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Nanoimprinted reflecting gratings for long-range surface plasmon polaritons
Microelectronic Engineering 84 (2007) 895–898 www.elsevier.com/locate/mee
Nanoimprinted reflecting gratings for long-range surface plasmon polaritons R.H. Pedersen a, A. Boltasseva b, D.M. Johansen a, T. Nielsen a, K.B. Jørgensen K. Leosson d, J.E. Østergaard c, A. Kristensen a,* a
a,c
,
MIC – Department of Micro and Nanotechnology, Nano•DTU, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark b COM•DTU, Nano•DTU, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark c University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark d Science Institute, University of Iceland, Dunhagi 3, IS-107 Reykjavik, Iceland Available online 2 February 2007
Abstract We present a novel design, fabrication, and characterization of reflecting gratings for long-range surface plasmon polaritons (LRSPPs) at telecom wavelengths. LR-SPP waveguides consisting of a thin (12 nm) gold film embedded in a thick (45 lm) layer of dielectric polymer cladding are structured by nanoimprint lithography to form a reflecting Bragg grating. By performing spectrally resolved transmission measurements pronounced Bragg grating behaviour is observed, with the transmission dip increasing (up to 12 dB) with the increasing grating length. 2007 Elsevier B.V. All rights reserved. Keywords: Nanoimprint lithography; Long-range surface plasmon polaritons; Reflecting grating
1. Introduction Recently, surface plasmon photonics (plasmonics) has received considerable interest [1]. The unique properties of surface plasmon polaritons (SPPs), surface-bound electromagnetic excitations propagating along a metal-dielectric interface, can be harnessed to improve light extraction from light-emitting diodes, to realize highly surface-sensitive detectors, to surpass the conventional diffraction limit in integrated optics, etc. However, the applicability of SPP waveguides to integrated optics is somewhat limited because SPPs suffer from strong damping due to absorption in metals, limiting SPP propagation to the range of hundreds of microns. One way to overcome this problem is to utilize long-range SPPs (LR-SPPs), sup-
*
Corresponding author. Tel.: +45 4525 6331; fax: +45 4588 7762. E-mail addresses: [email protected] (R.H. Pedersen), [email protected] (A. Kristensen). 0167-9317/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.110
ported by a thin (sub-20 nm) metal stripe embedded in a thick dielectric cladding material [2,3]. In such a structure, the SPPs associated with the bottom and top metal-dielectric interfaces couple, forming a symmetric mode with a very low field intensity within the metal layer. Thus, a substantial increase in the propagation length is achievable. Such structures have previously been fabricated using glass–polymer [4] or polymer–polymer sandwiches [5], leading to new device designs compatible with standard optical fibre technology, including devices containing subwavelength features patterned with electron-beam lithography (EBL) [6]. However, EBL-based fabrication of these devices is complicated, expensive and time-consuming. In the present work, a fabrication process for devices based on LR-SPP waveguides compatible with nanoimprint lithography is presented. This technique is well suited for mass production of components with nm-scale features and allows integration of LR-SPP waveguide components into more complex device geometries, e.g. including microfluidic channels for sensor applications.
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R.H. Pedersen et al. / Microelectronic Engineering 84 (2007) 895–898
3. Fabrication
Fig. 1. Schematic of a long-range surface plasmon polariton waveguide containing a reflecting grating. Dimensions are exaggerated for clarity. The imprint protrusion width w is varied across the design, from 40 nm to 380 nm.
2. Design As a demonstrator device based on nanoimprinted LR-SPP waveguides, we chose a reflecting Bragg grating. The device design is shown in Fig. 1. A thin metal stripe waveguide is embedded in a thick layer of thermoplastic polymer and structured by nanoimprint lithography to form a grating. As a cladding material, we use mr-I T85 nanoimprint resist from micro resist technology GmbH. This material has an excellent chemical resistance, low water absorption, and high optical transparency, and is compatible with UV-lithography and metal deposition [7]. The refractive index of the material is 1.53 [8], and the glass transition temperature is approximately 80 C. In accordance with the Bragg condition, a pitch of 500 nm was chosen for the gratings, which would result in the grating operating at around 1550 nm. In order to test grating performance for different structural parameters, the protrusion width w in the stamp (cf. Fig. 1) was varied from 40 nm to 380 nm, corresponding to a grating duty cycle ranging from 0.08 to 0.76. The grating length was also varied from 0.25 to 4 mm. The total device length containing nanoimprinted stripe waveguides was 10 mm. Finally, stripe waveguides without gratings used for reference measurements were also included in the design.
