1 Introduction

Following the invention of the laser in 1960s, it was recognized that intense laser beams can easily damage delicate optical instruments, especially human eyes. Thus, protection from lasers is not only a scientific subject but also a potential public safety issue [1,2,3]. The protection of such instrumentation has led to comprehensive and extensive research in the field of optical limiting (OL). A successful optical limiter should strongly attenuate intense optical beams while exhibiting high transmittance for low-intensity ambient light levels and therefore efficiently protects human eyes and optical sensors from accidental or hostile laser pulses. More recently, several materials and device configurations, including fullerenes [4, 5], phthalocyanines [6, 7], porphyrins [8, 9], carbon nanotubes [10, 11], graphene [12, 13], and metal complexes [14, 15], have been proposed and developed to meet this challenge. Among these materials, organic and organometallic compounds with nonlinear optical (NLO) properties have emerged as promising candidates for limiting the intense laser irradiation because of their large nonlinearities, inherently fast response time, broadband spectral response and ease of processing. Porphyrins (Pors) and phthalocyanines (Pcs) are especially attractive because their NLO properties can be tailored by suitable structural modifications, including varying the central metal atom or the axial substituted groups. The OL behavior of Pcs has been widely studied in liquid and solid matrices, and they have shown some promising results [16]. Conversely, few reports are available on the OL behavior of Pors, especially in the solid-state matrix.

Porphyrins are ‘the pigments of life,’ with an extensively delocalized two-dimensional conjugated π-electron system consisting of four pyrrole subunits linked by four methine bridges, showing a large extinction coefficient in the visible-light region, predictable rigid structures, and prospective photochemical electron-transfer ability [16]. A great number of structural changes including the variation of the central atom and/or peripheral substituents at the free pyrrolic positions can be carried out on Pors without altering their chemical stability. Other interesting properties of this molecule and many of its derivative products are their versatility, architectural flexibility, and high environmental stability, which are very important requirements for implementing in photo-electronic applications. Previous studies showed Pors and metallo-Pors were effective optical limiters because their ground-state absorption is confined to narrow regions, which allows high transmission between Soret and Q bands. In addition, Pors exhibit a large excited-state cross section and a long triplet excited-state lifetime. These properties render Pors as a potential candidate for OL applications. The literature also reveals that peripheral substitution on the meso- or β-positions and central atom would significantly influence the OL behavior of Pors [16]. However, up to know, most of the studies have been conducted in a liquid-state matrix. Despite the studies being of significant fundamental importance, from the point of practical application, it would be more important to introduce Pors in a solid-state matrix and fabricate Pors-doped composites for the design of OL devices.

One attractive approach to obtain solid-state materials is the sol–gel technique. Sol–gel-derived silica glasses exhibit excellent optical, thermal, and mechanical properties, high threshold of laser damage, and ideal transparency in the UV region. Furthermore, sol–gel-derived inorganic composites can provide low synthesis temperatures and mixtures between guest molecules and host matrices without arming the physical properties of the doping component [17, 18]. This enables the encapsulation of OL materials and has been widely used to incorporate a variety of organic and organometallic molecules in inorganic hosts. For example, Zhan et al. [19,20,21] reported the improved OL properties of different peripherally modified palladium phthalocyanine encapsulated in silica gel glass obtained using the sol–gel technique.

In the present study, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin Cobalt(II) (CoPor) was introduced into ORMOSIL gel glass using a single-step sol–gel technique. Scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), and UV–Vis spectroscopy were performed to investigate the morphology, structure, thermal stability, and linear optical properties of the ORMOSIL gel glasses. The NLO properties of CoPor-doped ORMOSIL gel glasses were investigated using a nanosecond open-aperture (OA) Z-scan technique at 532 nm.

