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Ultrafast Optical Modulation of Rationally Engineered Photonic− Plasmonic Coupling in Self-Assembled Nanocrystalline Cellulose/ Silver Hybrid Material Guang Chu,† Hang Yin,‡ Haijing Jiang,† Dan Qu,† Ying Shi,‡ Dajun Ding,*,‡ and Yan Xu*,† †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China ‡ Institute of Atomic and Molecular Physics, Jilin University, 2699 Qianjin Street, Changchun 130012, China S Supporting Information *

ABSTRACT: Controlling light−matter interactions in optoplasmonic structures has recently attracted considerable attention due to their potential applications in metamaterials and metasurfaces. Here we present a bottom-up self-assembly method for large-scale organization of plasmonic silver nanorods (SNRs) into photonic liquid-crystalline cellulose nanocrystal matrices, exhibiting rationally engineered photonic−plasmonic coupling across the nearultraviolet spectra range. In these metamaterials, we show that the resonant coupling of the photonic−plasmonic mode has a strong effect not only on the stationary optical response of guest SNRs but also on controlling light in ultrashort time scales. By using the time-resolved femtosecond pump−probe technique, we experimentally investigate the relaxation dynamics of SNRs embedded in hybrid photonic−plasmonic systems as the photonic band gap of the host matrix provides a varying local density of optical states to manipulate the radiative lifetimes of SNRs. The close correlation between the structure and optical properties allows for rational design of optoplasmonic composites with tailored plasmonics and light processing, and it also paves the way for a welldefined field enhancement substrate with applications in ultrasensitive spectroscopies.

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INTRODUCTION

actions in nanoscale structures is to design and fabricate the coupled photonic−plasmonic hybrid systems.19 Photonic−plasmonic coupling is the combination of plasmonic modes in metallic nanostructures and localized photonic modes in dielectric optics,20−22 and it can be used to obtain an intense localized electric field and manipulate the photonic local density of optical states (LDOS), providing degrees of freedom for creating optical fields with a preengineered phase and amplitude that are absent in conventional single photonic or plasmonic nanostructures.23 Light−matter interaction in photonic−plasmonic resonant structures is very fascinating due to its high-performance enhancement in sensing, catalysis, energy storage, and optics.24−26 Recent research efforts have shown great success in building photonic−plasmonic hybrid devices through self-assembly techniques, such as introducing flat noble metal films or plasmonic nanoparticles into a dielectric photonic crystal,27−30 integration of plasmonic nanoparticles into periodic arrays with the nanoparticles diffractively coupled,31 and liquid-crystal -dispersed long-range ordered fluid composites composed of

We live in a world of waves, and waves are all around us. The quest for trigger, control, and engineer waves in time and space drives the development of metamaterials with novel properties and promising applications.1−4 Surface plasmon is a kind of surface electromagnetic wave that occurs at the metal− dielectric interface in noble metal nanostructures.5 In surface plasmons, the collective oscillation of free electrons in the conduction bands of metals couples with the electromagnetic field of incident light, resulting in a stronger oscillation of the conduction electrons, which is commonly known as surface plasmon resonance (SPR) modes. The SPR modes in noble metal nanostructures can concentrate the electric field at the metal surface, endowing them rather unique physical properties and important uses in fluorescence resonance energy transfer,6 catalysis,7 sensing,8 negative-index material,9 nonlinear optics,10 surface-enhanced Raman scatterings,11−15 and so forth. A common strategy for manipulating SPR waves in metal nanostructures depends on their size, shape, phase, and composition (chemical synthetic control)16 as well as the collective distribution, orientation, and interparticle spacing (physical dynamic control),17,18 and so forth. In parallel, another exciting prospect for controlling light−matter inter© 2016 American Chemical Society

Received: September 7, 2016 Revised: November 4, 2016 Published: November 7, 2016 27541

