Adiabatic surface of self-trapping excitons

The problem of the self-trapping of the Frenkel exciton interacting with phonons is considered in the adiabatic approximation. The self-trapping is assumed to take place at a single site. The exciton scattering by phonons at this site and the nearest neighbouring sites is taken into account. Analytical expressions for the adiabatic surface of self-trapping excitons, in particular, for the barrier height separating almost free and self-trapped states, are found. The condition for the applicability of the model to the describing of the self-trapping barrier is determined. The adiabatic surface is shown to have several minima separated by a barrier, i.e. the coexistence of several types of self-trapped excitons is possible.Dedicated to Professor Miroslav Trlifaj on the occasion of his sixtieth birthday.     

极性晶体中极化子效应对界面强耦合激子性质的影响

在Huybrechts关于强耦合极化子的模型基础上,采用LLP变分法研究了极性晶体中激子与IO声子强耦合、与LO声子弱耦合体系的基态能量,推导出了激子的自陷能和诱生势的表达式,并以AgCl/AgBr晶体为例进行了数值计算,结果表明,激子的自陷能不仅与激子的坐标z有关,而且电子-空穴间距离ρ对激子自陷能的影响也十分显著;激子的诱生势不仅与电子-空穴间距离ρ有关,而且激子距离晶体界面的位置z对诱生势的影响也十分显著.     

多原子极性晶体中表面激子的自陷条件

本文研究多原子极性晶体中表面激子的性质,采用微扰法导出表面激子的有效哈密顿量.在计及反冲效应中不同波矢的声子之间的相互作用时,讨论对电子、空穴间的相互作用势、表面激子的自陷能和自陷条件的影响.     

强耦合表面激子内部激发态的性质

采用线性组合算符和幺正变换方法,研究极性晶体中强耦合表面激子内部激发态的性质.计算了表面激子的激发态能量、激发能量和平均声子数.     

Internal-motion dependence of self-trapping of a Wannier exciton

The phase diagram of a Wannier exciton in the phonon fields is presented for 1s, 2s and 2p states of the internal (relative) motion on the basis of the adiabatic approximation. Differences in self-trapping among these states are revealed for an exciton with strong electron-hole Coulomb binding.     

Two-phonon resonant Raman scattering at the indirect exciton in silver chloride

Resonant Raman scattering in AgCl is reported for the first time. With excitation in the indirect absorption edge at 1.8K the observed scattering processes involve pairs of momentum-conserving TA(L) and LA(L) phonons with energies of 8.2 meV and 12.9 meV, respectively. The dependence of the scattered intensity on excitation energy suggests that, in contrast to AgBr, intravalley scattering by long wavelength acoustic phonons and intervalley scattering are negligibly small, the main relaxation mechanism of the free exciton being self-trapping. Some enhancement in the 2LO scattering intensity observed is probably due to resonance with the lowest direct exciton.     

Femtosecond study of self-trapped vibrational excitons in crystalline acetanilide

Femtosecond IR spectroscopy of delocalized NH excitations of crystalline acetanilide confirms that self-trapping in hydrogen-bonded peptide units exists and does stabilize the excitation. Two phonons with frequencies of 48 and 76 cm (-1) are identified as the major degrees of freedom that mediate self-trapping. After selective excitation of the free exciton, self-trapping occurs within a few 100 fs. Excitation of the self-trapped states disappears from the spectral window of this investigation on a 1 ps time scale, followed by a slow ground state recovery of the hot ground state within 18 ps.     

Localized self-trapping in two-dimensional molecular lattice with interaction between Wannier-Mott excitons and phonon lattice

We investigate the interactions of lattice phonons with Wannier-Mott exciton, the exciton that has a large radius in two-dimensional molecular lattice, by the method of continuum limit approximation, and obtain that the self-trapping can also appear in two-dimensional molecular lattice with a harmonic and nonlinear potential. The exciton effect on molecular lattice does not distort the molecular lattice but only makes it localized and the localization can also react, again through phonon coupling, to trap the energy and prevents its dispersion.     

On Exciton Decoherence in Quantum Dots

The effects resulting due to dressing of an exciton with phonons are analyzed as the source of unavoidable decoherence of orbital degrees of freedom in quantum dots. The dressing with longitudinal optical phonons results in energetic shift of order of a few meV even of the ground state of exciton in a state-of-the-art InAs/GaAs dot and the mediating role of longitudinal acoustical phonons is essential in this process. The characteristic time needed for dressing of the exciton with optical phonons is of a picosecond order. That time can be regarded as the lower limit for decoherence for optically driven quantum gates employing self-assembled quantum dot structures.     

抛物量子点中弱耦合激子的光学声子平均数

研究了电子-体纵光学声子弱耦合情况下,抛物量子点中激子的性质。在有效质量近似下,采用线性组合算符和幺正变换方法研究了抛物量子点中弱耦合激子的基态能量和光学声子平均数。以GaAs半导体为例进行了数值计算,结果表明:弱耦合情况下,激子的光学声子平均数基态的能量和量子点受限强度的增大而减小,随量子点半径的增大而增大。     

Jahn-Teller effect at exciton self-trapping

In the adiabatic approximation two limiting cases of exciton self-trapping have been considered for a degenerate atomic term creating the bands of Frenkel excitons, which interact with all crystal deformations admitted by symmetry: a case of a large radius excitation on the self-trapping barrier and a small-radius case. In the first case wave functions and deformations have been found which correspond to the barrier, in the second case the adiabatic surface of self-trapping excitons have been obtained. It is shown that the interaction with non-totally symmetric deformations leads to a lowering of the crystal point symmetry in a state corresponding to the lowest self-trapping barrier, that is the manifestation of the Jahn-Teller effect applied to a self-trapping exciton.     

