Publications

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A two-dimensional phononic crystal (PnC) that can operate in the GHz range is created in a freestanding silicon substrate using NanoFIBrication (using a focused ion beam (FIB) to fabricate nanostructures). First, a simple cubic 6.75 x 6.75 ?m array of vias with 150 nm spacing is generated. After patterning the vias, they are backfilled with void-free tungsten scatterers. Each via has a diameter of 48 nm. Numerical calculations predict this 2D PnC will generate a band gap near 22 GHz. A protective layer of chromium on top of the thin (100 nm) silicon membrane confines the surface damage to the chromium, which can be removed at a later time. Inspection of the underside of the membrane shows the vias flaring out at the exit, which we are dubbing the 'trumpet effect'. The trumpet effect is explained by modeling the lateral damage in a freestanding membrane.

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Phononic crystals (or acoustic crystals) are the acoustic wave analogue of photonic crystals. Here a periodic array of scattering inclusions located in a homogeneous host material forbids certain ranges of acoustic frequencies from existence within the crystal, thus creating what are known as acoustic (or phononic) bandgaps. The vast majority of phononic crystal devices reported prior to this LDRD were constructed by hand assembling scattering inclusions in a lossy viscoelastic medium, predominantly air, water or epoxy, resulting in large structures limited to frequencies below 1 MHz. Under this LDRD, phononic crystals and devices were scaled to very (VHF: 30-300 MHz) and ultra (UHF: 300-3000 MHz) high frequencies utilizing finite difference time domain (FDTD) modeling, microfabrication and micromachining technologies. This LDRD developed key breakthroughs in the areas of micro-phononic crystals including physical origins of phononic crystals, advanced FDTD modeling and design techniques, material considerations, microfabrication processes, characterization methods and device structures. Micro-phononic crystal devices realized in low-loss solid materials were emphasized in this work due to their potential applications in radio frequency communications and acoustic imaging for medical ultrasound and nondestructive testing. The results of the advanced modeling, fabrication and integrated transducer designs were that this LDRD produced the 1st measured phononic crystals and phononic crystal devices (waveguides) operating in the VHF (67 MHz) and UHF (937 MHz) frequency bands and established Sandia as a world leader in the area of micro-phononic crystals.

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Phononic crystals are the acoustic wave analogue of photonic crystals. Here a periodic array of scattering inclusions located in a homogeneous host material forbids certain ranges of acoustic frequencies from existence within the crystal, thus creating what are known as acoustic bandgaps. The majority of previously reported phononic crystal devices have been constructed by hand, assembling scattering inclusions in a viscoelastic medium, predominantly air, water or epoxy, resulting in large structures limited to frequencies below 1 MHz. Recently, phononic crystals and devices have been scaled to VHF (30-300 MHz) frequencies and beyond by utilizing microfabrication and micromachining technologies. This paper reviews recent developments in the area of micro-phononic crystals including design techniques, material considerations, microfabrication processes, characterization methods and reported device structures. Micro-phononic crystal devices realized in low-loss solid materials are emphasized along with their potential application in radio frequency communications and acoustic imaging for medical ultrasound and nondestructive testing. The reported advances in batch micro-phononic crystal fabrication and simplified testing promise not only the deployment of phononic crystals in a number of commercial applications but also greater experimentation on a wide variety of phononic crystal structures. © 2009 IOP Publishing Ltd.

The combination of Proximity-field nanoPatterning (PnP) and graded temperature ALD has enabled the synthesis of robust three dimensional nanostructures. The PnP process uses a simple elastomeric optical phase mask to generate a complex three dimensional interference pattern in photopolymer 1. Once the photopolymer structure has been obtained, it is subsequently used as a template for graded temperature ALD. The graded temperature ALD chemistry is used to coat and lock-in the designed nanostructure without melting the template. This process generates a thermally robust nanostructure for further, higher temperature, ALD surface treatments. The ALD chemistry is performed at various (increasing) temperatures to secure the nanostructure and to reduce the macroscopic stress of the structure as higher temperature depositions are performed. Three methods for nanostructure characterization have been useful in interrogating these structures: quartz crystal microbalance (QCM), optical interference, and focused ion beam scanning electron microscopy (FIB-SEM). This paper will cover the fabrication process for generating PnP nanostructures. Details of the graded temperature ALD chemical process for AI2O3 will be covered. Also, structural characterizations using SEM and optical interference will be used to quantify the degree of deposition and the thermal stability of these interesting structures. © The Electrochemical Society.

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The three-dimensional confinement inherent in InAs self-assembled quantum dots (SAQDs) yields vastly different optical properties compared to one-dimensionally confined quantum well systems. Intersubband transitions in quantum dots can emit light normal to the growth surface, whereas transitions in quantum wells emit only parallel to the surface. This is a key difference that can be exploited to create a variety of quantum dot devices that have no quantum well analog. Two significant problems limit the utilization of the beneficial features of SAQDs as mid-infrared emitters. One is the lack of understanding concerning how to electrically inject carriers into electronic states that allow optical transitions to occur efficiently. Engineering of an injector stage leading into the dot can provide current injection into an upper dot state; however, to increase the likelihood of an optical transition, the lower dot states must be emptied faster than upper states are occupied. The second issue is that SAQDs have significant inhomogeneous broadening due to the random size distribution. While this may not be a problem in the long term, this issue can be circumvented by using planar photonic crystal or plasmonic approaches to provide wavelength selectivity or other useful functionality.

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