KEYWORDS: Luminescence, Spectroscopy, Process control, Electromagnetism, Metamaterials, Composites, Biophysics, Quantum optics, Nanostructures, Resonance energy transfer
The control of photoluminescence processes, via the design of composite materials with engineered electromagnetic properties, is of great interest for the development of many application areas ranging from biophysics to quantum optical technologies. Approaches providing broadband enhancements of emission, not limited to resonant nanostructures, are particularly advantageous. We discuss how various photoluminescence processes, including conventional and dipolar-forbidden spontaneous emission, as well as Förster resonance energy transfer, are altered nearby and inside plasmonic hyperbolic metamaterials. They provide a flexible platform for engineering broadband Purcell enhancements due to their peculiar electromagnetic mode structure controlled by the nonlocal response of the metamaterial.
Broadband coherent anti-Stokes Raman scattering (B-CARS) in a hybrid 2-color/3-color excitation regime has been shown to be a photon-efficient method of generating CARS across the entire vibrational region of interest (fingerprint and C-H stretch). We extend here our spectral interferometric polarized CARS (SIP-CARS) approach to a hybrid 2-color/3-color excitation regime, demonstrate that the power/polarization dependence for both 2- color and 3-color excitation agrees with theory, explore the application of SIP-CARS for the special case of anisotropic, birefringent materials (using collagen) and present in vivo lipid imaging of different mutations of the nematode Caenorhabditis elegans with modified lipid distributions. The extension of SIP-CARS to a hybrid excitation scheme makes possible a fast, all-optical and quantitative method for Raman imaging.
The control of spontaneous emission via the design of composite materials with engineered electromagnetic properties is important for the development of new faster and brighter sources of illumination with applications ranging from biophysics to quantum optical technologies. In particular, the fabrication of nanostructures leading to broadband enhancement of emission is of great interest. Hyperbolic plasmonic metamaterials have recently emerged as a very flexible platform for this purpose as they provide a high local density of electromagnetic states available for the radiative relaxation of emitters. This is due to their peculiar mode structure governed by both the structural nonlocal response and the dispersion properties.
Here, we investigate the modification of the spontaneous emission rate and intensity enhancement of emitters located inside a nanorod-based hyperbolic metamaterial. We experimentally show the coupling of the radiated emission to the waveguided mode of a planar hyperbolic metamaterial with finite thickness. The emitters located inside this planar hyperbolic metamaterial waveguide exhibit an almost 50-fold reduction of the decay rate and 3-fold intensity enhancement of the fluorescence coupled to the mode. We also discuss the effect of nanostructuring the nanorod-based metamaterial on the spontaneous emission properties of emitters located inside it, where suitable designs can lead to further enhancement of the radiative rate and improved light extraction of the emission coupled to the high-wavevector modes of the metamaterial to the far-field, useful for the development of efficient and fast free-space light-emitting devices.
Fluorescence-based processes are strongly modified by the electromagnetic environment in which the emitters are placed. Hence, the design of nanostructured materials with appropriate electromagnetic properties opens up a new route in the control of, for instance, the spontaneous rate of emission or the energy transfer rate in donor-acceptor pairs. In particular, hyperbolic plasmonic metamaterials have emerged as a very flexible and powerful platform for these applications as they provide a high local density of electromagnetic states due to their peculiar mode structure which is governed by both the structural nonlocal response and the dispersion properties. Here, we will discuss an experimental and theoretical study of the influence of a hyperbolic metamaterial comprised of an array of gold nanorods on the radiative properties of quantum emitters and the energy-transfer processes between them.
