Biosensors and Bioelectronics 68 (2015) 719–725
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Fano coupling between Rayleigh anomaly and localized surface plasmon resonance for sensor applications Feifei Liu, Xinping Zhang n Institute of Information Photonics Technology and College of Applied Sciences, Beijing University of Technology, Beijing 100124, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 October 2014 Received in revised form 13 January 2015 Accepted 30 January 2015 Available online 31 January 2015
Fano coupling between Rayleigh anomaly and localized surface plasmon resonance has been observed in diffractive grating structures consisting of aluminum nanolines deposited on the top surface of photoresist with each nanoline composed of tightly aggregated aluminum nanoparticles. Localized surface plasmon resonance is excited both in the nanoparticles and in the nanolines by differently polarized light. The surface propagation mode excited by the first- and second-order Rayleigh diffraction anomaly is strongly scattered and diffracted by the plasmonic aluminum grating structures, producing light rays in the same direction as the reflected light beam with the same spectral feature as the Rayleigh anomaly. The narrow-band diffracted and scattered light appears as sharp dips in the broad-band reflective optical extinction spectrum of plasmon resonance, which is recognized as a kind of Fano coupling. This kind of coupled mode is utilized successfully in refractive-index-sensor devices with excellent sensitivity. & 2015 Elsevier B.V. All rights reserved.
Keywords: Fano coupling Rayleigh anomaly Plasmon resonance Plasmonic scattering Refractive-index sensors
1. Introduction Fano resonance has been extensively investigated in nanostructured semiconductors (Fan et al., 2014), photonic crystals (Shuai et al., 2013), and plasmonic metamaterials (Lovera et al., 2013; Moritake et al., 2014). Interaction between plasmonic (Vakevainen et al., 2013; Wang et al., 2014) and photonic resonance modes has been one of the most important mechanisms for such optical Fano-coupling processes. These mechanisms lead to the development of optoelectronic devices and sensors (Shuai et al., 2013; Walsh and Negro, 2013; Zhang et al., 2011, 2012). Diffractions of light by periodical arrays of metallic and dielectric nanostructures are responsible for the excitation of resonance modes in different photonic crystal lattices. Rayleigh anomaly is one of the most important features observed in diffraction gratings when the incident light beam is diffracted by a grating into grazing orders (Anishur Rahman et al., 2012; Hessel and Oliner, 1965; Iberi et al., 2014; McMahon et al., 2007; Mohammad et al., 2014; Rayleigh, 1907; Savoia et al., 2013), leading to an abrupt change in the reflection or transmission spectrum. This kind of diffraction anomaly is dependent strongly on the grating parameters and on the environmental refractive indices. Sensors have been developed making use of Rayleigh anomaly in photonic devices (Feng et al., 2012; Gordon et al., 2008; Savoia et al., 2013; Zhang et al., 2013). n
Corresponding author. E-mail address: [email protected] (X. Zhang).
http://dx.doi.org/10.1016/j.bios.2015.01.071 0956-5663/& 2015 Elsevier B.V. All rights reserved.
For gratings made of dielectric materials with low refractive indices as compared with the environmental medium, the spectroscopic modulation by Rayleigh anomaly is generally very weak due to the small modulation depth of refractive index. Therefore, using metals to construct the grating structures is effective to overcome this problem. Furthermore, surface plasmon polaritons may be excited in the metallic nanostructures, providing new chances to enrich the photophysics and to extend the functions of the resultant device (McMahon et al., 2007; Wan et al., 2014). In this work, we report Fano-coupling effects in nanogratings consisting of thermally evaporated aluminum nanolines on the top surface of a dielectric grating. Interaction between localized surface plasmon resonance of aluminum nanolines and Rayleigh anomaly leads to a Fano-like resonance mode in the reflective extinction spectrum, which appears as a narrow dip in the broadband optical extinction spectrum. Meanwhile, we have also achieved large-area plasmonic aluminum nanoline arrays with excellent homogeneity using thermal evaporation, where the control of the duty cycle of the master dielectric gratings made of photoresist is crucial for realizing isolated aluminum nanolines. This kind of Fano coupling is important for the sensitivity enhancement in sensor devices that utilize localized surface plasmon resonance or Rayleigh anomaly, which is verified by refractiveindex-sensing experiments on glucose/water solutions with different concentrations.
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2. Experimental methods Interference lithography was employed to fabricate the photoresist (PR) gratings, where a He–Cd laser at 325 nm was used as the UV light source and photoresist S1805 was used as the recording medium. A spin-coating speed of 2000 rpm was employed to fabricate the S1805 film before the exposure process. The atomic force microscopic (AFM) images have been measured using the AFM unit incorporated with the WITec Alpha300S scanning near-field optical microscope system. The scanning electron microscopic (SEM) images have been measured using a S-4800 Field Emission Electron Microscope. The spectroscopic performance of the aluminum grating and the sensor device was characterized using a USB4000 spectrometer (Ocean Optics), which has a resolution of about 2 nm and an operation band from 350 to 1000 nm. The optical extinction spectrum was evaluated by log10[IR(λ)/I0(λ)], where IR(λ) is the reflection spectrum and I0(λ) is the spectrum of the white-light source. The light is polarized parallel and perpendicular to the grating lines for TE and TM polarizations, respectively. A thermal evaporator was used to deposit aluminum onto the photoresist template grating structures. A Rudolph Refractometer J257 was used to measure the refractive indices of the glucose solutions with difference concentrations.
3. Microscopic and spectroscopic performance of the gratings consisting of thermally evaporated aluminum nanolines on the top surface of photoresist By controlling the exposure and development processes in the stage of interference lithography, it is possible to produce photoresist grating structures with a duty cycle (d/D) as small as 15%, as illustrated by the scanning electron microscopic (SEM) image in Fig. 1(a). Fig. 1(b) shows the atomic force microscopic (AFM) image of the PR grating, where the modulation depth of the grating is measured to be about 193 nm. Fig. 1(c) shows the SEM image of the cross-section profile of the grating structures after the PR grating is thermally evaporated with aluminum at a speed of about 3.6 nm/min. Due to the steep edge of the PR grating, the aluminum is thinner on the side surface than on the top surfaces of the grating lines and grating grooves. On the top surface, isolated aluminum nanolines are produced with a mean width of about 160 nm and a mean height of about 200 nm, as marked by a dotted circle in yellow. This is the most important mechanism how localized surface plasmon resonance was observed with the aluminum-evaporated gratings. According to Fig. 1(c), the interference lithography did not develop to the PR/ITO interface, where a PR layer as thick as 100 nm remains between the grating and the 200nm-thick ITO layer. Fig. 2(a) shows the SEM image of the aluminum grating structures that have been produced by evaporating aluminum onto the surface of the PR gratings and Fig. 2(b) shows an enlarged view for a local area. The narrow aluminum nanolines can be identified to stand out of the aluminum film covering the grating grooves. A duty cycle of about 20% can still be measured for the aluminum grating. The grating period is measured to be about 580 nm. We can observe from Fig. 2(a) and (b) that the evaporated aluminum film is in fact composed of small aluminum nanoparticles, which are in a size smaller than 80 nm in diameter according to a rough evaluation. These aluminum nanoparticles also induce localized surface plasmon resonance. Fig. 2(c) and (d) shows the reflective optical extinction spectra by the aluminum gratings with a period of D¼580 nm for TM and TE polarizations, respectively, with the incident angle (θ) increased from 15° to 36°.
