3PA-induced optical limiting in pure and barium borate decorated MoS₂ nanocomposites

Abstract

Pure MoS₂ and barium borate nanorods embedded MoS₂ microspheres were successfully synthesized by adopting an environmental-friendly hydrothermal method. Powder XRD data confirmed the formation of BBO: MoS₂ nanocomposite made of barium borate [2 $\theta$ = 22^°, (018)] along with the MoS₂ [2 $\theta$ = 14^°, (002)]. The presence of few layers MoS₂ was ascertained from the measured difference in characteristic $E_{2g}^{1}$ (379 cm⁻¹) and A_1g (407 cm⁻¹) Raman modes of MoS₂. FESEM images portrayed the formation of microspheres for pure MoS₂ due to reduction in the surface energy induced by citric acid. In the nanocomposite, barium borate embedded themselves as nanorods upon MoS₂ microsphere forming an urchin-like structure. Ground-state absorption studies revealed a hyperchromic shift in absorption peak due to the overlapping of transition states between highly transparent barium borate and strongly absorbing MoS₂ in the visible region. Femtosecond laser pulses (800 nm, 150 fs, 80 MHz) were employed to perform the open-aperture Z-scan experiments revealing that both pure MoS₂ and BBO: MoS₂ nanocomposites possessed reverse saturable absorption (RSA), attributed to both 2PA and 3PA processes. Interestingly, pure MoS₂ exhibited sequential 3PA (1PA + 2PA) due to the presence of near-resonant energy states, while the dominance of transparent BBO in the nanocomposite induced genuine/instantaneous 3PA process. In closed-aperture Z-scans (for determining the sign and magnitude of nonlinear refraction), the pattern switched from self-focusing (MoS₂) to self-defocusing (BBO: MoS₂) nature. Both pure MoS₂ and BBO: MoS₂ nanocomposites exhibited strong optical limiting with a lower onset limiting threshold (0.11 µJ/cm²) due to synergetic effects of nonlinear absorption (3PA) and nonlinear refraction (self-defocusing). BBO:(0.02 M) MoS₂ nanocomposite possessing a urchin structure depicted strong NLO coefficients with $\gamma_{3PA} = 2.12 \times 10^{ - 21}$ m³/W², $n_{2} = - 11.1 \times 10^{ - 17} {\text{m}}^{2} /{\text{W}}$ and χ⁽³⁾ = $14.0 \times 10^{ - 19} {\text{m}}^{2} /{\text{V}}^{2}$, which were higher than pure MoS₂ and other composites.

1 Introduction

A wide variety of inorganic and organic materials are being extensively investigated to achieve efficient nonlinear optics (NLO)-driven devices for applications such as in laser medicine, signal processing, defence and precision measurement. Over the past few years, graphene, two-dimensional transition metal dichalcogenides (TMDC), topological insulators (TI), metal organic frameworks (MOF) have been investigated extensively as a promising optical limiting material for laser safety devices and saturable absorber for ultrafast fibre lasers [1,2,3]. Among TMDCs, molybdenum disulphide (MoS₂) has garnered increasing attention, primarily because of its peculiar band structure with exotic properties and represents an abundant, geographically ubiquitous and potentially cheap analogue of graphene [4]. It is a typical transition metal sulphide with a layered structure of covalently bonded S–Mo–S, which is bonded to adjacent layers by weak van der Waals (vdW) interactions between neighbouring S–S layers [5]. It possesses layer-dependent optical, electrical and thermal properties, i.e. the bandgap of MoS₂ decreases with increase in the number of layers due to the quantum confinement effects (direct band gap: 1.9 eV to indirect band gap: 1.2 eV) [6]. The ultimate crossover from direct to indirect bandgap nature can be accounted for a combined effect of quantum confinement and long-range columbic effects [7]. Further, thermal conductivity of the MoS₂ decreases from bulk material to a lower dimension nanostructures [8]. Earlier works on the investigation of NLO properties of pure MoS₂ revealed a saturable absorption (SA) behaviour, i.e. for higher input powers the absorption becomes nonlinear and the material exhibited high transmittance [9]. An interesting switch-over in the nonlinear absorption behaviour of MoS₂ saturable absorption to reverse saturable absorption was demonstrated through nanocomposites formation [10]. Especially, incorporating ZnO [11], TiO₂ [12] and organic polymers like PMMA [13] yielded reverse saturable absorption (RSA) due to two-photon absorption (TPA) based optical limiting in different MoS₂ nanocomposites. Optical limiting experiments on ZnO and PMMA composites with MoS₂ demonstrated low onset optical limiting threshold of 1.1 J cm⁻², 2.3 J cm⁻², respectively [11]. Impressed by the interesting NLO behaviour of MoS₂ nanocomposites, this article attempts to replace the above mentioned decorative materials with a well-known inorganic NLO material, beta barium borate (BBO). The chosen decorative exhibits many interesting properties such as wide transparency (190–3500 nm), high laser damage threshold, large birefringence and good mechanical properties suitable for NLO devices [14]. Therefore, by incorporating BBO, the major concern of lower linear transmittance in the visible-NIR region can be circumvented in MoS₂, which in turn makes them suitable for optical limiting applications. Often, these nanocomposites are expected to possess strong thermal stabilities which are essential for performing optical limiting action using intense ultrashort pulses. Additionally, advanced photonic materials made up of materials having extreme nonlinear absorption (reverse saturable and saturable absorption) are attractive for laser photonic devices such as optical limiters and mode lockers [15]. Thus, the combination of saturable absorbing MoS₂ with reverse saturable absorbing BBO nanostructures can be a potential system to investigate nonlinear absorption-based optical limiting for widely developing femtosecond IR lasers. These materials can be utilized to protect optical sensors from damage against the intense laser pulses and interrupt the transmission of a powerful laser pulse to eye-safe levels. Thus, the NLO properties of pure MoS₂ and BBO decorated on different molar concentration of MoS₂ were studied by open-aperture (OA) and closed-aperture (CA) Z-Scan technique using femtosecond laser pulses (800 nm, 150 fs, 80 MHz). Such an investigation helps us to further the understanding of NLO responses at nanoscale and provide avenues to develop new promising optical limiters for highly dangerous ultrafast IR laser pulses.

