Optical technology is poised to revolutionize short-reach interconnects. The leading candidate technology is silicon photonics, and the workhorse of such an interconnect is the optical modulator. Modulators have been improved dramatically in recent years, with a notable increase in bandwidth from the megahertz to the multigigahertz regime in just over half a decade. However, the demands of optical interconnects are significant, and many questions remain unanswered as to whether silicon can meet the required performance metrics. Minimizing metrics such as the device footprint and energy requirement per bit, while also maximizing bandwidth and modulation depth, is non-trivial. All of this must be achieved within an acceptable thermal tolerance and optical spectral width using CMOS-compatible fabrication processes. This Review discusses the techniques that have been (and will continue to be) used to implement silicon optical modulators, as well as providing an outlook for these devices and the candidate solutions of the future.

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Silicon optical modulators

An overview is presented of the current state-of-the-art in silicon nanophotonic ring resonators. Basic theory of ring resonators is discussed, and applied to the peculiarities of submicron silicon photonic wire waveguides: the small dimensions and tight bend radii, sensitivity to perturbations and the boundary conditions of the fabrication processes. Theory is compared to quantitative measurements. Finally, several of the more promising applications of silicon ring resonators are discussed: filters and optical delay lines, label-free biosensors, and active rings for efficient modulators and even light sources.

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Silicon microring resonators

We examine the current performance and future demands of interconnects to and on silicon chips. We compare electrical and optical interconnects and project the requirements for optoelectronic and optical devices if optics is to solve the major problems of interconnects for future high-performance silicon chips. Optics has potential benefits in interconnect density, energy, and timing. The necessity of low interconnect energy imposes low limits especially on the energy of the optical output devices, with a ~ 10 fJ/bit device energy target emerging. Some optical modulators and radical laser approaches may meet this requirement. Low (e.g., a few femtofarads or less) photodetector capacitance is important. Very compact wavelength splitters are essential for connecting the information to fibers. Dense waveguides are necessary on-chip or on boards for guided wave optical approaches, especially if very high clock rates or dense wavelength-division multiplexing (WDM) is to be avoided. Free-space optics potentially can handle the necessary bandwidths even without fast clocks or WDM. With such technology, however, optics may enable the continued scaling of interconnect capacity required by future chips.

Device Requirements for Optical Interconnects to Silicon Chips

We show a scheme for achieving high-speed operation for carrier-injection based silicon electro-optical modulator, which is optimized for small size and high modulation depth. The performance of the device is analyzed theoretically and a 12.5-Gbit/s modulation with high extinction ratio >9dB is demonstrated experimentally using a silicon micro-ring modulator.

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12.5 Gbit/s carrier-injection-based silicon micro-ring silicon modulators

The design and performance of next-generation chip multiprocessors (CMPs) will be bound by the limited amount of power that can be dissipated on a single die We present photonic networks-on-chip (NoC) as a solution to reduce the impact of intra-chip and off-chip communication on the overall power budget A photonic interconnection network can deliver higher bandwidth and lower latencies with significantly lower power dissipation We explain why on-chip photonic communication has recently become a feasible opportunity and explore the challenges that need to be addressed to realize its implementation We introduce a novel hybrid micro-architecture for NoCs combining a broadband photonic circuit-switched network with an electronic overlay packet-switched control network We address the critical design issues including: topology, routing algorithms, deadlock avoidance, and path-setup/tear-down procedures We present experimental results obtained with POINTS, an event-driven simulator specifically developed to analyze the proposed idea, as well as a comparative power analysis of a photonic versus an electronic NoC Overall, these results confirm the unique benefits for future generations of CMPs that can be achieved by bringing optics into the chip in the form of photonic NoCs

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Photonic Networks-on-Chip for Future Generations of Chip Multiprocessors

We experimentally demonstrate cascaded silicon micro-ring modulators as the key components of a WDM interconnection system. We show clean eye-diagrams when each of the four micro-ring modulators is modulated at 4 Gbit/s. We show that optical inter-channel crosstalk is negligible with a channel spacing of 1.3 nm.

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Cascaded silicon micro-ring modulators for WDM optical interconnection.

