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Formation of Enhanced Uniform Chiral Fields in Symmetric Dimer Nanostructures.

Tian X, Fang Y, Sun M - Sci Rep (2015)

Bottom Line: Plasmonic nanostructures have been proposed to realize such super chiral fields for enhancing weak chiral signals.However, most of them cannot provide uniform chiral near-fields close to the structures, which makes these nanostructures not so efficient for applications.It is especially useful in Raman optical activity measurement and chiral sensing of small quantity of chiral molecule.

View Article: PubMed Central - PubMed

Affiliation: College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, China.

ABSTRACT
Chiral fields with large optical chirality are very important in chiral molecules analysis, sensing and other measurements. Plasmonic nanostructures have been proposed to realize such super chiral fields for enhancing weak chiral signals. However, most of them cannot provide uniform chiral near-fields close to the structures, which makes these nanostructures not so efficient for applications. Plasmonic helical nanostructures and blocked squares have been proved to provide uniform chiral near-fields, but structure fabrication is a challenge. In this paper, we show that very simple plasmonic dimer structures can provide uniform chiral fields in the gaps with large enhancement of both near electric fields and chiral fields under linearly polarized light illumination with polarization off the dimer axis at dipole resonance. An analytical dipole model is utilized to explain this behavior theoretically. 30 times of volume averaged chiral field enhancement is gotten in the whole gap. Chiral fields with opposite handedness can be obtained simply by changing the polarization to the other side of the dimer axis. It is especially useful in Raman optical activity measurement and chiral sensing of small quantity of chiral molecule.

No MeSH data available.


Related in: MedlinePlus

Length-dependent optical chirality in the gap of Au block-dimers (L × 60 nm × 30 nm, gap d = 5 nm) on glass in water environment.(a) Extinction spectra. (b) Volume averaged optical chirality in the gap. Red curves are for the case of RCP excitation. (c and d) Super chiral near-field distributions at the dipole resonant wavelength excited by linearly polarized light (c) and RCP light (d). Rows (in (a) and (b)) or columns (in (c) and (d)) of i, ii and iii correspond to L = 30 nm, L = 60 nm and L = 90 nm respectively. The x-y slices are cut from middle of the height; x–z slices are cut from the middle position of the gap.
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f6: Length-dependent optical chirality in the gap of Au block-dimers (L × 60 nm × 30 nm, gap d = 5 nm) on glass in water environment.(a) Extinction spectra. (b) Volume averaged optical chirality in the gap. Red curves are for the case of RCP excitation. (c and d) Super chiral near-field distributions at the dipole resonant wavelength excited by linearly polarized light (c) and RCP light (d). Rows (in (a) and (b)) or columns (in (c) and (d)) of i, ii and iii correspond to L = 30 nm, L = 60 nm and L = 90 nm respectively. The x-y slices are cut from middle of the height; x–z slices are cut from the middle position of the gap.

Mentions: Figure 6 shows the effect of block length on the optical chirality of the fields in the gap. From Fig. 6a it can be seen that when the length increases, extinction cross section becomes larger, which means the dipole momentum becomes stronger. Figure 6b,c show that, when the length increases, the enhancement of the optical chirality also increases, and the chiral field becomes more uniform. In Fig. 6c–i, the chiral field is not so uniform because the length (defined in y axis) of the particle is less than the width, which causes the coupled dipoles mainly oscillating in x direction even if the excitation is in 45° direction. In such an oscillation situation, the chiral field with both ‘+’ and ‘−’ signs exist on the longer sides of the block for single block. When bring such two blocks together, chiral fields with opposite signs will overlap. As the two dipoles are parallel standing side by side, the middle of the block will be weak, and the side by side bonding mods will cause dramatically change of the field in the gap. RCP excitation in comparison shows non-uniform chiral field as well (Fig. 6d). For the case of longer length, the volume averaged chiral field enhancement reaches 30, which is as strong as the helical structure41.


