Henry I. Smith

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

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Publications (329)591.65 Total impact

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    Christel Zanke, Minghao Qi, Henry I. Smith
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    ABSTRACT: We designed and analyzed a "mesh-stack" three-dimensional photonic crystal of a 12.4% bandgap with a dielectric constant ratio of 12∶1. The mesh-stack consists of four offset identical square-lattice air-hole patterned membranes in each vertical period that is equal to the in-plane period of the square lattice. This design is fully compatible with the membrane-stacking fabrication method, which is based on alignment and stacking of large-area single-crystal membranes containing engineered defects. A bandgap greater than 10% is preserved as long as the membranes are subjected to in-plane misalignment less than 3% of the square period. By introducing a linear defect with a nonsymmorphic symmetry into the mesh-stack, we achieved a single-mode waveguide over a wide bandwidth.
    Optics Letters 11/2012; 37(22):4726-8. · 3.39 Impact Factor
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    ABSTRACT: Accurate conversion of wideband multi-GHz analog signals into the digital domain has long been a target of analog-to-digital converter (ADC) developers, driven by applications in radar systems, software radio, medical imaging, and communication systems. Aperture jitter has been a major bottleneck on the way towards higher speeds and better accuracy. Photonic ADCs, which perform sampling using ultra-stable optical pulse trains generated by mode-locked lasers, have been investigated for many years as a promising approach to overcome the jitter problem and bring ADC performance to new levels. This work demonstrates that the photonic approach can deliver on its promise by digitizing a 41 GHz signal with 7.0 effective bits using a photonic ADC built from discrete components. This accuracy corresponds to a timing jitter of 15 fs - a 4-5 times improvement over the performance of the best electronic ADCs which exist today. On the way towards an integrated photonic ADC, a silicon photonic chip with core photonic components was fabricated and used to digitize a 10 GHz signal with 3.5 effective bits. In these experiments, two wavelength channels were implemented, providing the overall sampling rate of 2.1 GSa/s. To show that photonic ADCs with larger channel counts are possible, a dual 20-channel silicon filter bank has been demonstrated.
    Optics Express 02/2012; 20(4):4454-69. · 3.55 Impact Factor
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    ABSTRACT: We report the fabrication of a reconfigurable wide-band twenty-channel second-order dual filterbank, defined on a silicon-on-insulator (SOI) platform, with tunable channel spacing and 20 GHz single-channel bandwidth. We demonstrate the precise tuning of eleven (out of the twenty) channels, with a channel spacing of 124 GHz (~1 nm) and crosstalk between channels of about -45 dB. The effective thermo-optic tuning efficiency is about 27 μW/GHz/ring. A single channel of a twenty-channel counter-propagating filterbank is also demonstrated, showing that both propagating modes exhibit identical filter responses. Considerations about thermal crosstalk are also presented. These filterbanks are suitable for on-chip wavelength-division-multiplexing applications, and have the largest-to-date reported number of channels built on an SOI platform.
    Optics Express 01/2011; 19(1):306-16. · 3.55 Impact Factor
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    ABSTRACT: We demonstrate a monolithic photonic integration platform that leverages the existing state-of-the-art CMOS foundry infrastructure. In our approach, proven XeF2 post-processing technology and compliance with electronic foundry process flows eliminate the need for specialized substrates or wafer bonding. This approach enables intimate integration of large numbers of nanophotonic devices alongside high-density, high-performance transistors at low initial and incremental cost. We demonstrate this platform by presenting grating-coupled, microring-resonator filter banks fabricated in an unmodified 28 nm bulk-CMOS process by sharing a mask set with standard electronic projects. The lithographic fidelity of this process enables the high-throughput fabrication of second-order, wavelength-division-multiplexing (WDM) filter banks that achieve low insertion loss without post-fabrication trimming.
    Optics Express 01/2011; 19(3):2335-46. · 3.55 Impact Factor
