When Louis de Broglie proposed in 1924 that the wave-particle duality applies not only to photons but also to electrons, he forged a metaphoric link that has had profound effects on the way we understand the physical world. De Broglie's insight, which suggested that electrons might be better understood if they were treated more like photons, made possible Schrödinger's wave equation, the basis of quantum mechanics. The beauty of metaphors, of course, is that they go both ways: we can reverse the direction of de Broglie's insight and try to develop our knowledge of photons by treating them as electrons. Recent work at the University of Würzburg in Germany continues the extension of de Broglie's rich scientific metaphor in just this way.
The research has its antecedents in structures known as electronic quantum dots, which were first fabricated in the 1980s. These tiny conducting islands, which can be as small as tens of nanometers across, are formed using a variety of techniques (see "Science Observer," July–August 1996). These structures can confine a specific number of electrons, much as individual atoms confine their electrons. As a consequence, quantum dots are also known as "artificial atoms."
Alfred Forchel and his colleagues at the University of Würzburg have been working with quantum dots that confine not electrons but photons. Their research group drew on calculations made by two American physicists, Thomas Reinecke of the Naval Research Laboratory in Washington D.C., and Peter Knipp, of Christopher Newport University in Virginia. The structures involved were first created in 1996 at Würzburg and at a laboratory operated by France Telecom. Forchel's group makes their "photonic atoms" by sandwiching a 7-nanometer-thick "active" layer of indium gallium arsenide (InGaAs) between two thicker layers of another semiconductor, gallium arsenide (GaAs). They sandwich this stack between thin, alternating layers of aluminum arsenide (AlAs) and GaAs. Finally, they etch the entire layered material to leave a box-like structure a few microns on each side with particular electromagnetic properties.
When stimulated with visible light, the active InGaAs layer emits infrared photons. These photons are kept in the dot horizontally because of the difference in refractive index between GaAs and the surrounding air. They are confined vertically by the alternating AlAs and GaAs layers, which form a structure known as a Bragg mirror. By manipulating the dot's size, the physicists are able to control the exact energy of the confined photons—just as the length of a taut string determines the frequency and wavelength of the sound it produces, the size of a photonic dot affects the same characteristics of the light it emits. Tuning the dot's size makes it favor certain photonic wave structures, or modes, over others.
The Würzburg group's real innovation was to extend the "artificial atom" metaphor still further. If they could make individual atoms that contained electrons or photons, why not construct "artificial molecules?" Artificial molecules built from electronic dots have been around in various forms since the early 1990s. Manfred Bayer, one of Forchel's colleagues, thought to try it with photonic dots. Bayer proposed a "photonic molecule" in which two photonic atoms would be connected by a narrow segment between them. The group etched these structures and then studied the results of varying the length of the connection.
Using spectroscopy, Bayer and colleague Thomas Gutbrod measured the energy of the photons emitted by the InGaAs layer (although most of the photons are confined to the molecule, some leak out and can be measured). Photons with particular energies correspond to specific peaks on the spectroscopic graph. When the photonic dots were far apart, the physicists found a spectroscopic emission peak around 891 nanometers (nearly the same wavelength the dots would emit if there were no connecting segment between them). As the dots were brought closer together, this single peak split into two—one indicating lower energy than the original and the other higher energy. The photonic modes of the two dots were apparently interacting, so that the initially equivalent modes were replaced by two unequal ones.
This is strikingly similar to what happens when real atoms approach each other and form a covalent bond. When atoms are far apart, their electrons occupy independent atomic orbitals and do not interact. As the atoms approach each other, this changes. The electron interaction "splits" the initially equivalent orbitals into two orbitals with different energies—one lower and one higher than the original orbital energy—just as the photonic modes are split in the photonic atom.
In real atomic interactions, when the phase of one atom's electron matches that of the other atom (phase is a property that indicates whether the electron's wave amplitude is positive or negative in a given region), the two electrons can be shared across both atoms, creating a molecular orbital with lower energy than the original. Because this orbital has a lower energy than the one created when the electrons are out of phase, the electrons "prefer" to occupy it. The lower energy orbital is a bonding molecular orbital, whereas the higher energy orbital is an antibonding one. To test further the analogy with their photonic molecule, Bayer and colleagues from the Russian Academy of Sciences measured the angular distribution of photons with particular energies to get a sense of the area they occupied within the photonic molecule. They found that photons of different wavelengths occupied areas remarkably similar to those occupied by electrons of different energies in diatomic molecules such as hydrogen (H2). The lowest-energy photons occupied an area analogous to a bonding molecular orbital, whereas photons with higher energies occupied antibonding regions, as well as other higher-energy orbitals (Figure 1).
Forchel hopes that, in addition to providing a nice illustration of the parallels between photons and electrons, the work will have some practical use. The group has been experimenting, for example, with larger photonic molecules (Figure 2), which might be used to make waveguides that steer certain wavelengths of light down particular paths—a function useful in optical telecommunications.
Whether photonic chemistry will ever match the productivity or richness of electronic chemistry remains to be seen. Since photonic atoms are less dynamic than real atoms—they are, after all, fixed to a semiconductor substrate—it isn't clear how one could produce real-time "reactions." But even if practical applications are elusive, the most lasting effect of the work may be that it provokes further creative extensions of a scientific metaphor with an illustrious past and a bright future.—Daniel B. Radov