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(in the hills above the Berkeley campus) in 2006, with money from the government’s Department of Energy. The Foundry’s mission is to help nanoscience researchers by distributing high-quality nanomaterials and providing access to state-of-the-art equipment and techniques, as well as staff who are experienced in working with nanoscale materials.

The marriage of nanowires and optics is an intriguing one. Yang and colleagues have shown, for example, that crystalline

oxide ‘nanoribbons’ can behave as subwavelength optical waveguides and nonlinear frequency converters2,3. Arrays of nanowires could potentially be used to channel beams of light inside integrated photonic circuits and to switch them on and off. One area that Yang is particularly excited about is photovoltaics. Traditional solar cells are made from very expensive polycrystalline silicon, but nanowire solar cells could offer a cheaper and more efficient alternative. “We are working on

the idea of a new generation of solar cells made from vertically oriented nanowire arrays,” explains Yang. “If you really want to enhance the energy-conversion efficiency of a photovoltaic cell, you need to optimize both the light absorption and the internal charge transport.” It turns out that in a nanowire-based solar cell, electron transport is about two orders of magnitude faster than in a traditional nanoparticle-based device, because electrons can flow through the device along direct electrical pathways provided by the nanowires4. And from a light-absorption perspective, Yang is looking to indium gallium nitride nanowires as a replacement for silicon. “We are trying to develop a material that offers better light absorption. Silicon is of course a useful material, but it has an indirect bandgap. A system like indium gallium nitride has the advantage of a direct bandgap and opens up other opportunities, such as direct solar-energy-to-fuel applications.”

For now, at about 2.5%, the efficiency of nanowire solar cells remains low, and it will be a while before people see nanowire panels installed on their rooftops. Nevertheless, Yang has a vision and, as with all his visions, he is pursuing it with energy and dedication. And now he has half a million dollars to help him along the way.

references1. Huang, M. et al. Science 292, 1897–1899 (2001).2. Law, M. et al. Science 305, 1269–1273 (2004).3. Nakayama, Y. et al. Nature 447, 1098–1102 (2007).4. Law, M., Greene, L. E., Johnson, J. C., Saykally, R. & Yang, P.

Nature Mater. 4, 455–459 (2005).

Figure 2 yang and his team have built intricate arrays of zinc oxide and gallium nitride nanowires, which could be used for the waveguiding of light in photonic devices, or in the next generation of solar cells.

Dietmar Knippis at the School of Engineering and Science, Jacobs University Bremen, 28759 Bremen, Germany.

e-mail: d.knipp@jacobs-university.de

S pectrometers — optical devices that detect, measure and analyse the spectral content of incoming

electromagnetic radiation — are of major importance to modern science and technology. Today, they are used in thousands of applications including

chemical and biological analysis, spectral imaging, inspection of food and industrial goods and optical metrology. On page 473 of this issue1, French researchers take a step towards shrinking these devices down to unprecedented limits.

A variety of different spectrometer concepts have been proposed and developed over the past 100 years, ranging from grating spectrometers to Fabry–Pérot and Fourier-transform

spectrometers and arrayed waveguide gratings. In all of these devices the incident electromagnetic radiation is split into a number of spectral components with different wavelengths (that is, dispersed) to enable spectral discrimination. Today, research into optical spectrometers is driven mainly by the need to improve the performance, size and cost of existing devices. As a consequence, several spectrometer designs have been successfully

Researchers uncover a new route to building ultrasmall spectrometers, by directly probing an optical standing wave.

NANophotoNiCS

Spectrometers shrink down

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transferred from the macro- to the microscale2.

In contrast to well-established techniques, the idea of harnessing standing waves in an optical spectrometer is relatively new3. But the underlying physics dates back to the nineteenth century. In 1890, Otto Wiener showed for the first time that a standing wave is formed when light is shone onto a mirror4, as a result of the superposition of the incident and reflected waves, and that it is characterized by a series of intensity maxima and minima separated by half a wavelength (Fig. 1). Just one year after the publication of Wiener’s work, Gabriel Lippmann demonstrated the first application of a standing wave, by exploiting the interference effect that yields a standing wave to create the first colour photograph5. For his work, Lippman was awarded the Nobel Prize in physics in 1908.