2 Experimental

2.1 Reagents

2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine Cobalt(II) (Fig. 1), 3-aminopropyltriethoxysilane [NH2(CH2)3Si(OC2H5)3, APTES], 3-glycidoxy-propyltrimethoxysilane [CH2OCHCH2O–(CH2)3–Si(OCH3)3, GPTMS], N,N-dimethylformamide (DMF), tetraethyloxysilane (TEOS), and ethanol were analytical grade, purchased from Sigma-Aldrich, and used as received without further purification.

Fig. 1
figure 1

Structure of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine Cobalt(II)

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2.2 Sample preparation

Optically transparent CoPor-doped ORMOSIL silica gel glasses were fabricated through hydrolysis and polycondensation of TEOS, GPTMS, and APTES. The molar ratio of (TEOS + GPTMS + APTES)/ethanol/distilled water in the precursor was 1:4:4; the molar ratio of TEOS/GPTMS/APTES was 7:2:1. In addition, DMF was introduced as a solvent and drying control chemical additive at a proportion of 0.6 DMF per ethanol volume ratio. Specifically, 8.0, 2.3, 1.2, 12.1, and 3.7 mL of TEOS, GPTMS, APTES, ethanol, and water, respectively, were mixed under ultrasonication for 30 min. Subsequently, 7.3 mL of DMF suspension containing different contents of CoPor was gradually added to the mixture, which was continuously ultrasonicated for 2 h. The mixture was divided into several parts with equal volumes, individually cast into polystyrene cells, sealed, and left to age and dry for several weeks.

The solutions of CoPor in DMF with molar concentrations of 4 × 10−5, 6 × 10−5, 1 × 10−4, and 2 × 10−4 mol were also made for reference. Meanwhile, silica gel glasses were also prepared for comparison.

2.3 Characterization

The morphology of CoPor-doped ORMOSIL was confirmed by field-emission scanning electron microscopy (FESEM: JEOL JSM-6700, JEOL Ltd., Tokyo, Japan). For the SEM observations, Au was deposited onto the freshly fractured surface of the sample by sputtering. FT-IR spectra were obtained using a ThermoNICOLET6700 spectrometer. The samples were prepared as KBr pellets. TGA was performed using a NETZSCH STA 449 F3 thermal analysis instrument (heating rate, 10 °C/min; nitrogen atmosphere protection). Data from 25 to 1000 °C were collected. UV–Vis spectra were detected using a Cary 50 spectrophotometer (UV-2600). The gel glasses were fixed vertically in the quartz cell using plasticine.

2.4 Z-scan measurement

The NLO behavior of CoPor-doped ORMOSIL gel glasses was examined by the OA Z-scan technique [22]. The excitation light source was an Nd:YAG laser (Brio 640, Quantel, Les Ulis, France) with a repetition rate of 1 Hz. The laser pulses (period, 4 ns; wavelength, 532 nm) were split into two beams by a mirror. The pulse energies in the front and at the back of the sample were monitored with energy detectors D1 and D2 (PE25, Ophir Optronics Solutions Ltd., Jerusalem, Israel). The laser beam waist was approximately 14.5 μm, and the energy of a single pulse was set at 200 μJ. All measurements were conducted at room temperature. Hybrid gel glasses were vertically fixed with a clamp. Each sample was mounted onto a computer-controlled translation stage that shifted the sample along the z-axis.

3 Results and discussion

3.1 Structure of CoPor-doped ORMOSIL gel glasses

The representative photograph in Fig. 2 shows that the CoPor-doped ORMOSIL samples are transparent gel glasses with good optical quality. The pink appearance confirms the successful incorporation of CoPor into the silica gel glasses. In addition, the color of the glasses changes from red to pink, depending on the doping concentration. With a uniform thickness and diameter of 1.3 and 25 mm, respectively, these samples meet the requirements of optical measurements without further processing. The FESEM images in Fig. 3 show a fracture surface of the SiO2 and CoPor-doped ORMOSIL gel glasses. It is noted that CoPor cannot be detected in the images because of the molecular doping level in the gel glasses. In Fig. 3, both samples were composed of round-like nanoparticles or aggregates. The granules of the CoPor-doped ORMOSIL gel glasses were smaller and more homogenous, and its surface was more condensed compared with those of the silica gel glasses. This is reasonable because during the hydrolysis and polycondensation, the presence of CoPor in the precursor would induce the silica granules to grow around them through inhomogeneous nucleation, which results in the formation of more homogenous silica granules. Furthermore, the addition of CoPor molecules would promote the hydrolysis and polycondensation procedure and then form a condensed surface through the fill effect.

