Spontaneous Raman scattering is a reliable technique for fast identification of single bacterial cells, when spectra are acquired in laboratory conditions where bacteria growth and state are controlled. We have developed a multi-modal system combining Raman spectroscopy and darkfield imaging, aiming at analysing environmental samples, typically in the field context of biological pathogens detection. Such samples are heterogeneous, both in terms of phenotype content and environmental matrix, even after a preliminary purification step. In this paper, we report a study on the identification of Bacillus Thuriengensis (BT) mimicing pathogen bacteria, embedded in a real-world matrix: a sample of surface water enriched with environmental bacterial species. The purpose is to evaluate both the detection limit of aging BT over time and the false alarm rate, in the conditions of our experiment.
We investigate the quadratic nonlinear optical response from metallic gold nanoparticles homogeneously dispersed in a
medium or deposited on glass substrates. The nanoparticles are prepared by the wet chemistry method in solution used
afterwards. The diameter of the gold nanoparticle is 80 nm. In a homogeneous medium, hyper Rayleigh scattering,
effectively incoherent second harmonic generation is used to determine the origin of the response. It is shown that for 80
nm diameter gold nanoparticles, the overall response stems from the deviation of the shape of the nanoparticle from that
of a perfect sphere and from retardation effects with a similar weight. The latter retardation effects occur because the
diameter of the nanoparticle is no longer vanishing before the wavelength of the incoming electromagnetic field. For
deposited nanoparticles, the sample is illuminated through the transparent glass slide and the light at the harmonic
frequency, produced through the second harmonic generation phenomenon, is observed in the retro-reflection. From the
collected SHG images, it can be unambiguously concluded that the origin of the nonlinearity in 80 nm diameter gold
nanoparticles stems from the substrate influence normal to the interface. It can also be concluded that the gold
nanoparticles can be used to map out the electromagnetic field in the focal volume.
The nonlinear optical modulation of a light beam (or Kerr effect) by a single gold metallic nanoparticle has been
obtained by the P-Scan method. The 80 nm diameter gold nanoparticles were spread on a glass substrate and investigated
with a tightly focused femtosecond laser beam at the fundamental wavelength of 840 nm for different incident
intensities. The retro-reflected intensity was recorded in the far field at the same wavelength of 840 nm. Clear deviations
from the linear regime were observed and attributed to both nonlinear refraction and nonlinear absorption of the gold
nanoparticles. Besides, the second harmonic signal was also observed for these nanoparticles allowing for the display of
the linear, quadratic and cubic susceptibility tensors χ<sup>(1)</sup>, χ<sup>(2)</sup> and χ<sup>(3)</sup> of the same substrate where are dispersed gold
Hyper Rayleigh Scattering (HRS) is used to determine the absolute first hyperpolarizability of gold nanorods with an
aspect ratio of 2.2 and 2.7. Two different long axis lengths are used, namely 25.5 nm and 64 nm. This allows for a
discussion of the size effect for these centrosymmetric nanoparticles. A comparison of the first hyperpolarizabilities
obtained with that of spherical nanoparticles, also centrosymmetric particles, is then made to discuss the role of the shape
of these particles on the origin of the response. For the smallest nanorods, a strong hyperpolarizability normalized per
atom is determined underlining the role of the interface and the shape in determining such a large absolute value. For the
larger nanorods, the first hyperpolarizability per atom is smaller than that of the smaller nanorods but remains larger than
the one obtained for gold nanospheres with a similar volume indicating that the shape and the surface response still
continue to play a major role. These results are in agreement with the multipolar theory for the first hyperpolarizability of
centrosymmetric nanoparticles but show that for nanorods, the surface regime pertains over a longer size range.
The Second Harmonic Generation (SHG) response from Tyrosine-containing peptides at the air-water interface is presented. First, the quadratic hyperpolarizability of the aromatic amino acid Tyrosine obtained by Hyper Rayleigh Scattering is reported, demonstrating its potentiality as an endogenous molecular probe for SHG studies. Then, the single Tyrosine antimicrobial peptide Mycosubtilin is monitored at the air-water interface and compared to another peptide, Surfactin, lacking a Tyrosine residue. Adsorption kinetics and polarization analysis of the SHG intensity for the peptide monolayers clearly demonstrate that the SHG response from Mycosubtilin arises from Tyrosine. Besides, it confirms that indeed Tyrosine can be targeted as an endogenous molecular probe.
The first hyperpolarizability of the aromatic amino acid Tyrosine has been obtained by Hyper Rayleigh Scattering (HRS) in aqueous environment and under non resonant conditions. The rather large value determined is in agreement with previous HRS studies and confirms that Tyrosine can be targeted as an endogenous molecular probe for the second harmonic studies of biological molecules. The same experiment was also performed in an aqueous environment containing 20 nm diameter gold metallic nanoparticles. The resulting first hyperpolarizability measured with the fundamental wavelengths of 815 nm and 1050 nm in these conditions for Tyrosine is reduced as compared to that obtained in absence of metallic nanoparticles.