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OPTICS LETTERS / Vol. 38, No. 23 / December 1, 2013
Increased detection sensitivity of surface plasmon sensors using oblique induced resonant coupling Wan-Shao Tsai,1,* Kuang-Li Lee,2 Ming-Yang Pan,2,3 and Pei-Kuen Wei2,4,5 1
Department of Applied Materials and Optoelectronics Engineering, National Chi Nan University, No.1, University Road, Puli, Nantou County 54561, Taiwan 2 Research Center for Applied Sciences, Academia Sinica, No. 128, Section 2, Academic Road, 11529 Taipei, Taiwan 3
Institute of Photonics Technologies, National Tsing Hua University, 300 Hsinchu, Taiwan 4
5
Institute of Biophotonics, National Yang-Ming University, Taipei, Taiwan National Taiwan Ocean University, Department of Optoelectronics, Keelung, Taiwan *Corresponding author: [email protected] Received August 26, 2013; revised October 15, 2013; accepted October 15, 2013; posted October 25, 2013 (Doc. ID 196038); published November 20, 2013
Increased detection sensitivity was achieved by adjusting the incident angle on periodic gold nanostructures that induced a resonant coupling between surface and substrate surface plasmon modes. For 500 nm-period gold nanoslits, a small incident angle, 7°, resulted in 2.64 times narrower linewidth and a 1.8 times increase in the figure of merit as compared to normal incidence. Furthermore, the intensity sensitivity was increased 4.5 times due to the change in the resonant coupling and redshift of the surface plasmon mode. © 2013 Optical Society of America OCIS codes: (280.4788) Optical sensing and sensors; (050.6624) Subwavelength structures; (240.6680) Surface plasmons. http://dx.doi.org/10.1364/OL.38.004962
The surface plasmon resonance (SPR)-based optical sensor has been known to be a very important tool due to its label-free, real-time, and sensitive detection capabilities [1,2]. Conventionally, surface plasmon polaritons were excited using the Kretschmann configuration with a prism or other waveguide couplers [3,4]. The bulky and expensive setup with large sample-volume requirement limited high-throughput and chip-based detection. Recently, sensors based on periodically metallic nanostructures have been demonstrated to utilize SPR without the need for a prism [5–8]. Although the wavelength sensitivity is usually not as high as that based on prism coupling, without the need for extra coupling structures, compact sensing arrays enable high-throughput and multiplexed applications [9–11]. Nanohole or nanoslit array structures were commonly chosen as high-density chip-based SPR biosensors [12–15]. For high-throughput measurement, a monochromatic light was directly illuminated on the sensing arrays, and the transmitted intensity was detected through a charge-coupled-device (CCD) camera [9,10]. The performance of SPR sensors is evaluated based on a few parameters. Since the SPR biosensors are sensitive to surface environmental change, wavelength sensitivity S λ , defined as the ratio of the resonance wavelength shift to the environmental index change, is commonly used to characterize the sensor [16,17]: S λ ∂λres ∕∂n;
(1)
where λres is defined as the resonance wavelength. However, to detect small wavelength shift, the resonance linewidth of the transmission peak Δλ, defined as the full width half-maximum (FWHM) of the transmission spectrum at the resonance wavelength, is also crucial for spectral resolution [8]. A figure of merit (FOM S λ ∕Δλ), 0146-9592/13/234962-04$15.00/0
defined as the quotient of wavelength sensitivity and linewidth, is a more meaningful parameter to estimate the sensing capability of the sensor [17,18]. The FOM depends on both wavelength sensitivity and resonance linewidth to characterize a sensor. In addition to the FOM and S λ , the intensity sensitivity S I is another important parameter. It is defined as the relative intensity change over the refractive index change at a fixed wavelength near resonance: SI
∂I∕∂n : I0
(2)
For the kinetic studies of biointeractions in a microarray, the real-time measurement of multiple resonant wavelength shifts is difficult. On the other hand, using a single wavelength light source combined with a CCD camera to capture the intensity images of the microarray can simultaneously record the real-time response of each microspot. In principle, the intensity sensitivity is proportional to the FOM value of the sensor [19,20]. Several works have attempted to increase the sensitivity by fine tuning the nanostructures [21,22]. However, the linewidth was sacrificed in the meantime, which made the overall performance of the sensor remain unchanged [23]. For the nanostructures with broadband light illumination, the transmission (or reflection) signals were usually collected at normal incidence [9,10]. Some research groups have demonstrated using angular dependent transmitted (or reflective) spectra to narrow the linewidth of 2D nanostructures [8,11]. For the normal transmission spectrum of 1D nanoslits, the SPRs at both the top and bottom surfaces of the gold film have different resonant conditions. This results in a lower transmission and a larger linewidth. In this work, a chip-based optical sensor based on gold nanoslit arrays was © 2013 Optical Society of America
