Highly Sensitive Photoacoustic NO2Measurement System Based on an Optimized Ring-Shaped Resonant Cell

This work proposes the feasibility of an ultraviolet (UV) photoacoustic (PA) gas sensing system based on an innovative resonant PA cell structure. The system design exploits the resonant characteristics of a ring-shaped cell whose design has been optimized through analytical and finite element method (FEM) analysis to meet the requirements of the target application. In particular, the device characteristics in terms of size, frequency response, limit of detection (LOD), and related metrological characteristics were taken into account. The overall designed sensing system is made up of low-cost and commercially available components and it represents a scalable solution for monitoring different gas species. The performance of the sensing system toward NO2 was experimentally assessed, showing a relative sensitivity of 0.38%/ppm NO2 and a LOD of 500 ppb with response and recovery times in the order of a few seconds. Moreover, the cross-sensitivity of the PA sensor toward CO, CH4, and NH3 was evaluated

Low-temperature electronic transport measurements on a gated delta -doped GaAs sample: magnetoresistance, quantum Hall effect and conductivity fluctuations

We present magnetotransport measurements (up to 7 T) performed at very low temperatures (down to 20 mK) on a GaAs sample containing two parallel delta -doped layers whose carrier concentration can be varied by means of a gate electrode. With increasing negative gate voltage the resistance becomes more strongly temperature-dependent, indicating a more localized electron system. The magnetoresistance is found to be strongly anisotropic. When the field is parallel to the layers we find a large positive magnetoresistance which we attribute to orbital shrinking of the strongly localized donor wavefunction. In contrast, in the perpendicular orientation, we observe a strong negative magnetoresistance at low fields whose origin remains unclear, and the quantum Hall effect at larger fields. At low gate voltages both delta -layers are in the quantum Hall state whereas at larger negative voltages the layer adjacent to the gate becomes insulating. In the case of strong depletion the high-ohmic sample shows reproducible conductivity fluctuations as a function of either the gate voltage or the magnetic field. The fluctuations diminish at higher temperatures and larger measuring currents

Privacy Preservation over Untrusted Mobile Networks

Abstract. The proliferation of mobile devices has given rise to novel user-centric applications and services. In current mobile systems, users gain access to remote servers over mobile network operators. These operators are typically assumed to be trusted and to manage the information they collect in a privacy-preserving way. Such information, however, is extremely sensitive and coveted by many companies, which may use it to improve their business. In this context, safeguarding the users ’ privacy against the prying eyes of the network operators is an emerging requirement. In this chapter, we first present a survey of existing state-of-the-art protection mechanisms and their challenges when deployed in the context of wired and wireless networks. Moreover, we illustrate recent and ongoing research that attempts to address different aspects of privacy in mobile applications. Furthermore, we present a new proposal to ensure private communication in the context of hybrid mobile networks, which integrate wired, wireless and cellular technologies. We conclude by outlining open problems and possible future research directions.