Anisotropically Enhanced Nonlinear Optical Properties of Ensembles

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Anisotropically Enhanced Nonlinear Optical Properties of Ensembles of Gold Nanorods Electrospun in Polymer Nanofiber Film Hang Zhang, Zhongliang Hu, Zhijun Ma,* Mindaugas Gecevičius, Guoping Dong, Shifeng Zhou, and Jianrong Qiu* State Key Laboratory of Luminescent Materials and Devices and Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, South China University of Technology, Guangzhou, 510640, China S Supporting Information *

ABSTRACT: Polymeric nanofibers containing gold nanorods (GNRs) are aligned in a uniform orientation through electrospinning. The dispersive and absorptive parts of the third-order optical nonlinear optical refractive index of the composite film measured by polarization dependent z-scan method are demonstrated to be anisotropically enhanced. Anisotropic optical response of the aligned GNRs and its connection with the ultrafast electron dynamics are discussed in light of the results of resonant femtosecond pump−probe experiments. The significant appearance of anisotropic nonlinear optical properties of ensembles of GNRs is attributed to the sensitive excitation of longitudinal surface plasmon resonance (LSPR) of highly aligned GNRs. For the macroscopic applications of ensembles of GNRs, such as passive mode-locking and all-optical switching, the experimental results demonstrate that the alignment of GNRs through electrospinning should be very high efficient, and economic. KEYWORDS: anisotropic optical properties, gold nanorods, surface plasmon resonance, electron−phonon coupling include template adhering,14 photothermal depletion,15,16 filmstretching,17,18 self-assembly based fabrication19,20 and solution drawing.21 Electrospinning technique, which is productive, costeffective, and easily controllable,22,23 has been introduced for aligning the GNRs in polymeric nanofibers.24,25 Linear optical absorption measurement revealed very large anisotropy in the macroscopically aligned long-range electrospun fibers containing GNRs. However, the nonlinear optical properties of such aligned GNRs embedded in nanofibers have not been fully investigated. In this study, we employ electrospinning to fabricate aligned polymeric nanofibers containing GNRs with orientation parallel to the axis of nanofibers. The measured coefficient of nonlinear absorption and nonlinear refractive index are anisotropically enhanced by two and 1 order of magnitude, respectively. Proposed as a saturable absorber for passive mode-locking experiments, the modulation depth and saturation intensity of the composite film are performed much more superior at the orientation parallel to the laser polarization. Ultrafast carrier dynamics related to electron−phonon scattering of LSPR mode are measured by resonant pump−probe configuration in several picoseconds time scale. The observed dependence between the time scale of electron−phonon coupling and pump power is described by the two-temperature model.

1. INTRODUCTION Femtosecond laser-induced anisotropic changes in isotropic media have been proved very useful in linear and nonlinear optical applications, such as waveplates,1 optical data storage,2 and ultrafast measurements.3 On the other hand, it has been demonstrated that the alignment of nanomaterials could show significant anisotropic optical nonlinearities without external intense-laser field.4,5 Recently, the nonlinear optical properties of gold nanospheres have been widely investigated.6,7 Compared with nanospheres, the structure of gold nanorods (GNRs) shows nonspherical symmetry, resulting in the surface plasmon resonance (SPR) modes splitting into longitudinal (LSPR) and transversal (TSPR) parts.8,9 In general, the microscopic anisotropic optical properties of a single GNR in randomly distributed ensembles of GNRs may be averaged to be a macroscopically isotropic presentation. The observation of saturable absorption of LSPR mode of GNRs, even when they are naturally randomly oriented, is due to the relatively strong response of LSPR rather than TSPR mode.10−13 Only the light with resonance frequency polarized strictly along the longitudinal axis would largely excite the LSPR of a single GNR. Therefore, only a small portion of GNRs with certain orientations in the random system can respond to the laser excitation. If the GNRs can be collectively aligned in a desired direction, the microscopically anisotropic optical properties would be dramatically amplified to a collective anisotropy in macroscopic scale. Currently developed techniques for aligning plasmonic metal nanoparticles mainly © XXXX American Chemical Society

