Metal-Dielectric Composite Nanostructures for Fano Resonance

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Metal-Dielectric Composite Nanostructures for Fano ResonanceBased Highly Sensitive SECARS from Visible to Deep-UV Kwang-Hyon Kim, and Myong-Hyok Ri J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03970 • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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The Journal of Physical Chemistry

Metal-Dielectric Composite Nanostructures for Fano Resonance-Based Highly Sensitive SECARS from Visible to Deep-UV Kwang-Hyon Kim*,† and Myong-Hyok Ri‡ †

Institute of Lasers, State Academy of Sciences, Unjong District, Pyongyang, Democratic People’s Republic of Korea

‡

Department of Physics, University of Science, Unjong District, Pyongyang, Democratic People’s Republic of Korea

ABSTRACT: The nanostructures of noble metals are known to support Fano resonances, in most cases, in the infrared. Blue-shifting Fano resonant spectral range is of great importance for practical applications. Here we propose to use metal-dielectric composite nanostructures for this purpose. In the nanostructures containing noble metal nanoparticles with moderate sizes, Fano resonances appear in the visible due to the composite effect. By changing the host medium and the filling factor of metal nanoparticles in the composite nanostructures, Fano resonant spectral region can be tuned in a wide range. These composite nanostructures exhibit field enhancement significantly high that they are applicable for single molecule detection based on surface-

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enhanced coherent anti-Stokes Raman spectroscopy (SECARS). In particular, the dielectric composite nanostructures containing Mg and Al nanoparticles support Fano resonances, providing unprecedentedly high SECARS enhancement factors in the near- and deep-UV regions.

INTRODUCTION Over the past decades, noble metals have been employed as the main ingredient of plasmonics. Their structures, however, exhibit plasmon resonances mainly in the visible and near-infrared. In order to extend the spectral region of plasmonic effects, various alternative materials have recently been proposed,1 including poor metals (e. g. Al, Mg, Rh, and Ga, etc.),2-5 heavily-doped semiconductors,6 materials supporting 2-dimensional electronic gases such as graphene7,8 and topological insulators.9,10 By employing the above materials, one might extend the spectral range of plasmonic applications to the visible, ultraviolet (UV), mid-infrared, and terahertz regions. The main issue with these materials, however, is high losses, limiting their practical applicability. Another important issue is related to the magnitudes of their dielectric constants: even though their real parts are negative, the magnitudes should be in an appropriate range in order to be able to obtain the desired plasmonic effects in their nanostructures of moderate sizes available by using the up-to-date nanotechnology. These problems strongly motivate further explorations for novel plasmonic materials applicable in a wide spectral range. Fano resonances in plasmonic nanostructures11-14 have extensively been investigated over the past decade due to the interesting physical background as well as their numerous promising applications, including refractive index sensing,15-19 enhanced nonlinear optical processes,20-24


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spectroscopic applications,25-28 spectral-band demultiplexing,29 perfect absorption,30 and lowthreshold spasing.31 This phenomenon has been investigated theoretically and experimentally in diverse plasmonic nanostructures.32-39 It has been experimentally demonstrated that Fano resonant plasmonic nanostructures can be used for single molecule detection based on surfaceenhanced coherent anti-Stokes Raman spectroscopic approaches (SECARS)27,28, since they can generate significantly high field enhancement effect over a wide spectral range. For practical applications, the development of the nanostructures supporting Fano resonances in the ranges of shorter wavelengths is of great importance. Recently, the applications of Al nanostructures for surface-enhanced resonance Raman spectroscopy (SERRS) in the deep-UV40,41 have been reported, but the investigations of SERRS in this range is still in the initial stage. The dielectric composites containing metal nanoparticles have recently been revealed to be promising materials for the various applications.42-52 The main purpose of this work is to show that the metal-dielectric composite materials can be used as a new category of materials for plasmonic applications in a wide range from visible to deep-UV, taking SECARS based on Fano resonances in their nanostructures as an example. We numerically demonstrate that the nanostructures of metal-dielectric composite can exhibit prominent Fano resonances. The nanostructures of noble metals support Fano resonances typically in the infrared, while the dielectric composite nanostructures containing nanoparticles of the same noble metals with the same geometrical structures and size scales exhibit Fano resonances in the visible, which is attributed to the optical responses of the composites significantly different from the pure metals. By changing the host medium and the filling factor of metal nanoparticles in the composites, the spectral range exhibiting Fano resonances is tunable in a wide range. These composite nanostructures exhibit significantly high field enhancement, enabling their applications for single


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molecule detection based on SECARS. In particular, the nanostructures made of dielectric composites of Mg and Al nanoparticles support Fano resonances in the near- and deep-UV regions and they possess unprecedentedly high SECARS enhancement factors in these regions.

