Coherent Excitation Spectroscopy on Inhomogeneous Exciton Ensembles

Abstract

A new spectroscopic technique is demonstrated on three characteristically different semiconductor quantum-well samples. Coherent excitation spectroscopy reveals coherent coupling and effectively suppresses the signatures of inhomogeneous broadening and incoherent transfer between various excitonic states in quantum wells. Coherently coupled homogeneous subsystems are analyzed in an inhomogeneously broadened excitonic ensemble. PACS numbers: 78.30.Ly, 71.35.Cc Optical spectroscopy is a powerful tool to study electronic properties of atoms, molecules, and organic as well as inorganic solids. Single isolated absorbers can be prepared in special cases [1] but, in general, macroscopic ensembles constitute the available samples. All these ensembles are subject to the influence of disorder, and thus their optical properties are modified by small variations of the environment. The most obvious consequence of this disorder is the inhomogeneous broadening of optical spectra. This broadening not only masks the intrinsic, homogeneous linewidth of the optical transitions, but also obscures disorder-induced changes of electronic and excitonic properties.Sophisticated techniques have been developed to overcome the problems introduced by inhomogeneous broadening. The three main experimental approaches are coherent, photon-echo spectroscopy [2,3], experiments with high spatial resolution [4], and energy-selective techniques [5,6]. Most promising are combinations of these approaches, such as high spatial resolution in connection with energy-selective spectroscopy [7-10] or energyselective combined with coherent spectroscopy [11,12]. Optical excitations are subject to complex interactions in most solid state materials. Diffusive motion, hopping, and tunneling processes lead to the loss of energy and coherence due to scattering events, in an inhomogeneous ensemble of excitons. Therefore, even the combinations of spectroscopic techniques often do not provide exhaustive information. The results are either influenced by incoherent relaxation and migration processes [9,10], or they yield only information on the energetically lowest state of excitons [10,11] or of exciton molecules [12].We will show in this Letter that the combination of energy-selective and coherent spectroscopy can be extended to achieve detailed information on complex optical resonances caused by coherently coupled exciton states. The basic idea is to perform coherent excitation spectroscopy (CES). Our new concept is demonstrated on excitons in different kinds of semiconductor quantum-well structures. These systems can be tailored such that certain couplings are known to exist, which allows us to clearly demonstrate the advantages of our concept. CES, however, has a much wider range of applications, including polymers.Our approach is based on partially nondegenerate fourwave mixing (FWM) [13]. In detail, we perform a transient FWM experiment in direction 2k 2 2 k 1 with a first 2-3 ps pulse in direction k 1 and a second 100 fs pulse in direction k 2 . Pulse 2 is sp...

Determination of homogeneous and inhomogeneous broadening in semiconductor nanostructures by two-dimensional Fourier-transform optical spectroscopy

“…For the analysis of excitons in semiconductor quantum wells, 2D coherentexcitation spectroscopy ͑CES͒ has been used. 17,34,35 This measurement is based on partially nondegenerate FWM 36 using a temporally long pulse and a short pulse, i.e., a spectrally narrow pulse and a broad pulse. Therefore, it provides mainly spectral and little temporal information.…”

Polarization-dependent optical 2D Fourier transform spectroscopy of semiconductors

“…Recently, there has been significant progress in translating multidimensional NMR techniques into the infrared and optical domains for the study of vibrations (13) and electronic excitations (14-16) in molecules. Although the usefulness of adding a second dimension was recognized in TWFM studies of semiconductors (17)(18)(19)(20), only the intensity of the emitted signal, not the phase-resolved electric field, was measured. The transient absorption experiments clearly show that detecting only the real part of the emitted field is advantageous (21,22), but the effects of inhomogeneous broadening cannot be removed.…”

2d Fourier spectroscopy of disordered quantum wells

“…The susceptibility of the 2d spectroscopy to the manybody correlations suggests its usage for resolving the longstanding problem of the coherent coupling between exciton states residing in different parts of disordered QWs [6][7][8]. However, in order to understand the manifestation of the two-fold effect of disorder, the formation of the new states and random scattering, it is necessary to have a theoretical background for the 2d Fourier spectroscopy in disordered QWs.…”