Electronics Lightens Up

NEWS & VIEWS

Optoelectronics Electronics lightens up

A demonstration of electronic operations on photons in an excitonic integrated circuit shows a possible route towards nanoscale optoelectronics.

Electric field Richard J. Warburton _ _ is in the Department of Physics, School of Engineering and Physical Sciences, Heriot-Watt University, Edingburgh EH14 4AS, UK. e-mail: [email protected] _ ight is ideal for communications applications. It is fast and can, under + + L the correct conditions, travel for large + distances with very little scattering and absorption by the surrounding medium. The insensitivity to its environment, however, poses the question: how could light pulses be manipulated to make a photonic transistor? This question has remained largely unanswered and, so far, only cumbersome infrastructures have been developed for Figure 1 Energy-band diagrams (energy versus position) for light–matter interaction in semiconductors. converting an electronic signal into a light a, An electron absorbs a photon and moves to the conduction band, leaving a positively charged carrier, a hole, signal and vice versa. Given the fundamental behind. The bound electron–hole pair is an elementary excitation called an exciton. b, In a double quantum well, role communications have in our society, the electrons and holes can be spatially separated, thus creating a spatially indirect exciton, with the benefits of this is more than an academic challenge. longer lifetimes and a permanent dipole moment. The emergence of nanotechnology has provided access to new physical processes that may provide a solution. However, although there have been experimental charge — in the valence band. The bound be increased substantially by pulling the realizations of nanoscale photonic storage electron–hole pair is called an exciton electron and hole apart. This is done by devices1,2 and individual transistors3, the and corresponds to the lowest energy applying an electric field, which essentially experimental demonstration of a functional state that an optical photon can create. ‘tilts’ the energy-band diagram, spatially and tunable photonic system has so far Conversely, when an electron jumps from separating the electrons and holes into proved elusive. Not so anymore: in Science4, the conduction to the valence band, a different wells owing to their opposite charge Alex High and colleagues present the photon is emitted (Fig. 1a). This transition (Fig. 1b). Under the correct conditions, the realization of a nanoscale integrated circuit is the fundamental basis of conventional Coulomb force is strong enough to bind based on three photonic semiconductor semiconductor light-emitting diodes. the electron and hole to each other, creating transistors in which the signal processing is Is it possible to build a photon processor a so-called spatially indirect exciton. The done by excitons — essentially bound pairs using excitons? At first sight, the exciton spatial separation of the electron and hole of electrons and holes. Light is converted does not seem to be very suitable, because not only slows down spontaneous emission to excitons, the excitons are electrically in typical bulk semiconductors it self- but also introduces a dipole moment. This manipulated in the circuit, then converted destructs in just a nanosecond or so by means that the exciton’s energy can be back to a light-based output. spontaneous emission, leaving no time for manipulated by electric fields via the Stark Manipulating photons is challenging manipulation and processing. However, effect, a crucial aspect of optoelectronics, because photons have no mass and no semiconductors can be tailored in many which can be implemented readily by charge. On the other hand, electronics ways at the nanoscale. For example, applying bias voltages to gate electrodes in exploits the electron charge. One thing is heterostructures containing thin layers of a normal integrated circuit. This is the basic therefore clear: a photonic circuit needs different materials can be engineered with principle behind the excitonic circuit now some sort of medium. Semiconductors high precision. A powerful building block realized by High and colleagues4. Photons are a natural choice because direct-gap is a double quantum well consisting of two are transformed into excitons at the circuit’s semiconductors have a strong light–matter closely spaced potential wells. Although input and excitons back into photons at interaction. A photon with the correct carrier motion is frozen in one direction, the output. In between, the exciton flux is energy can be absorbed by the material, carriers can assemble and move in a two- controlled by voltages applied at carefully lifting an electron from the valence band dimensional plane. positioned gate electrodes. The long lifetime to the conduction band, leaving behind The main interest in the double quantum of the excitons enables them to perform a hole — a quasiparticle with positive well is that the longevity of the exciton can simple operations such as directional

