Tailoring Nanohole Plasmonic Resonance with Light-Responsive

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Tailoring Nanohole Plasmonic Resonance with Light-Responsive Azobenzene Compound Guanqiao Zhang, Chungen Hsu, Chuwen Lan, Rui Gao, Yongzheng Wen, and Ji Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17258 • Publication Date (Web): 20 Dec 2018 Downloaded from http://pubs.acs.org on December 22, 2018

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ACS Applied Materials & Interfaces

Tailoring Nanohole Plasmonic Resonance with LightResponsive Azobenzene Compound

Guanqiao Zhang1, Chungen Hsu2, Chuwen Lan3, Rui Gao4, Yongzheng Wen1, Ji Zhou1,*

1

State Key Lab of New Ceramics and Fine Processing, School of Materials Science

and Engineering, Tsinghua University, Beijing 100084, China 2

Department of Chemical Engineering, Laboratory of Advanced Materials (MOE),

Tsinghua University, Beijing 100084, China 3

Beijing Laboratory of Advanced Information Networks & Beijing Key Laboratory of

Network System Architecture and Convergence, School of Information and Communication Engineering, Beijing University of Posts and Telecommunications, Beijing 100876, China 4

High Temperature Thermochemistry Laboratory, Department of Mining and

Materials Engineering, McGill University, Montreal, Quebec H3A 0C5, Canada

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Metal-based nanohole structures, featuring a continuous matrix and discrete voids, have seen a wide spectrum of practical applications, ranging from plasmonic sensing to extraordinary optical transmission (EOT). It would not be uncommon to pursue the further enhancement of their optical tunability, and the incorporation with other functional materials offers an intriguing lead. In this study, the first steps involve the colloidal lithography fabrication of gold-based, short-range ordered (SRO) nanohole structures on glass substrate with varying geometrical parameters. Plasmonic resonance in optical waveband is readily achieved from the coupling between bonding surface plasmons and nanohole lattices. Resonant features observed in transmission measurements could also be well reproduced both from numerical simulations as well as theoretical calculations based on the grating coupling mechanism. With the introducing of a thin layer of azobenzene compound by spin-coating, comes the critical transformation that not only alters optical performances by impacting the surface environment, but also bestows the structures with light-responsiveness. After 488 nm laser irradiation, it is observed that the structures underwent cross polarization conversion, which could be attributed to the photo-alignment behaviors from trans-cis isomerization within the azobenzene layer, yielding further optical tunability regarding linearly polarized probe light compared to the pre-irradiated state. The tuning of plasmonic resonances through light-stimuli paves a non-contacting path for achieving desired optical responses with potentially high spatial and temporal resolution. This work may sever as a fountainhead for future efforts on optically tailorable photonic devices associated with nanohole plasmonics.

KEYWORDS nanoholes, plasmonic resonance, light-responsive, azobenzene compound, trans-cis isomerization


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INTRODUCTION Light-matter interactions involving metal-based systems offers an intriguing path for realizing the manipulation of optical performances, much owing to the phenomenon known as the surface plasmon resonance (SPR), which describes the collective oscillation of free electrons induced by external electromagnetic fields.1 In order to take the full practical advantage, researchers have dedicated their efforts to a wide range of approaches, including the surface-enhanced Raman scattering (SERS),2 surfaceenhanced fluorescence (SEF),3 subwavelength waveguiding,4 plasmonic sensing,5 as well as metamaterials and metasurfaces,6 attaining a great spectrum of desired properties and functionalities. Nanoparticles with different geometrical configurations, such as nanorods,7 nanodisks8 and nanostars,9 have been exploited for tailoring localized surface plasmon resonance (LSPR), which confines electron oscillations into subwavelength volume.10 Besides the aforementioned discrete plasmonic structures, nanoholes which serve as a complementary counterpart have nonetheless attracted much interest, given their unique characters.11-23 The continuous metal-based matrix and discrete void space are particularly well-suited for certain applications that would require rational interaction volume with incident light. Without loss of generality, it is reasonable to classify the nanohole structures into three geometrical levels, namely single nanohole, short-range ordered (SRO) nanoholes, and finally, long-range ordered (LRO) nanoholes.11 For an individual nanohole, its plasmonic resonance wavelength increases by enlarging the hole diameter.12 The exact relation between the resonance wavelength and the geometrical size of the targeted nanohole can be understood by applying the plasmonic hybridization method, which resorts to the solving of plasmon mode dispersions.13 For SRO nanoholes, calculations of surface waves and waveguided modes in planar films are demonstrated to predict resonant behaviors through grating coupling mechanism.14,15 Different metal materials are compared regarding optical properties, illustrating the role of localized interband transition and plasmon damping effect.16 For LRO nanoholes, studies on extraordinary optical transmission (EOT) have been picking up popularity ever since the discovery of this intriguing phenomenon,17 leading to more ensuing efforts associated with the physical explanations18 as well as specific techniques on how to minimize spectral linewidth.19,20 Another promising kind of LRO nanoholes is the fishnet structure, which supports negative-refractive-index response in optical waveband, broadening the horizons in the emerging field of metamaterials.21 To sum up, plasmonic nanohole structures enable the realization of



