Exciton Branches Research Articles

Observation of Fabry-Pérot modes in the upper branch of the polariton in ZnSe-GaAs epilayers.

We have studied the reflectance properties of ZnSe epilayers deposited on GaAs substrates by metal-organic vapor-phase epitaxy. For layer thicknesses in the range of a micrometer, reflectance oscillations are observed, and are due to Fabry-P\'erot modes in the upper branch of the polariton. This observation is made possible due to the large exciton Rydberg energy in ZnSe and due to the subsequent long-energy extension of this branch of the polariton before its coupling with the 2s-related exciton branch.

Multiple-branch exciton energy spectra in the FQHE regime

Spectrum of excitons in a FQHE regime depends on the ratio h/l, h is a distance between electron and hole confinement planes, l is the magnetic length. It comprises a single exciton branch and continuum at h≤ l 2 and a multiplicity of branches at h> l 2 . New branches arise due to charge fractionalization. Their quantum numbers are angular momenta of the internal motion in an exciton. Connection of excitons to the low-energy sector of the quasielectron Hilbert space is established. Projecting rules from a spherical into plane geometry are generalized for composite particle spectra.

Nonlinear optics of CuCl under uniaxial stress

Abstract Stress-induced shifts and splittings of resonances on the transverse polariton and longitudinal exciton branches in CuCl are studied by nonlinear optical measurements. Owing to the small linewidth, shifts and splittings are clearly resolved. Thus the deformation potentials are determined with high accuracy.

Nonlinear spectroscopy of 1S-excitons in CuCl under uniaxial stress

Resonances on the transverse polariton and longitudinal exciton branches of CuCl are studied under unixial stress by nonlinear optical spectroscopy. Stress-induced shifts and splittings of resonances are clearly resolved for uniaxial stress up to 1.6 kbar. Due to the small linewidth the deformation potentials are determined with higher accuracy than with the classical methods of reflection or absorption spectroscopy.

Dispersion of the Exciton Polariton Branches in CuCl and Their k‐Dependent Splitting in a Magnetic Field

AbstractThe method of three‐photon difference‐frequency generation is used to measure resonances on the lower polariton branch in CuCl. Due to the small linewidth of the resonances (smallest width 80 μeV) it is possible to determine the splitting of the lower branch in a magnetic field at different k‐values for the first time. Resonances on the upper polariton branches and on the longitudinal exciton branches can also be excited with the use of two‐ and three‐photon sum‐frequency generation and two‐photon absorption. The rather peculiar wavevector‐dependent splitting of the lower polariton branch is explained quantitatively by taking into account the field induced mixing of the Z3‐ and Z1,2‐excitons.

Reflection spectrum of the benzimidazole crystal at 1.8 K

The polarized reflection spectra from an ab face of a benzimidazole single crystal at 1.8 K are reported. When taken with results from earlier reflection and two-photon spectra, three of the four exciton branches are now identified. The direction of the transition moment for the lowest energy system is rotated 30°±6° from the longer in-plane axis of the molecule towards the NH group in the five-membered ring.

Electronic spectroscopy and second harmonic generation in the benzimidazole crystal

The first excited electronic state of benzimidazole in the pure crystal at about 1.8 K has been studied using several techniques. Fluorescence is essentially b polarized and consists of a spectrum of doublets spaced 6 cm−1 apart. The doublets arise from two emitting states, one of which is the bottom of the lowest energy of the four exciton branches at 35 844 cm−1, and 6 cm−1 below it is some trap of unknown origin. The polarized reflection spectrum from a freshly cleaved bc face located the energies of two of the exciton branches; a c polarized origin band was observed at 35 844 cm−1 with a stronger b polarized band 34 cm−1 to the blue. The two-photon excitation spectrum was strong in (aa), (bb), and (cc) polarizations yielding an origin band at 35 844 cm−1, clearly marking this as the a1 Davydov component. There was a more intense line in the (cc) spectrum at 35 865 cm−1 arising from a resonance enhanced second harmonic generation (SHG) process in the solid; the frequency of the line varied as the crystal section was rotated about the b axis. Nonresonant SHG was observed in a bc section of benzimidazole at room temperature with ω equal to 15 533 cm−1. Making use of the fact that the energies of the exciton branches depend on the direction of the transition moment, an attempt was made to determine the position of the in-plane moment; the present data were insufficient to locate accurately the orientation of the moment in this way.