DOI: 10.1021/acs.jpcc.6b09052 J. Phys. Chem. C 2016, 120, 27541−27547

Article

The Journal of Physical Chemistry C

Figure 1. (a) TEM image of CNCs prepared from fast evaporation of a dilute suspension, which exhibits individual crystallites. The inset in (a) shows the high-magnification image of CNCs. (b) SEM image of SNRs with low magnification. The inset is the TEM image of the SNRs. (c) Static absorption spectra of a SNR suspension at a concentration of 0.01 mM. (d) Representative FIB image of the PC sample in the cross-sectional view, showing an extensive periodic structure with a counterclockwise twisted arrangement of CNCs. (e) FIB image cut of the sample with an oblique view at the interfaces between SNRs and CNCs, showing vertical and lateral gaps of SNRs in a layered CNC matrix. (f) POM image of a SNR-doped composite film with characteristic marble-like birefringence textures under crossed polarizers (black double arrows), indicating the high-quality longrange alignment.

anisotropic plasmonic nanoparticles.32−34 However, manufacturing the photonic−plasmonic coupled metamaterials in a cost-effective way still remains a significant challenge, especially for their mass production. The most important structural component in plants, cellulose, is an inexhaustible biopolymer on earth with fascinating applications and properties.35 A cellulose nanocrystal (CNC) can be extracted from woody biomass by controlled acid hydrolysis.36 It is a rod-like nanoparticle with excellent dispersibility in water. When a CNC’s suspension is above critical concentration, CNCs can self-assemble into a liquid-crystalline lyotropic phase with chiral nematic or nematic ordering.37 What’s more, such ordering can be preserved in solid CNC films after water evaporation,38 generating a robust matrix for assembling guest functional nanoparticles.39−42 Notably, the helicoidal ordering of the chiral nematic structured CNC films produces brilliant colors with a photonic band gap (PBG) that can be engineered via changing the conditions of film preparation.43,44 The directed self-co-assembly of functional nanoparticles using liquid-crystalline CNCs as soft templates enables precise control of the nanoparticle’s spatial distribution at nanoscale level,45−47 offering a simple “bottomup” method for the fabrication of functional hybrid materials with greatly enhanced physical properties. Therefore, by combining the photonic mode in the CNC’s matrix and the SPR mode in plasmonic nanoparticles, it is possible to achieve controlled photonic−plasmonic coupling in CNC-based hybrid materials. Recently, much attention has been paid to the study of interaction forces in photonic−plasmonic hybrid devices. It shows that the resonant coupling in these hybrid materials has a strong effect on the stationary optical response of guest plasmonic nanoparticles;19,29,48 however, little is known about the transient optical properties in photonic−plasmonic hybrid devices, and the formation of hybrid light−matter states in photonic−plasmonic structures as well as the intrinsic photophysics and dynamics are still far from being understood. The

strong coupling effects between photonic crystals and plasmonic nanoparticles are on the femtosecond or picosecond time scales, and previous relevant experiments were all performed under nonresonant excitation conditions without detailed information on the hybrid states.49,50 Therefore, the question that naturally arises from a chemist’s point view is, what is the inherent property of such a hybrid material? Can we modulate the optical response of the photonic−plasmonic device in the ultrafast transient region? In this article, we demonstrate that the LDOS in a dielectric photonic CNC matrix can be employed to fully manipulate the SPs propagation in guest metallic silver nanorods (SNRs). For this purpose, we have prepared a series of rigorous couplingdependent hybrid devices composed of CNCs and SNRs. The optical coupling between a photonic CNC matrix and guest SNR gives rise to generation of modulated SPR properties. Additionally, we also find that the transient optical properties of SNRs can be strongly tailored by the varying degree of photonic−plasmonic coupling, so that it is possible to achieve controlled modification of the ultrafast optical response in this hybrid material. We have also investigated the dynamics of the hybrid photonic−plasmonic states under resonant excitation, giving details of coherent coupling in these hybrid devices.