certain exotic photonic properties with promising potentials. Natural materials sensitive to the external photo-stimuli play a vital part in the fields of optical communication and signal processing. In the vast ocean of stimuli-responsive polymers, azobenzene, triphenylmethane, and spiropyran-based organics stand out as potential candidates for photo-switching devices in consideration of their fast-speed photo-triggered isomerization.24 For azobenzene, which contains two phenyl rings connected by a nitrogen-nitrogen double bond,25 the observation of its photochemical isomerization can be traced back to the year of 1937.26 The light-responsive properties of azobenzene boil down to photo-induced motions that occur in both nanoscale and macroscale levels.27 What lies in the heart of the stimuli-responsive properties of azobenzene is the presence of trans-cis isomerism in the molecule, with the trans isomer being thermodynamically stable, and the cis form as a meta-stable state. The trans to cis conversion can be easily induced upon ultraviolet (UV) irradiation, while exposure to heat or visible light illumination enables the reverse process,28 as shown in Figure 1a. Such a process involves a drastic change in the distance between the para carbon atoms, accompanied by the alteration of the corresponding dipole moment.29 Furthermore, upon continued irradiation with linearly polarized light, azobenzene molecules exhibit a peculiar photo-alignment behavior due to the repeated trans-cistrans isomerization,30 as demonstrated in Figure 1b. To elaborate on the mechanism, the stimuli experienced by the trans form during isomerization varies according to the molecular orientation, with almost no torque experienced when vertically aligned with the electric field of incident light, and the maximum possible torque when being parallel to the electric field. As a result, only those azobenzene groups that are not vertical to incident electric field vector would undergo trans-cis isomerization, followed by the spontaneous relaxation back to the trans form from the resulting cis isomer. Eventually, all azobenzene molecules would settle in a vertical alignment with respect to the orientation of the incident electric field, which gives rise to optical anisotropy and birefringence, as schematically shown in Figure 1c. Such anisotropy could be erased with circularly polarized light or unpolarized light.28 The exotic optical properties of azobenzene compound have attracted much attention and a plethora of applications have been demonstrated, such as photo-mechanical devices,31 surface-relief gratings,32 holographic data storage,33 optical switches,34 nonlinear optical materials,35 just to name a few. The combination between metal-based nanostructures and photo-sensitive polymers