Formation of Enhanced Uniform Chiral Fields in Symmetric Dimer Nanostructures.

Tian X, Fang Y, Sun M - Sci Rep (2015)

Length-dependent optical chirality in the gap of Au block-dimers (L × 60 nm × 30 nm, gap d = 5 nm) on glass in water environment.(a) Extinction spectra. (b) Volume averaged optical chirality in the gap. Red curves are for the case of RCP excitation. (c and d) Super chiral near-field distributions at the dipole resonant wavelength excited by linearly polarized light (c) and RCP light (d). Rows (in (a) and (b)) or columns (in (c) and (d)) of i, ii and iii correspond to L = 30 nm, L = 60 nm and L = 90 nm respectively. The x-y slices are cut from middle of the height; x–z slices are cut from the middle position of the gap.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4664915&req=5

f6: Length-dependent optical chirality in the gap of Au block-dimers (L × 60 nm × 30 nm, gap d = 5 nm) on glass in water environment.(a) Extinction spectra. (b) Volume averaged optical chirality in the gap. Red curves are for the case of RCP excitation. (c and d) Super chiral near-field distributions at the dipole resonant wavelength excited by linearly polarized light (c) and RCP light (d). Rows (in (a) and (b)) or columns (in (c) and (d)) of i, ii and iii correspond to L = 30 nm, L = 60 nm and L = 90 nm respectively. The x-y slices are cut from middle of the height; x–z slices are cut from the middle position of the gap.
Mentions: Figure 6 shows the effect of block length on the optical chirality of the fields in the gap. From Fig. 6a it can be seen that when the length increases, extinction cross section becomes larger, which means the dipole momentum becomes stronger. Figure 6b,c show that, when the length increases, the enhancement of the optical chirality also increases, and the chiral field becomes more uniform. In Fig. 6c–i, the chiral field is not so uniform because the length (defined in y axis) of the particle is less than the width, which causes the coupled dipoles mainly oscillating in x direction even if the excitation is in 45° direction. In such an oscillation situation, the chiral field with both ‘+’ and ‘−’ signs exist on the longer sides of the block for single block. When bring such two blocks together, chiral fields with opposite signs will overlap. As the two dipoles are parallel standing side by side, the middle of the block will be weak, and the side by side bonding mods will cause dramatically change of the field in the gap. RCP excitation in comparison shows non-uniform chiral field as well (Fig. 6d). For the case of longer length, the volume averaged chiral field enhancement reaches 30, which is as strong as the helical structure41.

Bottom Line: Plasmonic nanostructures have been proposed to realize such super chiral fields for enhancing weak chiral signals.However, most of them cannot provide uniform chiral near-fields close to the structures, which makes these nanostructures not so efficient for applications.It is especially useful in Raman optical activity measurement and chiral sensing of small quantity of chiral molecule.

View Article: PubMed Central - PubMed

Affiliation: College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, China.

ABSTRACT
Chiral fields with large optical chirality are very important in chiral molecules analysis, sensing and other measurements. Plasmonic nanostructures have been proposed to realize such super chiral fields for enhancing weak chiral signals. However, most of them cannot provide uniform chiral near-fields close to the structures, which makes these nanostructures not so efficient for applications. Plasmonic helical nanostructures and blocked squares have been proved to provide uniform chiral near-fields, but structure fabrication is a challenge. In this paper, we show that very simple plasmonic dimer structures can provide uniform chiral fields in the gaps with large enhancement of both near electric fields and chiral fields under linearly polarized light illumination with polarization off the dimer axis at dipole resonance. An analytical dipole model is utilized to explain this behavior theoretically. 30 times of volume averaged chiral field enhancement is gotten in the whole gap. Chiral fields with opposite handedness can be obtained simply by changing the polarization to the other side of the dimer axis. It is especially useful in Raman optical activity measurement and chiral sensing of small quantity of chiral molecule.

No MeSH data available.


Related in: MedlinePlus