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RESULTS AND DISCUSSION

CNCs and SNRs were prepared by a slight modification of the previously reported methods (Supporting Information).51,52 The as-prepared CNCs are negatively charged (−60 ± 0.5 mV) with rod-like morphology, showing an average length of 200− 300 nm and diameter of 15−20 nm (Figure 1a). Such a primitive CNC suspension is known to form a chiral nematic structured liquid-crystalline phase during evaporation due to a synergetic interplay of the negatively charged groups on CNCs and the twisted ribbon shape of the rod-like colloids. The chemically grown SNRs have an average length of 6.2 μm and an average diameter of 60 nm (Figure 1b), with two distinct SPR peaks of the in-plane dipole and quadrupole plasmon 27542


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The Journal of Physical Chemistry C coupling53 centered at 382 and 350 nm (Figure 1c), respectively. The photonic−plasmonic hybrid materials are prepared by evaporation-induced self-co-assembly of SNRs and CNCs using a modified procedure based on the previous report,32 with the details in the Supporting Information. Briefly, 0.5 mL of SNR suspension (0.01 mM) was mixed with 5 mL of CNC aqueous suspension (3.5 wt %) and stirred at room temperature for 30 min to ensure homogeneity. Then, this homogeneous suspension was transferred into a 60 mm polystyrene Petri dish and allowed to dry for 3 days at ambient conditions to cast the hybrid film. Finally, the resulting freestanding, large-area (cm2-sized) composite film was obtained with a thickness of around 200 μm, showing strong structural colored iridescence and plasmonic activity (photonic−plasmonic coupled composite (PC) samples). The microscopic structural details of sample PC1 were obtained from the focused ion beam (FIB) images prior to optical experiments. It shows an counterclockwise twisted layered structure of the left-handed chiral nematic organization of CNC nanorods (Figure 1d), which is responsible for the photonic properties of the CNC matrix.54Figure 1e is the oblique view of sample PC1 with low magnification. Obviously, the SNRs have been embedded in a layer-structured CNC matrix with some vertical and lateral crosses, suggesting the existence of photonic−plasmonic coupling. Retention of chiral nematic ordering in the PC samples is also confirmed by polarized optical microscopy (POM). The POM image of PC1 shows a strong birefringence property with marble-like patterns or planar textures, indicative of a chiral nematic structured composite (Figure 1f).44 In order to better understand the hybrid photonic− plasmonic coupled superstructure, we have also investigated the chiroptical activity of the PC composite. Figure 2 is the

electrolyte in the SNR suspension. The reason for choosing PC4 as a representative is that the plasmonic bands of SNRs in PC4 are located far away from the corresponding PBG. Therefore, we can distinguish the differences in chirality between the photonic matrix and guest SNRs. The SNR-free CNC films exhibit a positive CD peak at ∼700 nm, typical of a left-handed nature of the chiral nematic matrix. The CD spectrum of PC4 shows a series of CD bands: two strong positive CD signals at 400 and 356 nm, which can be ascribed to the chiral plasmonic mode of SNRs (Figure 2 inset), and an intense CD band at around 700 nm that corresponds to the chiral photonic matrix. This combined nature of chiroptical activity in PC samples not only indicates the photonic− plasmonic coupling in the hybrid superstructure but also provides solid evidence for the chirality in both CNC matrices and guest SNRs. Figure 3 illustrates the schematic of the hybrid PC samples. In such an architecture, the host CNC nanorods self-assemble

Figure 3. Sketch of the photonic−plasmonic coupled hybrid composite illustrating the extended capability of light processing.