Fig. 1. SEM (a) and AFM (b) images of the photoresist (PR) grating with a period of about 580 nm before aluminum evaporation. (c) The SEM image of the cross-section of the grating structures after being deposited with aluminum. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
For TM polarization, two features are clearly observed: a relatively broad extinction peak centered at about 466 nm, which does not shift with changing the angle of incidence, and a sharp peak centered at 730 nm for an incident angle of θ ¼15°, which shifts to the red with increasing the angle of incidence, as shown in Fig. 2 (c). The spectral feature centered at 466 nm results from localized surface plasmon resonance of the aluminum nanolines, which has a bandwidth of about 80 nm. The narrow spectral features at wavelengths longer than 730 nm can be attributed to Rayleigh anomaly of the grating diffraction, where the center wavelength is determined by λ ¼D(1 þsin θ), where D is the grating period. For TE polarization, the spectroscopic response becomes completely different from that for TM polarization, which is dominated by a strong and broad extinction spectrum that shifts from 580 to 620 nm with the incident angle increased from 15° to 36°. This strong optical extinction spectrum results from the localized surface plasmon resonance of the randomly distributed and randomly aggregated aluminum nanoparticles. The mechanisms for the broad-band and red-shifted plasmon resonance for TE polarization
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Fig. 2. (a) and (b) SEM images of aluminum-evaporated gratings with a period of about 580 nm and an enlarged view, respectively. (c) and (d) Reflective optical extinction spectra of aluminum grating with a period of 580 nm for TM and TE polarizations, respectively, with the incident angle increased from 15° to 36°.
can be understood by considering aluminum nanolines on the top surface of the photoresist, which extend in the same direction as the grating lines. The shift of the extinction spectrum can be understood by referring to the investigations in our previous publication (Liu et al., 2014), where we need to consider that TE polarized light excites plasmon resonance not only in the aluminum nanoparticles, but also in a large amount of random aggregates between them along the grating lines, as compared with TM polarization. The most important spectral feature for TE polarization is that dips in the extinction spectra are observed tunable with changing the angle of incidence, as shown in Fig. 2(d). In the shorter-wavelength range, the dips are observed at 428 nm (at θ ¼ 24°), 448 nm (at θ ¼ 27°), 465 nm (at θ ¼30°), 483 nm (at θ ¼33°), and 500 nm (at θ ¼36°), as indicated by downward arrows. For the longer-wavelength range, the dips are observed at 742 nm (at θ ¼15°), 786 nm (at θ ¼ 18°), 821 nm (at θ ¼ 21°), 851 nm (at θ ¼24°), and 892 nm (at θ ¼ 27°), as indicated by the obliquely downward arrows. However, for wavelength longer than 900 nm, it is difficult to resolve spectral features with similar performance. This is limited by the spectrometer and by the light source that has much lower intensity for wavelengths longer than 900 nm. Furthermore, interesting features in the spectrum correspond to larger angles of incidence than 36°, which led to additional modification on the spectroscopic response due to the structural parameters of the grating. Nevertheless, these available data are already sufficient to identify the mechanisms. The spectral dips in the longer-wavelength range (λ1) correspond to the first-order Rayleigh anomaly, whereas, those in the shorter-wavelength range (λ2) correspond to the second-order process with λ2 E λ1/2. Extinction dips imply enhanced reflection within a narrow band, instead of energy loss due to diffraction anomalies. Therefore, these dips in the optical extinction spectra in Fig. 2(d) result from
Fano-like coupling between plasmon resonance of aluminum nanostructures and the diffraction anomaly of the grating. However, the experimental results in Fig. 2(c) and (d) shows small overlap between plasmon resonance and diffraction anomalies and weak coupling between them. This can be improved by changing the grating period to tune the spectral position of the diffraction anomalies.
4. Fano coupling between second-order Rayleigh anomalies and plasmon resonance of the aluminum nanolines 4.1. Spectroscopic characterization To achieve better overlapping between plasmon resonance and Rayleigh diffraction anomaly, we fabricated aluminum gratings deposited on photoresist with a period of 650 nm, as shown by the SEM image in Fig. 3(a) and (b). Similarly, each aluminum nanoline consists of nanoparticles. The corresponding duty cycle was increased slightly to about 23%. Fig. 3(c) and (d) shows the optical extinction spectroscopic characterization on the reflected beam by the aluminum grating structures with a period of 650 nm, where the spectrum of the white-light source has been used as the blank for the calculation of the extinction spectra. The angle of incidence has been increased from 15° to 36° in steps of 3°, so that the tuning performance of the Fano coupling spectrum can be investigated. Due to the increased grating period, the Rayleigh anomaly is observed at longer wavelengths for a fixed angle of incidence according to λ ¼ D (1 þsin θ). For TM polarization, the first-order Rayleigh anomaly is observed at 818 nm, which is exactly equal to D(1 þsin θ), for an incident angle of θ ¼15° and a grating period of D ¼650 nm, as
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Fig. 3. (a) and (b) SEM images of aluminum-evaporated gratings with a period of about 650 nm and an enlarged view, respectively. (c) and (d) Reflective optical extinction spectra of aluminum grating with a period of 650 nm for TE and TM polarizations, respectively, with the incident angle increased from 15° to 36°.
