K. Meissner

Excitonic n-string in linear chains

Abstract The bound states of n Frenkel excitons ( n = 2,3,…), which are called excitonic n-strings, are theoretically shown to exist in contrast with a system of Wannier excitons. The electronic structure and nonlinear optical responses of these excitonic n-strings are clarified. The first evidence for the excitonic 2- and 3-strings is provided from comparison of the calculated and observed differential transmission spectrum of an Anthracene-PMDA crystal pumped by femtosecond laser pulses.

A new class of collective excitations: Exciton strings

Optical excitation in a strongly neutral quasi‐one‐dimensional mixed‐stack charge‐transfer solid results in an exciton state, in which the electron and the hole are bound by electrostatic Coulomb interactions that are large compared to the one‐electron hopping. We present a joint theoretical–experimental demonstration of a new class of collective excitations, multiexcitons or exciton strings, consisting of a string of several (more than two) bound excitons, in a prototype neutral charge‐transfer solid. The stability of the multiexciton states arise from the combined effects of one dimensionality and strong Coulomb interactions. Theoretically, we show that in narrow band one‐dimensional semiconductors with long range Coulomb interactions, the occurrence of stable 2‐exciton string (biexciton) necessarily implies stable higher multiexcitons. Experimentally, evidence for the multiexciton strings is demonstrated by femtosecond pump–probe spectroscopy of anthracene pyromellitic acid dianhydride. Excellent quali...

EXCITON-TO-BIEXCITON TRANSITION IN QUASI-ONE-DIMENSIONAL ORGANICS

In the previous paper we demonstrated novel multiexcitons in a neutral mixed‐stack charge‐transfer solid. The lowest multiexciton, the biexciton, has recently been of interest also in the context of quasi‐one‐dimensional organic materials that are different from the mixed‐stack solids. The nature and strength of the optical transition from the exciton to the two‐exciton states is of importance in understanding photoinduced absorption as well as two‐photon absorption. We show that within the diverse theoretical models that describe these different classes of materials, the excited state absorption from the optical exciton to the two‐exciton states changes in a fundamental way upon the formation of the biexciton. The identical nature of the exciton absorption within these models is a consequence of one dimensionality.

Band-gap renormalization and optical gain formation in highly excited CdSe

Abstract This paper reports the direct femtosecond observation of the dynamical bandgap renormalization and optical gain formation in a highly excited CdSe crystal. Interband excitation with 60 fs pulses results in broadening of the exciton at 0 fs, exciton bleaching and bandgap renormalization at 80 fs, and development of optical gain below the exciton level after 80 fs with rise time of 100 fs. At 380 fs, the optical gain reaches its maximum value and subsequently decreases slowly. Stimulated plasma luminescence has a peak around the energy where maximum gain is observed. The data are discussed in terms of plasma cooling, screening, state filling, and band-gap renormalization.

Experimental and theoretical investigation of femtosecond carrier relaxation in CdSe

Abstract A femtosecond spectral hole burning technique is employed to study the relaxation of nonequilibrium carriers via carrier-carrier and carrier-LO-phonon scattering. Excitation by 70-fs laser pulses several LO phonon energies above the exciton resonance at 10 K in CdSe results in a transient spectral hole that disappears in less than 100 fs. The experiments are compared with a theory that involves numerical evaluation of the semiconductor Bloch equations in the presence of carrier-carrier and carrier-LO phonon scattering.

Ultrafast transient gain in type II multiple quantum wells

Gain for less than one picosecond is observed in a type II GaAs/AlAs multiple quantum well and compared to a type I multiple quantum well under the same conditions. A sophisticated three‐beam experiment is used to unambiguously determine the internal gain of the samples. This method is capable of measuring very small amounts of gain and applicable to a wide variety of situations. The observations are well explained by a simple carrier scattering model.

Femtosecond optical nonlinearities under resonant excitation of excitons in CdSe

Abstract Femtosecond pump-and-probe spectroscopy was utilized to study the optical nonlinearities of excitons in CdSe. Under resonant excitation of the A-exciton, a 42 meV energy broadening of the B-exciton structure was observed. This broadening is caused by collisions between the A-exciton and the B-exciton. This is a rather unique observation of the scattering process between different kinds of excitons. Analysis indicates that the A-exciton-B-exciton scattering time is 31 fs. This time is well explained by a simple theory. Under resonant excitation of the B-exciton, a fast recovery of the bleaching was observed at the B-exciton. The recovery time of 0.9 ps is explained as the transformation time of B-excitons into A-excitons emitting LO phonons. The observed time constant agrees with the calculated scattering time based on the Frohlich interaction within an order of magnitude.