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attila mekisis at Luxtera, 2320 Camino Vida Roble, Carlsbad, California 92011, USA.

e-mail: amekis@luxtera.com

T he demand for data bandwidth seems to be insatiable. Time and time again bandwidth limitations

are reached as rapidly emerging new applications swell the stream of data that flows between computers. If only optical-fibre cables replaced copper wires from local networks all the way to the chips inside computers, access to an almost unlimited amount of bandwidth would be possible; but the prerequisite for this is efficient and cost-effective electro–optic conversion. On page 433 of this issue, Jifeng Liu and co-workers from the Massachusetts Institute of Technology and BAE Systems report a tiny, power-efficient modulator that may bring us another step closer to achieving this goal1.

In data networks, long-distance communication is sent over optical fibres as photons, but over short distances, where copper wires abound, data travels in the form of electrons. Optical fibres are capable of transmitting trillions of bits of data each second in contrast to copper cables, which reach their limits at the speed of ‘mere’ billions of bits per second. What has kept optical solutions from penetrating the marketplace for short-distance data interconnects is a mundane reason — economics. Converting an electrical signal into an optical one is expensive. This is because the materials necessary to create a fast optical signal modulator, such as lithium niobate, gallium arsenide and indium phosphide, are more expensive and more difficult to process than silicon, the primary material used in well-established CMOS fabrication processes found in mainstream semiconductor factories. Optical components also typically comprise many parts, which translates to high assembly costs.

One answer to these challenges is to exploit semiconductor manufacturing

techniques to create an efficient and cheap electro–optic converter on a silicon material platform. Silicon is known to be suitable for light guiding and transmission at telecommunications wavelengths, and university groups, including Cornell2, as well as several companies, among them Luxtera3 and Intel4, have demonstrated the feasibility of silicon photonics modulators. Building these modulators in a standard CMOS process potentially enables integration of the electronic driver circuitry on the same silicon chip, along with myriad other functions. Such a chip, a CMOS photonics chip with monolithically integrated optics and electronics, is expected to revolutionize how data communication is done today5.

Indeed, Liu et al. have fabricated their modulator in a CMOS semiconductor foundry, on a wafer that

also contains transistors. They report that the transistors were unaffected by the modulator’s fabrication, which is promising news for monolithic integration of driver circuits in the future.

As for the modulator itself, Liu’s design takes a different approach to those demonstrated before. Previously, electro–optic signal converters based on silicon photonics have commonly been of two types: the Mach–Zehnder interferometry (MZI) modulator and the ring modulator, as illustrated in Fig. 1a and b, respectively. In an MZI modulator, light is split into two and then recombined. By modulating the phase of the light in the two arms, a temporal interference pattern is created at the output, reproducing the electrical signal. In a ring modulator, the resonance wavelength of the ring can be changed by modulating the phase of the light within the ring. Whenever the ring resonance is tuned to the light wavelength, light is deflected into the ring and extinguished in the waveguide.

The modulator developed by Liu et al. operates on an alternative principle of electro-absorption — a phenomenon widely used in modulators made from III–V materials, but an effect difficult to create in silicon. Unlike the MZI or the ring modulators, this device does not rely on changes in the phase of the light to achieve amplitude modulation, but instead uses an electric field to change the strength of a material’s absorption of light. The absorber can be placed directly in the light path, as shown in Fig. 1c, which simplifies device architecture and opens the way to smaller and more cost-effective devices.

In their approach, germanium was used as the absorber because of its compatibility with CMOS processes. However, bulk germanium absorbs light at wavelengths below 1,600 nm, including the C-band data-transmission window near 1,550 nm. By mixing a tiny amount of silicon into the germanium, Liu et al. were able to shift this critical wavelength, called the absorption

A tiny GeSi electro-absorption modulator with energy consumption at the femtojoule-per-bit level represents a step towards bringing photonics ever closer to computer chips.

silicoN PHoToNics

Lighting up the chip

PM

AM

PM

Figure 1 Three types of silicon photonics modulators. a, an mZi modulator relies on splitting light, altering its phase through phase modulation (Pm), and recombining the two beams to achieve amplitude modulation. b, in a ring modulator, the resonance wavelength of the ring next to the waveguide must be tuned using Pm. c, The Gesi electro-absorption modulator demonstrated by liu et al. turns light on and off directly inside the waveguide, using amplitude modulation (am) to create the 1 and 0 bits.

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edge, to just below 1,520 nm so that C-band light can pass through.

To achieve modulation, Liu et al. use a waveguide-integrated design containing a vertical p–i–n heterojunction that acts as a capacitor and controls the electric field inside the waveguide. The result is a tiny GeSi electro-absorption modulator that is 50 μm in length, 600 nm in width, and 400 nm in height, giving an active device area of only 30 μm2. Modulation is achieved by applying a reverse voltage across the junction, causing a shift in the absorption edge and leading to light absorption near 1,550 nm.

