Graphene, the single-atom-thick form of carbon, holds promise for many applications, notably in electronics where it can complement or be integrated with silicon-based devices. Intense efforts have been devoted to develop a key enabling device, a broadband, fast optical modulator with a small device footprint. Now Liu et al. demonstrate an exciting new possibility for graphene in the area of on-chip optical communication: a graphene-based optical modulator integrated with a silicon chip. This new device relies on the electrical tuning of the Fermi level of the graphene sheet, and achieves modulation of guided light at frequencies over 1 gigahertz, together with a broad operating spectrum. At just 25 square micrometres in area, it is one of the smallest of its type. Integrated optical modulators with high modulation speed, small footprint and large optical bandwidth are poised to be the enabling devices for on-chip optical interconnects1,2. Semiconductor modulators have therefore been heavily researched over the past few years. However, the device footprint of silicon-based modulators is of the order of millimetres, owing to its weak electro-optical properties3. Germanium and compound semiconductors, on the other hand, face the major challenge of integration with existing silicon electronics and photonics platforms4,5,6. Integrating silicon modulators with high-quality-factor optical resonators increases the modulation strength, but these devices suffer from intrinsic narrow bandwidth and require sophisticated optical design; they also have stringent fabrication requirements and limited temperature tolerances7. Finding a complementary metal-oxide-semiconductor (CMOS)-compatible material with adequate modulation speed and strength has therefore become a task of not only scientific interest, but also industrial importance. Here we experimentally demonstrate a broadband, high-speed, waveguide-integrated electroabsorption modulator based on monolayer graphene. By electrically tuning the Fermi level of the graphene sheet, we demonstrate modulation of the guided light at frequencies over 1 GHz, together with a broad operation spectrum that ranges from 1.35 to 1.6 µm under ambient conditions. The high modulation efficiency of graphene results in an active device area of merely 25 µm2, which is among the smallest to date. This graphene-based optical modulation mechanism, with combined advantages of compact footprint, low operation voltage and ultrafast modulation speed across a broad range of wavelengths, can enable novel architectures for on-chip optical communications.

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A graphene-based broadband optical modulator

Metal interconnections are expected to become the limiting factor for the performance of electronic systems as transistors continue to shrink in size. Replacing them by optical interconnections, at different levels ranging from rack-to-rack down to chip-to-chip and intra-chip interconnections, could provide the low power dissipation, low latencies and high bandwidths that are needed. The implementation of optical interconnections relies on the development of micro-optical devices that are integrated with the microelectronics on chips. Recent demonstrations of silicon low-loss waveguides, light emitters, amplifiers and lasers approach this goal, but a small silicon electro-optic modulator with a size small enough for chip-scale integration has not yet been demonstrated. Here we experimentally demonstrate a high-speed electro-optical modulator in compact silicon structures. The modulator is based on a resonant light-confining structure that enhances the sensitivity of light to small changes in refractive index of the silicon and also enables high-speed operation. The modulator is 12 micrometres in diameter, three orders of magnitude smaller than previously demonstrated. Electro-optic modulators are one of the most critical components in optoelectronic integration, and decreasing their size may enable novel chip architectures.

Micrometre-scale silicon electro-optic modulator

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

The pace of the development of silicon photonics has quickened since 2004 due to investment by industry and government. Commercial state-of-the-art CMOS silicon-on-insulator (SOI) foundries are now being utilized in a crucial test of 1.55-mum monolithic optoelectronic (OE) integration, a test sponsored by the Defense Advanced Research Projects Agency (DARPA). The preliminary results indicate that the silicon photonics are truly CMOS compatible. RD however, lasing has not yet been attained. The new paradigm for the Si-based photonic and optoelectric integrated circuits is that these chip-scale networks, when suitably designed, will operate at a wavelength anywhere within the broad spectral range of 1.2-100 mum, with cryocooling needed in some cases

The Past, Present, and Future of Silicon Photonics

A Raman ring resonator based laser and wavelength converter method and apparatus. In one aspect of the present invention, the disclosed method includes directing a first optical beam of a first wavelength and a first power level into a first ring resonator defined in a semiconductor material. Emission of a second optical beam of a second wavelength is caused in the first ring resonator by propagating the first optical beam around the first ring resonator. The first power level is sufficient to cause the emission of the second optical beam. The first optical beam is directed out of the first ring resonator after a round trip of the first optical beam around the first ring resonator. The second optical beam is recirculated around the first ring resonator to further stimulate the emission of the second optical beam in the first ring resonator.

Method and apparatus for Raman ring resonator based laser/wavelength converter

A polarization beam splitter and combiner and a polarization insensitive modulating and switching method and apparatus. In one aspect of the present invention, the disclosed apparatus a first optical waveguide disposed in a semiconductor material layer. A second optical waveguide is also disposed in the semiconductor material layer. An insulating region is disposed between the first and second optical waveguides to provide a coupling region in the semiconductor material layer between the first and second optical waveguides. The coupling region has a first coupling length for a first polarization mode of an optical beam directed through one of the first and second optical waveguides into the coupling region. The coupling region has a second coupling length for a second polarization mode of the optical beam.

Polarization insensitive method and apparatus for switching and modulating an optical beam

An apparatus and method for modulating a phase of optical beam. In one embodiment, an apparatus according to embodiments of the present invention includes a first region of semiconductor material disposed in a second region of semiconductor material. An insulating region is disposed between the first and second regions of semiconductor material defining a plurality of interfaces between the insulating region and the first and second regions of semiconductor material. An angle of incidence between an optical beam and the plurality of interfaces is substantially equal to Brewster's angle. A plurality of charge modulated regions proximate to the plurality of interfaces in the first and second regions of semiconductor are included such that the optical beam directed through the plurality of interfaces and the plurality of charge modulated regions is phase shifted in response to the plurality of charge modulated regions.

Method and apparatus for phase shifting an optical beam in an optical device

An optical switching method and apparatus. In one aspect of the present invention, the disclosed apparatus includes first and optical waveguides disposed in a semiconductor substrate layer. An insulating layer disposed between the first and second waveguides in a coupling region in the semiconductor substrate layer to isolate the first optical waveguide from the second optical waveguide. Modulated charge layers proximate to the insulating layer in the coupling region are employed to control an optical coupling strength between the first and second optical waveguides.

Method and apparatus for switching an optical beam between first and second waveguides in a semiconductor substrate layer

An apparatus and method for modulating a phase of optical beam. In one embodiment, an apparatus according to embodiments of the present invention includes a plurality of alternating layers of semiconductor material and insulating material. The apparatus further includes a plurality of conductors, each of the conductors coupled to a respective one of the layers of semiconductor material to modulate charge layers proximate to the layers of insulating material. An optical path is included along the plurality of alternating layers of semiconductor material and insulating material and through the modulated charge layers. A phase of an optical beam directed along the optical path through the modulated charge layers is modulated in response to the modulated charge layers.

Remus Nicolaescu

Papers

Method and apparatus for Raman ring resonator based laser/wavelength converter

Polarization insensitive method and apparatus for switching and modulating an optical beam

Method and apparatus for phase shifting an optical beam in an optical device

Method and apparatus for switching an optical beam between first and second waveguides in a semiconductor substrate layer

Method and apparatus for modulating an optical beam in an optical device