4 research outputs found
Dynamics of Strongly Coupled Modes between Surface Plasmon Polaritons and Photoactive Molecules: The Effect of the Stokes Shift
We
have investigated the dynamics of strongly coupled modes of
surface plasmon polaritons (SPPs) and fluorescent molecules by analyzing
their scattered emission polarization. While the scattered emission
of SPPs is purely transverse magnetic (TM) polarized, the strong coupling
with molecules induces transverse electric (TE) polarized emission
via the partial molecular nature of the formed SPP–molecule
polariton mode. We observe that the TM/TE ratio of the polariton emission
follows the contribution of the molecular excited states in this hybrid
mode. By using several types of molecules, we observe that, in addition
to the coupling strength, which determines the contribution of the
molecular excited states, also the Stokes shift of the molecule fluorescence
influences the polarization of the emission: the larger the shift,
the lower the TE-polarized emission. We argue that due to random orientation
of the molecules, the emission of a fully coherent SPP–molecule
polariton should be purely TM-polarized, like SPP. However, as a result
of the unique microenvironments of the molecules in combination with
thermal motion, this symmetry may break for individual excitations,
providing a route to TE emission. The experimental results agree qualitatively
with this model, including the symmetry breaking. Furthermore, the
relaxation rate of the polariton correlates with the Stokes shift,
so that TE emission can occur only if the Stokes shift is small and
consequently the lifetime is long. Our results suggest that taking
into account microscopic details of the molecules in SPP–molecule
polaritons is important for a thorough understanding of the molecular
dynamics of molecules under strong coupling with light modes. Theoretical
models that include these details will be essential to systematically
exploit strong coupling for plasmonics or even controlling chemical
reactions
Toward Single Electron Nanoelectronics Using Self-Assembled DNA Structure
DNA
based structures offer an adaptable and robust way to develop
customized nanostructures for various purposes in bionanotechnology.
One main aim in this field is to develop a DNA nanobreadboard for
a controllable attachment of nanoparticles or biomolecules to form
specific nanoelectronic devices. Here we conjugate three gold nanoparticles
on a defined size TX-tile assembly into a linear pattern to form nanometer
scale isolated islands that could be utilized in a room temperature
single electron transistor. To demonstrate this, conjugated structures
were trapped using dielectrophoresis for current–voltage characterization.
After trapping only high resistance behavior was observed. However,
after extending the islands by chemical growth of gold, several structures
exhibited Coulomb blockade behavior from 4.2 K up to room temperature,
which gives a good indication that self-assembled DNA structures could
be used for nanoelectronic patterning and single electron devices
Plasmonic Coupling and Long-Range Transfer of an Excitation along a DNA Nanowire
We demonstrate an excitation transfer along a fluorescently labeled dsDNA nanowire over a length of several micrometers. Launching of the excitation is done by exciting a localized surface plasmon mode of a 40 nm silver nanoparticle by 800 nm femtosecond laser pulses <i>via</i> two-photon absorption. The plasmonic mode is subsequently coupled or transformed to excitation in the nanowire in contact with the particle and propagated along it, inducing bleaching of the dyes on its way. <i>In situ</i> as well as <i>ex situ</i> fluorescence microscopy is utilized to observe the phenomenon. In addition, transfer of the excitation along the nanowire to another nanoparticle over a separation of 5.7 μm was clearly observed. The nature of the excitation coupling and transfer could not be fully resolved here, but injection of an electron into the DNA from the excited nanoparticle and subsequent coupled transfer of charge (Dexter) and delocalized exciton (Frenkel) is the most probable mechanism. However, a direct plasmonic or optical coupling and energy transfer along the nanowire cannot be totally ruled out either. By further studies the observed phenomenon could be utilized in novel molecular systems, providing a long-needed communication method between molecular devices
Core–Shell Nanorod Columnar Array Combined with Gold Nanoplate–Nanosphere Assemblies Enable Powerful In Situ SERS Detection of Bacteria
Development
of a label-free ultrasensitive nanosensor for detection of bacteria
is presented. Sensitive assay for Gram-positive bacteria was achieved
via electrostatic attraction-guided plasmonic bifacial superstructure/bacteria/columnar
array assembled in one step. Dynamic optical hotspots were formed
in the hybridized nanoassembly under wet–dry critical state
amplifying efficiently the weak vibrational modes of three representative
food-borne Gram-positive bacteria, that is, Staphylococcus
xylosus, Listeria monocytogenes, and Enterococcus faecium. These
three bacteria with highly analogous Raman spectra can be effectively
differentiated through droplet wet–dry critical SERS approach
combined with 3D PCA statistical analysis so that highly sensitive
discrimination of bacterial species and samples containing mixtures
of bacteria can be achieved