into a left-handed chiral nematic ordering with the refractive index changes periodically, making the matrix behave as a onedimensional photonic crystal. The reflection peak wavelength can be termed as λ = navg × P, where P represents the helical pitch and navg is the average refractive index.55 Meanwhile, the guest SNRs are self-embedded into the chiral nematic CNC matrix with random distribution, leading to a significant optical modulation of the hybrid composite. When the incident light passes through the hybrid composite, light admission into the composite interior can be mediated by SP polaritons of SNRs, which are diffractively coupled to the propagating light.22 Therefore, compared with the SNR-free composites, plasmonic-assisted light processing in the hybrid PC composites can provide additional polarization, amplitude, frequency, and direction-dependent selectivity of light coupling to a photonic CNC matrix, leading to a synergy effect between photonic and plasmonic resonances. As we suspect, the preparation of CNCSNR-based metamaterials with a spatially engineered profile that combines two resonant subsystems could help us realize new concepts of transformation optics that allow for manipulation of the flow of the light beam to open doors to light-based technology. In chiral nematic ordering, the position of the PBG is optically tunable from visible to near-infrared regions by changing its helical pitch. By tuning the electrostatic repulsions between CNC nanorods prior to film casting, we can obtain chiral nematic CNC films with varying PBGs. Therefore, a relative question takes for granted whether we can manipulate

Figure 2. CD spectra of the sample of PC4 and its corresponding SNR-free films. The inset is the zoomed-in CD spectra of the chiral plasmonic bands in PC4. The spectra appear cut off because the peaks are greater than 3000 mdeg, which is the maximum detectable signal on the spectropolarimeter that was used.

circular dichroism (CD) spectra of the sample of PC4, along with the spectrum of a SNR-free CNC film. The corresponding UV−vis spectra are presented in Figure S1; both PC4 and the SNR-free composite show a significant PBG at 700 nm, which indicates the existence of chiral nematic ordering. Compared with the SNR-free sample, the PBG position of PC4 is a little red-shifted, which may be due to the addition of unremoved 27543


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Figure 4. Static UV−vis absorption spectra of the PC samples (a) PC4, (b) PC3, (c) PC2, and (d) PC1 with varying degrees of photonic−plasmonic coupling, exhibiting distinct LDOS induced optical modulation.

strongly overlapped with its SPR peaks, so that the plasmon bands are nearly completely enhanced with the highest optical intensity (Figure 4c). However, for PC1 with the plasmon band at the red end of the PBG (positioned at 295 nm) in the host matrix, things get more interesting (Figure 4d). It is worth noting that the spectrum shape changed with the modulation of photonic−plasmonic coupling and the SPR peak at 356 nm got stronger than that at 400 nm. These phenomena that we observed imply strong photonic−plasmonic coupling in hybrid materials and selectively induced PBG modulation of the SPR bands. As we know, the nature of surface plasmons changes when they propagate along the metal surface in a modulated photonic surrounding medium.19,23 In the photonic−plasmonic coupled hybrid materials, when the frequency of surface plasmons is within a band gap, the LDOS of the plasmonic mode is zero and no plasmonic mode can be supported. When the frequency is at the band edge, the plasmonic mode dispersion is flat and the LDOS of the plasmonic mode is high, so that there is a significant increase in the associated field enhancement, leading to selective modulation of the plasmon bands of SNRs. These results demonstrate that the resonant coupling of the localized plasmonic mode and the photonic mode can strongly tailor the stationary optical response of SNRs.56 However, the static absorption spectra only reveal evidence for strong interactions between the surface plasmon mode and the photonic cavity mode but tell us nothing about the nature (intrinsic photophysical interactions and dynamics) of these hybrid states. Under a strong coupling regime, the photonic− plasmonic coupled interactions are so strong that new hybrid states are formed by rapid electron−electron scattering and electron−photon scattering; thus, they populate transiently.57 The great advantage of transient absorption measurements is their ability to distinguish the small changes in absorbance,

