Física del estado sólido

Un excitón es una cuasipartícula (o excitación elemental) de los sólidos formada por un electrón y un hueco ligados a través de la interacción coulombiana. Se da únicamente en semiconductores y aislantes.- ................................:http://es.wikipedia.org/w/index.php?title=Especial:Libro&bookcmd=download&collection_id=ffc663c7d5a5626ef9e2fe7d87bb1f16a5b0721e&writer=rdf2latex&return_to=Excit%C3%B3n

a, In the absence of X–X interactions, excitation of a single electron–hole pair (single exciton) per nanocrystal (NC) on average does not produce optical gain but results in optical transparency—that is, the regime for which stimulated emission (Em.) is exactly compensated by absorption (Abs.). e, Electron; h, hole. b, The balance between stimulated emission and absorption is broken if one accounts for X–X interactions that spectrally displace the absorbing transition with respect to the emission band. The latter effect can be interpreted in terms of the transition Stark shift (Δ_S = Δ_XX) induced by a local electric field (E) associated with a single-exciton state. If the transition shift is greater than the ensemble line width, optical gain can occur in the single-exciton regime. c, In the case of large Δ_S (Δ_S  Γ), stimulated emission in singly excited NCs (n_x is their fraction in the NC ensemble) competes only with absorption in unexcited NCs (fraction (1-n_x); here, we neglect multiexcitons). The stimulated-emission cross-section of a singly excited NC is one half of the absorption cross-section of the lowest-energy NC transition. Based on these considerations, the optical-gain threshold can be found from the condition n_x/2 = (1-n_x), which indicates that the single-exciton gain onset corresponds to the situation where two-thirds of the NCs are excited with single excitons.

Manipulation of Light-Matter Interactions Using Plasmonic and Photonic Nanostructures

Activities in this area:

• Address the challenge of limited absorbtivity of thin-film photovoltaic structures.
• Study field-enhancement effects in plasmonic structures as a means for increasing absorption cross-sections of proximal semiconductor nanostructures or thin semiconductor films.
• Use photonic gratings for efficient coupling of light into planar photovoltaic layers to effectively increase the light propagation length.
• Explore broad-band plasmonic waveguides in the context of improved light coupling into photovoltaic structures.

This work builds on extensive expertise of Rice University participants in nanoplasmonics and expertise and experimental capabilities of Los Alamos National Laboratory (LANL) team members in semiconductor nanostructures.

Carrier Multiplication and Competing Energy-Loss Mechanisms

Activities in this area:

• Study the mechanism of this phenomenon to establish the factors that control its efficiency and spectral onset.
• Address a controversy regarding observed multi-exciton yields in different classes of nanocrystalline materials by developing reliable experimental techniques for detecting carrier multiplication, quantifying its yield, and isolating it from extraneous effects.
• Attempt to manipulate electron-electron and electron-photon interactions in nanoscale semiconductor and metal structures as a means to approach energy conservation-defined limits in both efficiency and threshold for multi-exciton generation.

Carrier multiplication—or creating two excitons with one high-energy photon—is shown schematically on an energy ladder (top) and within the crystal lattice (bottom).

Spectroscopic studies in this area represent a joint effort between LANL, the National Renewable Energy Laboratory (NREL), and the University of Colorado and will take advantage of complementary capabilities and expertise in a wide range of spectroscopies including femtosecond techniques, nonlinear coherent methods, and terahertz and single-nanostructure spectroscopies. The colloidal synthesis effort primarily resides at LANL. The theory component takes advantage of LANL facilities for high-performance computing, as well as advanced analytic methods.

Control of Excited-State Dynamics

Activities in this area:

• Study the mechanism for energy losses in semiconductor nanostructures to develop practical approaches for controlling hot-electron relaxation. Understanding and eventual control of excited-state dynamics in nanostructures are directly relevant to solar-energy conversion, especially in the context of multi-exciton generation and "hot electron" extraction.
• Explore various means for slowing down intra-band carrier relaxation, including control of electron-hole coupling and interactions with molecular surface species.
• Attempt "up-conversion" of photogenerated carriers via high-efficiency Auger recombination, which could help extract greater voltage following absorption of low-energy solar photons.

The proposed work is conducted as a joint spectroscopic effort between LANL and NREL, supported by theoretical work at LANL and by external consultants.