408 research outputs found
Level repulsion in hybrid photonic-plasmonic microresonators for enhanced biodetection
We theoretically analyse photonic-plasmonic coupling between a high Q
whispering gallery mode (WGM) resonator and a core-shell nanoparticle. Blue and
red shifts of WGM resonances are shown to arise from crossing of the photonic
and plasmonic modes. Level repulsion in the hybrid system is further seen to
enable sensitivity enhancements in WGM sensors: maximal when the two resonators
are detuned by half the plasmon linewidth. Approximate bounds are given to
quantify possible enhancements. Criteria for reactive vs. resistive coupling
are also established
Resonant Detection of Nano to Microscopic Objects Using Whispering Gallery Modes
A micron sized glass sphere is able to confine light to its interior volume. The trapped light describes an orbital trajectory circumnavigating just below the microsphere surface. Whenever the light ray tries to escape it is sent back on its circular path by total internal reflection. The light orbit closes in on itself several thousand times and thus creates an optical resonance. The unprecedented narrow linewidth of such a microsphere resonance (Q factors of up to 3 x 10 ) allows precise measurement of its frequency. Dielectric microspheres of very high Q are thus the ideal choice for a resonant molecular sensor. Although the resonance is stealth, an evanescent field extends from the microsphere surface the distance of a wavelength into the surrounding medium. This thesis demonstrates how label-free molecules binding to the microsphere surface perturb the optical resonance by interaction with this evanescent field. The effect is demonstrated by surface adsorption of a protein (serum albumin). The general use as a biosensor is shown by specific detection of streptavidin binding to biotin. A first order perturbation theory describing the linear response of the sensor is presented. Molecular perturbation leads to a wavelength shift that can be measured with such high precision that single molecule detection seems theoretically possible. The experimental approach is extended to the multiplexed measurement of D N A hybridization using two microsphere resonators. This differential measurement allows the detection of a single nucleotide mismatch with a high signal to noise ratio. The effect of larger Mie particles such as bacteria and polystytrene nanospheres perturbing the cavity resonance is examined experimentally and theoretically. For such larger objects it is necessary to include the decay length of the evanescent field in the theoretical analysis. The Q spoiling which occurs for such large Mie particles is described by an analytic formula. Furthermore, a pairing effect is observed for polystyrene nanospheres with diameters of ~ a quarter wavelength polarized in the evanescent field of the microsphere resonance. A novel mechanism might be involved since the coupling cannot be explained by simple dipole-dipole interactions
Impurity-induced step interactions: a kinetic Monte-Carlo study
A one-dimensional continuum description of growth on vicinal surfaces in the
presence of immobile impurities predicts that the impurities can induce step
bunching when they suppress the diffusion of adatoms on the surface. In the
present communication we verify this prediction by kinetic Monte-Carlo
simulations of a two-dimensional solid-on-solid model. We identify the
conditions where quasi one-dimensional step flow is stable against island
formation or step meandering, and analyse in detail the statistics of the
impurity concentration profile. The sign and strength of the impurity-induced
step interactions is determined by monitoring the motion of pairs of steps.
Assemblies containing up to 20 steps turn out to be unstable towards the
emission of single steps. This behavior is traced back to the small value of
the effective, impurity-induced attachment asymmetry for adatoms. An analytic
estimate for the critical number of steps needed to stabilize a bunch is
derived and confirmed by simulations of a one-dimensional model.Comment: 9 pages, 8 figure
Cavity ring-up spectroscopy for ultrafast sensing with optical microresonators
Spectroscopy of whispering-gallery mode (WGM) microresonators has become a
powerful scientific tool, enabling detection of single viruses, nanoparticles,
and even single molecules. Yet the demonstrated timescale of these schemes has
been limited so far to milliseconds or more. Here we introduce a novel scheme
that is orders of magnitude faster, capable of capturing complete spectral
snapshots of WGM resonances at nanosecond timescales: cavity ring-up
spectroscopy (CRUS). Based on sharply-rising detuned probe pulses, CRUS
combines the sensitivity of heterodyne measurements with the highest possible,
transform-limited acquisition rate. As a demonstration we capture spectra of
microtoroid resonators at time intervals as short as 16 ns, directly monitoring
sub-microsecond dynamics of their optomechanical vibrations, thermorefractive
response and Kerr nonlinearity. CRUS holds promise for the study of fast
biological processes such as enzyme kinetics, protein folding and light
harvesting, with applications in other fields such as cavity QED and pulsed
optomechanics.Comment: 6 pages, 4 figure
Label-free optical detection of single enzyme-reactant reactions and associated conformational changes
Monitoring the kinetics and conformational dynamics of single enzymes is
crucial in order to better understand their biological functions as these
motions and structural dynamics are usually unsynchronized among the molecules.