indicated by an upward arrow in Fig. 3(c). This first-order process shifts to the red with increasing the angle of incidence and becomes difficult to identify for wavelengths longer than 900 nm. The second-order process defined by D(1 þsin θ)/2 becomes pronounced for incident angles (θ) larger than 21°, which exhibits as narrow dips in the broad optical extinction band of localized surface plasmon resonance of the aluminum nanolines. This is clearly recognized as a kind of Fano coupling. The plasmon resonance is centered at about 488 nm and the dips in the extinction spectrum recognized as the second-order coupled mode shift from about 434 to 511 nm as θ is increased from 21° to 36°. This tuning properties basically agrees with the definition by D(1 þsin θ)/2. For TE polarization, much broader extinction spectrum is again observed for the plasmon resonance of the aluminum nanostructures of aggregated nanoparticles, as shown in Fig. 3(d). At θ ¼15°, the plasmon resonance is centered at about 587 nm and has a bandwidth of about 210 nm at FWHM. The first-order Rayleigh anomaly and its coupling with plasmon resonance become weak, which can be identified at about 853 nm for θ ¼15° and is located at the longer-wavelength edge of plasmon resonance. However, much stronger second-order coupling process can be observed for incident angles larger than 18°, where the coupled mode identified by a dip in the optical extinction spectrum shifts from about 450 nm to 545 nm as the incident angle is increased from 18° to 36°. The spectral positions of these second-order processes agree well with those for TM polarization that are shown in Fig. 3(c). Therefore, Fano-like coupling between the plasmonic scattering of narrow-band surface propagation mode induced by Rayleigh diffraction anomaly and the broad-band plasmon resonance is account for the spectroscopic performance shown in Fig. 3. The plasmon resonance of the aluminum nanostructures have enhanced two processes: strong optical extinction by localized surface plasmons and strong Rayleigh diffraction anomalies, which in
principles introduce optical loss in the reflection spectrum and can be recognized by optical extinction peaks. However, scattering of the surface propagation mode in the direction of the reflection beam has reversed the contribution of Rayleigh anomaly to the reflection spectrum. Meanwhile, secondary diffraction of the Rayleigh anomaly is exactly in the direction of the reflected beam, inducing similar effects as the directly scattered light rays. As a result, narrow-band “dips” are observed in the relatively broadband optical extinction spectrum of plasmon resonance of the aluminum nanostructures. 4.2. Mechanisms for Fano coupling between Rayleigh anomalies and plasmon resonance Fig. 4 illustrates schematically the basic principles for the observed Fano coupling process in the thermally evaporated aluminum gating structures. The white-light beam (➊) incident at an angle of θ excites both localized surface plasmon resonance in the aluminum nanostructures and the surface propagation mode (the leftward thick arrows) due to the Rayleigh anomaly in the diffraction process. The surface propagation mode (❸) corresponds to the Rayleigh anomaly wavelength at λ1 ¼D(1 þsin θ) for the firstorder and at λ2 ¼D(1 þsin θ)/2 for the second-order process. Plasmonic scattering of the surface propagation mode by the aluminum nanopatrticles and nanolines produces light rays in different directions, including that in the same direction as the reflected beam, as indicated by green arrows. Meanwhile, secondary diffraction of the surface mode of Rayleigh anomaly excites new light rays also in the same direction of the reflected beam through D(sin90° þsin θ) ¼ λ or λ ¼ D(1 þ sin θ) for the first-order and D(sin 90° þ sin θ)¼2λ or λ ¼D(1 þsin θ)/2 for the second-order processes. Thus, the reflected beam (❷) contains both scattered and diffracted light rays, which interfere constructively with the reflected beam, leading to a Fano-like feature in the reflected
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Fig. 4. Mechanisms for Fano coupling processes involved in the aluminum-evaporated gratings: ❶ incident light beam; ❷ reflected light beam; ❸ Rayleigh anomaly; and ❹ diffracted and scattered light rays.
spectrum. The beams ❹ in Fig. 4 are used to denote both the scattered and diffracted light rays inside the reflected beam, which have the same wavelength as the Rayleigh anomaly. The stronger the plasmon resonance of the aluminum nanostructures, the stronger the optical extinction will be in its resonance band, and the stronger the diffraction by the grating will become due to the stronger grating modulation. However, stronger diffraction induces stronger surface propagation mode and stronger plasmonic scattering, which is an inverse process to the optical extinction by plasmon resonance. As a result, as narrow-band enhanced reflection, which is the same as the Rayleigh anomaly in wavelength, appears in the reflection spectrum. In the optical extinction spectrum, a narrow dip is observed within the broad-band plasmon-resonance band, which is recognized as a Fano-like coupling. The wavelength of the coupled mode shifts to the red with increasing the angle of incidence. The above mechanisms explain the optical spectroscopic observations in both Figs. 2 and 3, giving insights into such a kind of Fano resonance.
5. Sensor applications using Fano coupling between secondorder diffraction anomalies and localized surface plasmon resonance The grating structures shown in Fig. 3 were used to perform the sensor measurements, where TM-polarization mode was employed. The same system as reported in Zhang et al. (2011) is employed to detect the change in the environmental refractive index as a result of the change in the concentration of glucose/ water solutions. However, the sensor chip now consists of aluminum nanograting structures instead of waveguide gold photonic crystals. Fig. 5 shows the measurement results using the reflective mode for incident angles (θ) of 20° (Fig. 5(a) and (b)), 24° (Fig. 5 (c) and (d)), and 28° (Fig. 5(e) and (f)) with the concentration of the detected glucose/water solution increased from 1%, 3%, 7%, to 10%. For the measurement results on the left panel (Fig. 5(a), (c), (e)), the spectrum of the white light source was used as the blank in the calculation of the optical extinction spectra; however, for those on the right panel (Fig. 5(b), (d), (f)) the transmission spectrum through pure water (0% concentration) has been used as the blank. The amplitude of the sensor signal (A) is defined by the dip-to-peak difference of the sensor signal, as shown in the inset of Fig. 5(b) and as described in detail in our previous publications (Zhang et al., 2011, 2012, 2014). On the left panel of Fig. 5, the vertical dotted line on the left is
