We are interested in the application of arrays of electron field emitters, which can be achieved from a variety of materials, for the preparation of compact and coherent X-ray sources via inverse Compton scattering.  Field emission of electrons is commonly achieved by applying a static electric field or optical illumination to sharp metal tips. Sub-wavelength nanostructures can provide geometry-dependent electric field enhancement for both methods. For applications in coherent X-ray sources, the field emitter arrays should be able to emit short electron pulses, typically on the femtosecond timescale, which is difficult using conventional electrical circuits. Therefore, optical triggering, whereby a femtosecond laser is used to stimulate electron emission, has been considered.

We have simulated optical fields around various field emitter structures using COMSOL finite element software.  Among them, we are particularly interested in the “bullet” structure illustrated in Figure 1.  Conical tip structures are widely used to achieve both electrostatic and optical field enhancement in field emitters; however, uniform conical structures pose significant challenges in nanoscale fabrication due to their tapered geometry.  Arrays of metallic “bullet” structures, as shown in Figure 1, may be fabricated with a high areal density via positive-tone electron beam lithography with a PMMA resist.

We have designed a fabrication process for the preparation of arrays of optical-field emitters, based on the metallic “bullet” structure shown in Figure 2 (a).  The thin SiO₂ layer shown in Figure 2 (a) is used to prevent electron emission from the bulk silicon substrate by acting as an electrically insulating barrier.  The thickness of the SiO₂ layer has yet to be optimized. As a proof of concept, we have fabricated an array of Au nanoparticle emitters with an aspect-ratio of 1 (see Figure 2 (b)).  Further optimization of the fabrication process is currently underway.

1. D. Temple, “Recent progress in field emitter array development for high performance applications,” Materials Science and Engineering: R: Reports, vol. 24, pp. 185-293, Jan. 1999.
2. O. J. F. Martin and C. Girard, “Controlling and tuning strong optical field gradients at a local probe microscope tip apex,” Applied Physics Letters, vol. 70, pp. 705-707, Feb. 1997.
3. L. Novotny, R. X. Bian, and X. S. Xie, “Theory of nanometric optical tweezers,” Physical Review Letters, vol. 79, pp. 645-648, July 1997.

Figure 1: a). Schematic of a 2-bit position array by inductive current splitting. The detectors are biased at I_B. L_S is the choke inductor. L_K is the total kinetic inductance of the nanowires, including the active nanowires and the nanowire-inductors. R_K is the on-chip Ti/Au resistor. L_B, C, and Z₀ are the inductance of bonding wire, coupling capacitor, and input impedance of the readout circuit, respectively. The orange arrows show the current distribution when the second detector D2 is fired. The width of each arrow is proportional to the amplitude of the current. b). The simulated waveforms of the differential output, V_L-V_R. The position information can be clearly read by the amplitude of the pulses. c). The current fluctuation in the unfired detectors when the second detector D2 is fired. All the detectors are biased at 0.8 I_B/I_C. The maximum overshot is 3% of I_C showing the array can work steadily without being affected much by the fired detector.

Single pixels of superconducting nanowire single photon detector (SNSPD) have high detection efficiency, low dark-count rate, and low timing jitter. To achieve a large active area, spatial resolution, or photon-number resolution, an SNSPD array is a promising approach for applications in quantum-information processing, single-photon imaging, and deep-space communication. However, designing a readout scheme is a serious challenge for multi-pixel detector arrays. Two groups have previously used single flux quantum (SFQ) to multiplex the array and have reported some experiment results on single detector ^{[1] [2]} . This method is useful for large-scale SSPD array but Josephson junctions are needed to be fabricated on the same chip and increase the complexity of the fabrication.

For a medium sized SNSPD array, we propose a simple design using only an on-chip integrated inductor and resistor network. The positions and number of detectors that have received a photon are read by the combination of difference and sum signals of the pulses in the two outputs. The maximum size of the array is limited by the amplitude of the differential output signal, the electric noise, the sensitivity of the readout circuit, and the circuit element variability. To overcome these limitations and increase the size of the array, we made improvements over conventional superconducting nanowire single photon detectors in two ways. First, we redesigned the detector to operate by a multi-stage avalanche mechanism, which increases the output signal dramatically. Second, we added a cryogenic amplifier with a low-loss connection to the readout circuit, increasing the signal-to-noise ratio by a factor of ~4. Additionally, to construct the array, we have designed and tested on-chip resistors. We also simulated and analyzed the array in a lumped circuit element and electro-thermal model. Based on simulation results, the size of an array resulting from this approach could be over 100 elements.

