High Sensitivity Localized Surface Plasmon Resonance Sensing

ARTICLE pubs.acs.org/JPCC

High Sensitivity Localized Surface Plasmon Resonance Sensing Using a Double Split NanoRing Cavity Shao-Ding Liu,*,†,‡ Zhi Yang,† Rui-Ping Liu,† and Xiu-Yan Li† † ‡

Department of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, People's Republic of China Department of Physics, Wuhan University, Wuhan 430072, People's Republic of China

bS Supporting Information ABSTRACT: Practical implementations of biosensing with metallic nanostructures often suffer from the large line width of the plasmon resonances induced by large radiative damping. A double split nanoring cavity is designed to suppress the radiative damping. The coupling between the superradiant quadrupole mode of a split nanoring with one gap and the subradiant quadrupole mode of a split nanoring with two gaps leads to splitting of the modal energies into bonding and antibonding quadrupolequadrupole modes. The radiative damping is suppressed effectively, leading to a narrow line width for both bonding and antibonding quadrupolequadrupole modes. Calculation results show that bulk refractive index sensitivities exceeding 1200 nm/RIU with a figure of merit exceeding 8.5 in the near-infrared are obtained with a Au double split nanoring cavity. The large cavity volumes and uniform electric fields inside the cavity make the double split nanoring cavity a good platform for surface-enhanced molecular sensing.

1. INTRODUCTION Localized surface plasmon resonances (LSPRs) in metallic nanostructures have attracted much attention in the past decades.1,2 Many applications were developed using metallic nanostructures, such as waveguiding devices below the diffraction limit,3 plasmon lasers,4 surface-enhanced Raman scattering (SERS),5 invisibility cloak,6,7 slow light,8 and biosensing.911 LSPRs are highly sensitive to the size and shape of nanostructures. There are higherorder resonances for nanostructures with symmetry breaking, and the effect of symmetry breaking is an important topic in the field of plasmonics in recent years.1215 Besides the studies of individual nanostructures, people are more interested in complex nanostructures, which are found to be useful for spectral line shaping applications.16,17 The plasmon hybridization (PH) model was proposed to describe the plasmon response of complex nanostructures of arbitrary shape.1820 According to the PH theory, all coupling between individual plasmon modes leads to splitting of the modal energies into bonding and antibonding combinations. Fano resonances in metallic nanostructures caused by the interaction of narrow dark modes with broad bright modes have gained much attention in recent years.2128 For strong interactions and neardegenerate levels, the coupling can lead to a plasmon-induced transparency,29,30 which is an effect similar to the electromagnetically induced transparency (EIT) phenomenon in atomic physics.31 LSPRs depend on the refractive index (RI) of the surrounding medium. This property can be used for label-free biosensing by monitoring the resonance wavelength, which follows the changes in local RI due to the adsorption of biomolecules on the nanostructures. r 2011 American Chemical Society

The spectral shift of the surface plasmon (SP) resonance should be high and the line width should be narrow to reach the best sensing performance. The spectral shift per RI unit (RIU) and figure of merit (FoM = (δλ/δn)/line width)32 were used to characterize the two properties. Many nanostructures have been investigated to get a good sensing performance, such as nanodisks,33 nanocubes,32,34 nanostars,35 nanobipyramids,36 and nanorings (NRs).37 A lot of new methods have also been proposed and demonstrated to improve the sensing performance, for example, reducing the substrate effect with a pillar,38 angledependent resonance,39 detuned electrical dipole,40 infrared perfect absorber,41 propagating surface plasmons,42 surface lattice resonances,43,44 planar metamaterial analogue of EIT,10 and using coupling subradiant mode and Fano resonance.4547 Among these methods, RI sensing with a coupling subradiant mode and Fano resonance is one of the most promising methods: the response is fast, the resonance is easily tunable, and there is substantial reduction in line width. Recently, Verellen et al. demonstrates the coherent coupling of bright and dark plasmon modes in a gold nanocross and nanobar, where bulk RI sensitivities exceeding 1000 nm/RIU with an FoM reaching 5 in the near-infrared (NIR) were obtained.48 Ring-shaped nanostructures have gained great interest for their promising properties.4953 It was found that, under oblique incident excitation, there were higher-order resonances of perfect Received: June 15, 2011 Revised: November 11, 2011 Published: November 15, 2011 24469

dx.doi.org/10.1021/jp209754m | J. Phys. Chem. C 2011, 115, 24469–24477

The Journal of Physical Chemistry C nanorings (NRs).50 Higher-order modes can also be excited for NRs with symmetry breaking, that is, split NRs (SNRs).51,52 NRs and SNRs have a large cavity volume and a homogeneous field enhancement, and it is a good platform for sensing applications.5356 Experimental results show that RI sensitivity exceeding 800 nm/RIU can be obtained with Au NRs, but the FoM is only about 2 for the large radiative damping and the inhomogeneous broadening.37 To get a larger FoM, the line width has to be reduced, and a ring-disk cavity was proposed and demonstrated for RI sensing applications.46,47 The RI sensitivities of complete dielectric filling of the cavity of Fano and dipole bonding resonances exceed, respectively, 500 and 1100 nm/RIU for a Ag ring-disk cavity, and the corresponding FoMs exceed 8 and 7.46 For a Au ring-disk cavity, LSPR shifts of the dipole bonding resonances exceed, respectively, 300 and 1100 nm/RIU for partial and complete dielectric filling of the cavity, and the corresponding FoMs are about 0.7 and 1.9.47 Recently, a Au double nanopillar has been studied for RI sensing applications, and an FoM exceeding 18 can be obtained.57 The ring-disk cavity is promising for RI sensing applications, but for the weak resonance of the quadrupole mode of perfect NRs, the Fano resonance of the ring-disk cavity is relatively weak compared with the bonding resonance. Sensing with a subradiant bonding mode is a good method for line width reduction, and to get a better sensing performance, the radiative damping should be reduced more effectively. In this paper, a double SNR (DSNR) cavity is studied for sensing applications. The coupling between the quadrupole modes of the two rings leads to splitting of the modal energies into bonding and antibonding quadrupole quadrupole modes. The scattering quantum yield is reduced with the quadrupole modes coupling, and the radiative damping has been suppressed effectively, leading to a narrow line width for both quadrupolequadrupole coupling modes. DSNRs have a large cavity volume, and the resonance of the quadrupole quadrupole coupling modes shows a large dependence on the surrounding medium, making this geometry promising for chemical and biomedical sensing applications.

