Nanophotonics concerns the interaction of light with objects on the nanometer scale. For visible light, these are smaller than its diffraction limit, and standard lenses cannot focus on spot sizes on this scale. Standard microscopy approaches would mean it would be impossible to resolve objects on the nanometer length scale.
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It is possible, however, to channel light through nanometer structures and antennas with certain materials. These nanophotonic effects rely on the very specific construction of the nanomaterial. Particular elements, typically metals, are used where there are precise interactions between the light and the surface of the material.1 The challenge is finding fabrication methods that have sufficient control and precision to engineer structures to have the desired types of interaction.
The ability to structure and control light with nanophotonics devices has become of increasing interest in fields such as optical communications and robotics. For robotics, this is because nanophotonic structures can be used to visualize sub-diffraction limit structures and to manipulate and control systems of this size. In this context, light may offer the most precise surgical tool to date, one that can be used to perform surgery on single cells.2
For optical communications, nanophotonics may open up routes to device miniaturization and the ability to code greater amounts of information in transmitted light by controlling properties such as its polarization.3
Nanoparticles and Polarization
Nanoparticles are widely used as nanoplasmonic devices as they can be manufactured in a variety of sizes, with different chemical substituents and structures. It is also possible to ‘twist’ nanoparticles to make helical structures that are chiral in nature. This can be done either through the nanofabrication process or through the addition of chiral ligands, such as amino acids and peptides.
Chiral molecules have a unique interaction with circularly polarized light. For many of the biological species that are chiral, such as DNA and amino acids, researchers make use of the fact that chiral species interact differently with left and right-handed circularly polarized light to confirm a structure is chiral in nature.
Characterizing chiral nanoparticles is challenging as these structures are very small, and the resulting chiroptical signals are incredibly weak, leading to poor signal-to-noise in their detection.
However, a recent development at the University of Bath now offers a straightforward experimental method for investigation of the interaction of chiral light with nanoparticles using a nonlinear chiroptical response, third-harmonic(hyper) Rayleigh scattering optical activity (THRS OA).4
Nanoplasmonics have been used to try and enhance the inherently weak signals generated from the chiral response of materials. This signal enhancement arises from the interaction of the light with surface plasmons of that nanomaterial.
Nonlinear optical interactions often occur at high peak intensities and can be used to drive effects such as harmonic generation, where a number of photons at a single wavelength are annihilated to produce photons of a higher order of the fundamental wavelength. Nanoplasmonics can also be used to enhance the magnitude of nonlinear optical processes.
While second harmonic scattering from a nanoplasmonic material had been observed,5 higher-order harmonics of the process never had. Many materials will only produce odd or even harmonic sequences due to the material's structure. For metal particles in a liquid, the odd harmonics could not propagate along the direction of light travel and so were forbidden.
However, by looking instead at the associated scattering signals rather than direct harmonic generation, the team at Bath found an experimental signature that was sensitive to the chirality of the system. They also found that the third harmonic Rayleigh scattering signal was a way to probe hyperpolarizabilities that have remained elusive in experimental studies.
Silicon Nanophotonics: turn off the dark | Ritesh Agarwal | TEDxPSU
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As the hyperpolarizability signal is a probe of the dipole length along the helical structure of the nanoparticle in solution, it is sensitive to the helix's length and the number of twists in the structure. Further work needs to be done to examine what type of nanostructures give rise to the greatest THRS OA signals to determine what type of nanoplasmonics give the greatest chiroptical signal enhancements.
The detection of THRS OA signals also represents an essential validation of a theoretical prediction and tool for better understanding the interaction of molecules with light, particularly how the electrical and magnetic dipolar and multipolar contributions affect the optical properties. All of this is essential knowledge of designing new nanophotonics to manipulate light as desired and for providing new spectroscopic tools for the rapid screening and testing of new devices.
The new developments by the team at Bath will open the door for higher-order harmonic signal detection and better characterization tools for nanoparticle systems for the development of highly sensitive molecular recognition.
References and Further Reading
Wang, J. (2014) A review of recent progress in plasmon-assisted nanophotonic devices. Frontiers of Optoelectronics, 7(3), 320–337. Available at: https://doi.org/10.1007/s12200-014-0469-4
Hayakawa, T., Maruyama, H., & Arai, F. (2017). Optically driven micro- and nanorobots. In Light Robotics-Structure-Mediated Nanobiophotonics. Elsevier Ltd. Available at: https://doi.org/10.1016/B978-0-7020-7096-9.00007-0
Benedikovic, D., et al.(2021) Silicon–germanium receivers for short-wave-infrared optoelectronics and communications. Nanophotonics, 10(3), 1059–1079. Available at: https://doi.org/10.1515/nanoph-2020-0547
Ohnoutek, L., et al. (2021) Optical Activity in Third-Harmonic Rayleigh Scattering: A New Route for Measuring Chirality. Laser and Photonics Reviews. Available at: https://doi.org/10.1002/lpor.202100235
Collins, J. T., et al. (2019) First Observation of Optical Activity in Hyper-Rayleigh Scattering. Physical Review X, 9(1), 11024. Available at: https://doi.org/10.1103/PhysRevX.9.011024