The fabrication process for the LR-SPP device is shown in Fig. 2. First, alignment marks were pre-structured on a silicon stamp by standard UV-lithography and reactive ion etching (RIE). Then the gratings were patterned using 100 kV electron beam lithography (JEOL JBX9300FS) in the negative tone e-beam resist TEBN-1 from Tokuyama corp., Japan, and etched to a depth of approximately 20 nm in a combined SF6/O2/CHF3 RIE process [9]. Finally, the stamp was coated with an antistiction layer deposited from a C4F8 plasma [10]. A substrate was prepared by spin-coating a silicon substrate with mr-I T85 at 2000 RPM, resulting in a film thickness of 23 lm. After spincoating, thermal nanoimprint lithography was performed in EVG520HE equipment at 160 C and 5 kN, for a duration of 5 min. To define the LR-SPP stripe waveguides, standard UV-lithography was performed on top of the mr-I-T85. 12 nm thick waveguides were defined by gold deposition (e-beam evaporation, Alcatel SCM600) and lift-off. The pitch and depth of the gratings were confirmed by AFM. To form the top polymer cladding, a borofloat glass wafer was spincoated with a mr-I T85 layer identical to the substrate, and the two wafers were thermally bonded in the imprint machine at 100 C and 10 kN, for 10 min. Finally, the devices were diced into individual samples and cleaned before optical characterization. 4. Results To perform transmission measurements, a setup as illustrated in Fig. 3 was constructed. Light was launched from a broadband ASE source (around 1550 nm wavelength), through a polarization controller into a polarization-maintaining fibre. This is required, because the LR-SPP waveguides only supports TM propagation. The light was coupled directly from the fibre to the waveguides using the end-fire coupling technique. The output facet was first imaged with a Vidicon camera for imaging and alignment purposes. To obtain transmission spectra, the light from the waveguide was coupled into a standard single-mode output fibre and launched into an optical spectrum analyzer.
Fig. 2. Fabrication process for LR-SPP waveguides with nanoimprinted gratings: (a) spincoating of a 23 lm thick layer of mr-I T85 resist on a silicon substrate; (b) stamp fabrication by electron beam lithography and etching and thermal nanoimprint lithography of grating features; (c) UV-lithography and metal deposition on top of the mr-I T85 layer; (d) lift-off to define waveguide dimensions; (e) spincoating of mr-I T85 on borofloat glass and thermal bonding to finalize device.
R.H. Pedersen et al. / Microelectronic Engineering 84 (2007) 895–898
897
Fig. 3. A schematic of the setup used for optical characterization of the devices. Light is carried from a broadband ASE source through a polarization controller and polarization-maintaining fibre and end-fire coupled to the waveguide. At the end facet, the output can be visualized by a camera or collected by a spectrometer for analysis.
Fig. 4. Measurements on waveguides without imprinted gratings. The propagation loss is measured to 8 dB/cm @ 1500 nm. Insert shows an image of the LR-SPP mode. The mode is observed to be highly symmetric and has a mode field diameter of 12 lm.