KEYWORDS: Metamaterials, Plasmonics, Fluorescence resonance energy transfer, Electromagnetism, Resonance energy transfer, Luminescence, Energy transfer, Molecules, Molecular energy transfer, Biosensing
The control of the Förster resonance energy transfer (FRET) rate between molecules has recently received a lot of interest, opening opportunities in the development of sources of incoherent illumination, photovoltaics and biosensing applications. The design of nanostructured materials with appropriate electromagnetic properties, particularly with the engineered local density of electromagnetic states (LDOS), allows the enhancement of the spontaneous emission rate of emitters in their vicinity. However, the question of the influence of the LDOS on the energy transfer rate between emitters remains controversial. To date, several contradicting theoretical and experimental studies involving emitters on metallic surfaces and plasmonic metamaterials as well as in optical cavities and plasmonic antennas have been reported. In this work we study the influence of the LDOS on the energy transfer between donor-acceptor pairs placed inside the anisotropic metamaterial. The study of the emission kinetics of both the donor and the acceptor allow us to experimentally compare FRET efficiencies in different electromagnetic environments including dielectric and plasmonic substrates as well as metamaterials.
Full analysis of signal contributions in broadband CARS shows that the broadband fields give rise to significant resonant spectral artefacts, in addition to the non-resonant background (NRB). These modify vibrational line amplitudes such that broadband CARS does not give accurate concentration information. We have used a spectral interferometry method (Spectral Interferometric Polarisation CARS) to examine the resonant background contributions directly, and find that with certain conditions on the Stokes spectrum resonant artefacts can be avoided in the C-H stretch spectral region, but not in the fingerprint region. A preprocessing step, however, permits fully quantitative B-CARS measurements.
In vivo lipid saturation maps of microscopic nematodes (Caenorhabditis elegans) have been produced using our novel Spectral Interferometric Polarisation Coherent anti-Stokes Raman Scattering (SIP-CARS) imaging technique. This technique employs simple passive polarisation optics and a balanced homodyne detection scheme to exploit symmetries in the CARS polarisation response resulting in the complete cancellation of the non-resonant background (NRB) and real component of the CARS signal (with no prior or post assumptions as regards to their form). The remaining imaginary component of the CARS response is linear with analyte concentration and directly relatable to the spontaneous Raman spectrum [1]. Furthermore, the resonant CARS signal is interferometrically amplified by the non-resonant response, a necessity for rapid imaging at biologically relevant powers [2].
This technique permits acquisition of a broad NRB-free spectrum, in excess of 1800cm-1, in a single exposure at each pixel. This allows simultaneous determination of lipid droplet saturation, from the fingerprint region, and lipid order, from the C-H stretch region from which maps can be readily constructed. Additionally exploiting the dispersive nature of our signal collection two-photon autofluorescence can be isolated and images subsequently produced.
We have successfully applied this technique to identify differences in lipid saturation distributions in selective C. elegans mutants and demonstrated that the technique is sufficiently sensitive to detect the effects of lipid metabolism altering drugs on wild type C. elegans.
[1] Littleton et al, Phys Rev Lett, 111, 103902 (2013)
[2] Parekh et al, Biophys J, 99, 2695–2704 (2010)
We have developed a new method for quantitative broadband CARS spectral imaging, which uses passive polarisation optics combined with spectrally-resolved balanced homodyne detection to remove the non-resonant background (NRB) in a single exposure. Independent measurement of the Stokes spectrum is not required for NRB removal, and the resulting spectra are amplified by the non-resonant response, vary linearly with concentration, and can be directly related to polarized spontaneous Raman spectra. The technique has relaxed requirements on spectral phase and instrument stability, is suitable for any laser system capable of generating CARS, and has been successfully applied to rapid hyperspectral Raman imaging.
The Raman spectrum of a fluorescent chromophore typically has many spectral features, which differ markedly between
dyes even if their electronic spectra are similar. This high information content makes it possible to distinguish
biomarkers based on their Raman spectra. Coherent anti-Stokes Raman scattering may therefore allow for the
simultaneous measurement of more biomarkers than is possible with fluorescent imaging, while avoiding bleaching and
sample autofluorescence. We have built a broadband CARS microspectrometer to demonstrate the principle of CARS
multiplexing and investigate the potential to apply the system to studies of biological samples.
