19 research outputs found
Plasmon dispersion in metal nanoparticle chains from angle-resolved scattering
We present angle and frequency resolved optical extinction measurements to
determine the dispersion relation of plasmon modes on Ag and Au nanoparticle
chains with pitches down to 75 nm. The large splitting between transverse and
longitudinal modes and the band curvature are inconsistent with reported
electrostatic near-field models, and confirm that far-field retarded
interactions are important, even for -sized structures. The data
imply that lower propagation losses, larger signal bandwidth and larger maximum
group velocity then expected can be achieved for wave vectors below the light
line. We conclude that for the design of optical nanocircuits coherent
far-field couplings across the entire circuit need to be considered, even at
subwavelength feature sizes.Comment: 4 pages, 4 figures, colo
Dissipative eigenvalue problems for a Sturm-Liouville operator with a singular potential
We present a study of optical signatures of salmon lice and the ability to distinguish them from a reference zooplankton species. This forms the basis for developing an instrument for detecting salmon lice in situ
Accurate distance control between a probe and a surface using a microcantilever
We demonstrate a method to accurately control the distance between a custom
probe and a sample on a {\mu}m to nm scale. The method relies on the
closed-loop feedback on the angular deflection of an in-contact AFM
microcantilever. High performance in stability and accuracy is achieved in this
method by taking advantage of the small mechanical feedback path between
surface and probe. We describe how internal error sources that find their
origin in the microcantilever and feedback can be minimized to achieve an
accurate and precise control up to 3 nm. In particular, we investigated how
hysteresis effects in the feedback caused by friction forces between tip and
substrate, can be minimized. By applying a short calibration procedure,
distance control from contact to several micrometers probe-sample distance can
be obtained with an absolute nanometer-scale accuracy. The method presented is
compatible with any probe that can be fixed on a microcantilever chip and can
be easily built into existing AFM systems
Quantitative Determination of Dark Chromophore Population Explains the Apparent Low Quantum Yield of Red Fluorescent Proteins
The fluorescence quantum yield of four representative red fluorescent proteins mCherry, mKate2, mRuby2, and the recently introduced mScarlet was investigated. The excited state lifetimes were measured as a function of the distance to a gold mirror in order to control the local density of optical states (LDOS). By analyzing the total emission rates as a function of the LDOS, we obtain separately the emission rate and the nonradiative rate of the bright states. We thus obtain for the first time the bright state quantum yield of the proteins without interference from dark, nonemitting states. The bright state quantum yields are considerably higher than previously reported quantum yields that average over both bright and dark states. We determine that mCherry, mKate2, and mRuby2 have a considerable fraction of dark chromophores up to 45%, which explains both the low measured quantum yields of red emitting proteins reported in the literature and the difficulties in developing high quantum yield variants of such proteins. For the recently developed bright mScarlet, we find a much smaller dark fraction of 14%, accompanied by a very high quantum yield of the bright state of 81%. The presence of a considerable fraction of dark chromophores has implications for numerous applications of fluorescent proteins, ranging from quantitative fluorescence microscopy to FRET studies to monitoring protein expression levels. We recommend that future optimization of red fluorescent proteins should pay more attention to minimizing the fraction of dark proteins.</p
Atomic-scale confinement of optical fields
In the presence of matter there is no fundamental limit preventing
confinement of visible light even down to atomic scales. Achieving such
confinement and the corresponding intensity enhancement inevitably requires
simultaneous control over atomic-scale details of material structures and over
the optical modes that such structures support. By means of self-assembly we
have obtained side-by-side aligned gold nanorod dimers with robust
atomically-defined gaps reaching below 0.5 nm. The existence of
atomically-confined light fields in these gaps is demonstrated by observing
extreme Coulomb splitting of corresponding symmetric and anti-symmetric dimer
eigenmodes of more than 800 meV in white-light scattering experiments. Our
results open new perspectives for atomically-resolved spectroscopic imaging,
deeply nonlinear optics, ultra-sensing, cavity optomechanics as well as for the
realization of novel quantum-optical devices
Photonic effects on the fluorescence lifetimes of dyes in thin PVA layers
In this paper we investigate the expected change in fluorescent decay rate when a fluorophore in aqueous solution is moved to a thin spin-coated layer of poly(vinyl alcohol). We take into account the local field effect due to the change in the refractive index of the medium around the fluorophore and the photonic effect due to the layers. The obtained results are compared with experimental results for the organic dye Atto565 and the fluorescent protein mCherry. We find that the effects for the organic dye can be well described with the model, for the fluorescent protein (FP) the model is less accurate. We discuss the possible explanations for this