Received: October 30, 2015 Accepted: January 5, 2016

A

DOI: 10.1021/acsami.5b10411 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

2. SAMPLE PREPARATION AND OPTICAL EXPERIMENT SETUP 2.1. Preparation of Aligned Polymeric Nanofiber Film Containing GNRs. GNRs with LSPR peaked around 800 nm were produced using the seed mediated growth method reported by Murray et al.26 Freshly prepared ice-cold NaBH4 (0.01 M, 0.6 mL) aqueous solution was added into an aqueous solution consisting of HAuCl4 (0.01 M, 0.25 mL) and CTAB (cetyltrimethyl Ammonium Bromide, 0.1 M, 9.5 mL). The growth solution was prepared by adding 6.15 g of CTAB and 1.543 g of NaOL to 250 mL of Milli-Q water at first and then AgNO3 (4 mM, 12 mL) and HAuCl4 (1 mM, 250 mL). After the solution changed from yellow to colorless, HCl (1.0 M, 4 mL) and ascorbic acid (0.064 M, 1.25 mL) were added under gentle mixing. Finally, the seed solution (1.6 mL) was added to the growth solution and left undisturbed for at least 12 h. The inset of Figure 1a shows the TEM imaging of the prepared GNRs with an average aspect ratio of 4.0.

speed up to 3000 rpm, was used as a collector. To make the composite film transparent, another material with refractive index similar to PVA was selected to fill in the space between PVA fibers. In detail, the tailored composite film was immersed into 0.05 g/mL PVP alcoholic solution and pulled-out. The step was repeated three times with 10 min intervals. The collected PVA nanofibers exhibited very high degree of alignment, as evidenced by SEM observations. The TEM image, which is shown in the inset of Figure 1b, shows the axis of longitudinal axis of GNRs is parallel to the aligned axis of PVA nanofibers. The spectrum of original GNRs shows two peaks located at 789 and 502 nm, which were attributed to the absorptions of LSPR and TSPR mode, respectively, as shown in Figure 1c. Within comparison to the original GNRs, the LSPR absorption peak of composite film has a 50 nm-redshift and broadening, which can be attributed to the increase of dielectric constant around GNRs in the composite film.27 2.2. Optical Experiment Setup. A commercial Ti:sapphire regenerative amplifier system, which emitted femtosecond laser pulses with central wavelength of 800 nm, pulse duration of 100 fs, and repetition rate of 1 kHz, was used as the laser source. The nonlinear absorption and nonliinear refractive index coefficients of the composite film were measured by using open- and close-aperture z-scan technique.28 A half-waveplate, which was placed in front of the focusing lens, was used to control the laser polarization angle relative to the axis of GNRs in the film. To investigate the ultrafast optical dynamics of GNRs in the composite film, a pump−probe experimental configuration was used. In this measurement, the polarization of pump beam was set parallel to the axis of the aligned GNRs. The anisotropic transient transmission was detected by varying the polarization of the probe beam.

3. RESULTS AND DISCUSSION 3.1. Anisotropically Enhanced Nonlinear Optical Properties. Open-aperture (OA) z-scan was performed under different crossing angles between polarization and the axis of nanofibers (Figure 2a). The laser peak intensity at the focus point was fixed at 25 GW/cm2. Sharp and narrow peak was observed in the OA z-scan curve showing the characteristic of saturable absorption in the prepared composite film. No saturable absorption was observed by measuring the PVA nanofiber film substrate (without GNRs), which confirmed the observed saturable absorption was originated from GNRs. The saturable absorption was the most obvious when the incident laser polarization was parallel to the aligned axis of the nanofibers (as well as the longitudinal axis of GNRs). Then, it decreased very fast when the crossing angle increased to 40°. When the laser polarization varied near to the perpendicular axis of the aligned nanofibers, there could not be seen any saturable absorption phenomenon in the composite film. In metal nanoparticles, the electron−hole pairs could be efficiently created in the sp conduction band because of the local field enhancement and the nonradiative plasmon decay. The energy of the pump photon is resonant with the longitudinal SP, but is smaller than the energy gap between the d-band and the Fermi level, meaning that direct interband transitions are negligible. Therefore, the observed saturable absorption was originated from the LSPR mode of GNRs, corresponding to one-photon absorption in the sp band.29 The measured OA z-scan curves in Figure 2a were fitted by the following expression in ref 28.

