiconductor saturable absorption mirror mode-locked technology was used as a pump light source to provide a center wavelength of 1550 nm, a repetition rate of 18.01 MHz, and a pulse duration of 820 fs. An attenuator adjusts the laser pulse and is then equally divided by an optical coupler (OC). All the SAs studied in this work are inserted into one branch that is linked to a circulator (CIR). Both input and output powers are recorded by a double-channel power meter. The balanced twin-detector measurement system was constructed with single-mode optical fiber devices; therefore, the incident light spot diameter is 9 μm, corresponding to the size of the fiber core. To ensure the accuracy of experimental results, three different points on the PPy-based SA surfaces were selected to measure nonlinear reflection curves. The nonlinear reflection curves of the SAs are plotted in Figure 6, and I_sat values are calculated by fitting the nonlinear reflection curves using the following function [19,28]:

$$\alpha (I)={\displaystyle \frac{{\alpha }_S}{1+I/I_{sat}}}+{\alpha }_{\mathrm{N}S}$$

in which α_NS and α_S and are nonsaturable and saturable absorptions. As shown in Figure 6b, the nonlinear reflection curve of the Py10 SA exhibits an increased reflectivity with the power intensity increasing from 0 to 0.88 MW/cm² due to the saturable absorption process, and the two-photon absorption can explain the decrease in the reflectivity for the Py10 SA. Saturable absorption processes are also found for the nonlinear reflection curve of Py50, Py100, and Py50/Gr. SAs, while no two-photon absorption process is observed in the measured power intensity range, which is connected with the increased amount of absorbers. By the curve fitting method, ΔT and I_sat are calculated to be (0.41 ± 0.04)% and (0.35 ± 0.02) MW/cm² for the Py10 SA. With the solution volume increased up to 50 drops, the ΔT and I_sat increase up to (1.08 ± 0.02)% and (0.42 ± 0.02) MW/cm² for the Py50 SAs, indicating that the capacity of the nonlinear absorption is improved due to the increased amount of absorbers in the Py50 SA. The Py100 SA has a smaller ΔT and an increase in I_sat, which is not beneficial for the generation of ultrashort laser pulses. The structural defect is induced by the larger volume of the dropped PPy solution. The optical scattering and carrier losses associated with the structural defect consume a portion of the incident light’s energy. After the transfer of the graphene layer, a considerable improvement in nonlinear optical characteristics is achieved. The ΔT of the Py50/Gr. SA increases by 2.5 times. The SA was reduced by a third compared with that of the Py50 SA. These optimizations in nonlinear optical characteristics are ascribed to the additional optical absorption and the π-π interaction between PPy nanoplates and the graphene layer.

It has been confirmed that the nonlinear optical characteristics of PPy materials mainly originate from the ultrafast and lossless excitation of highly charge-correlated π-electron states. Therefore, the conjugation and distribution of electron states determine the nonlinear optical absorption characteristics of PPy materials. The electrons located at chains of PPy nanoplates resulted in a decreased energy level due to the formation of holes. The additional energy band above the valence band is the main reason for the linear and nonlinear absorption at 1550 nm. Under the irradiation of the fs laser pulse, the additional energy band of PPy nanoplates absorbs the photons and generates the carriers. The photon-generated carriers rapidly occupy sites on the additional energy band due to the slower interband recombination relaxation. When the power intensity reaches the I_sat, the sites on the additional energy band are entirely taken up, and the SA achieves absorption saturation, making the reflectivity rise. The amount of the absorber strongly influences the nonlinear optical characteristics. For example, the ΔT and I_sat of the graphene SAs can be effectively controlled by adjusting the number of graphene layers. Therefore, the Py50 SA with the higher coverage rate of PPy nanoplates exhibits larger ΔT and I_sat. However, for the ultrafast laser, the larger ΔT and the smaller I_sat, which are conducive to the generation of high-performance laser pulses, are highly desired. To further optimize the nonlinear optical characteristics of the PPy SA, the hybrid Py50/Gr. SA was obtained by transferring the graphene layer onto the Py50 SA.