Fig. 2
figure 2

Representative photograph of CoPor-doped ORMOSIL silica gel glass

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Fig. 3
figure 3

SEM images of silica ORMOSIL gel glass (a) and CoPor-doped silica ORMOSIL gel glass (b) with a concentration of 1 × 10−4 mol

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The FT-IR spectrum of CoPor-doped ORMOSIL gel glass is shown in Fig. 4. Compared with the blank sample without CoPor, incorporation of CoPor does not visibly affect the overall IR spectral features of the gel glass. The bands at 1090, 790, 550, and 460 cm−1 correspond to the vibration of Si–O–Si groups [23], which are formed after the hydrolysis and condensation of TEOS, GPTMS, and APTES. There is also a shallow band at 960 cm−1 caused by the vibration of the Si–OH group [23], which is a byproduct of the incomplete condensation of silanes. Additionally, the bands at 2970 and 2881 cm−1 are attributed to the C–H stretching vibration [24]. The broad band at approximately 3440 cm−1 and the sharp band at 1670 cm−1 are assigned to stretching and bending of O–H, respectively [23]. The FT-IR spectrum strongly suggests that the silanes are thoroughly hydrolyzed and polymerized and indicates that the formation of a silica gel glass network during the sol–gel process was successful.

Fig. 4
figure 4

FT-IR spectra of silica ORMOSIL gel glass (a) and CoPor-doped silica ORMOSIL gel glass (b)

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3.2 Thermal stability of CoPor-doped ORMOSIL gel glasses

Figure 5 shows the typical TGA curves of CoPor-doped ORMOSIL gel glasses with doping concentration of 1 × 10−4 mol, in which four stages can be classified. In the first stage (room temperature to 120 °C), the mass loss is approximately 17%; the result is attributed to the evaporation of the physically adsorbed water and ethanol. In the second stage (120–370 °C), the mass loss is found to be approximately 10% because of the organic solvents with low melting points, such as DMF. A subsequent and sharp mass decrease of 23% occurs from 370 to 650 °C, which is attributed to the thermal decomposition of organic solvents. A mass loss of approximately 3% is observed in the fourth stage (650–1000 °C) because of the dehydration and evaporation of chemisorbed water. The weight losses of CoPor-doped ORMOSIL gel glasses decrease compared to non-doped SiO2 in all stages. The results reveal that the thermal stability of the composite gel glasses slightly improves after CoPor is added into the SiO2 because CoPor promotes the hydrolysis and polycondensation, and hence, results in less presence of residual organic solvent and ethanol in the silica network and a more condensed structure.

Fig. 5
figure 5

TGA curves of silica ORMOSIL gel glasses and CoPor-doped silica ORMOSIL gel glasses with a concentration of 1 × 10−4 mol

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3.3 Linear properties of CoPor-doped ORMOSIL gel glasses

