171 research outputs found
Efficient Generation of Two-Photon Excited Phosphorescence from Molecules in Plasmonic Nanocavities.
Nonlinear molecular interactions with optical fields produce intriguing optical phenomena and applications ranging from color generation to biomedical imaging and sensing. The nonlinear cross-section of dielectric materials is low and therefore for effective utilisation, the optical fields need to be amplified. Here, we demonstrate that two-photon absorption can be enhanced by 108 inside individual plasmonic nanocavities containing emitters sandwiched between a gold nanoparticle and a gold film. This enhancement results from the high field strengths confined in the nanogap, thus enhancing nonlinear interactions with the emitters. We further investigate the parameters that determine the enhancement including the cavity spectral position and excitation wavelength. Moreover, the Purcell effect drastically reduces the emission lifetime from 520 ns to <200 ps, turning inefficient phosphorescent emitters into an ultrafast light source. Our results provide an understanding of enhanced two-photon-excited emission, allowing for optimization of efficient nonlinear light-matter interactions at the nanoscale
Accelerated Molecular Vibrational Decay and Suppressed Electronic Nonlinearities in Plasmonic Cavities through Coherent Raman Scattering
Molecular vibrations and their dynamics are of outstanding importance for
electronic and thermal transport in nanoscale devices as well as for molecular
catalysis. The vibrational dynamics of <100 molecules are studied through
three-colour time-resolved coherent anti-Stokes Raman spectroscopy (trCARS)
using plasmonic nanoantennas. This isolates molecular signals from four-wave
mixing (FWM), while using exceptionally low nanowatt powers to avoid molecular
damage via single-photon lock-in detection. FWM is found to be strongly
suppressed in nm-wide plasmonic gaps compared to plasmonic nanoparticles. The
ultrafast vibrational decay rates of biphenyl-4-thiol molecules are accelerated
ten-fold by a transient rise in local non-equilibrium temperature excited by
enhanced, pulsed optical fields within these plasmonic nanocavities. Separating
the contributions of vibrational population decay and dephasing carefully
explores the vibrational decay channels of these tightly confined molecules.
Such extreme plasmonic enhancement within nanogaps opens up prospects for
measuring single-molecule vibrationally-coupled dynamics and diverse molecular
optomechanics phenomena
Nanoscopy through a plasmonic nanolens.
Plasmonics now delivers sensors capable of detecting single molecules. The emission enhancements and nanometer-scale optical confinement achieved by these metallic nanostructures vastly increase spectroscopic sensitivity, enabling real-time tracking. However, the interaction of light with such nanostructures typically loses all information about the spatial location of molecules within a plasmonic hot spot. Here, we show that ultrathin plasmonic nanogaps support complete mode sets which strongly influence the far-field emission patterns of embedded emitters and allow the reconstruction of dipole positions with 1-nm precision. Emitters in different locations radiate spots, rings, and askew halo images, arising from interference of 2 radiating antenna modes differently coupling light out of the nanogap, highlighting the imaging potential of these plasmonic "crystal balls." Emitters at the center are now found to live indefinitely, because they radiate so rapidly.We acknowledge EPSRC grants EP/N016920/1, EP/L027151/1, and NanoDTC EP/L015978/1. OSO acknowledges support of Rubicon fellowship from the Netherlands Organisation for Scientific Research, and RC thanks support from Trinity College Cambridge
A Search for Variability in Exoplanet Analogues and Low-Gravity Brown Dwarfs
We report the results of a -band survey for photometric variability in a
sample of young, low-gravity objects using the New Technology Telescope (NTT)
and the United Kingdom InfraRed Telescope (UKIRT). Surface gravity is a key
parameter in the atmospheric properties of brown dwarfs and this is the first
large survey that aims to test the gravity dependence of variability
properties. We do a full analysis of the spectral signatures of youth and
assess the group membership probability of each target using membership tools
from the literature. This results in a 30 object sample of young low-gravity
brown dwarfs. Since we are lacking in objects with spectral types later than
L9, we focus our statistical analysis on the L0-L8.5 objects. We find that the
variability occurrence rate of L0-L8.5 low-gravity brown dwarfs in this survey
is . We reanalyse the results of Radigan 2014 and find that
the field dwarfs with spectral types L0-L8.5 have a variability occurrence rate
of . We determine a probability of that the samples are
drawn from different distributions. This is the first quantitative indication
that the low-gravity objects are more likely to be variable than the field
dwarf population. Furthermore, we present follow-up and
observations of the young, planetary-mass variable object PSO 318.5-22 over
three consecutive nights. We find no evidence of phase shifts between the
and bands and find higher amplitudes. We use the lightcurves
to measure a rotational period of hr for PSO 318.5-22.Comment: accepted for publication in MNRA
The Hawaii Infrared Parallax Program. VI. The Fundamental Properties of 1000+ Ultracool Dwarfs and Planetary-mass Objects Using Optical to Mid-IR SEDs and Comparison to BT-Settl and ATMO 2020 Model Atmospheres
We derive the bolometric luminosities () of 865 field-age
and 189 young ultracool dwarfs (spectral types M6-T9, including 40 new