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opens up a new world of possibilities for the tailoring of plasmonic resonant behaviors. In essence, it is the alteration of the surrounding dielectric environment that empowers the tuning of optical performances concerning the plasmonic structures. Metal mirrors stacked by photo-switchable polymer PS-SPO composite have been exploited to realize perfect absorbers.36 Gold nanoparticles coated with PNIPAM polymer have been demonstrated for laser-triggered nano-oscillators.37 Silver nanoantennas located on SPI-PMMA composite have been proven to enable the active tuning of spectral linewidth.38 Moreover, chiral plasmonic metasurfaces supporting polarization control are realized via changing the coupling condition with ethyl red layer through green light irradiation.39 Such mentioning of a few notable highlights has merely scratched the surface of the immense and ongoing efforts devoted to illustrating the underlying mechanism regarding the plasmonic resonance tuned by light-responsive polymers. In this paper, we experimentally demonstrate the tuning of plasmonic resonance by combining nanohole structures with azobenzene compound (IA-Chol). Gold-based nanoholes on glass substrate are fabricated by taking the advantage of colloidal lithography, which enables the fabrication of two-dimensional ‘ polycrystalline ’ nanohole arrays over relatively large areas. Transmission spectra are employed to indicate where the plasmonic resonances could occur. It is found that the resonance wavelength, which corresponds to the position of the transmission dip, shows a strong correlation to the geometrical size of the nanohole arrays. The resonant features are further validated through numerical simulations and theoretical calculations, which contains grating coupling mechanism associated with surface wave eigenmodes.14 The thin layer of azobenzene spin-coated on the nanohole structures greatly modifies the resonant behaviors, as compared to the transmission spectra of the nanohole samples without azobenzene layer. The resonant positions further shift after irradiation with a monochromatic 488 nm laser, along with the observation of cross polarization conversion, confirming the photo-induced anisotropy of azobenzene compound. Concisely speaking, the tailoring of plasmonic resonance through light-stimuli displays aspects of a promising approach, in the light of its potential to be endowed with high spatial and temporal resolution, accompanied by a relatively fast-speed response. We envision this work with potential applications on all-optical switches, as well as other photonic devices related to nanohole plasmonics.



RESULTS AND DISCUSSION Colloidal lithography has been proven an effective and cost-efficient method for producing hexagonal arrays over relatively large dimensions. The geometrical parameters of the ultimate structure can be controlled by selecting nanospheres of rational size, followed by proper micro-nano fabrication techniques. The possible incorporation of tilted sputtering mode may further enable shapes and patterns with greater complexity as well as distinct symmetry features. Here we shall utilize the colloidal lithography technique for the fabrication of nanohole structures, with the procedure schematically illustrated in Figure 2. Polystyrene (PS) nanospheres with monolayer character are self-assembled on glass substrates through a well-established approach,40 forming close-packed hexagonal arrays. Two different sizes are chosen for the PS nanospheres, with diameters at 360 nm and 600 nm, respectively. After which, reactive ion etching (RIE) is carried out, with the purpose of utilizing O2 plasma to shrink the size of the nanospheres isotropically. At this stage, PS nanospheres are separated by some distances. A thin layer of Au, 40 nm thick, is deposited on all samples afterward, followed by a lift-off procedure for removing nanospheres thoroughly. The continuous mesh structures maintain their basic hexagonal features, though the overall geometrical characters can vary according to a variety of factors in the fabrication process. Azobenzene compound can then be spin-coated on the nanohole structures, forming a thin layer on top with some portions filling the void space. Figure 3 shows the scanning electron microscope (SEM) images of four samples with different size parameters prior to the spin-coating of azobenzene layer. Sample 1# and 2# are fabricated using the 360nm diameter nanospheres, while the 600 nm ones are used in sample 3# and 4#. The average periodicity measured in sample 1# and 2# are both around P = 370 nm, whereas the average nanohole radius r are 140 nm and 115 nm, respectively. For sample 3# and 4#, P measures approximately 630 nm, with r being 210 nm and 150 nm, respectively. Table 1 summarizes the size parameters of four fabricated samples in Figure 3. The periodicity is slightly larger than the original nanosphere diameter, which might be attributed to the expansion between adjacent nanospheres during the plasma etching process. All samples maintain hexagonal arrays in short-range, though the symmetry does not prevail over larger areas, due to the existence of defects. Therefore, the samples can be treated as ‘polycrystals’, which possess translational symmetry inside each ‘grain’, yet with the orientation of each ‘grain’ being different. Transmission spectrum of each sample is also shown in Figure