2. METHODS The finite-difference time-domain (FDTD) method is used to calculate the spectra.58 In the calculation of FDTD, the simulation area is discretized by a grid mesh, and the electromagnetic constants of the target and surrounding medium are assigned over the grid points. With finite-difference approximations in both time and space, the Maxwell’s curl equations are subsequently discretized. The change of the electric field in time is dependent on the change of the magnetic field across space; that is, at any point in the simulation area, the updated value of the electric field in time is dependent on the numerical curl of the local distribution of the magnetic field in space as well as the stored value of the electric field. The magnetic field is timestepped in a similar manner as the electric field. The simple simulation procedure makes FDTD a powerful method to model lightmatter interactions in complex systems. The software FDTD Solutions 6.5.11 is used to simulate the systems, gold is chosen as the material due to its amenability for surface functionalization with a viewpoint to molecular sensing, the unit cell size is 2.2 2.2 2.2 nm3, the individual cavity structures are illuminated with a plane-wave pulse, and the dielectric response of Au is modeled with a Drude fit to the experimental data (background dielectric constant ε∞ = 10.094, plasma frequency

ARTICLE

ωp = 1.36422 1016 rad/s, and collision frequency νc = 1.093 1014 Hz).48,59

3. RESULTS AND DISCUSSION Multipole LSPRs of SNRs. Figure 1a shows the geometry of a DSNR cavity, where a simple perfect cylindrical shape is used. The DSNR cavity is constructed with a split NR with one gap (SNR-I) and a split NR with two gaps (SNR-II). The parameters used to characterize the geometry are the following: the outer radius R (r) of SNR-I (SNR-II), the width W, the gap length g, the thickness T, and the center offset d. The optical properties of SNR-I and SNR-II are investigated before the study of the DSNR cavity. The extinction and absorption spectra of an SNR-I under normal incident excitation are shown in Figure 1b, where the surrounding medium is supposed to be a vacuum, the incident polarization is along the y axis, and there are multipolar SPRs due to symmetry breaking.51 The resonances around 1149 and 783 nm are, respectively, the quadrupole and octupole modes, and it should be noted that the quadrupolar mode is, in fact, the third-order resonances of SNR-I.51 There will be the dipole resonance when the circumference of SNR-I equals approximately a half of the corresponding SP wavelength of the bend nanorod, and calculation results show that the resonance of the dipole mode of SNR-I is around 3335 nm. Figure 1c illustrates the field enhancement distribution of the quadrupole mode at the cross section of SNR-I, the maximum field enhancement value is about 44, and the current density vectors are indicated by arrows. It is the linear colorscale of the field enhancement, and the color scale is the same for all panels. Unlike the quadrupole mode of a perfect NR, which couples weakly to dipolar radiation, the coupling between the quadrupole mode of SNR-I and the external field is strong due to symmetry breaking, and there is an asymmetry charge distribution on SNR-I. The scattering quantum yield will be used to characterize the scattering efficiency60 Q Sca η¼ ð1Þ Q Ext Res

which is the ratio of scattering efficiency to the extinction efficiency at their respective resonance maxima. The scattering quantum yield is about 0.723 for the quadrupole mode of SNR-I, and the line width characterized by the full width at half-maximum (fwhm) is about 172 nm. It is known that radiative damping is stronger for a larger nanostructure, but it is not the main reason leading to the large line width of the quadrupolar mode of SNR-I. The main scattering channels of the quadrupolar mode of SNR-I is associated with the dipole moment, there is a large scattering efficiency, and the quadrupole mode of SNR-I is superradiant.61,62 The dotted line in Figure 1d is the extinction spectrum of an SNR-II under normal incident excitation, where only one resonance around 908 nm is excited. The field enhancement distribution and current density vectors of this mode are shown in the inset of Figure 1e. The current flow directions of the two parts are different, and it is the dipole mode of SNR-II. The solid line in Figure 1d is the extinction spectrum of the SNR-II when illuminating from the side at grazing incident; the quadrupole resonance around 1153 nm appears in the spectrum as a result of retardation. The field enhancement distribution at the cross section is shown in Figure 1e, and the maximum field enhancement 24470

dx.doi.org/10.1021/jp209754m |J. Phys. Chem. C 2011, 115, 24469–24477

The Journal of Physical Chemistry C

ARTICLE

Figure 1. (a) Geometry of a DSNR cavity. The geometry parameters are the following: outer radius R (r) of the rings, the width W, the gap length g, the thickness T, the center of the rings o and o0 , and the center distance d. (b) Spectra of a gold SNR-I that is under normal incident excitation. (c) Field enhancement (|E|/|Einc|) of the quadrupole mode at the cross section of SNR-I. The current density vectors are indicated by arrows. (d) Spectra of an SNR-II when illuminating from the side at grazing incident, where the dotted line is the extinction spectrum that is under normal incident excitation, and the dashed-dotted line is the extinction spectrum when the wave vector and polarization are along the x axis and y axis, respectively. (e) Field enhancement of the quadrupole mode at the cross section of SNR-II, and the inset shows the field enhancement of the dipole mode around 908 nm. The structures are placed in a vacuum; the geometry parameters are R = 170 nm, r = 100 nm, W = 40 nm, g = 30 nm, and T = 30 nm.