强耦合多原子极性晶体中的表面激子

本文研究多原子极性晶体中表面激子的性质.采用线性组合算符和拉格朗日乘子法,导出强耦合多原子极性晶体中表面激子的有效哈密顿量,得到了强耦合表面激子的重正化质量.     

Localized self-trapping in the two-dimensional discrete molecular lattice with the interaction between Frenkel excitons and phonons

We investigate the interactions of lattice phonons with Frenkel exciton, which has a small radius in a two-dimensional discrete molecular lattice, by the virtue of the quasi-discreteness approximation and the method of multiple-scale, and obtain that the self-trapping can also appear in the two-dimensional discrete molecular lattice with harmonic and nonlinear potential. The excitons' effect on the molecular lattice does not distort it but only causes it to localize which enables it to react again through phonon coupling to trap the energy and prevent its dispersion.     

Exciton self-trapping in molecular media with an elastic dipole moment

Exciton self-trapping in a molecular medium is considered within a self-consistent model taking into account the change in the dipole moment as a result of a displacement of molecules and the resonance interaction with an electromagnetic field. New mechanisms and specific features of the formation of localized structures in quasi-one-dimensional molecular structures are investigated. It is shown that the dependence of the dipole moment on molecular vibrations leads to new conditions for exciton self-trapping and intrinsic optical bistability, as well as for the formation of electromagnetically induced transparency. The theory proposed is used to explain the specific features of exciton self-trapping in a conjugated polymer and J aggregates of dyes under the action of an external laser field.     

Localization of optically excited states by self-trapping

We discuss the localization of the surface exciton at the Si(111)- (2x1) surface due to self-trapping, which leads to a characteristic temperature-dependent linewidth of the optical response and to a significant Stokes shift of the luminescence. Self-trapping results in this case from a structural relaxation in the excited state, caused by the interplay between electronic and geometric degrees of freedom. The most significant contribution to this effect comes from one single geometric deformation mode which is driven by the internal electronic charge transfer in the self-trapped exciton.     

Energy of K-momentum dark excitons in carbon nanotubes by optical spectroscopy

Phonon sideband optical spectroscopy determines the energy of the dark K-momentum exciton for (6,5) carbon nanotubes. One-phonon sidebands appear in absorption and emission, split by two zone-boundary (K-point) phonons. Their average energy locates the E11 K-momentum exciton 36 meV above the E11 bright level, higher than available theoretical estimates. A model for exciton-phonon coupling shows the absorbance sideband depends sensitively on the K-momentum exciton effective mass and has minimal contributions from zone-center phonons, which dominate the Raman spectra of carbon nanotubes.     

Effects of confined longitudinal and interface optical phonons on excitons in an asymmetric quantum well

The interaction between exciton and confined longitudinal optical (LO) phonons, interface optical (IO) phonons in an asymmetric Ga 1 x Al x As/GaAs/Ga 0.7 Al 0.3 As square quantum well is investigated. By applying the LLP-like transformation and variational approach, the numerical results are obtained as functions of the well width and asymmetric-degree of well. The exciton-optical phonons interaction-energy has a minimum value with the increase of the well width. It is demonstrated that the LO-phonon energ...     

Influence of Electron-Phonon Interactions on the Physical Properties of an Interface Polaron

Taking into account the interactions of both bulk longitudinal optical (LO) pltonons and surface optical (SO) pltonons and using Haga's perturbatioit method, we derive an effective Hamiltonian for the interface magnetopolaron in polar crystals at zero temperature. We illustrate in details how to solve this effective Hamiltonian analytically and we obtain explicit formulae not on1y for the electron cyclotron mass associated with the Landau levels but also for the self-trapping energy of the magnetopolaron with respect to its first three quantum states in the direction normal to the interface of the system. Numerical results are calculated for some Ⅱ-Ⅵ and Ⅲ-Ⅴ semiconductor compounds,indicting that the bulk LO phonons and SO phonons do have different trends of effects on the electron in different quantum states.     

Coherent generation of acoustic phonons in an optical microcavity

Ultrafast coherent generation of acoustic phonons is studied in a semiconductor optical microcavity. The confinement of the light pulse amplifies both the generation and the detection of phonons. In addition, the standing wave character of the photon field modifies the generation and detection phonon bandwidth. Coherent generation experiments in an acoustic nanocavity embedded in an optical microcavity are reported as a function of laser energy and incidence angle to evidence the separate role of the optical and exciton resonances. Amplified signals and phonon spectra modified by the optical confinement are demonstrated.     

Self-trapping of hot and thermalized excitons in solid xenon

The first direct measurement of the self-trapping rate, Γ_ST, of thermalized excitons in solid Xe is presented. The temperature dependence of Γ_ST is explained by thermal assisted tunneling through the self-trapping barrier separating free and self-trapped exciton states. The results agree with recent theories on self-trapping. The ratio of luminescence intensity of free and self-trapped states disagrees with the measured self-trapping rates and displays the role of self-trapping of hot excitons.