Figure 1. (a) Schematic sketch of preparation of aligned nanofiber film containing gold nanorods (GNRs). (b) SEM image of the aligned nanofibers. The inset shows the TEM image of a single nanofiber. The red arrow denotes the orientation of nanofibers. (c) Linear absorption spectra of GNRs and the composite film.

The prepared GNRs solution was purified by centrifugation (10000 rpm for 10 min) to remove excess CTAB. The concentrated GNRs were then added to 7 wt % PVA aqueous solution under vigorous stirring to fully dispersed GNRs. The resulting purple-brown homogeneous precursor solution was loaded into a 20 mL plastic syringe and was pumped to the needle by a syringe infusion pump at a flow rate of 1.0 mL/h. During electrospinning, high voltages of 10.0 and −8.0 kV were applied to the needle and the nail, respectively, as shown in Figure 1a (more details about electrospinning device can be found in Supporting Information). A high speed roller, highest B


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Figure 2. (a) Nonlinear absorption and (b) nonlinear refractive measured by open- and close-aperture z-scan method, respectively, at different crossing angles between incident laser polarization and longitudinal axis of GNRs. (c) Nonlinear absorption coefficient β and refractive index coefficient γ extracted from panels a and b at different crossing angles. (d) The modulation depth and saturation intensity of the composite film at different crossing angles. ∞

T (z , S = 1) =

∑ m=0

[−q0(z)]m (m + 1)3/2

T=1+ (1)

2(x 2 + 3) 4x ΔΦ − ΔΨ (x 2 + 9)(x 2 + 1) (x 2 + 9)(x 2 + 1) (2)

where q0(z) = βI0Leff/(1 + z2/zR2), zR is the Rayleigh length of the incident femtosecond laser beam, I0 is the on-axis intensity at the beam waist, Leff is the effective length, and β is the nonlinear absorption coefficient. By fitting the curves, the values of β at different crossing angles were obtained (Figure 2c). The values of β decreased as the crossing angle increased. A maximum value of β was about −1.47 × 10−6 cm/W when the crossing angle was 0°. The corresponding values of β could not be detected when the crossing angle was larger than 40°, showing a significantly anisotropic saturable absorption originated from the aligned GNRs. The nonlinear absorption coefficient β measured at 0° was about 2 orders of magnitude larger than those measured at crossing angles larger than 40°. Meanwhile, we measured the nonlinear absorption of GNRs which were randomly distributed. The macroscopic response of LSPR mode of randomly distributed GNRs shows isotropic behavior (see Figure S3a). By the alignment, the saturable absorption coefficient can be enhanced about 1 order of magnitude (see Figure S3b). Simultaneously, close-aperture (CA) z-scan measurements were performed, as shown in Figure 2b. In the CA z-scan curve, a valley followed by a peak as the value of z changed from negative to positive, showing the nonlinear refractive index in the composite film was positive. Similar to the results of OA zscan measurements, the nonlinear refraction was the most obvious when the polarization of the incident laser was parallel to the axis of GNRs aligned in the composite film. That means the nonlinear phase shift reflected by the peak−valley separation distance in the CA z-scan curve was the largest, which was due to the fully resonant excitation of LSPR mode of aligned GNRs. The CA z-scan curves at different excitation angles were simulated by30

