We present experiments where a single subwavelength scatterer is used to examine and control the back-scattering induced coupling between counterpropagating high-Q modes of a microsphere resonator. Our measurements reveal the standing wave character of the resulting symmetric and antisymmetric eigenmodes, their unbalanced intensity distributions, and the coherent nature of their coupling. We discuss our findings and the underlying classical physics in the framework common to quantum optics and provide a particularly intuitive explanation of the central processes.The radiative properties of atoms can be strongly modified by coupling them to resonators [1]. A historical corner stone of this field of research, known as Cavity Quantum Electrodynamics (CQED), was set in 1946 by E. M. Purcell who proposed that the radiation rate of an oscillating dipole at wavelength λ can be enhanced by a factor F = 3Qλ 3 /4π 2 V m in a resonant cavity of quality factor Q and mode volume V m [1]. This socalled Purcell effect holds in the dissipative weak coupling regime where the cavity finesse is small so that the atomic radiation remains dominated by its coupling to the bath of the electromagnetic modes. In the strong coupling regime, coherent exchange of energy between the atom and the resonator causes the atomic resonance to lose its identity and to become replaced by a doublet. These phenomena have been studied for more than three decades [2,3,4,5] although the in-situ manipulation of a single emitter in a single mode of a high-Q microresonator remains a challenge [4,5]. In this Letter, we consider the controlled coupling of a classical nano-object to a high-finesse whispering-gallery mode (WGM) microresonator. We discuss both theoretically and experimentally the resulting coherent coupling between two degenerate counterpropagating WGMs and the modification of the Rayleigh scattering rate. Our findings show that the concepts of the strong and weak coupling play a central role even in this fully classical system. * Electronic address: oliver.benson@physik.hu-berlin. The resonators in our work consist of microspheres melted at the end of silica fibers [6]. Such spheres support very high-Q WGMs and have been studied by several groups [7,8,9]. About ten years ago, it was discovered that the high-Q resonances of these cavities are often composed of doublets [10]. Such a mode splitting has been since discussed in conjunction with various WGM resonators [8,9,11,12,13]. It turns out that mode splitting has been observed in other ring resonators and has been explained as the result of the coupling between the electric fields E c and E cc of the degenerate clockwise (c) and counter clockwise (cc) modes via back scattering. The new superpositions states (+) and (−) are described byHere a and b are complex coefficients. In the simplest case, the coupling between E c and E cc can be caused by a reflector [14,15]. In the case of WGM resonators, however, it has been suggested that backscattering from a distribution of residual subwavelength in...

In this Letter we report on the investigation of the upconversion emission of single NaYF(4) nanocrystals codoped with Yb(3+) and Er(3+). Single nanocrystals on a coverslip are excited with continuous wave laser light at 973 nm in a confocal setup and the upconversion fluorescence is analyzed with a spectrometer. With the help of an atomic force microscope the size of the nanocrystals is simultaneously determined. A strong size-dependence of the spectral properties of the upconversion signal of individual nanocrystals is observed. We attribute this to a differing number of available phonons in the individual crystals for multiphonon relaxation processes, depending on their size. We believe that this result provides a new strategy in the synthesis of upconversion nanoparticles with different spectral properties by changing only their size as it is well-known from the case of semiconductor quantum dots.

We investigate the effects of two-dimensional confinement on the lasing properties of a classical random laser system operating in the incoherent feedback (diffusive) regime. A suspension of 250 nm rutile (TiO2) particles in a rhodamine 6G solution was inserted into the hollow core of a photonic crystal fiber generating the first random fiber laser and a novel quasi-one-dimensional random laser geometry. A comparison with similar systems in bulk format shows that the random fiber laser presents an efficiency that is at least 2 orders of magnitude higher.

Implementing nonlinear optical components in nanoscale photonic devices is challenged by phase matching conditions requiring thickness in the order of hundreds of wavelengths and disadvantaged by the short optical interaction depth of nanometer-scale materials and weak photon-photon interactions. Here we report that ferroelectric NbOI2 nanosheets exhibit giant SHG with conversion efficiencies that are orders of magnitude higher than commonly reported nonlinear crystals. The nonlinear response scales with layer thickness and is strain-and electrical-tunable; a record >0.2 % absolute SHG conversion efficiency and an effective NL susceptibility 𝜒 !"" ($) in the order of 10 −9 m V -1 are demonstrated at average pump intensity of 8 kW/cm 2 . Due to the interplay between anisotropic polarization and excitonic resonance in NbOI2, the spatial profile of the polarized SHG response can be tuned by the excitation wavelength. Our results represent a new paradigm for ultrathin, efficient NL optical components.

We realize controlled cavity-mediated photon transfer between two single nanoparticles over a distance of several tens of micrometers. First, we show how a single nanoscopic emitter attached to a near-field probe can be coupled to high-Q whispering-gallery modes of a silica microsphere at will. Then we demonstrate transfer of energy between this and a second nanoparticle deposited on the sphere surface. We estimate the photon transfer efficiency to be about six orders of magnitude higher than that via free space propagation at comparable separations. Typeset by REVT E X 1If two dipolar emitters are separated by a distance r much less than the transition wavelength λ, they can undergo strong coherent dipole-dipole coupling, leading to sub-and superradiance 1,2 . If their transitions are broadened, dipole-dipole coupling becomes incoherent as in the case of Fluorescence Resonant Energy Transfer (FRET), where the energy from a "donor" is transferred to an "acceptor", provided there is sufficient overlap between the former's emission spectrum and the latter's absorption line. The efficiency of FRET 3 is proportional to (1 + (r/r 0 ) 6 ) −1 and falls to 50% already at r = r 0 ∼ 10 nm. For r > λ, optical communication between the two emitters takes place via propagating photons, while the coupling drops as 1/r 2 . At a distance of 50 µm, the efficiency of one emitter absorbing

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light–matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

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Plasmonic nanoparticles are ideal candidates for hot-electron-assisted applications, but their narrow resonance region and limited hotspot number hindered the energy utilization of broadband solar energy. Inspired by tree branches, we designed and chemically synthesized silver fractals, which enable self-constructed hotspots and multiple plasmonic resonances, extending the broadband generation of hot electrons for better matching with the solar radiation spectrum. We directly revealed the plasmonic origin, the spatial distribution, and the decay dynamics of hot electrons on the single-particle level by using ab initio simulation, dark-field spectroscopy, pump–probe measurements, and electron energy loss spectroscopy. Our results show that fractals with acute tips and narrow gaps can support broadband resonances (400–1100 nm) and a large number of randomly distributed hotspots, which can provide unpolarized enhanced near field and promote hot electron generation. As a proof-of-concept, hot-electron-triggered dimerization of p-nitropthiophenol and hydrogen production are investigated under various irradiations, and the promoted hot electron generation on fractals was confirmed with significantly improved efficiency.