****NON-TECHNICAL ABSTRACT**** This individual investigator award supports a project directed towards experimental studies of fundamental optical and electronic properties of structures made of thin layers of semiconductors. A special design of the structures on a length scale of a billionth part of a meter allows the creation of new entities called indirect excitons. The indirect excitons are unique because they can be cooled down to ultralow temperatures. Furthermore, the indirect excitons are also unique because they can be electronically controlled like electrons in electronic devices and can shine optical signals that electrons cannot do by themselves. The project is directed towards increasing our understanding of the physics of ultracold indirect excitons. In particular it will address the transport and optical properties of the indirect excitons. The understanding gained through this research may lead to the exciting possibility of developing advanced devices that make use of electronic as well as optical properties, i.e. optoelectronic devices. The students involved with this project will have the opportunity to perform exploratory research on the cutting edge of contemporary physics. The potential impact of the project is in development of knowledge in condensed matter physics, increase of fundamental understanding of the optical and electronic properties of materials, and in development of materials control.
This individual investigator award supports a project directed towards experimental studies of ultracold gases of indirect excitons in coupled quantum well, semiconductor structures. Excitons are composite bosons. Due to their long lifetime and high cooling rate, the indirect excitons can be cooled below the temperature of quantum degeneracy. Furthermore, the indirect excitons have a built-in dipole moment and their energy can be controlled by an applied voltage. This allows creating in-plane potential profiles for indirect excitons by the voltage pattern. Studies will include exciton transport through various potential profiles and as well as exciton pattern formation and spontaneous coherence at ultralow temperatures. The research will be performed by students and will be integrated with education. The potential impact of the project is in development of knowledge in condensed matter physics, increase of fundamental understanding of the optical and electronic phenomena of materials, and in development of materials control.
An exciton is a bound pair of an electron and a hole. Excitons mediate the absorption and emission of light in semiconductors and transfer energy in materials. We achieved the breakthrough in the emergence of quantum phenomena in excitons by using specially designed excitons – the indirect excitons. An indirect exciton is composed of an electron and a hole confined in separated layers. Indirect excitons have unique properties: they have long lifetime and spin-relaxation time, can travel over large distances, and have a built-in dipole moment. Within this NSF project on indirect excitons, we found new fundamental phenomena, including: - Spontaneous coherence of excitons [Nature 2012]; - Condensation of excitons in a trap [Nano Letters 2012]; - Spin currents and spin textures originating from the formation of a coherent exciton state – a new mechanism to suppress the spin relaxation [Physical Review Letters 2013]. We designed artificially structured materials where superfluidity and superconductivity can emerge at high temperatures due to indirect excitons [arXiv 2014]. We measured basic characteristics of indirect excitons, including kinetics, excitation energy dependence, and pattern formation for the inner ring [Physical Review B 2009, Physical Review B 2012, Physical Review B 2013] and kinetics for the external ring [Physical Review B 2010] in exciton emission patterns. We developed new excitonic devices, including: - Excitonic switches operating at temperatures two orders of magnitude higher than the previous record [Nature Photonics 2009]; - The first electrostatic conveyer for excitons, excitonic CCD [Physical Review Letters 2011]; - The first excitonic ramp, excitonic diode, with no energy-dissipating voltage gradient [Applied Physics Letters 2012]; - New traps for excitons – the diamond-shaped electrostatic traps [Physical Review Letters 2009]; - The first all-optical excitonic transistors and routers [Optics Letters 2010, Applied Physics Letters 2014]. The performed research increased our knowledge of the basic properties of light and matter. The studies of exciton coherence and condensation, spin currents and spin textures, excitonic conveyers, traps, ramps, and rings in the emission pattern contributed to the physics of cold bosons. The developed instrumentation for imaging and interferometric spectroscopy at ultralow temperatures allowed accessing the previously unavailable range of temperatures for imaging and interferometric spectroscopy. Using this instrumentation has already led to finding new phenomena described above. It will also allow studies of new phenomena in the future. The performed research also resulted in the development of the control of excitons, essential for creating novel optoelectronic devices. The potential advantages of excitonic devices for signal processing include compact footprint and high interconnection speed. The performed research had a strong educational component. 11 graduate and 2 undergraduate students of the PI group have been involved in the research of the project. 5 students of the PI group involved in the project have defended their Ph.D. and graduated. The results were disseminated in meetings, including in particular, in 4 plenary, 1 key-note, 8 invited, and 17 oral talks at conferences, 1 colloquium and 10 seminars at universities and research labs, and lectures at 4 summer schools. Educational webpage on indirect excitons was continuously supported. Lab tour and research presentations were given to upper-level physics majors of University of San Diego, a small liberal arts school. Lectures, lab tours, and experiment demonstrations were given to middle and high school students in the University of California San Diego Young Physicists Program. Highlights on the results obtained within the NSF project were done in media, including APS Physics Synopsis, Nature Nanotechnology, Nature Photonics, and Science.
