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Surface plasmon polariton coupling induced transmission of subwavelength metallic grating with waveguide layer
Microelectronic Engineering 87 (2010) 1297–1299
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Surface plasmon polariton coupling induced transmission of subwavelength metallic grating with waveguide layer Zhen-Cheng Xu a, Biqin Dong b, Jing Xue a, Rong Yang a, Bing-Rui Lu a,c, Shaoren Deng a, Zhi-Feng Li d, Wei Lu d, Yifang Chen c,1, Ejaz Huq c, Xin-Ping Qu a, Ran Liu a,* a
State Key Lab of ASIC and System, Department of Microelectronics, Fudan University, Shanghai 200433, China Department of Physics, Fudan University, Shanghai 200433, China Micro and Nanotechnology Centre, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK d State Key Lab of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China b c
a r t i c l e
i n f o
Article history: Received 14 September 2009 Received in revised form 17 December 2009 Accepted 20 December 2009 Available online 28 December 2009 Keywords: Surface plasmon resonance Subwavelength metallic grating Nanoimprint lithography
a b s t r a c t In this paper, we present the nanofabrication of a potentially coupled waveguide–surface plasmon resonance biosensor (CWSPRBs) by nanoimprint lithography. Subwavelength metallic gratings (SWMGs) with the pitches of 300 and 500 nm were fabricated by direct imprinting on PMMA layer which subsequently covered with a layer of Au. The key issue in this device is the coupling of surface plasmon with waveguide modes, which has been carefully investigated by measuring the coupling induced enhancement of light transmission. Both our measurement and simulation results indicate that the resonant coupling does exist for both 300 and 500 nm pitched gratings. This proves that the developed nanoimprint lithography is applicable for the CWSPRBs sensors which has significant advantages over traditional ones with a prism. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Surface plasmon polariton (SPP) is a coupled oscillation between free electrons at metal surface and external electromagnetic field. One of the most important characteristics of the SPPs is their ability to detect field interactions at the interfaces between metal and dielectric [1]. Recently fruitful areas of research on SPP in the fields of biosensors and nano-science have been published [2,3]. SPP have been studied to attract a great deal of attention as a quite unique possibility of the electromagnetic field localization and corresponding substantial enhancement of the electromagnetic field near the surface. Since this electromagnetic field decays exponentially away from the surface, if excited SPPs can be converted to photons with high efficiency, the effective emission rate of the emitter is expected to be enhanced [4]. One of the standard methods to allow SPPs to couple with photons is to introduce a corrugated surface like gratings, which allows the compensation of the momentum mismatch between SPPs and photons [5]. The coupling of SPPs with photons by a grating results in the well-defined directional emission with a characteristic polarization as well as the
* Corresponding author. Address: School of Microelectronics, Fudan University, 220 Handan Road, Shanghai, China. Tel./fax: +86 21 55664548. E-mail addresses: [email protected] (Y. Chen), [email protected] (R. Liu). 1 Tel.: +44 (0)1235 5159; fax: +44 (0)1235 6283. 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.12.074
enhancement of emission rate [6]. Specifically, SPPs in sandwiched structures have attracted much interest as a promising way to carry out a sensitive surface plasmon resonance sensors [7]. Many of the key properties of such biosensors, using multi-layer sandwiched structures, are determined by the coupling interaction between a waveguide and SPP modes [8]. In this paper, we demonstrated sandwiched multi-layer subwavelength gratings including the dielectric waveguide layer coupled SPP modes by nanoimprint lithography to present the enhancement transmission through gold film by the coupling with SPPs. To confirm the surface plasmonic effect, the enhancement of electromagnetic field was simulated and verified using the finitedifference time-domain (FDTD) method. 2. Sample fabrication The silicon grating templates used in nanoimprint lithography was first patterned by electron beam lithography (EBL), followed by metal deposition and lift-off process. High aspect-ratio grating templates with pitches of 300 and 500 nm were fabricated. In the imprint process, a 250 nm thick layer of SU-8 was spin-coated onto Pyrex and soft baked at 95 °C on a hotplate for 5 min. It was then cured by a UV exposure under 365 nm UV light for 2 min. After the curing, a very thin layer of PMMA was spin-coated onto SU-8 and soft baked at 180 °C on a hotplate for 30 min for the subse-
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Fig. 1. Schematic diagram of the NIL process of fabricated subwavelength metallic grating.