December 1, 2013 / Vol. 38, No. 23 / OPTICS LETTERS
measured with a small oblique incident light, which enabled the coupling of SPR modes between top and bottom gold films. The resonant coupling caused a narrower linewidth and a higher intensity sensitivity than the case of normal incidence. In the experiments, the periodic nanoslit pattern was first defined on a silicon substrate using electron-beam lithography with ZEP-520 resist, and then transferred to silicon substrate using a reactive ion etching method. 1 cm × 1 cm large-area nanoslit arrays with 500 nm period, 150 nm depth, and 60 nm slit width were made. The pattern was then transferred to a polycarbonate (PC) substrate using a thermal-annealing-assisted templatestripping method [24]. A 50 nm-thick gold film was deposited on a clean silicon template using an electron gun evaporator. A 178-μm-thick PC film was then placed on the gold-coated silicon template, with an additional polyethylene terephthalate (PET) thin film used as the sealing film. They were placed on a heating plate and heated at a temperature of 170°C to soften the PC substrate. Nitrogen gas was introduced into the chamber to produce a pressing pressure over the film. It pressed the silicon mold and PC substrate with large-area uniformity, which made the gold film uniformly stuck to the softened PC film. After peeling off from the template and PET thin film, the PC substrate with metallic nanostructures was made [25]. Figure 1(a) shows the scanning electron microscopy (SEM) images of the silicon mold and gold nanolsits. The sample was then encapsulated with acrylic plates on the top and bottom with UV glue and formed as a microfluidic device for the measurement of various environmental indices. Aqueous liquid with different concentrations of glycerin and water was mixed for various refractive indices. The refractive index values were measured with a refractometer. Figure 1(b) shows the measurement setup for full angular transmission spectra. A 150 W white-light source was coupled to the gold nanoslit arrays with a fiber cable and a fiber lens for light collimation. The incident polarization was controlled at TM with a linear polarizer. The transmission light was collected through a fiber lens focused on a fiber cable and then detected by a CCDarray-based spectrometer. To control the incident angle, the sample was put on a rotational stage controlled with a stepping motor. Transmission spectra with continuous angular variation can then be detected and recorded by the spectrometer simultaneously. Based on the grating equation, an oblique light incident on the nanoslits can be expressed as [4,14,26]
Fig. 1. (a) SEM images of the silicon mold (upper) and gold nanoslits (lower). (b) Measurement setup for full angular transmission spectra.
2π 2π 2π sin θ m ksp Re λ0 Λ λ0
r εm εd ; εm εd
4963
(3)
where λ0 is the incident wavelength, θ is the incident angle, Λ is the grating period, m is the order of diffraction, ksp is the propagation constant of the SPR mode, and εm and εd are the dielectric constants of the metal and the dielectric, respectively. Assume m 1 and −1, which represent the forward (ksp ) and backward (−ksp ) propagating SPR modes, respectively. Details can be seen in Fig. 2(a). For the top SPR mode, the dielectric is the environmental medium, while for the bottom SPR mode, the dielectric is the substrate. Resonance occurs when the phase velocity of incident light matches with that of the SPR mode, where the wavelength is defined as the resonance wavelength (λ0 λres ).
Fig. 2. (a) SPR wave propagation on a metallic nanoslit array. (b) Measured transmission diagram of gold nanoslits with period 500 nm under the refractive index of 1.3435 at full angular incidence compared with theoretical calculation (dashed lines). (c)–(e) Transmission spectra of gold nanoslits at incident angle of (c) 0°, (d) 3°, and (e) 7°. (f) Resonance linewidth (FWHM) and peak intensity change as a function of incident angle.
