Chirality, essential for distinguishing molecular enantiomers, impacts various scientific fields. Circular dichroism (CD) spectroscopy, which measures differential absorption of circularly polarized light, is limited by weak chiral-optical interactions, yielding minimal signals. Superchiral metasurfaces, enhancing optical chirality, offer improved CD signal and interaction with chiral molecules, necessitating advanced development for accurate analysis. This study presents a novel two-stage design methodology for optimizing superchiral metasurfaces, employing a neural network-based optimizer and adjoint topology optimization for enhanced optical chirality density. The results provide a platform for sensitive CD spectroscopy, facilitating fast optimization of metasurface unit cells for better chiral molecule analysis.
Contemporary literature lacks a classical explanation for the infrared and vibrational circular dichroism (VCD) spectra of chiral molecules. VCD spectra are primarily used to determine the absolute configuration of a chiral molecule by comparing experimental data with quantum mechanical calculations. However, classical treatments of IR and VCD spectra can offer some insights, such as the observation that the area of all bands of wavenumber-normalized absorbance above zero must be equal to the area below zero. Advanced techniques like dispersion analysis, which uses wave optics and dispersion theory, can be used to analyze the spectra of chiral substances.
Directional single-photon sources are of great interest for quantum communication and wireless optical nanocircuits. So far, directional nanoantennas are mostly fabricated by top-down approaches, typically expensive, time-consuming, non-scalable, and require state-of-the-art nanofabrication facilities. On the other hand, bottom-up approaches are relatively fast, inexpensive, and have great scalability. In particular, DNA origami method has been developed into a flexible, designable, and deterministic method for arranging plasmonic nanoparticles with molecular-level precision. These features make DNA origami an attractive method for the fabrication of directional optical nanoantennas. This work exploits DNA origami technology to construct single optical nanoantennas driven by single fluorescent molecules to obtain direction single-photon emission. The morphology and spatial arrangement of nanosized antenna elements are precisely controlled, and single quantum emitters are accurately attached to the hot spot of the antennas to provide local driving power in the optical frequency range. The presented method is a deterministic, precise, convenient, and cost-effective bottom-up fabrication method for mass production of the directional single-photon source.
Plasmonic nanoantennas mediate far and near optical fields and confine the light to subwavelength dimensions. Directional plasmonic antennas increase the emission and collection efficiency of optical networks. To obtain directionality, the spatial organization of nanoantenna elements is critical as it affects the interelement coupling and determines the resultant antenna mode. Moreover, to drive the antenna from its near field, near-field sources such as quantum emitters are required and their distance with respect to antenna elements is the main determinant in coupling between the main antenna mode and the emitter. To couple quantum emitters to optical antennas, high precision on the order of a few nm with respect to the antenna is necessary. As an emerging nanofabrication technique, DNA origami has proven itself to be a robust nanobreadboard to obtain sub-5 nm positioning precision for a diverse range of materials. Here, we use DNA origami to assemble a nanoantenna composed of three gold nanoparticles with sub-5 nm interparticle gaps to break the symmetry and scatter light in one direction very strongly compared to the rest of the directions. To drive this antenna, we place a single emitter, Atto647N dye molecule, exactly at one of the hotspots of the trimer. Here we present the first time realization of a unidirectional plasmonic nanoantenna driven by a single quantum emitter assembled by DNA origami.
We propose a novel microscopy method, called chiral structured illumination microscopy, for fast imaging fluorescent chiral domains at sub-wavelength resolution. This method combines three main techniques, namely structured illumination microscopy, fluorescence-detected circular dichroism and optical chirality engineering. By generating the moir`e effect based on the circular dichroism response of the sample, chiral SIM is able to restore the high spatial frequency information on the chiral domains and reconstruct an image with super resolution. We establish the theoretical framework of chiral SIM and present a numerical demonstration that indicates its superior resolving power over that of diffraction-limited wide-field imaging methods. We also discuss the possibility of applying nanostructures that form the superchiral near fields to boost the circular dichroism response in the proposed chiral SIM method.
Metasurfaces comprising 3D chiral structures have shown great potential in chiroptical applications such as chiral
optical components and sensing. So far, the main challenges lie in the nanofabrication and the limited operational
bandwidth. Here, we present an effective nanofabrication method to create a 3D chiral metasurface with far- and nearfield
broadband chiroptical properties. A focused ion beam is used to cut and stretch nanowires into 3D Archimedean
spirals from stacked films. The spiral exhibits far- and near-field broadband chiroptical responses from 2 to 8 μm. With
circularly polarized light (CPL), the spiral shows superior far-field transmission dissymmetry and handedness-dependent
near-field localization. The reported method and chiral metasurface may find applications in broadband chiral optics and
chiral sensing.
KEYWORDS: Particles, Plasmonics, Near field optics, Optical tweezers, Optical spheres, Surface plasmons, Dielectrics, Plasmons, Optical manipulation, Near field
Optical manipulation of small particles has long been challenging mainly due to reduced gradient force. Rotation of particles by light is even more difficult since that requires the particle to be absorbing or to exhibit large polarizability and optical anisotropy. Otherwise, the optical field has to carry orbital angular momentum. Recently surface-plasmonenhanced optical near field has been used to effectively trap small particles. However, rotation and spinning of isotropic dielectric particles by light has not been demonstrated, not to mention a single device capable of multiple functions. Here, we report the first demonstration of selective trapping or rotation of isotropic dielectric micro-particles using one single plasmonic device, a plasmonic Archimedes spiral. Such functionality is of great interest and may find applications in various fields, such as protein folding analysis and local mixing in microfluidic channels.
Recent advances in optical manipulation have made it an ideal tool to create one, two, and three dimensional periodic optical potential. Such periodic potentials have found interesting technological and fundamental applications such as micro particle sorting and optical fractionation. Plasmon enhanced optical trapping techniques using metallic nanostructures can overcome the diffraction limits of far-field optical trap techniques and therefore permit trapping of nanoparticle with deep sub wavelength dimensions. Here we report the trapping of nanoparticles for a plasmon-enhanced two dimensional optical lattice integrated with microfluidic chip. We observe the trapping of nanoparticles over such an optical lattice. Such an integrated device allows the directional control of nano particles and provides a suitable platform for stochastic transport experiment such as nanoscale optical sorting.
We introduce the possibility of performing two-pulse correlation measurements in order to probe the dynamics of twophoton
photoluminescence in Au nanostructures. Our preliminary results obtained from single-crystal Au nanorods are
consistent with the two-step model for the photoluminescence process.
We explore the possibility to control the polarization state of light confined into sub-diffraction volumes by means of
plasmonic optical antennas. To this aim, we describe a resonant cross antenna, constituted of two perpendicular two-wire
antennas sharing the same gap, which is able to maintain the polarization state in the plane of the antenna. We also
discuss how, by proper tuning of the arm length in a slightly off-resonance cross antenna, it is possible to effectively
realize a nanoscale quarter-waveplate antenna. We present experimental results for the preparation of individual cross
antennas by means of focused ion beam milling starting from single-crystalline Au microflakes, and finally show
preliminary characterization results based on two-photon photoluminescence confocal imaging with linearly-polarized
light.
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