Circular Differential Scattering Research Articles

Unraveling the Chirality Transfer from Circularly Polarized Light to Single Plasmonic Nanoparticles.

Due to their broken symmetry, chiral plasmonic nanostructures have unique optical properties and numerous applications. However, there is still a lack of comprehension regarding how chirality transfer occurs between circularly polarized light (CPL) and these structures. Here, we thoroughly investigate the plasmon-assisted growth of chiral nanoparticles from achiral Au nanocubes (AuNCs) via CPL without the involvement of any chiral molecule stimulators. We identify the structural chirality of our synthesized chiral plasmonic nanostructures using circular differential scattering (CDS) spectroscopy, which is correlated with scanning electron microscopy imaging at both the single-particle and ensemble levels. Theoretical simulations, including hot-electron surface maps, reveal that the plasmon-induced chirality transfer is mediated by the asymmetric distribution of hot electrons on achiral AuNCs under CPL excitation. Furthermore, we shed light on how this plasmon-induced chirality transfer can also be utilized for chiral growth in bimetallic systems, such as Ag or Pd on AuNCs. The results presented here uncover fundamental aspects of chiral light-matter interaction and have implications for the future design and optimization of chiral sensors and chiral catalysis, among others.

Unraveling the Chirality Transfer from Circularly Polarized Light to Single Plasmonic Nanoparticles

AbstractDue to their broken symmetry, chiral plasmonic nanostructures have unique optical properties and numerous applications. However, there is still a lack of comprehension regarding how chirality transfer occurs between circularly polarized light (CPL) and these structures. Here, we thoroughly investigate the plasmon‐assisted growth of chiral nanoparticles from achiral Au nanocubes (AuNCs) via CPL without the involvement of any chiral molecule stimulators. We identify the structural chirality of our synthesized chiral plasmonic nanostructures using circular differential scattering (CDS) spectroscopy, which is correlated with scanning electron microscopy imaging at both the single‐particle and ensemble levels. Theoretical simulations, including hot‐electron surface maps, reveal that the plasmon‐induced chirality transfer is mediated by the asymmetric distribution of hot electrons on achiral AuNCs under CPL excitation. Furthermore, we shed light on how this plasmon‐induced chirality transfer can also be utilized for chiral growth in bimetallic systems, such as Ag or Pd on AuNCs. The results presented here uncover fundamental aspects of chiral light‐matter interaction and have implications for the future design and optimization of chiral sensors and chiral catalysis, among others.

Trace detection of chiral J-aggregated molecules adsorbed on single Au nanorods.

Trace detection of chiral molecules, which is of great significance in chemical, biological, medical and pharmaceutical sciences, requires microscopic techniques at the single-particle or single-molecule level. Although ensemble experiments show that the circular dichroism of chiral molecules can be amplified by plasmonic nanocrystals, trace detection of small chiral molecules remains challenging due to weak signals that are far below the detection limit. Herein, we demonstrate trace detection of chiral J-aggregated molecules adsorbed on individual Au nanorods (NRs) using single-particle circular differential scattering (CDS) spectroscopy. Through measuring the single-particle CDS spectra, we identified dip-peak bisignatures and further determined the chirality by matching them with calculations modelled with chiral media. We therefore find that plasmonic nanocrystals can dramatically amplify the circular dichroism of strongly coupled molecules to a detectable level so that the detection limit is as low as 3.9 × 103 molecules on an individual plasmonic nanoparticle, whereas 2.5 × 1012 molecules free in solution are barely detectable using a commercial circular dichroism instrument, suggesting a significant amplification factor of 108. Our method provides a promising strategy with a high amplification factor, shedding light on the trace detection of chiral molecules using optical microscopic methods.

Laser‐Directed Asymmetric Growth of Plasmonic Chiral Ensembles

AbstractChiral plasmonic system due to its enhanced chiroptic activity in the visible region has attracted numerous interests in the last decade. Although various strategies have been developed for the fabrication of chiral plasmonic nanostructures, the limitations of cost‐effectiveness and controllability have been the main obstacle for their practical applications. Here, continuous wave laser is used to direct the growth of chiral ensembles of gold nanoparticles (AuNPs). These AuNPs are grown from the Au‐doped titanium dioxide (TiO2) films in a ring‐shaped arrangement under photoreduction, which can then evolve into nanotetramers, nanotrimers, and nanodimers at different irradiation time, showing good controllability and reproducibility. Especially, the nanodimers and nanotrimers show circular differential scattering (CDS) over 20%. The optical chirality of the nanotrimers presents a prominent superchiral field, which is demonstrated for the optical sensing of chiral molecules (approximately × 10−3 m) with large spectral dissymmetric factors. Such an unprecedented growth strategy via laser direct writing is facile, tailorable, location‐selective, and ligand‐free, which presents significant implications for on‐chip integration of chiral nano‐optic devices and surface‐enhanced Raman optical activity (SEROA).

Plasmon-Directed On-Wire Growth of Branched Silver Nanowires with Chiroptic Activity.

Silver nanowires (Ag NWs) present prominent waveguiding properties of subwavelength light due to their nanoconfinement with propagating surface plasmons, which is of great importance for on-chip integration of nanophotonic devices and optical computation. Such propagating plasmons also exert plasmonic forces, which can be utilized to manipulate nanoparticles (NPs) beyond the diffraction limit. However, such controllability is spatially limited to the near fields, whereas a large portion of uncontrolled particles are randomly deposited on the chips, which could be detrimental to the integrated optical devices. Herein we shine continuous wave laser at one end of the Ag NW immersed in AgNO3 solution to launch the propagating surface plasmons. The laser irradiation also induces the photoreduction of Ag+ ions to locally generate tiny Ag NPs, which evolve into large Ag flake branches closer to the other end of the Ag NW. Such a peculiar growth is due to the synergistic effect of plasmonic forces and the thermophoretic/thermo-osmosis forces induced by temperature gradient. These branched Ag NWs with sharp angles are intrinsically chiral, which can be partially controlled by changing the irradiation location, forming plasmonic chiral enantiomers. The circular differential scattering (CDS) response of these branched Ag NWs can be as large as 40%, which can be used for chiral enantiomer sensing with spectral dissymmetric factor up to 4 nm induced by phenylalanine. This plasmon-directed on-wire growth not only offers a facile approach for generating plasmonic chiral nanostructures with remote controllability, but also provides significant insights on the synergistic effect of plasmonic forces and thermal-induced forces, which has great implications for self-assembly and integration of on-chip optics.