The GeSi modulator demonstrated by Liu et al. combines the merits of the MZI modulator, with its relatively wide operational bandwidth, and the merits of the ring modulator, with its small size and low power consumption. Although the capacitor must be charged up and discharged for each bit, owing to the very low capacitance of the junction, only a minute amount of energy, 50 fJ, is required to generate each bit of information. This device also has a relatively low dynamic

voltage swing, 3 V, matched by only a handful of MZI devices3.

Some challenges still remain in implementing this device in a useful application. One of the issues is that the large 15-kΩ series resistance of the junction causes the device to take longer to charge up the capacitor that imprints the electrical signal onto the light wave, resulting in relatively slow modulation — a 3-dB bandwidth of 1.2 GHz. The researchers argue that this limitation is not intrinsic and can be removed by process optimization, and thus with smaller series resistance the modulation speed can be increased.

Another challenge is how to achieve operation across the wide temperature range (around 70 °C) required by commercial applications. The current reported operating range is limited to 20 °C by the temperature sensitivity of the absorption edge of the GeSi alloy. This property is, unfortunately, intrinsic to the material itself, which means that the chip temperature must be carefully controlled, negating the advantages of low power consumption.

Finally, the modulator’s fabrication process, although compatible with a 180-nm-linewidth CMOS process, cannot easily be moved to the latest processes used for computer chips, as they involve finer, 90-nm or 65-nm, linewidth nodes and have much stricter thermal budgets. The problem is that the long, high-temperature anneals needed to ensure a good material quality for the GeSi alloy make the fabrication process incompatible with many current CMOS process nodes. Although this approach may not provide the ultimate solution for integrated optics and electronics on the same silicon chip, Liu and colleagues’ tiny and efficient modulator is a valuable contribution to the field of CMOS photonics.

References1. Liu, J. et al. Nature Photon. 2, 433–437 (2008).2. Xu, Q., Manipatruni, S., Schmidt, B., Shakya, J. & Lipson, M.

Opt. Express 15, 430–436 (2007).3. Pinguet, T. et al. 2007 4th IEEE Int. Conf. Group IV Photon.,

Tokyo 186–188 (2007).4. Liu, A. et al. 2007 4th IEEE Int. Conf. Group IV Photon., Tokyo

198–200 (2007).5. Mekis, A. et al. Optoelectronic Integrated Circuits X, Proc. SPIE

6897, L1–L14 (2008).

John spenceis in the Department of Physics, Arizona State University, Tempe, Arizona 85287-1504, USA.

e-mail: jspence@asu.edu

S cience often advances in bursts. These can be due to new sources or detectors or the introduction of

new paradigms from theoretical research. We may be living through one such burst at present: the recent pace of X-ray laser development is quite remarkable. Leading the way is Germany’s ‘FLASH’ facility at DESY, which now boasts the ability to generate 8-fs pulses of 7-nm-wavelength radiation (soon to be 3 nm), each containing about 1012 photons. To put this in perspective, the time taken for a single period of atomic nuclear vibration is about 100 fs, whereas the finest detail visible in an image is normally larger than the wavelength

used for the imaging. So can we expect to make movies of atomic motion during chemical reactions? Unfortunately nuclei, unlike electrons, don’t diffract ordinary X-rays. ‘Seeing atoms’ directly requires the shorter wavelengths of hard X-rays; however, these intense pulses vaporize any sample, and X-ray lenses with atomic resolution are hard to find. So there are problems. Nevertheless, on page 415 of this issue, Anton Barty et al. present an experimental X-ray movie of an exploding object with just 10 picoseconds between each frame and a shutter speed of 10 fs at a spatial resolution better than 50 nm using 13.5-nm X-rays1.

How is it done? To deal with the lack of lenses, Barty et al. used a new lens-free diffractive imaging method2,3, which replaces lenses with a computer to synthesize an image directly from the scattering of radiation from a

non-crystalline sample. Remarkably, it has been predicted that sufficiently short pulses could terminate before appreciable electron motion occurs, so that no radiation-damage effects appear in the images (at least on the 27-nm scale of detail or resolution examined here). At the same time, enough photons can now be packed into each pulse to provide sufficient scattered X-rays to form a useful image4. Many identical samples are used, each destroyed in turn by a laser pulse. After picoseconds of delay, this same laser then triggers one pulse of a free-electron X-ray laser (FEL). The resulting diffraction pattern is read out, and the process repeated for the next identical sample. The delay is increased for successive samples, so that each sample provides one frame of the movie, as shown in Fig. 1. The frames track and image the exploding

Combining optical and X-ray lasers enables imaging with high temporal and spectral resolution. By taking pictures of a succession of exploding targets, a movie can be made charting the dynamics of the solid material on a 10-ps timescale.

X-Ray imaGiNG

ultrafast diffract-and-destroy movies

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