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used to mark the spectral position of the Fano-like coupling feature in the glucose/water solutions and the dashed arrow at a lower position is used to indicate the shift of the spectral feature when the environmental medium is changed from air to glucose/ water solutions. The vertical dotted line on the right is used to mark the spectral peak of the modulated plasmon resonance of the aluminum nanolines in glucose/water solutions, whereas, the dashed arrow at an upper position is used to indicate the spectral shift of the plasmonic feature when the environmental medium is changed from air to glucose/water solutions. For θ ¼ 20°, the Fano-like coupling is located at about F−C F−C ¼443 nm in air and shifts to about λ solution ¼555 nm in gluλ air cose/water solutions. However, the Fano-like coupling left a narrow-band plasmon-resonance mode with steepened spectral P ¼499 nm in air and shifted to edges, which is peaked at about λ air P about λ solution ¼662 nm in solutions, as shown in Fig. 5(a). Here the Fano-like coupling takes place between the second-order Rayleigh F−C ¼D(1 þsin θ)/2 ¼ anomaly and plasmon resonance with λ air F−C ¼ D(n þsin θ)/ 650 (1 þsin 20°)/2 E436 nm in air and λ solution 2 E650 (1.34 þsin 20°)/2 E547 nm in water, which basically agrees with the measurements results. With changing of the concentration of the glucose/water solution from 1% to 10%, both the coupled mode and the plasmon resonance shifts to the red and these spectral shifts are small due to the small change in the refractive index of the solution. However, the sensor signal can be calculated by −log10[IR(λ)/I0(λ)], where I0(λ) is the reflected spectrum when the sensor is placed in pure water or in the solution with 0% concentration and IR(λ) is the reflected spectrum by the grating for different solution concentrations. As shown in Fig. 5(b), the sensor signal increases in amplitude (A) with increasing of the concentration, where A was measured to be 0.0244, 0.0729, 0.1598, and 0.2322 for solution concentrations of 1%, 3%, 7%, and 10%, respectively. For θ ¼24°, the spectral shift of the Fano-coupling mode is from F−C F−C ¼446 nm to about λ solution ¼569 nm when the sensor device λ air is placed from air into glucose/water solutions, as shown in F−C ¼D(1 þsin θ)/2¼ 650 (1 þsin 24°)/ Fig. 5(c). Theoretically, λ air F−C ¼ D(n þ sinθ)/2E 650 (1.34 þ 2 E457 nm in air and λ solution sin 24°)/2E 567 nm in water. Although the discrepancy between F−C , excellent agreement can theory and experiment is large for λ air F−C be observed for λ solution . According to Fig. 5(d), the amplitude of the sensor signal A is measured to be 0.0355, 0.066, 0.1793, 0.2558 for solution concentrations of 1%, 3%, 7%, and 10%, respectively. When the incident angle is increased to θ ¼28°, much change can be observed for both the amplitude and the spectral shape of the optical extinction spectrum of plasmon resonance. It becomes difficult to identify the spectral position and bandwidth of plasmonic resonance, as shown in Fig. 5(e). However, the spectral shift of the Fano-coupling mode can still be observed from F−C F−C ¼498 nm to about λ solution ¼ 607 nm, when the sensor device λ air is placed from air into glucose/water solution. Calculations found F−C F−C ¼650 (1 þsin 28°)/2E 477 nm in air and λ solution E650 λ air (1.34 þsin 28°)/2E 588 nm. The large discrepancy between experiment and theory may result from the complex configuration of the aluminum grating lines when seen from different angles, where the interaction between the light beam and the aluminum grating lines becomes more sensitive to large incident angles. According to Fig. 5(f), the amplitude of the sensor signal A is measured to be 0.0116, 0.0321, 0.0651, 0.0089 for solution concentrations of 1%, 3%, 7%, and 10%, respectively. Fig. 6 summarizes the measurement results in Fig. 5 and replots the amplitude of sensor signal (A) as a function of the solution concentration at incident angles of 20° (yellow), 24° (red), and 28° (blue). The dashed lines are linear fits to the measurement data. Fig. 6 shows basically linear relationship between A and the
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Fig. 5. Sensor measurements on glucose/water solutions with concentrations increased from 0% to 10% for incident angles of (a) and (b) 20°; (c) and (d) 24°; (e) and (f) 28°. The extinction spectra on the left panel have been calculated using the spectrum of the light source as the blank; however, those on the right panel have been calculated using the transmission spectrum through pure water (0% concentration) as the blank.
solution concentration, implying excellent performance of the sensor device. The curve at θ ¼ 24° exhibits a largest slope, implying a highest sensitivity of the sensor. According to Fig. 5, the Rayleigh anomaly overlaps better the plasmon resonance at θ ¼24°, leading to stronger Fano-like coupling between them, as compared with θ ¼20° and θ ¼28°. This does not only confirm the mechanisms that support the operation of the sensor device, but also indicate that the sensitivity of the sensor can be improved by optimizing the angle of incidence. Fig. 6 also shows the variation of refractive index of the glucose/water solution with concentration using the plot of green squares, where an excellent linear relationship can be observed. Using the data for an incident angle of 24° and a concentration change from 3% to 10%, the sensitivity of the sensor device can be evaluated by the change in amplitude of the sensor signal over the change in the environmental refractive index as (0.2558–0.066)/ (1.3454–1.3363) ¼0.1898/0.0091 ¼20.86 OD per refractive index units (RIU). This value is much higher than that reported for the
waveguide metallic photonic crystals (Zhang et al., 2012), where a sensitivity smaller than 10 OD/RIU was achieved. To evaluate the reproducibility and reliability of the sensor device, we performed a series of sensor measurements at an optimized incident angle of 24° on the glucose/water solution with a concentration of 0%, 1%, 3%, 7%, and 10%, as shown in Fig. S1 in the Supporting information. The sensor system was cleaned by circulating pure water in the whole channels three times with each time for 10 min before each measurement on the glucose/water solution with a specified concentration. Measurement ➀ corresponds to the results shown in Fig. 5(d). Measurement ➁ was performed about 4 months later than measurement ➀, and measurements ➂ and ➃ were performed about 3 and 6 h after ➁, respectively. During the period between measurements ➀ and ➁, a number of different other experiments were performed using this sensor system. Fig. S1 shows excellent consistence between the presented 4 measurement results, although the measurement ➀ deviates relatively large from ➁, ➂, and ➃. Fig. S2 plots the average
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show excellent performance in the detection of refractive-index change in the surrounding environment.
Acknowledgments We acknowledge the 973 program (2013CB922404) and the National Natural Science Foundation of China (11274031) for the support.
Appendix A. Suplementary information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.bios.2015.01.071. Fig. 6. Sensor-signal amplitude as a function of the glucose/water solution concentration for incident angles of θ¼ 20° (yellow circles), 24° (red circles), and 28° (blue circles). Green squares: refractive index as a function of the solution concentration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
amplitude of the sensor signal (A) as a function of solution concentration with the scale bars showing deviations of the four presented measurements. The small deviation values imply again excellent consistence between multiple measurement results. The data in Figs. S1 and S2 not only confirm the reproducibility of the sensor signals, but also verify the homogeneity of the large-area photonic structures of aluminum nanolines, the excellent reproducibility of the angle-resolved tuning properties, and the reliability of the whole sensor system.