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Figure 2(b) demonstrates the measured transmission diagram of Au nanoslits with period 500 nm under the refractive index of 1.3435 at full angular incidence. Theoretical calculation using Eq. (3) was also plotted together in Fig. 2(b) for comparison. The green dashed lines show calculated top SPR modes, while the blue dashed lines show bottom SPR modes, with forward (ksp ) and backward (−ksp ) propagation, as indicated on the figure. The transmission spectra of incident angles 0°, 3°, and 7° were plotted in Figs. 2(c)–2(e), respectively. As can be seen in Fig. 2(c), two resonance peaks occur for the normal incidence. The higher resonance wavelength lies at 680 nm with resonance linewidth 34.5 nm. The transmission spectra show four resonant modes at incident angle of 3°. Forward and backward propagation of SPR modes can be seen more clearly. For the oblique incidence of θ 7°, there are sharp resonant points, due to the resonant coupling of forward and backward SPR modes on the top and bottom surfaces. The resonance wavelength redshifts to 730 nm with resonance linewidth only 13.06 nm, which is 2.64 times narrower than the case of normal incidence. The resonance linewidth (FWHM) and peak intensity changes as a function of incident angle were analyzed as shown in Fig. 2(f). As can be seen, linewidth reaches minimum at an oblique incident angle of 7°, where the intensity also reaches its peak value. The resonance system can be better explained with the field distribution based on FDTD calculation as shown in Fig. 3. As can be seen in Fig. 3(a), for normal incidence, the energy of surface plasma stays on the top metallic surface, when light is incident from above. However, for the case of oblique incidence, surface plasma waves exist on both the top and bottom metallic surfaces, as demonstrated in Fig. 3(b). The resonance coupling of top and bottom SPR modes makes a resonant system that preserves more SPR energy and thus narrows the linewidth. Figure 3(c) demonstrates the off-resonance condition at oblique incidence, when the environmental index is changed from water to index 1.45. In order to know the FOM of this sensor, wavelength sensitivity was measured under aqueous solution with various refractive index values. Figures 4(a) and 4(b) demonstrate the transmission spectra of gold nanoslits under various index liquids at normal and oblique incident angles of 0° and −7°, respectively. The corresponding wavelength sensitivities were demonstrated in Fig. 4(c) by estimating the resonance wavelength changes per refractive index unit (RIU). For a thorough study, gold nanoslits immersed in glycerin solutions from 0% to 100% (v/v) in concentration correspond to indices from 1.333 to 1.48 that were measured and shown in the insets of Figs. 4(c) and 4(d). As can be seen from the inset
Fig. 3. Simulated field transmission under water environment at (a) normal and (b) oblique incidence. (c) Off-resonance condition at oblique incidence under the refractive index of 1.45.
Fig. 4. Transmission spectra of gold nanoslit arrays under various environmental refractive index liquids at incident angle of (a) 0° and (b) −7°. (c), (d) Estimated (c) wavelength sensitivity and (d) intensity sensitivity near resonance wavelength from measured spectra in (a) and (b) [Insets: (c) resonance wavelength response and (d) intensity at resonant wavelength under glycerin solution from 0% to 100%.]
