In Brillouin scattering imaging, rejection of background noise due to elastic scattering and reflections from optical components is crucial. This is because Brillouin signal is weak, and the signal frequency shift compared with source laser line is very small. Therefore the line of interest is very easy to be contaminated. Whereas physical blocking of undesired component in a dispersed spectrum is used, to filter out background optically provides better stability. Conventional optical filter techniques, such as dielectric-stack filters, holographic volume filters, Lyot fitlers etc. normally have a stopband-width (bandstop filter) or edge-width (edge filter) ranging from a few nanometers to tens of nanometers despite high rejection rate. They cannot be implemented in Brillouin imaging due to the small wavelength shift (< 1 pm).
We report a Fabry-Perot etalon-based notch filter for background cleaning in Brillouin imaging. The notch filer takes advantage of multiple reflections of the light beam with a Fabry-Perot etalon to achieve high rejection with narrow bandwidth. The theoretical rejection rate is multiple time of the rejection of one reflection of the etalon. We demonstrated a laser line suppression of > 40 dB while with < 40% power loss with experiments. Width of the stopband at -30 dB rejection level is ~ 1 GHz. This method is not wavelength specific. One etalon may be implanted to a wide spectrum of laser wavelengths. Furthermore, it does not require heating as gaseous notch filters. Our method can also be implemented to Raman scattering, fluorescent imaging and other imaging techniques in which line of interest is close to the laser source.
Keratoconus is a degenerative disorder of the eye characterized by human cornea thinning and morphological change to a more conical shape. Current diagnosis of this disease relies on topographic imaging of the cornea. Early and differential diagnosis is difficult. In keratoconus, mechanical properties are found to be compromised. A clinically available invasive technique capable of measuring the mechanical properties of the cornea is of significant importance for understanding the mechanism of keratoconus development and improve detection and intervention in keratoconus.
The capability of Brillouin imaging to detect local longitudinal modulus in human cornea has been demonstrated previously. We report our non-contact, non-invasive, clinically viable Brillouin imaging system engineered to evaluate mechanical properties human cornea in vivo. The system takes advantage of a highly dispersive 2-stage virtually imaged phased array (VIPA) to detect weak Brillouin scattering signal from biological samples. With a 1.5-mW light beam from a 780-nm single-wavelength laser source, the system is able to detect Brillouin frequency shift of a single point in human cornea less than 0.3 second, at a 5μm/30μm lateral/axial resolution. Sensitivity of the system was quantified to be ~ 10 MHz. A-scans at different sample locations on a human cornea with a motorized human interface. We imaged both normal and keratoconic human corneas with this system. Whereas no significantly difference were observed outside keratocnic cones compared with normal cornea, a highly statistically significantly decrease was found in the cone regions.
Femtosecond laser ablation of a gold plate in aqueous solutions has been used to form colloidal gold nanoparticles. Using different chemical environments during the fabrication, this method makes it possible to functionalize nanomaterials by an appropriate capping ligand. In particular, we were able to control the size and the reactivity of gold particles by using different polymers (dextran and polyethylene glycol). The size of the nanoparticles, measured by transmission electron microscopy (TEM), was found to be as low as 5 nm, in some cases. The addition of these capping agents also significantly improved the long term stability of gold particles. The produced nanoparticles exhibited a strong absorption band near 520 nm due to the surface plasmon resonance and a photoluminescence signal in the violet-blue spectral range. The functionalized nanoparticles produced are of significance in view of their bio-imaging and bio-sensing applications.
Femtosecond laser ablation technique has been used to process Si and Au targets in vacuum, air and water environment. The threshold of ablation was found to be much lower for Si compared to Au and that was related to much better radiation absorption of Si. The values of the threshold were almost identical for vacuum, air and water in the case of Si (0.4 J/cm2 0.2 J/cm2 in the single and multi-pulse irradiation regime, respectively) and Au (0.9 J/cm2 and 0.3 J/cm2). Craters on the surface of Si and Au were essentially similar for low fluences, suggesting an involvement of the same radiation-related mechanism of material removal, whereas for high fluences significant differences could take place. In particular, quite different crater morphologies were observed during the laser ablation in water, including ones with nanoporous layers for Si and ones with concentric spheres for Au. The differences of morphologies for high laser fluences were explained by the involvement of plasma-related effects under the processing in relatively dense media.
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