6. Conclusions Fano coupling between localized surface plasmon resonance and Rayleigh anomaly is revealed in a diffraction grating consisting of aluminum-nanoparticle-composed nanolines. Localized surface plasmon resonance is excited in both the aluminum nanoparticles and the aluminum nanolines, which shows different spectroscopic response for different polarizations. The Rayleigh anomaly that is determined by the grating period and the angle of incidence in spectroscopic response is scattered by aluminum nanostructures, which excites light rays with a narrow-band spectrum in the direction of the reflected beam. Interactions between these two processes induces Fano resonance mode in the reflective optical extinction spectrum. Localized surface plasmon resonance of aluminum nanostructures has been the common enhancement mechanisms for these processes. This kind of Fanoresonance mode is sensitive to the change both in the angle of incidence and in the environmental refractive index, providing opportunities to develop optical sensors. Sensor measurements
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Biosensors and Bioelectronics journal homepage: www.elsevier.com/locate/bios
Fano coupling between Rayleigh anomaly and localized surface plasmon resonance for sensor applications Feifei Liu, Xinping Zhang n Institute of Information Photonics Technology and College of Applied Sciences, Beijing University of Technology, Beijing 100124, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 October 2014 Received in revised form 13 January 2015 Accepted 30 January 2015 Available online 31 January 2015
Fano coupling between Rayleigh anomaly and localized surface plasmon resonance has been observed in diffractive grating structures consisting of aluminum nanolines deposited on the top surface of photoresist with each nanoline composed of tightly aggregated aluminum nanoparticles. Localized surface plasmon resonance is excited both in the nanoparticles and in the nanolines by differently polarized light. The surface propagation mode excited by the first- and second-order Rayleigh diffraction anomaly is strongly scattered and diffracted by the plasmonic aluminum grating structures, producing light rays in the same direction as the reflected light beam with the same spectral feature as the Rayleigh anomaly. The narrow-band diffracted and scattered light appears as sharp dips in the broad-band reflective optical extinction spectrum of plasmon resonance, which is recognized as a kind of Fano coupling. This kind of coupled mode is utilized successfully in refractive-index-sensor devices with excellent sensitivity. & 2015 Elsevier B.V. All rights reserved.
Keywords: Fano coupling Rayleigh anomaly Plasmon resonance Plasmonic scattering Refractive-index sensors
1. Introduction Fano resonance has been extensively investigated in nanostructured semiconductors (Fan et al., 2014), photonic crystals (Shuai et al., 2013), and plasmonic metamaterials (Lovera et al., 2013; Moritake et al., 2014). Interaction between plasmonic (Vakevainen et al., 2013; Wang et al., 2014) and photonic resonance modes has been one of the most important mechanisms for such optical Fano-coupling processes. These mechanisms lead to the development of optoelectronic devices and sensors (Shuai et al., 2013; Walsh and Negro, 2013; Zhang et al., 2011, 2012). Diffractions of light by periodical arrays of metallic and dielectric nanostructures are responsible for the excitation of resonance modes in different photonic crystal lattices. Rayleigh anomaly is one of the most important features observed in diffraction gratings when the incident light beam is diffracted by a grating into grazing orders (Anishur Rahman et al., 2012; Hessel and Oliner, 1965; Iberi et al., 2014; McMahon et al., 2007; Mohammad et al., 2014; Rayleigh, 1907; Savoia et al., 2013), leading to an abrupt change in the reflection or transmission spectrum. This kind of diffraction anomaly is dependent strongly on the grating parameters and on the environmental refractive indices. Sensors have been developed making use of Rayleigh anomaly in photonic devices (Feng et al., 2012; Gordon et al., 2008; Savoia et al., 2013; Zhang et al., 2013). n
Corresponding author. E-mail address: [email protected] (X. Zhang).
http://dx.doi.org/10.1016/j.bios.2015.01.071 0956-5663/& 2015 Elsevier B.V. All rights reserved.
For gratings made of dielectric materials with low refractive indices as compared with the environmental medium, the spectroscopic modulation by Rayleigh anomaly is generally very weak due to the small modulation depth of refractive index. Therefore, using metals to construct the grating structures is effective to overcome this problem. Furthermore, surface plasmon polaritons may be excited in the metallic nanostructures, providing new chances to enrich the photophysics and to extend the functions of the resultant device (McMahon et al., 2007; Wan et al., 2014). In this work, we report Fano-coupling effects in nanogratings consisting of thermally evaporated aluminum nanolines on the top surface of a dielectric grating. Interaction between localized surface plasmon resonance of aluminum nanolines and Rayleigh anomaly leads to a Fano-like resonance mode in the reflective extinction spectrum, which appears as a narrow dip in the broadband optical extinction spectrum. Meanwhile, we have also achieved large-area plasmonic aluminum nanoline arrays with excellent homogeneity using thermal evaporation, where the control of the duty cycle of the master dielectric gratings made of photoresist is crucial for realizing isolated aluminum nanolines. This kind of Fano coupling is important for the sensitivity enhancement in sensor devices that utilize localized surface plasmon resonance or Rayleigh anomaly, which is verified by refractiveindex-sensing experiments on glucose/water solutions with different concentrations.
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2. Experimental methods Interference lithography was employed to fabricate the photoresist (PR) gratings, where a He–Cd laser at 325 nm was used as the UV light source and photoresist S1805 was used as the recording medium. A spin-coating speed of 2000 rpm was employed to fabricate the S1805 film before the exposure process. The atomic force microscopic (AFM) images have been measured using the AFM unit incorporated with the WITec Alpha300S scanning near-field optical microscope system. The scanning electron microscopic (SEM) images have been measured using a S-4800 Field Emission Electron Microscope. The spectroscopic performance of the aluminum grating and the sensor device was characterized using a USB4000 spectrometer (Ocean Optics), which has a resolution of about 2 nm and an operation band from 350 to 1000 nm. The optical extinction spectrum was evaluated by log10[IR(λ)/I0(λ)], where IR(λ) is the reflection spectrum and I0(λ) is the spectrum of the white-light source. The light is polarized parallel and perpendicular to the grating lines for TE and TM polarizations, respectively. A thermal evaporator was used to deposit aluminum onto the photoresist template grating structures. A Rudolph Refractometer J257 was used to measure the refractive indices of the glucose solutions with difference concentrations.
3. Microscopic and spectroscopic performance of the gratings consisting of thermally evaporated aluminum nanolines on the top surface of photoresist By controlling the exposure and development processes in the stage of interference lithography, it is possible to produce photoresist grating structures with a duty cycle (d/D) as small as 15%, as illustrated by the scanning electron microscopic (SEM) image in Fig. 1(a). Fig. 1(b) shows the atomic force microscopic (AFM) image of the PR grating, where the modulation depth of the grating is measured to be about 193 nm. Fig. 1(c) shows the SEM image of the cross-section profile of the grating structures after the PR grating is thermally evaporated with aluminum at a speed of about 3.6 nm/min. Due to the steep edge of the PR grating, the aluminum is thinner on the side surface than on the top surfaces of the grating lines and grating grooves. On the top surface, isolated aluminum nanolines are produced with a mean width of about 160 nm and a mean height of about 200 nm, as marked by a dotted circle in yellow. This is the most important mechanism how localized surface plasmon resonance was observed with the aluminum-evaporated gratings. According to Fig. 1(c), the interference lithography did not develop to the PR/ITO interface, where a PR layer as thick as 100 nm remains between the grating and the 200nm-thick ITO layer. Fig. 2(a) shows the SEM image of the aluminum grating structures that have been produced by evaporating aluminum onto the surface of the PR gratings and Fig. 2(b) shows an enlarged view for a local area. The narrow aluminum nanolines can be identified to stand out of the aluminum film covering the grating grooves. A duty cycle of about 20% can still be measured for the aluminum grating. The grating period is measured to be about 580 nm. We can observe from Fig. 2(a) and (b) that the evaporated aluminum film is in fact composed of small aluminum nanoparticles, which are in a size smaller than 80 nm in diameter according to a rough evaluation. These aluminum nanoparticles also induce localized surface plasmon resonance. Fig. 2(c) and (d) shows the reflective optical extinction spectra by the aluminum gratings with a period of D¼580 nm for TM and TE polarizations, respectively, with the incident angle (θ) increased from 15° to 36°.
