517 research outputs found
Femtosecond phase-resolved microscopy of plasmon dynamics in individual gold nanospheres
The selective optical detection of individual metallic nanoparticles (NPs)
with high spatial and temporal resolution is a challenging endeavour, yet is
key to the understanding of their optical response and their exploitation in
applications from miniaturised optoelectronics and sensors to medical
diagnostics and therapeutics. However, only few reports on ultrafast pump-probe
spectroscopy on single small metallic NPs are available to date. Here, we
demonstrate a novel phase-sensitive four-wave mixing (FWM) microscopy in
heterodyne detection to resolve for the first time the ultrafast changes of
real and imaginary part of the dielectric function of single small (<40nm)
spherical gold NPs. The results are quantitatively described via the transient
electron temperature and density in gold considering both intraband and
interband transitions at the surface plasmon resonance. This novel microscopy
technique enables background-free detection of the complex susceptibility
change even in highly scattering environments and can be readily applied to any
metal nanostructure
Polarization-resolved extinction and scattering cross-section of individual gold nanoparticles measured by wide-field microscopy on a large ensemble
We report a simple, rapid, and quantitative wide-field technique to measure
the optical extinction and scattering
cross-section of single nanoparticles using wide-field microscopy enabling
simultaneous acquisition of hundreds of nanoparticles for statistical analysis.
As a proof of principle, we measured nominally spherical gold nanoparticles of
40\,nm and 100\,nm diameter and found mean values and standard deviations of
and consistent with previous literature.
Switching from unpolarized to linearly polarized excitation, we measured
as a function of the polarization direction, and used it to
characterize the asphericity of the nanoparticles. The method can be
implemented cost-effectively on any conventional wide-field microscope and is
applicable to any nanoparticles
Ultrafast exciton dephasing in PbS colloidal quantum dots
In this work, we have measured the ground state excitonic dephasing in PbS QDs of sizes from 3.7nm to 5.7nm diameter in the temperature range from 5K to 100K by transient degenerate four-wave mixing (FWM) using 100fs pulses. A combination of heterodyne and k-selection detection was implemented to increase sensitivity and enable 4 orders of magnitude dynamic range in the FWM field detection
Wide-field imaging of single nanoparticle extinction with sub-nm2 sensitivity
We report a highly sensitive wide-�eld imaging technique for quantitative measurement of the
optical extinction cross-section �ext of single nanoparticles. The technique is simple and high-speed,
and enables simultaneous acquisition of hundreds of nanoparticles for statistical analysis. Using
rapid referencing, fast acquisition, and a deconvolution analysis, a shot-noise limited sensitivity down
to 0.4nm2 is achieved. Measurements on a set of individual gold nanoparticles of 5nm diameter
using this method yield �ext = (10:0 � 3:1)nm2, consistent with theoretical expectations, and well
above the background
uctuations of 0.9nm2
Quantitative coherent Raman scattering microscopy for bioimaging
Optical microscopy is an indispensable tool that is driving progress in cell biology and is still the only practical means of obtaining spatial and temporal resolution within living cells and tissues. Coherent Raman Scattering (CRS) microscopy has attracted increasing attention as a powerful multiphoton microscopy technique which overcomes the need for fluorescent labelling and yet retains biomolecular specificity and intrinsic 3D resolution [1] . Over the past 15 years, our laboratory has developed and demonstrated a range of label-free CRS microscope set-ups featuring innovative excitation/detection schemes
Sparse sampling for fast hyperspectral coherent anti-Stokes Raman scattering imaging
We demonstrate a method to increase the acquisition speed in coherent anti-Stokes Raman scattering (CARS) hyperspectral imaging while retaining the relevant spectral information. The method first determines the important spectral components of a sample from a hyperspectral image over a small number of spatial points but a large number of spectral points covering the accessible spectral range and sampling the instrument spectral resolution at the Nyquist limit. From these components we determine a small set of frequencies needed to retrieve the weights of the components with minimum error for a given measurement noise. Hyperspectral images with a large number of spatial points, for example covering a large spatial region, are then measured at this small set of frequencies, and a reconstruction algorithm is applied to generate the full spectral range and resolution. The resulting spectra are suited to retrieve from the CARS intensity the CARS susceptibility which is linear in the concentration, and apply unsupervised quantitative analysis methods such as FSC3 [1]. We demonstrate the method on CARS hyperspectral images of human osteosarcoma U2OS cell, with a reduction in the acquisition time by a factor of 25. This method is suited also for other coherent vibrational microscopy techniques such as stimulated Raman scattering, and in general for hyperspectral imaging techniques with sequential spectral acquisition
Simultaneous microscopic imaging of thickness and refractive index of thin layers by heterodyne interferometric reflectometry (HiRef)
The detection of spatial or temporal variations in very thin samples has
important applications in the biological sciences. For example, cellular
membranes exhibit changes in lipid composition and order, which in turn
modulate their function in space and time. Simultaneous measurement of
thickness and refractive index would be one way to observe these variations,
yet doing it noninvasively remains an elusive goal. Here we present a
microscopic-imaging technique to simultaneously measure the thickness and
refractive index of thin layers in a spatially resolved manner using
reflectometry. The heterodyne-detected interference between a light field
reflected by the sample and a reference field allows measurement of the
amplitude and phase of the reflected field and thus determination of the
complex reflection coefficient. Comparing the results with the simulated
reflection of a thin layer under coherent illumination of high numerical
aperture by the microscope objective, the refractive index and thickness of the
layer can be determined. We present results on a layer of polyvinylacetate
(PVA) with a thickness of approximately 80~nm. These results have a precision
better than 10\% in the thickness and better than 1\% in the refractive index
and are consistent within error with measurements by quantitative differential
interference contrast (qDIC) and literature values. We discuss the significance
of these results, and the possibility of performing accurate measurements on
nanometric layers. Notably, the shot-noise limit of the technique is below
0.5~nm in thickness and 0.0005 in refractive index for millisecond measurement
times
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