Fig. 4 shows results of optical characterization of straight waveguides without gratings (unstructured stripes). The output intensity distribution from an LRSPP waveguide (Fig. 4, inset) was fitted to a Gaussian distribution yielding an LR-SPP mode field diameter of about 12 lm. This is comparable to the mode field diameter in the input fibre, measured to 10.8 lm. Additionally, the propagation loss was measured in the unstructured waveguides using the cutback technique. At 1500 nm, the propagation loss was measured to be approximately 8 dB/cm, which is comparable to previously reported metal-polymer LRSPP waveguides [5]. A selection of transmission spectra for gratings with different parameters is shown in Fig. 5. These spectra were normalized by subtracting the transmission through an unstructured waveguide from that through a grating-containing waveguide. The spectra indicate pronounced Bragg-grating behaviour, with the dip in transmission increasing with increasing grating length [11]. The maxi-
mum achieved dip in transmission was found to be about 12 dB for the longest grating of 4 mm length, decreasing to small values of the order of 1 dB for the shortest fabricated gratings of 0.25 mm length (transmission spectra for 0.25 mm long gratings not shown). The size of the transmission dip was found to strongly depend on the duty cycle (Fig. 6), with the strongest grating effect occurring at a duty cycle of 0.5 (where the protrusion width equals half of the grating period). Additionally, it was found that the position of the grating wavelength is independent of both grating length and duty cycle. The transmission dip was located at 1594 nm with a standard deviation of 1.3 nm. From the Bragg condition k0 = 2 Kneff an effective refractive index for a propagating LR-SPP was estimated to be
Fig. 6. Analysis of the size of the transmission dip as a function of the duty cycle (protrusion width divided by the grating period). The most efficient grating is found near a duty cycle of 0.5.
Fig. 5. A selection of measured transmission spectra after subtracting the reference transmission.
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R.H. Pedersen et al. / Microelectronic Engineering 84 (2007) 895–898
about 1.594, 4% higher than the refractive index of the surrounding mr-I T85 polymer.
grant no. 2106-05-0033. The partial support of the ECfunded project NaPa (Contract no. NMP4-CT-2003500120) is gratefully acknowledged.
5. Conclusion References We demonstrated that it is feasible to fabricate LR-SPP waveguide components with integrated Bragg gratings using a simple fabrication process involving thermal nanoimprint lithography in mr-I T85 resist and thermal polymer bonding. The fabricated grating devices show pronounced Bragg-grating behaviour with a transmission dip of up to 12 dB. The highest response was found for gratings with a duty cycle of about 0.5 (half-pitch). The presented technology platform is a building block for fabrication of more complex device geometries for application in e.g. chemical sensing. Acknowledgements This work is supported by the Danish Research Agency, the programme committee for nanoscience and technology, biotechnology and information technology (NABIIT),
[1] W.L. Barnes, A. Dereux, T.W. Ebbesen, Nature 424 (2003) 824– 830. [2] D. Sarid, Phys. Rev. Lett. 47 (1981) 1927. [3] P. Berini, Phys. Rev. B 61 (2000) 10484. [4] R. Charbonneau, N. Lahoud, G. Mattiussi, P. Berini, Opt. Exp. 13 (2005) 977–984. [5] A. Boltasseva, T. Nikolajsen, K. Leosson, K. Kjaer, M.S. Larsen, S.I. Bozhevolnyi, J. Lightwave Technol. 23 (2005) 413–422. [6] S.I. Bozhevolnyi, A. Boltasseva, T. Sondergaard, T. Nikolajsen, K. Leosson, Opt. Commun. 250 (2005) 328–333. [7] T. Nielsen, D. Nilsson, F. Bundgaard, P. Shi, P. Szabo, O. Geschke, A. Kristensen, J. Vac. Sci. Technol. B 22 (2004) 1770–1775. [8] G. Khanarian, Opt. Eng. 40 (6) (2001) 1024–1029. [9] B. Bilenberg, M. Schøler, P. Shi, M.S. Schmidt, P. Bøggild, M. Fink, C. Schuster, F. Reuther, G. Gruetzner, A. Kristensen, J. Vac. Sci.Technol. B 24 (2006) 1776–1779. [10] A.A. Ayo´n, D.-Z. Chen, R. Khanna, R. Braff, H.H. Sawin, M.A. Schmidt, Mat. Res. Soc. Symp. Proc. 605 (2000) 141–147. [11] A. Yariv, P. Yeh, Optical Waves in Crystals, Wiley, New York, 1984.