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    ABSTRACT: The secondary-electron signal levels of eight thiophenol-based self-assembled monolayers (SAMs) on gold (Au) are measured and compared against the signal level from bare gold between energies 1 and 2 keV. To enable accurate comparison, scanning electron micrographs of SAMs are taken with a Faraday cup and a reference sample. Most SAMs-on-gold produce a lower signal level than that from bare gold, with the exception of 3-methylthiophenol. Highest occupied molecular orbital and lowest unoccupied molecular orbital levels of the thiophenol derivatives are calculated and compared against the signal levels. Signal levels from bis[3-(triethoxysilyl)propyl]tetrasulfide, (4-chlorophenyl)-triethoxysilane, and amino-propyl-triethoxy-silane on titanium (Ti) and aluminum (Al) are also measured. All three SAMs on aluminum have lower signal levels than bare Al but this effect is reversed for the case of Ti, where SAMs deposited on Ti result in a higher signal level. A hybrid Ti/Al fiducial grid is fabricated and the point-spread function at 2 keV in the underlying resist is investigated.
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 01/2011; 29. · 1.36 Impact Factor
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    ABSTRACT: We fabricated 9–30 nm half-pitch nested Ls and 13–15 nm half-pitch dot arrays, using 2 keV electron-beam lithography with hydrogen silsesquioxane (HSQ) as the resist. All structures with 15 nm half-pitch and above were fully resolved. We observed that the 9 and 10-nm half-pitch nested Ls and the 13-nm-half-pitch dot array contained some resist residues. We obtained good agreement between experimental and Monte-Carlo-simulated point-spread functions at energies of 1.5, 2, and 3 keV. The long-range proximity effect was minimal, as indicated by simulated and patterned 30 nm holes in negative-tone resist.
    Microelectronic Engineering. 01/2011; 88(10):3070-3074.
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    ABSTRACT: The fabrication of complex three-dimensional (3D) structures at sub-100 nm resolution presents a difficult challenge. 3D photonic crystals that contain waveguides, resonant cavities, filters or other devices, and require deep-sub-100 nm dimensional control, are a particular example of this challenge. Multilayer 3D structures can be formed by stacking and bonding thin membranes that have been patterned in advance. This approach enables the full panoply of 2D planar-fabrication techniques to be employed. Membranes containing patterns that are not perfectly regular will exhibit in-plane distortion unless their intrinsic stress is zero. To minimize the effects of intrinsic stress we float individual membranes on the surface of a liquid. Thin single-crystal Si membranes on an oxide substrate are first patterned and then removed by etching the oxide in hydrofluoric acid. The freed Si membranes readily float on the liquid surface, aided by the hydrophobic nature of H-terminated Si. The authors describe methods for cleaning, patterning, manipulating, bonding and stacking such freely floating membranes.
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 01/2011; 29. · 1.36 Impact Factor
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    ABSTRACT: The authors describe an approach to fabricating high resolution, complex 3D structures based on the stacking of thin membranes that have been patterned in advance. The membranes are attached to a rigid frame by means of tethers that are strong enough to permit normal handling but can be cleaved after bonding. The tether shape was designed using finite-element analysis to enable clean cleavage at a specific location so that fragments are avoided that would interfere with the bonding of subsequent layers. The authors used 12 × 12 mm SiNx membranes, 350 nm thick, patterned with a square array of holes at 600 nm pitch and demonstrate the stacking of three layers.
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 01/2011; 29. · 1.36 Impact Factor
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    ABSTRACT: A novel dynamical slow light cell with a tunable group delay, fabricated in silicon-on-insulator, is demonstrated. It provides a tuning range of more than 1 ns, with a usable group delay of about 0-24 ps.
    07/2010;
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    ABSTRACT: A wide-band eleven-channel second-order filterbank fabricated on an SOI platform is demonstrated with tunable channel spacing and 20 GHz single-channel bandwidths. The tuning efficiency is ∼28 µW/GHz/ring.
    05/2010;
