Publications

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We have developed a pulsed optically pumped magnetometer (OPM) array for detecting magnetic field maps originated from an arbitrary current distribution. The presented magnetic source imaging (MSI) system features 24-OPM channels has a data rate of 500 S/s, a sensitivity of 0.8\mathrm {pT/}\sqrt {\mathrm {Hz}} , and a dynamic range of 72 dB. We have employed our pulsed-OPM MSI system for measuring the magnetic field map of a test coil structure. The coils are moved across the array in an indexed fashion to measure the magnetic field over an area larger than the array. The captured magnetic field maps show excellent agreement with the simulation results. Assuming a 2-D current distribution, we have solved the inverse problem using the measured magnetic field maps, and the reconstructed current distribution image is compared with that of the simulation.

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We describe a multichannel magnetoencephalography (MEG) system that uses optically pumped magnetometers (OPMs) to sense the magnetic fields of the human brain. The system consists of an array of 20 OPM channels conforming to the human subject's head, a person-sized magnetic shield containing the array and the human subject, a laser system to drive the OPM array, and various control and data acquisition systems. We conducted two MEG experiments: auditory evoked magnetic field and somatosensory evoked magnetic field, on three healthy male subjects, using both our OPM array and a 306-channel Elekta-Neuromag superconducting quantum interference device (SQUID) MEG system. The described OPM array measures the tangential components of the magnetic field as opposed to the radial component measured by most SQUID-based MEG systems. Herein, we compare the results of the OPM- and SQUID-based MEG systems on the auditory and somatosensory data recorded in the same individuals on both systems.

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We have developed a four-channel optically pumped atomic magnetometer for magnetoencephalography (MEG) that incorporates a passive diffractive optical element (DOE). The DOE allows us to achieve a long, 18-mm gradiometer baseline in a compact footprint on the head. Using gradiometry, the sensitivities of the channels are < 5 fT/Hz1/2, and the 3-dB bandwidths are approximately 90 Hz, which are both sufficient to perform MEG. Additionally, the channels are highly uniform, which offers the possibility of employing standard MEG post-processing techniques. This module will serve as a building block of an array for magnetic source localization.

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We design and fabricate arrays of diffractive optical elements (DOEs) to realize neutral atom micro-traps for quantum computing. We initialize a single atom at each site of an array of optical tweezer traps for a customized spatial configuration. Each optical trapping volume is tailored to ensure only one or zero trapped atoms. Specifically designed DOEs can define an arbitrary optical trap array for initialization and improve collection efficiency in readout by introducing high-numerical aperture, low-profile optical elements into the vacuum environment. We will discuss design and fabrication details of ultra-fast collection DOEs integrated monolithically and coaxially with tailored DOEs that establish an optical array of micro-traps through far-field propagation. DOEs, as mode converters, modify the lateral field at the front focal plane of an optical assembly and transform it to the desired field pattern at the back focal plane of the optical assembly. We manipulate the light employing coherent or incoherent addition with judicious placement of phase and amplitude at the lens plane. This is realized through a series of patterning, etching, and depositing material on the lens substrate. The trap diameter, when this far-field propagation approach is employed, goes as 2.44λF/#, where the F/# is the focal length divided by the diameter of the lens aperture. The 8-level collection lens elements in this presentation are, to our knowledge, the fastest diffractive elements realized; ranging from F/1 down to F/0.025. © 2012 SPIE.

This report describes a passive, optical component called resonant subwavelength gratings (RSGs), which can be employed as one element in an RSG array. An RSG functions as an extremely narrow wavelength and angular band reflector, or mode selector. Theoretical studies predict that the infinite, laterally-extended RSG can reflect 100% of the resonant light while transmitting the balance of the other wavelengths. Experimental realization of these remarkable predictions has been impacted primarily by fabrication challenges. Even so, we will present large area (1.0mm) RSG reflectivity as high as 100.2%, normalized to deposited gold. Broad use of the RSG will only truly occur in an accessible micro-optical system. This program at Sandia is a normal incidence array configuration of RSGs where each array element resonates with a distinct wavelength to act as a dense array of wavelength- and mode-selective reflectors. Because of the array configuration, RSGs can be matched to an array of pixels, detectors, or chemical/biological cells for integrated optical sensing. Micro-optical system considerations impact the ideal, large area RSG performance by requiring finite extent devices and robust materials for the appropriate wavelength. Theoretical predictions and experimental measurements are presented that demonstrate the component response as a function of decreasing RSG aperture dimension and off-normal input angular incidence.

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This project combined nanocomposite materials with microfabricated optical device structures for the development of microsensor arrays. For the nanocomposite materials we have designed, developed, and characterized self-assembling, organic/inorganic hybrid optical sensor materials that offer highly selective, sensitive, and reversible sensing capability with unique hierarchical nanoarchitecture. Lipid bilayers and micellar polydiacetylene provided selective optical response towards metal ions (Pb(II), Hg(II)), a lectin protein (Concanavalin A), temperature, and organic solvent vapor. These materials formed as composites in silica sol-gels to impart physical protection of the self-assembled structures, provide a means for thin film surface coatings, and allow facile transport of analytes. The microoptical devices were designed and prepared with two- and four-level diffraction gratings coupled with conformal gold coatings on fused silica. The structure created a number of light reflections that illuminated multiple spots along the silica surface. These points of illumination would act as the excitation light for the fluorescence response of the sensor materials. Finally, we demonstrate an integrated device using the two-level diffraction grating coupled with the polydiacetylene/silica material.

The design and on-going fabrication of an opto-electro-mechanical microsystem that acts as a four-function optical fiber switch will be presented. The four functions of the 2×2 optical switch include 1) Normal mode, where channel A and channel B pass light straight through, 2) Loopback mode, where light originating in channel A is detected in the B leg, 3) Monitor A mode, where a probe pulse is inserted into channel B and any reflections are detected in the A leg, and 4) Monitor B mode, the compliment of 3) above. The Monitor A and Monitor B modes allow the microsystem to operate as an Optical Time Domain Reflectometer (OTDR). High spatial frequency gratings etched in fused silica configure the light beams through free-space substrate-mode propagation. The design for an OTDR-mode transmission grating that normally passes light from an incidence angle of 45 degrees within the silica substrate as well as passes light from a normal incidence straight through the silica will be discussed. A miniature commercial drive motor, positioned with LIGA alignment plates, rotates the optical grating disk into one of the four implemented function positions. The impact of required tolerances and packaging limitations on the optics, LIGA alignment plates, and the complete microsystem will be presented.

We have designed and assembled two generations of integrated micro-optical systems that deliver pump light and detect broadband laser-induced fluorescence in micro-fluidic chemical separation systems employing electrochromatography. The goal is to maintain the sensitivity attainable with larger, tabletop machines while decreasing package size and increasing throughput (by decreasing the required chemical volume). One type of micro-optical system uses vertical-cavity surface-emitting lasers (VCSELs) as the excitation source. Light from the VCSELs is relayed with four-level surface relief diffractive optical elements (DOEs) and delivered to the chemical volume through substrate-mode propagation. Indirect fluorescence from dye-quenched chemical species is collected and collimated with a high numerical aperture DOE. A filter blocks the excitation wavelength, and the resulting signal is detected as the chemical separation proceeds. Variations of this original design include changing the combination of reflective and transmissive DOEs and optimizing the high numerical aperture DOE with a rotationally symmetric iterative discrete on-axis algorithm. We will discuss the results of these implemented optimizations.