of Fig. 4(c), the linear dynamic range of oblique incidence lies from n 1.333 to n 1.38, while the normal incidence has better linearity over the whole index range. For oblique incidence, when n > 1.38, decoupling occurs and the bottom SPR mode dominates. Therefore, a twostep response can be seen. For incident angle at −7° and 0°, the measured wavelength sensitivity is 230 and 320 nm∕RIU, respectively. The wavelength sensitivity decreases 0.69 times for oblique incidence, compared with normal incidence. This is due to the coupling between top and bottom SPR modes for oblique incidence, and only the top SPR mode senses the environmental index change. Therefore, the wavelength sensitivity of oblique incidence is not as high as normal incidence. However, as compared to the FOM, the value of oblique incident angle of −7° is still 1.8 times higher than that of normal incidence, due to the great improvement of linewidth. It is noted that the FOM and wavelength sensitivity can be further improved by using a longer period. As seen in Eq. (3), assuming m 1 and ksp , the wavelength sensitivity is proportional to the slit period: Sλ ≡
∂nsp ∂λ0 Λ ; ∂n ∂n
(4)
where nsp is the effective refractive index of the surface plasmon. The intensity sensitivities of normal (θ 0°) and oblique (θ −7°) incidence under various environmental indices were demonstrated in Fig. 4(d). For incident angle at −7° and 0, the intensity sensitivity is 2100 and 430%/RIU, respectively. The inset of Fig. 4(d) shows the intensity response under wider index variation from 1.333 to 1.48. As can be seen, the normal incidence is less linear than the oblique incidence. However, the oblique incidence has a narrower linear dynamic range (n 1.33 to n 1.36) compared with the normal incidence. The intensity sensitivity at oblique incidence is much higher
December 1, 2013 / Vol. 38, No. 23 / OPTICS LETTERS
than normal incidence. The higher intensity sensitivity is attributed to the detuning of coupling resonance between the top and bottom SPR modes. Only the top SPR mode experiences environmental index; the bottom SPR mode remains unchanged with the surface refractive index. As the refractive index changes, the decoupling makes a large decrease of the transmission intensity. The intensity sensitivity at oblique incidence increases 4.5 times over that in normal incidence. This S I improvement is much better than the FOM value, for which only the S λ and linewidth are considered. In addition to the sensitivity improvement, the oblique induced coupling method results in good linearity when n < 1.36. For large surface index change, a curve fitting or calibration can solve the nonlinearity at the larger index region, if the response curve can be measured in advance. In conclusion, periodic gold nanoslit arrays were illuminated with a broadband light at full angular incidence. Strong couplings were observed on the measured transmission spectra at normal (θ 0°) and oblique (θ 7°) incidence. Due to the coupling between top and bottom SPR modes during oblique incidence, a 2.64 times narrower linewidth can be achieved and the FOM is 1.8 times increased as compared with normal incidence. Furthermore, the intensity sensitivity at oblique incident angle (θ 7°) is increased to a factor of 4.5. The increased detection sensitivity at small oblique incidence provides a simple way to enhance the biomolecular detection by using a narrowband light source with real-time and highthroughput measurement. This work was supported by the National Science Council, Taipei, Taiwan, under Contract No. NSC- 1012221-E-260-021. References 1. J. N. Anker, W. P. Hall, O. Lyandres, N. C. Shah, J. Zhao, and R. P. V. Duyne, Nat. Mater. 7, 442 (2008). 2. X. Fan, I. M. White, S. I. Shopova, H. Zhu, J. D. Suter, and Y. Sun, Anal. Chim. Acta 620, 8 (2008). 3. J. Homola, Chem. Rev. 108, 462 (2008). 4. S. Roh, T. Chung, and B. Lee, Sensors 11, 1565 (2011).
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5. T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, Nature 391, 667 (1998). 6. F. J. Garcia-Vidal, L. Martin-Moreno, T. W. Ebbesen, and L. Kuipers, Rev. Mod. Phys. 82, 729 (2010). 7. H. Liu and P. Lalanne, Nature 452, 728 (2008). 8. A. A. Yanik, A. E. Cetin, M. Huang, A. Artar, S. H. Mousavi, A. Khanikaev, J. H. Connor, G. Shvets, and H. Altug, Proc. Natl. Acad. Sci. U.S.A. 108, 11784 (2011). 9. E. Özkumur, J. W. Needham, D. A. Bergstein, R. Gonzalez, M. Cabodi, J. M. Gershoni, B. B. Goldberg, and M. S. Ünlü, Proc. Natl. Acad. Sci. U.S.A. 105, 7988 (2008). 10. T. Y. Chang, M. Huang, A. A. Yanik, H. Y. Tsai, P. Shi, S. Aksu, M. F. Yanik, and H. Altug, Lab Chip 11, 3596 (2011). 