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    ABSTRACT: To achieve the maximum benefit of electronic-photonic integrated circuits wavelength-division multiplexing must be used. This requires the design and fabrication of a highly integratable photonic device, capable of performing multiplexing/demultiplexing operations with low loss and minimal crosstalk. A filter bank consisting of high-index-contrast microring-resonator filters, with accurately spaced resonant frequencies can meet these requirements. This paper describes the basic architecture of microring-resonator filter banks, and how to maximize performance while keeping fabrication challenges reasonable. The greatest challenge in fabricating such devices is achieving the dimensional precision, on the scale of tens of picometers, needed to attain accurately spaced resonant frequencies. To do this, a fabrication method based on varying the electron-beam dose during scanning-electron beam lithography is used. This approach is used to create a dual twenty-channel filter bank, comprised of second-order silicon-rich silicon nitride microring resonators. The average resonant frequency spacing is off from the target spacing by only 3 GHz, corresponding to a dimensional precision of 75 pm. This approach is also shown to be compatible with the fabrication process for silicon microring resonators. Furthermore, it is shown that any remaining resonant frequency errors can be corrected with postfabrication thermal tuning. Also, a method of using the contra-propagating mode of a microring-resonator filter is demonstrated, enabling a single filter bank to multiplex/demultiplex two signals at the same time.
    Journal of Nanoscience and Nanotechnology 03/2010; 10(3):2044-52. · 1.15 Impact Factor
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    ABSTRACT: The ground-state helium-4 beam employed by the microscopy technique discussed in this article interacts exclusively with the atoms in the topmost sample-monolayer. Its low-energy (tens of meV) and chemical inertness make the beam an almost ‘ideal’ imaging probe in the sense that a sample surface can be imaged without alteration by the probe. The microscopy technique therefore has promising applications in the imaging of fragile samples and metrology. In this article we present a fabrication process for the diffraction-based focusing element (Fresnel Zoneplate) in such a setup. Zoneplates made previously for this purpose have suffered from low transmission, a problem we have solved with our new process. In addition, we have measured the first-order diffraction efficiency of three zoneplates fabricated with this process. The efficiency of a zoneplate with 388 μm diameter was close to 70% of the theoretically predicted value. We believe the reduction stems mainly from misalignment between the writefields used to pattern the zoneplate. The fabricated zoneplates of 190 μm diameter, which we patterned using a single writefield, have close to the theoretically predicted transmission characteristics; a result that has not been achieved before for neutral atom Fresnel lenses.
    Microelectronic Engineering 01/2010; 87:1011-1014. · 1.22 Impact Factor
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    ABSTRACT: In the Poisson-spot experiment, waves emanating from a source are blocked by a circular obstacle. Due to their positive on-axis interference an image of the source �the Poisson spot� is observed within the geometrical shadow of the obstacle. In this paper we report the observation of Poisson’s spot using a beam of neutral deuterium molecules. The wavelength independence and the weak constraints on angular alignment and position of the circular obstacle make Poisson’s spot a promising candidate for applications ranging from the study of large molecule diffraction to patterning with molecules.
    Physical Review A 01/2009; 79:053823. · 3.04 Impact Factor
  • Hsin-Yu Tsai, Henry I Smith, Rajesh Menon
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    ABSTRACT: We experimentally verify the focusing characteristics of dichromats, a new class of circular-symmetric diffractive-optical lenses that generate, in the same focal plane, focal spots for one wavelength and ring-shaped spots with central nodes for another wavelength. Using a dichromat, we illuminate a thin photochromic layer and demonstrated point-spread-function compression of the transmitted focal spot.
    Optics Letters 01/2009; 33(24):2916-8. · 3.39 Impact Factor
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    ABSTRACT: We present a new monolithic silicon photonics technol- ogy suited for integration with standard bulk CMOS pro- cesses, which reduces costs and improves opto-electrical coupling compared to previous approaches. Our tech- nology supports dense wavelength-division multiplexing with dozens of wavelengths per waveguide. Simulation and experimental results reveal an order of magnitude better energy-efficiency than electrical links in the same technology generation. Exploiting key features of our photonics technology, we have developed a processor- memory network architecture for future manycore sys- tems based on an opto-electrical global crossbar. We illustrate the advantages of the proposed network archi- tecture using analytical models and simulations with syn- thetic traffic patterns. For a power-constrained system with 256 cores connected to 16 DRAM modules using an opto-electrical crossbar, aggregate network throughput can be improved by ! 8-10" compared to an optimized purely electrical network.