NEWS & VIEWS switching and merging. Although the basic operating temperature to room temperature? additional phenomena might include global idea sounds simple, the execution is not. Currently it is not clear if GaN can ever phase effects: Bose–Einstein condensation Is this a future technology and a viable match the incredibly high-quality GaAs layers and — who knows — a Josephson effect. way of information processing? It could be. produced by Art Gossard and colleagues Perhaps this experiment will end where it The advantages are that the exciton-based at UC Santa Barbara for this experiment. began, as a manifestation of elegant physics. systems can potentially be very fast and that Technology aside, what would clearly the device architecture can be defined with be fascinating to explore is the transport References 1. Zimmerman, S., Wixforth, A., Kotthaus, J. P., Wegscheider, W. standard lithography. A drawback however, at of excitons — which are bosons — in a & Bichler, M. Science 283, 1292–1295 (1999). least for the present device, is that operation tunable-potential landscape. In the case of 2. Lundstrom, T., Schoenfeld, W., Lee, H. & Petroff, P. M. Science at low temperatures is required to prevent electronic transport, electrons — which 286, 2312–2314 (1999). the excitons being shaken apart by lattice are fermions — can interfere in subtle 3. High, A. A., Hammack, A. T., Butov, L. V., Hanson, M. & Gossard, A. C. Opt. Lett. 32, 2466–2468 (2007). vibrations. Perhaps the use of GaN instead ways, which is explored in the field of 4. High, A. A., Novitskaya, E. E., Butov, L. V., Hanson, M. & of GaAs, the typical choice, could raise the mesoscopic physics. For excitonic bosons, Gossard, A. C. Science 321, 229–231 (2008). HISTORY OF SCIENCE Those names to remember

“The past is by no means definite. It is rather open”, wrote the German historian of science Ernst Peter Fischer in Die Welt on 24 July 2006. In his column, Fischer introduced the “zeroth theorem of the history of science”; a discovery named after a person, the theorem says, did not originate from that person. Take, for example, Avogadro’s ollege Dublin ollege

number, named after Amedeo Avogadro, C who asserted that there is the same rinity rinity T , ,

number of molecules contained in a s ic s

given volume of any gas at the same hy temperature and pressure. However, P it was not the Italian savant who first

estimated that number, but the Austrian of School scientist Johann Josef Loschmidt. Indeed, German-language texts sometimes in some specific contexts, but little is length contraction proportional to (v/c)2, refer to the number 6.022 × 1023 as known about their complete works. one of the earliest contributions to what ‘Loschmidt’s number’. Much depends on A particularly impressive example would lead to the theories of relativity. The who tells a story, and where and when. is that of the Irish physicist George effect is known as the Lorentz–FitzGerald Fischer sees his zeroth theorem as an Francis FitzGerald. Jackson discusses contraction, but in many places it is invitation to look with fresh eyes at the FitzGerald’s work on determining “the simply called Lorentz contraction history of science, and in particular at period of vibration of disturbances of (although FitzGerald’s priority, apparently, how discoveries got their names. electrification of the Earth”. Such modes was never questioned). A man of That thought has now been picked of electromagnetic waves trapped between many interests, in 1895 FitzGerald also up by J. David Jackson (Am. J. Phys. 76, the conducting Earth and the ionosphere undertook some of the early experiments 704–719; 2008). He has explored five are nowadays known as ‘Schumann in human flight, using a Lilienthal glider examples from physics that illustrate resonances’, after Winfried Otto Schumann, (pictured). And only months before his Fischer’s zeroth theorem, and discusses who worked in the 1950s on extremely death on 21 February 1901, at the age the broader issue of credit-giving, and low-frequency propagation in the Earth– of 49, FitzGerald put forward the idea where it gives rise to inappropriate ionosphere cavity. The phenomenon, that magnetism could be due to rotation attributions. Jackson’s five examples take however, had already been described by of electrons (Nature 62, 564; 1900) — a in various areas of physics, from the FitzGerald in 1893; he gave a good estimate quarter of a century before Ralph Kronig, Dirac delta function to the Weizsäcker– for the lowest mode and also noted that George Uhlenbeck and Samuel Goudsmit Williams method of virtual quanta, to thunderstorms are involved — today it conceived the concept of the electron the Bargmann–Michel–Telegdi equation is known that the resonances are indeed possessing intrinsic angular momentum, of spin dynamics. The journey includes excited by lightning discharges. or spin. encounters with big names such as FitzGerald showed similar foresight The past, indeed, is by no means Enrico Fermi or Nikola Tesla, but also in earlier studies, most notably when he definite. Much remains to be rediscovered with physicists whose biographies are hypothesized in 1889 that key aspects of and many stories to be told — an far less commonly known, such as the Michelson–Morley experiment could undertaking that is undoubtedly worth Oliver Heaviside, Llewellyn Thomas or be explained by assuming that a body the effort. Emil Wiechert. Their names are famous travelling at velocity v seems to experience a Andreas Trabesinger