1. S. Miki, H. Terai, T. Yamashita, K. Makise, M. Fujiwara, M. Sasaki, and Z. Wang, “Superconducting single photon detectors integrated with single flux quantum readout circuits in a cryocooler,” Applied Physics Letters, vol. 99, no. 11, pp. 111 108, 2011.
2. T. Ortlepp, M. Hofherr, L. Fritzsch, S. Engert, K. Ilin, D. Rall, H. Toepfer, H. G. Meyer, and M. Siegel, “Demonstration of digital readout circuit for superconducting nanowire single photon detector,” Opt. Express, vol. 19, no. 19, pp. 18 593-18 601, Sep. 2011.

Photolithography’s accuracy and scalability have made it the method for sub-micron-scale definition of single-crystal semiconductor devices for over half a century. Unfortunately, organic semiconductor devices are chemically incompatible with the types of resists, solvents, and etchants traditionally used. This work investigates the use of an uncommonly used chemically inert resist method ^{[1] [2] [3] [4] [5]} that relies on physical phase changes for lift-off patterning of thin films of organic semiconductors and metals.

The resist gas is flowed over a cryogenically cooled substrate, where it freezes solid as schematically shown in Figure 1. This layer an be patterned by thermal excitation in a number of ways to define the areas where the desired thin film is to remain.  After the desired thin film or films are deposited, the substrate is brought up above the resist material’s sublimation point, leaving behind only the intended pattern. All the unwanted regions are lifted-off by the subliming resist.

Creating and defining the shadow mask on the surface of the substrate in this manner allow for patterning it with a stamp or roller with micron-scale features without changing the process conditions.  In this work, carbon dioxide is used as the sublimable mask material, and prototype stamps have been fabricated using SU-8 photoresist. A mask and the subsequent organic thin film are shown in Figure 2. This process may provide an alternative to shadow masks and provide a manufacturing solution for large area organic electronics.

1. W. Johnson, R. Laibowitz, and C. Tsuei, “Condensed gas, in situ lithography,” IBM Technical Disclosure Bulletin, vol. 20, no. 9, Feb. 1978.
2. A. Han, D. Vlassarev, J. Wang, J. A. Golovchenko, and D. Branton, “Ice lithography for nanodevices,” Nano Letters, vol. 10, no. 12, pp. 5056-5059, Dec. 2010.
3. D. Branton, J. A. Golovchenko, G. M. King, W. J. MoberlyChan, and G. M. Schürmann, “Lift-off Patterning Processing Employing Energetically-stimulated Local Removal of Solid-condensed-gas Layers,” U.S. Patent 752443 B1, April 28, 2009.
4. G. M. King, G. Schürmann, D. Branton, and J. A. Golovchenko, “Nanometer patterning with ice,” Nano Letters, vol. 5, no. 6, pp. 1157-1160, June 2005.
5. J. Cuomo, C. Guarnieri, K. Saenger, and D. Yee, “Selective deposition with ‘dry’ vaporizable lift-off mask,” IBM Technical Disclosure Bulletin, vol. 35, no. 1, June 1992.

Figure 1: Detection efficiency (η) of a 30 nm width SNSPD as a function of normalized bias current (IB/Isw) measured at wavelength λ = 1.4 µm (blue curve), λ = 3.7 µm (red curve), and λ = 5 µm (black curve).

We report on the detection of single-photons in the middle-infrared (mid‑IR) range using superconducting nanowire single-photon detectors (SNSPDs). In 2011 our group managed to detect single-photons at a wavelength λ = 1.55 µm using SNSPDs based on ultra-narrow nanowires (30‑nm‑width) with a saturated detection efficiency of η = 20 % at a lower bias current than is possible for wider NbN nanowires[1] (I_B = 0.7 I_sw, where I_sw is the bias current after which the detectors switch to the normal state). These results suggest that ultra-narrow SNSPDs are suitable for detection of photons at longer wavelengths. Figure 1 shows the detection efficiency η measured as a function of the normalized bias current (I_B/I_sw) for λ = 1.4 µm (blue curve), λ = 3.6 µm (red curve), and λ = 5 µm (black curve). The detection efficiency measured at λ = 1.4 µm does not reach η = 20 % because the polarization of the photons could not be optimized to enhance the absorption in the nanowires as in reference ^[1] . The detection efficiency reaches a value of η = 4 % at λ = 3.7 µm, which is two orders of magnitude higher than what has been reported in the literature so far for this type of detector at this wavelength. The figure also shows that the detection efficiency reaches an appreciable value of η = 2 % at λ = 5 µm.