value is about 57. Like that of a pair of nanorods,29 the quadrupole mode of SNR-II is, in fact, the bonding dipole mode, and its resonance energy is lower than that of the dipole mode of SNR-II. The quadrupole mode of SNR-II couples weakly to the external field, the scattering quantum yield is only about 0.2, the line width is about 53 nm, and the quadrupole mode of SNR-II is subradiant. The dashed-dotted line in Figure 1d is the extinction spectrum of the SNR-II when the wave vector and polarization are along the x axis and y axis, respectively. It can be found the dipole mode of SNR-II cannot be excited under this polarization, and a new resonance around 670 nm appears in the spectrum. Field distributions reveal that the new resonance is a multipolar plasmon mode with six charge lobes around SNR-II. These properties imply that, if SNR-I and SNR-II are aligned together,

as shown in Figure 1a, the dipole mode of SNR-II would not interact with the quadrupole mode of SNR-I when the polarization is along the y axis. Optical Properties of the DSNR Cavity. As shown in Figure 1a, when the two kinds of split NRs are aligned together to form a DSNR cavity, one can expect that there will be a plasmonic-induced optical transparency phenomenon for the coupling between the superradiant and subradiant quadrupole modes.29,30 From the viewpoint of PH theory, the coupling between the two plasmon modes leads to splitting of the modal energies into bonding and antibonding resonances. Under normal incident excitation, Figure 2a shows the extinction spectra evolution versus the inner ring radius. For a DSNR cavity with a small SNR-II, for example, when r = 90 nm, there are two resonances in the 24471



ARTICLE

Figure 3. Fano fitting of the extinction spectrum of the DSNR cavity with an inner ring radius r = 100 nm, where the red thick solid line is the calculated spectrum, the black thin solid line is the two-oscillator Fano fit, and the blue dashed lines are the two oscillators used in the fit. The parameters used for the fitting are ar = 0.02605, b1 = 0.71483, Γ1 = 0.04032 eV, j1 = 0.05768 rad, ω1 = 0.9883 eV, b2 = 0.83755, Γ2 = 0.05233 eV, j2 = 0.0108 rad, and ω2 = 1.0855 eV.

Figure 2. (a) Extinction spectra of DSNR cavities with different inner ring sizes under normal incident excitation, where the structures are placed in a vacuum, R = 170 nm, W = 40 nm, g = 30 nm, T = 30 nm, and d = 0. Field enhancement distribution at the cross section of the DSNR cavity with (b) r = 90 nm, (c) r = 100 nm, and (d) r = 110 nm, where the left panel is the AQQ mode, the middle panel is the transparence dip, and the right panel is the BQQ mode. The current density vectors are indicated by arrows.

spectrum. For the resonance with a higher energy around 1056 nm, the left panel of Figure 2b shows the field enhancement distribution at the cross section, and the arrows indicate the current density vectors. One can find that it is the antibonding quadrupolequadrupole (AQQ) mode, and the resonance is weak due to the weak coupling between the two modes. Compared with those of the outer ring, the field enhancement and current density of the inner ring are stronger, leading to a narrow line width of the AQQ mode. The field enhancement distribution and current density vectors of the resonance with a lower energy around 1188 nm are shown in the right panel of Figure 2b. There is strong field enhancement between the two rings, and it is the bonding quadrupole quadrupole (BQQ) mode. The field enhancement and current density of the outer ring are much stronger than those of the inner ring, leading to a broad line width of the BQQ mode when the coupling is weak. The middle panel of Figure 2b represents the field enhancement distribution for the dip around 1081 nm, at where the excitation of the subradiant quadrupole mode of the inner ring (by the superradiant

quadrupole mode of the outer ring) is coupling back to the superradiant quadrupole mode of the outer ring, which would form a transparence window. Figure 2c shows the field enhancement distributions when the inner ring radius is increased to 100 nm. The AQQ mode around 1143 nm becomes stronger for the strong coupling between the two rings, the ratio of the current density amplitude of the outer ring to the inner ring becomes larger, and the line width is wider than that of r = 90 nm. For the BQQ mode, the radiative damping of the quadrupole mode of SNR-I is strongly suppressed because of the bonding coupling with the quadrupole mode of SNR-II, and the line width becomes narrower. With the near-degenerate levels of the quadrupole modes of SNR-I and SNR-II, a promising plasmonic-induced optical transparency appears around 1202 nm, and one can find from the middle panel of Figure 2c that the current density of SNR-I is very weak compared with that of SNR-II . When the size of the inner ring is enlarged to r = 110 nm, there is a large separation between the AQQ and BQQ modes because of the strong coupling, and field enhancement distributions of the AQQ mode, the transparence dip, and the BQQ mode are shown in Figure 2d. It is also noted that some higher-order modes below 1000 nm appear in the spectra when r > 110 nm, which will not be discussed here. To consistently quantify the resonance wavelength and line width of the AQQ and BQQ modes, a proper fitting function to the whole spectrum is required, and an analytical Fano interference model is used to fit the extinction spectra E(ω) = |e(ω)| 2 48 bj Γj eijj eðωÞ ¼ ar þ ð2Þ j ω ωj þ iΓj