Nanoimprinted reflecting gratings for long-range surface plasmon polaritons R.H. Pedersen a, A. Boltasseva b, D.M. Johansen a, T. Nielsen a, K.B. Jørgensen K. Leosson d, J.E. Østergaard c, A. Kristensen a,* a
a,c
,
MIC – Department of Micro and Nanotechnology, Nano•DTU, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark b COM•DTU, Nano•DTU, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark c University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark d Science Institute, University of Iceland, Dunhagi 3, IS-107 Reykjavik, Iceland Available online 2 February 2007
Abstract We present a novel design, fabrication, and characterization of reflecting gratings for long-range surface plasmon polaritons (LRSPPs) at telecom wavelengths. LR-SPP waveguides consisting of a thin (12 nm) gold film embedded in a thick (45 lm) layer of dielectric polymer cladding are structured by nanoimprint lithography to form a reflecting Bragg grating. By performing spectrally resolved transmission measurements pronounced Bragg grating behaviour is observed, with the transmission dip increasing (up to 12 dB) with the increasing grating length. 2007 Elsevier B.V. All rights reserved. Keywords: Nanoimprint lithography; Long-range surface plasmon polaritons; Reflecting grating
1. Introduction Recently, surface plasmon photonics (plasmonics) has received considerable interest [1]. The unique properties of surface plasmon polaritons (SPPs), surface-bound electromagnetic excitations propagating along a metal-dielectric interface, can be harnessed to improve light extraction from light-emitting diodes, to realize highly surface-sensitive detectors, to surpass the conventional diffraction limit in integrated optics, etc. However, the applicability of SPP waveguides to integrated optics is somewhat limited because SPPs suffer from strong damping due to absorption in metals, limiting SPP propagation to the range of hundreds of microns. One way to overcome this problem is to utilize long-range SPPs (LR-SPPs), sup-
*
Corresponding author. Tel.: +45 4525 6331; fax: +45 4588 7762. E-mail addresses: [email protected] (R.H. Pedersen), [email protected] (A. Kristensen). 0167-9317/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.110
ported by a thin (sub-20 nm) metal stripe embedded in a thick dielectric cladding material [2,3]. In such a structure, the SPPs associated with the bottom and top metal-dielectric interfaces couple, forming a symmetric mode with a very low field intensity within the metal layer. Thus, a substantial increase in the propagation length is achievable. Such structures have previously been fabricated using glass–polymer [4] or polymer–polymer sandwiches [5], leading to new device designs compatible with standard optical fibre technology, including devices containing subwavelength features patterned with electron-beam lithography (EBL) [6]. However, EBL-based fabrication of these devices is complicated, expensive and time-consuming. In the present work, a fabrication process for devices based on LR-SPP waveguides compatible with nanoimprint lithography is presented. This technique is well suited for mass production of components with nm-scale features and allows integration of LR-SPP waveguide components into more complex device geometries, e.g. including microfluidic channels for sensor applications.
896
R.H. Pedersen et al. / Microelectronic Engineering 84 (2007) 895–898
3. Fabrication
Fig. 1. Schematic of a long-range surface plasmon polariton waveguide containing a reflecting grating. Dimensions are exaggerated for clarity. The imprint protrusion width w is varied across the design, from 40 nm to 380 nm.
2. Design As a demonstrator device based on nanoimprinted LR-SPP waveguides, we chose a reflecting Bragg grating. The device design is shown in Fig. 1. A thin metal stripe waveguide is embedded in a thick layer of thermoplastic polymer and structured by nanoimprint lithography to form a grating. As a cladding material, we use mr-I T85 nanoimprint resist from micro resist technology GmbH. This material has an excellent chemical resistance, low water absorption, and high optical transparency, and is compatible with UV-lithography and metal deposition [7]. The refractive index of the material is 1.53 [8], and the glass transition temperature is approximately 80 C. In accordance with the Bragg condition, a pitch of 500 nm was chosen for the gratings, which would result in the grating operating at around 1550 nm. In order to test grating performance for different structural parameters, the protrusion width w in the stamp (cf. Fig. 1) was varied from 40 nm to 380 nm, corresponding to a grating duty cycle ranging from 0.08 to 0.76. The grating length was also varied from 0.25 to 4 mm. The total device length containing nanoimprinted stripe waveguides was 10 mm. Finally, stripe waveguides without gratings used for reference measurements were also included in the design.