2 Material preparation

Preparation of pure MoS₂ and decoration of BBO on the surface of MoS₂ structure were achieved by simple one-pot hydrothermal technique. Initially, sodium molybdate and thiourea were taken in ratio of 2:3 and were dissolved in 120 ml distilled water. To control the agglomeration in the MoS₂ system, citric acid was added as a surfactant. The homogeneous white solution obtained on addition of citric acid was transferred to 150 ml Teflon-lined autoclave set-up and maintained at 180 °C for 24 h [16]. Following heat treatment, the solution was brought to room temperature naturally. The acquired black colour suspension in an orange colour solution was washed several times using ethanol and water and filtered repeatedly to remove the excess surfactant from the suspension. Finally, the obtained powders were kept at 200 °C in an oven to remove the adsorbed water molecules. For decoration of BBO on MoS₂, barium chloride, boric acid and sodium hydroxide were dissolved in 80 ml distilled water and kept at stirring for one hour [17]. After 30 min, 0.01 M, 0.02 M and 0.03 M concentration of acquired MoS₂ powders were added to the solution and kept at 120 °C for 24 h. The acquired BBO: MoS₂ particles were repeatedly washed and dried at 400 °C for 3 h resulting in the formation of ash white colour powder.

3 Preliminary identification: XRD

Preliminary confirmation of the material formation was achieved through powder XRD studies employed with Rigaku Ultima IV XRD at the scan speed of 7.19 degrees/min. The recorded XRD pattern of prepared pure and barium borate decorated MoS₂ is as shown in Fig. 1. The formation of pure MoS₂ and BBO: MoS₂ nanocomposites was confirmed through indexing the observed XRD diffraction peaks using JCPDS card Nos. 065-1951 (MoS₂) and 015-0862 (BBO). Identified diffraction crystal planes of MoS₂ are (002), (004), (100), (102), (103), (105), (106), (110), (112) which corresponds to the angle 2 $\theta = 14$ ^°, $29$ ^o,$32^{\text{o}}$,$34^{\text{o}} , 39^{\text{o}} , 47^{\text{o}}$,$55^{\text{o}}$, $58^{\text{o}} ,$ $60^{\text{o}}$ and beta barium borate are (018), (110), (1010), (202), (205), (119), (1112), (218), (306), (220) that corresponds to the angle 2 $\theta = 22$ ^°, $24$ ^o,$26^{\text{o}}$,$28^{\text{o}}$,$32^{\text{o}}$, $34^{\text{o}}$, $37^{\text{o}}$, $42^{\text{o}}$, $45^{\text{o}}$, $50^{\text{o}}$, respectively. Literature reports show that MoS₂ has a hexagonal shape unit cell with P6₃/mmc space group, i.e. each layer of MoS₂ is composed of S–Mo–S stacks, where a single molybdenum atom is surrounded by six sulphur atoms. Similarly, beta barium borate crystallizes in the R3c space group with rhombohedral structure having lattice constant of a = b = 7.3 Å and C = 39 Å. In pure MoS₂, diffraction peaks of (002) and (004) are highly intense compared to the other peaks, which expose the presence of well stacked layered structure of MoS₂ [18]. As the concentration of MoS₂ varied as 0.01 M, 0.02 M and 0.03 M in the BBO: MoS₂ composite, major diffraction peak of MoS₂ (002) suffered a small shift in 2θ (= 15⁰) position due to the surfactant. However, the diffraction peaks of barium borate remain unchanged in different concentrations of MoS₂ which shows the dominance of crystalline nature of BBO in composite. Absence of any other peaks that corresponds to the by-products, impurities or other phases of title compound signifies the formation of pure and BBO decorated MoS₂. Thus, preliminary confirmation made using XRD ascertain the formation of BBO: MoS₂ composite without destructing its original crystal structure.