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3. Here the incident light is linearly polarized and the polarizer of the detector is set parallel with the incident electric field direction. All samples are in-plane isotropic given their ‘polycrystalline’ feature, indicating the insensitivity to the incident polarization. A clear transmission dip is observed for each sample in the targeted wavelength range, which corresponds to the plasmonic resonance. For sample 1# and 2# with the same periodicity, the resonance wavelength redshifts when decreasing the nanohole radius. The same trend can be found by comparing sample 3# and 4#. The resonant strength of sample 1# is weaker than sample 2#, which is associated with filling ratio of the void space. Sample 4# shows relatively low transmission across the selected wavelength range when compared to sample 3#. This is no extraordinary result considering the rather higher filling ratio of gold in sample 4# blocking even more transmitted light. The transmission measurements clear reveal the strong dependence of plasmonic resonance on size parameters concerning the nanohole structures, paving the way for later discussions on the coupling between nanohole plasmon and lightresponsive azobenzene compound. Table 1. Size parameters and resonance wavelengths of the four fabricated samples shown in Figure 3. Periodicity

Nanohole

(P, nm)

radius (r, nm)

Filling ratio

Resonance

Resonance

of

wavelength-

wavelength-

nanoholes

simulated

experimental

(nm)

(nm)

Sample 1#

370

140

0.52

572

585

Sample 2#

370

115

0.35

603

620

Sample 3#

630

210

0.40

823

825

Sample 4#

630

150

0.21

875

915

It is high time we enlisted the assistance of numerical simulations to further elaborate the underlying mechanism behind experimental observations from a quantitative standpoint. Figure 4a and 4c show the simulated transmission spectra for P = 370 nm and P =630 nm, respectively. Color maps are adopted to demonstrate multiple results for continuously varied nanohole radius r. Dash-dotted lines in the color plots trace the transmission minimum, which corresponds to where plasmonic resonance occurs. It is clear that the resonance wavelength blueshifts with the increase of nanohole radius at



fixed periodicity. Figure 4b and 4d show the transmission results corresponding to the actual size parameters in the experiment. The lines are extracted from Figure 4a and 4c for clear presentation. In Figure 4b, the positions of the major transmission dip well reproduce plasmonic resonance wavelength measured in Figure 3a and 3b. In Figure 4d, the major transmission dip for r = 210 nm also exhibits good consistency with the experiment result in Figure 3c, though for r = 150 nm, the predicted dip wavelength slightly blueshifts compared to the transmission measurement in Figure 3d. We also notice the undesired oscillations in simulations that are not actually observed in experimental measurements, as indicated by the shaded regions in Figure 4c and 4d. We attribute these oscillations to the following two factors. First, we are obliged to highlight the fact that the thickness of the substrate set in the simulations is a finite value in the same order of magnitude as the operating wavelength, which could cause Fabry-Perot resonance due to the index mismatch between air and glass. This is however sheerly due to software limitations, and with an actual thickness of the glass substrate being over 1mm, far greater than the operating wavelength, this interference effect is easily vanquished. Secondly, the metal-based nanohole structures with regular arrays are well-known for their EOT property as a result of the coupling between surface plasmon polariton (SPP) and grating lattice.18 In our simulations, periodical boundaries are exploited to mimic an infinite number of nanoholes. The transmission peaks in the shorter wavelength region of Figure 4d largely results from this very effect. Such peaks are not observed in experimental measurements due to the lack of longrange translational symmetry in the fabricated samples. To visualize how electromagnetic field evolves when coupling to the structures, we plot the near-field distributions of the electric field at the resonance wavelength in Figure 4e and 4f for the sample with parameters of P =370 nm and r = 115 nm, indicated by the green arrow in Figure 4b. The incident light is directed along the –z direction with electric field vector pointing towards the y direction. Figure 4e shows the magnitude of the electric field at resonance in xy-plane at the gold-glass interface, normal to the wavevector. The electric field is greatly enhanced at the verge of nanohole, forming the ‘hot-spot’ region. The magnitude attenuates when moving away from nanohole verge. Figure 4f plots the electric field vector distribution in yz-plane marked by the dashed line in Figure 4e. The vector plot shows dipole-like feature across the void region, which is caused by the induction of localized surface charges located along the rim of the nanohole.13 The plasmonic resonance that corresponds to the transmission dip can


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be viewed as the coupling effect between bonding surface plasmons and nanohole lattices. Grating coupling mechanism has been demonstrated to predict resonance wavelength for short-range ordered (SRO) nanoholes.14,15 In the case of multilayer system under normal illumination, the excitation of first-order resonance should satisfy the following conditions:14 2π P Re(k x )