METHODS Field distributions and scattering characteristics of composite nanostructures have been calculated by using the discrete-dipole approximation (DDA) software adda-1.2.53 In the DDA calculations, the composites comprising the nanostructures are dealt with as homogeneous materials52 whose dielectric constants are the same as effective dielectric functions obtained by using the Maxwell-Garnett model:54,55 = 1 − + ⁄1 − + , where is the filling factor of metal nanoparticles, = 3 / + 2 is the field enhancement factor for spherical nanoparticles, and are the dielectric functions of the host medium and the inclusion metal nanoparticles, respectively. Around the wavelength at which the condition Re1 − + = 0 is satisfied, the absolute value of significantly increases and the composite exhibits strong dispersion. Resultantly, the effective dielectric function of the composite may become negative,42 while much different from that of the inclusion metal nanoparticles. Thus, one can expect diverse plasmonic effects, including Fano resonances, with these composite materials in the spectral ranges different from the cases of the nanostructures made of pure metals.

RESULTS AND DISCUSSIONS Fano Resonances in Metal-Dielectric Composite Nanostructures.


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In what follows, we study Fano resonant responses of dielectric composite nanostructures containing noble metal nanoparticles in the visible. Here we take amorphous titania containing silver nanospheres which are randomly distributed in the host, as an example. The dielectric function of silver is calculated by using the extended Drude model = − / + , where is the angular frequency of light, is the dielectric function at infinite frequency, is the plasma frequency, and is the collision frequency, respectively. Here we use the Drude parameters of = 4.3378, = 1.3385 " 10#$ s-1, and = 1.2641 " 10#& s-1,56,57 which are obtained by fitting the experimental data for silver58 in the spectral range from 300 nm to 900 nm. The dielectric function of titania is taken from Ref.59. We assume that the inclusion particles in the composite have the sizes of a few nm, thus the nanostructure contains more than several thousands of silver nanoparticles, which enables us to replace the dielectric constant of the nanostructure with the effective dielectric function of the composite as in Refs.42,52.


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Figure 1. Fano resonance in a nanostructure made of titania composite containing silver nanospheres. In 1a, the real Re and imaginary parts Im of the effective dielectric function of the composite are shown. Filling factor is 0.35. Figure 1b shows the composite nanostructure to be examined. Scattering )*+, and absorption efficiencies ),* of the composite are shown in 1c, where the effective dielectric function is the same as in 1a. From 1c, one can see that Fano dip appears at 565 nm, while peaks at 545 nm and 598 nm, respectively. The distributions of the intensity enhancement (1d-1f) and the electric charge (1g-1i) are shown at 545 nm, 565 nm, and 598 nm, respectively. In the figures, the geometrical parameters are -# = 120 nm, - = 50 nm, . = 40 nm, ℎ = 40 nm, 0 = 20 nm, and 1 = 20 nm, respectively.

Figure 1a presents the effective dielectric function of titania composite containing silver nanospheres with a filling factor of 0.35, which shows that its real part Re has the negative sign at the wavelengths shorter than 640 nm (light blue colored area). One can, therefore, expect the plasmonic responses of nanostructures made of this composite in this wavelength range. As an example, we take the structure shown in Fig. 1b, Fano resonant characteristics of which are examined in