∑

where ar is the background amplitude and Γj, bj, jj and ωj characterize, respectively, the radiative damping, amplitude, phase, and resonant energy of the j different oscillators representing the interfering hybridized resonances. A two-oscillator (j = 1, 2) model is used to fit the spectra, and a good match was obtained for all the calculated and fitted spectra. For example, the red line in Figure 3 is the calculated spectrum with r = 100 nm, and the black curve shows the fitted line. The blue dashed lines represent the two fit oscillators. From low to higher energy, we 24472



Figure 4. (a) The relationship between resonance wavelength and inner ring size of DSNR cavities. (b) fwhm of BQQ and AQQ modes versus inner ring radius. (c) Scattering quantum yield value versus inner ring radius, where the dashed lines indicate the line width and the scattering quantum yield value of the quadrupole mode of SNR-I, and the geometry parameters are the same as those in Figure 2.

have the oscillator representing the BQQ and AQQ modes, respectively. The corresponding radiative damping will be used to define the line width of the resonances. The relationships between the resonance wavelength of the two coupling modes and the inner ring radius are shown in Figure 4a. The quadrupole resonance of SNR-II is red shifting with the increase of its radius, leading to a red shift of the AQQ mode. The BQQ mode is also red shifting for the same reason. Compared with that of the AQQ mode, the wavelength shift of the BQQ mode is more pronounced, especially for r > 105 nm. The resonance energy of the BQQ mode is determined by SP interaction between the two rings. There is a smaller space between the two rings for DSNRs with a larger r, leading to a strong SP coupling, and the resonance of the BQQ mode is red shifting dramatically. The circular points in Figure 4b show the relationship between the line width and the inner ring radius of the AQQ mode. As

ARTICLE

mentioned above, for a DSNR cavity with a small inner ring, the quadrupole resonance of the outer ring is not excited effectively, where the current density of the outer ring is weaker compared with that of the inner ring, leading to the narrow line width of the AQQ mode. When the size of the inner ring is enlarged(r < 105 nm), the quadrupole resonance of the outer ring is excited more effectively, the current density of the outer ring becomes stronger, and the line width of the AQQ mode is broadening. One may also understand the broadening of the line width of the AQQ mode from the evolution of the scattering quantum yield. The circular points in Figure 4c represent the scattering quantum yield of the AQQ mode versus the inner ring radius. Because of the antibonding coupling of the two individual quadrupole modes, it can be found that the scattering quantum yield of the AQQ mode does increase with the increase of the inner ring size (r < 105 nm), and the line width is broadening. However, the maximum value of the scattering quantum yield of the AQQ mode (around 1165 nm when r = 105 nm) is only about 0.557, which is smaller than that of the superradiant quadrupole resonance of the SNR-I (around 1149 nm with η = 0.723). The corresponding widest line width of the AQQ mode is only about 122 nm, which is also narrower than that of the SNR-I (172 nm). These phenomena indicate that the radiative damping of the outer ring is suppressed by the inner ring even for the antibonding coupling. When one continues to enlarge the inner ring (120 nm > r > 105 nm), the radiative damping of the outer ring is suppressed more effectively, the scattering quantum yield value is decreasing, and the line width of the AQQ mode becomes narrower. The square points in Figure 4b show the relationship between the line width and the inner ring radius for the BQQ mode. The scattering quantum yield versus the inner ring radius is represented in Figure 4c. The scattering quantum yield is decreasing with the increase of inner ring radius, and its value is only about 0.121 when r = 120 nm. This property may be used for absorber applications. On the other hand, there is not a simple relationship between the line width and the inner ring radius. It is well known that radiative losses can increase dramatically with particle size. On the contrary, radiative damping can be reduced with a bonding coupling between two individual resonances. As a result, there is a competition between the increase of radiative losses for the increase of particle size and the reduction of radiative damping for the bonding coupling of the BQQ mode. When r is increased from 85 to 100 nm, there is a small variation of the resonance wavelength (from 1178 to 1255 nm), the radiative damping is reduced by the bonding coupling, and the line width is decreased from 157 to 102 nm. When the inner ring size is in the range of 100 nm < r < 110 nm, the resonance wavelength is red shifting from 1255 to 1420 nm, there is almost an equal effect between the increase and reduction of radiative damping, and the variation of the line width is minute (decreased from 102 to 98 nm). When 110 nm < r < 120 nm, the increase of radiative losses with the increase of particle size is the main contribution, the resonance wavelength is red shifting from 1420 to 1760 nm, and the line width is broadening from 98 to 146 nm. The line width of the BQQ mode is also narrower than that of the superradiant quadrupole resonance of the SNR-I; even the resonance has been shifted to 1760 nm for r = 120 nm. From Figure 4b, one can find that the line width of the AQQ mode is very narrow for a DSNR cavity with a small inner ring, but the resonance is weak. For a DSNR cavity with a large inner ring, the line width of the BQQ mode is broadening. Therefore, 24473



ARTICLE

Table 1. Bulk RI Sensitivity of the DSNR Cavity with Different Offsets for the BQQ and AQQ Modes, Where Resonance Values Are Taken in Water (n = 1.33) d

resonance

mode

(nm)

(nm)

BQQ

15

1681

0.378

0

1642

0.431

15

1700

0.432

15 0

1506 1490

15

1483

AQQ

Figure 5. Extinction spectra of DSNR cavities with different offsets between the two rings, where the structures are placed in a vacuum, R = 170 nm, r = 100 nm, W = 40 nm, g = 30 nm, and T = 30 nm.

