Supporting Information The plasmonic pixel: large area, wide gamut ...
Supporting Information
The plasmonic pixel: large area, wide gamut color reproduction using aluminum nanostructures Timothy D. James1‡*, Paul Mulvaney2‡ and Ann Roberts1‡. 1
School of Physics, The University of Melbourne, Parkville, Victoria 3010, Australia
2
Bio 21 Institute and School of Chemistry, The University of Melbourne, Victoria
3010, Australia *E-mail: [email protected] KEYWORDS: Nanoplasmonics, aluminum plasmonics, nanoimprint lithography, electron-beam lithography, plasmonic printing, color printing
Materials and Methods
Fabrication: Figure S1(a) illustrates the flow of the fabrication process for producing the Plasmonic Pixel (PP) prototypes. Fused silica wafers were used as substrates. An 80nm film of PMMA A2 was spun onto the wafer at a spin speed of 1850 rpm, and baked at 180oC for 3min. After baking, a 30nm layer of Cr was evaporated onto the sample via electron-beam evaporation, to act as the charge dissipation layer for the electron beam lithography (EBL) step. EBL was carried out with a Vistec EBPG 5000 tool, equipped with a 100kV source, beam current of 10nA and dosage of 1850µC/cm2. Post exposure, the Cr layer was stripped with a commercial Cr wetetchant that has no effect on the PMMA layer. The PMMA was subsequently developed in a solution of MIBK : isopropanol in a ratio of 1:3 for 1min, then rinsed in isopropanol for 1min. After development, the Al device layer was evaporated, followed by the SiO2 capping layer via electron-beam evaporation. An alternative process for producing the PP devices is presented in Figure S1(b), suitable for large scale, rapid nano-imprint lithography. This simple process requires the substrate to be imprinted with the nano-featured master, the device layer of Al is evaporated, followed by an overcoating of polymer to complete the structure.
Figure S1 Cross-sectional diagrams of the (a) EBL process used for prototyping the PP, and (b) the NIL process for production.
PP Algorithm: The PP algorithm presented in the main text used to produce the pattern files for the EBL process was implemented in Python using the gds CAD package, an open source GDS scripting project. The resulting GDS file was then converted to an EBL tool readable GPF file using the BEAMER commercial software.
Optical Measurements: The reflectance measurements were taken with a custom-built, cage-mounted optical system, comprising a broadband white light source, microscope objective and high quantum efficiency spectrometer. The light source was an Ocean Optics HL-2000FHSA fiber coupled halogen source, which was polarized using a Thorlabs GlanThompson broadband polarizer. The light was focused onto, and collected from, the sample with an Olympus 20x 0.4 NA Plan N object objective, where the reflected light was measured with an Ocean Optics QE65000 high quantum efficiency fiber coupled spectrometer. Photographs were taken with a Nikon 1 mirrorless camera, which was mounted on a Nikon microscope with a Leica 5x 0.14NA N Plan objective for imaging small samples. Alternatively, a Sigma 50mm f/2.8 macro lens was mounted onto the camera using adaptors to photograph the larger feature shown in the last two figures of the manuscript.
Floating Dipole Mode Hybridization
Figure S2 Energy diagram of the hybridized localized plasmon resonances for a nano-antenna and coupled thin film, where the surface charge of each structure is shown at resonance.
One of these modes, the symmetric (ωS) mode, has an induced dipole in phase with that of the floating antenna, while that of the anti-symmetric (ωA) mode is 180° out of phase. The symmetric (ωS) and asymmetric (ωA) modes are frequency shifted from the individual resonant mode of the single antenna, due to the asymmetrically (symmetrically) ordered dipoles being attractive (repulsive) to each other, reducing (increasing) the restoring force of the mode, and therefore reducing (increasing) the frequency of the resonance over the single dipole. In the case of the PP then, the greater the coupling between the floating dipole and the thin film, the greater the reduction in the resonant frequency (or increase in resonant wavelength). The intensity of the dipole interaction of the two transversely coupled dipoles is described by 1:
V =γ
p1 ⋅ p2 4πε r ε 0 r 3
(1)
where V is the quasistatic interaction energy, p1 and p2 are the dipole moments, ε0 is the free-space permittivity, εr iis the dielectric function illumination wavelength of the PMMA matrix, and r is the distance between the centre of the dipoles [12]. From the perspective of this work, the important feature of this expression is the r3 dependence, as it highlights that the distance between the dipoles is critical to determining the coupling energy. The net result of the mode hybridization on the floating dipole design is observed is that there are color shifts caused by the increasing interaction, V, between the coupled dipoles of the antenna and the film. The resonant mode used for producing color effects in the plasmonic pixel is the asymmetric mode. With increasing interaction energy (reduced spacing), wavelength red-shifting occurs resulting in a drastic change in observed color. As shown in Figure 2 of the manuscript, at small separations between the antenna and film the observed color of the structure would be cyan, and not the desired magenta, due to the resonance redshifting caused by increased asymmetric dipole interaction.