60

for pure noble metal as its constituent material. Figure 1c presents the scattering

and absorption efficiencies in this composite nanostructure. The refractive index of the surrounding medium is set to be 1.33 throughout this work. The dipole size is taken as 2 nm for the DDA-based simulations in Fig. 1. The details of the simulations are described in Supporting information S1. As Fig. 1c shows, the composite nanostructure exhibits prominent Fano resonance in metallic regime (light blue colored area) of the composite with double peak at 545 nm and 598 nm and a dip at 565 nm. Note that at 565 nm the absorption efficiency ),* exhibits a peak, while the scattering efficiency )*+, has a reduced value corresponding to the Fano dip,


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which is the typical for Fano resonances in plasmonic nanostructures. Figures 1d, 1e, and 1f show the intensity enhancement distributions at the top of the nanostructure. The figures show that the maximum intensity enhancement factor reaches about 600 in the gap region between the short nanorods at 565 nm (Fig. 1e). The corresponding charge distributions are presented in Fig. 1g, 1h, and 1i, respectively, from which one can see that the mode at Fano dip (565 nm) is the result of destructive interference between the plasmonic modes similarly as in Fig. 1 of Ref.60. Here we calculate the charge distributions by using the Gauss’s law as in Ref.22, which are given by the difference between the normal components of the electric fields above and below the surface of the nanostructure. Metal nanostructures with the same sizes, however, support Fano resonances in longer wavelength ranges: as shown in Fig. S1 in Supporting information S1, for instance, silver nanostructure with the same structure and size supports Fano resonance in the near-infrared range. Although here we have presented the result only for the structure shown in Fig. 1b as an example, similar Fano resonant behaviors can be observed in composite nanostructures with other geometrical structures (see e. g. Fig. S2 in Supporting information S2 illustrating Fano resonance in a quadrumer of metal-dielectric composite nanodisks). From the practical point of view, filling factor = 0.35 of metal nanoparticles is relatively high. However, several techniques for fabricating metal-dielectric granular composites with high volume fractions have already been developed. By using the reactive co-sputtering, one can obtain ≈ 0.35~0.861-63 with percolation limit of up to about 0.82.63 The other technique is the deposition of dielectric-coated metal nanoparticles, which enables obtaining composite films with volume fractions higher than 0.6.64,65 The applicability of Maxwell-Garnett model as an effective medium theory for composite nanostructures with high filling factors should also be discussed. In order to be able to use this model, the sizes of the inclusion particles and the


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distances between them should be much smaller than the light wavelength and the gaps between them should be much larger than their sizes.42 Theoretical results, however, show that the last requirement is considerably relaxed and the upper limit of the filling factor, for which this model is applicable, reaches up to about 0.5 (see Fig. 6 in66) or even 0.74.42 In Supporting information S3, we have presented an example of scattering property of the composite nanostructure shown in Fig. 1 obtained without using the Maxwell-Garnett model, which perfectly agrees with the result obtained by using this model. It was also experimentally demonstrated that the MaxwellGarnett model can be applied for metal-dielectric composites with volume fractions of up to about 0.5.67 Thus, in this work we consider the granular composites of metal nanoparticles with filling factors less than 0.5.

Figure 2. Scattering efficiencies for different structural parameters -# (2a), - (2b), 1 (2c), and 0 (2d), in the insets of which their values are presented. Other structural and material parameters are the same as in Fig. 1.


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Then we study the dependencies of Fano resonant characteristics on the structural parameters of the composite nanostructures (Fig. 2). With increasing -# of longer rods the second peak redshifts, while the first peak red-shifts for the increase of the length - of shorter rods (Fig. 2a). The contrast of Fano profile has great importance for practical applications. From Figs. 2a-2d, one can see that the lengths of longer rods -# and shorter ones - affect the contrast of Fano profile stronger than the gaps 1 and 0 between the constituent nanorods. In particular, Fano profile becomes the most prominent if the condition -# = 2- + 0 is fulfilled (see Figs. 2a and 2b) as in Fig. 1c-1i, revealing that the coupling between the plasmonic modes becomes strongest under this condition.

Figure 3. Influence of refractive index 4* of the surrounding medium on Fano resonance in the nanostructure of titania composite containing silver nanospheres. In 3a, the scattering efficiency )*+, is shown for 4* = 1.23 (blue solid line), 1.33 (green dashed), and 1.43 (red dotted), respectively. In 3b, the dependence of wavelength 56 at which Fano dip appears on 4* is presented.