11. H. Gao, J. C. Yang, J. Y. Lin, A. D. Stuparu, M. H. Lee, M. Mrksich, and T. W. Odom, Nano Lett. 10, 2549 (2010). 12. R. Gordon, D. Sinton, K. L. Kavanagh, and A. G. Brolo, Acc. Chem. Res. 41, 1049 (2008). 13. K. L. Lee, C. W. Lee, W. S. Wang, and P. K. Wei, J. Biomed. Opt. 12, 044023 (2007). 14. B. K. Singh and A. C. Hillier, Anal. Chem. 78, 2009 (2006). 15. Y. Gao, Q. Gan, and F. J. Bartoli, IEEE J. Sel. Top. Quantum Electron. 20, 6900306 (2014). 16. K.-L. Lee, W.-S. Wang, and P.-K. Wei, Plasmonics 3, 119 (2008). 17. K. M. Mayer and J. H. Hafner, Chem. Rev. 111, 3828 (2011). 18. M. A. Otte and B. Sepulveda, in Nanoplasmonic Sensors, A. Dmitriev, ed. (Springer, 2012). 19. K.-L. Lee, S.-H. Wu, and P.-K. Wei, Opt. Express 17, 23104 (2009). 20. S.-H. Wu, S.-Y. Hsieh, K.-L. Lee, R.-H. Weng, A. Chiou, and P.-K. Wei, Appl. Phys. Lett. 103, 133702 (2013). 21. K.-L. Lee and P.-K. Wei, Appl. Phys. Lett. 99, 083108 (2011). 22. K.-L. Lee, Y.-H. Tai, and P.-K. Wei, Plasmonics 6, 483 (2011). 23. A. Lahav, M. Auslender, and I. Abdulhalim, Opt. Lett. 33, 2539 (2008). 24. W.-Y. Chang, J.-T. Wu, K.-H. Lin, S.-Y. Yang, K.-L. Lee, and P.-K. Wei, Microelectron. Eng. 88, 909 (2011). 25. K.-L. Lee, P.-W. Chen, S.-H. Wu, J.-B. Huang, S.-Y. Yang, and P.-K. Wei, ACS Nano 6, 2931 (2012). 26. J. Homola, in Surface Plasmon Resonance Based Sensors, J. R. Homola, ed. (Springer, 2006).
OPTICS LETTERS / Vol. 38, No. 23 / December 1, 2013
Increased detection sensitivity of surface plasmon sensors using oblique induced resonant coupling Wan-Shao Tsai,1,* Kuang-Li Lee,2 Ming-Yang Pan,2,3 and Pei-Kuen Wei2,4,5 1
Department of Applied Materials and Optoelectronics Engineering, National Chi Nan University, No.1, University Road, Puli, Nantou County 54561, Taiwan 2 Research Center for Applied Sciences, Academia Sinica, No. 128, Section 2, Academic Road, 11529 Taipei, Taiwan 3
Institute of Photonics Technologies, National Tsing Hua University, 300 Hsinchu, Taiwan 4
5
Institute of Biophotonics, National Yang-Ming University, Taipei, Taiwan National Taiwan Ocean University, Department of Optoelectronics, Keelung, Taiwan *Corresponding author: [email protected] Received August 26, 2013; revised October 15, 2013; accepted October 15, 2013; posted October 25, 2013 (Doc. ID 196038); published November 20, 2013
Increased detection sensitivity was achieved by adjusting the incident angle on periodic gold nanostructures that induced a resonant coupling between surface and substrate surface plasmon modes. For 500 nm-period gold nanoslits, a small incident angle, 7°, resulted in 2.64 times narrower linewidth and a 1.8 times increase in the figure of merit as compared to normal incidence. Furthermore, the intensity sensitivity was increased 4.5 times due to the change in the resonant coupling and redshift of the surface plasmon mode. © 2013 Optical Society of America OCIS codes: (280.4788) Optical sensing and sensors; (050.6624) Subwavelength structures; (240.6680) Surface plasmons. http://dx.doi.org/10.1364/OL.38.004962
The surface plasmon resonance (SPR)-based optical sensor has been known to be a very important tool due to its label-free, real-time, and sensitive detection capabilities [1,2]. Conventionally, surface plasmon polaritons were excited using the Kretschmann configuration with a prism or other waveguide couplers [3,4]. The bulky and expensive setup with large sample-volume requirement limited high-throughput and chip-based detection. Recently, sensors based on periodically metallic nanostructures have been demonstrated to utilize SPR without the need for a prism [5–8]. Although the wavelength sensitivity is usually not as high as that based on prism coupling, without the need for extra coupling structures, compact sensing arrays enable high-throughput and multiplexed applications [9–11]. Nanohole or nanoslit array structures were commonly chosen as high-density chip-based SPR biosensors [12–15]. For high-throughput measurement, a monochromatic light was directly illuminated on the sensing arrays, and the transmitted intensity was detected through a charge-coupled-device (CCD) camera [9,10]. The performance of SPR sensors is evaluated based on a few parameters. Since the SPR biosensors are sensitive to surface environmental change, wavelength sensitivity S λ , defined as the ratio of the resonance wavelength shift to the environmental index change, is commonly used to characterize the sensor [16,17]: S λ ∂λres ∕∂n;
(1)
where λres is defined as the resonance wavelength. However, to detect small wavelength shift, the resonance linewidth of the transmission peak Δλ, defined as the full width half-maximum (FWHM) of the transmission spectrum at the resonance wavelength, is also crucial for spectral resolution [8]. A figure of merit (FOM S λ ∕Δλ), 0146-9592/13/234962-04$15.00/0
