This project supports experimental work on a fascinating new state of matter, which has some aspects similar to a laser and some aspects similar to a superconductor. Like a laser, this system emits coherent light, but it is not a laser in the standard sense, because not only the emitted light but also the electrons in the solid are coherent. In other words, as in a superconductor, the electrons are correlated and have coherent wave-like properties. Superconductors do not emit light, however, and therefore their coherent properties must be deduced indirectly. The semiconductor structures used for this study consist of layers, each with a thickness of a few nanometers, and are fabricated at Bell Labs of Alcatel-Lucent. The structures are cooled down to very low (cryogenic) temperatures and studied with state-of-the-art optical spectroscopy and imaging. The goal is to understand this new state of matter. A unique technique, which uses stress to confine the coherent electrons in a small spot, developed at the University of Pittsburgh in previous work supported by NSF, provides an advantage for these studies relative to other work on similar systems. The students working on this project will receive training cutting edge scientific topics and in state-of-the-art techniques that will prepare them for future careers in academe, industry, or government.
This project supports experimental work on spontaneous coherence (Bose-Einstein condensation and related transitions) in two related systems: spatially indirect excitons in coupled quantum wells and exciton-polaritons in microcavities. Semiconductor heterostructures for both systems will be fabricated at Bell Labs of Alcatel-Lucent. In both cases inhomogeneous stress is used to confine the quasiparticles in a harmonic-potential trap analogous to atom traps, a method developed at the University of Pittsburgh in previous work supported by NSF. The studies on spatially indirect excitons will focus on the many-body renormalization effects which arise due to the exciton-exciton interaction, and which shift the critical temperature for condensation. Exciton-polaritons in microcavities have the advantage that they have extremely light effective mass, which implies a critical temperature for spontaneous coherence from tens to hundreds of Kelvin at experimentally achievable polariton densities. Recent work on exciton-polaritons in microcavities at Pittsburgh and other laboratories has shown multiple forms of evidence for spontaneous coherence in a state that can be viewed as a non-equilibrium Bose condensate. The students working on this project will receive training cutting edge scientific topics and in state-of-the-art techniques that will prepare them for future careers in academe, industry, or government.
"A new type of coherent light emission" Coherent light is light that has no random "noise": you can think of a coherent light source as like the sound from a bell with a pure tone, while incoherent light has random noise, like the sound of a radio tuned to no radio station. Most people are familiar with the technology of lasers as a source of coherent light. Lasers now have many uses, including sending light in a well-defined straight line, with high intensity. Our project has focused on a new system that emits coherent light but is not a laser. Our work joins with that of many experimental and theoretical groups around the world working on this new type of system. The initial systems all work at very low temperature, but there is great promise that this type of system will also be able to work at room temperature in the near future. One advantage of the new type of coherent light emission which has already been shown is that the coherent light emission in this system requires much lower input power than standard lasing. Our group was one of the first to show that this new light source is not the same as a laser. A major outcome of this project was to show clearly the two different behaviors, lasing and coherence without lasing, occurring in the same system under different conditions, so that we can clearly distinguish between the two. Our experimental approach uses a unique method of flexing a solid semiconductor, developed in our labs, to control the energy of the electrons in the solid. Another significant result was successful numerical modeling of the behavior of the electrons in the solid during the coherent light emission process. We have also recently designed new structures which allow channeling of the light in a solid structure over long distances, which may make this system very useful for communications. Our work on this project has allowed us to now have the expertise to design new structures of this type with very specific properties. The technical name for this new state of coherent light emission is "Bose-Einstein condensation of polaritons." It is similar in many ways to Bose-Einstein condensation of atoms, which has attracted much attention in the past decade as a fundamentally new state of matter. In both the atomic Bose-Einstein condensate and in our system, the matter itself acquires coherence, making it of fundamental interest in the theory of quantum mechanics. During the lifetime of this project, two of the graduate students involved received their Ph.D., with expertise in the cutting edge of laser and optics technology. One of these students received two international awards, and after a stay in the Philippines to help with education in that country, will be traveling to Cambridge University to continue work on this type of system. The other student has moved directly into the optics industry. This project also supported four undergraduate students to learn the methods of a modern optics laboratory.