Figure 6. Extinction spectra of the DSNR cavity with different surrounding meda: (a) d = 15 nm, (b) d = 0 nm, and (c) d = 15 nm. The insets show the resonance wavelength shift relative to water (n = 1.33).

the DSNR cavity with an inner ring r = 100 nm is chosen for the following studies, at where it has a relatively narrow line width and strong resonance for both AQQ and BQQ modes. The

fwhm

shift/RIU

(nm)

(nm)

FoM

142

1218

8.6

161

1169

7.3

170

1196

7.0

0.547 0.532

190 171

1061 1061

5.6 6.2

0.524

166

1042

6.3

η

resonance energy of AQQ and BQQ modes is determined by SP interactions between the inner and the outer rings, and the resonance can be modified by adjusting the relative position of the two rings. Figure 5 shows the extinction spectra of the DSNR cavity with different offsets d between the two rings. For the AQQ mode, with the increasing of the offset d, the restoring force is increased, and the resonance wavelength is blue shifted. For the BQQ mode, as can be seen from the right panels of Figure 2b,c, charge distributions on the two rings are different, there are strong SP coupling, and the field enhancement between the two rings is very strong. The SP coupling becomes stronger when the two rings are close to each other, and the resonance energy would be reduced, leading to a red shift of the BQQ resonance. There is the minimum SP coupling when d = 0, and the corresponding resonance wavelength of the BQQ mode is 1255 nm. A decrease or increase of the offset d would lead to a stronger SP coupling, and the BQQ resonance would be red shifted. LSPR Sensing with DSNR Cavity. Because of the suppression of radiative damping because of the quadrupolequadrupole mode coupling, one can expect that an improvement of FoM can be achieved for sensing applications with a DSNR cavity. The bulk RI sensitivity is a good indicator for the local sensitivity. Figure 6a represents the extinction spectra of the DSNR cavity with different surrounding mediums, where the offset between the two rings, d = 15 nm, and the inset shows a spectral shift relative to water (n = 1.33) versus the RI of the surrounding medium. A linear fit to the data gives a large RI sensitivity of 1218 and 1061 nm/RIU for the BQQ and AQQ modes, respectively. The extinction spectra of the DSNR cavity with an offset d = 0 and 15 nm surrounded with different mediums are shown in Figure 6b,c, respectively. The RI sensitivities with different offsets are summarized in Table 1, where a spectral shift larger than 1000 nm/RIU is achieved for both BQQ and AQQ modes with different offsets. When the line width is taken into account, FoMs of both modes are calculated, as shown in Table 1, where all line widths are obtained from a full Fano model fit. For the BQQ mode with d = 15 nm, the scattering quantum yield is only about 0.378, leading to a narrow line width (142 nm), and the corresponding FoM is about 8.6. When the offset is enlarged to d = 0, the scattering quantum yield is increased to 0.431, and the line width of the BQQ mode is broadening to 161 nm, leading to a smaller FoM = 7.3. When d = 15 nm, although the scattering quantum yield of the BQQ mode is not changed much compared with that of d = 0 nm, the resonance is red shifted, the line width is broadening to 170 nm, and the FoM is decreased to 7.0. As for the AQQ mode, the scattering quantum yield is reduced with the increase of the offset, leading to the decreasing of the line width, and the FoMs are about 5.6, 6.2, and 6.3 for d = 15, 0, 24474



ARTICLE

Table 2. Complete Filling RI Sensitivity of the DSNR Cavity with Different Offsets for the BQQ and AQQ Modes mode

d (nm)

shift/RIU (nm)

FoM

BQQ

15

566

4.0

0 15

503 513

3.1 3.0

15

292

1.5

0

287

1.7

15

293

1.8

AQQ

Figure 7. Extinction spectra of the DSNR cavity with different mediums and complete filling of the cavity, where the RI of the background medium is 1.33: (a) d = 15 nm, (b) d = 0 nm, and (c) d = 15 nm. The inset shows the resonance wavelength shift relative to water (n = 1.33).

and 15 nm, respectively. The spectral shift sensitivity and FoM sensitivity of the individual split ring components are also calculated. For SNR-I that is under normal incident excitation, the spectral shift sensitivity of the quadrupole mode is about 1083 nm/RIU, the line width is about 261 nm when n = 1.33, and the corresponding FoM is about 4.1. For SNR-II when illuminating from the side at grazing incident, the spectral shift sensitivity of the quadrupole mode is about 1072 nm/RIU, the line width is about 120 nm when n = 1.33, and the corresponding FoM is about 8.9, which is larger than that of the DSNR cavity. The quadrupole mode of SNR-II is a dark mode, which cannot be excited under normal incident excitation, and the resonance is weak when the incident angle is small. Next, we examine the sensitivity of the resonance spectra of the RI in the gap region between the two rings, where small molecules are expected to concentrate because of strong gradient forces induced by plasmons. Figure 7 represents the spectra variations with different surrounding mediums when the whole

Figure 8. Extinction spectra of the DSNR cavity with different mediums and partial filling of the cavity, where the RI of the background medium is 1.33: (a) d = 15 nm, (b) d = 0 nm, and (c) d = 15 nm. The inset shows the resonance wavelength shift relative to water (n = 1.33).

cavity is completely filled with a dielectric medium, where the background medium RI is 1.33. The RI sensitivities for both BQQ and AQQ modes are summarized in Table 2. A maximum RI sensitivity of 566 nm/RIU and FoM = 4.0 are obtained for the BQQ mode of the DSNR cavity with d = 15 nm. The spectra variations when the cavity is partially filled with a dielectric medium are represented in Figure 8. The RI sensitivities 24475



ARTICLE

Table 3. Partial Filling RI Sensitivity of the DSNR Cavity with Different Offsets for the BQQ and AQQ Modes mode

d (nm)

shift/RIU (nm)

FoM

BQQ

15

217

1.53

0 15

192 209

1.19 1.22

15

105

0.55

0

112

0.65

15

109

0.66

AQQ

for both BQQ and AQQ modes are summarized in Table 3. A maximum RI sensitivity of 217 nm/RIU and FoM = 1.53 is obtained for the BQQ mode when d = 15 nm. When the material is changed to Ag, which has a smaller absorption loss, a bulk RI sensitivity exceeding 1300 nm/RIU with an FoM reaching 18 in the NIR can be obtained; please refer to the Supporting Information for detailed information.