Fig. 1

XRD patterns of (a) pure MoS₂ and barium borate decorated (b) 0.03 M MoS₂ (c) 0.02 M MoS₂ (d) 0.01 M MoS₂ composites

4 Molecular vibrations analysis: Raman spectroscopy

Molecular structural arrangement of MoS₂ and barium borate decorated on different concentration of MoS₂ was studied by Raman spectroscopy in the spectral range of 100−1500 cm⁻¹. Two characteristic peaks were observed at low range for pure MoS₂ at 379 cm⁻¹ and 407 cm⁻¹ which corresponds to in-plane $E_{2g}^{1}$ and out-of-plane $A_{1g}$ vibrational modes of the MoS₂. At in-plane vibration mode, sulphur S atoms vibrate in one direction and molybdenum Mo atom vibrates in another direction and at out-of-plane vibration mode, sulphur S atoms only vibrate in a vertical direction. It is known that with increase in number of single layers the $A_{1g}$ mode shifts to higher frequencies and the $E_{2g}^{1}$ mode shifts to lower frequencies, i.e. difference between the corresponding peak frequency of $A_{1g}$ and $E_{2g}^{1}$ increases as a function of numbers of layer. Earlier report on Raman spectrum of MoS₂ exposes that single layer of MoS₂ shows $A_{1g}$ and $E_{2g}^{1}$ peaks at 405 cm⁻¹ and 384 cm⁻¹ [19]. Here in the present case, Raman spectrum of pure MoS₂ prepared by hydrothermal method shows $A_{1g}$ and $E_{2g}^{1}$ peaks at 407 cm⁻¹ and 379 cm⁻¹, respectively. Difference in the peak position (Fig. 2) of pure MoS₂ with earlier report [19] and shift in $A_{1g}$ peak towards higher wavenumber and $E_{2g}^{1}$ peak towards lower wavenumber can be attributed to the change in layer number obtained in surfactant assisted hydrothermal method. Also in the Raman spectrum of composites, $A_{1g}$ mode encountered a shift towards higher wavenumber 419 cm⁻¹ and it represents the increase in layer thickness during composite formation, while $E_{2g}^{1}$ mode exhibited relatively weak vibration due to the thermal exfoliation at 400 °C and it corresponds to the increase in crystal defect states [20]. Further the dominance of BBO, suppressed the $E_{2g}^{1}$ and $E_{g}^{1}$ vibrational peaks of MoS₂. Although minimal variations in intensity of BBO were observed in nanocomposites, Raman spectrum was not altered significantly due to dominance of BBO vibrations and lower concentration of MoS₂.The notable characteristic peaks of BBO in nanocomposites are 493 and 602 cm⁻¹ (intra–ring bending vibration of BO₃), 703 cm⁻¹ (symmetric breathing vibration of B–O), 812 cm⁻¹ (symmetric B–O–B bridge bond vibration), 913, 996, 1115 (asymmetric stretching of BO₄) and 1264 cm⁻¹ (symmetric vibration of B₂-O₃ group), 1325, 1389 and 1442 cm⁻¹ (stretching vibration of the terminal B-O₃). It is clearly inferred that almost all the BBO vibrations are observed in the Raman spectra of nanocomposite along with the characteristic vibrations of MoS₂, thereby confirming the formation of BBO: MoS₂ composite without any molecular defects.