(1)

1 P Im(k x )

(2)

where k x is the wavevector component parallel to the interfaces for surface waves in the multilayer system. We then perform theoretical calculations based on the principles outlined above to elucidate resonant features for our nanohole structures. Figure 5a shows the relation between mode wavelength ( 2π/ Re(k x ) ) and vacuum wavelength for a single gold film with a thickness of 40 nm located on glass substrate in air background. It is known that at normal incidence only bonding mode is able to couple with lattice momentum provided by nanohole structures.13 In Figure 5a, arrows are plotted at the mode wavelength of 370 nm and 630 nm, predicting the resonance wavelength at 620 nm and 970 nm, respectively. These results reproduce the resonance wavelength in sample 2# and 4# within a fairly reasonable margin of error. For sample 1# and 3#, which have relatively larger nanohole radius and a lower filling ratio of gold, the plasmonic resonances are strongly affected by the overlapping of electric field within individual nanoholes, resulting in a notable deviation of resonance feature from what the grating coupling theory predicts. Figure 5b plots the propagation length of bonding mode, with inset showing the enlarged view of the < 600 nm wavelength range. The propagation length could be deemed sufficient to support resonance at > 600 nm region. Figure 5c shows the magnetic field strength calculated at the middle wavelength in the selected range, which is equal to 750 nm. The bonding mode strength is much stronger than that of antibonding mode. This is closely related to the leaky and bound features of the hybridized modes concerned. Figure 5d plots the transmission spectra of the planar film system shown as inset by means of transfer matrix method.15 The transmission peak at around 500 nm is associated with the intrinsic property of gold. After addressing the feature of the plasmonic resonance related to nanohole structures, it is time to shed some light on how azobenzene compound can couple with nanohole



plasmons. To set a baseline for future reference, we take our first subject of study as the optical properties of a single layer of azobenzene spin-coated on glass substrate. The synthesis process of azobenzene compound (IA-Chol) in question is detailed in the Supporting Information. The thickness of the azo layer is estimated at about 600 nm by ellipsometry measurement. Optical anisotropy of azobenzene is induced with 30 min of irradiation under a monochromatic laser at 488 nm. Transmission spectra for the azobenzene layer in isotropic and anisotropic state are plotted in Figure 6a. The polarization direction of the pump light (laser) points along the y axis. The probe light is polarized at an angle of 45 with respect to the y axis. The detector polarizer is set parallel to the electric field of the incident probe light. Transmission is near-zero below 500 nm and relatively stable above 800 nm, indicating strong absorption in shorter wavelength region along with low absorption in longer wavelength region. Transmission of the azobenzene layer in isotropic state is higher than that of the anisotropic state, as the transmitted light after passing the anisotropic azobenzene layer is elliptically polarized, so that some portion of the energy has been blocked from the detector. Figure 6b shows the cross polarization conversion of the anisotropic azobenzene layer. Here the polarizer of the detector is set perpendicular to the electric field of incident probe light. The cross polarization conversion reaches maximum at around 580 nm and remains stable above 700 nm. This reaffirms our prediction that the azobenzene layer is anisotropic after laser irradiation since no cross polarization could be detected before irradiation. Figure 6c shows the fitted refractive index of the isotropic azobenzene layer by ellipsometry measurement. The azobenzene layer is almost fully transparent in the waveband between 700 nm to 1100 nm. Therefore, Cauchy dispersion model is applicable and thus employed for the determination of the optical constants. We also measure the refractive index across the whole targeted wavelength range (400 nm – 1100 nm) for another thinner azobenzene layer sample, demonstrating the absorption feature in the shorter wavelength region (see Supporting Information for details). Next, we spin-coat azobenzene compound onto the nanohole structures using the same rotation rate as in the previous paragraph. This could roughly ensure an identical 600 nm thickness of the azobenzene layer formed on nanoholes. Transmission spectra are shown in Figure 7a-7d for the four fabricated samples. The detector polarizer is again set parallel to the polarization direction of the incident probe light. It is clearly demonstrated that a layer of azobenzene could place a significant impact on the optical