Fig. 1. SEM (a) and AFM (b) images of the photoresist (PR) grating with a period of about 580 nm before aluminum evaporation. (c) The SEM image of the cross-section of the grating structures after being deposited with aluminum. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
For TM polarization, two features are clearly observed: a relatively broad extinction peak centered at about 466 nm, which does not shift with changing the angle of incidence, and a sharp peak centered at 730 nm for an incident angle of θ ¼15°, which shifts to the red with increasing the angle of incidence, as shown in Fig. 2 (c). The spectral feature centered at 466 nm results from localized surface plasmon resonance of the aluminum nanolines, which has a bandwidth of about 80 nm. The narrow spectral features at wavelengths longer than 730 nm can be attributed to Rayleigh anomaly of the grating diffraction, where the center wavelength is determined by λ ¼D(1 þsin θ), where D is the grating period. For TE polarization, the spectroscopic response becomes completely different from that for TM polarization, which is dominated by a strong and broad extinction spectrum that shifts from 580 to 620 nm with the incident angle increased from 15° to 36°. This strong optical extinction spectrum results from the localized surface plasmon resonance of the randomly distributed and randomly aggregated aluminum nanoparticles. The mechanisms for the broad-band and red-shifted plasmon resonance for TE polarization
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Fig. 2. (a) and (b) SEM images of aluminum-evaporated gratings with a period of about 580 nm and an enlarged view, respectively. (c) and (d) Reflective optical extinction spectra of aluminum grating with a period of 580 nm for TM and TE polarizations, respectively, with the incident angle increased from 15° to 36°.
can be understood by considering aluminum nanolines on the top surface of the photoresist, which extend in the same direction as the grating lines. The shift of the extinction spectrum can be understood by referring to the investigations in our previous publication (Liu et al., 2014), where we need to consider that TE polarized light excites plasmon resonance not only in the aluminum nanoparticles, but also in a large amount of random aggregates between them along the grating lines, as compared with TM polarization. The most important spectral feature for TE polarization is that dips in the extinction spectra are observed tunable with changing the angle of incidence, as shown in Fig. 2(d). In the shorter-wavelength range, the dips are observed at 428 nm (at θ ¼ 24°), 448 nm (at θ ¼ 27°), 465 nm (at θ ¼30°), 483 nm (at θ ¼33°), and 500 nm (at θ ¼36°), as indicated by downward arrows. For the longer-wavelength range, the dips are observed at 742 nm (at θ ¼15°), 786 nm (at θ ¼ 18°), 821 nm (at θ ¼ 21°), 851 nm (at θ ¼24°), and 892 nm (at θ ¼ 27°), as indicated by the obliquely downward arrows. However, for wavelength longer than 900 nm, it is difficult to resolve spectral features with similar performance. This is limited by the spectrometer and by the light source that has much lower intensity for wavelengths longer than 900 nm. Furthermore, interesting features in the spectrum correspond to larger angles of incidence than 36°, which led to additional modification on the spectroscopic response due to the structural parameters of the grating. Nevertheless, these available data are already sufficient to identify the mechanisms. The spectral dips in the longer-wavelength range (λ1) correspond to the first-order Rayleigh anomaly, whereas, those in the shorter-wavelength range (λ2) correspond to the second-order process with λ2 E λ1/2. Extinction dips imply enhanced reflection within a narrow band, instead of energy loss due to diffraction anomalies. Therefore, these dips in the optical extinction spectra in Fig. 2(d) result from
Fano-like coupling between plasmon resonance of aluminum nanostructures and the diffraction anomaly of the grating. However, the experimental results in Fig. 2(c) and (d) shows small overlap between plasmon resonance and diffraction anomalies and weak coupling between them. This can be improved by changing the grating period to tune the spectral position of the diffraction anomalies.
4. Fano coupling between second-order Rayleigh anomalies and plasmon resonance of the aluminum nanolines 4.1. Spectroscopic characterization To achieve better overlapping between plasmon resonance and Rayleigh diffraction anomaly, we fabricated aluminum gratings deposited on photoresist with a period of 650 nm, as shown by the SEM image in Fig. 3(a) and (b). Similarly, each aluminum nanoline consists of nanoparticles. The corresponding duty cycle was increased slightly to about 23%. Fig. 3(c) and (d) shows the optical extinction spectroscopic characterization on the reflected beam by the aluminum grating structures with a period of 650 nm, where the spectrum of the white-light source has been used as the blank for the calculation of the extinction spectra. The angle of incidence has been increased from 15° to 36° in steps of 3°, so that the tuning performance of the Fano coupling spectrum can be investigated. Due to the increased grating period, the Rayleigh anomaly is observed at longer wavelengths for a fixed angle of incidence according to λ ¼ D (1 þsin θ). For TM polarization, the first-order Rayleigh anomaly is observed at 818 nm, which is exactly equal to D(1 þsin θ), for an incident angle of θ ¼15° and a grating period of D ¼650 nm, as
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Fig. 3. (a) and (b) SEM images of aluminum-evaporated gratings with a period of about 650 nm and an enlarged view, respectively. (c) and (d) Reflective optical extinction spectra of aluminum grating with a period of 650 nm for TE and TM polarizations, respectively, with the incident angle increased from 15° to 36°.