4. CONCLUSIONS In conclusion, Au DSNR cavities are investigated for RI sensing applications. The coupling between the superradiant quadrupole mode of SNR-I and the subradiant quadrupole mode of SNR-II leads to splitting of the modal energies into BQQ and AQQ modes. The scattering quantum yield is decreased for the quadrupole mode coupling, and the radiative damping has been suppressed effectively, leading to a narrow line width for both BQQ and AQQ modes. Bulk RI sensitivities exceeding 1200 nm/ RIU with an FoM exceeding 8.5 in the NIR are obtained for the BQQ mode of the DSNR cavity. The large cavity volumes and uniform electric fields inside the cavity make the DSNR cavity a good platform for biosensing applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. Sensing performance of Ag DSNR cavities. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the fund of Taiyuan University of Technology for young teachers. The authors thank Zi-Peng Zhu and Hui Li for the helpful discussion. ’ REFERENCES (1) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Surface plasmon subwavelength optics. Nature 2003, 424, 824–830. (2) Maier, S. A. Plasmonics: Fundamentals and Applications, 1st ed.; Springer: New York, 2007. (3) Bozhevolnyi, S. I.; Volkov, V. S.; Devaux, E.; Laluet, J. Y.; Ebbesen, T. W. Channel plasmon subwavelength waveguide components including interferometers and ring resonators. Nature 2006, 440, 508–511. (4) Noginov, M. A.; Zhu, G.; Belgrave, A. M.; Bakker, R.; Shalaev, V. M.; Narimanov, E. E.; Stout, S.; Herz, E.; Suteewong, T.; Wiesner, U. Demonstration of a spaser-based naolaser. Nature 2009, 460, 1110–1113.

(5) Li, W.; Camargo, P. H. C.; Lu, X.; Xia, Y. Dimers of silver nanospheres: Facile synthesis and their use as hot spots for surfaceenhanced Raman scattering. Nano Lett. 2009, 9, 485–490. (6) Zhang, B.; Luo, Y.; Liu, X.; Barbastathis, G. Macroscopic invisibility cloak for visible light. Phys. Rev. Lett. 2011, 106, 033901. (7) Zhu, X.; Liang, B.; Kan, W.; Zou, X.; Cheng, J. Acoustic cloaking by a superlens with single-negative materials. Phys. Rev. Lett. 2011, 106, 014301. (8) Yannopapas, V.; Paspalakis, E.; Vitanov, N. V. Electromagnetically induced transparency and slow light in an array of metallic nanoparticles. Phys. Rev. B 2009, 80, 035104. (9) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Duyne, R. P. V. Biosensing with plasmonic nanosensors. Nat. Mater. 2008, 7, 442–453. (10) Liu, N.; Weiss, T.; Mesch, M.; Langguth, L.; Eigenthaler, U.; Hirscher, M.; S€onnichsen, C.; Giessen, H. Planar metamaterial analogue of electromagnetically induced transparency for plasmonic sensing. Nano Lett. 2010, 10, 1103–1107. (11) Jain, P. K.; Huang, X. H.; El-Sayed, I. H.; El-Sayed, M. A. Noble metals on the nanoscale: Optical and photothermal properties and some applications in imaging sensing, biology, and medicine. Acc. Chem. Res. 2008, 41, 1578–1586. (12) Christ, A.; Martin, O. J. F.; Ekinci, Y.; Gippius, N. A.; Tikhodeev, S. G. Symmetry breaking in a plasmonic metamaterial at optical wavelength. Nano Lett. 2008, 8, 2171–2175. (13) Wang, H.; Wu, Y.; Lassiter, B.; Nehl, C. L.; Hafner, J. H.; Nordlander, P.; Halas, N. J. Symmetry breaking in individual plasmonic nanoparticles. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10856–10860. (14) Demokritov, S. O.; Serga, A. A.; Demidov, V. E.; Hillebrands, B.; Kostylev, M. P.; Kalinikos, B. A. Experimental observation of symmetry-breaking nonlinear modes in an active ring. Nature 2003, 426, 159–162. (15) Fan, J. A.; Wu, C.; Bao, K.; Bao, J.; Bardhan, R.; Halas, N. J; Manoharan, V. N.; Nordlander, P.; Shvets, G.; Capasso, F. Self-assembled plasmonic nanoparticle clusters. Science 2010, 328, 1135–1138. (16) Brown, L. V.; Sobhani, H.; Lassiter, J. B.; Nordlander, P.; Halas, N. J. Heterodimers: Plasmonic properties of mismatched nanoparticle pairs. ACS Nano 2010, 4, 819–832. (17) Larsson, E. M.; Hao, F.; Eurenius, L.; Olsson, E.; Nordlander, P.; Sutherland, D. S. Plasmon hybridization in stacked double gold nanorings with reduced symmetry. Small 2008, 4, 1630–1634. (18) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. A hybridization model for the plasmon response of complex nanostructures. Science 2003, 302, 419–422. (19) Nordlander, P.; Prodan, E. Plasmon hybridization in nanoparticles near metallic surfaces. Nano Lett. 2004, 4, 2209–2213. (20) Davis, T. J.; G omez, D. E.; Vernon, K. C. Simple model for the hybridization of surface plasmon resonances in metallic nanoparticles. Nano Lett. 2010, 10, 2618–2625. (21) Mukherjee, S.; Sobhani, H.; Lassiter, J. B.; Bardhan, R.; Nordlander, P.; Halas, N. J. Fanoshells: Nanoparticles with built-in Fano resonances. Nano Lett. 2010, 10, 2694–2701. (22) Sonnefraud, Y.; Verellen, N.; Sobhani, H.; Vandenbosch, G. A. E.; Moshchalkov, V. V.; Dorpe, P. V.; Nordlander, P.; Maier, S. A. Experimental realization of subradiant, superradiant, and Fano resonances in ring/disk plasmonic nanocavities. ACS Nano 2010, 4, 1664–1670. (23) Verellen, N.; Sonnefraud, Y.; Sobhani, H.; Hao, F.; Moshchalkov, V. V.; Dorpe, P. V.; Nordlander, P.; Maier, S. A. Fano resonances in individual coherent plasmonic nanocavities. Nano Lett. 2009, 9, 1663–1667. (24) Luk’yanchuk, B.; Zheludev, N. I.; Maier, S. A.; Halas, N. J.; Nordlander, P.; Giessen, H.; Chong, C. T. The Fano resonance in plasmonic nanostructures and metamaterials. Nat. Mater. 2010, 9, 707–715. (25) Mirin, N. A.; Bao, K.; Nordlander, P. Fano resonances in plasmonic nanoparticle aggregates. J. Phys. Chem. A 2009, 113, 4028–4034. (26) Lassiter, J. B.; Sobhani, H.; Fan, J. A.; Kundu, J.; Capasso, F.; Nordlander, P.; Halas, N. J. Fano resonances in plasmonic nanoclusters: Geometrical and chemical tenability. Nano Lett. 2010, 10, 3184–3189. 24476