The authors thank E. Dauler and R. J. Molnar from Lincoln Laboratory for technical support and scientific discussions.

1. F. Marsili, F. Najafi, E. Dauler, F. Bellei, X. Hu, M. Csete, R.J. Molnar, and K.K. Berggren, “Single-photon detectors based on ultra-narrow superconducting nanowires,” Nano Letters, vol. 11, pp. 2048-53, May 2011.

In recent years, several ion-trapping groups have shown that microwave electrodes integrated into surface electrode ion traps can be used for state manipulation of trapped ions ^[1] . Our group has pursued integrating microwave electrodes into superconducting ion traps to perform microwave spectroscopy in both atomic and molecular ions. Large fields are required to perform state manipulation, and these fields can be locally amplified using resonators. This amplification comes at the expense of bandwidth on the spectroscopic signal.

We designed microwave-integrated ion traps and fabricated them with a niobium on A-plane sapphire process in the Nanostructures Laboratory at MIT. We deposited thin films between 50-200 nm of niobium on the sapphire with an in-house DC magnetron sputtering system. Once we completed the material stack, we patterned the front niobium layer in PR1-2000A resist using the Heidelberg uPG-101 pattern generator, developed it, and then reactive-ion etched it with CF4 and O2 to transfer the pattern into the niobium

We are currently testing to see if our integrated microwave signals can excite the microwave clock transition in ⁸⁷Sr⁺ ions. This experiment uses microwave signals to couple between two hyperfine levels in the ground state of ⁸⁷Sr⁺. A laser excites the atom from one of the ground states to an excited state; when the atom decays back to one of the ground states, photons are emitted, and they can be imaged. Without the applied microwaves, the ion would quickly shelve into a dark state.

1. Ospelkaus, C., Warring, U., Colombe, Y., Brown, K.R., Amini, J.M., Leibfried, D., Wineland, D.J., “Microwave quantum logic gates for trapped ions,” Nature, vol. 476, no. 7359, pp. 181-184, 2011.

Superconducting nanowire single-photon detectors (SNSPDs) ^[1] , based on 100-nm-wide, ~ 4-nm‑thick niobium nitride (NbN) nanowires, are unmatched in sensitivity ^[2] and timing accuracy ^[3] by other detector technologies at standard optical telecommunication wavelengths. The detection efficiency (η) of SNSPDs is the product of the optical absorption of the nanowire meander (A, which increases with the thickness of the nanowires) and the probability of photon induced resistive state formation in the nanowire (P_r, which increases with decreasing cross-sectional area of the nanowires ^[4] ).

Figure 1: (a) Cross-sectional view of an SNSPD based on 20-nm-wide and 10-nm-thick NbN nanowires with an integrated HSQ cavity. The pitch is 40 nm (the nanowires fill 50% of the detector area). The detector is illuminated from the back of the substrate with light polarized in parallel to the nanowires. The thickness of the cavity was optimized for maximum absorption in the NbN nanowires. The optimum cavity thickness was found to be 270 nm. (b) Simulated optical absorption at 1550-nm wavelength for the geometry shown in (a) as a function of cavity thickness.

We propose a high‑detection‑efficiency detector based on ultra-narrow (20-nm‑wide), thick (10‑nm‑thick instead of 4-nm-thick) nanowires arranged in a 50%-fill-factor meander. In this detector, we expect the decrease of P_r due to the thicker nanowire to be compensated by reducing the width of the nanowire down to 20 nm. When integrated with a quarter-wavelength optical cavity, as shown in Figure 1a, this detector is expected to have a peak optical absorption of 96.5% (Figure 1b, based on a COMSOL model reported in Ref. ^[5] ). The nanowire cross-sectional area (200nm²) of the proposed detector is comparable to the nanowire cross-sectional area of ultranarrow-nanowire SNSPDs (~ 150nm², as in Ref. ^[6] ), which have P_r ~ 0.9. If we assume the same P_r value for the proposed detector, it is expected to have a detection efficiency of η = P_{r *} A ~ 87%. A 15% variation in the cavity thickness is expected to result in a variation of < 1% in the absorption, further relaxing the fabrication constraints. We are currently working on the experimental implementation of this cavity-integrated detector.