Fig. 2

Raman spectra of (a) pure MoS₂ and barium borate decorated (b) 0.03 M MoS₂ (c) 0.02 M MoS₂ (d) 0.01 M MoS₂ composites (e) pure BBO [17]

5 Morphological analysis: SEM

The morphological images (Fig. 3) of pure and barium borate decorated MoS₂ were recorded using a scanning electron microscope (VEGA3 TESCAN). It can be clearly visualized that MoS₂ appears like a microsphere (Fig. 3a), while BBO decorating on MoS₂ microsphere appears like an urchin structure (Fig. 3b–d). In general, MoS₂ is a 2D layered structure material in which molybdenum (Mo⁺) and sulphur (S⁻) atoms are assembled in a same plane to form themselves like nanosheets. In the present case, the possible formation of MoS₂ microsphere is due to the reduction of surface energy induced by the citric acid surfactant. Here, the surfactant (citric acid) act as a reducing agent, which easily reduces Mo⁶⁺ to Mo⁴⁺, and then bond with S²⁻ [16]. Formed MoS₂ nanosheets roll themselves together and aggregate into marigold flower like structure with average diameter of 2.68 µm through an electrostatic interaction between the nucleates of MoS₂ and citric acid. In the BBO: MoS₂ nanocomposites, with increase in MoS₂ concentration, transformation of microsphere (pure and 0.03 M MoS₂) to urchin (0.02 M MoS₂) to layer (0.01 M MoS₂) like morphology was witnessed. Earlier report shows that BBO grows as elongated nanorod due to the prolonged heating at 120 °C for 24 h [17]. In the BBO: MoS₂ composite, BBO grow themselves as nanorod with an average length of 0.92 µm (0.03 M MoS₂), 1.21 µm (0.02 M MoS₂), 1.32 µm (0.01 M MoS₂) upon the MoS₂ microspheres. The possible mechanism is that barium nucleate themselves upon the surface of readily available MoS₂ microspheres and form themselves as BBO nucleates which then grow into BBO nanorods under prolonged heating (Fig. 3e). At higher concentration (0.03 M) of MoS₂, slightly agglomerated MoS₂ microsphere (due to thermal shocking applied at 400 °C for composite formation) with traces of smaller length BBO rods was observed confirming the transition of microsphere to urchin-like structure. Interestingly at 0.02 M MoS₂, these BBO nanorods strongly embed on the surface of the MoS₂ microsphere to appear like an urchin structure (0.02 M MoS₂) with a diameter 1.69 µm (MoS₂ microsphere) and length 1.2 µm (barium borate nanorod) (Fig. 3c). At lower concentration (0.01 M) of MoS₂, due to higher density of BBO nanorods the microspheres structure got destructed to form a sheet-like structure. This mainly arises because interlayer distance of MoS₂ in microsphere morphology was increased due to the incorporation of barium borate. Thus, the formation of peculiar urchin structure BBO: MoS₂ nanocomposite can be advantageous for laser applications due to the availability of spherical MoS₂ (higher stability) along with 1D BBO rods (confined optical excitations).

Fig. 3

Morphological images of (a) Pure MoS₂ and barium borate decorated (b) 0.03 M MoS₂ (c) 0.02 M MoS₂ (d) 0.01 M MoS₂ and (e) schematic of the pictorial growth mechanism of pure MoS₂ and barium borate on MoS₂

