We report selectively plasmon-mediated nonradiative energy transfer between quantum dot (QD) emitters interacting with each other via Forster-type resonance energy transfer (FRET) under controlled plasmon coupling either to only the donor QDs (i.e., donor-selective) or to only the acceptor QDs (i.e., acceptor-selective). Using layer-by-layer assembled colloidal QD nanocrystal solids with metal nanoparticles integrated at carefully designed spacing, we demonstrate the ability to enable/disable the coupled plasmon-exciton (plexciton) formation distinctly at the donor (exciton departing) site or at the acceptor (exciton feeding) site of our choice, while not hindering the donor exciton-acceptor exciton interaction but refraining from simultaneous coupling to both sites of the donor and the acceptor in the FRET process. In the case of donor-selective plexciton, we observed a substantial shortening in the donor QD lifetime from 1.33 to 0.29 ns as a result of plasmon-coupling to the donors and the FRET-assisted exciton transfer from the donors to the acceptors, both of which shorten the donor lifetime. This consequently enhanced the acceptor emission by a factor of 1.93. On the other hand, in the complementary case of acceptor-selective plexciton we observed a 2.70-fold emission enhancement in the acceptor QDs, larger than the acceptor emission enhancement of the donor-selective plexciton, as a result of the combined effects of the acceptor plasmon coupling and the FRET-assisted exciton feeding. Here we present the comparative results of theoretical modeling of the donor-and acceptorselective plexcitons of nonradiative energy transfer developed here for the first time, which are in excellent agreement with the systematic experimental characterization. Such an ability to modify and control energy transfer through mastering plexcitons is of fundamental importance, opening up new applications for quantum dot embedded plexciton devices along with the development of new techniques in FRET-based fluorescence microscopy. KEYWORDS: Localized plasmons, nonradiative energy transfer, excitons, metal nanoparticles, semiconductor quantum dots, plexcitons, layer-by-layer assembly T he Forster-type resonance energy transfer (FRET), an important proximity effect that can strongly modify the emission kinetics of fluorophores serving as donors and acceptors, is widely used in biotechnology especially as

We propose and demonstrate a nanocomposite localized surface plasmon resonator embedded into an artificial three-dimensional construction. Colloidal semiconductor quantum dots are assembled between layers of metal nanoparticles to create a highly strong plasmon-exciton interaction in the plasmonic cavity. In such a multilayered plasmonic resonator architecture of isotropic CdTe quantum dots, we observed polarized light emission of 80% in the vertical polarization with an enhancement factor of 4.4, resulting in a steady-state anisotropy value of 0.26 and reaching the highest quantum efficiency level of 30% ever reported for such CdTe quantum dot solids. Our electromagnetic simulation results are in good agreement with the experimental characterization data showing a significant emission enhancement in the vertical polarization, for which their fluorescence decay lifetimes are substantially shortened by consecutive replication of our unit cell architecture design. Such strongly plasmon-exciton coupling nanocomposites hold great promise for future exploitation and development of quantum dot plasmonic biophotonics and quantum dot plasmonic optoelectronics.

The Verwey transition in magnetite (Fe 3 O 4 ) is the first metal-insulator transition ever observed [1] and involves a concomitant structural rearrangement and charge-orbital ordering. Due to the complex interplay of these intertwined degrees of freedom, a complete characterization of the low-temperature phase of magnetite and the mechanism driving the transition have long remained elusive. It was demonstrated in recent years that the fundamental building blocks of the charge-ordered structure are three-site small polarons called trimerons [2]. However, electronic collective modes of this trimeron order have not been detected to date, and thus an understanding of the dynamics of the Verwey transition from an electronic point of view is still lacking. Here, we discover spectroscopic signatures of the low-energy electronic excitations of the trimeron network using terahertz light. By driving these modes coherently with an ultrashort laser pulse, we reveal their critical softening and hence demonstrate their direct involvement in the Verwey transition. These findings represent the first observation of soft modes in magnetite and shed new light on the cooperative mechanism at the origin of its exotic ground state.Along with his groundbreaking discovery in 1939, Verwey postulated the emergence of a charge ordering of Fe 2+ and Fe 3+ ions as the mechanism driving the dramatic conductivity drop at T V ∼ 125 K [1]. A vast number of subsequent experimental and theoretical investigations, including those by Anderson [3], Mott [4], and many others, have stimulated a still unresolved debate over a complete description of the Verwey transition [5,6]. In particular, several seemingly incompatible findings related to the intricate low-temperature phase of magnetite have been reported: the crucial role of Coulomb repulsion [7], the necessity of including electron-phonon coupling [4,8,9], small charge disproportionation [7,10,11], anomalous phonon broadening with the absence of a softening towards T V [12], and the observation of structural fluctuations that are connected to the Fermi surface nesting [13] and that persist up to the Curie transition temperature (T C ∼ 850 K) [14].The last decade witnessed significant progress in understanding the Verwey transition from a structural point of view. Most notably, a refinement of the lowtemperature charge-ordered structure as a network of three-site small polarons, termed trimerons, was given by x-ray diffraction [2] ( Fig. 1a). A trimeron consists of a linear unit of three Fe sites accompanied by distortions of the two outer Fe 3+ ions towards the central Fe 2+ ion. An orbital ordering of coplanar t 2g orbitals is also established on each ion within the trimeron (Fig. 1b). This picture of the trimeron order has been crucial for determining the correct noncentrosymmetric Cc space group of magnetite and explaining its spontaneous charge-driven ferroelectric polarization [2,6,15]. Nevertheless, despite extensive research, no soft modes of the trimeron order have been detected to date. ...