Microarrays have revolutionized the field of genomics and more recently proteomics and have proven to be an asset in high-throughput screening. As the demand for improved sensitivity and throughput of biomolecular assays increases, considerable research effort has been put into developing microelectronic and nanophotonic biosensors, which are presumably more sensitive than conventional fluorescencebased assays and have a faster response time. The lithographic technology for making the densely packed microelectronic devices in a high-throughput manner is already quite advanced and may be integrated to form biosensors. This is expected to increase the pace of research in early detection of disease biomarkers, discovering cell-signal transduction pathways, and in drug discovery. Denser arrays are also important for reducing reagent volume consumption and to improve sensitivity. Successful biomolecule patterning on sensor chip circuitry requires a number of important steps. First, selective immobilization of the probe and reduction in non-specific binding should be achieved for higher signal-tonoise ratios. Second, reduction in the sensor size reduces the background, enhancing the signal-to-noise ratio, and therefore biomolecule patterns on the same order as the sensors are desirable. Third, the biomolecule pattern should be aligned with the sensor circuitry, which becomes more difficult as sensor size decreases. Finally, a fabrication process should be formulated to ensure that the biomolecules are intact and functional, which is a challenge given the harsh microor nanofabrication processing steps. Here, we have demonstrated an electron-beam (e-beam)-based approach fulfilling the above requirements for patterning biological macromolecules that does not involve the use of resist, hence eliminating the exposure of these biomolecules to harsh resist-stripping processes that are normally employed to remove the resist. A non-biofouling poly(ethylene glycol) self-assembled monolayer (PEG-SAM) was selectively removed by e-beam and patterned with aldehyde-terminated polyamidoamine dendrimer (ald-PAMAM-SAM) in a layer-by-layer (LbL) manner to covalently immobilize the aminated oligonucleotide, which bind only to their complementary sequence targets and can be stripped and reprobed. The Generation-6 (G-6) PAMAM molecule, terminated with 256 primary amine groups and 6.7 nm in diameter, was used to increase the surface density of aldehyde functional groups to increase the oligonucleotide-immobilization efficiency. Current techniques for patterning biomolecules involve the use of polymer-based templates, which can be removed mechanically without the use of organic solvents after biomolecule immobilization. However, serious limitations exist in each case. Poly(dimethylsiloxane) (PDMS)-based soft-lithographic techniques cannot be used to create high-resolution patterns, as aligning the PDMS pattern with sub-micrometer features has been shown to work in a mix-and-match manner with an accuracy of only 2 lm. Although alignment is not an issue for biomolecule patterning based on polymer liftoff, as it is an integrated process, this method involves extra steps of deposition and etching a polymer film, which increases the complexity of the process. Challenges are also encountered as the size of the photolithographic patterns decrease due to the increase in line-edge roughness (LER) and the isotropic nature of oxygen plasma etch. Patterned gold has been used for creating protein patterns using thiolbased linkers, but gold surfaces cannot be tolerated in some biosensors as gold interferes with the optical signal or conductivity of the sensor. This technique also includes extra photolithographic and lift-off processing steps for patterning gold. Protein patterning using fluorescence-tagged proteins C O M M U N IC A IO N S

Dendrimer‐Scaffold‐Based Electron‐Beam Patterning of Biomolecules

We experimentally demonstrate an electro-optic modulator using a high-index-contrast silicon Fabry-Perot resonator. This compact high-speed device is a 1-D integrated cavity, only 4.53 ?m in length with an embedded p-i-n junction on a silicon-on-insulator platform.

Compact Electro-Optic Modulator on Silicon-on-Insulator Substrates Using Cavities with Ultra-Small Modal Volumes

We demonstrate experimentally cascaded silicon micro-ring modulators for WDM interconnection systems. We show clean eye-diagrams when each of the four micro-ring modulators coupled to a single waveguide is modulated at 4 Gbit/s

Brad Schmidt

Papers

12.5 Gbit/s carrier-injection-based silicon micro-ring silicon modulators

Cascaded silicon micro-ring modulators for WDM optical interconnection.

Dendrimer‐Scaffold‐Based Electron‐Beam Patterning of Biomolecules

Compact Electro-Optic Modulator on Silicon-on-Insulator Substrates Using Cavities with Ultra-Small Modal Volumes

WDM Silicon Modulators Based on Micro-ring Resonators