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

Interband and intraband optical transitions in InAs nanocrystal quantum dots: A pseudopotential approach

An atomistic pseudopotential method is used to investigate the electronic and optical properties of spherical InAs nanocrystals. Our calculated interband (valence-to-conduction) absorption spectra reproduce the features observed experimentally both qualitatively and quantitatively. The results relative to intraband (valence-to-valence and conduction-to-conduction) absorption successfully reproduce the recently measured photoinduced absorption spectra, which had so far been addressed only qualitatively. They exclude the hypothesis of a thermal activation process between dot-interior-delocalized hole states to explain the temperature dependence observed experimentally. Furthermore, based on the agreement of our data with the experimental valence intersublevel transitions and the almost complete overlap of the latter with scanning tunneling microscopic (STM) measurements, we question the simplistic attribution of the observed STM peaks obtained for negative bias

Electronic and optical properties of InAs nanocrystals

An atomistic pseudopotential method is used to investigate the electronic and opti- cal properties of spherical InAs nanocrystals. Our calculated interband (valence-to- conduction) absorption spectra reproduce the features observed experimentally both qualitatively and quantitatively. The results relative to intraband (valence-to-valence and conduction-to-conduction) absorption successfully reproduce the recently measured photoinduced absorption spectra, which had so far been addressed only qualitatively. They exclude the hypothesis of a thermal activation process between dot-interior delocalised hole states to explain the temperature dependence observed experimentally. Furthermore, based on the agreement of our data with the experimental valence inter- sublevel transitions and the almost complete overlap of the latter with STM measure- ments, we question the simplistic attribution of the observed STM peaks obtained for negative bias. Motivated by the excellent agreement of our calculated results with the STM, PLE and PIA spectra, we therefore extend our knowledge to a detailed theoretical investigation of the electronic structure and optical properties of InAs nanocrystals at the transition from spheres to rods. 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 ii Abstract iii 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 suggest 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. Additionally, the atomistic pseudopotential approach is also applied to a 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 0- to quasi-1-dimensional structures is characterised by a common monotonic behaviour of band gap, Stokes shift, degree of linear polarisation 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

Miniaturized triaxial optical fiber force sensor for MRI-guided minimally invasive surgery

Proceedings of: 2010 IEEE International Conference on Robotics and Automation (ICRA'10), May 3-8, 2010, Anchorage (Alaska, USA)This paper describes the design and construction of a miniaturized triaxial force sensor which can be applied inside a magnetic resonance imaging (MRI) machine. The sensing principle of the sensor is based on an optical intensity modulation mechanism that utilizes bent-tip optical fibers to measure the deflection of a compliant platform when exposed to a force. By measuring the deflection of the platform using this optical approach, the magnitude and direction of three orthogonal force components (Fx, Fy, and Fz) can be determined. The sensor prototype described in this paper demonstrates that it can perform force measurements in axial and radial directions with working ranges of +/- 2 N. Since the sensor is small in size and entirely made of nonmetallic materials, it is compatible with minimally invasive surgery (MIS) and safe to be deployed within magnetic resonance (MR) environments.European Community's Seventh Framework Progra

Novel miniature MRI-compatible fiber-optic force sensor for cardiac catherization procedures

Proceedings of: 2010 IEEE International Conference on Robotics and Automation (ICRA'10), May 3-8, 2010, Anchorage (Alaska, USA)This paper presents the prototype design and development of a miniature MR-compatible fiber optic force sensor suitable for the detection of force during MR-guided cardiac catheterization. The working principle is based on light intensity modulation where a fiber optic cable interrogates a reflective surface at a predefined distance inside a catheter shaft. When a force is applied to the tip of the catheter, a force sensitive structure varies the distance and the orientation of the reflective surface with reference to the optical fiber. The visual feedback from the MRI scanner can be used to determine whether or not the catheter tip is normal or tangential to the tissue surface. In both cases the light is modulated accordingly and the axial or lateral force can be estimated. The sensor exhibits adequate linear response, having a good working range, very good resolution and good sensitivity in both axial and lateral force directions. In addition, the use of low-cost and MR-compatible materials for its development makes the sensor safe for use inside MRI environments.European Community's Seventh Framework Progra