Excitonic reference state of a model semiconductor in the dynamic Stark regime.

The stationary electronic state of an electron-hole system driven by a strong monochromatic light source is studied within a mean-field approach. The problem can be formulated in terms of a classical static source dressed by a nonlinear Schr\odinger field of N electron-hole pairs. The linearized form reduces to the Coulomb-Schr\odinger field describing the internal motion of pairs with zero total momentum. While the respective homogeneous field equation has solutions for discrete energies only---the stationary bound states---the linearized field with source exhibits resonances at these points, but allows for solutions at any energy. These off-resonance states, for which N goes to zero with the source strength, may be characterized as a cloud of virtual excitons dressing the source. However, due to interactions between identical fermions, the solution changes its character evoking real exciton branches for all excitation energies above the exciton ground state. In this range approximations should be taken with care, as they might sensitively deform basic features of the solution. The possibility of a novel hysteresis effect is discussed.

Direct measurement of the vibrational relaxation of the 991 cm −1 mode of the benzene crystal

A picosecond time-resolved CARS experiment on the 991 cm −1 mode of the benzene crystal at low temperature is reported. The decay is single exponential with a decay time τ = 39 ± 2 ps. This is interpreted as the vibrational population relaxation time of the Ag( k ≈ 0) vibron state which is at the minimum of the lowest exciton branch.

Dynamics of localized excitations from energy and time resolved spectroscopy

The electronic structure and the excited state relaxation phenomena in condensed rare gases have been studied by absorption spectroscopy, photoelectron spectroscopy, and time resolved luminescence spectroscopy, all using selective excitation by monochromatized synchrotron radiation. Radiationless and radiative electronic transitions in excited states and the rearrangement of surrounding matrix atoms will be discussed for excited rare gas atoms in rare gas matrices. Furthermore, experimental results concerning the scattering of excitons within the free exciton branches, localization of excitons, relaxation within the self-trapped exciton states, and energy transfer to guest atoms and boundaries will be presented.

Exciton polaritons and hyper-Raman scattering in zinc-blende-type semiconductors: CuBr as an example

The dispersion of excitons and hence of excitonic polaritons in zinc-blende-type semiconductors reflects the complexity of the band structure of these materials. We calculate the dispersion of the exciton branches ${E}_{i}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}})$ of the eightfold ground state from an invariant expansion of the center-of-mass Hamiltonian, including exchange interaction, up to second order in the exciton total wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}}$. The excitonic polaritons are then constructed according to Hopfield's polariton theory, which is extended to the case of more than one oscillator. The polariton dispersion is experimentally studied for CuBr by hyper-Raman scattering via biexcitons. The selection rules for this process are calculated for different scattering configurations, whereby the symmetry of the biexciton ground state, having components of ${\ensuremath{\Gamma}}_{1}$, ${\ensuremath{\Gamma}}_{3}$, and ${\ensuremath{\Gamma}}_{5}$ symmetry, is considered. A self-consistent analysis of the observed hyper-Raman emission lines yields the dispersion of the multicomponent excitonic polaritons in CuBr.

Exciton Bands in AntiferromagneticCr2O3

We have calculated the energy dispersion of the lowest eight $^{2}E$ Frenkel exciton branches in the four-sublattice antiferromagnet ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$. This is the first such calculation for a magnetic insulator. The symmetry properties and $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ dependence of the interion exchange and Coulomb interactions which give rise to dispersion and Davydov splittings are presented in detail. Pairwise matrix elements of the interion Hamiltonian are treated as phenomenological parameters, and in most cases were determined from the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}=0$ energies analyzed in an earlier paper. Dispersion curves for five directions in the rhombohedral Brillouin zone, and the exciton density of states, are given. Confirmation of the main features of the calculated exciton bands is provided by a measurement of the exciton-magnon absorption band shape. In the presence of a number of simplifying assumptions, this band shape is given by the joint exciton-magnon density of states. Good agreement between the calculated and observed band shape is obtained.