1. G. N. Gol’tsman et al., Applied Physics Letters vol. 79, p. 705,  2001.
2. A. Korneev et al., IEEE T Appl Supercon vol. 15, pp. 571-574, 2005.
3. E. A. Dauler et al., IEEE T Appl Supercon, vol. 17, pp. 1051-8223, 2007.
4. A. Semenov et al., The European Physical Journal B, vol. 47, pp. 495-501,  2005.
5. V. Anant et al., Opt Express vol. 16, pp. 10750-10761 (2008)
6. F. Marsili et al., Nano Letters vol. 11, p. 2048, 2011.

In this research we seek to develop acousto-optic, guided-wave modulators in proton-exchanged lithium niobate for use in holographic and other high-bandwidth displays.  Guided-wave techniques make possible the fabrication of modulators that are higher in bandwidth and lower in cost than analogous bulk-wave acousto-optic devices.  In particular, we are investigating multichannel variants of these devices with an emphasis on maximizing the number of modulating channels to achieve large total bandwidths.  Efficient, low-cost, monolithic modulators capable of modulating billions of pixels/sec should be possible.

1. C. S. Tsai, Q. Li, and C. L. Chang, “Guided-wave two-dimensional acousto-optic scanner using proton-exchanged lithium niobate waveguide,” Fiber and Integrated Optics, vol. 17, pp. 57-166, 1998.
2. D. Smalley, “High-resolution spatial light modulation for holographic video,” Master’s thesis, Massachusetts Institute of Technology, Cambridge MA, 2008.
3. D. E. Smalley, Q. Y. J. Smithwick, and V. M. Bove, Jr., “Holographic video display based on guided-wave acousto-optic devices,” Proc. SPIE Practical Holography XXI, 2007, vol. 6488, p. 64880L.
4. J. Barabas, S. Jolly, D. E. Smalley, and V. M. Bove, Jr., “Diffraction specific coherent panoramagrams of real scenes,” Proc. SPIE Practical Holography XXV, 2011, vol. 7957, p. 795702.

The cost of silicon solar cells has fallen precipitously in recent years, primarily as a result of manufacturing improvements, increasing scale, and decreasing profit margin.  Further cost reductions will depend upon technical advances that increase cell efficiency and/or minimize the variable costs associated with module production.  One strategy to lower the cost of silicon photovoltaic modules is to dramatically reduce the amount of material required in a cell from the current 180-μm standard to 10 μm or less.  However, since the absorption length for longer-wavelength photons (red and infrared) is significantly larger than this thickness, thin cells must be designed to trap photons in the absorber layer very effectively to yield competitive efficiencies.  Light-trapping in conventional wafer-based photovoltaics is well understood ^[1] , and light-trapping structures are integrated into virtually all commercially available solar cells.  However, the physical basis associated with light-trapping in thin-film silicon cells is quite different than that for conventional silicon photovoltaics, both because the thinness of the material physically limits the dimensions of light-trapping features and because light-trapping based on geometric optics is less effective for very thin materials.  In this work, simulations based on the transfer matrix method were developed to identify optimal surface structures for light trapping.  As shown in Figure 1, a variety of potential geometries offer significant enhancement over a planar silicon surface.  Of the structures that are modeled in this work, the one offering the best combination of light-trapping effectiveness and manufacturability is a two-dimensional periodic array of inverted pyramids on a sub-micrometer pitch ^{[2] [3]} , as in Figure 2.  Theoretical calculations suggest that a 10 μm silicon film textured in this way can absorb as effectively as a flat film 300 μm thick.  Demonstration versions on SOI substrates are currently being fabricated in the Microsystems Technology Laboratories at MIT.

1. P. Campbell and M. A. Green, “Light-trapping of pyramidally textured surfaces,” Journal of Applied Physics, vol. 62, no. 1, pp. 243-249, July 1987.
2. S. E.  Han and G. Chen, “Toward the Lambertian limit of light-trapping in thin nanostructured silicon solar cells,” Nano Letters, vol. 10, pp. 4692-4696, Oct. 2010.
3. S. E. Han, A. Mavrokefalos, M. S. Branham, and G. Chen, “Efficient light-trapping nanostructures in thin silicon solar cells,” Proc. of  SPIE:  Micro- and Nanotechnology Sensors, Systems, and Applications III, 2011, vol. 8031, p. 80310T.

