Excitons resonant

The ability to create and observe coherent dynamics in heterostructures offers the intriguing possibility to control the dynamics of the charge carriers. Recent experiments have shown that control in such systems is indeed possible. For example, phase-locked laser pulses can be used to coherently amplify or suppress THz radiation in a coupled quantum well [5]. The direction of a photocurrent can be controlled by exciting a structure with a laser field and its second harmonic, and then varying the phase difference between the two fields [8,9]. Phase-locked pulses tuned to excitonic resonances allow population control and coherent destruction of heavy hole wave packets [10]. Complex filters can be designed to enhance specific characteristics of the THz emission [11,12]. These experiments are impressive demonstrations of the ability to control the microscopic and macroscopic dynamics of solid-state systems. 

Fig. 2.3. The Fourier-transformed (FT) intensity of coherent phonons as a function of the pump polarization angle ip for a GaAs/Alo.36Gao.64As MQW. The excitation wavelength is slightly above the n = 1 exciton resonance (left) and slightly above the n = 2 subband energy (right), ( -dependent component is attributed to ISRS, while the ( -independent component is to TDFS and forbidden Raman scattering. From [20]...

Transient transmittance of single-walled carbon nanotubes (SWNTs) in suspension was modulated at two periods of T40 and 21 fs, corresponding to the RBM and G mode, respectively [54,55]. The amplitude and the frequency of the coherent RBMs exhibited a clear excitation-wavelength dependence (Fig. 2.15) [54]. The different frequencies were attributed to SWNTs with different diameters coming to the excitonic resonance. The FT spectra of the coherent RBMs in Fig. 2.15 had noticeable differences from the resonant Raman spectra, such as the different intensities and better frequency resolution. 

Coherent optical phonons can couple with localized excitations such as excitons and defect centers. For example, strong exciton-phonon coupling was demonstrated for lead phtalocyanine (PbPc) [79] and Cul [80] as an intense enhancement of the coherent phonon amplitude at the excitonic resonances. In alkali halides [81-83], nuclear wave-packets localized near F centers were observed as periodic modulations of the luminescence spectra. 

We note at this point that, when the two fragments become identical, the charge transfer configurations and the locally excited configurations will have to be replaced by charge-resonance and exciton-resonance configurations, respectively. 

The interest in semiconductor QD s as NLO materials has resulted from the recent theoretical predictions of strong optical nonlinearities for materials having three dimensional quantum confinement (QC) of electrons (e) and holes (h) (2,29,20). QC whether in one, two or three dimensions increases the stability of the exciton compared to the bulk semiconductor and as a result, the exciton resonances remain well resolved at room temperature. The physics framework in which the optical nonlinearities of QD s are couched involves the third order term of the electrical susceptibility (called X )) for semiconductor nanocrystallites (these particles will be referred to as nanocrystallites because of the perfect uniformity in size and shape that distinguishes them from other clusters where these characteriestics may vary, but these crystallites are definitely of molecular size and character and a cluster description is the most appropriate) exhibiting QC in all three dimensions. (Second order nonlinearites are not considered here since they are generally small in the systems under consideration.)... 

R. D. Harcourt, G. D. Scholes and K. P. Ghiggino, Rate expressions for excitation transfer. II. Electronic considerations of direct and through-configuration exciton resonance interactions, J. Chem. Phys., 101 (1994) 10521-10525. 

Zhirko Yu.I. (1999) Investigation of the light absorption mechanisms near exciton resonance in layered crystals. N=1 state exciton absorption in InSe. 

One of the most obvious perturbations of the surface excitons by the bulk crystal is the short-range coulombic interactions causing surface-exciton transfer to the bulk. We discard these perturbations on the grounds both of theoretical calculations27 and of the experimental observations, which show the presence of a surface exciton (the second subsurface exciton S3 see Fig. 3.2) resolved at about 2cm-1 above the bulk-exciton resonance. 

In particular, we have only one polariton mode, with an excitonic resonance energy at about cAweA + CgOjeB, which is an average of the energies of the pure excitons A and B. 

