Raman excitonic scattering

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]...

When the frequency of a laser falls fully into an absorption band, multiple phonon processes start to appear. Leite et al 2° ) observed /7 h order ( = 1, 2. 9) Raman scattering in CdS under conditions of resonance between the laser frequency and the band gap or the associated exciton states. The scattered light spectrum shows a mixture of fluorescent emission and Raman scattering. Klein and Porto 207) associated the multiphonon resonance Raman effect with the fluorescent emission spectrum, and suggested a possible theoretical approach to this effect. 

The mechanism of control with multipulse excitation is likely due to dynamics of the carotenoid donor. The presumably incoherent EET process [1] would not support the observed dependence on the carrier phase via the parameter c. Furthermore, the control effect does not suffer from annihilation at higher excitation intensities [2], as would be characteristic for the delocalised excitons in the B850 ring [1], However, it is well known that femtosecond pulses populate higher ground state vibrational levels by impulsive Raman scattering (IRS) [4], and that the periodic phase modulation (Eq. 1) makes IRS selective for specific vibrations... 

We must remark that the amplitude of these processes is generally weak compared to the direct exciton-photon amplitude, owing to the small libration amplitudes (of the order of 1 °) at low temperatures. It is still smaller when the incident light polarization is parallel to the molecular transition dipole. For instance, in anthracene-crystal excitation, we expect the exciton-photon-phonon contribution to be more important for the a than for the b polarization. On the contrary, these processes become much more important in nonresonant excitations, in Raman scattering for instance (cf. Section II.D). 

The model of an isolated layer was refined by introducing substrate effects by coupling the surface 2D excitons to the bulk polaritons with coherent effects modulating the surface emission and incoherent k-dependent effects damping the surface reflectivity and emission, both effects being treated by a KK analysis of the bulk reflectivity. The excitation spectra of the surface emission allowed a detailed analysis of the intrasurface relaxation dominated by resonant Raman scattering, by vibron fission, and by nonlocal transfer of... 

In thick ( 300 pm) crystals of GaN electronic excitons of shallow dopants have been observed in far infrared absorption at 215 cm 1 [44], Interpreted as the ls-2p transition of a residual shallow donor, its binding energy was calculated to be (35.5 0.5) meV. Further modes at 149 and 242 cm 1 have been observed in mixed phase GaN/GaAs in Raman scattering and have been associated with electronic excitations of shallow donors in cubic and sphalerite GaN, respectively [45] see also [46], Far infared absorption at 23.2 cm 1 in magnetic fields has been used to determine the effective electron mass in GaN, m = 0.20 0.005 m, (corrected for polaron effects) in cyclotron resonance [47]. 

This section discusses applications of IR and Raman spectroscopy to materials with reasonable or even very high conductivity. These systems generally present special problems in addition to those described in earlier sections. Incident radiation interacts not only with the vibrational excitations of the material but also with the free carriers and with its electronic structure. These interactions may create phenomena such as free carrier absorption, excitation across the energy gap, exciton transitions, or light scattering by free electrons. Excitations are very often in the IR spectral range, particularly in the... 

The depolarization of light by atomic gases 462 and the use of Raman scattering to probe exciton-phonon coupling in molecular crystals 463 have been discussed. 

When the incident light as well as the scattered light is in resonance with electronic transitions [doubly-resonant Raman spectroscopy (DRRS)], a strong enhancement is observed, because both denominators in the expression for the Raman process resonate at the same time [251-253]. To realize the conditions for DRRS, the exciton energies have to be adjusted in a way that the difference of two interband transitions matches the phonon... 

TD Krauss, FW Wise. Raman-scattering study of exciton-phonon coupling in PbS nanocrystals. Phys Rev B 55 9860-9865, 1997. 

JE Zucker, A Pinczuk, DS Chemla, A Gossard, W Wiegmann. Raman-scattering resonant with quasi-2-dimensional excitons in semiconductor quantum wells. Phys Rev Lett 51 1293-1296,... 

A Frommer, Y Garini, A Ron, E Cohen. Resonant Raman-scattering within an inhomogeneously broadened exciton band in semiconductors. J Lumin 45 9-12, 1990. 

AV Fedorov, AV Baranov, K Inoue. Exciton-phonon coupling in semiconductor quantum dots— Resonant Raman-scattering. Phys Rev B 56 7491-7502, 1997. 

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