The first photoelectric devices to probe or sense a standing light wave were suggested in 1933 by Ives and Fry6. Nevertheless, it took more than half a century to actually build them, as the necessary technology was not available at that time7. Only recently have different groups developed such devices3,8–10 and actually put them to use. In most cases the standing wave is probed by a semitransparent optical sensor, which is significantly thinner than the wavelength of the light involved, and is inserted in the path of the standing wave. The detection principle is very simple and the approach enables the overall size of the spectrometer to be reduced substantially.

By directly probing the intensity of a standing wave, researchers have successfully built standing-wave interferometers8,9 and standing-wave Fourier-transform spectrometers operating in the near-infrared3 and the visible part of the optical spectrum10. However, a major drawback with all of these devices is the fact that the semi-transparent sensor has to be repeatedly moved to scan the standing wave. Now, an approach to standing-wave spectrometers that eliminates the need for moving, tunable parts has been suggested and implemented by le Coarer and co-workers1. Their method enables the volume of the spectrometer to be reduced to just a few hundred cubic wavelengths.

Instead of probing the intensity of the standing wave directly, the authors suggest probing the evanescent field of a standing wave formed inside a waveguide, using a periodic arrangement of nanosensors. In their set-up, le Coarer and colleagues use a commercial light source, which emits a broadband spectrum in the infrared,

to evaluate the performance of their spectrometer. The waveguide is made from a silicon-on-insulator substrate, in a fabrication process that is CMOS-compatible. Despite the fact that the proposed submicroscopic nanodetectors do not yet exist, the authors offer a preliminary demonstration of the underlying principle using a scattering scanning near-field optical microscope (s-SNOM) to probe the standing wave with nanometric resolution instead. The incident spectrum is recovered by computing the discrete Fourier transform of the stationary-field profile extracted from the s-SNOM images.

Although SNOM tips are ideal probes with which to sample the stationary field with nanometre-scale resolution, they are not compatible with the idea of compact integrated and monolithic optical devices. The authors therefore present an alternative approach, in which the spectrum of the incident radiation is recovered using a far-field method that does not involve a moving scanning tip. Specifically, they deposit a comb of gold nanowires on top of the waveguide at equally spaced (3 µm) intervals that can be easily resolved. The nanowires are narrow enough to act as point-source scattering centres and the spectrum of incoming light can be reconstructed by detecting the scattered light with a simple infrared camera. With this technique, the

spectrometer has a spectral resolution of 4 nm over a working spectral range of approximately 100 nm, centred around 1,550 nm. Moreover, the measured spectrum is in good agreement with a reference spectrum detected by a spectrum analyser.

Even though considerable efforts are still needed to translate the ideas of the French team into truly viable spectrometer technology — in particular, new optical nanosensors are needed to probe the standing wave every quarter wavelength — these results demonstrate that standing-wave spectrometers hold huge potential for future applications. The work of le Coarer et al. sets the scene for ultrasmall, integrated optical spectrometers that can offer high resolution without the need for moving parts. If realized, these devices will find applications in chemical and biological analysis, metrology and beyond.

references1. le Coarer, E. et al. Nature Photon. 1, 473–478 (2007).2. Wolffenbuttel, R. F. IEEE Trans. Instrum. Meas. 53, 197–202 (2004).3. Kung, H. L., Bhalotra, S. R., Mansell, J. D., Miller, D. A. B. &

Harris, J. S. IEEE Sel. Top. Quant. Electron. 8, 98–105 (2002).4. Wiener, O. Ann. Phys. (Leipzig) 40, 203–243 (1890).5. Lippmann, G. Compte Rendus de l’Académie des Sciences (Paris)

112, 274–275 (1891).6. Ives, H. & Fry, T. J. Opt. Soc. Am. 23, 73–83 (1933).7. Toitskii, Y. V. Opt. Spectrosc. 25, 309–313 (1968).8. Sasaki, M., Mi, X. & Hane, K. Appl. Phys. Lett. 75, 4–6 (1999).9. Stiebig, H. et al. Appl. Phys. Lett. 83, 12–14 (2003).10. Stiebig, H., Knipp, D. & Bunte, E. Appl. Phys. Lett. 88,

083509 (2006).

Incident wave Standing wave

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Figure 1 the counterpropagative interference effect is exploited in the standing-wave spectrometer presented by le Coarer and colleagues1. a, A schematic representation of a standing-wave front in front of a mirror. b, A schematic plot of the intensity in front of a mirror. I0 and Is represent the intensities of the incoming and standing waves, respectively.

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