Single Plasmonic Oligomer Chiral Spectroscopy

Abstract

dispersion. In general, these two quantities are Kramers-Kronig related. [3] Both effects can be utilized to study chiral molecules, with circular dichroism spectroscopy being the most commonly used. Chirality is a powerful method to analyze the 3D conformation of molecules as it is an intrinsically 3D property. CD spectra can help to unravel the secondary structure of proteins, which is ultimately responsible for the interaction of proteins and for the biologically and physiologically relevant processes. [4] That said, it is obvious that the exact conformation of a molecule and the differences between the molecules of an ensemble are of paramount interest. However, in chemistry and biology the CD of many molecules is measured in a solution with a CD spectrometer, as the CD response of a single molecule appears to be too small to be determined. This measurement scheme obstructs the contribution of the individual molecule and thus does not allow for single molecule based secondary structure investigations. In recent years, researchers have been intensely studying chiral plasmonic systems. [5][6][7][8][9][10][11][12] A plasmon is the collective oscillation of the quasi-free conduction electrons in a metallic nanoparticle. Such plasmon resonances have large resonant light interaction cross sections which allow for the detection and measurement of single nanostructures. What is more, the locally enhanced near-fields associated with the plasmon resonance allow to couple adjacent nanoparticles and create plasmonic molecules of nearly arbitrary complexity. Due to advances in the top-down techniques related to micro-and nanofabrication, we are able to structure such systems in two and three dimensions nearly at will. [13][14][15] Additionally, bottomup techniques, such as DNA-mediated self-assembly, [16][17][18][19][20] even allow for switchable chiral structures. [21][22][23][24][25][26][27][28] Two aspects are of particular importance here: On the one hand, the tunability of these systems allows studying the properties of chirality in great detail while having full control over the chiral plasmonic structures, [29,30] which is in stark contrast to molecular systems whose structures cannot be manipulated easily. On the other hand, researchers are searching for interactions between chiral (bio)-molecules and chiral plasmonic molecules which could lead to decreased detection limits. [31][32][33][34][35][36] In all the mentioned cases, it is of large interest to study single structures and unravel the influence of even minute conformational differences or to track changes in time on a single structure level. Dark-field scattering spectroscopy has been widely used to measure the (chiral) optical response of single Chirality plays a crucial role in our everyday lives. Many biomolecules are handed and the associated biological processes rely on their handedness. Thus, chirality is investigated intensely, also because of the fundamental and inherent interest in the concept of chirality. However, virtually all studies are perfo...

Ultrasensitive hyperspectral imaging and biodetection enabled by dielectric metasurfaces

“…Each MRS is then hyperspectrally imaged by 275 x 275 CMOS pixels with water as top medium. The different tuning parameters, aided by intrinsic fabrication variances 24 , yield sensors with a wide range of densely distributed resonance wavelengths that respond to changes in the top media refractive index identically. Moreover, since the footprint of an individual MRS is small, sensor arrays over large imaging areas can be imaged in a single snapshot for multiplexed detection (see Figs.…”

Optical Helicity and Optical Chirality in Free Space and in the Presence of Matter

“…Thus, metallic nanoparticles exhibiting a left-or right-handed chiral geometry can effectively dissipate optical chirality and generate an optical chirality flux [66,83,86,96]. A myriad of research efforts have, therefore, realized metallic nanostructures with complex chiral geometries, such as metallic helices, pyramids, dimers, and oligomers [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] (see also review articles [25,[39][40][41] In dielectric nanostructures, the conduction and polarization currents are considerably smaller than in metals [98]. Further, in the absence of primary sources, an intrinsic magnetic dipole moment arises from the magnetization current.…”

“…The interaction between chiral electromagnetic plane waves, such as circularly polarized light (CPL), and matter is inherently limited in sensitivity due to their bounded spatial distribution. Rapidly-evolving research efforts in the field of chiral nanophotonics aim to address this challenge by the tailored design of metallic [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] and dielectric [29][30][31][32][33][34][35][36][37][38] nanostructures, arranged periodically in sub-wavelength metamaterials and metasurfaces, or in colloidal dispersions, achieving highly concentrated electromagnetic chirality in their evanescent field (see also review articles [25,[39][40][41][42]). However, the rational design of enhanced electromagnetic chirality in the presence of matter requires the definition of physical observables by which to quantify the chirality of light.…”

Chiroptical spectroscopy of a freely diffusing single nanoparticle

“…In contrast, single-particle measurements only need one particle to provide rich information as has been demonstrated, for instance, by orientation-sensing of single Au nanorods with linearly polarized light 9 , 10 . In recent years, single nanoparticles and their differential interaction with circularly polarized light has been studied with darkfield microscopy 11 – 14 and luminescence 15 . In these measurements the signal for left- and right-circularly polarized light is collected in two separate experiments.…”