Nanometer-Resolved Collective Micromeniscus Oscillations through Optical Diffraction

We study the dynamics of periodic arrays of micrometer-sized liquid-gas menisci formed at superhydrophobic surfaces immersed into water. By measuring the intensity of optical diffraction peaks in real time we are able to resolve nanometer scale oscillations of the menisci with sub-microsecond time resolution. Upon driving the system with an ultrasound field at variable frequency we observe a pronounced resonance at a few hundred kHz, depending on the exact geometry. Modeling the system using the unsteady Stokes equation, we find that this low resonance frequency is caused by a collective mode of the acoustically coupled oscillating menisci.Comment: 4 pages, 5 figure

Large bandwidth, highly efficient optical gratings through high index materials

We analyze the diffraction characteristics of dielectric gratings that feature a high index grating layer, and devise, through rigorous numerical calculations, large bandwidth, highly efficient, high dispersion dielectric gratings in reflection, transmission, and immersed transmission geometry. A dielectric TIR grating is suggested, whose -1dB spectral bandwidth is doubled as compared to its all-glass equivalent. The short wavelength diffraction efficiency is additionally improved by allowing for slanted lamella. The grating surpasses a blazed gold grating over the full octave. An immersed transmission grating is devised, whose -1dB bandwidth is tripled as compared to its all-glass equivalent, and that surpasses an equivalent classical transmission grating over nearly the full octave. A transmission grating in the classical scattering geometry is suggested, that features a buried high index layer. This grating provides effectively 100% diffraction efficiency at its design wavelegth, and surpasses an equivalent fused silica grating over the full octave.Comment: 15 pages, 7 figure

Polar magneto-optical Kerr effect for low-symmetric ferromagnets

The polar magneto-optical Kerr effect (MOKE) for low-symmetric ferromagnetic crystals is investigated theoretically based on first-principle calculations of optical conductivities and a transfer matrix approach for the electrodynamics part of the problem. Exact average magneto-optical properties of polycrystals are described, taking into account realistic models for the distribution of domain orientations. It is shown that for low-symmetric ferromagnetic single crystals the MOKE is determined by an interplay of crystallographic birefringence and magnetic effects. Calculations for single and bi-crystal of hcp 11-20 Co and for a polycrystal of CrO_2 are performed, with results being in good agreement with experimental data.Comment: 14 pages, 7 figures, accepted for publication in Phys. Rev.

Superhydrophobic surfaces : from fluid mechanics to optics

In this thesis optical diraction was used to study the static and dynamic properties of microscopic liquid-gas interfaces that span between adjacent ridges of a superhydrophobic surface. An observed interference phenomenon at grazing incident angle led to the development of optical gratings with a large spectral bandwidth, and an observed sensitive response of the liquid-gas interfaces to ultrasound led to the development of a superhydrophobic ber optical micro cavity that enables interferometric detection of the motion of a single microscopic meniscus

Microscopic shape and contact angle measurement at a superhydrophobic surface

We have studied the microscopic shape, contact angle and Laplace law behavior of the liquid–gas interfaces at a superhydrophobic surface. A superhydrophobic surface is immersed in water, and the radius of liquid gas menicsi that span between adjacent ridges of the surface texture is measured. The surface pattern consists of rectangular grooves, such that the sample is simultaneously an optical grating. The diffraction properties encode the shape of the menisci. The shape of the menisci is determined by measuring the intensity of several diffraction orders as a function of the incident angle, and fitting the data to numerical calculations of the diffraction. The uncertainty of the determined menisci deflections is a few nanometres. Observing the deflection as a function of externally controlled hydrostatic pressure, Laplace's law is probed for the menisci on the micrometre scale. The microscopic contact angle is determined by measuring the radius of the menisci prior to collapse. Close agreement with the macroscopic Young angle is found. A stability limit for the superhydrophobic-to-impregnated transition is given. The measurement is a microscopic analogue of ‘bubble’ and ‘sessile drop’ type methods

Symmetry Assumptions, Kramers-Kronig Transformation and Analytical Continuation in Ab Initio Calculations of Optical Conductivities

Abstract The limit of infinite relaxation of the Kubo formula and analytical and numerical properties of the Kramers-Kronig transformation and analytical continuation used in ab initio calculations of the optical conductivity tensor are considered. Essential symmetry assumptions used in magneto optics are pointed out and their validity for some classes of important systems is shown. It is shown that for an energy dependent relaxation time, the optical conductivity can always be calculated with desired numerical accuracy by applying a Kramers-Kronig transformation and analytical continuation to the result obtained in the limit of infinite relaxation time instead of calculating it directly from the Kubo formula with a finite relaxation time. Consequently the difference between the two approaches is reduced to the difference between the Brillouin zone integration techniques

Fabrication, mechanical testing and application of high-pressure glass microreactor chips

The design, fabrication and high-pressure performance of several in-plane fiber-based interface geometries to microreactor chips for high-pressure chemistry are discussed, and an application is presented. The main investigated design parameters are the geometry of the inlet/outlet structure, the manner in which top and bottom wafer are bonded and the way the inlets/outlets turn over into the microfluidic channels.\ud Destructive pressure experiments with H2O and liquid CO2 showed that the maximum pressure that the proposed inlet/outlet structures can withstand is in the range of 180–690 bar. The optimal geometry for high-pressure microreactor chips is a tubular structure that is etched with hydrofluoric acid (HF) and suitable for fibers with a diameter of 110 μm. These inlets/outlets can withstand pressures up to 690 bar. On the other hand, small powderblasted inlets/outlets that are smoothened with HF and with a sharp transition towards the flow channels are adequate for working pressures up to 300 bar.\ud Microreactor chips with tubular inlet/outlet geometries were used for studying the formation of the carbamic acid of N-benzylmethylamine and CO2. These chips could be used for pressures up to 400 bar without problems/failure, thereby showing that these micromachined microreactor chips are attractive tools for performing high-pressure chemistry in a fast and safe way.\ud \u