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performances of nanohole structures. Major plasmonic resonance positions, which are marked by the blue arrows in each plot, experience redshift when compared to the spectra in Figure 3. For sample 1# and 2#, the plasmonic resonance wavelengths shift for about 50 nm and 85 nm, respectively. The transmission is near-zero at the < 500 nm region, which is closely related to the strong absorption effect of azobenzene layer. In other wavelength regions, the transmission amplitudes also decrease slightly. For sample 3# and 4#, the plasmonic resonance wavelengths shift for about 85 nm and 95 nm, respectively. Though the overall transmission is relatively low, sample 4# exhibits the most obvious change of plasmonic resonance among the four samples. Therefore, the azobenzene layer in its isotropic state is indeed a suitable candidate for tuning the nanohole plasmon resonance. We then irradiate the four samples with 488 nm laser for the duration as in the treatment of solo azobenzene layer, in order to reveal how anisotropy can affect the overall optical properties. Figure 7e-7h show the transmission spectra and cross polarization conversion after laser illumination. The probe light is polarized at an angle of 45º with respect to the polarization direction of pump laser. For sample 1# and 2#, the resonance wavelength slightly blueshifts compared to the results in Figure 7a and 7b. Meanwhile, cross polarization conversion for each sample is clearly observed, suggesting that the anisotropy is induced inside the azobenzene layer. The wavelength of the maximum cross polarization conversion closely matches the transmission peak position, whereas for the plasmonic resonance wavelength the cross polarization conversion reaches local minimum. For sample 3# and 4#, the plasmonic resonance wavelengths exhibit slight redshifts compared to Figure 7c and 7d. The cross polarization conversion remains at a relatively low level due to the overall low transmission amplitude. At the same time, it also exhibits complicated correlation to the transmission spectra. The transmission measurements prove that optically induced anisotropy can be harnessed to manipulate plasmonic resonance within the nanohole structures. The optical tuning effect may even be further enhanced by means of optimizing the design of nanohole size parameters, along with the choice of better suited plasmonic constituent materials. Moreover, the vast kinds of azobenzene compounds and their derivatives provide a broad platform for the selection of optically induced anisotropy with desired response time and polarization conversion efficiency. The marriage between long-range regular nanohole arrays with azobenzene compound opens up yet another promising gateway toward the tailoring of optical responses, considering that



the EOT peak is sensitive to the surface dielectric environment and thus readily tunable. Numerical simulations are adopted to elucidate how azobenzene layer can shape the plasmonic resonance of the nanohole structures. As has been discussed, the azobenzene layer strongly absorbs light in the visible wavelength region, especially when the wavelength is below 500 nm. Unfortunately, introducing absorption term into our simulation only complicates the calculation process and might yield unrealistic results due to model and software limitations. There are ways around the issue, however, which is exactly why we turn our attention to the samples with P = 630 nm, where the azo layer displays negligible loss above the wavelength of 700 nm. The simulated transmission spectra for nanohole structures with a fixed periodicity of 630 nm is shown in Figure 8a. The azobenzene layer is assumed with an isotropic refractive index of 1.7. The 700 nm – 900 nm region is riddled with undesired oscillations which are caused by Fabry-Perot interference and EOT effect in simulations. Therefore, the 900 nm – 1100 nm is selected as the targeted wavelength range. The plasmonic resonance wavelength can be seen blueshifting with the increase of nanohole radius. Figure 8b shows the results when azobenzene layer is modeled in its anisotropic state. The accurate determination of the anisotropic refractive within irradiated azobenzene layer, across the whole spectrum, remains a challenge. Here we assume a reasonable set of anisotropic refractive index as nx = nz = 1.73 and ny = 1.67, where z-axis is normal to the surface. In Figure 8d and 8f, we extract the data for r = 210 nm and r = 150 nm from the color plots of Figure 8a and 8b for better viewing the resonance shift induced by anisotropy. Transmission spectra are plotted in dB scale for better comparison. It can be seen that the resonance wavelength redshifts when azobenzene layer changes from isotropic state to anisotropic state, coinciding with the trends observed in Figure 7. Figure 8e and 8g convert the data in Figure 8d and 8f into normal scale. The simulated lines qualitatively reproduce the lineshapes in Figure 7, which validates that the assumption of the azobenzene layer properties in simulation is reasonable. We also simulate the cross polarization conversion for the nanohole structures coated with anisotropic azobenzene layer, as shown in Figure 8c. The positions where cross polarization conversion reaches minimum follows the evolution of plasmonic resonance wavelength, which is indicated by the dash-dotted line in each color plot. Figure 8h plots the data for r = 210 nm and r = 150 nm extracted from Figure 8c. The simulation accuracy may be further enhanced with an optimized set of size parameters, as well as by resorting to a better, more realistic set of optical parameters for the


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constituent materials.

