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We present a comprehensive overview of chirality and its optical manifestation in plasmonic nanosystems and nanostructures. We discuss top-down fabricated structures that range from solid metallic nanostructures to groupings of metallic nanoparticles arranged in three dimensions. We also present the large variety of bottom-up synthesized structures. Using DNA, peptides, or other scaffolds, complex nanoparticle arrangements of up to hundreds of individual nanoparticles have been realized. Beyond this static picture, we also give an overview of recent demonstrations of active chiral plasmonic systems, where the chiral optical response can be controlled by an external stimulus. We discuss the prospect of using the unique properties of complex chiral plasmonic systems for enantiomeric sensing schemes.
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Waveguide-integrated plasmonics is a growing field with many innovative concepts and demonstrated devices in the visible and near-infrared. Here, we extend this body of work to the mid-infrared for the application of surface-enhanced infrared absorption (SEIRA), a spectroscopic method to probe molecular vibrations in small volumes and thin films. Built atop a silicon-on-insulator (SOI) waveguide platform, two key plasmonic structures useful for SEIRA are examined using computational modeling: gold nanorods and coaxial nanoapertures. We find resonance dips of 80% in near diffraction-limited areas due to arrays of our structures and up to 40% from a single resonator. Each of the structures are evaluated using the simulated SEIRA signal from poly(methyl methacrylate) and an octadecanethiol self-assembled monolayer. The platforms we present allow for a compact, on-chip SEIRA sensing system with highly efficient waveguide coupling in the mid-IR.
Recently, we proposed a metasurface design for chiral sensing that (i) results in enhanced chiroptical signals by more than two orders of magnitude for ultrathin, subwavelength, chiral samples over a uniform and accessible area, (ii) allows for complete measurements of the total chirality (magnitude and sign of both its real and imaginary part), and (iii) offers the possibility for a crucial signal reversal (excitation with reversed polarization) that enables chirality measurements in an absolute manner, i.e., without the need for sample removal. Our design is based on the anisotropic response of the metasurface, rather than the superchirality of the generated near-fields, as in most contemporary nanophotonic-based chiral sensing approaches. Here, we derive analytically, and verify numerically, simple formulas that provide insight to the sensing mechanism and explain how anisotropic metasurfaces, in general, offer additional degrees of freedom with respect to their isotropic counterparts. We provide a detailed discussion of the key functionalities and benefits of our proposed design and we demonstrate practical measurement schemes for the unambiguous determination of an unknown chirality. Last, we provide the design principles towards broadband operation - from near-infrared to near-ultraviolet frequencies - opening the way for highly sensitive nanoscale chiroptical spectroscopy.
Selective configuration control of plasmonic nanostructures using either top-down or bottom-up approaches has remained challenging in the field of active plasmonics. We demonstrate the realization of DNA-assembled reconfigurable plasmonic metamolecules, which can respond to a wide range of pH changes in a programmable manner. This programmability allows for selective reconfiguration of different plasmonic metamolecule species coexisting in solution through simple pH tuning. This approach enables discrimination of chiral plasmonic quasi-enantiomers and arbitrary tuning of chiroptical effects with unprecedented degrees of freedom. Our work outlines a new blueprint for implementation of advanced active plasmonic systems, in which individual structural species can be programmed to perform multiple tasks and functions in response to independent external stimuli.
Engineering the transport of radiation and its interaction with matter using non-Hermiticity, particularly through spectral degeneracies known as exceptional points(EPs), is an emerging field that has both fundamental and practical implications. Chiral behavior in the vicinity of EPs opens new opportunities in radiation control, such as unidirectional reflection or lasing with potential applications in areas ranging from cavity quantum electrodynamics and spectral filtering to sensing and thermal imaging. However, tuning and stabilizing a system to a discrete EP in parameter space is a challenging task: either the system is operated close to an EP rather than directly at the EP or the true power of EP is obscured by stability issues. Here, we circumvent this challenge by designing a photonic system that operates on a surface of exceptional points, known as an exceptional surface (ES). We achieve this by using a waveguide-coupled optical resonator with an external feedback loop that induces a nonreciprocal coupling between its frequency degenerate clockwise (CW) and counterclockwise (CCW) traveling modes. By operating the system at critical coupling on the ES, we demonstrate, for the first time, the effect of chiral and degenerate coherent perfect absorption (CPA) where input waves in only one direction are perfectly absorbed. This novel type of CPA-EP is revealed through the observation of the predicted and long-sought squared Lorentzian absorption lineshapes. We expect our results and approach to serve as a bridge between non-Hermitian physics and other fields that rely on radiation engineering.
We present a comprehensive review of recent developments in the field of chiral plasmonics. Significant advances have been made recently in understanding the working principles of chiral plasmonic structures. With advances in micro- and nanofabrication techniques, a variety of chiral plasmonic nanostructures have been experimentally realized; these tailored chiroptical properties vastly outperform those of their molecular counterparts. We focus on chiral plasmonic nanostructures created using bottom-up approaches, which not only allow for rational design and fabrication but most intriguingly in many cases also enable dynamic manipulation and tuning of chiroptical responses. We first discuss plasmon-induced chirality, resulting from the interaction of chiral molecules with plasmonic excitations. Subsequently, we discuss intrinsically chiral colloids, which give rise to optical chirality owing to their chiral shapes. Finally, we discuss plasmonic chirality, achieved by arranging achiral plasmonic particles into handed configurations on static or active templates. Chiral plasmonic nanostructures are very promising candidates for real-life applications owing to their significantly larger optical chirality than natural molecules. In addition, chiral plasmonic nanostructures offer engineerable and dynamic chiroptical responses, which are formidable to achieve in molecular systems. We thus anticipate that the field of chiral plasmonics will attract further widespread attention in applications ranging from enantioselective analysis to chiral sensing, structural determination, and in situ ultrasensitive detection of multiple disease biomarkers, as well as optical monitoring of transmembrane transport and intracellular metabolism.
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