Chiral nanostructures studied using polarization-dependent NOLES imaging

Jeremy W. Jarrett, Patrick J. Herbert, Scott Dhuey, Adam M. Schwartzberg, Kenneth Knappenberger

Research output: Contribution to journalArticle

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Abstract

The Nonlinear Optical Localization using Electromagnetic Surface fields (NOLES) imaging technique was used to generate optical images in which the position of a chiral object could be determined with nanometer precision. Asymmetric gold bowtie nanostructures were used as a model system with 2D chirality. The bowties functioned as a chiral nonlinear medium that converted the fundamental of a Ti:sapphire laser to its second harmonic frequency. The bowties consisted of two lithographically prepared equilateral triangles (base = 75 nm, height = 85 nm, thickness = 25 nm) separated by a 20 nm gap. Asymmetric bowties were formed by lateral displacement of one triangle by 10 nm, yielding C2 point group symmetry. The chirality of the bowtie nanostructures was confirmed via nonzero second-harmonic generation circular dichroism (SHG-CDR) ratios, which came from single-particle SHG measurements. The SHG-CDR ratios were validated using numerical finite difference time domain simulations that quantified the relative magnitudes of gap-localized electromagnetic fields at the harmonic frequency resulting from excitation by left and right circularly (LCP and RCP) and linearly polarized fundamental waves. The relative electric dipolar and magnetic dipolar contributions to the SHG responses were determined using single-particle continuous polarization variation (CPV) SHG measurements. The spatial localization precision obtainable for individual chiral nanostructures was determined by statistical analysis of the SHG image point spread function. Our results demonstrated that both the chiral image contrast, which resulted from LCP and RCP excitation, and the corresponding localization precision was dependent upon the relative magnetic dipole/electric dipole ratio (G/F). A localization precision of 1.13 ± 0.13 nm and left-to-right image enhancements of 400% were obtained for bowties with the highest G/F ratios using 5 s frame exposure times. The polarization dependence and magnetic dipole amplification confirmed here demonstrate that the NOLES imaging technique is a powerful method for studying chiral specimens with high spatial precision.

Original languageEnglish (US)
Pages (from-to)8393-8401
Number of pages9
JournalJournal of Physical Chemistry A
Volume118
Issue number37
DOIs
StatePublished - Sep 18 2014

Fingerprint

Nanostructures
Chirality
Polarization
Harmonic generation
electromagnetism
Imaging techniques
polarization
Point groups
Image enhancement
Aluminum Oxide
Optical transfer function
Crystal symmetry
magnetic dipoles
chirality
triangles
imaging techniques
Gold
Electromagnetic fields
dichroism
Amplification

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry

Cite this

Jarrett, Jeremy W. ; Herbert, Patrick J. ; Dhuey, Scott ; Schwartzberg, Adam M. ; Knappenberger, Kenneth. / Chiral nanostructures studied using polarization-dependent NOLES imaging. In: Journal of Physical Chemistry A. 2014 ; Vol. 118, No. 37. pp. 8393-8401.
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abstract = "The Nonlinear Optical Localization using Electromagnetic Surface fields (NOLES) imaging technique was used to generate optical images in which the position of a chiral object could be determined with nanometer precision. Asymmetric gold bowtie nanostructures were used as a model system with 2D chirality. The bowties functioned as a chiral nonlinear medium that converted the fundamental of a Ti:sapphire laser to its second harmonic frequency. The bowties consisted of two lithographically prepared equilateral triangles (base = 75 nm, height = 85 nm, thickness = 25 nm) separated by a 20 nm gap. Asymmetric bowties were formed by lateral displacement of one triangle by 10 nm, yielding C2 point group symmetry. The chirality of the bowtie nanostructures was confirmed via nonzero second-harmonic generation circular dichroism (SHG-CDR) ratios, which came from single-particle SHG measurements. The SHG-CDR ratios were validated using numerical finite difference time domain simulations that quantified the relative magnitudes of gap-localized electromagnetic fields at the harmonic frequency resulting from excitation by left and right circularly (LCP and RCP) and linearly polarized fundamental waves. The relative electric dipolar and magnetic dipolar contributions to the SHG responses were determined using single-particle continuous polarization variation (CPV) SHG measurements. The spatial localization precision obtainable for individual chiral nanostructures was determined by statistical analysis of the SHG image point spread function. Our results demonstrated that both the chiral image contrast, which resulted from LCP and RCP excitation, and the corresponding localization precision was dependent upon the relative magnetic dipole/electric dipole ratio (G/F). A localization precision of 1.13 ± 0.13 nm and left-to-right image enhancements of 400{\%} were obtained for bowties with the highest G/F ratios using 5 s frame exposure times. The polarization dependence and magnetic dipole amplification confirmed here demonstrate that the NOLES imaging technique is a powerful method for studying chiral specimens with high spatial precision.",
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Chiral nanostructures studied using polarization-dependent NOLES imaging. / Jarrett, Jeremy W.; Herbert, Patrick J.; Dhuey, Scott; Schwartzberg, Adam M.; Knappenberger, Kenneth.

In: Journal of Physical Chemistry A, Vol. 118, No. 37, 18.09.2014, p. 8393-8401.

Research output: Contribution to journalArticle

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