Experimental observation of enhanced reverse saturable absorption in Bi 2 Se 3 nanoplates doped PMMA thin film

By employing the ultrafast Z-scan technique, we characterize the nonlinear absorption property of PMMA/Bismuth Selenide (Bi 2 Se 3) composite with varying concentrations. We report the fabrication of bismuth selenide (Bi 2 Se 3) nanoplate (topological insulator (TI)) doped poly methyl methacrylate (PMMA) thin film with varying doping concentrations. The effect of Bi 2 Se 3 on structural and linear properties of PMMA thin film has been investigated through UV-Vis spectroscopy, scanning electron microscope (SEM), and Energy dispersive x-ray spectroscopy (EDS) elemental mapping techniques. Furthermore, the nonlinear optical absorption property of PMMA and PMMA/Bi 2 Se 3 composites have been performed employing a single beam open aperture z-scan technique under femtosecond laser excitation at 750 nm. The z-scan results exhibit an enhancement of reverse saturable absorption (RSA) property with an increased nonlinear absorption coefficient (β) of the PMMA/Bi 2 Se 3 composites compared to pure PMMA measured with intensity at 320 GW cm−2. The RSA response gets enhanced with the increase in doping concentration also. Our experimental observations reveal that PMMA/Bi 2 Se 3 composite can provide a promising platform to realize photonic devices such as optical limiters, optical switches, and efficient protectors from high power sources.


Introduction
In recent years, there has been tremendous development in the combined fields of metamaterial and plasmonics domain. Researchers have invested great effort to understand the complex physics in that domain and realized them in a multitude of nanophotonic applications [1,2]. The strong localization of electromagnetic energy in the nanoscale of plasmonic structure triggers to employ of it with various functionalities such as photovoltaic devices [3], optical sensors [4], optical filter [5,6], optical ring resonator [7], and optically assisted magnetic data storage technology [8]. Surface plasmon polaritons (SPP) has been exploited effectively to realize information carrier for ultra-compact inter-chip interconnects and all-optical data processing [9][10][11]. However, despite immense advancement, plasmonic and metamaterial devices suffer fundamental limitations owing to the high energy dissipation in plasmonic media which prevents to employ especially in the visible to ultraviolet (UV) spectral range [12]. Traditional plasmonic materials (such as gold and silver) suffer from strong dissipation due to the interband electronic transitions and Drude losses [13]. Hence, there has been a surge of research interest aiming to explore alternative low-loss plasmonic materials in the high-frequency spectral range. Researchers are investigating materials such as metallic alloys and oxides, highly doped semiconductors, two-dimensional materials, and more recently Topological insulators (TIs) materials [14,15]. Recent years have witnessed a growing interest in the study of TI materials as an alternative promising platform capable of overcoming these issues [16].
TIs are a new class of electronic materials that are gaining considerable scientific and technical attention among the scientific community due to their intrinsic properties over other materials [17][18][19]. TI materials exhibit conducting states at the edge and surface containing a Dirac cone with a helical spin in the momentum Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. space while showing bulk insulating properties characterized by a gaped band structure [20,21]. The fascinating behavior of TIs triggers to employ it in a multitude of applications which includes quantum computing [19], dissipation less electronics [22], spintronics [23,24], enhanced thermoelectric [25], optical recording [26], highperformance field effect transistor [27], near-infrared flexible electrodes [28], thermoelectric and infrared applications [29], laser photonics and high-speed optoelectronic devices [30][31][32], etc.
Owing to the strong light-matter interaction of TI materials, investigation of linear optical properties as well as nonlinear optical (NLO) properties are attracting a considerable amount of research interest [33][34][35][36]. NLO properties of TIs and their different composites have been reported by several groups employing the z-scan technique [36][37][38][39]. An intensive amount of work has been reported demonstrating ultrashort pulse generation employing TIs as saturable absorber [40][41][42][43][44]. Among all other TIs, Bi 2 Se 3 which has a relatively large bulk band gap (0.3 eV) and tunable surface band gap (controlled by layer thickness), provides a promising platform to realize optical devices for room temperature application [45,46]. In accordance with the electronic properties, researchers are exploring the nonlinear optical properties of the novel material [39,[47][48][49][50]. The Bi 2 Se 3 film exhibits excellent saturation absorption (SA) property and has been employed effectively as a saturable absorber in achieving mode lock laser [40,51]. The SA property of different layered Bi 2 Se 3 films has been investigated using the Z-scan method revealing the influence of the second surface state (SS) and the thickness on the SA property of the material [52]. The work provides an effective approach where controllable SA using Bi 2 Se 3 could be achieved by properly tailoring the thickness or the excitation wavelength. Very recently, an effective technique has been reported where the nonlinear optical property of the Bi 2 Se 3 can be well-tailored using ion irradiation by controlling the defects in Bi 2 Se 3 which have been introduced intentionally [53]. There are ample scopes to realize the TI:Bi 2 Se 3 material in a myriad of applications. On the other hand, Poly-methylmethacrylate (PMMA) is a commonly used polymer for the preparation of thin film as most of the material gets decomposed into PMMA [54][55][56]. In addition to this, PMMA has moderate properties such as low-cost, hightransparency window with good flexibility, good physical and chemical properties, and easy handling capabilities, which stem researchers to use it in a wide range of applications [57][58][59].
In this study, we demonstrate the fabrication of TI:Bi 2 Se 3 doped PMMA thin film with varying doping concentrations. The structural and linear properties of the PMMA and PMMA/Bi 2 Se 3 composites have been studied through ultraviolet-visible (UV-Vis), SEM, profilometry, and energy-dispersive x-ray (EDX) elemental mapping techniques. Furthermore, the reverse saturation absorption (RSA) property of pure PMMA and PMMA/Bi 2 Se 3 composites has been investigated by employing the ultrafast open aperture (OA) z-scan technique. We observe an enhancement of the RSA property of the PMMA/Bi 2 Se 3 composites compared to pure PMMA. RSA increases effectively with the increase in doping concentration.

