Strong coupling between an atom and a cavity is the foundation of quantum photonic devices, which are hard to be integrated into a microchip due to the alignment. Here, DNA strands are used to construct a nanocavity and simultaneously integrate a few fluorophores to form a strong coupling unit. This method produces high cavity yield and strong coupling yield. Combination of this method with e-beam lithography can further position strong coupling units on a specific location of a microchip. The detuned polariton energy can fit into three sets of two-level systems, which means vibronic modes may involve the strong coupling.
Quantum strong coupling between emitters and cavities generates hybrid modes which provide a platform for quantum devices. The atom based systems require precise control over the position of atoms within the cavity and are difficult to be integrated on a chip. The quantum dots-photonic crystal system is limited to the cryogenic temperature. On the contrary, the molecule-plasmonic cavity is a good candidate for chip scale, room temperature operating strong coupling units due to the extremely small mode volume of plasmonic nanocavities.
However, to precisely position a single or a few molecules into a plasmonic nanocavity is challenging. In this work, a few molecules are integrated into the nanocavity through oligonucleotides. The clear Rabi splitting is observed and the anti-crossing curve shows a clear verification of coupling. The number of fluorophore integrated into the nanocavity is estimated to be one. The deterministic strong coupling may be realized based on this configuration.
KEYWORDS: Plasmonics, Chemical species, Molecules, Control systems, Single photon, Solid state physics, Photonic crystals, Quantum dots, Cryogenics, Nanoparticles
Strong coupling between quantum emitters and cavities is of particular interest because of the potential application in quantum devices such as quantum gates and single photon sources. The quantum gate based on the strong coupling between a single atom and a cavity has been realized. The atoms based systems require precise control of the atoms which are difficult to be integrated and scaled up on a single chip. For the solid state system, strong coupling between a photonic crystal cavity and a single quantum dot has been demonstrated at cryogenic temperature. Recently, the plasmonic nanocavity provides a platform for strong coupling at the ambient temperature due to its extremely small mode volume. However, to precisely position a single emitter into a high field region of a plasmonic nanocavity is still challenging.
In this work, a few fluorophores are embedded into a plasmonic nanocavity through the oligonucleotides. A plasmonic nanocavity consists of a functionalized nanoparticle and a metal film. Among 45 fluorophore-embedded nanocavities we measured, 20% of them show clear mode splitting. On the contrary, for the controls, none of the nanocavities shows mode splitting. We believe that some of the molecules have been strongly coupled to the plasmonic nanocavity. The EM simulation shows the mode volume is extremely small, which means only a few molecules can be located in the high field region. With the improvement of the molecule design, the deterministic strong coupling can be realized based on this configuration for quantum devices.
GBM (Glioblastoma Multiforme), a fatal brain tumor, is highly infiltrative and difficult to be completely removed by the surgery. In this work, the Raman tags based on the plasmonic core-satellite assemblies with ~1 nm internal gap accompanied by extremely high gap field have been fabricated and applied to GBM cell labeling. The brightness of the Raman tags is comparable to the fluorophores. The GBM cells with overexpression of EGFR are labeled with these Raman tags and can be distinguished from the normal cells through Raman imaging.
Plasmonic structures are attractive due to their optical properties in the near-field and far-field. In the near-field, the enhanced field they generated strongly interacts with materials in proximity to the surface and even produces the quantum hybrid states in the strong coupling regime. In the far-field, the larger scattering cross section of plasmonic particles provides better contrast for tissue imaging. In addition, the strong absorption can generate substantial amount of heat for cancer cell elimination. These optical properties are usually engineered through tuning the size and morphology of individual nanoparticles by various chemical synthesis methods. The alternative way is to use coupled structure based on existing particles. The molecule-linked structure is a common way for 3D plasmonic materials.
However, to produce a stable coupled structure in the solution phase is challenging. The formation of linkage between linker molecules is usually time-consuming and at low efficiency. Increasing the concentration of linker molecules may raise the reaction speed but also result in the random aggregation of particles. In this work, a polyelectrolyte coating is used to connect spherical nanoparticles of different sizes to form core-satellite assemblies (CSA). The coupled core-satellite structure is formed almost immediately after the solutions of two particles are mixed. The output efficiency is nearly 100%. The CSA is robust under the additional silica shell coating and strong laser illumination. The stability of this CSA is confirmed by the Raman spectra and this assembly can potentially be used as Raman tags.