quent imprint process. Fig. 1 illustrates the whole imprint process. Before heating PMMA above its glass transition temperature (Tg) in oven, a very thick PDMS buffer layer was placed between the templates and imprint tools to uniform the pressure. The depth of imprint was strictly controlled by the pressure to ensure that a very critical thickness control should be applied to make the continuous gold layer. Then a PVD (physical vapor deposition) technique was applied to deposit a 100 nm thick gold layer. In this structure, the refractive index of bottom Pyrex is 1.44, while the waveguide layer SU-8 is 1.58 and cladding layer PMMA is 1.45. Metallic gratings with two different pitches in subwavelength regime were fabricated and Fig. 2 shows the top view of scanning electron microscope (SEM) image of the surface. In the optical measurement, the zeroth-order transmission of the sample at a
Fig. 2. Top view of the subwavelength metallic grating. (a) 300 nm pitch and (b) 500 nm pitch.
Fig. 3. Experimental and calculated transmission spectrum of subwavelength metallic gratings. (a) 300 nm pitch with 100 nm Au cladding (b) 500 nm pitch with 100 nm Au cladding (c) measured TE mode and TM mode intensity of 300 nm pitch with 100 nm Au cladding (d) measured TE mode and TM mode intensity of 500 nm pitch with 100 nm Au cladding.
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the thickness of gold layer is 100 nm, which is much larger than the gold skin depth at the wavelength of visible light, TE mode energy were hardly transmitted through the structure expect for the intrinsic peak. Additional transmission maximum occurs in TM polarized component depends strongly on the wavelength of the incident radiation, suggesting that the enhancement is truly cased by the resonant coupling between the photos and polaritons. Fig. 4 presents a simulated field distribution of x–z direction at the transmission valley and peak of normal incidence. Fig. 4a and b shows the electric field distribution of total components simulated by the finite-difference time-domain (FDTD). The simulation software is Lumerical’s FDTD solution. The mesh override region at metal layer is 5 nm at x-axis, 10 nm at y-axis and 5 nm at z-axis, while the whole structure mesh region is of the default high accuracy setting. The source was plane wave without polarization whose wavelength is from 400 to 800 nm. Note that on the cross-section of 3D simulation, z = 0 is defined as the waveguide layer between the cladding layer and the Pyrex substrate. It can be seen that at the transmission valley the electric field is greatly depressed and the incidence energy was trapped in the cladding layer and transmitted to the substrate with severe attenuation. It can also be seen that the electric field is greatly enhanced as a result of surface plasmon excitation at the interface. As a result, the wavelength bandwidth of the transmission spectrum is enhanced by coupling with waveguide modes in the dielectric layer. 4. Conclusion
Fig. 4. Calculated field distribution of x–z direction of 500 nm pitch and 100 nm Au cladding. (a) Transmission valley and (b) transmission peak.