Fabrication of PMMA/Bi 2 Se 3 composites
Bi 2 Se 3 nanoplates were synthesized by a solvothermal method, wherein BiCl 3 , Se, NaOH, and polyvinyl pyrrolidone (PVP) powders, and ethylene glycol solutions were used as precursors. For the synthesis, suitable amounts of BiCl 3 and Se powders were first dissolved in an already prepared solution of PVP and ethylene glycol. To this solution, another solution made by dissolving NaOH in distilled water was added. The resulting solution was stirred for 30 minutes and subsequently sealed in a 50 ml autoclave. The autoclave was then heated to 180 0 C in a furnace for 36 hours and then cooled naturally to room temperature. The black-colored product was then washed several times in de-ionized water and alcohol and finally dried in a vacuum.
The solutions of Bi 2 Se 3 and PMMA with varying concentrations of Bi 2 Se 3 in PMMA were prepared. A stock solution of Bi 2 Se 3 /DMF (55% w/v) was prepared. To this solution, we added the high molecular weight (Mw = 996,000 g mol −1 ) PMMA (by Sigma Aldrich) and Dimethylformamide(DMF) to obtain the varying concentration ratio of Bi 2 Se 3 to PMMA, keeping the concentration of PMMA (7% w/v) in all final solutions fix. The solutions were stirred for 1 hour to obtain a homogeneous solution. The concentration of Bi 2 Se 3 with respect to PMMA varied to 0.36 and 0.73 wt% in the final solution. The High concentration and high molecular weight of PMMA provide the necessary viscosity which facilitates the formation of desired film thicknesses via spin-coating. The solution was spin-coated on the vitreous quartz substrate at 1000 R.P.M. to form a thin film with thicknesses of about 1 μm. Next, the thin film of PMMA/Bi 2 Se 3 was dried in an oven for 4 hours at 40 0 C to remove any residue of DMF in the thin film. Finally, the different solutions of pristine PMMA and Bi 2 Se 3 /PMMA were used for the analysis.
The UV-Vis absorption spectra of the prepared samples were measured between 200 nm to 1000nm range by absorption spectrometer and are shown in figure 1. The absorption peak for Bi 2 Se 3 doped thin films appears at 225 nm. The morphology of pure and doped PMMA thin films was investigated, which is shown in figure 2. The synthesized Bi 2 Se 3 nanoplates are shown in figure 2(a) with considerable magnification. The uniform morphology of PMMA thin film on quartz substrate deposited by the spin coating method is shown in