Porphyrins usually exhibit characteristic electronic absorption in the visible and near-infrared region (Q-band at ~600 nm) including α- and β-bands and near-ultraviolet region (Soret band at ~400 nm), both arising from π → π*transitions. Figure 6a displays the UV–Vis absorption spectrum of Pors in a DMF solution. The curve has a prominent band around 390 nm (Soret band) together with a shoulder at 415 nm and a less remarkable band around 550 nm with a shoulder center at 520 nm assigned to vibration of the α- and β-bands in Q-band. The shoulder at 415 nm corresponding to the dimerization band of Pors implies that the Pors molecules aggregate to a certain degree in DMF solution. Figure 6b shows the UV–Vis absorption spectra of gel glasses, which rise with the increase in the CoPor concentration, and all of them show Soret and Q bands, indicating the successful introduction of CoPor into silica gel glasses and retention of the ground-state optical properties of CoPor after being incorporated in the ORMOSIL silica gel glasses. It is obvious that the shoulder at 415 nm disappears while the β-band in Q-band becomes obvious after CoPor is introduced into ORMOSIL gel glasses. The reason is that the special structure of CoPor with an extensively delocalized two-dimensional conjugated π-electron system together with the cross-linked silica network has what is known as a ‘cage protection effect’ [25], which effectively hinders the movement of the doping CoPor molecules. This results in a steric hindrance effect, which offers opportunities to reduce the intermolecular interactions, preventing two or more rings from close proximity and therefore suppressing dimerization, and therefore reducing the probability of a collision and suppressing aggregation to a certain extent. The Soret band at 390 nm has a slight blueshift with increasing concentration of CoPor, which might be attributed to the increasing interaction between CoPor molecules and the rigid silica matrix.

Fig. 6
figure 6

UV–Vis spectra of CoPor in DMF solution (a) and CoPor-doped silica ORMOSIL gel glasses with different concentration (b)

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3.4 Nonlinear properties of CoPor-doped ORMOSIL gel glasses

OA Z-scan measurements were carried out in the nanosecond regime at 532 nm to investigate the NLO properties of CoPor. Figure 7a presents the OA results of CoPor in a DMF solution and gel glass composites with the same linear transmittance of 60% at 532 nm. In the OA Z-scan, the transmittance of the sample was determined during its translation through the focal plane of a tightly focused beam. As the sample moves closer to the focus, the beam intensity increases and the nonlinear effect occurs, which then results in decreasing transmittance because of reverse saturable absorption (RSA), two-photon absorption, and nonlinear scattering. The depth of the valley in the Z-scan curve directly determines the NLO performance. Clearly, CoPor-doped ORMOSIL gel glass shows better NLO performance than CoPor in DMF solution. Meanwhile, the OL effects of CoPor in a DMF solution and gel glass composites, manifested by plotting the normalized transmittance versus the input energy density, were calculated from the OA Z-scan measurements, as shown in Fig. 7b. Both of the samples present a similar trend, that is, the transmittance remains the same at low input fluence and then decreases as the fluence increases. For quantitative comparison, values of OL threshold, which is defined as the input fluence at which transmittance decreases to 50% of the linear transmittance because of nonlinearity, were extracted from the OL curves. The optimal limiting thresholds of CoPor in a DMF solution and gel glass composites are at 3.83 and 2.42 J cm2, respectively. The demonstration of the improved NLO behavior of CoPor in solid silica gel glasses indicates its great potential for solid OL devices.

Fig. 7
figure 7

OA Z-scan curves (a) and OL curves (b) of CoPor in DMF solution and CoPor-doped silica ORMOSIL gel glasses with a concentration of 1 × 10−4 mol

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To demonstrate the potential advantage of CoPor-doped ORMOSIL silica gel glasses in nonlinear optics field, their OL performance was also compared to that of C60 in toluene under the same experimental conditions and the results are shown in Fig. 8. The depth of the valley in a Z-scan curve directly indicates the extent of OL behavior. The minimum transmittance of CoPor-doped ORMOSIL silica gel glasses at Z = 0 is comparable to that of C60 solution, implying that CoPor-doped ORMOSIL silica gel glasses can be a good candidate for OL applications.

Fig. 8
figure 8

OA Z-scan curves of CoPor-doped silica ORMOSIL gel glasses with a concentration of 6 × 10−5 mol and C60 in toluene solution

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It has been widely accepted that the OL mechanism of porphyrin in a liquid matrix under 532 nm irradiation results from RSA [16], in which the absorbing material has an excited-state absorption cross section, σ ex, larger than the ground-state absorption cross section, σ g. As the optical excitation intensity increases, more molecules are promoted to the excited state, thus giving rise to higher absorption at intense light excitation. The enhancement of OL in CoPor-doped ORMOSIL gel glasses may be attributed to CoPor molecules being introduced into the solid-state gel glass. The surrounding rigid matrix can effectively decrease the probability of non-radiative relaxation and therefore increase the population of S 1 followed by the increase in intersystem crossing and population of T 1. The result is an increase in excited absorption and the improvement of OL properties.