normal incidence was collected by a fiber spectrometer using a Xenon light source. 3. Results and discussion The measured and calculated transmission spectrum through subwavelength metallic gratings are presented in Fig. 3. As shown in Fig. 3a, the transmission spectrum at normal incidence through the sample with 300 nm pitch and 100 nm Au cladding was recorded. The peak at the wavelength of 500 nm belongs to the intrinsic transmission of gold material. We further found another transmission peak at about 680 nm, which agrees with the experimental result. Fig. 3b shows the transmission spectrum at normal incidence of 500 nm pitch and 100 nm Au cladding. One can see that the transmission peak was shifted to 770 nm and the transmissivity was critically up to over 80%, which is attributed to the surface plasmon resonant transmission. Fig. 3c and d shows TM mode and TE mode intensity from the structures with 300 and 500 nm pitch, respectively. At the wavelength of 500 nm, TE mode and TM mode have the same intensity of the intrinsic peak. Since
In summary, using nanoimprint lithography combined with conventional micro- and nano-processing, we have successfully fabricated the sandwiched multi-layer subwavelength gratings, which have promising potential of the coupled waveguide–surface plasmon resonance biosensor (CWSPRB). Optical measurement results show that the subwavelength metallic gratings give rise to an extra transmission peak at the wavelength corresponding to the grating pitch. The measured and simulation results have shown the excited surface plasmon polariton coupling between waveguides and metallic gratings. This encourages us to further develop the CWSPRB sensors which is much more sophisticated than the conventional surface Plasmon resonant sensor with a prism on the top. Acknowledgements The work was supported by Shanghai Municipal of Science and Technology (08QH14002), the National Basic Research Program of China (2006CB302703), the Seed Funds for Key Science and Technology Innovation Projects of MOE (708020) and the ‘‘985” Micro/Nanoelectronics Science and Technology Platform. References [1] William L. Barnes, Alain Dereux, Thomas W. Ebbesen, Nature 424 (2003) 824– 830. [2] A.V. Zayats, I.I. Smolyaninov, A.A. Maradudin, Phys. Rep. 408 (2005) 131. [3] Jian Jim Wang, Lei Chen, Steven Kwan, Feng Liu, Xuegong Deng, J. Vac. Sci. Technol. B 23 (2005) 6. [4] J. Kalkman, H. Gersen, L. Kuipers, A. Polman, Phys. Rev. B 73 (2006) 075317. [5] Jiri Homola, Sinclair S. Yee, Gunter Gauglitz, Sens. Actuators B 54 (1999) 3–15. [6] Askin Kocabas, Gulay Ertas, S. Seckin Senlik1, Atilla Aydinli, Opt. Express 16 (17) (2008) 12469. [7] J. Homola, Anal. Bioanal. Chem. 377 (2003) 528–539. [8] F.-C. Chien, C.-Y. Lin, J.-N. Yih, K.-L. Lee, C.-W. Chang, P.-K. Wei, C.-C. Sun, S.-J. Chen, Biosens. Bioelectron. 22 (2007) 2737–2742.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Surface plasmon polariton coupling induced transmission of subwavelength metallic grating with waveguide layer Zhen-Cheng Xu a, Biqin Dong b, Jing Xue a, Rong Yang a, Bing-Rui Lu a,c, Shaoren Deng a, Zhi-Feng Li d, Wei Lu d, Yifang Chen c,1, Ejaz Huq c, Xin-Ping Qu a, Ran Liu a,* a
State Key Lab of ASIC and System, Department of Microelectronics, Fudan University, Shanghai 200433, China Department of Physics, Fudan University, Shanghai 200433, China Micro and Nanotechnology Centre, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK d State Key Lab of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China b c
a r t i c l e
i n f o
Article history: Received 14 September 2009 Received in revised form 17 December 2009 Accepted 20 December 2009 Available online 28 December 2009 Keywords: Surface plasmon resonance Subwavelength metallic grating Nanoimprint lithography
a b s t r a c t In this paper, we present the nanofabrication of a potentially coupled waveguide–surface plasmon resonance biosensor (CWSPRBs) by nanoimprint lithography. Subwavelength metallic gratings (SWMGs) with the pitches of 300 and 500 nm were fabricated by direct imprinting on PMMA layer which subsequently covered with a layer of Au. The key issue in this device is the coupling of surface plasmon with waveguide modes, which has been carefully investigated by measuring the coupling induced enhancement of light transmission. Both our measurement and simulation results indicate that the resonant coupling does exist for both 300 and 500 nm pitched gratings. This proves that the developed nanoimprint lithography is applicable for the CWSPRBs sensors which has significant advantages over traditional ones with a prism. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Surface plasmon polariton (SPP) is a coupled oscillation between free electrons at metal surface and external electromagnetic field. One of the most important characteristics of the SPPs is their ability to detect field interactions at the interfaces between metal and dielectric [1]. Recently fruitful areas of research on SPP in the fields of biosensors and nano-science have been published [2,3]. SPP have been studied to attract a great deal of attention as a quite unique possibility of the electromagnetic field localization and corresponding substantial enhancement of the electromagnetic field near the surface. Since this electromagnetic field decays exponentially away from the surface, if excited SPPs can be converted to photons with high efficiency, the effective emission rate of the emitter is expected to be enhanced [4]. One of the standard methods to allow SPPs to couple with photons is to introduce a corrugated surface like gratings, which allows the compensation of the momentum mismatch between SPPs and photons [5]. The coupling of SPPs with photons by a grating results in the well-defined directional emission with a characteristic polarization as well as the
* Corresponding author. Address: School of Microelectronics, Fudan University, 220 Handan Road, Shanghai, China. Tel./fax: +86 21 55664548. E-mail addresses: [email protected] (Y. Chen), [email protected] (R. Liu). 1 Tel.: +44 (0)1235 5159; fax: +44 (0)1235 6283. 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.12.074
enhancement of emission rate [6]. Specifically, SPPs in sandwiched structures have attracted much interest as a promising way to carry out a sensitive surface plasmon resonance sensors [7]. Many of the key properties of such biosensors, using multi-layer sandwiched structures, are determined by the coupling interaction between a waveguide and SPP modes [8]. In this paper, we demonstrated sandwiched multi-layer subwavelength gratings including the dielectric waveguide layer coupled SPP modes by nanoimprint lithography to present the enhancement transmission through gold film by the coupling with SPPs. To confirm the surface plasmonic effect, the enhancement of electromagnetic field was simulated and verified using the finitedifference time-domain (FDTD) method. 2. Sample fabrication The silicon grating templates used in nanoimprint lithography was first patterned by electron beam lithography (EBL), followed by metal deposition and lift-off process. High aspect-ratio grating templates with pitches of 300 and 500 nm were fabricated. In the imprint process, a 250 nm thick layer of SU-8 was spin-coated onto Pyrex and soft baked at 95 °C on a hotplate for 5 min. It was then cured by a UV exposure under 365 nm UV light for 2 min. After the curing, a very thin layer of PMMA was spin-coated onto SU-8 and soft baked at 180 °C on a hotplate for 30 min for the subse-
1298
Z.-C. Xu et al. / Microelectronic Engineering 87 (2010) 1297–1299
Fig. 1. Schematic diagram of the NIL process of fabricated subwavelength metallic grating.
quent imprint process. Fig. 1 illustrates the whole imprint process. Before heating PMMA above its glass transition temperature (Tg) in oven, a very thick PDMS buffer layer was placed between the templates and imprint tools to uniform the pressure. The depth of imprint was strictly controlled by the pressure to ensure that a very critical thickness control should be applied to make the continuous gold layer. Then a PVD (physical vapor deposition) technique was applied to deposit a 100 nm thick gold layer. In this structure, the refractive index of bottom Pyrex is 1.44, while the waveguide layer SU-8 is 1.58 and cladding layer PMMA is 1.45. Metallic gratings with two different pitches in subwavelength regime were fabricated and Fig. 2 shows the top view of scanning electron microscope (SEM) image of the surface. In the optical measurement, the zeroth-order transmission of the sample at a
Fig. 2. Top view of the subwavelength metallic grating. (a) 300 nm pitch and (b) 500 nm pitch.
Fig. 3. Experimental and calculated transmission spectrum of subwavelength metallic gratings. (a) 300 nm pitch with 100 nm Au cladding (b) 500 nm pitch with 100 nm Au cladding (c) measured TE mode and TM mode intensity of 300 nm pitch with 100 nm Au cladding (d) measured TE mode and TM mode intensity of 500 nm pitch with 100 nm Au cladding.