with ΔΦ = kγI0Leff ,

ΔΨ = βI0Leff /2

where ΔΦ and ΔΨ, respectively, are the nonlinear phase shifts due to the nonlinear refraction and the nonlinear absorption, kis the wave vector, and x = z/zR indicates the dimensionless relative position from the waist. The third-order nonlinear optical refractive index coefficient γ was extracted from the fitted curves of CA z-scan measurements, as shown in Figure 2(c). The value of γ is about 26.41 × 10−12 and 3.52 × 10−12 cm2/W at the laser polarization parallel and perpendicular to the orientation of GNRs, respectively. By the alignment, the third-order nonlinear refractive coefficient was enhanced by about 7.5 times. By fitting the saturable absorption curves (see Figure S4), the saturable absorption parameters were obtained (Figure 2d). The modulation depth and saturation intensity at the crossing angle of 0° were 30.6% and 11.7 GW/cm2, respectively. Though the modulation depths increased a little when the crossing angle varied from 10° to 20°, the saturation intensities increased a lot. For the larger crossing angle, the saturation intensities decreased fast as well as the modulation depths. For the application of passive mode-locking, the saturable absorbers with lower saturation intensity but much larger modulation depth, which help the self-starting of mode-locking and the decreasing of threshold respectively, are much more beneficial. 3.2. Anisotropic Transient Optical Response. The transient optical response of ensembles of aligned GNRs in the composite film was examined by femtosecond pump−probe experiments. The appearance of anisotropic optical response was measured by varying the polarization of probe beam, as shown in the inset of Figure 3a. In gold the interband transition energy is ∼2.4 eV, in close proximity with the TSP peak but quite off from the LSP peak C


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Figure 3. (a) Probe beam polarization dependent on ultrafast optical response in the composite film. The polarization of pump beam was set parallel to the axis of the nanofibers as well as the GNRs. θ denoted the crossing angle between the probe and pump beam. (b) The maximum value of transient transmittance (max. ΔT/T) at different probe polarization angles, as well as the linear transmission (which were measured without the pump beam).

Figure 4. (a) Pump power dependence of transient transmittance (ΔT/T) in the resonant pump−probe experiments. The polarizations of pump and probe beams and the aligned axis of GNRs were paralleled to each other. The experimental data were fitted by a single exponentially decaying function. (b) The pump power dependence of the maximum ΔT/T and ultrafast electron−phonon coupling times. A linear fitting is done for τe−ph at relative low pump power intensity range.

(1.46−1.88 eV). The energy of the pump photon is resonant with the LSP but is smaller than the energy gap between the dband and the Fermi level, meaning that direct interband transitions could be neglected. In previous works, the transient photo bleaching was observed at excitation wavelengths either in resonance or shorter than the LSPR of GNRs.31 Similarly, in our measurements, the polarity of the transient differential transmission (ΔT/T) signals near zero delay is positive, that is, arising from ground state photo bleaching for the aligned GNRs with ELSP smaller than the probe photon energy Eprobe. By increasing the polarization angle of probe beam, the fitted bleach recover time were maintained at about 5 ps, which were independent of the probe beam polarization. However, the transient transmission (ΔT/T) decreased very fast as probe polarization angle increased from 0° to 90°. This could be attributed to that the pump beam induced saturable absorption of ensembles of aligned GNRs was the most efficient at θ = 0°. The bleach signals at zero time delay (the maximum ΔT/T) are plotted in Figure 3b with the comparison of linear transmission of the probe beam. The maximum ΔT/T appeared when the polarization of probe beam was parallel to that of pump beam as well as the orientation of GNRs. Meanwhile, the value of linear transmission decreased to the lowest due to the strongly absorption of LSPR mode of GNRs. As promoted to be an ultrafast all-optical switching configuration based on the ensembles of aligned GNRs, the ratio of switching signal to background could be enhanced about 66 times. 3.3. Ultrafast Electron−Phonon Relaxation. We set the polarization of probe beam parallel to that of pump beam, as well as the longitudinal axis of aligned GNRs. Figure 4a shows photobleaching curves measured at different pump power