defined as the quotient of wavelength sensitivity and linewidth, is a more meaningful parameter to estimate the sensing capability of the sensor [17,18]. The FOM depends on both wavelength sensitivity and resonance linewidth to characterize a sensor. In addition to the FOM and S λ , the intensity sensitivity S I is another important parameter. It is defined as the relative intensity change over the refractive index change at a fixed wavelength near resonance: SI
∂I∕∂n : I0
(2)
For the kinetic studies of biointeractions in a microarray, the real-time measurement of multiple resonant wavelength shifts is difficult. On the other hand, using a single wavelength light source combined with a CCD camera to capture the intensity images of the microarray can simultaneously record the real-time response of each microspot. In principle, the intensity sensitivity is proportional to the FOM value of the sensor [19,20]. Several works have attempted to increase the sensitivity by fine tuning the nanostructures [21,22]. However, the linewidth was sacrificed in the meantime, which made the overall performance of the sensor remain unchanged [23]. For the nanostructures with broadband light illumination, the transmission (or reflection) signals were usually collected at normal incidence [9,10]. Some research groups have demonstrated using angular dependent transmitted (or reflective) spectra to narrow the linewidth of 2D nanostructures [8,11]. For the normal transmission spectrum of 1D nanoslits, the SPRs at both the top and bottom surfaces of the gold film have different resonant conditions. This results in a lower transmission and a larger linewidth. In this work, a chip-based optical sensor based on gold nanoslit arrays was © 2013 Optical Society of America
December 1, 2013 / Vol. 38, No. 23 / OPTICS LETTERS
measured with a small oblique incident light, which enabled the coupling of SPR modes between top and bottom gold films. The resonant coupling caused a narrower linewidth and a higher intensity sensitivity than the case of normal incidence. In the experiments, the periodic nanoslit pattern was first defined on a silicon substrate using electron-beam lithography with ZEP-520 resist, and then transferred to silicon substrate using a reactive ion etching method. 1 cm × 1 cm large-area nanoslit arrays with 500 nm period, 150 nm depth, and 60 nm slit width were made. The pattern was then transferred to a polycarbonate (PC) substrate using a thermal-annealing-assisted templatestripping method [24]. A 50 nm-thick gold film was deposited on a clean silicon template using an electron gun evaporator. A 178-μm-thick PC film was then placed on the gold-coated silicon template, with an additional polyethylene terephthalate (PET) thin film used as the sealing film. They were placed on a heating plate and heated at a temperature of 170°C to soften the PC substrate. Nitrogen gas was introduced into the chamber to produce a pressing pressure over the film. It pressed the silicon mold and PC substrate with large-area uniformity, which made the gold film uniformly stuck to the softened PC film. After peeling off from the template and PET thin film, the PC substrate with metallic nanostructures was made [25]. Figure 1(a) shows the scanning electron microscopy (SEM) images of the silicon mold and gold nanolsits. The sample was then encapsulated with acrylic plates on the top and bottom with UV glue and formed as a microfluidic device for the measurement of various environmental indices. Aqueous liquid with different concentrations of glycerin and water was mixed for various refractive indices. The refractive index values were measured with a refractometer. Figure 1(b) shows the measurement setup for full angular transmission spectra. A 150 W white-light source was coupled to the gold nanoslit arrays with a fiber cable and a fiber lens for light collimation. The incident polarization was controlled at TM with a linear polarizer. The transmission light was collected through a fiber lens focused on a fiber cable and then detected by a CCDarray-based spectrometer. To control the incident angle, the sample was put on a rotational stage controlled with a stepping motor. Transmission spectra with continuous angular variation can then be detected and recorded by the spectrometer simultaneously. Based on the grating equation, an oblique light incident on the nanoslits can be expressed as [4,14,26]
Fig. 1. (a) SEM images of the silicon mold (upper) and gold nanoslits (lower). (b) Measurement setup for full angular transmission spectra.
