Spectroscopy and Biosensing with Optically Resonant Dielectric Nanostructures

Dr. A. Krasnok, Prof. Dr. A. Alù Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas 78712, USA E-mail: alu@mail.utexas.edu (A. A.), akrasnok@utexas.edu (A. K.) Martín Caldarola Huygens-Kamerlingh Onnes Laboratory, Leiden University, Leiden, Netherlands Dr. Nicolas Bonod Aix-Marseille Univ, CNRS, Centrale Marseille, Institut Fresnel, Marseille, France

Keywords Resonant dielectric nanoparticles, nanophotonics, fluorescence spectroscopy, surface- enhanced Raman scattering, biosensing, lab-on-a-chip technology, Purcell effect, strong coupling, hybrid exciton-polariton systems

Abstract Resonant dielectric nanoparticles (RDNs) made of materials with large positive dielectric permittivity, such as Si, GaP, GaAs, have become a powerful platform for modern light science, enabling various fascinating applications in nanophotonics and quantum optics. In addition to light localization at the nanoscale, dielectric nanostructures provide electric and magnetic resonant responses throughout the visible and infrared spectrum, low dissipative losses and optical heating, low doping effect and absence of quenching, which are interesting

2 for spectroscopy and biosensing applications. In this review, we present state-of-the-art applications of optically resonant high-index dielectric nanostructures as a multifunctional platform for light-matter interactions. Nanoscale control of quantum emitters and applications for enhanced spectroscopy including fluorescence spectroscopy, surface-enhanced Raman scattering (SERS), biosensing, and lab-on-a-chip technology are surveyed. We describe the theoretical background underlying these effects, overview realizations of specific resonant dielectric nanostructures and hybrid excitonic systems, and outlook the challenges in this field, which remain open to future research.

1. Introduction The operation principles of plasmonic nanoantennas and nanostructures are based on the optical properties of metals (e.g., Au and Ag). They have a profound impact on nanophotonics, as they provide efficient means to manipulate light and enhance light-matter interactions at the nanoscale1–10. These nanostructures can significantly enhance the interactions between quantum emitters (e.g., quantum dots, defect centers in solid, molecules) and their surrounding photonic environment11–14, leading to giant luminescence enhancement9,15–24, ultrafast emission in the picosecond range25–27, strong coupling28,29, surface enhanced Raman scattering (SERS)30–32, optical interconnections33,34, and control of emission patterns35–38. Because of these advantages, plasmonic nanostructures are broadly used in many applications, including near-field microscopy39–41, biosensors42,43, photovoltaics44,45, photodetection46, and medicine47,48. However, the typical plasmonic materials, gold and silver, have finite conductivities at optical frequencies, leading to inherent dissipation of the electromagnetic energy. This dissipation is caused by interband transitions of valence electrons to the Fermi surface or the electrons near the Fermi surface to the next unoccupied states in the conduction band49, which leads to Joule heating of the structure and its local environment. For many applications, heat generation in the nanostructure and its surroundings are detrimental, since

3 the behavior of the molecules under study can be modified dramatically with temperature. Moreover, elevated temperatures may lead to reshaping and even complete destruction of the nanostructures. In addition, a quantum emitter placed in the nanometer proximity of a metallic nanostructure will be quenched due to the dominant non-radiative decay channels, leading to the necessity of various dielectric spacers that reduce the overall enhancement effects. The issue of optical losses in plasmonic nanostructures has been addressed in many recent studies49–55 and it is still an object of discussion. It has been shown that plasmonic resonators are always accompanied by losses because of their nature: in order to achieve the resonant behavior for subwavelength plasmonic resonators, part of the optical energy is stored in the kinetic energy of electrons53. Studies show that even high-Tc superconductors cannot be a good alternative to noble metals because they would have to operate at energies less than the superconductive gap, that is, in the THz or far-IR52. Using highly doped semiconductors in place of metals is also not a panacea, because the Fermi level decline necessarily means a decrease of the plasma frequency, thus impacting surface and localized plasmonic resonances53, at which noble metals demonstrate superior properties. We also note that there is a limit to the overall electric field enhancemen

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