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Next we study the influences of material parameters of the composite nanostructures. Figure 3a shows the spectral response of metal-dielectric composite nanostructure for different refractive indices 4* of the medium surrounding the nanostructure. The other geometrical and material parameters are the same as in Fig. 1. From the figure, one can see that the average spectral bandwidth (FWHM) of Fano dip amounts to about Δ589:; ≈ 21 nm. Figure 3b shows the spectral positions 56 of Fano dip for different values of 4* : 56 = 553 nm for 4* = 1.23 , 56 = 558.5 nm for 4* = 1.28, 56 = 564 nm for 4* = 1.33, 56 = 569 nm for 4* = 1.38, and 56 = 574 nm for 4* = 1.43, respectively. Here we evaluate the figure-of-merit (FoM) of this composite nanostructure as a refractive index sensor: >?@ = A̅/Δ589:;, where A̅ = 〈Δ56 /Δ4* 〉 is the spectral sensitivity, Δ56 is the spectral shift of 56 , and Δ4* is the variation of 4* , respectively. From Fig. 3b, one obtains A̅ ≈ 105 nm and, correspondingly, >?@ ≈ 5. This value of >?@ is relatively large compared with the preceding results reported in Fano resonant plasmonic structures in the visible, which reveals their potential applicability for highly sensitive biosensors operating in this spectral range.

Figure 4. Scattering efficiency )*+, for different filling factors (1a) and different host media (1b) in silver nanoparticles-doped dielectric nanostructure shown in Fig. 1b. In inset of 4a, filling factors of silver nanoparticles are shown: = 0.25 (blue solid line), 0.30 (green dashed), 0.35


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(red dotted), 0.40 (cyan dash-dotted), and 0.45 (magenta solid), respectively. In 4b, )*+, is shown for the different host media (silica: SiO2-blue solid line, alumina: Al2O3-green dashed, titania: TiO2-red dotted, and zinc sulfide: ZnS-cyan dash-dotted) of the composites. Geometrical and other material parameters are the same as in Fig. 1.

Figure 4a illustrates the influence of filling factor of silver nanoparticles in the composite nanostructure shown in Fig. 1. The other material parameters are the same as in Fig. 1. The figure shows that the Fano profile becomes further prominent with increasing filling factor, which originates from that the composite material becomes further metallic for the increase of . In the meantime, Fano resonant spectral position red-shifts with increasing : the spectral positions of Fano dips are given by 56 = 551 nm for = 0.25, 56 = 558 nm for = 0.3, 56 = 565 nm for = 0.35 , 56 = 571 nm for = 0.4 , and 56 = 577 nm for = 0.45 , respectively. Such tendency is attributed to the red-shift of spectral positions of the wavelengths at which the optical responses of metal-dielectric nanocomposites switch from metallic to dielectric (see. e. g. Fig. 3b in 48). Figure 4b presents the dependence of Fano resonance on the dielectric constant of the host medium, taking different dielectric materials as examples: silica, alumina, titania, and zinc sulfide. The dielectric constants of these dielectrics are shown in Supporting information S4. The structural parameters are the same as in Fig. 1. From the figure, one can see that Fano profile red-shifts and becomes more prominent with increasing the dielectric constant. From the results shown in Fig. 4, one can conclude that the Fano resonant ranges can fully cover the whole visible spectral region with metal-dielectric composite nanostructures by changing material parameters such as filling factor of metal nanoparticles and refractive index of the host.


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High SECARS Enhancement in Fano Resonant Composite Nanostructures in the Visible and Ultraviolet. Surface-enhanced Raman spectroscopy (SERS) and surface-enhanced coherent anti-Stokes Raman spectroscopy (SECARS) provide the detailed information for molecular structure.68 In comparison with spontaneous Raman scattering as a two photon process, coherent anti-Stokes Raman scattering (CARS) has the advantage of stronger dependence on the pump intensity due to the nature as a four-photon process, thus SECARS is capable to generate significantly higher enhancement factor than SERS.69 The preceding results27,28 show that SECARS enhancements sufficiently high for single molecule detection can be obtained with Fano resonant plasmonic nanostructures, exhibiting high field enhancements at all wavelengths of photons taking part in the CARS process. With up-to-date Fano resonant substrates for SECARS using noble metals as building blocks, however, single molecule detection is available mainly in the infrared. From the nature of dipole emission-based scattering, Raman cross-sections increase approximately in accordance with the rule of 5E& (see Ref.68) and one can, therefore, obtain stronger signals with shorter pump wavelengths.


