Optical sensing and determination of complex reflection coefficients of plasmonic structures using transmission interferometric plasmonic sensor

The combination of interferometry and plasmonic structure, which consists of gold nanoparticle layer, sputter coated silicon oxide spacer layer, and aluminum mirror layer, was studied in transmission mode for biosensing and refractive index sensing applications. Because of the interferometric nature of the system, the information of the reflection amplitude and phase of the plasmonic layer can be deduced from one spectrum. The modulation amplitude in the transmission spectrum, caused by the interference between the plasmonic particle layer and the mirror layer, increases upon the refractive index increase around the plasmonic particles due to their coherent backscattering property. Our proposed evaluation method requires only two light sources with different wavelengths for a stable self-referenced signal, which can be easily and precisely tuned by a transparent spacer layer thickness. Unlike the standard localized surface plasmon sensors, where a sharp resonance peak is essential, a broad band plasmon resonance is accepted in this method. This leads to large fabrication tolerance of the plasmonic structures. We investigated bulk and adsorption layer sensitivities both experimentally and by simulation. The highest sensitivity wavelength corresponded to the resonance of the plasmonic particles, but useful signals are produced in a much broader spectral range. Analysis of a single transmission spectrum allowed us to access the wavelength-dependent complex reflection coefficient of the plasmonic particle layer, which confirmed the reflection amplitude increase in the plasmonic particle layer upon molecular adsorption

Seclusion of molecular layers in a confined simple liquid

We provide theoretical evidence by using molecular dynamics that a nanoconfined film of octamethylcy-clotetrasiloxane divides into manifolds of secluded thermodynamic substates. We find equivalence between the splitting into substates and the formation of molecular layers. The seclusion of layers is validated in drainage experiments using an extended surface forces apparatus (eSFA). Furthermore, per-molecule evaluations of the configurational entropy provide evidence for an increased molecular packing upon confinement. The increasing trends in both layer seclusion and molecular packing with confinement explain the exponentially increasing surface forces measured in eSFA.ISSN:2643-156

Molecular liquid under nanometre confinement: density profiles underlying oscillatory forces

Ultrathin (<12 nm) films of tetrakis(trimethyl)siloxysilane (TTMSS) have been confined by atomically flat mica membranes in the presence and absence of applied normal forces. When applying normal forces, discrete film thickness transitions occur, each involving the expulsion of TTMSS molecules. Using optical interferometry we have measured the step size associated with a film thickness transition (7.5 Å for compressed, 8.4 Å for equilibrated films) to be smaller than the molecular diameter of 9.0 Å. Layering transitions with a discrete step size are commonly regarded as evidence for strong layering of the liquid's molecules in planes parallel to the confining surfaces and it is assumed that the layer spacing equals the measured periodicity of the oscillatory force profile. Using x-ray reflectivity (XRR), which directly yields the liquid's density profile along the confinement direction, we show that the layer spacing (10–11 Å) proves to be on average significantly larger than both the step size of a layering transition and the molecular diameter. We observe at least one boundary layer of different electron density and periodicity than the layers away from the surfaces

X-ray reflectivity theory for determining the density profile of a liquid under nanometre confinement

An X-ray reflectivity theory on the determination of the density profile of a molecular liquid under nanometre confinement is presented. The confinement geometry acts like an X-ray interferometer, which consists of two opposing atomically flat single-crystal mica membranes with an intervening thin liquid film of variable thickness. The X-rays reflected from the parallel crystal planes (of known structure) and the layered liquid in between them (of unknown structure) interfere with one another, making X-ray reflectivity highly sensitive to the liquid's density profile along the confinement direction. An expression for the reflected intensity as a function of momentum transfer is given. The total structure factor intensity for the liquid-filled confinement device is derived as a sum of contributions from the inner and outer crystal terminations. The method presented readily distinguishes the confined liquid from the liquid adsorbed on the outer mica surfaces. It is illustrated for the molecular liquid tetrakis(trimethyl)siloxysilane, confined by two mica surfaces at a distance of 8.6 nm