A metal film coupled with a metal nanoparticle is a simple and stable nanoantenna structure with plasmonic
characteristics. This film-coupled nanoparticle system also has potential for the signal enhancement due to the highly
confined field between the film and the nanoparticle. Recently, this structure has been used to probe the limit of the
enhanced field and the interaction with quantum emitters. The well-known mode in this nanoantenna structures is the gap
dipole mode. However, the high order modes become significant when the gap between the nanoparticle and the film is
reduced. In this work, the high order modes are investigated. The size of the whole nanoantenna structure is around λ/10.
In experiments, the far field scattering spectra/images under different excitation and collection conditions indicate the
influence and the existence of the high order modes. The calculated far-field scattering spectra and spatial intensity
profiles have good agreement with the experimental results. In addition, among these high order modes, the simulated
near-field distributions reveal distinguishable features which include the different symmetry of field distributions and the
various size of confined field. The investigation of these high order modes may provide the information for designing the
interaction between this nanoantenna structure and other plasmonic devices.
The plasmonic modes of a nano-antenna formed by a nanoparticle/thin film hybrid system are investigated. Plasmonic
nano-antennas are well-known for their capabilities to concentrate electromagnetic wave into extreme small region and
couple the emission from active materials in proximity to the antennas into far-field region. Previously, we have shown
through direct measurement of emission profile images that the nano-antennas not only enhance Raman emission but
also systematically direct inelastic emission to the far-field through the dipole mode. We also showed that high order
modes of the hybrid structure can be detected. Here, the higher order plasmonic modes are characterized through
imaging, variable angle linearly polarized excitation, and simulation. Through spectral simulation with improved
resolution, two distinct modes are found to contribute to the broad band high order mode. One has dipole-like behavior
and the other has quadrupole-like behavior. The modes are characterized both through near-field distribution and farfield
scattering profiles. The quadrupole-like mode can be excited by both p- and s-polarized light whereas the dipolelike
mode is only excited by p-polarized light. These high order modes are not as bright as the dipole mode in the farfield
spectrum but have substantial near field enhancement which can be utilized for surface-enhancing spectroscopy and
sensing. In addition, characterization of high order modes may serve to clarify the interaction between nano-antenna and
active materials and will lead to design rules for applications of active plasmonic structures in integrated optical circuits.
Metal nanoparticle assemblies of designed structure are investigated as substrates for polarization manipulation in the
near field region. Gold nanoparticles are known for their optical response due to the excitation of surface plasmons.
Surface plasmons in coupled particles can strongly modulate light either in the far or near field region. The most
common near field application of coupled particles is as field enhancing substrates for amplifying signals of molecules,
for example, Raman signals, IR signals or fluorescence signals. However, the capabilities of metal nanoparticle
assemblies can be extended beyond field amplification. Groups of particles can function as small antennas which convert
far field excitation into localized fields with specific polarization. Through simulations we demonstrate that the near
field polarization can be partially controlled through suitable design of nanoparticle configuration. The benefit of this
configuration is that no probe excitation or other localized excitation is needed. The far field signal is converted into
specific spots with designed polarization, which is not necessarily the same as excitation. Polarization is manipulated
through the coupling of different surface plasmon modes. This polarization modulation extends down to the few
nanometer scale and may provide us more control of interaction of light with nano-scale emitters or molecules.
Metal nanoparticle assemblies of well-defined structure are investigated as substrates for quantitative surface enhanced
Raman scattering (SERS). The ~100 nm structures are formed from oligonucleotide-functionalized gold core and
satellite particles. Raman scattering from Cy5 incorporated on the core particles is detected before and after formation of
the coupled plasmonic structures. The amplification of Raman scattering observed upon formation of the coupled
structures matches quantitatively the increase in the fourth power of the surface E-field associated with coupling
between particles. Raman scattering per core-satellite structure is determined by calibrating measured intensities using
methanol as an intensity standard. The number of molecules that contribute significantly to the Raman signal and the
mean cross section per adsorbed molecule is determined by analysis of the spatial non-uniformity of the core surface
field distribution. Comparison of the wavelength dependence of the near field and the scattering spectrum using
simulation reveals that the wavelengths of the maxima in near and far fields are more closely aligned for the coupled
structures than for isolated cores.
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