Gold nanoparticles that selectively emit left-/right-handed light

When chiral⁽¹⁾ gold nanoparticles⁽²⁾ are irradiated with near-infrared⁽³⁾ femtosecond pulses⁽⁴⁾, visible emission of luminescence is observed. In this study, this luminescence was found to yield high selectivity for left- or right-handed circularly polarized⁽⁵⁾ light, depending on the chirality of the nanoparticles, with a dissymmetry factor⁽⁶⁾ of approximately 0.7. This finding suggests the potential to elevate various applications using circularly polarized light to practical levels.

Abstruct

The research group led by Project Assistant Professor Dr. Hyo-Yong AHN, Dr. Khai Quang LE, Dr. Tetsuya NARUSHIMA, Research Assistant Professor Dr. Junsuke YAMANISHI, and Professor Dr. Hiromi OKAMOTO from the Institute for Molecular Science, and Dr. Ryeong Myeong KIM and Professor Dr. Ki Tae NAM from Seoul National University, found that the visible luminescence from chiral gold nanoparticles caused by irradiated with near-infrared femtosecond pulses depends on the chirality of the nanoparticles and yields high selectivity for left- or right-handed circularly polarized light. While the dissymmetry factor for circular polarization of luminescence in most chiral materials is typically of the order of 0.01 or below, the emission from these chiral gold nanoparticles exhibited a high dissymmetry factor of approximately 0.7.

This research was published in Advanced Optical Materials on May 30 in 2024.

Backgrounds

Chirality refers to a property of materials where their structure is not superimposable on a mirror image of itself. Light also has a chiral structure in the form of circular polarization, which can be left-handed or right-handed. Circularly polarized light has possible future applications in the fields of trace analysis of chiral substances, anti-counterfeiting, quantum information, screens or displays, and so forth.

A number of research papers on efficient generation methods for circularly polarized light have been published. One such method involves generating circular polarization through the luminescence from a material excited by light, where the wavelengths of the excitation and emitted light are different. While many studies have developed materials that generate circular polarization using this method, in most cases they provide only small dissymmetry factors (described below). That is, they produce mixed left- and right-handed circularly polarized light, with only a slight intensity difference between them.

The dissymmetry factor is an indicator of how much circular polarization is biased to be left- or right-handed.  It is determined by calculating the difference between the left- and right-circularly polarized intensity divided by their average. Pure circular polarization has a dissymmetry factor (g value) of ±2, and linear or unpolarized light has a g value of 0. Most traditional circularly polarized light-emitting materials have a dissymmetry factor of the order of 0.01 or less, and thus it has been difficult to reliably identify the generated circularly polarized light.

Achievements

The research group focused on the visible luminescence generated when chiral gold nanoparticles are irradiated with near-infrared femtosecond pulses. Although the incident light was non-chiral and linearly polarized, the emitted light was found to be highly selective for either left- or right-handed circular polarization. The dissymmetry factor was approximately 0.7, indicating a significantly higher degree of circular polarization compared to many other circularly polarized light-emitting materials used in previous studies (the dissymmetry factors are typically of the order of 0.01 or less). Additionally, theoretical calculations and analyses revealed the mechanism for this high selectivity.

Perspectives

This research demonstrates that chiral structured metal nanoparticles are useful materials for generating circularly polarized light, biased to left- or right-handed polarization. Understanding this mechanism also provides guidelines for more efficient circular polarization generation. This work paves the way for developing materials and devices that can efficiently generate circular polarization at various wavelengths and applications in anti-counterfeiting and quantum information using circularly polarized light.

Technical Terms

(1) Chiral, Chirality: When the structure of a matter or a physical phenomenon is not superposable on its mirror image, the structure or the phenomenon is chiral. The property when a matter or a phenomenon is unable to be superimposed with its mirror image is called chirality. For example, the right hand and the left hand are mirror copies of each other, but these structures are not superposable.  The right and left hand are thus chiral structures.

(2) Nanoparticles: Small particles with sizes of nanometer scale. Nanoparticles that are sufficiently smaller than half the wavelength of light can be exerted by the optical gradient force.

(3) Near-infrared region: Light with a slightly longer wavelength than visible light (in the range of approximately 0.75μm to 2μm).

(4) Femtosecond pulse light: Flashes with an extremely short duration of about ten trillionths of a second, obtained from special lasers. The light source used here produces pulses at a rate of about 80 million times per second.

(5) Circular polarization: When the electric and magnetic fields of light circularly rotate in a plane perpendicular to the direction of propagation, the light is called circularly polarized light. There are two rotational directions, clockwise and counter-clockwise, which are called right- and left-circular polarizations, respectively. As the electric and magnetic fields associated with right- and left-circularly polarized light are of spiral structures that are mirror images to each other, circularly polarized light is chiral.

(6) Dissymmetry factor: A numerical indicator of how much circular polarization is biased to be left- or right-handed (how pure the left- or right-handed circular polarization is), defined as g = 2(IL-IR)/(IL+IR). Here, IL and IR are the intensities of the left- and right-handed circularly polarized components, respectively. Pure left-handed circular polarization has g=2, pure right-handed circular polarization has g=-2, and linear or unpolarized light has g=0.

Information of the paper

Authors: Hyo-Yong Ahn, Khai Q. Le, Tetsuya Narushima, Junsuke Yamanishi, Ryeong Myeong Kim, Ki Tae Nam, Hiromi Okamoto
Journal Name: Advanced Optical Materials
Journal Title: “Highly Chiral Light Emission Using Plasmonic Helicoid Nanoparticles”
DOI: 10.1002/adom.202400699

Research Group

Institute for Molecular Science
Seoul National University

Financial Supports

This work was supported in part by Grants-in Aid for Scientific Research (JP21H04641, JP21K18884, JP16H06505, and JP22H05135) from JSPS and MEXT.

Contact Person

Hiromi OKAMOTO
Professor, Center for Mesoscopic Sciences, Institute for Molecular Science / Professor, The Graduate University for Advanced Studies, SOKENDAI
E-mail: aho_at_ims.ac.jp (Please replace the "_at_" with @)

Hyo-Yong AHN
Project Assistant Professor, Center for Novel Science Initiatives, National Institutes of Natural Sciences; Center for Mesoscopic Sciences, Institute for Molecular Science
Phone: +81-564-55-7320 / Fax: +81-564-54-2254