Detecting the enzyme-reactant interactions and associated conformational
changes of the enzyme on a single molecule basis, however, remain as a
challenge with established optical techniques due to the commonly required
labeling of the reactants or the enzyme itself. The labeling process is usually
non-trivial and the labels themselves might skew the physical properties of the
enzyme. Here we demonstrate an optical, label-free method capable of observing
enzymatic interactions and the associated conformational changes on the single
molecule level. We monitor polymerase/DNA interactions via the strong
near-field enhancement provided by plasmonic nanorods resonantly coupled to
whispering gallery modes in microcavities. Specifically, we employ two
different recognition schemes: one in which the kinetics of polymerase/DNA
interactions are probed in the vicinity of DNA-functionalized nanorods, and the
other in which these interactions are probed via the magnitude of
conformational changes in the polymerase molecules immobilized on nanorods. In
both approaches we find that low and high polymerase activities can be clearly
discerned via their characteristic signal amplitude and signal length
distributions. Furthermore, the thermodynamic study of the monitored
interactions suggests the occurrence of DNA polymerization. This work
constitutes a proof-of-concept study of enzymatic activities via plasmonically
enhanced microcavities and establishes an alternative and label-free method
capable of investigating structural changes in single molecules
Kosmischer Staub im Nano-Labor : ein Blick in die Kinderstube des Sonnensystems
Staubwolken sind im Universum die Geburtsstätten neuer Sterne. Dort wiederholen sich Prozesse, die vor 4,56 Milliarden Jahren auch zur Entstehung unseres Sonnensystems geführt haben. Noch heute gibt es Zeugen aus dieser Zeit: Kometenstaub, Sternenstaub und interstellarer Staub. Die »Stardust-Mission« hat sie eingefangen, und Frankfurter Geowissenschaftler haben darin – dank modernster Labor-Analytik – erstaunliche Funde gemacht
Lasing in localized modes of a slow light photonic crystal waveguide
We demonstrate lasing in GaAs photonic crystal waveguides with InAs quantum
dots as gain medium. Structural disorder is present due to fabrication
imperfection and causes multiple scat- tering of light and localization of
light. Lasing modes with varying spatial extend are observed at random
locations along the guide. Lasing frequencies are determined by the local
structure and occur within a narrow frequency band which coincides with the
slow light regime of the waveguide mode. The three-dimensional numerical
simulation reveals that the main loss channel for lasing modes located away
from the waveguide end is out-of-plane scattering by structural disorder.Comment: 8 pages, 4 figure
Advances in optoplasmonic sensors – combining optical nano/microcavities and photonic crystals with plasmonic nanostructures and nanoparticles
AbstractNanophotonic device building blocks, such as optical nano/microcavities and plasmonic nanostructures, lie at the forefront of sensing and spectrometry of trace biological and chemical substances. A new class of nanophotonic architecture has emerged by combining optically resonant dielectric nano/microcavities with plasmonically resonant metal nanostructures to enable detection at the nanoscale with extraordinary sensitivity. Initial demonstrations include single-molecule detection and even single-ion sensing. The coupled photonic-plasmonic resonator system promises a leap forward in the nanoscale analysis of physical, chemical, and biological entities. These optoplasmonic sensor structures could be the centrepiece of miniaturised analytical laboratories, on a chip, with detection capabilities that are beyond the current state of the art. In this paper, we review this burgeoning field of optoplasmonic biosensors. We first focus on the state of the art in nanoplasmonic sensor structures, high quality factor optical microcavities, and photonic crystals separately before proceeding to an outline of the most recent advances in hybrid sensor systems. We discuss the physics of this modality in brief and each of its underlying parts, then the prospects as well as challenges when integrating dielectric nano/microcavities with metal nanostructures. In Section 5, we hint to possible future applications of optoplasmonic sensing platforms which offer many degrees of freedom towards biomedical diagnostics at the level of single molecules
New insights into the effects on blood pressure of diets low in salt and high in fruits and vegetables and low-fat dairy products
Results from the recent Dietary Approaches to Stop Hypertension (DASH)-Sodium trial provide the latest evidence concerning the effects of dietary patterns and sodium intake on blood pressure. Participants ate either the DASH diet (high in fruits, vegetables and low-fat dairy products, and reduced in saturated and total fat) or a typical US diet. Within each diet arm, participants ate higher, intermediate, and lower sodium levels, each for 30 days. The results indicated lower blood pressure with lower sodium intake for both diet groups. Although some critics would argue otherwise, these findings provide important new evidence for the value of the DASH diet and sodium reduction in controlling blood pressure
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