[show abstract][hide abstract] 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.
[show abstract][hide abstract] 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.
[show abstract][hide abstract] 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.
[show abstract][hide abstract] 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.
[show abstract][hide abstract] 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.
[show abstract][hide abstract] 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
[show abstract][hide abstract] 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.
[show abstract][hide abstract] 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.
[show abstract][hide abstract] 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.
[show abstract][hide abstract] 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
[show abstract][hide abstract] 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.
[show abstract][hide abstract] 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.
[show abstract][hide abstract] ABSTRACT: Physically intuitive coupled-mode and equivalent electrical-circuit theories are described for synthesis of nanophotonic resonator/interferometer circuits, including a new phase law for general 4-ports. Synthesis of self-adaptive (highly-nonlinear) optomechanical systems based on light forces is introduced.
[show abstract][hide abstract] ABSTRACT: Thermal tuning with an efficiency of 40uW/GHz/ring is demonstrated in silicon-rich silicon nitride second-order microring resonators. Open-loop thermal stability of the resonant frequency is measured to be within 400MHz for these resonators.
[show abstract][hide abstract] ABSTRACT: We present the first experimental demonstration of recently proposed loop-coupled resonator device concepts, with characteristic transmission zeros, enabling optimally sharp passbands for channel add-drop filter applications. Fourth-order SiN-core and Si-core strong-confinement microring-resonator designs are described. OCIS codes: (130.3120) Integrated optics devices; (230.5750) Resonators; (120.2440) Filters; (230.7370) Waveguides. Microphotonic circuits have enabled the integrated implementation of flat-top, high-order coupled-cavity filters , analogous to thin-film filters available in bulk optics, suitable for chip-scale wavelength routing applications [1-3]. These designs have – because they lack transmission zeros – limitations such as being suboptimally sharp, and being minimum-phase filters so that their amplitude and phase responses are related by the Kramers-Kronig condition. The latter implies that a flat-top-amplitude design is necessarily dispersive . However, planar microphotonic chips accommodate other new device topologies not easily implemented in bulk optics. For example, interferometer-embedded-resonator filters  permit responses with transmission zeros, but their extinction ratios are highly sensitive to designing a precise 3dB coupler. Recently, a new class of loop-coupled-resonator optical filters has been proposed that enables the realization of optimally sharp bandpass filters for a given number of cavities, and dispersionless (maximally linear phase) flat-top filters and optical delay lines . These designs are robust, and provide transmission zeros by coupling cavities in a loop as in Fig. 1(a). The four microrings coupled in this case have a layout geometry defined by a 4-side polygon connecting their centers – determined by their four mutual-coupling gaps and the angles of the polygon [Fig. 1(a)]. A new degree of freedom in this geometry is a tilt angle that can be applied to this polygon without changing the chord lengths (coupling gaps). This angle determines a new electromagnetic parameter of loop-coupled resonant structures, the loop-coupling phase (LCP), Φ, which is the phase accumulated by one propagation around the loop [5,6]. The LCP turns out to be directly related to the position of transmission zeros, as shown in Fig. 1(b). Proper choice of LCP allows optimally sharp or non-minimum-phase optimal linear-phase filters. While the present microring structures have analogues in the microwave domain [7,8], they are more general in practice. Standard, reciprocal microwave devices can only have LCP Φ = 0 or π, while the optical microring structures, because they are intrinsically 4-port reflectionless "hybrids" , can have arbitrary LCP (in principle reproducible only in non-reciprocal 2-ports). This gives greater control of zero placement and opens the possibility of novel response designs enabled by these geometries, see Fig. 1(b) . In this paper, we show the first experimental demonstration of loop-coupled resonators, designed to provide an optimally sharp 4 th -order add-drop filter, including the confirmation of transmission null interference mediated by the LCP. Two designs are used: a static microring filter in Si-rich SiN, and a thermally tunable Si microring filter. Fig. 1. (a) Layout of a 4 th -order SiN loop-coupled-resonator filter, with LCP Φ = π (8 μm microring radius, ring and bus cross-sections same as , gaps shown); (b) bandpass response of a Si device (waveguides same as ) and pole-zero constellation vs. the LCP, Φ.
[show abstract][hide abstract] ABSTRACT: Propagation losses are paramount to the performance of microphotonic devices. In silicon photonics, the expected contribution of known propagation-loss mechanisms is often insufficient to account for all the observed loss. Here, we identify a loss mechanism that we believe has not yet been reported in the literature. We observe loss reaching 70 dB / cm in silicon wire waveguides patterned in proximity of metals with low temperatures of silicide formation. The loss is attributed to formation of a dilute silicide at the waveguide sidewalls during reactive-ion etching. Sputtered metal atoms originate from exposed metal on the wafer surface or from the reactive-ion etcher chamber and react with the bare silicon of the waveguide sidewall being formed.
[show abstract][hide abstract] ABSTRACT: We describe silicon microring-resonator-based microphotonic circuits that support complete (amplitude and phase) disabling of resonant states, enabling novel capabilities: truly-hitless switching/tuning of high-order, telecom-grade channel add-drop filters, dispersionless FSR multiplication, and "hot-swapping" of photonic subsystems.
[show abstract][hide abstract] ABSTRACT: A structure including a grating and a semiconductor nanocrystal layer on the grating, can be a laser. The semiconductor nanocrystal layer can include a plurality of semiconductor nanocrystals including a Group II-VI compound, the nanocrystals being distributed in a metal oxide matrix. The grating can have a periodicity from 200 nm to 500 nm.