CONCLUSIONS We have demonstrated the possibility of plasmonic resonance tailoring in nanoholes structures by incorporating light-responsive azobenzene compound. Fabricated via colloidal lithography, the nanohole samples have displayed plasmonic resonances in transmission spectra measurements, which stems from the coupling between the surface plasmon bonding mode and nanohole gratings. The azobenzene layer spin-coated onto the nanohole structures significantly modified the resonance wavelengths as expected, since the plasmonic system displays sensitivity to the surrounding dielectric environment. Furthermore, optical anisotropy could be induced in the azobenzene layer upon irradiation from 488 nm laser, providing extra tunability to the resonant features. Cross polarization conversion measurements confirm the photo-induced anisotropy as the nanohole structures only possess in-plane isotropic symmetry. The ability to tune the plasmonic resonance with light signals offers an efficient, non-contact route for controlling optical performance. The azobenzene-based plasmonic system may find promising prospects for photonic devices that favors light-responsive properties.

METHODS Optical performance measurements.

The transmission spectra, cross polarization

conversion and ellipsometry characterization are all performed on a Woollam spectroscopic VASE ellipsometer. Transmission mode is selected for transmission and cross polarization measurements. Solid state detector with rotating analyzer is utilized to control the polarization of the received transmission signal. Beam diameter is estimated at around 200 μm. Laser irradiation setup for photo-induced anisotropy.

A solid state 488 nm laser

generates linearly polarized beam, which then passes through a convex lens in order to achieve a parallel beam. Central part of the beam is selected by a diaphragm with a diameter of 8 mm. A half wavelength plate is utilized to generate linearly polarized light. The sample surface is placed normal to the incident laser. Light intensity is estimated at about 250 mW/cm2. Numerical simulations.

The commercial software CST Microwave Studio is

adopted for the task of calculating transmission spectra and cross polarization



conversion. Unit cell boundary is used with the aid of Frequency Domain Solver. The refractive index of the glass substrate is set as 1.5 without lossy part. Optical constant of gold in previous reference41 is adopted. Cross polarization conversion is computed by selecting the proper port modes.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on ACS Publications website. Synthesis of azobenzene compound (IA-Chol), ellipsometry characterization method for azobenzene layer, addition simulation results on size parameters in nanohole structure systems, theory of surface wave calculation. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Guanqiao Zhang: 0000-0001-9274-9358 Rui Gao: 0000-0002-7619-2382 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Basic Science Center Project of NSFC under Grant No. 51788104, as well as National Natural Science Foundation of China under Grant Nos. 51532004 and 11704216.


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Figure 1. (a) Isomerization of azobenzene. The transformation from trans form to cis form can be optically induced by irradiation of ultraviolet light, whereas the reverse process requires external heat or visible light illumination. (b) Photo-alignment of azobenzene molecules. Upon linearly polarized light irradiation, azobenzene molecules would orientate vertically with the incident electric field vector after repeated trans-cis-trans isomerization. (c) Schematic view of the photo-induced anisotropy of azobenzene, which is the macroscopic result of the photo-alignment.



Figure 2. Procedure for fabricating hexagonal nanohole arrays by using colloidal lithography. Geometrical sizes of the nanohole structures are controlled by selecting nanospheres of different diameter and by properly adjusting etching time. Azobenzene compound can then be spin-coated on top of the nanoholes, forming a thin layer with photo-responsive performance.


