April 3, 2009
Friday, 3:00PM
PHO 211
Cosponsored by ECE, Physics, and the Center for Nanotechnology |
Dr. Arto Nurmikko
Division of Engineering
Brown University
Colloidal Semiconductor Quantum Dots as Active Optical Elements: From Single Photons Sources to Lasers?
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| Abstract:
Colloidal semiconductor nanocrystals (quantum dots) represent an approach to quantum confined electronic structures using synthesis techniques not uncommon in a wet chemistry laboratory. To date, II-VI compound semiconductor QDs, as well as those based on InP and Si have been reported as sources of relatively efficient nanophotonic luminescence. The CdSe/ZnS core-shell construct, for example, is already in relatively common use as a stable fluorescence label in imaging for cellular biology.
A broader and a more challenging question concerns the possibility of using such solution-based synthesis methods to produce QDs for practical optoelectronic devices. Then, in addition to creating highly efficient nanophotonic point sources, one must consider the optical and electronic material environment adjacent to the QDs. This is required so that a suitable device-compatible electronic and optical “housing” can also be built in-situ by the solution-based or other compatible techniques. A key material science staring point is the fact that practically all colloidal QDs to date require an organic cladding to stabilize their electronic/optical quality. Therefore, matching the QDs with compatible solid state host materials implies an understanding of the electronic excitation and charge transfer across “hybrid” inorganic/organic interfaces. Broadly, energetics at interfaces between electronically dissimilar materials under excitation or bias can be separated to processes such as excitation and charge transfer, the former referring to (charge-neutral) excitons as the basic elementary excitations.
In this presentation we consider how it is possible to adjoin the organic and inorganic partners including cases where resonance-type effects can enhance the excitation/charge transfer. In the language of ‘bandoffsets’, for example, a resonant condition might ensue if the valence/ conduction bandgap in the inorganic semiconductor is matched with the HOMO-LUMO gap in the organic semiconductor. In case of excitons (of Frenkel and Wannier type in the organic and inorganic semiconductors, respectively), electromagnetic dipolar coupling has been shown to be very effective for resonant energy transfer in selected material systems, in excess of 90% efficiency, demonstrated here using a combination of II-VI quantum dots and J-aggregate polymers as monolayer stacks in thin film heterostructures. Given such “material impedance” matching we discuss the prospects of creating optical sources at two polar opposites – single photon sources to lasers.
Biography:
Professor Nurmikko carries out research in experimental laser sciences, optoelectronics, condensed matter physics, and neuroengineering. Topics range from basic condensed matter physics to the development of new optoelectronic devices, to methods of recording brain signals. In his basic research, one example includes the use of ultrashort pulse laser techniques (to femtosecond timescale) for study of the dynamics of nanostructured electronic materials such as semiconductor quantum dots. He also studies means to manipulate states of magnetization in very thin ferromagnetic films. Professor Nurmikko is a Fellow of the American Physical Society, a Fellow of the Optical Society of America, a Fellow of the IEEE. a Simon Guggenheim Fellow, and he was elected to the American Academy of Arts and Sciences. |
