ABSTRACT
As basic optical elements, waveplates with anisotropic electromagnetic responses are imperative for manipulating light polarization. Conventional waveplates are manufactured from bulk crystals (e.g., quartz and calcite) through a series of precision cutting and grinding steps, which typically result in large size, low yield, and high cost. In this study, a bottom-up method is used to grow ferrocene crystals with large anisotropy to demonstrate self-assembled ultrathin true zero-order waveplates without additional machining processing, which is particularly suited for nanophotonic integration. The van der Waals ferrocene crystals exhibit high birefringence (Δn (experiment) = 0.149â ± â 0.002 at 636 nm), low dichroism Δκ (experiment) = -0.0007 at 636 nm), and a potentially broad operating range (550 nm to 20 µm) as suggested by Density Functional Theory (DFT) calculations. In addition, the grown waveplate's highest and the lowest principal axes (n1 and n3 , respectively) are in the a-c plane, where the fast axis is along one natural edge of the ferrocene crystal, rendering them readily usable. The as-grown, wavelength-scale-thick waveplate allows the development of further miniaturized systems via tandem integration.
ABSTRACT
Cholesterol is believed to induce the formation of membrane domains, "rafts", which are implicated in a range of natural and pathologic membrane processes. Therefore, it is important to understand the role that cholesterol plays in the formation of these structures. Here, we use label-free spectroscopic imaging to investigate cholesterol fractioning in supported bilayer membranes at nanoscale. Scattering-type scanning near-field optical microscopy (s-SNOM) was used to visualize the formation of cholesterol-induced domains in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) membranes. Our results revealed the coexistence of phase separated domains in DMPC lipids with 10 mol % cholesterol content, whereas a mostly homogeneous bilayer was found at low (5 mol %) and high (15 mol %) cholesterol content. Near-field nano-FTIR spectroscopy was used to identify the cholesterol-rich domains based on their qualitative chemical compositions. It was determined that cholesterol binds to phosphodiester and alkyl glycerol ester moieties, likely via hydrogen bonding of the alcohol to either of the ester oxygens. The results also confirm the existence of an ideal cholesterol-lipid mixture ratio (â¼15:85) with a geometrically defined packing. At lower cholesterol content there is phase separation between liquid ordered and almost neat DMPC domains. Thus, the liquid ordered phase exists at an energy minimum at a given lipid-cholesterol ratio.
Subject(s)
Cholesterol/chemistry , Dimyristoylphosphatidylcholine/chemistry , Lipid Bilayers/chemistry , Nanostructures/chemistry , Glycerol/chemistry , Hydrogen Bonding , Microscopy , Oxygen/chemistry , Phase Transition , Spectroscopy, Fourier Transform Infrared , Surface PropertiesABSTRACT
Here we report the results of shear-mode thicknesses and absorption coefficient measurements made on neat membranes using scanning near-field optical microscopy (SNOM). Biomimic neat membranes composed of two different types of phoshpholipid molecules: 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2- dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were found to exhibit different absorption coefficients under the SNOM. The localization of the lipids could be identified and correlated to the morphology of the membrane domains indicating that SNOM can be an effective and accurate approach for the label-free characterization of the structure-function relationships in cell membranes.
ABSTRACT
Plasmonic gold nanorod instability and reshaping behavior below melting points are important for many future applications but are yet to be fully understood, with existing nanoparticle melting theories unable to explain the observations. Here, we have systematically studied the photothermal reshaping behavior of gold nanorods irradiated with femtosecond laser pulses to report that the instability is driven by curvature-induced surface diffusion rather than a threshold melting process, and that the stability dramatically decreases with increasing aspect ratio. We successfully utilized the surface diffusion model to explain the observations and found that the activation energy for surface diffusion was dependent on the aspect ratio of the rods, from 0.6 eV for aspect ratio of 5 to 1.5 eV for aspect ratio less than 3. This result indicates that the surface atoms are much easier to diffuse around in larger aspect ratio rods than in shorter rods and can induce reshaping at any given temperature. Current plasmonics and nanorod applications with the sharp geometric features used for greater field enhancement will therefore need to consider surface diffusion driven shape change even at low temperatures.