6 Linear optical studies: absorption spectroscopy

The recorded linear optical absorption spectrum (Fig. 4) of pure and BBO decorated MoS₂ was studied by UV–Visible spectrophotometer (ELICO-SL159) in the range of 190–800 nm. The absorption maximum of MoS₂ was observed at 271 nm with absorption edge at 350 nm, which can be attributed to the excitonic features of MoS₂ microspheres. It has a strong absorption at visible to near infrared (400 –800 nm) region due to the direct transition of excitons at K point of Brillouin zone. In BBO: MoS₂ composite, the absorption maximum of MoS₂ was blue shifted to 266 nm from 271 nm when decorating the barium borate nanoparticles upon MoS₂. Literature reports reveal that absorption maxima of beta barium borate nanorods were found at 201 nm and exhibit complete transparency throughout the visible region [14]. Here, the absorption peak of barium borate was observed at 222 nm which was strongly red shifted compared to its bulk form due to the influence of 1D morphology and MoS₂ interaction. Also, in the composite the intensity of absorbance increases with increase in the concentration of MoS₂. The absorbance of BBO: MoS₂ composite is found to be low throughout the visible spectral region when compared with pure MoS₂. This hypochromic shift in the absorbance of BBO: MoS₂ clearly shows that the high linear transmittance (transparent properties) of composite arises from the contribution of highly transparent barium borate. Hyperchromic shift in absorption peak was due to the overlapping of transition states between highly transparent BBO and strongly absorbing MoS₂ in visible region. Similar observation was made for the BBO: rGO nanocomposites in which the maximum absorption peak of BBO (207 nm) was red shifted due to the restored electronic conjugation within the sheets of rGO and inclusion of barium borate [21]. Importantly, the visible and infrared region absorption of MoS₂ was greatly reduced due to the decorated transparent barium borate. Thus, the change in elemental composition along with morphology has created minor alterations in band structure which open up the possibility to explore different nonlinear absorption processes in the materials. The linear absorption coefficient of nanocomposites @ 800 nm (Fig. 4f) showed a nonlinear dependence at lower concentration of MoS₂ which can be attributed to the dominance of BBO and change in morphology of composite. Here, BBO: (0.01 and 0.02 M) MoS₂ composite possesses high linear transmittance in NIR region (800 nm) imitating the absorbance pattern of BBO, while BBO:0.03 M MoS₂ composite shows high absorbance as like pure MoS₂. It is also to be admitted that as hydrothermal method was adopted, control of MoS₂ concentration was quiet challenging. Thus due to the inclusion of transparent BBO, the linear transmittance suffered a considerable change and among the samples, BBO: (0.01 M MoS₂) composite possess high linear transmittance in NIR region (800 nm) making them superior for optical limiting action in the NIR spectral region.

Fig. 4

Absorbance spectra of (a) pure MoS2 and barium borate decorated (b) 0.03 M MoS2 (c) 0.02 M MoS2 (d) 0.01M MoS2 composites (e) pure barium borate [17] and (f) linear absorbance @800 nm

7 Nonlinear optical studies: Z-Scan

Z-Scan technique is an effective tool to understand the characteristic features of nonlinear interactions and measure third-order nonlinear optical properties such as nonlinear absorption (NLA) and nonlinear refraction (NLR) coefficients of the materials. Open-aperture and closed-aperture Z-scan patterns were recorded for both pure and BBO: MoS₂ nanocomposites using femtosecond laser pulses (Ti: Sapphire, 800 nm, 150 fs, 80 MHz) as an excitation source. Since the experiments were performed using high repetition rate pulses, thermal effects will also contribute to the observed NLO processes. In the typical experiment, the prepared samples were individually dispersed in ethylene glycol having 65% linear transmittance and were taken in a 1-mm-thick cuvette. The transmittance was measured as a function of sample position by moving the sample along the focussed beam. A graph was drawn between normalized transmittance (T_n) and position (Z) of the sample. A theoretical fit for the obtained experimental data was made using a nPA equation as proposed by Sheik-Bahae et al. [22]. The obtained open-aperture and closed-aperture patterns are shown in Figs. 5 and 7, respectively, in which solid lines represent the theoretical fits and the scattered data (circles) denote the experimental data. The following formulae from Sheik–Bahae formalism are used to estimate the normalized transmittance (nPA) of theoretically and are given by [23],

Fig. 5

Open-aperture Z-scan data of (a) pure MoS₂ and barium borate decorated (b) 0.03 M MoS₂ (c) 0.02 M MoS₂ (d) 0.01 M MoS₂ composites, (e) pure barium borate [17]. The excitation was with femtosecond pulses (800 nm, 150 fs, 80 MHz) with a peak intensity of 295 MW/cm². Symbols are experimental data points, and the solid lines are theoretical fits to the data

For an open aperture, $T_{nPA} = \frac{1}{{\left[ {1 + \left( {n - 1} \right)\beta_{n} L\left( {\frac{{I_{0} }}{{1 + \left( {\frac{z}{{z_{0} }}} \right)^{2} }}} \right)^{n - 1} } \right]^{{\frac{1}{n - 1}}} }}$

For the closed aperture, $T_{\text{CA}} = 1 \pm \frac{{4\Delta \emptyset \left( {\frac{z}{{z_{0} }}} \right)}}{{\left[ {1 + \left( {\frac{z}{{z_{0} }}} \right)^{2} } \right]\left[ {9 + \left( {\frac{z}{{z_{0} }}} \right)^{2} } \right]}}$where $\beta_{n}$ is the nonlinear absorption coefficient with n as order of the nonlinear absorption (n = 1, 2, 3…,), $L$ is the effective thickness of the sample, $I_{0}$ is the incident intensity of the laser beam at the focal point, $\left( {z_{0} = \frac{{\pi \omega_{0}^{2} }}{\lambda }} \right)$ is the Rayleigh length, and $\Delta \emptyset$ is the phase distortion of the transmitted laser beam due to refraction.

