Photonic Crystal Enhances Brightness of Quantum Dots
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current in molecular wires with high efficiency, yet is remarkably resistant to the deleterious effects of vibronic coupling. As reported in the September 21 issue of Physical Review Letters (DOI: 10.1103/ PhysRevLett.99.126802), Franco, Brumer, and Shapiro chose ω and 2ω far-off resonance, well below interband transition frequencies, thereby relying on Stark shifts to couple ground and excited electronic states adiabatically. The researchers used a model composed of a trans-polyacetylene (PA) oligomer, the ends of which are connected to macroscopic metallic leads. They employed the so-called tight-binding model (the full Hamiltonian of the system is approximated by the Hamiltonian of an isolated atom centered at each lattice point and the atomic orbitals are assumed to be very small at distances exceeding the lattice constant). The energy of the nuclei and electrons of the oligomer were represented with the well-known Su-SchriefferHeeger (SSH) Hamiltonian, which successfully reproduces the electronic structure and the dynamics of electronic excitations in PA. The electronic structure of the PA chain consists of 50 doubly occupied valence π orbitals and 50 empty π* states separated by an energy gap of 1.3 eV. The leads were treated as rigid, semi-infinite chains. The researchers developed a Hamiltonian for the nanojunctions, treating each as a one-dimensional lattice. The ω + 2ω field was turned on and off smoothly during a 100-fs interval, and the amplitude, 6.1 × 10–3 V –1, was kept constant for 400 fs. The photoinduced, electron-vibrational dynamics were followed within a mean-field, mixed quantum-classical approximation. For a relative pulse phase of φ2ω–2φω= 0 and averaging more than 1000 trajectories, the researchers found that the spectrum displayed considerable Stark shifts and narrowing of the energy gap, causing frequent crossings between ground and excited states in individual trajectories. In addition, charge bursts were deposited in the leads when the electron population was transferred from the valence to the conduction band. The researchers observed that a minimum in the energy gap was concomitant with the maximum in the field strength, but the Stark effect was only sufficiently strong to close the energy gap when the field had positive amplitudes. For φ 2ω –2φ ω =π/2 the researchers found that the Stark shifts were equally strong for positive and negative field amplitudes, resulting in no net current; the direction of the current can therefore be controlled by the relative phase of the laser. Plotting the net rectification as a function of the laser phase, the researchers showed that up to 90% of the electrons can participate in the net current, the mechanism is resistant to decoherence effects, and the currents observed are phonon-assisted. Franco, Brumer, and Shapiro said that their prediction of ultrafast currents “could lead to the development of molecular switches that operate on a femtosecond timescale.” STEVEN TROHALAKI
Photonic Crystal Enhances Brightness of Quantum Dots N. Ganesh, B.T. Cun
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