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Polarization-independent actively tunable colour generation on imprinted plasmonic surfaces.

Franklin D, Chen Y, Vazquez-Guardado A, Modak S, Boroumand J, Xu D, Wu ST, Chanda D - Nat Commun (2015)

Bottom Line: Structural colour arising from nanostructured metallic surfaces offers many benefits compared to conventional pigmentation based display technologies, such as increased resolution and scalability of their optical response with structure dimensions.A large range of colour tunability is achieved over previous reports by utilizing an engineered surface which allows full liquid crystal reorientation while maximizing the overlap between plasmonic fields and liquid crystal.In combination with imprinted structures of varying periods, a full range of colours spanning the entire visible spectrum is achieved, paving the way towards dynamic pixels for reflective displays.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Physics, University of Central Florida, 4111 Libra Drive, Physical Sciences Building 430, Orlando, Florida 32816, USA [2] NanoScience Technology Center, University of Central Florida, 12424 Research Parkway Suite 400, Orlando, Florida 32826, USA.

ABSTRACT
Structural colour arising from nanostructured metallic surfaces offers many benefits compared to conventional pigmentation based display technologies, such as increased resolution and scalability of their optical response with structure dimensions. However, once these structures are fabricated their optical characteristics remain static, limiting their potential application. Here, by using a specially designed nanostructured plasmonic surface in conjunction with high birefringence liquid crystals, we demonstrate a tunable polarization-independent reflective surface where the colour of the surface is changed as a function of applied voltage. A large range of colour tunability is achieved over previous reports by utilizing an engineered surface which allows full liquid crystal reorientation while maximizing the overlap between plasmonic fields and liquid crystal. In combination with imprinted structures of varying periods, a full range of colours spanning the entire visible spectrum is achieved, paving the way towards dynamic pixels for reflective displays.

No MeSH data available.


Liquid crystal orientation states and plasmonic modes.(a) Schematic top and cross-sectional views of the nanostructure unit cell. The green and orange planes represent x–z and x–y cross-sections, respectively. (b,c) FEM-computed liquid crystal orientation on a 300 nm period nanostructure (b) without an applied electric field (OFF) and (c) with a field of 10 V μm−1 (ON). (d) FDTD-computed electric field intensity (/E/2) spatial cross-section of the first order plasmonic resonance at λ=600 nm, showing penetration of the fields into the liquid crystal region. The 300 nm period structure is excited with y-polarized light. (e) FDTD-predicted reflectance spectrum as a function of surrounding index for a structure of period 300 nm. White dashed lines indicate the effective surrounding index for the OFF and ON states, respectively. Black dashed lines show the analytical dispersion relation for grating-coupled surface plasmon modes. (f) FDTD predicted reflectance spectrum as a function of structure periodicity for the anisotropic effective index given by the ON liquid crystal orientation state, [nxnynz]=[1.55 1.55 1.97]. Black dashed lines show excellent agreement with the analytical dispersion relation.
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f2: Liquid crystal orientation states and plasmonic modes.(a) Schematic top and cross-sectional views of the nanostructure unit cell. The green and orange planes represent x–z and x–y cross-sections, respectively. (b,c) FEM-computed liquid crystal orientation on a 300 nm period nanostructure (b) without an applied electric field (OFF) and (c) with a field of 10 V μm−1 (ON). (d) FDTD-computed electric field intensity (/E/2) spatial cross-section of the first order plasmonic resonance at λ=600 nm, showing penetration of the fields into the liquid crystal region. The 300 nm period structure is excited with y-polarized light. (e) FDTD-predicted reflectance spectrum as a function of surrounding index for a structure of period 300 nm. White dashed lines indicate the effective surrounding index for the OFF and ON states, respectively. Black dashed lines show the analytical dispersion relation for grating-coupled surface plasmon modes. (f) FDTD predicted reflectance spectrum as a function of structure periodicity for the anisotropic effective index given by the ON liquid crystal orientation state, [nxnynz]=[1.55 1.55 1.97]. Black dashed lines show excellent agreement with the analytical dispersion relation.