The Journal of Physical Chemistry C (27) Bardhan, R.; Mukherjee, S.; Mirin, N. A.; Levit, S. D.; Nordlander, P.; Halas, N. J. Nanosphere-in-a-nanoshell: A simple nanomatryushka. J. Phys. Chem. C 2010, 114, 7378–7383. (28) Fan, J. A.; Bao, K.; Wu, C.; Bao, J.; Bardhan, R.; Halas, N. J.; Manoharan, V. N.; Shvets, G.; Nordlander, P.; Capasso, F. Fano-like interference in self-assembled plasmonic quadrumer clusters. Nano Lett. 2010, 10, 4680–4685. (29) Zhang, S.; Genov, D. A.; Wang, Y.; Liu, M.; Zhang, X. Plasmon-induced transparency in metamaterials. Phys. Rev. Lett. 2008, 101, 047401. (30) Liu, S. D.; Yang, Z.; Liu, R. P.; Li, X. Y. Plasmonic-induced optical transparency in the near-infrared and visible range with double split nanoring cavity. Opt. Express 2011, 19, 15363–15370. (31) Alzar, C. L. G.; Martinez, M. A. G.; Nussenzveig, P. Classical analog of electromagnetically induced transparency. Am. J. Phys. 2002, 70, 37–41. (32) Sherry, L. J.; Chang, S. H.; Schatz, G. C.; Duyne, R. P. V; Wiley, B. J.; Xia, Y. Localized surface plasmon resonance spectroscopy of single silver nanocubes. Nano Lett. 2005, 5, 2034–2038. (33) Hanarp, P.; K€all, M.; Sutherland, D. S. Optical properties of short range ordered arrays of nanometer gold disks prepared by colloidal lithography. J. Phys. Chem. B 2003, 107, 5768–5772. (34) Galush, W. J.; Shelby, S. A.; Mulvihill, M. J.; Tao, A.; Yang, P.; Groves, J. T. A nanocube plasmonic sensor for molecular binding on membrane surfaces. Nano Lett. 2009, 9, 2077–2082. (35) Nehl, C. L.; Liao, H.; Hafner, J. H. Optical properties of starshaped gold nanoparticles. Nano Lett. 2006, 6, 683–688. (36) Chen, H.; Kou, X.; Yang, Z.; Ni, W.; Wang, J. Shape- and sizedependent refractive index sensitivity of gold nanoparticles. Langmuir 2008, 24, 5233–5237. (37) Larsson, E. M.; Alegret, J.; K€all, M.; Sutherland, D. S. Sensing characteristics of NIR localized surface plasmon resonances in gold nanorings for application as ultrasensitive biosensors. Nano Lett. 2007, 7, 1256–1263. (38) Dmitriev, A.; H€agglund, C.; Chen, S.; Fredriksson, H.; Pakizeh, T.; K€all, M.; Sutherland, D. S. Enhanced nanoplasmonic optical sensors with reduced substrate effect. Nano Lett. 2008, 8, 3893–3898. (39) Gao, H.; Yang, J. C.; Lin, J. Y.; Stuparu, A. D.; Lee, M. H.; Mrksich, M.; Odom, T. W. Using the angle-dependent resonances of molded plasmonic crystals to improve the sensitivities of biosensors. Nano Lett. 2010, 10, 2549–2554. (40) Evlyukhin, A. B.; Bozhevolnyi, S. I.; Pors, A.; Nielsen, M. G.; Radko, I. P.; Willatzen, M.; Albrektsen, O. Detuned electrical dipoles for plasmonic sensing. Nano Lett. 2010, 10, 4571–4577. (41) Liu, N.; Mesch, M.; Weiss, T.; Hentschel, M.; Giessen, H. Infrared perfect absorber and its application as plasmonic sensor. Nano Lett. 2010, 10, 2342–2348. (42) Svedendahl, M.; Chen, S.; Dmitriev, A.; K€all, M. Refractometric sensing using propagating versus localized surface plasmons: A direct comparison. Nano Lett. 2009, 9, 4428–4433. (43) Kravets, V. G.; Schedin, F.; Kabashin, A. V.; Grigorenko, A. N. Sensitivity of collective plasmon modes of gold nanoresonators to local environment. Opt. Lett. 2010, 35, 956–958. (44) Offermans, P.; Schaafsma, M. C; Rodriguez, S. R. K.; Zhang, Y.; Crego-Calama, M.; Brongersma, S. H.; Rivas, J. G. Universal scaling of the figure of merit of plasmonic sensors. ACS Nano 2011, 5, 5151–5157. (45) Zhang, S.; Bao, K.; Halas, N. J.; Xu, H.; Nordlander, P. Substrate-induced Fano resonances of a plasmonic nanocube: A route to increased-sensitivity localized surface plasmon resonance sensors revealed. Nano Lett. 2011, 11, 1657–1663. (46) Hao, F.; Sonnefraud, Y.; Dorpe, P. V.; Maier, S. A.; Halas, N. J.; Nordlander, P. Symmetry breaking in plasmonic nanocavities: Subradiant LSPR sensing and a tunable Fano resonance. Nano Lett. 2008, 8, 3983–3988. (47) Hao, F.; Nordlander, P.; Sonnefraud, Y.; Dorpe, P. V.; Maier, S. A. Tunability of subradiant dipolar and Fano-type plasmon resonances in metallic ring/disk cavities: Implications for nanoscale optical sensing. ACS Nano 2009, 3, 643–652.

