Abstract: Intramolecular distances in proteins and other biomolecules can be studied in the living cell by means of fluorescence resonance energy transfer (FRET) in steady state or pulsed-excitation experiments. The major uncertainty originates from the unknown orientation between the optical dipole moments of the fluorescent markers, especially when the molecule undergoes thermal fluctuations in physiological conditions. We introduce a statistical method for the interpretation of fluorescence decay dynamics in donor-acceptor FRET pairs that allows us to retrieve both the orientation and the extent of directional fluctuations of the involved dipole moments. We verify the method by applying it to donor-acceptor pairs controllably attached to DNA helices and find that common assumptions such as complete rotational freedom or fully hindered rotation of the dipoles fail a physical interpretation of the fluorescence decay dynamics. This methodology is applicable in single molecule and ensemble measurements of FRET to derive more accurate distance estimates from optical experiments, without the need for more complex and expensive NMR studies.
"Increased color conversion efficiency in hybrid light emitting diodes utilizing non-radiative energy transfer"
Abstract: An efficient hybrid color-conversion light-emitting device consisting of colloidal nanocrystal quantum dots (NQDs) and a surface-patterned GaN-based LED is demonstrated (see figure). Excitation in a surface-patterned LED is efficiently transferred to NQD emitters via non-radiative energy transfer. A twofold enhancement of the NQD emission is achieved.
“Photocurrent enhancement in hybrid nanocrystal quantum dot/p-i-n photovoltaic devices”Physical Review Letters, 102, 077402 (2009)
Abstract:We fabricate a hybrid nanocrystal quantum-dot patterned p-i-n structure that utilizes nonradiative energy transfer from highly absorbing colloidal nanocrystal quantum dots to a patterned semiconductor slab to demonstrate a sixfold increase of the photocurrent conversion efficiency compared to the bare p-i-n semiconductor device.
"Efficient light harvesting in hybrid CdTe nanocrystal/bulk GaAs p-i-n photovoltaic devices"
“Temperature Dependence of Exciton Transfer in Hybrid Quantum well/Nanocrystal Heterostructures”
Applied Physics Letters 91, 092126 (2007)
Abstract: The authors investigate the temperature dependence of exciton transfer from a single InGaN quantum well (QW) donor to colloidal CdS nanocrystal quantum dot acceptors and obtain an optimum transfer efficiency of 65% at 60 K. Time and spectrally resolved measurements reveal that the transfer efficiency is dominated by the interplay between exciton localization and nonradiative recombination intrinsic to the QW.
“Room temperature exciton storage in elongated semiconductor nanocrystals”
Abstract:The excited state of colloidal nanoheterostructures consisting of a spherical CdSe nanocrystal with an epitaxially attached CdS rod can be perturbed effectively by electric fields. Field-induced fluorescence quenching coincides with a conversion of the excited state species from the bright exciton to a metastable trapped state (dark exciton) characterized by a power-law luminescence decay. The conversion is reversible so that up to 10% of quenched excitons recombine radiatively post turn-off of a 1 μs field pulse, increasing the delayed luminescence by a factor of 80. Excitons can be stored for up to 105 times the natural lifetime, opening up applications in optical memory elements.
“Room temperature polariton lasing in semiconductor microcavities”Abstract:We observe a room-temperature low-threshold transition to a coherent polariton state in bulk GaN microcavities in the strong-coupling regime. Nonresonant pulsed optical pumping produces rapid thermalization and yields a clear emission threshold of 1 mW, corresponding to an absorbed energy density of 29 u.J cm.2, 1 order of magnitude smaller than the best optically pumped (In,Ga)N quantumwell surface-emitting lasers (VCSELs). Angular and spectrally resolved luminescence show that the polariton emission is beamed in the normal direction with an angular width of .10 deg. and spatial size around 5 u.m.
“Exciton accumulation in π-conjugated wires encapsulated by light-harvesting macrocycles”