Third-order nonlinear optical phenomena lie at the heart of all-optical signal processing, where laser pulses are used to switch, modulate, or gate another laser pulse. Here we report that the solidified silk film possesses extremely large third-order susceptibilities (~10−9 esu) in a wide near-infrared spectral region. The nonlinearity of silk protein can be substantially enhanced by inducing stronger molecular interactions from van der Waals forces and hydrogen bonding. The biocompatibility and capability of nanofabrication techniques will enable us to pave new ways for nonlinear bioimaging and biosensing applications.
© 2016 Optical Society of America
Silk fibroin, a well-known old and noble fabric material with a rich history, is resurfacing in the context of high technology. The unique mechanical robustness, optical transparency, biocompatibility, controllable degradation, and ease of functionalization of silk fibroin have opened up sophisticated applications in bio-optics, electronics, and medical engineering [1–5]. To date, various optical elements have been demonstrated by employing silk fibroin, including photonic crystals , lasers [6,7], and plasmonic crystals [8,9]. These applications prove that silk fibroin is a good optical material when used in conjunction with nanofabrication techniques and functional materials. Recently, unique material traits of silk have been moving its stage to an optically functional material from a base material. For example, hydrogel properties of silk fibroin allow for tunable and stimuli-responsive nature of a nanoplasmonic resonator . These unusual material properties of silk fibroin are based on the aggregation of peptides or polypeptides. The aggregation may be considered as a new class of the self-assembled biological materials that could be useful in various optical applications as the highly nonlinear materials reported here.
Third-order optical nonlinearity of materials gives rise to a wide variety of phenomena, such as self-focusing, four-wave mixing, optical bistability, and optical Kerr effect, and offers fundamental material information that can be utilized for efficient nonlinear optical devices [11–13]. In particular, polymers, consisting of chains of repeating chemical groups, has led to their increasing use due to processibility, cost-effectiveness, durability, and enormous versatility in chemical methods. To obtain large third-order nonlinear optical effects, many conjugated polymers with delocalized π-electrons have been previously synthetized [14–18]. However, instability in air environments and low transmission in the visible and the near-infrared regions are intrinsic limitations to apply most polymers in optics and optoelectronics . Thus, polymers have been playing a limited role as a matrix for nanomaterials or as a molecular species to enhance the third-order optical nonlinearity. Recently, the optical nonlinearity of biopolymers such as DNA and amyloid fibrils have been investigated, expecting to happen unusual effects by complex molecular interactions [20,21]. Furthermore, the nonlinear absorption of individual silk fibroin molecules, dispersed in water, was studied at high laser intensity level where the multiphoton absorption occures .
In this study, we report the discovery that the solidified silk fibroin thin films exhibit large third-order optical nonlinearity which can be further enhanced by increasing a secondary molecular structure, called the β-sheet. Z-scan and optical Kerr gate (OKG) measurements were conducted in a wide near-infrared (near-IR) spectral range to investigate third-order susceptibility (χ(3)) that directly corresponds to the third-order optical nonlinearity of materials [23,24]. The measured χ(3) values fell within the range of 10−10–10−9 esu (electrostatic units), which is the highest experimental value reported for a polymer. Inducing the secondary molecular structure of a silk film could lead to a four-fold increase in the χ(3) value due to stronger intermolecular interactions. Along with the large χ(3) values, silk fibroin films are very stable in ambient air and show negligible linear/nonlinear absorption (optically transparent) up to GW-level incident intensities . Our findings will help to design all-protein-based nonlinear optical devices for applications in bio-optics, nanofabrication, and optoelectronics.
2. Experimental methods
A regenerated silk fibroin aqueous solution was prepared as reported previously . Briefly, Bombyx mori cocoons were boiled in 0.02 M sodium carbonate solution to remove the sericin glue, and then dissolved in lithium bromide solution. The solution was dialyzed against water to obtain the silk fibroin aqueous solution. A 200-nm-thick silk film was prepared on quartz substrates by spin-coating the silk aqueous solution. To induce the β-sheet secondary molecular structure, a spin-coated silk film with an amorphous molecular structure was chemically treated using methanol and water vapor .
The third-order optical nonlinearity of the samples was characterized using Z-scan technique at wavelengths of 720, 800, 1220, and 1540 nm (Fig. 1). The Z-scan measurement system based on a Ti:sapphire laser operating in a wide tunable wavelength range from 710 to 950 nm and delivering 100 fs pulses at a repetition rate of 80 MHz and a synchronously pumped optical parametric oscillator (OPO) tunable between 1090 and 1600 nm with a pulse width of 200 fs was used to investigate the third-order nonlinear optical properties. The output beam was focused on the sample by a plano-convex lens with a focal length of 50 mm. The sample was translated along the Z-axis (the propagation direction) around the focus controlled by a motorized stage. Open-aperture and closed-aperture Z-scan measurements were performed in the present study. In the open aperture Z-scan, the aperture behind the sample was fully open and the transmission change was only caused by intensity-dependent nonlinear absorption of sample. In the close-aperture Z-scan, the transmission through the aperture behind the sample was 40% and the transmission change caused by intensity-dependent nonlinear absorption and refraction was measured.
