Dan E. Angelescu

Thermal conductivity and thermal boundary resistance of nanostructures

AbstractWe present a fabrication process of low-cost superlattices and simulations related with the heat dissipation on them. The influence of the interfacial roughness on the thermal conductivity of semiconductor/semiconductor superlattices was studied by equilibrium and non-equilibrium molecular dynamics and on the Kapitza resistance of superlattices interfaces by equilibrium molecular dynamics. The non-equilibrium method was the tool used for the prediction of the Kapitza resistance for a binary semiconductor/metal system. Physical explanations are provided for rationalizing the simulation results.PACS68.65.Cd, 66.70.Df, 81.16.-c, 65.80.-g, 31.12.xv

Optical trapping and binding of particles in an optofluidic stable Fabry–Pérot resonator with single-sided injection

In this article, microparticles are manipulated inside an optofluidic Fabry-Pérot cylindrical cavity embedding a fluidic capillary tube, taking advantage of field enhancement and multiple reflections within the optically-resonant cavity. This enables trapping of suspended particles with single-side injection of light and with low optical power. A Hermite-Gaussian standing wave is developed inside the cavity, forming trapping spots at the locations of the electromagnetic field maxima with a strong intensity gradient. The particles get arranged in a pattern related to the mechanism affecting them: either optical trapping or optical binding. This is proven to eventually translate into either an axial one dimensional (1D) particle array or a cluster of particles. Numerical simulations are performed to model the field distributions inside the cavity allowing a behavioral understanding of the phenomena involved in each case.

Black silicon with sub-percent reflectivity: Influence of the 3D texturization geometry

In this paper we study the impact of the three-dimensional geometry of a micro/nanostructured silicon surface on its reflectivity under incident electromagnetic (EM) illumination. We simulate the optical reflectance of 3D micro/nano silicon cones of different dimensions. Based on the favorable simulation results, maskless textured silicon, called “black silicon” is processed by deep reactive ion etching (DRIE) under cryogenic temperatures. By varying the process parameters, we fabricate conical black silicon substrates with excellent anti-reflective behavior. Notable among the results, one of the samples exhibits the lowest reflectivity in the optical wavelength published to date for plasma-etched black-silicon.

A replaceable, low thermal mass hot stage for scanning probe microscopy

We describe the design, construction, and characterization of a hot stage for use in scanning probe microscopy. The hot stage incorporates a heater and thermometer on a single 10×10×0.5 mm silicon chip, allowing rapid thermal response, uniform heat distribution, and low power operation. This design facilitates the incorporation of microfabricated features on the hot stage surface, which we illustrate with a SiNx step edge 30 nm high. Samples to be imaged can also be applied or fabricated directly on the chip. Individual chips can be easily inserted into and removed from a small sample holder, which provides spring contact electrodes to an external temperature controller; wire bonding is not required. The chip and holder combined are 15×15×12 mm.

Enhanced Microgas Chromatography Using Correlation Techniques for Continuous Indoor Pollutant Detection

We present for the first time a proof-of-concept system implementing the stochastic injection techniques within a silicon-based microgas chromatograph (μGC) which differs from standard laboratory chromatographs by its small size, shorter column and corresponding elution times, and potential low cost when batch manufactured in high volumes. We demonstrate that stochastic injection techniques can enable the continuous detection of pollutants or toxic gases, with high temporal resolution (5 s) and order-of-magnitude improvements in limit of detection compared to a standard single-injection technique, thus greatly improving performance of air quality monitoring devices. Since micro-GC systems have the potential to 1 day become ubiquitous in indoor environments, such stochastic injection techniques could enable faster detection of toxic compounds at lower concentrations in both industrial and residential settings.

Simultaneous measurement of liquid absorbance and refractive index using a compact optofluidic probe

We present a novel optical technique for simultaneously measuring the absorbance and the refractive index of a thin film using an infrared optofluidic probe. Experiments were carried on two different liquids and the results agree with the bibliographical data. The ultimate goal is to achieve a multi-functional micro-optical device for analytical applications.

Yield-stress fluids foams: flow patterns and controlled production in T-junction and flow-focusing devices

We study the formation of yield-stress fluid foams in millifluidic flow-focusing and T-junction devices. First, we provide a phase diagram for the unsteady operating regimes of bubble production when the gas pressure and the yield-stress fluid flow rate are imposed. Three regimes are identified: a co-flow of gas and yield-stress fluid, a transient production of bubble and a flow of yield-stress fluid only. Taking wall slip into account, we provide a model for the pressure at the onset of bubble formation. Then, we detail and compare two simple methods to ensure steady bubble production: regulation of the gas pressure or flow-rate. These techniques, which are easy to implement, thus open pathways for controlled production of dry yield-stress fluid foams as shown at the end of this article.

Volume refractometry of liquids using stable optofluidic Fabry–Pérot resonator with curved surfaces

Abstract. This work reports a simple, miniaturized optical sensing module for liquid refractometry. It is based on a stable Fabry–Pérot resonator consisting of two silicon cylindrical mirrors with a cylindrical lens in between. The lens is formed by a capillary tube through which the analyte passes. This setup enables volume refractometry, where light propagates through the sample realizing high-interaction depth. The cylindrical surfaces achieve light confinement, reducing the light escaping loss encountered in classical cavities with straight mirrors; hence, a high-quality factor (Q) over 1000 is attained. Exploiting this high Q, we adopt the refractive index (RI) measurement criterion: operating at a fixed wavelength and detecting the power drop as a consequence to the spectral shift with RI change. This technique showed that measuring RI change Δn above the RI of the reference solution can be determined for 0.0023<Δn<0.0045. Sensitivity up to 4094  dBm/RIU is achieved. A wider range is still achievable by the conventional method of tracing the shift in peak wavelengths: a range of Δn=0.0163  RIU can be scanned, with a sensitivity of 221  nm/RIU. Error analysis has been accomplished, and the device’s design parameters are discussed to evaluate the performance.

Highly integrated microfluidic sensors

We have developed fabrication techniques for creating suspended electrically addressable MEMS structures in microfluidic channels, as well as monolithic integration of sensors within microfluidic devices. As we will demonstrate, creative use of state-of-the-art MEMS fabrication techniques allows the integrated manufacturing of a number of sensors, for simultaneous measurement of, for example, flow velocity, thermal conductivity and normal stress. We will demonstrate the versatility of these techniques with an example of capillary viscosity sensor integrating independent flowrate, temperature, and pressure drop sensors.

SUB-0.05° precision optofluidic dual-axis inclinometer

This paper details a low-power bi-axial miniaturized inclinometer based on a mobile mass (spherical ball or fluidic droplet) positioned on a precision curved surface that is generated using a novel MEMS process. The detection of the mobile mass was implemented through an external optical system, using a quadrant photodetector. Nanotopography and chemical treatment of the curved surface have been implemented to increase accuracy when using a fluidic mobile mass, by tailoring wetting properties and minimizing contact angle hysteresis. We achieve a range of ±1° with a true linear bi-axial measurement of precision better than 0.05°.