Open-aperture Z-scan technique was used to evaluate the nonlinear absorption of the material. Narrow and sharp valley pattern observed for pure and BBO decorated MoS₂ demonstrates the occurrence of reverse saturable absorption (RSA) (Fig. 5), where the transmittance of light decreased with increase in light intensity. Theoretical fits made on the experimental data revealed the observed reverse saturable absorption can be ascribed due to two-photon absorption (2PA) and three-photon absorption (3PA) processes. It is interesting to observe that the experimental data of both pure MoS₂ and BBO: MoS₂ composite show best fit for both 2PA and 3PA equations. In the UV–Visible pattern, pure MoS₂ possesses absorption in visible and NIR region and thus avails electronic states for 1PA (800 nm, 1.55 eV) and 3PA (271 nm, 4.58 eV) processes directly. The literature shows the indirect bandgap value of MoS₂ changes with the number of layers, and band structure calculated between K and Γ points in the Brillouin zone can be used to display the possible transitions responsible for nonlinear absorption [1]. Strong absorption in the wavelength of excitation (800 m) suggests that the observed 3PA is most likely to be a sequential (1PA + ESA induced genuine 2PA) process rather than simultaneous (genuine 3PA) process. The possible electronic transition involved in observed sequential 3PA of MoS₂ (Fig. 6a) is as follows: under IR excitation, an electron absorbs a photon from the ground state (E₀) and excite themselves to first excited state (E₁, 1.55 eV—inter band transition of MoS₂ between ground state and $1s_{A}$ state). Then, it simultaneously absorbs two photons to transit themselves to the third excited state (E₃, 4.57 eV—quasi continuum state of MoS₂) leading to the possible sequential 3PA through 1PA + genuine 2PA process.

Fig. 6

Energy-level diagram demonstrating sequential 3PA for (a) Pure MoS₂, (b) genuine 2PA-induced ESA for BBO: MoS₂ nanocomposite and (c) genuine 2PA for BBO [17]. Solid lines represent the available energy states and possible electron transitions during laser excitation. And virtual states are represented in dotted lines

However, in the BBO: MoS₂ (Fig. 6b) composite due to the dominance of BBO, the material shows negligible absorption in NIR region. Earlier reports show that nonlinear absorption in pure BBO nanostructures excited with similar laser pulses exhibited 2PA process [17]. Unlike pure MoS₂, due to the non-availability of near-resonant state of excitation, BBO: MoS₂ composite undergoes genuine 3PA process (via virtual states and because sufficient peak intensity is available in the femtosecond pulses used). Here, the possible electronic transition responsible for genuine 3PA is as follows: an electron in ground state (E₀) absorbs three photons simultaneously to transit themselves to available first excited state (E₃, 4.66 eV—quasi continuum state of MoS₂) through genuine 3PA. In pure barium borate excited under similar laser (ultrashort pulse) excitation, the electrons in the lowest state could possibly be excited to the self-trapped exciton state by simultaneously absorbing two photons leading to genuine 2PA (Fig. 6c) [17]. Therefore, alteration in band structure resulted in sequential 3PA in MoS₂ and genuine 3PA in BBO: MoS₂. It is be mentioned here that to resolve and confirm the postulates of proposed mechanism, intensity-dependent Z-scan studies and transient absorption spectroscopic studies with femtosecond pulses are absolutely necessary. The estimated 2PA and 3PA absorption coefficients (β_2PA and γ_3PA) of both pure and BBO: MoS₂ composite from the theoretical fits are summarized in Table 1. Considering the possible factors that can affect the experimental data like focal spot size, peak intensities, fitting procedures and calibration of intensity, the error value was fixed as ~ 5% for the chosen experimental condition. Similar error values are reported elsewhere in the literature and as the error correction is employed uniformly and the change in nonlinearity was directly compared. The error bars indicated in Figs. 5, 7 and 8 are indicative of possible inaccuracies in the data collection. We anticipate the maximum error resulting from such factors in the evaluated values of NLO coefficients to be 5%. Similar reports on Z-scan measurement with error values are already available in the literature. It is interesting to observe that BBO: MoS₂ composite possesses higher 3PA and 2PA coefficients than pure MoS₂. However, 2PA coefficient of pure BBO is higher than bare MoS₂ and BBO: MoS₂ composite. In the composite, BBO: (0.02 M) MoS₂ composite possesses higher value due to various factors like urchin morphology with microsphere structure of MoS₂, 1D structure of barium borate and overlapping the transition states of barium borate and MoS₂.