2π 2π 2π sin θ m ksp Re λ0 Λ λ0
r εm εd ; εm εd
4963
(3)
where λ0 is the incident wavelength, θ is the incident angle, Λ is the grating period, m is the order of diffraction, ksp is the propagation constant of the SPR mode, and εm and εd are the dielectric constants of the metal and the dielectric, respectively. Assume m 1 and −1, which represent the forward (ksp ) and backward (−ksp ) propagating SPR modes, respectively. Details can be seen in Fig. 2(a). For the top SPR mode, the dielectric is the environmental medium, while for the bottom SPR mode, the dielectric is the substrate. Resonance occurs when the phase velocity of incident light matches with that of the SPR mode, where the wavelength is defined as the resonance wavelength (λ0 λres ).
Fig. 2. (a) SPR wave propagation on a metallic nanoslit array. (b) Measured transmission diagram of gold nanoslits with period 500 nm under the refractive index of 1.3435 at full angular incidence compared with theoretical calculation (dashed lines). (c)–(e) Transmission spectra of gold nanoslits at incident angle of (c) 0°, (d) 3°, and (e) 7°. (f) Resonance linewidth (FWHM) and peak intensity change as a function of incident angle.
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Figure 2(b) demonstrates the measured transmission diagram of Au nanoslits with period 500 nm under the refractive index of 1.3435 at full angular incidence. Theoretical calculation using Eq. (3) was also plotted together in Fig. 2(b) for comparison. The green dashed lines show calculated top SPR modes, while the blue dashed lines show bottom SPR modes, with forward (ksp ) and backward (−ksp ) propagation, as indicated on the figure. The transmission spectra of incident angles 0°, 3°, and 7° were plotted in Figs. 2(c)–2(e), respectively. As can be seen in Fig. 2(c), two resonance peaks occur for the normal incidence. The higher resonance wavelength lies at 680 nm with resonance linewidth 34.5 nm. The transmission spectra show four resonant modes at incident angle of 3°. Forward and backward propagation of SPR modes can be seen more clearly. For the oblique incidence of θ 7°, there are sharp resonant points, due to the resonant coupling of forward and backward SPR modes on the top and bottom surfaces. The resonance wavelength redshifts to 730 nm with resonance linewidth only 13.06 nm, which is 2.64 times narrower than the case of normal incidence. The resonance linewidth (FWHM) and peak intensity changes as a function of incident angle were analyzed as shown in Fig. 2(f). As can be seen, linewidth reaches minimum at an oblique incident angle of 7°, where the intensity also reaches its peak value. The resonance system can be better explained with the field distribution based on FDTD calculation as shown in Fig. 3. As can be seen in Fig. 3(a), for normal incidence, the energy of surface plasma stays on the top metallic surface, when light is incident from above. However, for the case of oblique incidence, surface plasma waves exist on both the top and bottom metallic surfaces, as demonstrated in Fig. 3(b). The resonance coupling of top and bottom SPR modes makes a resonant system that preserves more SPR energy and thus narrows the linewidth. Figure 3(c) demonstrates the off-resonance condition at oblique incidence, when the environmental index is changed from water to index 1.45. In order to know the FOM of this sensor, wavelength sensitivity was measured under aqueous solution with various refractive index values. Figures 4(a) and 4(b) demonstrate the transmission spectra of gold nanoslits under various index liquids at normal and oblique incident angles of 0° and −7°, respectively. The corresponding wavelength sensitivities were demonstrated in Fig. 4(c) by estimating the resonance wavelength changes per refractive index unit (RIU). For a thorough study, gold nanoslits immersed in glycerin solutions from 0% to 100% (v/v) in concentration correspond to indices from 1.333 to 1.48 that were measured and shown in the insets of Figs. 4(c) and 4(d). As can be seen from the inset
Fig. 3. Simulated field transmission under water environment at (a) normal and (b) oblique incidence. (c) Off-resonance condition at oblique incidence under the refractive index of 1.45.