The linear (ID) absorption specua of all models are presented in Fig. 4. Model A shows five well-resolved one-exciton lines. In model B the lines 2 and 3 are poorly resolved due to the increased homogeneous broadening. Diagonal disorder in models C and D further broadens the spectra. Since off-diagonal disorder induces state delocalization, the one-exciton resonances shift for models E and F and become s = 1578 cm, 2 = 1605 cm-1, 3 = 1618 cm-1, = 1652 cm-1, and 5 = 1679 cm-1. 

Furthermore, on the excited surface both formation of a complex, to the stabilization of which both an exciton resonance term and an electron transfer term contribute, and full electron transfer to yield a radical ion pair may be envisaged [3], Diffusion of the charged species leads to free solvated radical ions (FRI), but for a sizeable fraction of the systems which will be discussed in the following, the reaction takes place at the stage of the contact ion pair (CIP), and then distinction between the properties of the polar exciplex and of the radical ion pair may not be unambiguous. 

Change in solvent polarity has been shown to alfect the relative contribution of exciton resonance and charge transfer to the stabilisation of excited complexes (Eunice et al., 1979). It was found, for example, that the quenching of the fluorescence of anthracene by amines and phosphines in nonpolar solvents showed a better correlation between log and the singlet energy of the quencher than with the oxidation potential of the quencher. The reverse is true when polar solvents are used, showing, as had been postulated in earlier work (Davidson and Lambeth, 1969), that charge transfer is important in such solvents. 

Figure 14. Schematic diagram of the stabilization of the CDMA excimer due to exciton resonance interaction. An ideal sandwich-like conformation with opposite orientation of the two monomers is assumed for the excimer. The values of the excited-state energies and the corresponding transition dipole moments were taken from Ref [33cj. The exciton splitting of the Lb state is only 39 cm owing to the low transition dipole moment corresponding to the Lb <— So emission. The much larger A/nu value for the La — So fluorescence makes the exciton splitting of the La state the dominant stabilization. The in-phase and the out-of-phase combinations of the exciton resonance states are labeled L and L, respectively. The unknown contributions of the charge resonance interaction are indicated by a question mark. Reproduced with permission from Ref [92a].

Davydov splitting for exciton resonance in anthracene, and for the first time obtained reasonable agreement with available experimental data. He used a dipole approximation for the intermolecular interaction and the only ingredients in his theory were the resonance frequencies and oscillator strength. In contrast to quantum theory described in this chapter the classical dipole theory does not take into account the contribution of the nondipole interaction, which are important in the majority of solids. It is clear that also multiexciton states including states with few quantum of excitations on the same molecule (what is forbidden for the two-level model) in classical harmonic oscillator theory contribute to the energy of excitons. However, in the framework of the classical theory it is impossible to develop the estimation of corrections which we discussed here. 

The tensor tij for the region of exciton resonances was first calculated using microscopic theory by Pekar (18). 

THE DIELECTRIC TENSOR OF CRYSTALS IN THE REGION OF EXCITONIC RESONANCES... 

Before discussing this new method it is useful to recall briefly the methods which we have already discussed. Note, first of all that calculations of the dielectric tensor must be based, as is known, upon a microscopic theory Such a theory for ionic crystals was first developed by Born and Ewald (2) for the infrared spectral region. The application of this approach for the region of exciton resonances has also been demonstrated in (3). In an approach identical to that of Born and Ewald (2) the mechanical excitons (see Section 2.2) are taken as states of zeroth-approximation. In the calculation of these states the Coulomb interaction between charges has to be taken into consideration without the contribution of the long-range macroscopic part of the longitudinal electric field. If this procedure can be carried out, then the Maxwell total macroscopic fields E and H can be taken as perturbations. In the first order of perturbation theory, we find... 

Polariton states in the calculation of the dielectric tensor in the region of Frenkel exciton resonances... 

The consideration of the strong dependence of dissipation of polaritons near exciton resonances will be performed below with the use of transverse dielectric tensor ej y(w, k). This tensor will be calculated assuming that the excitonic states, with complete account of the Coulomb interaction, are known. Since the derivation of the expression for k) is given in the monograph (3), see... 

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