Table 1 2PA & 3PA-induced NLO coefficients of Pure and BBO: MoS₂ composites

Fig. 7

Closed-aperture pattern of (a) pure MoS₂ and barium borate decorated (b) 0.03 M MoS₂ (c) 0.02 M MoS₂ (d) 0.01 M MoS₂ composites (e) pure barium borate [17]. The excitation was with femtosecond pulses (800 nm, 150 fs, 80 MHz) with a peak intensity of 295 MW/cm². Symbols are experimental data points, and the solid lines are theoretical fits to the data

Fig. 8

Optical limiting curves of (a) pure MoS₂ and barium borate decorated (b) 0.03 M MoS₂ (c) 0.02 M MoS₂ (d) 0.01 M MoS₂ composites (e) pure barium borate [17]. The excitation was with femtosecond pulses (800 nm, 150 fs, 80 MHz) with a peak intensity of 295 MW/cm². Symbols are experimental data points, and the solid lines are theoretical fits to the data

Nonlinear refractive index (n₂) of the material was obtained from the closed-aperture Z-scan method. Pure MoS₂ shows the valley followed by a peak pattern (Fig. 7a) indicating self-focusing behaviour with positive nonlinear refractive index. Self-focusing behaviour switches to the self-defocusing (peak followed by a valley as shown in Figs. 7b–d) nature as the sign is reversal and is associated with negative nonlinear refractive index due to the inclusion of barium borate in BBO: MoS₂ composite. It is fascinating to note that the sign of nonlinear refractive index switches from negative to positive. Earlier reports show that pure barium borate nanoparticles exhibit negative nonlinear refraction [17]. In this experiment, thermo-optic effect cannot be completely neglected because high repetition rate of 80 MHz was used in excitation source, and thus, the observed nonlinear refraction can be explained through thermal lens model. Under 800 nm, 150 fs, 80 MHz laser excitation, MoS₂ showed self-focusing effect (positive nonlinear refraction leading to converging of laser), while BBO exhibited self-defocusing effect (negative nonlinear refraction leading to diverging of laser). Thus, the observed self-defocusing effect in BBO: MoS₂ composite clearly exposed the dominance of BBO in the complex system. A change in refractive index as function of temperature induced during laser excitation creates two different lenses in MoS₂ (Convex lens like) and BBO (Concave lens like). Therefore, due to dominance of BBO in BBO: MoS₂ composite, a change in the sign of nonlinear refractive index was observed. The estimated nonlinear refractive index (n₂) of pure and barium borate decorated MoS₂ is given in Table 1. Nonlinear refractive index of BBO: MoS₂ was higher than the pure MoS₂ and BBO. The superiority of MoS₂ in nonlinear refraction arises predominantly from high thermal conductivity capability of MoS₂ (2.3 Wm⁻¹K⁻¹ for few layer MoS₂) [24] with which it can transform the induced local heating during excitation (high repetition rate) resulting in stronger thermal lens model. Also, third-order nonlinear optical susceptibility of the all pure and compositing materials was estimated to be 2.84 × 10⁻¹⁹ m²/ $V^{2}$, 9.91 × 10⁻¹⁹ m²/ $V^{2}$, 14.0 × 10⁻¹⁹ m²/ $V^{2}$ and 3.08 × 10⁻¹⁹ m²/ $V^{2}$ for pure MoS₂, BBO: 0.03 M MoS2, BBO: 0.02 M MoS₂, BBO:0.01 M MoS₂, respectively. Among all the samples, BBO decorated on 0.02 M concentration of MoS₂ possesses a higher NLO coefficient such as nonlinear absorption ($\gamma_{3PA} = 2.12 \times 10^{ - 21} {\text{m}}^{3} /{\text{W}}^{2} , \beta_{2PA} = 5.73 \times 10^{ - 10} {\text{m}}/{\text{W}}$), nonlinear refractive index ($n_{2} = - 11.1 \times 10^{ - 17} {\text{m}}^{2} /{\text{W}}$) and third-order NLO susceptibility ($\chi^{\left( 3 \right)} = 14 \times 10^{ - 19} {\text{m}}^{2} /{\text{V}}^{2}$ due to urchin morphology with the availability of spherical MoS₂ (higher thermal stability against high repetition rate femtosecond laser) and 1D BBO rod (confined optical excitations).