The fabrication process for the LR-SPP device is shown in Fig. 2. First, alignment marks were pre-structured on a silicon stamp by standard UV-lithography and reactive ion etching (RIE). Then the gratings were patterned using 100 kV electron beam lithography (JEOL JBX9300FS) in the negative tone e-beam resist TEBN-1 from Tokuyama corp., Japan, and etched to a depth of approximately 20 nm in a combined SF6/O2/CHF3 RIE process [9]. Finally, the stamp was coated with an antistiction layer deposited from a C4F8 plasma [10]. A substrate was prepared by spin-coating a silicon substrate with mr-I T85 at 2000 RPM, resulting in a film thickness of 23 lm. After spincoating, thermal nanoimprint lithography was performed in EVG520HE equipment at 160 C and 5 kN, for a duration of 5 min. To define the LR-SPP stripe waveguides, standard UV-lithography was performed on top of the mr-I-T85. 12 nm thick waveguides were defined by gold deposition (e-beam evaporation, Alcatel SCM600) and lift-off. The pitch and depth of the gratings were confirmed by AFM. To form the top polymer cladding, a borofloat glass wafer was spincoated with a mr-I T85 layer identical to the substrate, and the two wafers were thermally bonded in the imprint machine at 100 C and 10 kN, for 10 min. Finally, the devices were diced into individual samples and cleaned before optical characterization. 4. Results To perform transmission measurements, a setup as illustrated in Fig. 3 was constructed. Light was launched from a broadband ASE source (around 1550 nm wavelength), through a polarization controller into a polarization-maintaining fibre. This is required, because the LR-SPP waveguides only supports TM propagation. The light was coupled directly from the fibre to the waveguides using the end-fire coupling technique. The output facet was first imaged with a Vidicon camera for imaging and alignment purposes. To obtain transmission spectra, the light from the waveguide was coupled into a standard single-mode output fibre and launched into an optical spectrum analyzer.
Fig. 2. Fabrication process for LR-SPP waveguides with nanoimprinted gratings: (a) spincoating of a 23 lm thick layer of mr-I T85 resist on a silicon substrate; (b) stamp fabrication by electron beam lithography and etching and thermal nanoimprint lithography of grating features; (c) UV-lithography and metal deposition on top of the mr-I T85 layer; (d) lift-off to define waveguide dimensions; (e) spincoating of mr-I T85 on borofloat glass and thermal bonding to finalize device.
R.H. Pedersen et al. / Microelectronic Engineering 84 (2007) 895–898
897
Fig. 3. A schematic of the setup used for optical characterization of the devices. Light is carried from a broadband ASE source through a polarization controller and polarization-maintaining fibre and end-fire coupled to the waveguide. At the end facet, the output can be visualized by a camera or collected by a spectrometer for analysis.
Fig. 4. Measurements on waveguides without imprinted gratings. The propagation loss is measured to 8 dB/cm @ 1500 nm. Insert shows an image of the LR-SPP mode. The mode is observed to be highly symmetric and has a mode field diameter of 12 lm.
