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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

Formation schematic of enhanced chiral near-fields with uniform optical chirality in the gap of a coupled point dipole dimer (a) and Au spherical nanoparticle (10 nm diameter) dimer (b). (a) Analytically calculated chiral near-fields distributions of (i) one dipole, (ii) two dipoles with a large gap d of 0.06λ, and (iii–iv) two dipoles with a small gap d of 0.04λ. Black arrows show the dipole momentum. Signs of ‘+’ and ‘–‘ in the scale bar indicate the field is left- and right- handed, respectively, which applies to all figures in the following. (b) Numerically calculated chiral near-fields distributions of (i) one sphere, (ii) two spheres with a large gap d of 10 nm, and (iii–iv) two dipoles with a small gap d of 2 nm. (c) Corresponding electric fields distribution of the case (b)-iii. The right coordinates gives incident polarization and direction. (d) Schematic of the directions of incident fields and scattered fields by a dipole dimer.
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f1: Formation schematic of enhanced chiral near-fields with uniform optical chirality in the gap of a coupled point dipole dimer (a) and Au spherical nanoparticle (10 nm diameter) dimer (b). (a) Analytically calculated chiral near-fields distributions of (i) one dipole, (ii) two dipoles with a large gap d of 0.06λ, and (iii–iv) two dipoles with a small gap d of 0.04λ. Black arrows show the dipole momentum. Signs of ‘+’ and ‘–‘ in the scale bar indicate the field is left- and right- handed, respectively, which applies to all figures in the following. (b) Numerically calculated chiral near-fields distributions of (i) one sphere, (ii) two spheres with a large gap d of 10 nm, and (iii–iv) two dipoles with a small gap d of 2 nm. (c) Corresponding electric fields distribution of the case (b)-iii. The right coordinates gives incident polarization and direction. (d) Schematic of the directions of incident fields and scattered fields by a dipole dimer.

Mentions: Figure 1 illustrates the formation principle of strong chiral fields in the gap of a dipole dimer. Schäferling has analytically shown that, for a Hertzian dipole driven by a linearly polarized external field, the chiral near-field around the dipole is a four lobes with alternating sign of optical chirality around the scatter, here shown in Fig. 1a–i, due to the interaction of the incident magnetic field and the scattered electric field by the dipole40. We will show that when two such dipoles are put together with the same oscillation direction, if we choose the excitation direction so as that the two lobs with the same chirality overlapped, an enhanced chiral field with the same and uniform chirality will be formed in the gap between the dipoles, as shown in Fig. 1a(ii–iv). To better understand the interaction and interference of the two dipoles, we first analyze the model of two coupled dipoles with coupled-dipole approximation method (see Supporting Information for details)4546. The dipole moments of the two coupled dipoles can be expressed as47


Formation of Enhanced Uniform Chiral Fields in Symmetric Dimer Nanostructures.

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

Formation schematic of enhanced chiral near-fields with uniform optical chirality in the gap of a coupled point dipole dimer (a) and Au spherical nanoparticle (10 nm diameter) dimer (b). (a) Analytically calculated chiral near-fields distributions of (i) one dipole, (ii) two dipoles with a large gap d of 0.06λ, and (iii–iv) two dipoles with a small gap d of 0.04λ. Black arrows show the dipole momentum. Signs of ‘+’ and ‘–‘ in the scale bar indicate the field is left- and right- handed, respectively, which applies to all figures in the following. (b) Numerically calculated chiral near-fields distributions of (i) one sphere, (ii) two spheres with a large gap d of 10 nm, and (iii–iv) two dipoles with a small gap d of 2 nm. (c) Corresponding electric fields distribution of the case (b)-iii. The right coordinates gives incident polarization and direction. (d) Schematic of the directions of incident fields and scattered fields by a dipole dimer.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Formation schematic of enhanced chiral near-fields with uniform optical chirality in the gap of a coupled point dipole dimer (a) and Au spherical nanoparticle (10 nm diameter) dimer (b). (a) Analytically calculated chiral near-fields distributions of (i) one dipole, (ii) two dipoles with a large gap d of 0.06λ, and (iii–iv) two dipoles with a small gap d of 0.04λ. Black arrows show the dipole momentum. Signs of ‘+’ and ‘–‘ in the scale bar indicate the field is left- and right- handed, respectively, which applies to all figures in the following. (b) Numerically calculated chiral near-fields distributions of (i) one sphere, (ii) two spheres with a large gap d of 10 nm, and (iii–iv) two dipoles with a small gap d of 2 nm. (c) Corresponding electric fields distribution of the case (b)-iii. The right coordinates gives incident polarization and direction. (d) Schematic of the directions of incident fields and scattered fields by a dipole dimer.
Mentions: Figure 1 illustrates the formation principle of strong chiral fields in the gap of a dipole dimer. Schäferling has analytically shown that, for a Hertzian dipole driven by a linearly polarized external field, the chiral near-field around the dipole is a four lobes with alternating sign of optical chirality around the scatter, here shown in Fig. 1a–i, due to the interaction of the incident magnetic field and the scattered electric field by the dipole40. We will show that when two such dipoles are put together with the same oscillation direction, if we choose the excitation direction so as that the two lobs with the same chirality overlapped, an enhanced chiral field with the same and uniform chirality will be formed in the gap between the dipoles, as shown in Fig. 1a(ii–iv). To better understand the interaction and interference of the two dipoles, we first analyze the model of two coupled dipoles with coupled-dipole approximation method (see Supporting Information for details)4546. The dipole moments of the two coupled dipoles can be expressed as47

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