OKG measurements, enabling a relative measure of pure nonlinearity without thermal contributions, were additionally investigated to compare with and to confirm the third-order nonlinear optical properties at 800 nm measured by the Z-scan method. In our OKG experiment setup, a linear polarized laser beam was divided by a beam splitter into a pump and probe beam with a 10:1 intensity ratio by a beam splitter. The polarization of the probe beam was set to 45° with respect to that of the pump beam using a half wave plate. The separated two beams were focused on the sample by a plano-convex lens with a focal length of 70 mm. The time delay between the two beams was controlled by a motorized stage located within the optical path of the pump beam. A temporal refractive index change in the sample occurred only in the direction parallel to the polarization direction of the pump beam, so the linear polarization of the probe beam overlapped with the pump beam at the sample was changed to elliptical polarization due to the birefringence.
3. Results and discussion
The normalized transmission of the open aperture (OA) Z-scan trace using the fully opened aperture behind the sample showed the nonlinear absorption property of the material. As shown in Fig. 2(a)-2(d), the silk fibroin films showed negligible nonlinear absorption or saturation (very small imaginary part (χI(3)) of the χ(3)) in the wide wavelength range. This result indicates that silk fibroin is useful to demonstrate self-acting (self-focusing) nonlinear optical devices, derived only from the intensity dependence of refractive index, instead of two-photon absorption and third harmonic generation based on nonlinear absorption . Note that the measurements were performed at incident intensities of 11.5 GW/cm2 at 720 nm, 8.0 GW/cm2 at 800 nm, 4.1 GW/cm2 at 1220 nm, and 3.0 GW/cm2 at 1540 nm. Up to these intensity-level, no nonlinear absoption was observed.
To estimate the real part of χ(3) (χR(3)), a closed aperture (CA) Z-scan measurement was performed using a 40%-opened aperture behind the sample as shown in Fig. 2(e)-2(h). We prepared three different silk films, i.e. untreated silk film (U-SF), water-vapor treated silk film (WV-SF), and methanol treated silk films (M-SF). Valley-to-peak trace configurations in the normalized transmission spectra indicated positive nonlinear refraction. The M-SF showed a four-fold enhanced signal, whereas the WV-SF also showed two-fold enhancement at all wavelengths. To obtain the real part of susceptibility, we used the following equations: γ = Δϕ0/kI0Leff and χR(3) = n02cγ/120π2, where Δϕ0 is the relative phase shift of the transmission, I0 is the intensity of light at the focus, Leff is effective thickness, n0 is the linear refractive index, c is the speed of light in a vacuum, ω is angular frequency, γ is the nonlinear index (m2/W), and k is the wave number . The calculated values for the 200-nm-thick silk films, as an example, are shown in Fig. 3. Notably, the susceptibility values for all samples were unexpectedly large, and inducing the secondary molecular structure enhanced their susceptibilities.
As shown in Table 1, a survey of the reported χ(3) values for various materials ranked the silk films among the largest in polymers.
Figure 4 shows OA and CA Z-scan results achieved at four different intensities of 8.0, 13.2, 26.4, and 51.8 GW/cm2. For these measurements enabling the investigation of the intensity-dependence of χ(3), we used the same 200-nm-thick M-SF film and the wavelength of the pulses was fixed at 800 nm. The estimated χI(3) and χR(3) values are listed in the inset of Fig. 4. With increasing the incident laser intensity, the nonlinear absorption starts to emerge above a certain intensity-level. Above this threshold, χI(3) was logically increasing while decreasing χR(3). Up to the maximum applied intensity, we did not observe any damage of the sample. Such intensity-dependent control of large χR(3) and χI(3) values proves that the solidified silk fibroin would be useful for application as novel nonlinear optical switching devices .
In order to ensure that the large χ(3) values originate from the pure material traits of silk fibroin, we performed optical Kerr gate (OKG) measurements because the focused laser beam at a high repetition rate might cause heat accumulation in the Z-scan measurements and therefore induce additional thermal nonlinearities. The OKG measurement, a kind of pump-probe methods, traces birefringence by the probe laser beam for the fixed pumping power and is free from thermal contribution to the electronic nonlinearity . Figure 5 shows the OKG measurement results performed at the wavelength of 800 nm. The fitted Gaussian curves show symmetric forms about the zero-time delay, and these instantaneous signals mean that the nonlinear response time of the silk fibroin films are comparable or even faster than the measurement resolution related to the laser pulse duration.