Experimental setup and results
The RSA measurement has been carried out by the open aperture (OA) single beam z-scan technique. The schematic of the OA z-scan setup is shown in figure 4. The OA z-scan has been performed using an output of  750 nm from a commercial optical parametric amplifier (TOPAS PRIME, Light Conversion Inc.) which is seeded with a Ti: sapphire regenerative amplifier (Libra He, Coherent Inc.) laser pulse at 808 nm, 150 fs, at 1 kHz repetition rate. The fs laser pulses are focused with a beam waist of 35 ± 2 μm using a plano-convex lens of  15 cm. The sample is placed in a cuvette of 2 mm path length. A motorized translational stage (GTS 150 Newport) which is connected with a motion controller (ESP 301 Newport), has been employed for the movement of the sample across the focal point. The transmitted beam is collected by Si photodiode (Thorlabs, PDA100A) to recover the OA signal. To avoid saturation, we have placed variable neutral density filters before the photodiode. The OA signals are connected with the lock-in amplifiers (Signal Recovery 7225), which are also triggered by a 1 kHz transistor-transistor logic pulse from the delay generator (SDG Elite) of the laser. We also incorporate another reference signal to avoid pulse to pulse fluctuation where the reference signal is collected by a photodiode (PD1) after passing through a beam splitter (BS1) and focusing lens (L1), and is fed to the lock-in amplifiers. For more precision in the OA experiment, we collect the scattered light signal through a photodetector which is placed at an angle of 5 0 to the z-axis. The lock-in amplifier and motorized translational stage are connected with LabVIEW 2014 environment for automation and averaging the experimental data.
The NLO properties of pure PMMA and Bi 2 Se 3 /PMMA composites have been investigated by OA z-scan method at 750 nm wavelength with a photon energy of 1.65 eV (2.64 × 10 −19 J). The input intensity has been fixed at 320 GW cm −2 . The experimental results are shown in figure 5 which comprises a normalized transmittance of the doped and undoped films. The normalized transmittance of the three curves exhibits a symmetric valley about the focus and lowest value at z = 0, which demonstrates the RSA property of the three samples. Figure 5  for pure PMMA, PMMA/Bi 2 Se 3 0.36 wt% and PMMA/Bi 2 Se 3 0.73 wt%, respectively. In the qualitative interpretation of the experimentally observed OA z-scan results, we have carried out an analysis based on intensity-dependent absorption [60]. A single Gaussian beam profile with a beam waist w 0 can be expressed as, where α(I) is the intensity-dependent absorption coefficient. The total absorption coefficient combining the SA coefficient and the two-photon absorption (TPA) coefficient, can be expressed as, where I and I sat are laser radiation intensity and saturation intensity, respectively. β is the nonlinear absorption coefficient. The first term on the right-hand side describes negative nonlinear absorption such as SA, whereas the second term represents positive nonlinear absorption such as RSA, TPA, etc. To obtain the transmitted intensity of the incident light beam, equation (3) has been substituted in equation (2) and finally, the intensity equation has been solved using the Crank-Nichelson method. The transmission curve obtained from the OA z-scan experiment has been fitted theoretically. The nonlinear absorption coefficient β and I sat have been evaluated from the theoretical fittings of the experimental data. Since, the three samples have the absorption peaks in the wavelength range 200 nm-250 nm (figure 1), the origin of RSA could be attributed to the excited state absorption (ESA) or a two-photon absorption (TPA) process [61]. The calculated I sat and β values for pure and doped PMMA composites have been summarized in table 1. It is observed from the table that β values have been significantly increased with higher concentrations in doped PMMA than in pure PMMA composites. Furthermore, to characterize the optical limiting property of the samples, we have placed the samples at the focal point. The optical limiting behavior is measured at 750 nm wavelength and shown in figure 6, where

Conclusion
In conclusion, we have demonstrated the RSA of pure and Bi 2 Se 3 -doped PMMA composites with the OA Z-scan technique at 750 nm wavelength in the femtosecond time scale. The structural and linear properties of pure and doped composites have been investigated by UV-Vis spectroscopy, SEM, and EDS elemental mapping techniques. Our experimental results revealed that compared to pure PMMA, Bi 2 Se 3 -doped PMMA composites exhibit a stronger RSA response, and the response also gets enhanced with increased doping concentration. The experimental data were fitted perfectly by the Z-scan theory. Our observations demonstrate that Bi 2 Se 3 -doped PMMA with high concentration could provide a very promising platform to realize optical devices such as optical limiters and optical shutters.

Data availability statement
The data that support the findings of this study are available upon reasonable request from the authors.