indicated by an upward arrow in Fig. 3(c). This first-order process shifts to the red with increasing the angle of incidence and becomes difficult to identify for wavelengths longer than 900 nm. The second-order process defined by D(1 þsin θ)/2 becomes pronounced for incident angles (θ) larger than 21°, which exhibits as narrow dips in the broad optical extinction band of localized surface plasmon resonance of the aluminum nanolines. This is clearly recognized as a kind of Fano coupling. The plasmon resonance is centered at about 488 nm and the dips in the extinction spectrum recognized as the second-order coupled mode shift from about 434 to 511 nm as θ is increased from 21° to 36°. This tuning properties basically agrees with the definition by D(1 þsin θ)/2. For TE polarization, much broader extinction spectrum is again observed for the plasmon resonance of the aluminum nanostructures of aggregated nanoparticles, as shown in Fig. 3(d). At θ ¼15°, the plasmon resonance is centered at about 587 nm and has a bandwidth of about 210 nm at FWHM. The first-order Rayleigh anomaly and its coupling with plasmon resonance become weak, which can be identified at about 853 nm for θ ¼15° and is located at the longer-wavelength edge of plasmon resonance. However, much stronger second-order coupling process can be observed for incident angles larger than 18°, where the coupled mode identified by a dip in the optical extinction spectrum shifts from about 450 nm to 545 nm as the incident angle is increased from 18° to 36°. The spectral positions of these second-order processes agree well with those for TM polarization that are shown in Fig. 3(c). Therefore, Fano-like coupling between the plasmonic scattering of narrow-band surface propagation mode induced by Rayleigh diffraction anomaly and the broad-band plasmon resonance is account for the spectroscopic performance shown in Fig. 3. The plasmon resonance of the aluminum nanostructures have enhanced two processes: strong optical extinction by localized surface plasmons and strong Rayleigh diffraction anomalies, which in
principles introduce optical loss in the reflection spectrum and can be recognized by optical extinction peaks. However, scattering of the surface propagation mode in the direction of the reflection beam has reversed the contribution of Rayleigh anomaly to the reflection spectrum. Meanwhile, secondary diffraction of the Rayleigh anomaly is exactly in the direction of the reflected beam, inducing similar effects as the directly scattered light rays. As a result, narrow-band “dips” are observed in the relatively broadband optical extinction spectrum of plasmon resonance of the aluminum nanostructures. 4.2. Mechanisms for Fano coupling between Rayleigh anomalies and plasmon resonance Fig. 4 illustrates schematically the basic principles for the observed Fano coupling process in the thermally evaporated aluminum gating structures. The white-light beam (➊) incident at an angle of θ excites both localized surface plasmon resonance in the aluminum nanostructures and the surface propagation mode (the leftward thick arrows) due to the Rayleigh anomaly in the diffraction process. The surface propagation mode (❸) corresponds to the Rayleigh anomaly wavelength at λ1 ¼D(1 þsin θ) for the firstorder and at λ2 ¼D(1 þsin θ)/2 for the second-order process. Plasmonic scattering of the surface propagation mode by the aluminum nanopatrticles and nanolines produces light rays in different directions, including that in the same direction as the reflected beam, as indicated by green arrows. Meanwhile, secondary diffraction of the surface mode of Rayleigh anomaly excites new light rays also in the same direction of the reflected beam through D(sin90° þsin θ) ¼ λ or λ ¼ D(1 þ sin θ) for the first-order and D(sin 90° þ sin θ)¼2λ or λ ¼D(1 þsin θ)/2 for the second-order processes. Thus, the reflected beam (❷) contains both scattered and diffracted light rays, which interfere constructively with the reflected beam, leading to a Fano-like feature in the reflected
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Fig. 4. Mechanisms for Fano coupling processes involved in the aluminum-evaporated gratings: ❶ incident light beam; ❷ reflected light beam; ❸ Rayleigh anomaly; and ❹ diffracted and scattered light rays.
spectrum. The beams ❹ in Fig. 4 are used to denote both the scattered and diffracted light rays inside the reflected beam, which have the same wavelength as the Rayleigh anomaly. The stronger the plasmon resonance of the aluminum nanostructures, the stronger the optical extinction will be in its resonance band, and the stronger the diffraction by the grating will become due to the stronger grating modulation. However, stronger diffraction induces stronger surface propagation mode and stronger plasmonic scattering, which is an inverse process to the optical extinction by plasmon resonance. As a result, as narrow-band enhanced reflection, which is the same as the Rayleigh anomaly in wavelength, appears in the reflection spectrum. In the optical extinction spectrum, a narrow dip is observed within the broad-band plasmon-resonance band, which is recognized as a Fano-like coupling. The wavelength of the coupled mode shifts to the red with increasing the angle of incidence. The above mechanisms explain the optical spectroscopic observations in both Figs. 2 and 3, giving insights into such a kind of Fano resonance.
5. Sensor applications using Fano coupling between secondorder diffraction anomalies and localized surface plasmon resonance The grating structures shown in Fig. 3 were used to perform the sensor measurements, where TM-polarization mode was employed. The same system as reported in Zhang et al. (2011) is employed to detect the change in the environmental refractive index as a result of the change in the concentration of glucose/ water solutions. However, the sensor chip now consists of aluminum nanograting structures instead of waveguide gold photonic crystals. Fig. 5 shows the measurement results using the reflective mode for incident angles (θ) of 20° (Fig. 5(a) and (b)), 24° (Fig. 5 (c) and (d)), and 28° (Fig. 5(e) and (f)) with the concentration of the detected glucose/water solution increased from 1%, 3%, 7%, to 10%. For the measurement results on the left panel (Fig. 5(a), (c), (e)), the spectrum of the white light source was used as the blank in the calculation of the optical extinction spectra; however, for those on the right panel (Fig. 5(b), (d), (f)) the transmission spectrum through pure water (0% concentration) has been used as the blank. The amplitude of the sensor signal (A) is defined by the dip-to-peak difference of the sensor signal, as shown in the inset of Fig. 5(b) and as described in detail in our previous publications (Zhang et al., 2011, 2012, 2014). On the left panel of Fig. 5, the vertical dotted line on the left is