the degrees of photonic−plasmonic coupling by altering the PBG of the CNC matrix. Figure 4 shows static absorption spectra of the hybrid PC samples consisting of a plasmonic SNR and photonic CNC matrix. A series of broad SP bands is observed at around 400 nm, and the absorption due to interband transitions of silver is not seen in this spectral range. As can be clearly seen, the SPR peaks of the SNRs are slightly shifted to longer wavelengths (356, 400 nm) as compared to those of the isotropic SNR suspensions (Figure 1c). This is a result of the increase of refractive index in the surrounding medium from that of water (n = 1.33) to the CNC matrix (n = 1.56). The PBGs in PC samples are optically tunable from nearultraviolet to visible ranges with their positions centered at 295, 400, 550, and 700 nm. The sample of PC4 can be used as a reference because its PBG is positioned far away from the plasmon bands of SNRs, so that no photonic−plasmonic coupling occurs (Figure 4a). It shows a series of absorption bands, among which the three strong absorption peaks at 700, 400, and 356 nm can be attributed to the PBG of the CNC host matrix, the coupling of the SNRs’ dipole, and quadrupole plasmon resonances, respectively. In Figure 4a, from another point of view, it also should be noted that the SPR intensity at 400 nm is stronger than that at 356 nm, and its spectrum shape is similar to that of the sample of SNRs embedded in a nonphotonic matrix (Figure S2). When the PBG in the host matrix is blue-shifted to a smaller wavelength (from 550 to 295 nm), there occurs a strong coupling between the photonic mode in the CNC matrix and the plasmonic mode in guest SNRs, exhibiting varying degrees of coupling intensity. For the sample of PC3, whose plasmon bands are located in the bandedge of its PBG (positioned at 550 nm), the SPR peaks at 400 and 356 nm have a stronger optical intensity than that of PC4 and a similar spectrum shape (Figure 4b). For the sample of PC2, the PBG (positioned at 400 nm) in the host matrix is 27544


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Figure 5. Transient spectra recorded at different times after excitation at 400 nm of the varying degrees of the photonic−plasmonic coupled hybrid composite, (a) PC4 (no coupling) and (b) PC2 (full coupling).

implying photonic−plasmonic intermediate transient optical changes. These results demonstrate that resonant coupling in photonic−plasmonic hybrid materials not only enables us to tailor their stationary optical response of plasmonic nanoparticles but also can control light on an ultrashort time scale. The dynamics of the transient optical response are also affected by the strong photonic−plasmonic coupling. Figure 6

which provides robust information about the ground state and generation of the excited state in photonic−plasmonic coupled materials. To verify this, various PC samples were analyzed with a pump−probe laser setup. The time-resolved transient absorption spectrum is taken by a 50 fs laser at very low pump intensities (2 μJ/pulse) to avoid destroying the photonic− plasmonic hybrid materials, and the excitation pump pulse wavelength is fixed at 400 nm, under the resonant conditions of in-plane dipole plasmon coupling of SNRs. The FIB images of the PC samples after ultrafast laser irradiation are presented in Figure S4. No evident morphological changes are observed, indicating less damage to the hybrid structure. First, we studied the transient optical response of the hybrid materials without photonic−plasmonic coupling. Figure 5a shows the temporal evolution of transient absorption spectra for sample PC4. It can be seen that the transient spectral evolution steeply rises (redshifted) following the ultrafast pump pulse, reaches a maximum value at 470 nm within 1 ps, and then relaxes back (blueshifted) to equilibrium within a few picoseconds. It can also be noticed that the spectral shape of the PC4 shows a positive transient broad band from 450 to 600 nm, extending over a large spectral range. The observed red shift of the plasmon band in our study can be interpreted in terms of the nonlinear dielectric function of guest SNRs, namely, the changes in the real part of the dielectric constant of silver nanocrystals.58 However, when the photonic mode is fully matched with the plasmonic mode of SNRs, the transient spectral profile of the SNRs can be strongly affected by this coupling. Figure 5b shows transient absorption spectra for PC2 with the strongest photonic−plasmonic coupling. Compared with PC4, the spectral signals of PC2 show a massive difference. The coupling leads to an obvious sharpening of the modulation spectral profile around its maximum, with positive transient peaks positioned at 455 nm. The intensity of these transient peaks is smaller than that of PC4. These results underline the importance of photonic−plasmonic coupling to change the ultrafast optical modulation of plasmonic metal nanoparticles. Thus, we can conclude that strong photonic−plasmonic coupling can result in sharpening and spectral shifting of its transient optical response. The sharpening may result from restriction of absorption to the PBG in the CNC host matrix, and the shifting is related to light trapping of both the pump and probe fields in the photonic cavity.49 In addition, it is worth noting that the transient absorption spectra of PC1 and PC3 (Figure S3) with an imperfect photonic−plasmonic coupling mode have shown a transient trace between PC2 and PC4,