Expectedly, significant improvements in nonlinear optical characteristics have been obtained in the Py50/Gr. SA, which can be assigned to a combination of various interaction mechanisms. In the hybrid Py50/Gr. SA, the transferred graphene layer, as an additional absorber, can not only strongly improve the linear optical properties due to its broadband absorption but also increase the ΔT and decrease the I_sat. The extended π-electron states of PPy nanoplates could be effectively enhanced by forming the hybrid heterostructure due to the confirmed π-π interaction, as proved in the results of the Raman and XPS measurements. It strongly boosts the additional energy band, leading to an increase in the ΔT. In addition, the graphene is originally an excellent SA with broadband absorption and fast carrier relaxation. Under the irradiation of the fs laser pulse, the transferred graphene layer also exhibits saturable absorption characteristics, which is another reason for the increase in the ΔT. The carriers generated in the graphene layer under irradiation rapidly transfer via multi-dimensional channels and fill the carrier sites on the additional energy band of PPy nanoplates, promoting the saturable absorption process and decreasing I_sat. Moreover, for the PPy-based SA studied in this work, no reverse saturable absorption process is found, indicating that the PPy nanoplates obtained by the surface-grafting method are suitable for the generation of ultrashort laser pulses.

3.3. Passively SA Mode-Locked Laser

Passively SA mode-locked technology is a low-cost and straightforward method to generate 1550 nm ultrashort laser pulses, whose performance strongly depends on the nonlinear optical absorption characteristics of SAs. The 1550 nm mode-locked fiber laser cavity is designed as shown in Figure 7. The pump energy is provided by a 976 nm distributed feedback semiconductor laser, and the gain in the fiber laser cavity is obtained from a 0.75 m high-gain EDF. The wavelength division multiplexer (WDM), polarization independent isolator (PI-ISO), polarization controller (PC), OC, and CIR are combined into the fiber laser cavity, as shown in Figure 7. The PPy and Py50/Gr. SAs are inserted into the fiber laser cavity at the one port of CIR, respectively, and a spectrometer (Anritsu MS9740A), a spectrum analyzer (Keysight N9322C), and an autocorrelator (FR-103XL) record output characteristics of the mode-locked EDF laser. The mode-locked operations are achieved by inserting the Py10, Py50, and Py50/Gr. SAs into the fiber laser cavity and adjusting the PC at the proper position, but no laser pulse is observed after inserting the Py100 SA due to the significant optical losses. Figure 8 shows the output characteristics of the Py50/Gr. SA mode-locked fiber laser.

Stable mode-locked operations are observed at the pump power, ranging from 168 to 500 mW, in which the output power of the mode-locked fiber laser increases from 1.24 to 2.51 mW, as shown in Figure 8a. The center wavelength of the output spectrum for the mode-locked operation at 168 mW is at around 1559 nm, corresponding to a 3 dB width of about 5.6 nm (Figure 8b). The pulse sequence exhibits a time interval between two mode-locked laser pulses of 306.7 ns, which is in agreement with the repetition frequency of 3.26 MHz, as shown in Figure 8c,d. According to the time interval and repetition frequency of mode-locked laser pulses, the length of the fiber laser cavity is estimated to be approximately 92 m. Figure 8e shows a high signal-to-noise ratio of 59 dB at the pump power of 168 mW, suggesting that the mode-locked operation is highly stable [29,30]. The autocorrelation trace recorded the pulse duration of the Py50/Gr. SA mode-locked laser to be 815 fs by fitting with the self-correlation function, confirming the potential utilization of the PPy/graphene hybrid heterostructure SA to develop ultrafast lasers.

4. Conclusions

In conclusion, we prepared PPy-based SAs with different PPy solution volumes, which were dropped using a simple spin-coating method. The formation of PPy nanoplates was confirmed by employing SEM, Raman, and XPS measurements. The Py50 SA exhibits a higher surface coverage rate and better optical absorption in the NIR range. To further improve the optical properties of PPy nanoplates, a new type of PPy/graphene hybrid heterostructure was developed by transferring the graphene onto the Py50 SA. After the transfer process, the additional optical absorption and the strong π-π interaction between PPy nanoplates and the graphene layer was demonstrated, which significantly enhanced the PPy SA’s optical properties, including a ~7.2% increase in linear optical absorption, a 2.5-fold increase in ΔT, and a third decrease in I_sat by the increased amount of electron states and the transferred photon-generated carriers. Based on the hybrid heterostructure, a stable 1550 nm mode-locked fiber laser was achieved, with a pulse duration of 815 fs and a repetition frequency of 3.26 MHz. The present results clearly demonstrate that the PPy/graphene heterostructure SA is a promising nonlinear optical material for ultrafast fiber lasers.