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Figure 5. Enhancement factors of SECARS in titania composite nanostructure containing silver nanoparticles for different filling factors. The structural and material parameters are the same as in Fig. 1. In insets of 5a and 5b, the values of filling factors are shown. In 5a, the maximum intensity enhancement factors FG,,I in the gap region between short nanorods (see Fig. 1b) are shown as the functions of light wavelength. In 5b, the maximum values JKLMNOK,,I of SECARS enhancement factors are shown as the functions of Raman shift ΔPO . Figure 5c illustrates the distribution of enhancement factor of SECARS signal at the top of the nanostructure for filling factor = 0.35. The figures show that SECARS-based single molecule detection is realizable with the Fano resonant composite nanostructure.

As shown in Figs. 1d-1f, with Fano resonant metal-dielectric composite nanostructures one can obtain significantly high field enhancement in the gap region between the short rods. For the detailed study, we calculate the maximum field enhancement factor FG,,I = max|TU | / |TVU+ | in the gap region of the structure shown in Fig. 1, where TU and TVU+ are the enhanced and incident field in the vicinity of the composite nanostructure. The calculated FG,,I is shown in Fig. 5a as the function of wavelength. The enhancement factor JKLMNOK of SECARS signal is calculated

by &

using

the

formula

&

JKLMNOK = WTU X5 YW |TU 5K | |TU 5N | /WTVU+ X5 YW |TVU+ 5K | |TVU+ 5N | ,27 where 5 , 5K , and 5N are the pump, Stokes, and anti-Stokes wavelengths, respectively. In Ref.27, it was experimentally demonstrated that single molecule detection can be realized by using Fano resonance in metallic nanocluster with enhancement factor JKLMNOK in the order of 10-11 for pumping at 800 nm, even for molecules with much small Raman cross-sections. Taking the


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wavelength dependence of Raman cross-sections into account (see above),68 ZKLMNOK for the pump wavelength 5 = 532 nm is about 5 times larger than that for 5 = 800 nm. Thus for pumping at 532 nm, single molecule detection can be realized for JKLMNOK in the orders of 9-10. Figure 5b presents the maximum values of JKLMNOK as the functions of Raman shift ΔPO = 5E# − E# E# 5E# K = 5N − 5 for different filling factors, where 5 = 532 nm. From the figure, one can see

that JKLMNOK reaches up to more than 1 " 10## for ΔPO below 1500 cm-1, which is order-ofmagnitude larger than the above mentioned threshold necessary for single molecule detection. Figure 5c presents the distribution of JKLMNOK at the top of composite nanostructure for 5 = 532 nm and ΔPO = 1070 cm-1. The above results reveal that Fano resonant metal-dielectric composite nanostructures can be used as highly sensitive substrates for single molecule detection by SECARS with pumping in the visible.

Figure 6. Fano resonances (6a and 6d) and maximum SECARS enhancement factors in Si3N4 composite nanostructures doped with Mg and Al nanoparticles with geometrical structures


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similar to Fig.1. In the figures, filling factors are taken as = 0.4. Figures 6b and 56e show the maximum intensity enhancement factors FG,,I in the gap regions. In 6c and 6f, the maximum enhancement factors of SECARS signals are shown for excitations at 375 nm (6c) and 270 nm (6f), respectively. The filling factors of metal nanoparticles are 0.4. Figures 6a-6c correspond to Mg nanoaprticles-doped Si3N4 composite nanostructure and the geometrical parameters are -# = 90 nm, - = 37.5 nm, . = 30 nm, ℎ = 30 nm, 0 = 15 nm, and 1 = 15 nm, respectively. Figures 6d-6f show the results for Al nanoparticles-doped Si3N4 composite nanostructure and the geometrical parameters are -# = 48 nm, - = 20 nm, . = 16 nm, ℎ = 16 nm, 0 = 8 nm, and 1 = 8 nm, respectively.