Optical limiting curves were extracted from the open-aperture Z-scan data and using a fluence equation $(4\sqrt {\ln 2} E_{\text{in}} /\pi^{{{\raise0.7ex\hbox{$3$} \!\mathord{\left/ {\vphantom {3 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}}}} \omega \left( z \right)^{2}$) are shown in Fig. 8. The graphs represent a nonlinear pattern in which output transmittance changes nonlinearly with input fluence. And the deviation point at which the nonlinearity begins is termed as onset optical limiting behaviour of the material. Thus, the prepared materials can act as optical limiters that are transmitting a low intensity while blocking the high input intensity. Here, the observed optical limiting arises mainly due to nonlinear absorption (2PA and 3PA) and nonlinear refraction (self-defocusing). Among the samples, onset optical limiting threshold of BBO decorate on 0.02 M of MoS₂ (0.11 µJ/cm²) and pure MoS₂ (0.16 µJ/cm²) is lower than other composites. Further, these values are found to be lower than the reported values of other materials like copper niobite (0.21 µJ/cm²) [23], bismuth (2.16 µJ/cm²) [25] and zinc oxide (128 µJ/cm²) [26] excited with similar laser. As optical limiting threshold is an important parameter in OL device fabrication, an extrapolation of OA data was done and the input fluence at which normalized transmittance becomes 50% was estimated to be few tens of m $J/{\text{cm}}^{2}$ for all samples. A comparison of NLO coefficients of various composites excited with similar laser is presented in Table 2. Synergetic effects of 2PA/3PA- and self-defocusing-induced optical limiting with lower onset optical limiting threshold and higher NLO coefficients make BBO: MoS₂ nanocomposite preferable choice for laser safety devices for ultrashort IR laser pulses.

Table 2 Summary of the NLO coefficients of various composites excited with Ti: Sapphire laser pulses (at 800 nm)

8 Conclusions

In summary, MoS₂ microspheres and barium borate nanorods decorated MoS₂ microspheres with urchin-like structure were prepared by hydrothermal method. Both MoS₂ and BBO: MoS₂ exhibited reverse saturable absorption due to 2PA and 3PA. Sequential and genuine 3PA processes occurred for pure MoS₂ and barium borate decorated MoS₂, respectively, and was confirmed with the aid of available energy states from absorption spectrum. Nonlinear refraction of pure MoS₂ depicted self-focusing behaviour which switched to self-defocusing in BBO: MoS₂ composite. The dynamic processes involved in the observed nonlinearity of MoS₂, BBO and BBO: MoS₂ composites are identified as sequential 3PA [ ${\text{E}}_{ 0} {\mathop{\longrightarrow}\limits^{1PA}} {\text{E}}_{1}$ (interband transition, 1.55 eV) ${\mathop{\longrightarrow}\limits^{{{\text{genuine}} 2PA}}}$ E₃ (continuum state, 4.58 eV)], genuine 2PA [E₀ ${\text{E}}_{ 0} {\mathop{\longrightarrow}\limits^{{{\text{genuine}} 2PA}}}{\text{E}}_{ 2}$ E₂ (self-trapped exciton state of BBO, 3.10 eV)] and genuine 3PA [ ${\text{E}}_{0} {\mathop{\longrightarrow}\limits^{{{\text{genuine}} 3PA}}}{\text{E}}_{3}$ E₃ (continuum state of MoS₂, 4.66 eV)], respectively. The tunability of NLO properties achieved by varying the content of MoS₂ and BBO: (0.02 M) MoS₂ nanocomposite possess high nonlinear absorption coefficient (β), nonlinear refractive index (n₂) and third-order NLO susceptibility ($\chi^{\left( 3 \right)} = 14.0 \times 10^{ - 19} {\text{m}}^{2} /{\text{V}}^{2}$). Multiphoton (nPA, n = 2,3)-induced optical limiting action in femtosecond IR domain was demonstrated both for pure MoS₂ and BBO: MoS₂ composite and lower onset optical limiting threshold of ~ μJ/cm² opens the possibility of utilizing BBO:(0.02 M) MoS₂ for laser safety devices and as an energy stabilizer in microjoule lasers based surgery.