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Figure 3. (a)-(d) Scanning electron microscope (SEM) images of the fabricated nanohole structures and their corresponding transmission spectra before the spin-coating of azobenzene layer. The scale bars in all SEM images are 2 μm. Refer to Table 1 for details on size parameters. For transmission measurement, the incident light is linearly polarized, with the polarizer of the detector aligning parallel to the incident electric field direction. All samples exhibit plasmonic resonance features in the targeted wavelength range.



Figure 4. Numerical simulation results for the nanohole structures without azobenzene layer. (a) Color map for the transmission spectra as a function of the nanohole radius r at fixed periodicity P = 370 nm. (b) Transmission spectra for r = 140 nm and r = 115 nm extracted from (a). (c) Color map for the transmission spectra as a function of the nanohole radius r at fixed periodicity P = 630 nm. (d) Transmission spectra for r = 210 nm and r = 150 nm extracted from (c). The dash-dotted lines in (a) and (c) mark the evolution of transmission minima. Shaded regions in (b) and (d) indicate undesired oscillations, which are associated with Fabry-Perot interference and EOT effect in simulations that are not observed in the actual experimental measurements. (e) Electric field magnitude distribution at the resonance wavelength marked by green arrow in (b). The plot plane is at the gold-glass interface. (f) Electric field vector distribution corresponding to (e). The plot plane is indicated by the dashed line in (e). In simulations ypolarized light is considered.


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Figure 5. Theoretical calculations on surface plasmon modes of the 40 nm gold film on glass substrate in air background. (a) Dispersion relations of bonding and antibonding mode. Photon dispersions of air and glass are also plotted for reference. Arrows plotted at mode wavelength of 370 nm and 630 nm are shown to predict resonance wavelength according to the grating coupling mechanism. (b) Propagation length of the bonding mode. Inset shows the enlarged view for the wavelength region below 600 nm. (c) Magnetic field distribution at 750 nm. The region of air and glass are indicated by different colors. (d) Transmission spectrum of the system considered calculated by transfer matrix method.



Figure 6. Optical properties of a 600 nm azobenzene compound layer spin-coated on glass substrate. (a) Transmission spectra of the azobenzene layer in isotropic (before laser illumination) and anisotropic (after laser illumination) state. Inset shows the polarization configuration for pump (laser) and probe light. (b) Cross polarization conversion of the anisotropic azobenzene layer. The polarizer of the detector is set perpendicular to the electric field of incident probe light. (c) Fitted refractive index of the isotropic azobenzene layer based on ellipsometry measurement.


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Figure 7. Experimental results of the transmission performance for the nanohole structures with azobenzene layer. (a)-(d) Transmission spectra of sample 1# to 4# before 488 nm laser irradiation, which means that the azobenzene layer is in isotropic state. (e)-(h) Transmission spectra and cross polarization conversion (red dashed line) of sample 1# to 4# after 488 nm laser irradiation, which means that the azobenzene layer is in anisotropic state. Major transmission dips are marked with blue arrows. Thickness of azobenzene layer is about 600 nm. For transmission spectra measurement, the probe light is polarized at an angle of 45 with respect to the polarization direction of pump laser, which coincides with the configuration in Figure 6. For cross polarization conversion measurement, the polarizer of the detector is set perpendicular to the polarization direction of incident probe light.



Figure 8. Numerical simulation results for the nanohole structures with periodicity P = 650 nm coated with a layer of azobenzene. (a) Color map for the transmission spectra as a function of the nanohole radius r when azobenzene layer is assumed as isotropic with a refractive index n of 1.7. (b) Transmission spectra when azobenzene layer is assumed as anisotropic with nx = nz = 1.73 and ny = 1.67. Z axis is normal to the sample surface. (c) Cross polarization conversion when azobenzene layer is assumed as anisotropic. In all color plots, local minima are marked by dash-dotted lines. (d),(e) Transmission spectra for r = 210 nm extracted from (a) and (b) represented with dB scale (d) and normal scale (e). The conversion formula is T(dB) = 10lg(T). (f),(g) Transmission spectra for r = 150 nm extracted from (a) and (b) represented with dB scale (d) and normal scale (e). (h) Cross polarization conversion for r = 210 nm and r = 150 nm extracted from (c).


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For TOC only


Tailoring Nanohole Plasmonic Resonance with Light-Responsive

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