discoveries presented here) by directly integrating flux-calibrated optical to
mid-IR spectral energy distributions (SEDs). The SEDs consist of low-resolution
( 150) near-IR (0.8-2.5 m) spectra (including new spectra for 97
objects), optical photometry from the Pan-STARRS1 survey, and mid-IR photometry
from the CatWISE2020 survey and Spitzer/IRAC. Our
calculations benefit from recent advances in parallaxes from Gaia, Spitzer, and
UKIRT, as well as new parallaxes for 19 objects from CFHT and Pan-STARRS1
presented here. Coupling our measurements with a new uniform
age analysis for all objects, we estimate substellar masses, radii, surface
gravities, and effective temperatures () using evolutionary
models. We construct empirical relationships for and
as functions of spectral type and absolute magnitude,
determine bolometric corrections in optical and infrared bandpasses, and study
the correlation between evolutionary model-derived surface gravities and
near-IR gravity classes. Our sample enables a detailed characterization of
BT-Settl and ATMO 2020 atmospheric model systematics as a function of spectral
type and position in the near-IR color-magnitude diagram. We find the greatest
discrepancies between atmospheric and evolutionary model-derived
(up to 800 K) and radii (up to 2.0 ) at
the M/L transition boundary. With 1054 objects, this work constitutes the
largest sample to date of ultracool dwarfs with determinations of their
fundamental parameters.Comment: Resubmitted to The Astrophysical Journal (ApJ) after a positive
referee report. 51 pages, 29 figures, 7 tables. Data presented in this work:
https://doi.org/10.5281/zenodo.8315643. Scripts associated with methods:
https://github.com/cosmicoder/HIPPVI-Cod
Room-Temperature Optical Picocavities below 1 nm3 Accessing Single-Atom Geometries.
Reproducible confinement of light on the nanoscale is essential for the ability to observe and control chemical reactions at the single-molecule level. Here we reliably form millions of identical nanocavities and show that the light can be further focused down to the subnanometer scale via the creation of picocavities, single-adatom protrusions with angstrom-level resolution. For the first time, we stabilize and analyze these cavities at room temperatures through high-speed surface-enhanced Raman spectroscopy on specifically selected molecular components, collecting and analyzing more than 2 million spectra. Data obtained on these picocavities allows us to deduce structural information on the nanoscale, showing that thiol binding to gold destabilizes the metal surface to optical irradiation. Nitrile moieties are found to stabilize picocavities by 10-fold against their disappearance, typically surviving for >1 s. Such constructs demonstrate the accessibility of single-molecule chemistry under ambient conditions
Resolving sub-angstrom ambient motion through reconstruction from vibrational spectra.
Metal/organic-molecule interactions underpin many key chemistries but occur on sub-nm scales where nanoscale visualisation techniques tend to average over heterogeneous distributions. Single molecule imaging techniques at the atomic scale have found it challenging to track chemical behaviour under ambient conditions. Surface-enhanced Raman spectroscopy can optically monitor the vibrations of single molecules but understanding is limited by the complexity of spectra and mismatch between theory and experiment. We demonstrate that spectra from an optically generated metallic adatom near a molecule of interest can be inverted into dynamic sub-Å metal-molecule interactions using a comprehensive model, revealing anomalous diffusion of a single atom. Transient metal-organic coordination bonds chemically perturb molecular functional groups > 10 bonds away. With continuous improvements in computational methods for modelling large and complex molecular systems, this technique will become increasingly applicable to accurately tracking more complex chemistries.We acknowledge financial support from EPSRC grant EP/G060649/1, EP/L027151/1, EP/G037221/1, EP/R013012/1, EPSRC NanoDTC, and EU grant THOR 829067 and ERC starting grant BioNet 757850.
B.d.N. acknowledges support from the Leverhulme Trust and Isaac Newton Trust. We acknowledge use of the Rosalind computing facility at King’s College London. We are grateful to the UK Materials and Molecular Modelling Hub for computational resources, which is partially funded by EPSRC 397 (EP/P020194/1)
Giant optomechanical spring effect in plasmonic nano- and picocavities probed by surface-enhanced Raman scattering
Molecular vibrations couple to visible light only weakly, have small mutual
interactions, and hence are often ignored for non-linear optics. Here we show
the extreme confinement provided by plasmonic nano- and pico-cavities can
sufficiently enhance optomechanical coupling so that intense laser illumination
drastically softens the molecular bonds. This optomechanical pumping regime
produces strong distortions of the Raman vibrational spectrum related to giant
vibrational frequency shifts from an optical spring effect which is
hundred-fold larger than in traditional cavities. The theoretical simulations
accounting for the multimodal nanocavity response and near-field-induced
collective phonon interactions are consistent with the experimentally-observed
non-linear behavior exhibited in the Raman spectra of nanoparticle-on-mirror
constructs illuminated by ultrafast laser pulses. Further, we show indications
that plasmonic picocavities allow us to access the optical spring effect in
single molecules with continuous illumination. Driving the collective phonon in
the nanocavity paves the way to control reversible bond softening, as well as
irreversible chemistry
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