Exciton

About: Exciton is a research topic. Over the lifetime, 31603 publications have been published within this topic receiving 810642 citations.

Quantum-Confined Stark Effect in Ge/SiGe Quantum Wells on Si for Optical Modulators

Abstract: We present observations of quantum confinement and quantum-confined Stark effect (QCSE) electroabsorption in Ge quantum wells with SiGe barriers grown on Si substrates, in good agreement with theoretical calculations. Though Ge is an indirect gap semiconductor, the resulting effects are at least as clear and strong as seen in typical III-V quantum well structures at similar wavelengths. We also demonstrate that the effect can be seen over the C-band around 1.55-mum wavelength in structures heated to 90degC, similar to the operating temperature of silicon electronic chips. The physics of the effects are discussed, including the effects of strain, electron and hole confinement, and exciton binding, and the reasons why the effects should be observable at all in such an indirect gap material. This effect is very promising for practical high-speed, low-power optical modulators fabricated compatible with mainstream silicon electronic integrated circuits

Free carrier and many-body effects in absorption spectra of modulation-doped quantum wells

Abstract: The temperature-dependent optical absorption and luminescence spectra of GaAs/AlGaAs and InGaAs/InAlAs n-doped modulation-doped quantum wells is discussed with emphasis on the peak seen at the edge of the absorption spectra of these samples A many-body calculation of the electron-hole correlation enhancement is presented, which identifies this peak with the Mahan exciton-the result of the Coulomb interaction between the photoexcited hole in the valence band and the sea of electrons in the conduction band This calculation accounts for the strong dependence of the absorption edge peak on both the temperature and carrier concentration, in good qualitative agreement with experimental data and with previously published results The changes induced by the carriers on the subband structure through self-consistent calculations are also analyzed, and it is concluded that in these symmetric structures, the changes are small for achievable carrier densities >

Intrinsic origin of visible light emission from silicon quantum wires: Electronic structure and geometrically restricted exciton.

Abstract: We theoretically investigate excitonic effects on the optical properties of silicon quantum wires, based on ab initio electronic structure calculations. The Si wires have a direct, allowed band gap in the visible energy range and exhibit a strong optical anisotropy. Taking into account the electron-hole Coulomb interaction, the geometrical restriction of excitons dramatically enhances the oscillator strength of the optical transitions. Comparisons with recent experimental results are also made.

Exciton delocalization and dynamics in helical π-stacks of self-assembled perylene bisimides

Abstract: Whilst the excitonic properties of J-aggregates have been investigated in great detail, those of H-aggregates have not been systematically investigated yet. In this regard, we have explored the exciton dynamics and excited-species formation processes in columnar H-aggregates of planar PBI dyes that are stacked in a helical fashion by various spectroscopic techniques such as time correlated single-photon counting and femtosecond pump–probe measurements with anisotropy changes. The outcome of this study is that photogenerated excitons in helically stacked PBI dyes experience complicated relaxation processes that involve excited-state interactions such as exciton delocalization and excimer formation. To scrutinize the exciton dynamics in the helically stacked aggregates, we have also included distorted bay-substituted PBI dyes as reference molecules that exhibit either no or only relatively small-sized dimeric aggregate structures. The comparative study revealed that the excited-state interactions in the large-sized helically stacked aggregates extend beyond two PBI units, leading to a final excimer (here, excimer means not only an “excited dimer” but an “excited multimer”) trap state within ∼50 ps. Although in competition with this relaxation path into the excimeric trap state, exciton diffusion has been revealed by exciton–exciton annihilation processes, occurring at high excitation power. Whilst the excimer formation process interrupts the direct observation of exciton diffusion in these columnar PBI aggregates, the exciton migration distance could be estimated by the incorporation of non-fluorescent PBI quencher molecules. From this analysis we can conclude that the exciton diffusion can reach a length of about 10 monomer units. Although this value appears to be shorter than those values observed for J-aggregates, this result shows that columnar PBI stacks might still be useful for optoelectronic applications if the relaxation process leading to excimer traps is prevented, e.g. by structural modifications of the molecules.