We present a novel approach to create spinodal-like structures appropriately modulating the instability of the solid state dewetting: many materials, indeed, featuring anomalous suppression of density fluctuations over large length scales are emerging systems known as disordered hyperuniform. The underlying hidden order renders them appealing for several applications, as light management and topologically protected electronic states. These applications require scalable fabrication, which is hard to achieve with available top-down approaches. These spinodal materials are used by a hybrid top-down/bottom-up approach based on sol-gel dip-coating and nano-imprint lithography for the faithful reproduction of the disordered metasurfaces in metal oxides (SiO2 and TiO2).
Non-linear optical microscopy proves to be an indispensable tool in natural sciences and becomes more and more attractive for clinical applications. Coherent Raman scattering, for instance, has the potential to become an in-vivo fast label-free histology tool as its chemical selectivity provides quantitative information on lipids and proteins locations and concentrations in tissues. Along with these techniques, second-harmonic generation of collagen and 2-photon excitation fluorescence broaden even more the non-linear imaging ability as collagen fibers represent an important role in human body construction. Whilst 2-photon excitation fluorescence allows to study auto-fluorescence (ex. NADH and NADHP molecules), and to excite a vast range of chromophores. However, absorption and scattering limit significantly the nonlinear imaging depth into tissues. As a solution, we offer a flexible, compact, and multimodal nonlinear endoscope (2.2 mm outer diameter, 35 mm rigid length) based on a resonantly piezo scanned hollow-core negative curvature double-clad fiber. The fiber design allows distortion-less, background-free delivery of femtosecond excitation pulses and the back-collection of nonlinear signals through the same fiber. The double-cladding of this fiber attends 10^5μm of silica collection surface which allows for a 4-fold collection improvement compared to previously used Kagomé hollow core fibers. Having a good control on the resonantly scanning fiber the endoscope can perform nonlinear imaging up to 8 frames per second over a field of view of 400μm. We demonstrate 2photon, SHG and CARS imaging in ex vivo gastric human tissue samples and in-vivo 2-photon fluorescence imaging of GFP-labeled neurons in mouse brain.
Metamaterials are artificial structures consisting of periodic sub-wavelength elements, they produce a particular electromagnetic response which often cannot be obtained from conventional media. One class of metamaterials called hyperbolic metamaterials (HMM) is widely studied nowadays due to their various applications such as subwavelength imaging and local density of states engineering.1 In this work, HMM consisting of alternating gold, copper, and niobium pentoxide layers was designed and fabricated with plasma assisted electron beam deposition technique. For the accurate description of this material opto-geometrical parameters of thin layers composing it were determined separately using reflection and transmission measurements. Afterward, the produced material was structured using focused ion beam milling and its influence on the Alexa Fluor 647 biomolecules spontaneous emission was studied experimentally with fluorescence correlation spectroscopy and time-correlated single photon counting techniques. Combining these techniques enable the measurement of the fluorescence lifetime reduction together with the brightness enhancement per molecule. The same measurements were previously done for the structured gold films3 which let us compare the results. HMM has shown significant emission enhancement compatible with the one produced by a single gold aperture which is known for its good plasmonic properties.
Minimizing the luminescence lifetime while maintaining a high emission quantum yield is paramount in optimizing the excitation cross-section, radiative decay rate, and brightness of quantum solid-state light sources, particularly at room temperature, where non-radiative processes can dominate. In that sense, plasmon-based optical nanoantennas can feature strongly enhanced and confined optical fields to enhance excitation probabilities and fluorescence decay rates. Their morphology and their coupling to luminescent emitters can be engineered to minimize non-radiative losses and optimize their overall brightness.
We demonstrate here that short DNA strands are an excellent template to introduce individual fluorescent molecules in dimers of gold nanoparticles in order to achieve single photon emission with decay rates enhanced by more than two orders of magnitude (M. P. Busson et al, Nat. Commun. 3, 962 (2012)). The coupling between single dye molecules and plasmonic gap antennas can be further optimized by selecting nanostructures where the transition dipole of the emitter is aligned with the gold particle dimer axis (M. P. Busson & S. Bidault, Nano Lett. 14, 284 (2014)). Furthermore, by using dimers of 60 and 80 nm diameter gold particles, we demonstrate the assembly of nanostructures exhibiting single-photon emission with lifetimes that can fall below 10 ps and typical quantum yields in a 45−70% range (S. Bidault et al, ACS Nano 10, 4806 (2016)). These data are in excellent agreement with theoretical calculations and demonstrate that millions of bright fluorescent nanostructures, with radiative lifetimes below 100 ps, can be produced in parallel.
Resolving the various interactions of lipids and proteins in the plasma membrane of living cells with high spatiotemporal resolution is of upmost interest [1]. Here we introduce an innovative design of plasmonic nanogap antennas to monitor single-molecule events on model biological membranes at physiological relevant concentrations by means of fluorescence correlation spectroscopy. Our design involves the fabrication of in-plane plasmonic nanogap antennas arrays embedded in nanometric-size boxes to provide full surface accessibility of the hotspot-confined region. Using these antennas we recently reported fluorescence enhancement factors of 104-105 times on individual molecules diffusing in solution, together with nanoscale detection volumes in the zeptoliter range [2]. In principle, the planarity of these antennas should enable similar studies on biological membranes without unwanted membrane curvature effects.