ARTICLE

(48) Verellen, N.; Dorpe, P. V.; Huang, C.; Lodewijks, K.; Vandenbosch, G. A. E.; Lagae, L.; Moshchalkov, V. V. Plasmon line shaping using nanocrosses for high sensitivity localized surface plasmon resonance sensing. Nano Lett. 2011, 11, 391–397. (49) Habteyes, T. G.; Dhuey, S.; Cabrini, S.; Schuck, P. J.; Leone, S. R. Theta-shaped plasmonic nanostructures: Bringing “dark” multipole plasmon resonances into action via conductive coupling. Nano Lett. 2011, 11, 1819–1825. (50) Hao, F.; Larsson, E. M.; Ali, T. A.; Sutherland, D. S.; Nordlander, P. Shedding light on dark plasmons in gold nanorings. Chem. Phys. Lett. 2008, 458, 262–266. (51) Sheridan, A. K.; Clark, A. W.; Glidle, A.; Cooper, J. M.; Cumming, D. R. S. Multiple plasmon resonances from gold nanostructures. Appl. Phys. Lett. 2007, 90, 143105. (52) Liu, S. D.; Zhang, Z. S.; Wang, Q. Q. High sensitivity and large field enhancement of symmetry broken Au nanorings: Effect of multipolar plasmon resonance and propagation. Opt. Express 2009, 17, 2906–2917. (53) Aizpurua, J.; Hanarp, P.; Sutherland, D. S.; K€all, M.; Bryant, G. W.; de Abajo, F. J. G. Optical properties of gold nanorings. Phys. Rev. Lett. 2003, 90, 057401. (54) Clark, A. W.; Glidle, A.; Cumming, D. R. S.; Cooper, J. M. Plasmonic split-ring resonators as dichroic nanophotonic DNA biosensors. J. Am. Chem. Soc. 2009, 131, 17615–17619. (55) Lahiri, B.; Khokhar, A. Z.; De La Rue, R. M.; McMeekin, S. G.; Johnson, N. P. Asymmetric split ring resonators for optical sensing of organic materials. Opt. Express 2009, 17, 1107–1115. (56) Cubukcu, E.; Zhang, S.; Park, Y. S.; Bartal, G.; Zhang, X. Split ring resonator sensors for infrared detection of single molecular monolayers. Appl. Phys. Lett. 2009, 95, 043113. (57) Kubo, W.; Fujikawa, S. Au double nanopillars with nanogap for plasmonic sensor. Nano Lett. 2011, 11, 8–15. (58) Taflove, A.; Hagness, S. C. Computational Electrodynamics: The Finite-Difference Time-Domain Method; Artech House: Boston, 2005. (59) Johnson, P. B.; Christy, R. W. Optical constants of noble metals. Phys. Rev. B 1972, 6, 4370. (60) Lee, K. S.; El-Sayed, M. A. Dependence of the enhanced optical scattering efficiency relative to that of absorption for gold metal nanorods on aspect ratio, size, end-cap shape, and medium refractive index. J. Phys. Chem. B 2005, 109, 20331–20338. (61) Nielsen, M. G.; Pors, A.; Nielsen, R. B.; Boltasseva, A.; Albrektsen, O.; Bozhevolnyi, S. I. Demonstration of scattering suppression in retardation-based plasmonic nanoantennas. Opt. Express 2010, 18, 14802–14811. (62) Pors, A.; Willatzen, M.; Albrektsen, O.; Bozhevolnyi, S. I. From plasmonic nanoantennas to split-ring resonators: Tuning scattering strength. J. Opt. Soc. Am. B 2010, 27, 1680–1687.

24477


High Sensitivity Localized Surface Plasmon Resonance Sensing

Recommend Documents