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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

Polarization-dependent optical chirality in the gap of Au block dimers (60 nm × 60 nm × 30 nm, gap d = 5 nm) on glass in water environment.(a) Extinction spectra. (b) Volume averaged optical chirality in the gap. Insets show chiral near-field distributions in a plane parallel to the gap at the resonant peak positions, cut from the middle position of the gap. Red curves are for the case of RCP excitation. (c) Corresponding optical chiral near-field distributions in x-y plane at the dipole resonant wavelength, cut from middle of the height. The scale bar applies to all images except the inset images in b–i and b–v, whose intensity is magnified by 5 times to get a better view.
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f4: Polarization-dependent optical chirality in the gap of Au block dimers (60 nm × 60 nm × 30 nm, gap d = 5 nm) on glass in water environment.(a) Extinction spectra. (b) Volume averaged optical chirality in the gap. Insets show chiral near-field distributions in a plane parallel to the gap at the resonant peak positions, cut from the middle position of the gap. Red curves are for the case of RCP excitation. (c) Corresponding optical chiral near-field distributions in x-y plane at the dipole resonant wavelength, cut from middle of the height. The scale bar applies to all images except the inset images in b–i and b–v, whose intensity is magnified by 5 times to get a better view.

Mentions: In order to get deeper understanding of this system, polarization-dependent chiral fields of the block dimer are investigated, shown in Fig. 4. Only fields in dimer gaps are considered, because strongest chiral fields are confined here. The extinction spectra show the variation trend of the longitudinal and transverse modes of the dimer under different polarizations. To well mimic the real experiment, volume averaged optical chirality defined as is also considered in gaps. Unsurprisingly, 0° or 90° excitation gives near zero chiral field value (shown in Fig. 4b(i,v),c(i,v). This is because at 90° the scattered electric field has the same oscillating direction with the incident polarization, resulting in always orthogonal electric and magnetic field components, and at 0° the scattered electric field in the gap is near zero. When polarization of the incident field is off the two symmetry axes (x and y axis), the excited bonding mode and orthogonal incident field component offer parallel electric and magnetic components with delayed phase, which results in non-zero optical chirality, as shown in Fig. 4b(ii–iv), c(ii–iv). It is easy to understand that 45° excitation gives the strongest chiral field. The volume averaged chirality enhancement reaches 18 times for 45°, which is a very big value.


Formation of Enhanced Uniform Chiral Fields in Symmetric Dimer Nanostructures.

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

Polarization-dependent optical chirality in the gap of Au block dimers (60 nm × 60 nm × 30 nm, gap d = 5 nm) on glass in water environment.(a) Extinction spectra. (b) Volume averaged optical chirality in the gap. Insets show chiral near-field distributions in a plane parallel to the gap at the resonant peak positions, cut from the middle position of the gap. Red curves are for the case of RCP excitation. (c) Corresponding optical chiral near-field distributions in x-y plane at the dipole resonant wavelength, cut from middle of the height. The scale bar applies to all images except the inset images in b–i and b–v, whose intensity is magnified by 5 times to get a better view.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Polarization-dependent optical chirality in the gap of Au block dimers (60 nm × 60 nm × 30 nm, gap d = 5 nm) on glass in water environment.(a) Extinction spectra. (b) Volume averaged optical chirality in the gap. Insets show chiral near-field distributions in a plane parallel to the gap at the resonant peak positions, cut from the middle position of the gap. Red curves are for the case of RCP excitation. (c) Corresponding optical chiral near-field distributions in x-y plane at the dipole resonant wavelength, cut from middle of the height. The scale bar applies to all images except the inset images in b–i and b–v, whose intensity is magnified by 5 times to get a better view.
Mentions: In order to get deeper understanding of this system, polarization-dependent chiral fields of the block dimer are investigated, shown in Fig. 4. Only fields in dimer gaps are considered, because strongest chiral fields are confined here. The extinction spectra show the variation trend of the longitudinal and transverse modes of the dimer under different polarizations. To well mimic the real experiment, volume averaged optical chirality defined as is also considered in gaps. Unsurprisingly, 0° or 90° excitation gives near zero chiral field value (shown in Fig. 4b(i,v),c(i,v). This is because at 90° the scattered electric field has the same oscillating direction with the incident polarization, resulting in always orthogonal electric and magnetic field components, and at 0° the scattered electric field in the gap is near zero. When polarization of the incident field is off the two symmetry axes (x and y axis), the excited bonding mode and orthogonal incident field component offer parallel electric and magnetic components with delayed phase, which results in non-zero optical chirality, as shown in Fig. 4b(ii–iv), c(ii–iv). It is easy to understand that 45° excitation gives the strongest chiral field. The volume averaged chirality enhancement reaches 18 times for 45°, which is a very big value.

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