Specific loss power optimization in metal ferrite nanocrystals for radiofrequency stimulation of neurons

Thesis: M. Eng., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, February 2013.Cataloged from PDF version of thesis. "September 2012."Includes bibliographical references (pages 83-90).Neurons serve as the basic unit of computation within the nervous system. As the nervous system is involved with the encoding, transmission, processing, and decoding of information at every level, characterization of the nervous system is of the utmost interest to neuroscience. However, techniques for probing the nervous system have previously focused primarily of characterizing single cell behavior, which does not provide insight as to the functioning of the system as a whole. This is further complicated by the fact that functional network of neurons are typically spatial interwoven, rendering spatially-limited stimulation techniques ineffective. The desire to characterize the system in its entirety necessitates the development of neuronal probes that can target functional subpopulations of cells. A proposed system for such stimulation is the genetic targeting of neurons via expression of gated ion channels, and the selective stimulation of them using a transmitter-receiver pair. This thesis describes the design and optimization of such a transmitter-receiver pair that activates ion channels via the dissipation of heat. Magnetic losses in superparamagnetic metal ferrite nanocrystals are modeled to determine the optimal operating parameters for dissipation of heat. Optimal nanocrystals are then synthesized via high-temperature thermolysis of a mixed metal oleate precursor, and stabilized in the aqueous phase through functionalization with polyethylene glycol. A solenoid is designed and constructed to serve as a radiofrequency excitation source, and subsequently optimized to maximize the power transfer from solenoid to magnetic nanocrystals. A susceptometer and lock-in amplifier are designed for characterization of colloidal nanocrystals in the aqueous phase. The constructed susceptometer is then used to measure magnetic losses in metal ferrite nanocrystals and compare their performance with the modeled behavior.by Nathan S. Lachenmyer.M. Eng

Measurements of electric field noise and light-induced charging in Al and Cu surface electrode ion traps at cryogenic temperatures

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 85-89).Ion traps provide an excellent tool for controlling and observing the state of a single trapped ion. For this reason, ion traps have been proposed as a possible system for large-scale quantum computation. However, many obstacles must be overcome before quantum computing can become a reality. In particular, perturbations in the electric field due to noise and electrode charging must be reduced to increase coherence of the motional quantum state. Gold has been a popular choice in the past due to its inert properties; however, it is undesirable due its incompatibility with CMOS technology. This has led to increased research into alternative CMOS-compatible materials, such as aluminum and copper. This thesis presents measurements of electric field noise and light-induced charging in aluminum, copper, and gold surface electrode traps. In addition, the effect of oxide growth on field noise and electrode charging is explored by controlling the thickness of aluminum oxide on several aluminum traps. The measurements show that electric field noise can be suppressed in aluminum traps to approximately 10-18 V2 cm-2 Hz-1, matching the noise exhibited in gold traps, and that copper traps exhibit noise within an order of magnitude of that in aluminum and gold. However, the natural oxide of aluminum poses many problems towards high-performance aluminum ion traps. The electric field noise is shown to be strongly dependent on the oxide thickness, increasing the noise by a factor of about 10 until saturation at a thickness of 13 nm. Charging of surface electrodes is shown to be highly dependent upon the material, but the model presented does not match the experimental data and is found to be incomplete. These results indicate that ion traps made out of CMOS-compatible materials can perform as well as more traditional traps fabricated from gold with respect to heating and charging as long as methods are developed for controlling oxide growth.by Nathan S. Lachenmyer.S.B

Laser-induced charging of microfabricated ion traps

Electrical charging of metal surfaces due to photoelectric generation of carriers is of concern in trapped ion quantum computation systems, due to the high sensitivity of the ions' motional quantum states to deformation of the trapping potential. The charging induced by typical laser frequencies involved in doppler cooling and quantum control is studied here, with microfabricated surface electrode traps made of aluminum, copper, and gold, operated at 6 K with a single Sr$^+$ ion trapped 100 $\mu$m above the trap surface. The lasers used are at 370, 405, 460, and 674 nm, and the typical photon flux at the trap is 10$^{14}$ photons/cm$^2$/sec. Charging is detected by monitoring the ion's micromotion signal, which is related to the number of charges created on the trap. A wavelength and material dependence of the charging behavior is observed: lasers at lower wavelengths cause more charging, and aluminum exhibits more charging than copper or gold. We describe the charging dynamic based on a rate equation approach.Comment: 8 pages, 8 figure