1. Liu, N.; Giessen, H. Angewandte Chemie International Edition 2010, 49, (51), 9838-9852.
The plasmonic pixel: large area, wide gamut color reproduction using aluminum nanostructures Timothy D. James1‡*, Paul Mulvaney2‡ and Ann Roberts1‡. 1
School of Physics, The University of Melbourne, Parkville, Victoria 3010, Australia
2
Bio 21 Institute and School of Chemistry, The University of Melbourne, Victoria
3010, Australia *E-mail: [email protected] KEYWORDS: Nanoplasmonics, aluminum plasmonics, nanoimprint lithography, electron-beam lithography, plasmonic printing, color printing
Materials and Methods
Fabrication: Figure S1(a) illustrates the flow of the fabrication process for producing the Plasmonic Pixel (PP) prototypes. Fused silica wafers were used as substrates. An 80nm film of PMMA A2 was spun onto the wafer at a spin speed of 1850 rpm, and baked at 180oC for 3min. After baking, a 30nm layer of Cr was evaporated onto the sample via electron-beam evaporation, to act as the charge dissipation layer for the electron beam lithography (EBL) step. EBL was carried out with a Vistec EBPG 5000 tool, equipped with a 100kV source, beam current of 10nA and dosage of 1850µC/cm2. Post exposure, the Cr layer was stripped with a commercial Cr wetetchant that has no effect on the PMMA layer. The PMMA was subsequently developed in a solution of MIBK : isopropanol in a ratio of 1:3 for 1min, then rinsed in isopropanol for 1min. After development, the Al device layer was evaporated, followed by the SiO2 capping layer via electron-beam evaporation. An alternative process for producing the PP devices is presented in Figure S1(b), suitable for large scale, rapid nano-imprint lithography. This simple process requires the substrate to be imprinted with the nano-featured master, the device layer of Al is evaporated, followed by an overcoating of polymer to complete the structure.
Figure S1 Cross-sectional diagrams of the (a) EBL process used for prototyping the PP, and (b) the NIL process for production.
PP Algorithm: The PP algorithm presented in the main text used to produce the pattern files for the EBL process was implemented in Python using the gds CAD package, an open source GDS scripting project. The resulting GDS file was then converted to an EBL tool readable GPF file using the BEAMER commercial software.
Optical Measurements: The reflectance measurements were taken with a custom-built, cage-mounted optical system, comprising a broadband white light source, microscope objective and high quantum efficiency spectrometer. The light source was an Ocean Optics HL-2000FHSA fiber coupled halogen source, which was polarized using a Thorlabs GlanThompson broadband polarizer. The light was focused onto, and collected from, the sample with an Olympus 20x 0.4 NA Plan N object objective, where the reflected light was measured with an Ocean Optics QE65000 high quantum efficiency fiber coupled spectrometer. Photographs were taken with a Nikon 1 mirrorless camera, which was mounted on a Nikon microscope with a Leica 5x 0.14NA N Plan objective for imaging small samples. Alternatively, a Sigma 50mm f/2.8 macro lens was mounted onto the camera using adaptors to photograph the larger feature shown in the last two figures of the manuscript.
Floating Dipole Mode Hybridization
Figure S2 Energy diagram of the hybridized localized plasmon resonances for a nano-antenna and coupled thin film, where the surface charge of each structure is shown at resonance.
One of these modes, the symmetric (ωS) mode, has an induced dipole in phase with that of the floating antenna, while that of the anti-symmetric (ωA) mode is 180° out of phase. The symmetric (ωS) and asymmetric (ωA) modes are frequency shifted from the individual resonant mode of the single antenna, due to the asymmetrically (symmetrically) ordered dipoles being attractive (repulsive) to each other, reducing (increasing) the restoring force of the mode, and therefore reducing (increasing) the frequency of the resonance over the single dipole. In the case of the PP then, the greater the coupling between the floating dipole and the thin film, the greater the reduction in the resonant frequency (or increase in resonant wavelength). The intensity of the dipole interaction of the two transversely coupled dipoles is described by 1:
V =γ
p1 ⋅ p2 4πε r ε 0 r 3
(1)
where V is the quasistatic interaction energy, p1 and p2 are the dipole moments, ε0 is the free-space permittivity, εr iis the dielectric function illumination wavelength of the PMMA matrix, and r is the distance between the centre of the dipoles [12]. From the perspective of this work, the important feature of this expression is the r3 dependence, as it highlights that the distance between the dipoles is critical to determining the coupling energy. The net result of the mode hybridization on the floating dipole design is observed is that there are color shifts caused by the increasing interaction, V, between the coupled dipoles of the antenna and the film. The resonant mode used for producing color effects in the plasmonic pixel is the asymmetric mode. With increasing interaction energy (reduced spacing), wavelength red-shifting occurs resulting in a drastic change in observed color. As shown in Figure 2 of the manuscript, at small separations between the antenna and film the observed color of the structure would be cyan, and not the desired magenta, due to the resonance redshifting caused by increased asymmetric dipole interaction.
1. Liu, N.; Giessen, H. Angewandte Chemie International Edition 2010, 49, (51), 9838-9852.