    IEEE Micro 01/2009; 29:8-21. · 2.39 Impact Factor
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    ABSTRACT: Periodic structures, such as gratings and grids, are required in a variety of applications including spectroscopy, photonic and phononic devices, and as substrates for basic studies in materials science. Interference lithography readily forms periodic patterns in photoresist, but conventional approaches, using a Lloyd’s mirror or Mach–Zehnder configuration, suffer from a number of shortcomings including difficulty in aligning patterns with respect to pre-existing structures on a substrate and difficulty in precisely repeating a given spatial period. Coherent diffraction lithography (CDL), a mask-based approach, utilizes the well-known Talbot effect to accurately replicate the one- or two-dimentional pattern on a mask by reimaging the mask pattern in photoresist. Moreover, with appropriate alignment marks on the mask, one can align the replicated pattern relative to pre-existing patterns on the substrate. The authors describe the design, construction, and utilization of a dedicated CDL apparatus that permits replication, at a well-defined mask-substrate gap, of the periodic structure of a phase mask. The system also incorporates interferometric-spatial-phase imaging for aligning the replicated pattern relative to fixed fiducials on a substrate. They obtained high quality replications of a mask pattern, consisting of a 600 nm period grating, from the 1st to the 52nd plane of reimaging, i.e., from 1.55 to 40.16 μm.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 01/2009; 27(6). · 1.36 Impact Factor
  • Thomas B. OReilly, Henry I. Smith
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    ABSTRACT: Control of linewidth in patterns written with interference lithography (IL) is an important part of process control for many applications, requiring an understanding of how exposure parameters, such as dose and dose modulation, vary across the exposed area, and how a given photoresist will respond to changes in those parameters. This article presents a model of linewidth variation in a common IL system, Lloyd’s mirror, along with some results derived from that model that relate to optimizing the linewidth uniformity that can be achieved for a given application. An extension to a previously reported resist-characterization method that makes it possible to directly measure how changes in dose modulation affect linewidth is also discussed.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 12/2008; · 1.36 Impact Factor
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    ABSTRACT: We report results on the synthesis of silicon nanostructures that were fabricated using a combination of interference lithography and catalytic etching. With this technique, we were able to create nanostructures that are perfectly periodic over very large areas (1 cm(2) or more), where the cross-sectional shapes and the array ordering can be varied. Furthermore this technique can readily and independently control the sizes and spacings of the nanostructures down to spacings of 200 nm or less. These characteristics cannot be achieved using other known techniques.
    Nano Letters 12/2008; 8(11):3799-802. · 13.03 Impact Factor
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    ABSTRACT: Frequency mismatch in high-order microring-resonator filters is investigated. We demonstrate that this frequency mismatch is caused mainly by the intrafield distortion of scanning-electron-beam-lithography (SEBL) used in fabrication. The intrafield distortion of an SEBL system is measured, and a simple method is also proposed to correct this distortion. By applying this correction method, the average frequency mismatch in second-order microring-resonator filters was reduced from -8.6 GHz to 0.28 GHz.
    Optics Express 10/2008; 16(20):15958-63. · 3.55 Impact Factor

Publication Stats

4k Citations
591.65 Total Impact Points

Institutions

  • 1969–2013
    • Massachusetts Institute of Technology
      • • Research Laboratory of Electronics
      • • Department of Electrical Engineering and Computer Science
      • • Department of Materials Science and Engineering
      Cambridge, Massachusetts, United States
  • 1989–1995
    • Princeton University
      • Department of Electrical Engineering
      Princeton, NJ, United States
    • Osaka Prefecture University
      Sakai, Ōsaka, Japan