As shown in Table 2, we obtained χ(3) values quite close to the Z-scan results after a comparison with carbon disulfide (CS2) reference data , which are evident for the reliability of our Z-scan measurements and indicate, again, notably large values.
Our results for silk fibroin stand out in the polymer-based optical materials since there are no natural biopolymers with such large χ(3) values and its enhancement by modulating molecular structures, reported until now. It was reported that amyloid fibrils exhibited enhanced multiphoton absorption directly related to fibrillization . However, this does not mean that the absolute values were comparable to other reported synthesized polymers. The amino acid sequence of the polypeptide chain of silk fibroin is composed of 45.9% glycine (Gly), 30.3% alanine (Ala), 12.1% serine (Ser), 5.3% tyrosine (Tyr), 1.8% valine (Val), 0.5% tryptophan (Trp), and only 4.2% of the other 14 amino acid types . The relatively small number of aromatic amino acids (Tyr and Trp), known in optically responsive molecules, and, hence, generally large intermolecular separation in the monomer protein structure make it difficult to deduce the mechanism for the large χR(3) values. However, a key to understanding the large χ(3) values and their enhancement is by increasing the strength of intermolecular interactions (van der Waals or hydrogen bonding between silk molecules) in the solidified silk film. As shown in Fig. 6(a), the solidified silk film exhibits two distinguishable absorption peaks at 228 nm and 277 nm, which correspond to the π→π* transitions of the Tyr and Trp aromatic rings . In particular, it is an interesting point that the absorption at 228 nm is distinguishable even in the only 5% aromatic amino acids containing protein when considering that the absorption is not distinguishable for the silk aqueous solution. This aggregation-induced enhancement of UV absorption can be understood in terms of the cooperative effects of intra- and intermolecular charge transfer transitions, and hence, may be also responsible for the large nonlinear χ(3) values, as reviewed and discussed recently .
From the enhancement of optical responses in the aggregated protein, it is plausible that stronger intermolecular interactions induce more enhanced optical nonlinearity. Treating the spin-coated silk film with methanol and water-vapor induced the structural transition to the β-sheet, mediated by hydrogen bonding, from the amorphous molecular structure. The intermolecular bonding strength of hydrogen bonding is stronger than that of van der Waals bonding forces and therefore make the silk film obtaining hydrogel properties . Figure 6(b) shows the Fourier transform infrared (FT-IR) spectra for the U-SF, the WV-SF, and the M-SF. The amide I (1600–1700 cm−1) and amide II (1450–1600 cm−1) regions were selected to monitor the formation of the β-sheets . In the amorphous state, there was no peak centered at 1626 cm−1, the main absorption of β-sheet in the amide I region. A large amount of β-sheet structure (~56%) was generated for the M-SF, whereas a small amount was generated for the WV-SF (~14%). As expected, the increase in β-sheet structuring of the silk film was strongly related to increasing the χ(3) value, along with the UV absorption. In contrast to the dendrimers and porphyrins [38,39], relatively well-defined polymers, it has been difficult to understand the exact microscopic mechanisms that determine the overall optical responses including third-order nonlinearity due to the complexity and general lack of structural information on silk fibroin. Our optical nonlinearity results of changes in the silk fibroin molecular structure present a strong clue to understand the enhancement of optical nonlinearity and prove that a natural protein can be reinvented as a new bio-compatible nonlinear optical material.
In conclusion, we investigated the third-order nonlinear optical responses of the solidified silk fibroin films and obtained their χ(3) values in a broad spectral range. The observed χ(3) values from the OA and CA Z-scan measurements were very large compared to those of other nonlinear polymers. This is mainly attributed to additional intermolecular interactions of the aromatic amino acids in the solidified silk fibroin. We confirmed that inducing the β-sheet secondary molecular structure enhanced nonlinearity, which was also strongly related to enhancing transition absorption of the aromatic acids. Interestingly, the real part of χ(3) was dominant in the observations up to GW-level incident intensities. Our results provide the solidified silk fibroin as a remarkable new nonlinear optical material for applications in all-optical signal processing devices, switching and controlling optical beams through light-induced changes in the refractive index. In addition, the biocompatibility and the nanofabrication capability of silk fibroin promise nonlinear bio-optical imaging and sensing devices that will be applicable in vivo.
This work was supported by the National Research Foundation (NRF) of Korea grants funded by the Korea government (MSIP: 2011-0017494, CAMM-2014M3A6B3063709, 2014R1A1A1008080, 2014K2A1B8048519, 2009-0082580).
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