To show their applicability, we recorded the diffusion of individual molecules inserted in multi-component lipid bilayers as a simple mimetic system that recapitulates some of the most important features of cell membranes. We prepared membranes of different compositions: saturated phospholipids, sphingolipids and cholesterol and used antennas of different gap sizes (10-45 nm). The diffusion of individual molecules on membranes consisting of phospholipids and/or in a mixture with sphingolipids resulted Brownian, confirming homogenous lipid distribution. Interestingly, the strong confinement of antennas revealed the formation of transient (<1ms lifetime) nanoscopic domains of ~11 nm in size upon cholesterol addition. These results indicate that in-plane antennas represent a highly promising non-invasive tool to investigate the nanoscale dynamic organization of biological membranes and its impact in biological function.
References:
[1] D. Lingwood, K. Simons, Science 327, 46 (2010).
[2] V. Flauraud et al, submitted.
The development of nonlinear fiber-endoscopes capable of imaging deeper in tissues and accessing internal organs represents a very attractive perspective for application of nonlinear optical microscopes to in-vivo research and diagnostics. The transmission of ultra-short laser pulses within a fiber is a critical issue in the development of such endoscopes. For instance, self-phase modulation (SPM), four-wave mixing (FWM) and Raman scattering occurring in conventional fibers severely affect transmitted pulses profiles in the time and frequency domains. Hollow-core (HC) fibers bring a solution to the problem, since propagation of the pulses in the air core limits nonlinear interactions. We employ here a novel double clad Kagomé-lattice HC fiber for the delivery of ultrafast pulses across a large spectral window (~400nm) with no pulse distortion. The epi-collection of the signal generated at the sample is efficiently performed with a specially designed outer multimode cladding. The fiber is incorporated in a prototype endoscope using a four-quartered piezo-electric tube to scan the laser beam on the sample. The low numerical aperture of the hollow-core (0.02) is efficiently increased by means of a dielectric microsphere attached to the fiber face. This results in tight focusing (~1 micron) of the beam at the HC fiber output. Resonant scanning of the fiber tip allows imaging over a field of 300 microns using low driving voltages. High-resolution images with different contrast mechanisms, such as SHG and TPEF, acquired with the prototype endoscope illustrate the potential of these fibers for nonlinear imaging in regions otherwise inaccessible to conventional optical microscopes.
Single nanoparticles made of noble metals are strongly appealing to develop practical applications to detect fluorescent molecules in solution. Here, we detail the use of a single gold nanoparticle of 100 nm diameter to enhance the detection of single Alex Fluor 647 fluorescent molecules at high concentrations of several micromolar. We discuss the implementation of fluorescence correlation spectroscopy, and provide a new method to reliably extract the enhanced fluorescence signal stemming from the nanoparticle near-field from the background generated in the confocal volume. The applicability of our method is checked by reporting the invariance of the single molecule results as function of the molecular concentration, and the experimental data is found in good agreement with numerical simulations.
Microspheres made of high refractive index melamine resin are shown to enhance the fluorescence from single
molecules in solution by seven-fold, and simultaneously reduce the observation volume by thirteen-fold, as compared
to state-of-the-art confocal microscopy. This fluorescence enhancement is demonstrated to dramatically
increase the signal-to-noise ratio in fluorescence correlation spectroscopy and reduce the experiment integration
time by fifty-fold. We also provide the first description of dual-color fluorescence cross-correlation spectroscopy
(FCCS) enhanced by a dielectric microsphere, and report comparable enhancement factors as for the single color
case.
KEYWORDS: Luminescence, Molecules, Quantum efficiency, Gold, Aluminum, Fluorescence correlation spectroscopy, Signal detection, Metals, Fluorescence spectroscopy, Signal to noise ratio
Nanoapertures milled in opaque metallic films offer a simple and robust photonic tool to significantly enhance
the fluorescence of single molecules. We provide a detailed physical characterization of this phenomenon
for apertures milled in gold and aluminum, and discuss its application to biophotonics. For the first time,
the most general figures are provided to predict the awaited enhancement factors for almost every kind of
fluorescent molecule. This knowledge is essential to discuss the ability to detect low-quantum yield species.
We also report the first demonstration of single metal nanoapertures to perform DNA hybridization sensing,
and measure similar enhancement factors as for experiments on diffusing molecules.
We discuss the compound set of two dielectric microspheres to confine light in a three dimensional region of dimensions
on the order of the wavelength when the spheres are illuminated by a plane wave. This simple configuration enables the
reduction of the longitudinal dimension of so called photonic jets, together with a strong focusing effect. The beam
shaped in that way is suitable for applications requiring high longitudinal resolutions and/or strong peak intensities.
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