SEMICONDUCTORS Quantum optics by design
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SEMICONDUCTORS Quantum optics by design CLAIRE GMACHL is at the Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA e-mail: firstname.lastname@example.org There may be a little bit of the alchemists’ dream in each materials scientist, and it is certainly found in a new breed of semiconductor researchers who turn to nanostructures, namely sophisticated stacks of many hundreds of ultrathin layers of semiconductors, to fi nd the sparkle of quantum optics in their earthy materials. Semiconductors are the workhorse materials for many of today’s technologies. Cost-eff ective and abundant, they more and more replace other materials, especially in photonics with applications such as light sources and detectors. Nevertheless, from the fundamental scientist’s point of view, semiconductors are oft en considered rather dull and down-to-earth materials. Instead of exhibiting distinct energy levels that beautifully show off their quantum nature, they have broad energy bands, and much of the apparent width of these bands is determined by disorder and temperature. As a result, semiconductors are not widely considered suitable materials to see the quantum nature of the light– matter interactions or the eff ects of quantum optics in general. Now, this view may be changing with the advent of engineered semiconductor nanostructures. On page 175 of this issue, Frogley et al. report the fi rst observation of one of the ‘gold standards’ of quantum optics, namely gain without inversion (GWI), in specially designed semiconductor quantum well structures 1 . With this they open up a range of possibilities for active semiconductor devices that are based on quantum optics. Quantum optics describes light and light– matter interactions in a fully quantum mechanical description 2 . It explains eff ects that are only explainable when the photon fi eld and electronic states are allowed to fully mix and form new states. Strong laser fi elds interact with electronic transitions altering the states of the systems in a way not explainable classically with statistics or thermodynamics. Th e resulting phenomena include electromagnetic-induced transparency 3 or GWI 4 . GWI is a particular case in point for quantum optics, as it defi es the dogma of population inversion (when the number of electrons in the excited state is higher than in the ground state) being necessary for gain or laser action; it will render a material fi rst transparent and then amplify the light fi eld at a wavelength where it usually is opaque. Th e eff ects of quantum optics are most cleanly seen in appropriate atomic or molecular systems, where well-defi ned energy level cascades can be prepared or ‘dressed’ with narrowband and powerful pump laser fi elds, and probed with other narrowband lasers. Th e success of these measurements depends then on fi nding the most suitable, nature-given level cascades, and excellent examples have been published for vapours 3,4 and more recently Bose–Einstein condensates 5,6 . Yet, bulk semiconductors have played hardly any role in this fi eld. Th eir natural energy levels are too blurred for the subtle energy-level shift s and level splittings to be easily observable most of the time, with few exceptions 7 . Nevertheless, it would be of technological importance and — admittedly, just plainly exciting — if semiconductors and quantum optics could be brought together. Th is is where semiconductor nanostructures — in particular, coupled quantum wells that consist of layers a few atoms thin of one semiconductor material, interleaved with similarly thin barrier layers of another material — come to the rescue. In these semiconductor nanostructures, the energy bands split into subbands with energy separations of NEWS & VIEWS The observation of quantum optical effects in engineered semiconductor structures creates the opportunity to harvest these phenomena in designer devices. nature materials | VOL 5 | MARCH 2006 | www.nature.com/naturematerials 169 ©2006 Nature Publishing Group |3〉 |2〉 |1〉 Figure 1 Energy-band structure of a designer semiconductor heterostructure demonstrating GWI. The thin quantum wells fl anking the wide well allow the energy levels to be tailored ‘just right’ for GWI to be possible. (Reproduced from ref. 1).
Magazine: SEMICONDUCTORS Quantum optics by design