In addition, the OA Z-scan results of CoPor-doped ORMOSIL silica gel glasses with different concentration and irradiated under different input laser intensity are shown in Fig. 9a, b, respectively. Using the Crank–Nicolson finite-difference scheme, the value of the nonlinear extinction coefficient β can be fitted numerically to the following transmission equation for a third-order nonlinear process [22]:

Fig. 9
figure 9

OA Z-scan curves of CoPor-doped silica ORMOSIL gel glasses with different concentrations (a) and OA Z-scan curves of CoPor-doped silica ORMOSIL gel glasses with a concentration of 1 × 10−4 mol at different input laser intensity

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$$ T(z,s = 1) = \frac{1}{{\sqrt \pi q_{0} (z,0)}}\int_{ - \infty }^{\infty } {\ln \left[ {1 + q_{0} (z,0)e^{{ - r^{2} }} } \right]} {\text{d}}r . $$
(1)

Here, \( q_{0} (z,0) = \beta I_{0} L_{\text{eff}} \), where \( I_{0} \) is the on-axis peak intensity at the focus (\( z = 0 \)), \( L_{\text{eff}} = {{[1 - \exp ( - \alpha l)]} \mathord{\left/ {\vphantom {{[1 - \exp ( - \alpha l)]} \alpha }} \right. \kern-0pt} \alpha } \) is the effective thickness of the sample, α is the linear absorption coefficient, and l is the sample thickness. The β values of gel glasses with different CoPor concentration and irradiated with different input laser intensity are fitted by Eq. 1 and together with the linear transmittance (T L) are shown in Fig. 10. β gradually increases, whereas T L is reduced with the increase in CoPor, suggesting that a higher CoPor concentration leads to better NLO response at 532 nm and the NLO of the composite gel glasses can be optimized by tuning the concentration of CoPor in the gel glass matrix. Meanwhile, the β gradually increases with the increase in input laser intensity, which verified the advantage of the composite system in nonlinear optics fields.

Fig. 10
figure 10

Dependence of nonlinear extinction coefficient (β) and linear transmittance (T L) in ORMOSIL gel glasses on CoPor concentrations (a) and relationship between β and input laser intensity (b)

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4 Conclusion

CoPor was introduced into solid-state ORMOSIL by an easy sol–gel technique. SEM, TGA, FT-IR, and UV–Vis spectroscopy were performed to investigate the morphology, composition, structure, thermal stability, and linear optical properties of the composite gel glasses. The FT-IR spectrum strongly indicated the formation of a silica gel glass network during the sol–gel process. The UV–Vis spectra confirmed the successful encapsulation of CoPor in ORMOSIL silica gel glasses. The introduction of guest CoPor molecular can result in more condensed surface characteristics of silica-based gel glasses owing to the promotion of hydrolysis and polycondensation, and hence better thermal stability compared to blank silica gel glasses. Meanwhile, the dimerization phenomenon in a liquid matrix can be effectively suppressed in a silica solid-state matrix as a result of the ‘cage protection effect.’ These results prove that the sol–gel technique effectively produces Pors-based solid-state bulky composites. The NLO properties of CoPor gel glasses were investigated using the open-aperture Z-scan technique at 532 nm. The NLO performance in CoPor-doped solid-state silica gel glasses improved in comparison with those dispersed in DMF solution. More significantly, the NLO properties of CoPor-doped ORMOSIL gel glasses can be controlled by adjusting the concentration of the CoPor molecules. Such homogeneously CoPor-doped silica gel glasses with the combination of convenience, thermal stability, and good NLO properties indicate that CoPor-doped ORMOSIL gel glasses have potential applications in optics, all-optical switching, and related optical devices.