Professor Ed Boyden uses light to precisely control neural activity.  His lab has invented safe, effective ways to deliver light-gated membrane proteins to neurons and other excitable cells (e.g., muscle, immune cells, pancreatic cells, etc.) in an enduring fashion, thus making the cells permanently sensitive to being activated or silenced by millisecond-timescale pulses of blue and yellow light, respectively ^[1] .  This ability to modulate neural activity with a temporal precision that approaches that of the neural code itself holds great promise for human health, and his lab has developed animal models of epilepsy and Parkinson’s disease to explore the use of optical control to develop new therapies.

We have recently developed mass-fabricatable multiple light guide microstructures produced using standard microfabrication techniques to deliver light to activate and silence neural target regions along their length as desired ^[2] .  Each probe is a 100- to 150-micron-wide insertable micro-structure with many miniature lightguides running in parallel and delivering light to many points along the axis of insertion.  Such a design maximizes the flexibility and power of optical neural control while minimizing tissue damage.  We are currently developing 2-D arrays of such probes so multiple colors of light can be delivered to 3-dimensional patterns in the brain, at the resolution of tens to hundreds of microns, thus furthering the causal analysis of complex neural circuits and dynamics.  Such devices will allow the substrates that causally contribute to neurological and psychiatric disorders to be systematically analyzed via causal neural control tools.  Given recent efforts to test such reagents in nonhuman primates, these devices may also enable a new generation of optical neural control prosthetics, contributing directly to the alleviation of intractable brain disorders.

The initial light-guide structures have been fabricated from silicon oxynitride clad with silicon dioxide, and tests show excellent transmission of light with no visible loss in the taper and bend regions of the patterns ^[2] .  Significantly, the novel 90˚ bend invented to direct light laterally out the side of the narrow probe functions as designed ^[2] .  The optical sources for initial tests with the probe are independent laser modules coupled to one end of a fiber-optic ribbon cable (see Figure 2).  The other end of the ribbon cable is butt-coupled to the inputs of the probe via a standard fiber-optic connector ferrule.  This allows for increased modularity and control in initial probe testing.

We are now utilizing transgenic mice, which express optogenetic activators and silencers in cortical pyramidal neurons, to demonstrate optogenetic control of neural circuits in a fashion appropriate for in vivo circuit mapping or brain machine interface prototyping.  Our goal is to explore the degree to which this technology can be used to functionally map neural network connectivity over large, multi-region circuits in the brain, and to subserve a new generation of neural control prosthetics.

1. X. Han and E. S. Boyden, “Multiple-color optical activation, silencing, and desynchronization of neural activity, with single-spike temporal resolution,” PLoS ONE, vol. 2, no. 3, p. e299, Mar. 2007.
2. A. N. Zorzos, E. S. Boyden, and C. G. Fonstad, “A multi-waveguide Implantable probe for light delivery to distributed brain targets,” Applied Optics Letters vol. 35, no. 12, pp. 4133-4135, Dec. 2010.

Optogenetics is commonly used for precision modulation of the activity of specific neurons within neural circuits ^[1] , but assessing the impact of optogenetic neuromodulation on the neural activity of local and global circuits remains difficult. Our collaborative team recently initiated a project (Scholvin et al., SFN 2011) to design and implement 3-D silicon-micromachined electrode arrays with customizable electrode locations, targetable to defined neural substrates distributed in a 3-D pattern throughout a neural network in the mammalian brain and compatible with simultaneous use of a diversity of existing light-delivery devices.

We have developed a series of innovations aimed at facilitating the scalability aspect of these probes, i.e., aspects of probe design that should enable them to scale to 1000 channels of neural recording or more.  First, we have developed streamlined electrode fabrication methodologies that enable micromachined probes to be first fabricated using conventional silicon micromachining, then rapidly assembled into custom 3-D arrays, with semi-automated formation of the necessary electrical connections and mechanical constraints.  Second, we have developed a set of surgical and insertion technologies towards the goal of enabling the insertion of electrode arrays with a high number of electrode shanks into the brain, while minimizing probe insertion damage.  Finally, to facilitate scaling of the channel count beyond what is feasible with external amplifiers, we are exploring new approaches for integration of amplifier circuits directly on the probe arrays themselves, to remove bottlenecks associated with connecting of probes to the outside world.

1. X. Han and E. S. Boyden, “Multiple-color optical activation, silencing, and desynchronization of neural activity, with single-spike temporal resolution,” PLoS ONE, vol. 2, no. 3, p. e299, Mar. 2007.