Circularly polarized luminescence, or CPL, is a luminescence phenomenon that provides the differential emission intensity of right and left circularly polarized light, thereby providing information on the excited state properties of the chiral molecular systems. In recent years, there has been a growing interest toward the development of organic chromophores capable of circularly polarized emission due to their potential applications in sensors, asymmetric synthesis as well as display and optical storage devices. The major drawback with organic molecules is the low dissymmetric factors exhibited by these systems. One of the recent strategies adopted for the improvement in luminescence dissymmetry of organic systems is through the controlled self-assembly of chromophores. In this Perspective, we highlight the recent experimental and theoretical developments in the field of chiral organic chromophoric systems and their self-assembly, that has produced promising results toward the enhancement of glum values ...

Circularly Polarized Luminescence in Chiral Molecules and Supramolecular Assemblies.

One of the most intuitive ways to classically understand the generation of natural optical activity in chiral media is provided by the coupled oscillator model of Born and Kuhn consisting of two identical, vertically displaced, coupled oscillators. We experimentally realize and discuss its exact plasmonic analog in a system of corner-stacked gold nanorods. In particular, we analyze the arising circular dichroism and optical rotatory spectra in terms of hybridized electromagnetic modes and retardation. Specifically, we demonstrate how tuning the vertical distance between the nanorods can lead to a selective excitation of the occurring bonding and antibonding chiral plasmonic modes.

Interpreting Chiral Nanophotonic Spectra: The Plasmonic Born−Kuhn Model

This paper reviews the recent advances in experiment and theory of the induction of chiroptical effects, primarily circular dichroism (CD), at the plasmonic and excitonic resonances of achiral inorganic nanocrystals (NCs) capped and/or formed with chiral molecules. It also addresses stronger chiroptical effects obtained in intrinsically chiral inorganic nanostructures obtained from growing enantiomeric excess of intrinsically chiral NCs or arranging achiral plasmonic particles in chiral configurations. The accumulated experimental data and theory on various CD induction mechanisms provide an extended set of tools to properly analyze and understand the electromagnetic influence of chiral molecules on inorganic particles and obtain new general insights into the interaction of capping molecules with inorganic NCs. Among the field-induced CD mechanisms developed recently one can name the Coulomb (near-field, dipolar) mechanism for nanostructures much smaller than the wavelength, and for larger nanostructures, the electromagnetic (effective chiral medium), and intrinsically chiral plasmonic mechanisms.

Chirality and chiroptical effects in inorganic nanocrystal systems with plasmon and exciton resonances

Chirality arises universally across many different fields. Recent advancements in artificial nanomaterials have demonstrated chiroptical responses that far exceed those found in natural materials. Chiroptical phenomena are complicated processes that involve transitions between states with opposite parities, and solid interpretations of these observations are yet to be clearly provided. In this review, we present a comprehensive overview of the theoretical aspects of chirality in light, nanostructures, and nanosystems and their chiroptical interactions. Descriptions of observed chiroptical phenomena based on these fundamentals are intensively discussed. We start with the strong intrinsic and extrinsic chirality in plasmonic nanoparticle systems, followed by enantioselective sensing and optical manipulation, and then conclude with orbital angular momentum-dependent responses. This review will be helpful for understanding the mechanisms behind chiroptical phenomena based on underlying chiral properties and useful for interpreting chiroptical systems for further studies. Strengthening the theoretical understanding of chirality is necessary for developing applications based on its phenomena. Junsuk Rho of Korea’s Pohang University of Science and Technology (POSTECH) reviewed with colleagues the theoretical aspects of chirality, a symmetry property that describes mirror-image objects or systems that cannot be superimposed. Chiral materials have attracted much attention due to their interesting interactions. Scientists are familiar with how geometrically chiral objects and systems interact with light. However, such ‘chiroptical effects’ can also be found in achiral systems, Rho and his colleagues explain. Also, globally achiral light can be locally chiral near nanostructures. Scientists need to extend their concepts and theoretical understandings of chiroptical systems in order to be able to further develop applications based on their phenomena, such as in metamaterials, sensing, spintronics and stereochemistry.

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Electromagnetic chirality: from fundamentals to nontraditional chiroptical phenomena

We manufacture large-area plasmonic structures featuring 3-dimensional chirality by colloidal nanohole lithography. By varying the polar rotating speed of the samples during gold evaporation, we can fabricate spiral-type ramp nanostructures. The optical properties show chiroptical resonances in the 100 to 400 THz frequency region (750 to 3000 nm), with circular dichroism values of up to 13%. Our method offers a simple low-cost manufacturing method of cm2-sized chiral plasmonic templates for chiroptical applications such as stereochemical enantiomer sensors.

Large-area 3D chiral plasmonic structures.

Chiral molecules are shown to induce circular dichroism (CD) at surface plasmon resonances of gold nanostructures when in proximity to the metal surface without direct bonding to the metal. By changing the molecule-Au separation, we were able to learn about the mechanism of plasmonic CD induction for such nanostructures. It was found that even two monolayers of chiral molecules can induce observable plasmonic CD, while without the presence of the plasmonic nanostructures their own CD signal is unmeasurable. Hence, plasmonic arrays could offer a route to enhanced sensitivity for chirality detection.

Omri Bar Elli

Papers

Amplification of chiroptical activity of chiral biomolecules by surface plasmons.