Mentions: The LC orientation within and near the nanostructured surface is vital in defining the spectral location of the plasmonic modes and ultimately their potential for being tuned. To understand the structure's topographical influence on the LC, FEM calculations on a unit cell of the surface is performed. The numerical simulation uses a Q-tensor method to minimize the Landau-de Gennes-free energy functional for a given set of boundary conditions, LC parameters and external applied fields30. The LC will take the orientation, which minimizes this internal energy, the unit cell and results of which can be seen in Fig. 2a–c. The LC depicted in Fig. 2b–c represents the average local LC orientation about a uniformly sampled grid. The LC is not drawn to scale as typical molecules are ∼2-nm long while the structure period is 300 nm. The simulations use periodic boundary conditions to imitate an infinite array of nanowells and use experimentally verified LC elastic coefficients (see Supplementary Fig. 1). Degenerate anchoring is applied for the aluminium surface, while the top surface anchoring energy is set to zero. The purpose of this is to isolate the aluminium surface's alignment properties from that of the top polyimide alignment layer. Without an external bias, the LC conforms to the profile of the aluminium surface and aligns diagonally with respect to the unit cell as can be seen in the FEM prediction of Fig. 2b. With the application of voltage, a Freedericksz transition is observed where the LC molecules start to reorient from their initial OFF state. Further increase in voltage continuously rotates the LCs vertically until they align along the electric field as shown in FEM prediction, Fig. 2c. This transition followed by a continuous tuning can be seen in Supplementary Fig. 2, where the experimental reflection spectra of a structured surface is tuned as a function of voltage. These orientation matrices along with the ne and no values of the LC produce an anisotropic index tensor, which can be used to predict properties of the surface's optical behaviour. Interestingly, the orientation states, and therefore index tensors, have symmetries which suggest polarization-independent behaviour from light polarized along the structure's orthogonal periodicity vectors. This is a useful property for reflective display elements illuminated with ambient white light as polarizers are not needed, reducing fabrication costs and increasing reflection efficiency. Lastly, it's important to note that the actual LC orientation within a device will also depend on the top alignment layer and the spacing between them. To maintain the system's polarization independence, the LC orientations in Fig. 2b,c must be preserved. For the present case, a relatively large cell gap of ∼4 μm (defined by the order coherence length of the specific LC) is used to reduce the effect of the top-rubbed-polyimide alignment layer on the anchoring of the aluminium surface. Polarization-dependent reflection is observed for cell gaps at ≤2 μm due to the strong influence of the top alignment layer. Cell gap measurements are obtained by fitting FTIR reflection spectra to a Febry–Perot analytical model (see Supplementary Fig. 3).


Polarization-independent actively tunable colour generation on imprinted plasmonic surfaces.

Franklin D, Chen Y, Vazquez-Guardado A, Modak S, Boroumand J, Xu D, Wu ST, Chanda D - Nat Commun (2015)

Liquid crystal orientation states and plasmonic modes.(a) Schematic top and cross-sectional views of the nanostructure unit cell. The green and orange planes represent x–z and x–y cross-sections, respectively. (b,c) FEM-computed liquid crystal orientation on a 300 nm period nanostructure (b) without an applied electric field (OFF) and (c) with a field of 10 V μm−1 (ON). (d) FDTD-computed electric field intensity (/E/2) spatial cross-section of the first order plasmonic resonance at λ=600 nm, showing penetration of the fields into the liquid crystal region. The 300 nm period structure is excited with y-polarized light. (e) FDTD-predicted reflectance spectrum as a function of surrounding index for a structure of period 300 nm. White dashed lines indicate the effective surrounding index for the OFF and ON states, respectively. Black dashed lines show the analytical dispersion relation for grating-coupled surface plasmon modes. (f) FDTD predicted reflectance spectrum as a function of structure periodicity for the anisotropic effective index given by the ON liquid crystal orientation state, [nxnynz]=[1.55 1.55 1.97]. Black dashed lines show excellent agreement with the analytical dispersion relation.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4490413&req=5

