The effect of small elongations on the electronic and optical signatures in InAs nanocrystal quantum dots

We present a detailed theoretical investigation of the electronic structure and optical properties of InAs nanocrystals at the transition from spheres to rods. Using a semiempirical pseudopotential approach, we predict that, despite the qualitative similarity of both intra- and inter-band optical spectra, for NCs with R > 15 °A even slight elongations should result in shifts of the order of hundreds of meV in the spacings between STM peaks measured in the positive bias regime, in the position of the intra- band absorption peaks associated with transitions in the conduction band and in the separation between the first and the fifth peak in PLE experiments. Our results show that, based on the spectroscopic data, it should be possible to discriminate between spherical and elongated NCs with aspect ratios of length over diameter as small as 1.2. Indeed our results suggest that many nominally spherical experimental samples contained a large fraction of slightly elongated structures

Analiza FPGA implementacije bilateralnih algoritama upravljanja za dodirnu teleoperaciju

This paper presents the FPGA implementation of sliding mode control algorithm for bilateral teleoperation, such that, the problem of haptic teleoperation is addressed. The presented study improves haptic fidelity by widening the control bandwidth. For wide control bandwidth, short control periods as well as short sampling periods are required that was achieved by the FPGA. The presented FPGA design methodology applies basic optimization methods in order to meet the required control period as well as the required hardware resource consumption. The circuit specification was performed by the high-level programing language LabVIEW using the fixed-point data type. Hence, short design times for producing the FPGA logic circuit can be achieved. The proposed FPGA-based bilateral teleoperation was validated by master-slave experimental device.Ovaj rad opisuje FPGA implementaciju algoritama upravljanja kliznim režimima za bilateralnu teleoperaciju, pri čemu je opisan problem haptičke teleoperacije. Prikazano istraživanje poboljšava dodirnu pouzdanost proširenjem upravljačkog propusnog pojasa. Za široki propusni pojas, potrebni su kratki upravljački periodi i brzo vrijeme uzorkovanja, što je postignuto primjenom FPGA sklopovlja. Prikazana metodologija za projektiranje FPGA sklopovlja koristi osnovne optimizacijske metode s ciljem postizanja potrebnih upravljačkih perioda i zahtijevane fizičke iskorištenosti sklopovlja. Specifikacije sklopovlja su provedene programskim jezikom visoke razine LabVIEW uz korištenje podataka s nepomičnim decimalnim zarezom. Stoga je moguće implementirati traženu logiku na FPGA sklopovlje u kratkom vremenu. Opisana bilateralna teleoperacija temeljena na FPGA slopovlju je testirana na eksperimentalnom postavu s nadre.enim i podre.enim čvorom

Monotonic Evolution of the Optical Properties in the Transition from Three- to Quasi-Two-Dimensional Quantum Confinement in InAs Nanorods

We present an atomistic pseudopotential study of the electronic and optical properties of InAs quantum rods as a function of increasing length-to-diameter ratio. We show that, as the aspect ratio increases, energy levels cross in both conduction and valence bands, reflecting their different dependence on confinement along a specific direction. Unlike in CdSe and InP quantum rods, however, the position of the crossover between highest occupied molecular orbitals with different symmetries is found to be size-dependent and the value of the aspect ratio at the crossing to increase with the rod diameter. We find that the level crossings at the top of the valence band are crucial to explain the evolution with elongation of all optical properties in these systems. Their transformation from zero- to quasi-one-dimensional structures is characterized by a common monotonic behavior of band gap, Stokes shift, degree of linear polarization, and radiative lifetime, closely linked to the variation with aspect ratio of the electronic structure of the nanocrystal valence band edge. This characteristic feature was not observed in elongated CdSe structures, whose optical properties exhibited instead a distinctive non-monotonic evolution with length, with a turning point associated with a crossover at the top of the valence band, similar to that found here between states with σ and π symmetries

Direct and inverse Auger processes in InAs nanocrystals: can the decay signature of a trion be mistaken for carrier multiplication?

A complete and detailed theoretical investigation of the main processes involved in the controversial detection and quantification of carrier multiplication (CM) is presented, providing a coherent and comprehensive picture of excited state relaxation in InAs nanocrystals (NCs). The observed rise and decay times of the 1S transient bleach are reproduced, in the framework of the Auger model, using an atomistic semiempirical pseudopotential method, achieving excellent agreement with experiment. The CM time constants for small core-only and core/shell nanocrystals are obtained as a function of the excitation energy, assuming an impact-ionization-like process. The resulting lifetimes at energies close to the observed CM onset are consistent with the upper limits deduced experimentally from PbSe and CdSe samples. Most interestingly, as the Auger recombination lifetimes calculated for charged excitons are found to be of a similar order of magnitude to those computed for biexcitons, both species are expected to exhibit the fast decay component in NC population dynamics so far attributed exclusively to the presence of biexcitons and therefore identified as the signature of CM occurrence in high-energy low-pump-fluence spectroscopic studies. However, the ratio between trions and biexcitons time constants is found to be larger than the typical experimental accuracy. It is therefore concluded that, in InAs NCs, it should be experimentally possible to discriminate between the two species and that the origin of the observed discrepancies in CM yields is unlikely to lay in the presence of charged excitons

Light Intensity-Modulated Bending Sensor Fabrication and Performance Test for Shape Sensing

© Springer Nature Switzerland AG 2019. Notable advancements in shape sensing for flexible continuum robot arms can be observed. With a keen interest to develop surgical and diagnostic tools that can advance further and further into inaccessible spaces along tortuous pathways, such as the human body, a need for the precise determination of the robot’s pose arises. Whilst there have been techniques developed that use external sensors to observe the advancing robot from the outside to determine its location and orientation in space, there is an observable trend towards using integrated, internal sensors to measure these positional parameters. Especially in the medical world with its tough requirements on robot size, e.g., catheter-type robots, most pose-sensing approaches to date make use of a technique called Fiber Bragg Grating (FBG). FBG sensors make use of fibers that are grated, and the amount of bending can be determined with an appropriate optical interrogator. Although these fiber sensors have been successfully employed to measure the deformation and through advanced signal processing the pose of continuum catheters, they have a major drawback which is their exorbitant cost. To address this issue a different design and fabrication process is proposed to produce an affordable shape sensor that is highly flexible and can detect bending. The method of operation involves a segmented flexible robot arm with three waveguides in a 120-degrees configuration. The segments are made of silicone elastomer with channels that encapsulate light propagating internally, with a photodiode and light-emitting diode (LED) embedded in each individual channel. The prototype was developed and characterized for strain, and bending response detection