Multi-objective optimization of end-to-end sutured anastomosis for robot-assisted surgery

Background Due to differences in surgical operations between free-hand and robot-assisted vessel anastomosis, there exist new challenges in applying the manipulation criteria of free-hand surgery to robot-assisted surgery in order to guarantee successful completion of the surgical procedure. Methods A mathematical model is established to optimize the process variables in vessel anastomosis. The distance between entry point and cross-section, suture tension and the number of individual sutures are selected as design variables. The allowable range of suture tension and the difference between longitudinal stresses of vessel tissue on transverse sections are used as the objective functions. Simulation experiments are carried out to obtain the allowable range of suture tension and tissue stress distribution, based on numerical analysis. Results For a vessel in anastomosis with 4 mm diameter, a larger distance between the entry point and the cross-section and/or more sutures can result in less tissue deformation and a tighter joint between the two vessel ends. The allowable range of suture tension is a function of the number of individual sutures and increases with the decrease of the distance between entry point and cross-section. The optimal designs providing the suture configuration of distance between entry point and cross-section and the number of individual sutures are presented in the case that the performance of robot-assisted anastomosis can be guaranteed without strong control of suture tension. Conclusions The work provides meaningful results for the optimal design of the suturing procedure in robot-assisted vascular anastomosis when the robotic system does not allow tactile feedback. Copyright © 2010 John Wiley & Sons, Ltd.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78069/1/347_ftp.pd

Mechanical analysis of end-to-end silk-sutured anastomosis for robot-assisted surgery

Background Robot-assisted anastomosis holds great promise for the future. To secure surgery quality, some key process factors, such as the force arrangement of sutures, should be provided because of the lack of haptic feedback in robotics systems Methods A model of anastomosis is presented to establish the mechanical relationship between vessel and sutures. Stress distribution of the vessel loaded by the suture was then achieved through finite-element simulations, based on the material property test results. Further, experiments were performed to validate the reliability of the FEM simulation of the anastomosis process. Results To avoid blood osmosis, the allowable lower limit of the suture tension was 0.05 N. To keep the tissue free from injury, the allowable upper limit of tension on the suture was 0.4 N. Conclusions The study provided meaningful results for directing the robot-assisted anastomosis procedure and design of the surgical tools. Copyright © 2009 John Wiley & Sons, Ltd.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/64523/1/276_ftp.pd

Wireless tissue palpation: Head characterization to improve tumor detection in soft tissue

For surgeons performing open procedures, the sense of touch is a valuable tool to directly access buried structures and organs, to identify their margins, detect tumors, and prevent undesired cuts. Minimally invasive surgical procedures provide great benefits for patients; however, they hinder the surgeon's ability to directly manipulate the tissue. In our previous work, we developed a Wireless Palpation Probe (WPP) to restore tissue palpation in Minimally Invasive Surgery (MIS) by creating a real-time stiffness distribution map of the target tissue. The WPP takes advantage of a field-based magnetic localization algorithm to measure its position, orientation, and tissue indentation depth, in addition to a barometric sensor measuring indentation tissue pressure. However, deformations of both the tissue and the silicone material used to cover the pressure sensors introduce detrimental nonlinearities in sensor measurements. In this work, we calibrated and characterized different diameter WPP heads with a new design allowing exchangeability and disposability of the probe head. Benchtop trials showed that this method can effectively reduce error in sensor pressure measurements up to 5% with respect to the reference sensor. Furthermore, we studied the effect of the head diameter on the device's spatial resolution in detecting tumor simulators embedded into silicone phantoms. Overall, the results showed a tumor detection rate over 90%, independent of the head diameter, when an indentation depth of 5 mm is applied on the tissue simulator

Survey of Visual and Force/Tactile Control of Robots for Physical Interaction in Spain

Sensors provide robotic systems with the information required to perceive the changes that happen in unstructured environments and modify their actions accordingly. The robotic controllers which process and analyze this sensory information are usually based on three types of sensors (visual, force/torque and tactile) which identify the most widespread robotic control strategies: visual servoing control, force control and tactile control. This paper presents a detailed review on the sensor architectures, algorithmic techniques and applications which have been developed by Spanish researchers in order to implement these mono-sensor and multi-sensor controllers which combine several sensors

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