Figure 6. Plot of the normalized kinetic traces extracted from the corresponding transient spectra recorded at 500 nm for the samples PC2 and PC4. The lifetime of the induced absorptions is fitted well with double-exponential decay curves with our instrument response function of 50 fs.

shows the normalized kinetic trace of samples PC2 and PC4 at 470 nm following 50 fs irradiation. Once the transient signal reaches its maximum in a very short time, it relaxes back to equilibrium. For silver nanocrystals, the dynamics of the transient relaxation shows two components after the athermal regime: (1) the fast component shows a characteristic time scale of a few of picoseconds, which corresponds to electron− phonon energy exchange, and (2) the slow component is related to heat releasing from the nanoparticles’ interface to the surrounding medium, which lasts for dozens of picoseconds.59,60 The lifetimes of the photoinjected SNRs are sensitive to coupling of photonic−plasmonic mode. For PC samples, the decay times are analyzed from the relevant decay curves; assuming a two-component exponential decay, they are of ∼1.1 ± 0.1 ps (fast), 14.4 ± 2.1 ps (slow) for PC2 and 3.9 ± 0.5 ps (fast), 28.4 ± 4.0 ps (slow) for PC4, respectively. The slow component of the relaxation dynamics is related to 27545


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Professor Xiao-An Zhang for valuable assistance in POM imaging analysis.

nanoparticle cooling as the inputted energy in PC4 is larger than that in PC2, with the larger heat to be released before reaching thermal equilibrium. The fast decay in the PC sample is related to the influence of the photonic−plasmonic coupling as the probe-induced photonic−plasmonic coupling generates an enhancement of the transient optical response. These significant changes in the kinetic traces indicate that there are different relaxation processes affected by the coherent photonic−plasmonic coupling.

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CONCLUSION In conclusion, we have obtained a series of rational engineered photonic−plasmonic coupled hybrid materials by self-coassembly of renewable liquid-crystalline CNCs and plasmonic SNRs. These materials combine metallic and dielectric components into morphologically well-defined hybrid structures that integrate the intrinsic merits of plasmonic and photonic materials through synergistic electromagnetic interactions. It shows that the LDOS in a photonic matrix can be fully utilized to manipulate the plasmonic property of guest SNRs. We have also systemically studied the stationary and ultrafast optical responses of such photonic−plasmonic hybrid materials. In the pump−probe experiments, we observed a very significant photonic−plasmonic coupling-induced ultrafast optical modulation not only in its spectral profile but also in the relaxation dynamics of the embedded nanoparticles. The highly impressive optical performances, together with the stacked layer structure, the abundance of initial raw materials, and the cost-efficient “bottom-up” self-assembly method, will make these photonic−plasmonic coupled hybrid devices suitable for application in integrated optophotonics.

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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/acs.jpcc.6b09052. Additional experimental details and figures (e.g., FIB images, static UV−visible spectra) as well as tables of preparation conditions (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.D.). *E-mail: [email protected] (Y.X.). ORCID

Yan Xu: 0000-0003-4590-660X Author Contributions

G.C. synthesized the PC composite films and carried out the POM, SEM, FIB, and steady-state UV−visible measurements. H.Y. and G.C. accomplished the ultrafast transient spectra. G.C. designed and led the project. 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 supported by the National Natural Science Foundation of China (21171067, 21373100), Jilin Provincial Talent Funds (802110000412), and Tang Aoqing Professor Funds of Jilin University (450091105161). The authors thank 27546


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