title: Exciton polarizability in semiconductor nanocrystals.
abstract: The response of charge to externally applied electric fields is an important basic property of any material system, as well as one critical for many applications. Here, we examine the behaviour and dynamics of charges fully confined on the nanometre length scale. This is accomplished using CdSe nanocrystals of controlled radius (1-2.5 nm) as prototype quantum systems. Individual electron-hole pairs are created at room temperature within these structures by photoexcitation and are probed by terahertz (THz) electromagnetic pulses. The electronic response is found to be instantaneous even for THz frequencies, in contrast to the behaviour reported in related measurements for larger nanocrystals and nanocrystal assemblies. The measured polarizability of an electron-hole pair (exciton) amounts to approximately 10(4) A(3) and scales approximately as the fourth power of the nanocrystal radius. This size dependence and the instantaneous response reflect the presence of well-separated electronic energy levels induced in the system by strong quantum-confinement effects.
authors.affiliation: Department of Physics, Columbia University, New York, New York 10027, USA.
authors.forename: Feng
authors.initials: F
authors.lastname: Wang
authors.forename: Jie
authors.initials: J
authors.lastname: Shan
authors.forename: Mohammad A
authors.initials: MA
authors.lastname: Islam
authors.forename: Irving P
authors.initials: IP
authors.lastname: Herman
authors.forename: Mischa
authors.initials: M
authors.lastname: Bonn
authors.forename: Tony F
authors.initials: TF
authors.lastname: Heinz