Two-exciton State Research Articles

Four-wave-mixing signals from excitons under linear-circular polarized excitation exhibit an elliptical polarization, depending on both the pulse delay and the spectral position. Besides the resonances corresponding to excitons and exciton-biexciton transitions, a breakup of the exciton line is found reflecting the influence of correlations on the four-point level. An analysis accounting for the exciton density, the bound biexciton, and the exciton-exciton scattering continuum reveals that these features are not due to an antibound two-exciton state. Instead, they result from an interference of the correlated exciton continuum with the exciton density. In addition, the modeling shows that the signals ellipticity is highly sensitive to the influences of different correlations and, therefore, allows for a discrimination of their contributions.

Two-exciton states with lattice relaxation in one-dimensional semiconductors

The paper deals with the energy and the optical spectra of Frenkel biexciton (bound two-exciton state, excitonic molecule) in molecular crystal. A very simple system of equations describing the biexciton states is obtained. The number of equations in this system is equal to the number of the nonzero matrix elements of dynamical exciton interaction. In contrast to the other papers, the kinematical interaction of excitons is included into the Green’s functions, which are equivalent to the Green’s functions for one exciton states in a crystal with a vacancy. The obtained system of equations is solved for one-, two-, and three-dimensional crystals taking into account only the isotropic nearest-neighbor interaction. It is shown that biexciton has anisotropic character in two- and three-dimensional crystals for certain quasimomentum directions, i.e., it can be stretched along one axis or plane. In the isotropic case the wave function of a biexciton is analogous to the s or d function of the hydrogen atom. This system of equations can be easily solved for more complex models as well. The transition rates from the ground state to the biexciton band (two-photon absorption) and from the one-exciton band (one-photon absorption) to the same states is investigated, too. It is shown, that transition is allowed only into the s biexciton state, and it has a resonance character analogous to the Rashba effect for the localized one-exciton state in impurity crystal.

Two-exciton emission of BIC J-aggregates

Excitonic nonlinearities in the weak-excitation limit condition are examined with time-integrated and time-resolved four-wave mixing (FWM). Below an excitation of 3\ifmmode\times\else\texttimes\fi{}${10}^{9}$ excitons/${\mathrm{cm}}^{2}$, coherent third-order processes prevail. Echo-type signals are observed in parallel and perpendicular polarization configurations. The coherent process induced by the two-photon coherence of the two-exciton state is attributable to signals in the perpendicular configuration. The polarization dependences of a temporal profile and the intensity of FWM signals are well explained with third-order perturbational analysis that takes into account the two-exciton states and inhomogeneity.

Analytical expressions for the exciton-exciton interaction potentials have been approximately derived in both 2D and 3D materials exhibiting explicit dependences on exciton momentum difference, momentum transfer, electron-hole effective mass ratio and two-exciton state spin symmetry. Numerical calculations show that the character of the exciton-exciton interaction is determined by all of the above-mentioned dependences.

Jahn-Teller effects in 4 T 1 g + 4 T 1 g excited ion pair in RbMnF 3 are studied under the assumption of the A 1 g - and E g -distortion. General expressions of the Jahn-Teller energy and the exciton-exciton interaction energy due to the lattice distortions are derived and examined in detail by using a simple lattice model. A positive large value of the exciton-exciton interaction energy is obtained for the lowest energy two-exciton state for certain mixing ratio of the two types of distortions, though it seems difficult at present to explain the large value observed experimentally (S. E. Stokowski, D. D. Sell and H. J. Guggenheim: Phys. Rev. B 4 (1971) 3141) by the simple model adopted here. The shape and the peak position of the two-exciton phonon sideband are discussed briefly.