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the thickness of gold layer is 100 nm, which is much larger than the gold skin depth at the wavelength of visible light, TE mode energy were hardly transmitted through the structure expect for the intrinsic peak. Additional transmission maximum occurs in TM polarized component depends strongly on the wavelength of the incident radiation, suggesting that the enhancement is truly cased by the resonant coupling between the photos and polaritons. Fig. 4 presents a simulated field distribution of x–z direction at the transmission valley and peak of normal incidence. Fig. 4a and b shows the electric field distribution of total components simulated by the finite-difference time-domain (FDTD). The simulation software is Lumerical’s FDTD solution. The mesh override region at metal layer is 5 nm at x-axis, 10 nm at y-axis and 5 nm at z-axis, while the whole structure mesh region is of the default high accuracy setting. The source was plane wave without polarization whose wavelength is from 400 to 800 nm. Note that on the cross-section of 3D simulation, z = 0 is defined as the waveguide layer between the cladding layer and the Pyrex substrate. It can be seen that at the transmission valley the electric field is greatly depressed and the incidence energy was trapped in the cladding layer and transmitted to the substrate with severe attenuation. It can also be seen that the electric field is greatly enhanced as a result of surface plasmon excitation at the interface. As a result, the wavelength bandwidth of the transmission spectrum is enhanced by coupling with waveguide modes in the dielectric layer. 4. Conclusion
Fig. 4. Calculated field distribution of x–z direction of 500 nm pitch and 100 nm Au cladding. (a) Transmission valley and (b) transmission peak.
normal incidence was collected by a fiber spectrometer using a Xenon light source. 3. Results and discussion The measured and calculated transmission spectrum through subwavelength metallic gratings are presented in Fig. 3. As shown in Fig. 3a, the transmission spectrum at normal incidence through the sample with 300 nm pitch and 100 nm Au cladding was recorded. The peak at the wavelength of 500 nm belongs to the intrinsic transmission of gold material. We further found another transmission peak at about 680 nm, which agrees with the experimental result. Fig. 3b shows the transmission spectrum at normal incidence of 500 nm pitch and 100 nm Au cladding. One can see that the transmission peak was shifted to 770 nm and the transmissivity was critically up to over 80%, which is attributed to the surface plasmon resonant transmission. Fig. 3c and d shows TM mode and TE mode intensity from the structures with 300 and 500 nm pitch, respectively. At the wavelength of 500 nm, TE mode and TM mode have the same intensity of the intrinsic peak. Since
In summary, using nanoimprint lithography combined with conventional micro- and nano-processing, we have successfully fabricated the sandwiched multi-layer subwavelength gratings, which have promising potential of the coupled waveguide–surface plasmon resonance biosensor (CWSPRB). Optical measurement results show that the subwavelength metallic gratings give rise to an extra transmission peak at the wavelength corresponding to the grating pitch. The measured and simulation results have shown the excited surface plasmon polariton coupling between waveguides and metallic gratings. This encourages us to further develop the CWSPRB sensors which is much more sophisticated than the conventional surface Plasmon resonant sensor with a prism on the top. Acknowledgements The work was supported by Shanghai Municipal of Science and Technology (08QH14002), the National Basic Research Program of China (2006CB302703), the Seed Funds for Key Science and Technology Innovation Projects of MOE (708020) and the ‘‘985” Micro/Nanoelectronics Science and Technology Platform. References [1] William L. Barnes, Alain Dereux, Thomas W. Ebbesen, Nature 424 (2003) 824– 830. [2] A.V. Zayats, I.I. Smolyaninov, A.A. Maradudin, Phys. Rep. 408 (2005) 131. [3] Jian Jim Wang, Lei Chen, Steven Kwan, Feng Liu, Xuegong Deng, J. Vac. Sci. Technol. B 23 (2005) 6. [4] J. Kalkman, H. Gersen, L. Kuipers, A. Polman, Phys. Rev. B 73 (2006) 075317. [5] Jiri Homola, Sinclair S. Yee, Gunter Gauglitz, Sens. Actuators B 54 (1999) 3–15. [6] Askin Kocabas, Gulay Ertas, S. Seckin Senlik1, Atilla Aydinli, Opt. Express 16 (17) (2008) 12469. [7] J. Homola, Anal. Bioanal. Chem. 377 (2003) 528–539. [8] F.-C. Chien, C.-Y. Lin, J.-N. Yih, K.-L. Lee, C.-W. Chang, P.-K. Wei, C.-C. Sun, S.-J. Chen, Biosens. Bioelectron. 22 (2007) 2737–2742.