intensities. The bleach intensity (ΔT/T) is initially enhanced as the pump power intensity increased. At higher pump power the bleach intensity levels off, when complete saturation of the excitonic transition is reached. The measured initial rise times (∼500 fs) are nearly same at different pump power intensities, which is attributed to the electron thermalization. The recovery times of photo bleaching under different pump power intensities were fitted by a single exponentially decaying function, ΔT = A1 + A2 exp(−t/τ). Figure 4b shows the pump power dependence of the maximum value of ΔT/T in the film. This is in agreement with the OA z-scan results in Figure 2c, confirming that the nonlinear absorption response of the aligned nanofiber film is dominated by a saturation process. The measured decay time increases from 1.6 to 4.4 ps as the pump power increases from 8.5 to 40 GW/cm2. The decay time of several picoseconds time scale indicates the electron− phonon scattering dominate in the decay processes. This is in agreement with previous research that the electron−phonon coupling is pump power dependent because of the temperature-dependent heat capacity of the electronic distribution.31−33 The dependence of the energy transfer time on the pump power is ascribed to the electron excitation dependence on the electron−lattice energy transfer kinetics. After establishment of an electron temperature, that is, a few hundred femtoseconds, the electron−lattice energy exchanges can be described by twotemperature model34 D


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ACS Applied Materials & Interfaces Ce(Te) Cl

∂Te = −Ge − ph(Te − Tl) ∂t

∂Tl ≈ Ge − ph(Te − Tl) − Gph − ph(Tl − T0) ∂t

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*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 11404114, 51302087, 51132004), the China Postdoctoral Science Foundation (Grant No. 2015T80903), Guangdong Natural Science Foundation (Grant No. S2011030001349), and open fund of State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics.

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where κ = G[1/Ce(Te) + 1/Cl(Tl)]. For the weak pump power, Ce(Te) can be approximated by Ce(T0), leading to an exponential decay of the electron temperature, with a time constant τ0e−ph ≡ 1/κ = γeT0/Ge‑ph. The reasonably accurate value of τ0e−ph can be determined by extrapolating τe−ph to zero pump intensity, as shown in Figure 4b at pump power intensity lower than 25 GW/cm2, giving an intrinsic electron−phonon relaxation time τ0e−ph = 1.31 ps. This implies an electron− phonon coupling constant Ge−ph = γeT0/τ0e−ph = 1.5 × 1016 W/ m3·K, in good agreement with the results reported for gold nanoparticles.36,37 For high pump power intensities, which are larger than 25 GW/cm2 here, the SPR oscillation is strongly excited, implying larger amplitude of coherent collective electron oscillation. When the pump power intensity was further increased to 55 GW/cm2, the electron−phonon scattering time was further increased to 5.6 ps. Meanwhile, a very fast decay of ∼360 fs was observed, attributing to the electron−electron scattering.10,33 As mentioned above, for the application of passive modelocking, the recovery time of saturable absorber is demanded to be short, which helps the emptying rate of state in conduction band for the next light absorbing.11,12

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REFERENCES

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4. CONCLUSION In conclusion, large-scale polymeric nanofiber film containing aligned GNRs was fabricated by electrospinning. The measurements of anisotropic nonlinear optical properties, such as nonlinear refractive index, modulation depth, and transient transmission, demostrates that the anisotropically enhanced nonlinear optical properties of ensambles of GNRs. Furthermore, the dynamics of hot electrons of the composite film were measured by pump−probe experiments. These experimental results revealed that the time scale for energy relaxation by electron−phonon coupling was strongly depedent on the pump power, which can be interpreted in terms of the twotemperature model.

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AUTHOR INFORMATION

Corresponding Authors

Here, Ce(Te) and Cl are the heat capacities of the electrons and lattice, respectively, and Ge−ph and Gph−ph are the coupling coefficients between the electrons and the lattice and between the lattice and surroundings, respectively. Following electron thermalization electrons cool by exchanging heat with the lattice. The electron heat capacity is temperature-dependent: Ce(Te) = γeTe, with γe = 71.5 J/m3·K2.35 By contrast, the lattice heat capacity Cl is nearly independent of temperature. For short time delays, the thermal coupling between the GNRs and the surroundings can be neglected. Equations 3 and 4, describing the evolution of the electron and lattice temperature occurring on picoseconds time scales, simplify to ∂(Te − Tl) ≈ −κ(Te − Tl) ∂t

Details of electrospinning device, nonlinear absorption measurement of randomly distributed GNRs, and descriptions for fitting open z-scan curves (PDF)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10411. E


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Anisotropically Enhanced Nonlinear Optical Properties of Ensembles

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