Current high-content screening (HCS) techniques involve the analysis of cellular assays using high-resolution
imaging combined with sophisticated algorithms for automated image analysis. Commercially available platforms
are invariably highly specialised and expensive. Here we present a novel assay utilising changes in fluorescence
lifetime in the vicinity of a rough Au film. A mammary carcinoma cell line was created expressing EGFP in the
membrane, and cells were plated onto multi-well slides covered with a 30 nm Au film. FLIM images show a large
reduction in lifetime for membrane-bound GFP in close proximity to the Au surface. Addition of a suitable ligand
leads to internalization of the GFP with a corresponding increase in lifetime. The degree of internalization can
be very quickly and easily checked using standard lifetime analysis techniques, with no need for image analysis.
We demonstrate the method by comparing the efficacies of two small molecule inhibitors interfering with the
internalization process.
We report on the resonant coupling between localized surface plasmon resonances (LSPRs) in nanostructured Ag
films, and an adsorbed monolayer of Rhodamine 6G dye. Hybridization of the plasmons and molecular excitons
creates new coupled polaritonic modes, which have been tuned by varying the LSPR wavelength. The resulting
polariton dispersion curve shows an anticrossing behavior which is very well fit by a simple coupled-oscillator
Hamiltonian, giving a giant Rabi-splitting energy of ~400 meV. The strength of this coupling is shown to be
proportional to the square root of the molecular density. The Raman spectra of R6G on these films show an
enhancement of many orders of magnitude due to surface enhanced scattering mechanisms; we find a maximum
signal when a polariton mode lies in the middle of the Stokes shifted emission band.
We have investigated the effects of tuning the localized surface plasmon resonance (LSPR) of a silver film on
the extinction spectrum, Raman signal, and fluorescence intensity from nearby fluorophores. We observe the
formation of hybridized modes due to strong coupling between the plasmonic and molecular excitations. The
Raman spectra of R6G on these films show an enhancement of many orders of magnitude due to surface enhanced
scattering mechanisms; we find a maximum signal when a hybridized mode lies in the middle of the Stokes shifted
emission band. The effect of fluorophore-film separation on fluorescence intensity has been investigated using an
alumina spacer layer. An enhancement in detected signal of up to 18× is observed relative to that detected from
a bare Ag film. Overall, we observe a greater than 40× increase in detected intensity from the alumina-coated
Ag film relative to fluorophores on glass; this is a result of increased collection efficiency and a greater radiative
emission rate.
Highly ordered periodic arrays of silver nanoparticles have been fabricated which exhibit surface plasmon resonances
in the visible spectrum. We demonstrate the ability of these structures to alter the fluorescence properties
of vicinal dye molecules by providing an additional radiative decay channel. Using fluorescence lifetime imaging
microscopy, we have created high resolution spatial maps of the molecular lifetime components; these show
an order of magnitude increase in decay rate from a localized volume around the nanoparticles, resulting in a
commensurate enhancement in the fluorescence emission intensity. Spatial maps of the Raman scattering signal
from molecules on the nanoparticles shows an enhancement of more than 5 orders of magnitude.
We investigate the mechanisms for fluorescence enhancement and energy transfer near a gold tip in apertureless
scanning near-field optical microscopy (ASNOM) and provide a demonstration of sub-diffraction tip-enhanced
fluorescence imaging. We have imaged the fluorescence from a single quantum dot cluster using ASNOM and find that
when a sharp gold tip is brought within a few nanometres from the sample surface, the resulting enhancement in
quantum dot fluorescence in the vicinity of the tip leads to a resolution of about 60 nm. We determine this enhancement
of the fluorescence to be about four-fold in magnitude, which is consistent with the value calculated with a simple
quasistatic model. Using this model we show that the observed enhancement of fluorescence results from a competition
between enhancement and quenching, dependent on a range of experimental parameters. We also demonstrate that
optical signals measured in ASNOM under ambient conditions are found to be affected significantly by the thin water
layer absorbed on the surface under investigation.
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