126 research outputs found
Molecular emission near metal interfaces: the polaritonic regime
The strong coupling of a dense layer of molecular excitons with
surface-plasmon modes in a metal gives rise to polaritons (hybrid light-matter
states) called plexcitons. Surface plasmons cannot directly emit into (or be
excited by) free-space photons due to the fact that energy and momentum
conservation cannot be simultaneously satisfied in photoluminescence. Most
plexcitons are also formally non-emissive, even though they can radiate via
molecules upon localization due to disorder and decoherence. However, a
fraction of them are bright even in the presence of such deleterious processes.
In this letter, we theoretically discuss the superradiant emission properties
of these bright plexcitons, which belong to the upper energy branch and reveal
huge photoluminescence enhancements compared to bare excitons. Our study
generalizes the well-known problem of molecular emission next to a metal
interface to collective molecular states and provides new design principles for
the control of photophysical properties of molecular aggregates using
polaritonic strategies.Comment: Replaced previous version, noticing that van Hove anomalies are only
observed in the direct and reflected contributions of photoluminescence, but
they cancel out when added up in the total photoluminescence. The correct
phenomenology is that enhancements of photoluminescence are still huge (not
infinite) and are near (not exactly at) the critical poin
Ultrafast Thermal Modification of Strong Coupling in an Organic Microcavity
There is growing interest in using strongly coupled organic microcavities to
tune molecular dynamics, including the electronic and vibrational properties of
molecules. However, very little attention has been paid to the utility of
cavity polaritons as sensors for out-of-equilibrium phenomena, including
thermal excitations. Here, we demonstrate that non-resonant infrared excitation
of an organic microcavity system induces a transient response in the visible
spectral range near the cavity polariton resonances. We show how these optical
response can be understood in terms of ultrafast heating of electrons in the
metal cavity mirror, which modifies the effective refractive index and
subsequently the strong coupling conditions. The temporal dynamics of the
microcavity are strictly determined by carriers in the metal, including the
cooling of electrons via electron-phonon coupling and excitation of propagating
coherent acoustic modes in the lattice. We rule out multiphoton excitation
processes and verify that no real polariton population exists despite their
strong transient features. These results suggest the promise of cavity
polaritons as sensitive probes of non-equilibrium phenomena
Tunable hyperbolic metamaterials utilizing phase change heterostructures
We present a metal-free tunable anisotropic metamaterial where the iso-frequency surface is tuned from elliptical to hyperbolic dispersion by exploiting the metal-insulator phase transition in the correlated material vanadium dioxide (VO2). Using VO2-TiO2 heterostructures, we demonstrate the transition in the effective dielectric constant parallel to the layers to undergo a sign change from positive to negative as the VO2 undergoes the phase transition. The possibility to tune the iso-frequency surface in real time using external perturbations such as temperature, voltage, or optical pulses creates new avenues for controlling light-matter interaction. (C) 2014 AIP Publishing LLC
Beyond Natural Numbers: Negative Number Representation in Parietal Cortex
Unlike natural numbers, negative numbers do not have natural physical referents. How does the brain represent such abstract mathematical concepts? Two competing hypotheses regarding representational systems for negative numbers are a rule-based model, in which symbolic rules are applied to negative numbers to translate them into positive numbers when assessing magnitudes, and an expanded magnitude model, in which negative numbers have a distinct magnitude representation. Using an event-related functional magnetic resonance imaging design, we examined brain responses in 22 adults while they performed magnitude comparisons of negative and positive numbers that were quantitatively near (difference <4) or far apart (difference >6). Reaction times (RTs) for negative numbers were slower than positive numbers, and both showed a distance effect whereby near pairs took longer to compare. A network of parietal, frontal, and occipital regions were differentially engaged by negative numbers. Specifically, compared to positive numbers, negative number processing resulted in greater activation bilaterally in intraparietal sulcus (IPS), middle frontal gyrus, and inferior lateral occipital cortex. Representational similarity analysis revealed that neural responses in the IPS were more differentiated among positive numbers than among negative numbers, and greater differentiation among negative numbers was associated with faster RTs. Our findings indicate that despite negative numbers engaging the IPS more strongly, the underlying neural representation are less distinct than that of positive numbers. We discuss our findings in the context of the two theoretical models of negative number processing and demonstrate how multivariate approaches can provide novel insights into abstract number representation
- …