f2: Liquid crystal orientation states and plasmonic modes.(a) Schematic top and cross-sectional views of the nanostructure unit cell. The green and orange planes represent x–z and x–y cross-sections, respectively. (b,c) FEM-computed liquid crystal orientation on a 300 nm period nanostructure (b) without an applied electric field (OFF) and (c) with a field of 10 V μm−1 (ON). (d) FDTD-computed electric field intensity (/E/2) spatial cross-section of the first order plasmonic resonance at λ=600 nm, showing penetration of the fields into the liquid crystal region. The 300 nm period structure is excited with y-polarized light. (e) FDTD-predicted reflectance spectrum as a function of surrounding index for a structure of period 300 nm. White dashed lines indicate the effective surrounding index for the OFF and ON states, respectively. Black dashed lines show the analytical dispersion relation for grating-coupled surface plasmon modes. (f) FDTD predicted reflectance spectrum as a function of structure periodicity for the anisotropic effective index given by the ON liquid crystal orientation state, [nxnynz]=[1.55 1.55 1.97]. Black dashed lines show excellent agreement with the analytical dispersion relation.
Mentions: The LC orientation within and near the nanostructured surface is vital in defining the spectral location of the plasmonic modes and ultimately their potential for being tuned. To understand the structure's topographical influence on the LC, FEM calculations on a unit cell of the surface is performed. The numerical simulation uses a Q-tensor method to minimize the Landau-de Gennes-free energy functional for a given set of boundary conditions, LC parameters and external applied fields30. The LC will take the orientation, which minimizes this internal energy, the unit cell and results of which can be seen in Fig. 2a–c. The LC depicted in Fig. 2b–c represents the average local LC orientation about a uniformly sampled grid. The LC is not drawn to scale as typical molecules are ∼2-nm long while the structure period is 300 nm. The simulations use periodic boundary conditions to imitate an infinite array of nanowells and use experimentally verified LC elastic coefficients (see Supplementary Fig. 1). Degenerate anchoring is applied for the aluminium surface, while the top surface anchoring energy is set to zero. The purpose of this is to isolate the aluminium surface's alignment properties from that of the top polyimide alignment layer. Without an external bias, the LC conforms to the profile of the aluminium surface and aligns diagonally with respect to the unit cell as can be seen in the FEM prediction of Fig. 2b. With the application of voltage, a Freedericksz transition is observed where the LC molecules start to reorient from their initial OFF state. Further increase in voltage continuously rotates the LCs vertically until they align along the electric field as shown in FEM prediction, Fig. 2c. This transition followed by a continuous tuning can be seen in Supplementary Fig. 2, where the experimental reflection spectra of a structured surface is tuned as a function of voltage. These orientation matrices along with the ne and no values of the LC produce an anisotropic index tensor, which can be used to predict properties of the surface's optical behaviour. Interestingly, the orientation states, and therefore index tensors, have symmetries which suggest polarization-independent behaviour from light polarized along the structure's orthogonal periodicity vectors. This is a useful property for reflective display elements illuminated with ambient white light as polarizers are not needed, reducing fabrication costs and increasing reflection efficiency. Lastly, it's important to note that the actual LC orientation within a device will also depend on the top alignment layer and the spacing between them. To maintain the system's polarization independence, the LC orientations in Fig. 2b,c must be preserved. For the present case, a relatively large cell gap of ∼4 μm (defined by the order coherence length of the specific LC) is used to reduce the effect of the top-rubbed-polyimide alignment layer on the anchoring of the aluminium surface. Polarization-dependent reflection is observed for cell gaps at ≤2 μm due to the strong influence of the top alignment layer. Cell gap measurements are obtained by fitting FTIR reflection spectra to a Febry–Perot analytical model (see Supplementary Fig. 3).

Bottom Line: Structural colour arising from nanostructured metallic surfaces offers many benefits compared to conventional pigmentation based display technologies, such as increased resolution and scalability of their optical response with structure dimensions.A large range of colour tunability is achieved over previous reports by utilizing an engineered surface which allows full liquid crystal reorientation while maximizing the overlap between plasmonic fields and liquid crystal.In combination with imprinted structures of varying periods, a full range of colours spanning the entire visible spectrum is achieved, paving the way towards dynamic pixels for reflective displays.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Physics, University of Central Florida, 4111 Libra Drive, Physical Sciences Building 430, Orlando, Florida 32816, USA [2] NanoScience Technology Center, University of Central Florida, 12424 Research Parkway Suite 400, Orlando, Florida 32826, USA.

ABSTRACT
Structural colour arising from nanostructured metallic surfaces offers many benefits compared to conventional pigmentation based display technologies, such as increased resolution and scalability of their optical response with structure dimensions. However, once these structures are fabricated their optical characteristics remain static, limiting their potential application. Here, by using a specially designed nanostructured plasmonic surface in conjunction with high birefringence liquid crystals, we demonstrate a tunable polarization-independent reflective surface where the colour of the surface is changed as a function of applied voltage. A large range of colour tunability is achieved over previous reports by utilizing an engineered surface which allows full liquid crystal reorientation while maximizing the overlap between plasmonic fields and liquid crystal. In combination with imprinted structures of varying periods, a full range of colours spanning the entire visible spectrum is achieved, paving the way towards dynamic pixels for reflective displays.

No MeSH data available.