This work demonstrates the comparative studies of non-radiative resonance energy transfer in bi-polymer nanoparticles based on fluorescent conjugated polymers. For this purpose, poly[(9,9-dihexylfluorene) (PF) as a donor (D) and poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as an acceptor (A) have been utilized, from which four different bi-polymer nanoparticle systems are designed and synthesized. Both, steady-state fluorescence spectra and time-resolved fluorescence measurements indicate varying energy transfer efficiencies from the host polymer PF to the acceptor polymer MEH-PPV depending on the D-A distances and structural properties of the nanoparticles. The first approach involves the preparation of PF and MEH-PPV nanoparticles separately and mixing them at a certain ratio. In the second approach, first PF and MEH-PPV solutions are mixed prior to nanoparticle formation and then nanoparticles are prepared from the mixture. Third and fourth approaches involve the sequential nanoparticle preparation. In the former, nanoparticles are prepared to have PF as a core and MEH-PPV as a shell. The latter is the reverse of the third in which the core is MEH-PPV and the shell is PF. The highest energy transfer efficiency recorded to be 35% is obtained from the last system, in which a PF layer is sequentially formed on MEH-PPV NPs.

Collective excitations of bound electron-hole pairs—known as excitons—are ubiquitous in condensed matter, emerging in systems as diverse as band semiconductors, molecular crystals, and proteins. Recently, their existence in strongly correlated electron materials has attracted increasing interest due to the excitons’ unique coupling to spin and orbital degrees of freedom. The non-equilibrium driving of such dressed quasiparticles offers a promising platform for realizing unconventional many-body phenomena and phases beyond thermodynamic equilibrium. Here, we achieve this in the van der Waals correlated insulator NiPS3 by photoexciting its newly discovered spin–orbit-entangled excitons that arise from Zhang-Rice states. By monitoring the time evolution of the terahertz conductivity, we observe the coexistence of itinerant carriers produced by exciton dissociation and a long-wavelength antiferromagnetic magnon that coherently precesses in time. These results demonstrate the emergence of a transient metallic state that preserves long-range antiferromagnetism, a phase that cannot be reached by simply tuning the temperature. More broadly, our findings open an avenue toward the exciton-mediated optical manipulation of magnetism.

Revealing the spin excitations of complex quantum magnets is key to developing a minimal model that explains the underlying magnetic correlations in the ground state. We investigate the lowenergy magnons in α-RuCl3 by combining time-domain terahertz spectroscopy under an external magnetic field and model Hamiltonian calculations. We observe two absorption peaks around 2.0 and 2.4 meV, which we attribute to zone-center spin waves. Using linear spin-wave theory with only nearest-neighbor terms of the exchange couplings, we calculate the antiferromagnetic resonance frequencies and reveal their dependence on an external field applied parallel to the nearest-neighbor Ru-Ru bonds. We find that the magnon behavior in an applied magnetic field can be understood only by including an off-diagonal Γ exchange term to the minimal Heisenberg-Kitaev model. Such an anisotropic exchange interaction that manifests itself as a result of strong spin-orbit coupling can naturally account for the observed mixing of the modes at higher fields strengths. arXiv:1909.00462v1 [cond-mat.str-el] 1 Sep 2019

Single-cycle THz fields induce free-induction decays from high-spin transition-metal complexes, yielding THz EPR spectra and zero-field splitting parameters from a simple tabletop measurement.

Colloidal semiconducting quantum dots ( QDs), also known as nanocrystals, offer a number of advantages in light generation, e.g., for solid-state lighting, biol ogical labeling , and bio -imaging. 3 These applications typically necessitate high quantum eff iciencies in film, preferably with polarized emission in certain applications . One of the major problems encountered when working with films of these QDs is the decrease in their fluorescence quantum efficiency in the solid for m. Another technical challenge is to obtain anis otropic emission from these QDs. Recently plasmonics has enabled a wide range of important applications related to nanocrystals including metal enhanced light generation and plasmonic biosensing.1,2 In this work, to address the se difficulties encountered with QD solids, we investigate and demonstrate anisotropic enhanced emission from semiconductor quantum dots placed in multilayered plasmon resonator nanocomposite s using strong plasmon-exciton interactions . In these metal nanoparticle (MNP) resonator architectures, we show that highly isotropic nature of QD emission c an desirably be altered through modifying emission kinetics by plasmon coupling , while also enhancing the overall QD emission. To date plasmo n coupling of such QDs has been widely studied using various methods including the self-assembly of a monolayer of QDs and a monolayer of M NPs. However, previous works studied only the plasmonic coupling of a QD film located either on top or bottom of the metal nanostructure layer , focusing on the resulting bilayer systems of MNPs and QDs . On the other hand , the emission polarization has been investigated and observed for only antisymmetric particle systems such as semiconductor nanorods. Different than th e previous reports, here our work focuses on the demonstration of plasmon -coupled QD solids in a three -dimensional construction by repeating QD -MNP unit cells to increase the quantum efficiency of the composite film and modify the isotropic emission nature of QDs in plasmonic cavities. In our bottom -up approach, the polarization properties are significantly modified , resulting in a 80% of emission in verti cal polarization, and a quantum efficiency of 30% is achieved for the se plasmonic QD-composite films. Plasmonic coupling of CdTe QD layer using Au nanoparticles in a bilayer system has been previously reported both for on-resonance 2 and off-resonance 3 spectral conditions. However , one of the most important conditions for strong plasmon-exciton interaction s is the spectral overlap between plasmon resonance wavelength of MNPs and emission wavelength of QDs. In our experiments, w e observe strong on-resonance plasmon coupling of CdTe QDs, which is also supported by o ur numerical calculation s. As depicted in Fig . 1(a)-(b) there is a significant red -shift in the localized plasmon resonance wavelength of MNPs when they are cast into a closely-packed film. In our design, w e 144 978-1-4244-5369-6/10/$26.00 ©2010 IEEE