Time-Resolved Intraband Relaxation of Strongly-Confined Electrons and Holes in Colloidal PbSe Nanocrystals

The relaxation of strongly-confined electrons and holes between 1P and 1S levels in colloidal PbSe nanocrystals has been time-resolved using femtosecond transient absorption spectroscopy. In contrast to II-VI and III-V semiconductor nanocrystals, both electrons and holes are strongly confined in PbSe nanocrystals. Despite the large electron and hole energy level spacings (at least 12 times the optical phonon energy), we consistently observe picosecond time-scale relaxation. Existing theories of carrier relaxation cannot account for these experimental results. Mechanisms that could possibly circumvent the phonon bottleneck in IV-VI quantum dots are discussed

Angular redistribution of near-infrared emission from quantum dots in 3D photonic crystals

We study the angle-resolved spontaneous emission of near-infrared light sources in 3D photonic crystals over a wavelength range from 1200 to 1550 nm. To this end PbSe quantum dots are used as light sources inside titania inverse opal photonic crystals. Strong deviations from the Lambertian emission profile are observed. An attenuation of 60 % is observed in the angle dependent radiant flux emitted from the samples due to photonic stop bands. At angles that correspond to the edges of the stop band the emitted flux is increased by up to 34 %. This increase is explained by the redistribution of Bragg-diffracted light over the available escape angles. The results are quantitatively explained by an expanded escape-function model. This model is based on diffusion theory and adapted to photonic crystals using band structure calculations. Our results are the first angular redistributions and escape functions measured at near-infrared, including telecom, wavelengths. In addition, this is the first time for this model to be applied to describe emission from samples that are optically thick for the excitation light and relatively thin for the photoluminesence light.Comment: 24 pages, 8 figures (current format = single column, double spaced

High-performance Bragg gratings in chalcogenide rib waveguides written with a modified Sagnac interferometer

This paper was published in Journal of the Optical Society of America B-Optical Physics and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/abstract.cfm?URI=josab-23-7-1323. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.Mehrdad Shokooh-Saremi, Vahid G. Ta'eed, Neil J. Baker, Ian C. M. Littler, David J. Moss, Benjamin J. Eggleton, Yinlan Ruan, and Barry Luther-Davie

Electron relaxation following UV excitation in CdSe nanocrystals: sub-picosecond-fast population of the 1PStates across a gap wider than 10 phonon energies

The typical energy separation between D-like and P-like electronic states in CdSe nanocrystals is often of similar magnitude to the S-P splitting in the conduction band and would therefore similarly preclude efficient electron decay via multiple phonon emission. Despite the puzzling observation of fast D-to-P intraband relaxation, however, little is known about the mechanisms governing it. This work shows that the same process responsible for fast P-to-S electron decay, namely Auger cooling, plays a fundamental role to allow fast population of the P state, from higher excited states, in case of high-energy excitations. Since in nanocrystals the latter are employed to initiate carrier multiplication, where multiple excitons are created from the absorption of a single photon, the D electron lifetimes calculated here could provide a reference for the estimate of the carrier multipli- cation time constant, the magnitude of which is still object of intense debate. These results also provide a further proof of the suitability of the Auger cooling model to explain electron relaxation in semiconductor nanocrystals