Fig. 4 shows results of optical characterization of straight waveguides without gratings (unstructured stripes). The output intensity distribution from an LRSPP waveguide (Fig. 4, inset) was fitted to a Gaussian distribution yielding an LR-SPP mode field diameter of about 12 lm. This is comparable to the mode field diameter in the input fibre, measured to 10.8 lm. Additionally, the propagation loss was measured in the unstructured waveguides using the cutback technique. At 1500 nm, the propagation loss was measured to be approximately 8 dB/cm, which is comparable to previously reported metal-polymer LRSPP waveguides [5]. A selection of transmission spectra for gratings with different parameters is shown in Fig. 5. These spectra were normalized by subtracting the transmission through an unstructured waveguide from that through a grating-containing waveguide. The spectra indicate pronounced Bragg-grating behaviour, with the dip in transmission increasing with increasing grating length [11]. The maxi-
mum achieved dip in transmission was found to be about 12 dB for the longest grating of 4 mm length, decreasing to small values of the order of 1 dB for the shortest fabricated gratings of 0.25 mm length (transmission spectra for 0.25 mm long gratings not shown). The size of the transmission dip was found to strongly depend on the duty cycle (Fig. 6), with the strongest grating effect occurring at a duty cycle of 0.5 (where the protrusion width equals half of the grating period). Additionally, it was found that the position of the grating wavelength is independent of both grating length and duty cycle. The transmission dip was located at 1594 nm with a standard deviation of 1.3 nm. From the Bragg condition k0 = 2 Kneff an effective refractive index for a propagating LR-SPP was estimated to be
Fig. 6. Analysis of the size of the transmission dip as a function of the duty cycle (protrusion width divided by the grating period). The most efficient grating is found near a duty cycle of 0.5.
Fig. 5. A selection of measured transmission spectra after subtracting the reference transmission.
898
R.H. Pedersen et al. / Microelectronic Engineering 84 (2007) 895–898
about 1.594, 4% higher than the refractive index of the surrounding mr-I T85 polymer.
grant no. 2106-05-0033. The partial support of the ECfunded project NaPa (Contract no. NMP4-CT-2003500120) is gratefully acknowledged.
5. Conclusion References We demonstrated that it is feasible to fabricate LR-SPP waveguide components with integrated Bragg gratings using a simple fabrication process involving thermal nanoimprint lithography in mr-I T85 resist and thermal polymer bonding. The fabricated grating devices show pronounced Bragg-grating behaviour with a transmission dip of up to 12 dB. The highest response was found for gratings with a duty cycle of about 0.5 (half-pitch). The presented technology platform is a building block for fabrication of more complex device geometries for application in e.g. chemical sensing. Acknowledgements This work is supported by the Danish Research Agency, the programme committee for nanoscience and technology, biotechnology and information technology (NABIIT),
[1] W.L. Barnes, A. Dereux, T.W. Ebbesen, Nature 424 (2003) 824– 830. [2] D. Sarid, Phys. Rev. Lett. 47 (1981) 1927. [3] P. Berini, Phys. Rev. B 61 (2000) 10484. [4] R. Charbonneau, N. Lahoud, G. Mattiussi, P. Berini, Opt. Exp. 13 (2005) 977–984. [5] A. Boltasseva, T. Nikolajsen, K. Leosson, K. Kjaer, M.S. Larsen, S.I. Bozhevolnyi, J. Lightwave Technol. 23 (2005) 413–422. [6] S.I. Bozhevolnyi, A. Boltasseva, T. Sondergaard, T. Nikolajsen, K. Leosson, Opt. Commun. 250 (2005) 328–333. [7] T. Nielsen, D. Nilsson, F. Bundgaard, P. Shi, P. Szabo, O. Geschke, A. Kristensen, J. Vac. Sci. Technol. B 22 (2004) 1770–1775. [8] G. Khanarian, Opt. Eng. 40 (6) (2001) 1024–1029. [9] B. Bilenberg, M. Schøler, P. Shi, M.S. Schmidt, P. Bøggild, M. Fink, C. Schuster, F. Reuther, G. Gruetzner, A. Kristensen, J. Vac. Sci.Technol. B 24 (2006) 1776–1779. [10] A.A. Ayo´n, D.-Z. Chen, R. Khanna, R. Braff, H.H. Sawin, M.A. Schmidt, Mat. Res. Soc. Symp. Proc. 605 (2000) 141–147. [11] A. Yariv, P. Yeh, Optical Waves in Crystals, Wiley, New York, 1984.