Surface-enhanced resonance Raman spectroscopy (SERRS) in the UV range is of great importance, which has still rarely studied. In particular, SERRS with pumping in the deep-UV region has the advantage since its signals are free from fluorescent background.40 As is shown above, with metal-dielectric nanocomposites one can obtain Fano resonances at the wavelengths shorter than those available with nanostructures made of pure metals. In the UV region, two enhancement mechanisms of Raman signals appear in addition to the plasmon-induced field enhancement:68 one is wavelength-dependent increase due to the dipolar nature of Raman scattering and the other one is due to the resonant Raman transitions, providing several ordersof-magnitude increase in Raman enhanement.41 Figure 6 presents Fano resonant responses and SECARS enhancement factors in composite nanostructures containing Mg and Al nanoparticles, in which we have taken Si3N4 as the host medium, since it is transparent in a broad UV range and has high refractive index70 necessary for obtaining prominent Fano resonance (see Fig. 4b). The effective dielectric constants of these composites have been presented in Supporting information


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S5. From Figs. 6a and 6d, one can see that Mg nanoparticles-doped nanostructure supports Fano resonance in the near-UV (from 300 nm to 400nm) and Al nanoparticles-doped one in the deepUV region (from 200 nm to 300 nm). Similarly to the case of nanostructure containing Ag nanoparticles (Figs. 1-5), they support Fano resonances in the ranges of shorter wavelengths compared with the nanostructures made of pure metal with the geometrical structures and size scales the same as the composite nanostructures (see Fig. S6 in Supporting information S6). The composite nanostructures exhibit high intensity enhancement (see Figs. 6b and 6e), leading to the values of SECARS enhancement factors in the order of 9 in Mg nanoparticles-doped Si3N4 composite nanostructure (Fig. 6c) and in the order of 10 in Al nanoparticles-doped one (Fig. 6f). Such SECARS enhancement factors in the near- and deep-UV regions are unprecedentedly high, to the best of our knowledge, although the several experimental demonstrations of highly enhanced SERS have been reported in these regions.40,41 Considering the additional enhancement mechanisms described above, one can conclude that these composite nanostructures can be applied for highly sensitive single molecule detection in the near-/deep-UV region of shorter wavelengths compared with the nanostructures of pure metals.

CONCLUSIONS In this work, we have shown that metal-dielectric composite nanostructures support prominent Fano resonances in the ranges from visible to deep-UV and can be applied for SECARS-based single molecule detection. In the nanostructures made of such composites, the plasmonic effects appear at shorter wavelengths compared with nanostructures of pure metals. By changing the host medium and the filling factor of metal nanoparticles in the composites, the spectral ranges exhibiting the Fano resonances can be tuned in a wide spectral range. The composite


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nanostructures containing silver nanoparticles generate significantly high SECARS enhancement factors for pumping at about 532 nm, which is order-of-magnitude higher than the threshold for single molecule detections based on SECARS technique. In particular, the nanostructures made of dielectric composites of Mg and Al nanoparticles exhibit Fano resonances in the near- and deep-UV regions where the inherent Raman cross-sections are much larger than in the visible and infrared due to the wavelength dependence and molecular resonances, thus supporting SECARS enhancement factors sufficiently high for single molecule detection. The findings presented in this work can find broad applications in bio-sensing based on spectroscopic approaches.

ASSOCIATED CONTENT Supporting Information. S1. Details of DDA-based simulations using software adda-1.2 and comparison with the result of FDTD. S2. Fano resonance in quadrumer of titania nanodisks containing silver nanoparticles. S3. Simulation result without using Maxwell-Garnett is compared with the result shown in Fig. 1. S4. Dielectric constants of silica, alumina, titania, and zinc sulfide. S5. Effective dielectric functions of Si3N4 composites containing Mg and Al nanoparticles shown in Fig. 6. S6. Fano resonances in the nanostructures of Mg and Al. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (K.-H. Kim).


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Basic Research Program of DPR Korea (No. 9-1-1).

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Metal-Dielectric Composite Nanostructures for Fano Resonance

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