Fig. 4. Transmission spectra of gold nanoslit arrays under various environmental refractive index liquids at incident angle of (a) 0° and (b) −7°. (c), (d) Estimated (c) wavelength sensitivity and (d) intensity sensitivity near resonance wavelength from measured spectra in (a) and (b) [Insets: (c) resonance wavelength response and (d) intensity at resonant wavelength under glycerin solution from 0% to 100%.]
of Fig. 4(c), the linear dynamic range of oblique incidence lies from n 1.333 to n 1.38, while the normal incidence has better linearity over the whole index range. For oblique incidence, when n > 1.38, decoupling occurs and the bottom SPR mode dominates. Therefore, a twostep response can be seen. For incident angle at −7° and 0°, the measured wavelength sensitivity is 230 and 320 nm∕RIU, respectively. The wavelength sensitivity decreases 0.69 times for oblique incidence, compared with normal incidence. This is due to the coupling between top and bottom SPR modes for oblique incidence, and only the top SPR mode senses the environmental index change. Therefore, the wavelength sensitivity of oblique incidence is not as high as normal incidence. However, as compared to the FOM, the value of oblique incident angle of −7° is still 1.8 times higher than that of normal incidence, due to the great improvement of linewidth. It is noted that the FOM and wavelength sensitivity can be further improved by using a longer period. As seen in Eq. (3), assuming m 1 and ksp , the wavelength sensitivity is proportional to the slit period: Sλ ≡
∂nsp ∂λ0 Λ ; ∂n ∂n
(4)
where nsp is the effective refractive index of the surface plasmon. The intensity sensitivities of normal (θ 0°) and oblique (θ −7°) incidence under various environmental indices were demonstrated in Fig. 4(d). For incident angle at −7° and 0, the intensity sensitivity is 2100 and 430%/RIU, respectively. The inset of Fig. 4(d) shows the intensity response under wider index variation from 1.333 to 1.48. As can be seen, the normal incidence is less linear than the oblique incidence. However, the oblique incidence has a narrower linear dynamic range (n 1.33 to n 1.36) compared with the normal incidence. The intensity sensitivity at oblique incidence is much higher
December 1, 2013 / Vol. 38, No. 23 / OPTICS LETTERS
than normal incidence. The higher intensity sensitivity is attributed to the detuning of coupling resonance between the top and bottom SPR modes. Only the top SPR mode experiences environmental index; the bottom SPR mode remains unchanged with the surface refractive index. As the refractive index changes, the decoupling makes a large decrease of the transmission intensity. The intensity sensitivity at oblique incidence increases 4.5 times over that in normal incidence. This S I improvement is much better than the FOM value, for which only the S λ and linewidth are considered. In addition to the sensitivity improvement, the oblique induced coupling method results in good linearity when n < 1.36. For large surface index change, a curve fitting or calibration can solve the nonlinearity at the larger index region, if the response curve can be measured in advance. In conclusion, periodic gold nanoslit arrays were illuminated with a broadband light at full angular incidence. Strong couplings were observed on the measured transmission spectra at normal (θ 0°) and oblique (θ 7°) incidence. Due to the coupling between top and bottom SPR modes during oblique incidence, a 2.64 times narrower linewidth can be achieved and the FOM is 1.8 times increased as compared with normal incidence. Furthermore, the intensity sensitivity at oblique incident angle (θ 7°) is increased to a factor of 4.5. The increased detection sensitivity at small oblique incidence provides a simple way to enhance the biomolecular detection by using a narrowband light source with real-time and highthroughput measurement. This work was supported by the National Science Council, Taipei, Taiwan, under Contract No. NSC- 1012221-E-260-021. References 1. J. N. Anker, W. P. Hall, O. Lyandres, N. C. Shah, J. Zhao, and R. P. V. Duyne, Nat. Mater. 7, 442 (2008). 2. X. Fan, I. M. White, S. I. Shopova, H. Zhu, J. D. Suter, and Y. Sun, Anal. Chim. Acta 620, 8 (2008). 3. J. Homola, Chem. Rev. 108, 462 (2008). 4. S. Roh, T. Chung, and B. Lee, Sensors 11, 1565 (2011).
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