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used to mark the spectral position of the Fano-like coupling feature in the glucose/water solutions and the dashed arrow at a lower position is used to indicate the shift of the spectral feature when the environmental medium is changed from air to glucose/ water solutions. The vertical dotted line on the right is used to mark the spectral peak of the modulated plasmon resonance of the aluminum nanolines in glucose/water solutions, whereas, the dashed arrow at an upper position is used to indicate the spectral shift of the plasmonic feature when the environmental medium is changed from air to glucose/water solutions. For θ ¼ 20°, the Fano-like coupling is located at about F−C F−C ¼443 nm in air and shifts to about λ solution ¼555 nm in gluλ air cose/water solutions. However, the Fano-like coupling left a narrow-band plasmon-resonance mode with steepened spectral P ¼499 nm in air and shifted to edges, which is peaked at about λ air P about λ solution ¼662 nm in solutions, as shown in Fig. 5(a). Here the Fano-like coupling takes place between the second-order Rayleigh F−C ¼D(1 þsin θ)/2 ¼ anomaly and plasmon resonance with λ air F−C ¼ D(n þsin θ)/ 650 (1 þsin 20°)/2 E436 nm in air and λ solution 2 E650 (1.34 þsin 20°)/2 E547 nm in water, which basically agrees with the measurements results. With changing of the concentration of the glucose/water solution from 1% to 10%, both the coupled mode and the plasmon resonance shifts to the red and these spectral shifts are small due to the small change in the refractive index of the solution. However, the sensor signal can be calculated by −log10[IR(λ)/I0(λ)], where I0(λ) is the reflected spectrum when the sensor is placed in pure water or in the solution with 0% concentration and IR(λ) is the reflected spectrum by the grating for different solution concentrations. As shown in Fig. 5(b), the sensor signal increases in amplitude (A) with increasing of the concentration, where A was measured to be 0.0244, 0.0729, 0.1598, and 0.2322 for solution concentrations of 1%, 3%, 7%, and 10%, respectively. For θ ¼24°, the spectral shift of the Fano-coupling mode is from F−C F−C ¼446 nm to about λ solution ¼569 nm when the sensor device λ air is placed from air into glucose/water solutions, as shown in F−C ¼D(1 þsin θ)/2¼ 650 (1 þsin 24°)/ Fig. 5(c). Theoretically, λ air F−C ¼ D(n þ sinθ)/2E 650 (1.34 þ 2 E457 nm in air and λ solution sin 24°)/2E 567 nm in water. Although the discrepancy between F−C , excellent agreement can theory and experiment is large for λ air F−C be observed for λ solution . According to Fig. 5(d), the amplitude of the sensor signal A is measured to be 0.0355, 0.066, 0.1793, 0.2558 for solution concentrations of 1%, 3%, 7%, and 10%, respectively. When the incident angle is increased to θ ¼28°, much change can be observed for both the amplitude and the spectral shape of the optical extinction spectrum of plasmon resonance. It becomes difficult to identify the spectral position and bandwidth of plasmonic resonance, as shown in Fig. 5(e). However, the spectral shift of the Fano-coupling mode can still be observed from F−C F−C ¼498 nm to about λ solution ¼ 607 nm, when the sensor device λ air is placed from air into glucose/water solution. Calculations found F−C F−C ¼650 (1 þsin 28°)/2E 477 nm in air and λ solution E650 λ air (1.34 þsin 28°)/2E 588 nm. The large discrepancy between experiment and theory may result from the complex configuration of the aluminum grating lines when seen from different angles, where the interaction between the light beam and the aluminum grating lines becomes more sensitive to large incident angles. According to Fig. 5(f), the amplitude of the sensor signal A is measured to be 0.0116, 0.0321, 0.0651, 0.0089 for solution concentrations of 1%, 3%, 7%, and 10%, respectively. Fig. 6 summarizes the measurement results in Fig. 5 and replots the amplitude of sensor signal (A) as a function of the solution concentration at incident angles of 20° (yellow), 24° (red), and 28° (blue). The dashed lines are linear fits to the measurement data. Fig. 6 shows basically linear relationship between A and the
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Fig. 5. Sensor measurements on glucose/water solutions with concentrations increased from 0% to 10% for incident angles of (a) and (b) 20°; (c) and (d) 24°; (e) and (f) 28°. The extinction spectra on the left panel have been calculated using the spectrum of the light source as the blank; however, those on the right panel have been calculated using the transmission spectrum through pure water (0% concentration) as the blank.
solution concentration, implying excellent performance of the sensor device. The curve at θ ¼ 24° exhibits a largest slope, implying a highest sensitivity of the sensor. According to Fig. 5, the Rayleigh anomaly overlaps better the plasmon resonance at θ ¼24°, leading to stronger Fano-like coupling between them, as compared with θ ¼20° and θ ¼28°. This does not only confirm the mechanisms that support the operation of the sensor device, but also indicate that the sensitivity of the sensor can be improved by optimizing the angle of incidence. Fig. 6 also shows the variation of refractive index of the glucose/water solution with concentration using the plot of green squares, where an excellent linear relationship can be observed. Using the data for an incident angle of 24° and a concentration change from 3% to 10%, the sensitivity of the sensor device can be evaluated by the change in amplitude of the sensor signal over the change in the environmental refractive index as (0.2558–0.066)/ (1.3454–1.3363) ¼0.1898/0.0091 ¼20.86 OD per refractive index units (RIU). This value is much higher than that reported for the
waveguide metallic photonic crystals (Zhang et al., 2012), where a sensitivity smaller than 10 OD/RIU was achieved. To evaluate the reproducibility and reliability of the sensor device, we performed a series of sensor measurements at an optimized incident angle of 24° on the glucose/water solution with a concentration of 0%, 1%, 3%, 7%, and 10%, as shown in Fig. S1 in the Supporting information. The sensor system was cleaned by circulating pure water in the whole channels three times with each time for 10 min before each measurement on the glucose/water solution with a specified concentration. Measurement ➀ corresponds to the results shown in Fig. 5(d). Measurement ➁ was performed about 4 months later than measurement ➀, and measurements ➂ and ➃ were performed about 3 and 6 h after ➁, respectively. During the period between measurements ➀ and ➁, a number of different other experiments were performed using this sensor system. Fig. S1 shows excellent consistence between the presented 4 measurement results, although the measurement ➀ deviates relatively large from ➁, ➂, and ➃. Fig. S2 plots the average
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show excellent performance in the detection of refractive-index change in the surrounding environment.
Acknowledgments We acknowledge the 973 program (2013CB922404) and the National Natural Science Foundation of China (11274031) for the support.
Appendix A. Suplementary information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.bios.2015.01.071. Fig. 6. Sensor-signal amplitude as a function of the glucose/water solution concentration for incident angles of θ¼ 20° (yellow circles), 24° (red circles), and 28° (blue circles). Green squares: refractive index as a function of the solution concentration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
amplitude of the sensor signal (A) as a function of solution concentration with the scale bars showing deviations of the four presented measurements. The small deviation values imply again excellent consistence between multiple measurement results. The data in Figs. S1 and S2 not only confirm the reproducibility of the sensor signals, but also verify the homogeneity of the large-area photonic structures of aluminum nanolines, the excellent reproducibility of the angle-resolved tuning properties, and the reliability of the whole sensor system.
6. Conclusions Fano coupling between localized surface plasmon resonance and Rayleigh anomaly is revealed in a diffraction grating consisting of aluminum-nanoparticle-composed nanolines. Localized surface plasmon resonance is excited in both the aluminum nanoparticles and the aluminum nanolines, which shows different spectroscopic response for different polarizations. The Rayleigh anomaly that is determined by the grating period and the angle of incidence in spectroscopic response is scattered by aluminum nanostructures, which excites light rays with a narrow-band spectrum in the direction of the reflected beam. Interactions between these two processes induces Fano resonance mode in the reflective optical extinction spectrum. Localized surface plasmon resonance of aluminum nanostructures has been the common enhancement mechanisms for these processes. This kind of Fanoresonance mode is sensitive to the change both in the angle of incidence and in the environmental refractive index, providing opportunities to develop optical sensors. Sensor measurements
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