Excitonic Component Research Articles

Inverse exciton series for observation of bipolariton coupling

We report the first observation of inverse polariton series (LP series) in CuCl, due to the radiative decay of excitonic molecules selectively excited at mode K m from the band 0⩽K m ⩽0.3×10 6 cm −1 . In order to explain the experimental data, the bipolariton model of an excitonic molecule is adapted to describe the LP ns lines ( n=1, 2, 3 and 4) in terms of the scattering scheme “two incoming polaritons → excitonic molecule → two outgoing polaritons, associated with 1s and ns excitonic states”. We show that the efficiency of the multi-branch polariton scattering is determined by the excitonic component of the final-state (outgoing) polaritons.

Temperature dependence of residual stress in epitaxial GaAs/Si(100) films determined from photoreflectance spectroscopy data

In the temperature range T=10–300 K, photoreflectance spectroscopy was used to study the temperature dependence of residual stress in epitaxial n-GaAs films (1–5 µm thick, electron concentration of 1016–1017 cm−3) grown on Si(100) substrates. A qualitative analysis showed that the photoreflectance spectra measured in the energy region of the E0 transition in GaAs had two components. They consisted of the electromodulation component caused by the valence subband |3/2; ±1/2〉-conduction band transition and the low-energy excitonic component. The magnitude of stress was determined from the value of the strain-induced energy shift of the fundamental transition from the subband |3/2; ±1/2〉 with respect to the band gap of the unstressed material E0(T)-E0|3/2; ±1/2〉(T). The increase in the energy shift E0-E0|3/2; ±1/2〉 from 22 ± 3 meV at 296 K to 29 ± 3 meV at 10 K, which was found in the experiments, gives evidence of an increase in biaxial stress with decreasing temperature.

LA Phonoritons inCu2O

We report the observation of LA-phonoriton eigenstates, which are a pump-induced coherent superposition of excitons, photons, and longitudinal acoustic (LA) phonons developing in up-conversion Brillouin scattering of an intense polariton. The excitonic component of the LA phonoritons in a ${\mathrm{Cu}}_{2}\mathrm{O}$ bulk crystal in the presence of a coherent orthoexcitonic polariton is studied by stimulated two-photon emission spectroscopy. For optical intensities of $30--200\mathrm{MW}/\mathrm{cm}{}^{2}$ the relevant scale of the pump-induced spectral changes is $5--10\ensuremath{\mu}\mathrm{eV}$. The theory of LA phonoritons in ${\mathrm{Cu}}_{2}\mathrm{O}$ quantitatively reproduces the experimental data.

Electric-Field Tuning of Spin-Dependent Exciton-Exciton Interactions in Coupled Quantum Wells

We have shown experimentally that an electric field decreases the energy separation between the two components of a dense spin-polarized exciton gas in a coupled double quantum well, from a maximum splitting of $\sim 4$ meV to zero, at a field of $\sim $35 kV/cm. This decrease, due to the field-induced deformation of the exciton wavefunction, is explained by an existing calculation of the change in the spin-dependent exciton-exciton interaction with the electron-hole separation. However, a new theory that considers the modification of screening with that separation is needed to account for the observed dependence on excitation power of the individual energies of the two exciton components.

Symmetry of excitons in GaN

Magnetoreflectivity measurements on GaN are used to resolve the symmetry of three excitonic resonances, arising from the crystal-field and spin-orbit effects on the valence band. The symmetry characteristic for each valence subband is directly seen in the observed Zeeman pattern. The linear magnetic-field effect for all exciton components is well accounted for by introducing only two coupling constants: the electronic g factor for the conduction band and the Luttinger $\ensuremath{\kappa}$ parameter for the valence band. The strength of the electron-hole exchange interaction is determined.

From Exciton Resonance to Frequency Mixing in GaAs Multiple Quantum Wells

Frequency mixing (2v1 2 v2) is demonstrated in femtosecond nondegenerate four-wave mixing (FWM) on GaAs multiple quantum wells using two synchronized, independently tunable lasers. The frequency mixing component in the spectrally resolved FWM coexists with the exciton resonance, and it dominates over the exciton component at high intensity and high detuning. This off-resonant component is present only for delays smaller than the temporal pulse width, whereas the resonant exciton component survives for longer delays. [S0031-9007(99)09078-X]

Analysis of the Low-Temperature (77 K) Near-Band-Edge Luminescence in Undoped Semi-Insulating GaAs

The dependences of the peak position and of the half-width of the near-band-edge luminescence band in nominally undoped semi-insulating GaAs crystals on the content of residual C and Si shallow impurities are studied at 77 K. It is shown that a rise in the impurity concentration leads to a shift of the peak position to lower energies and to an increase of the half-width. The observed effect is mainly connected with a broadening of the excitonic component of the near-band-edge luminescence band resulting from an increase in the efficiency of radiative recombination of free excitons due to their interaction with C and Si shallow impurities.

Strain-induced photoreflectance spectra in the vicinity of the E 0 transition in GaAs/Si and InP/Si heterostructures

A study is reported of the structure of photoreflectance (PR) spectra in the vicinity of the E0 transition from thin (d=1–5 µm) n-GaAs and n-InP films (n=1016–1017 cm−3) grown epitaxially on Si(001) substrates. A quantitative analysis of the spectra involving multi-component fitting shows that the electronic optical transition from the {3/2;±1/2} subband provides a dominant contribution to the intermediate-field electromodulation component in both systems. The splitting observed in the GaAS/Si PR spectra near the main peak are accounted for not by the strain-induced valence-band splitting but rather by a spectral superposition of the intermediate-field component due to the {3/2;±1/2} subband with a low-energy excitonic component. The analytically established transition energy E 0 3/2;±1/2 is used to calculate biaxial strains in epitaxial films.

Multiple pathways for ultrafast transduction of light energy in the photosynthetic reaction center of Rhodobacter sphaeroides.

A pathway of electron transfer is described that operates in the wild-type reaction center (RC) of the photosynthetic bacterium Rhodobacter sphaeroides. The pathway does not involve the excited state of the special pair dimer of bacteriochlorophylls (P*), but instead is driven by the excited state of the monomeric bacteriochlorophyll (BA*) present in the active branch of pigments along which electron transfer occurs. Pump-probe experiments were performed at 77 K on membrane-bound RCs by using different excitation wavelengths, to investigate the formation of the charge separated state P+HA-. In experiments in which P or BA was selectively excited at 880 nm or 796 nm, respectively, the formation of P+HA- was associated with similar time constants of 1.5 ps and 1. 7 ps. However, the spectral changes associated with the two time constants are very different. Global analysis of the transient spectra shows that a mixture of P+BA- and P* is formed in parallel from BA* on a subpicosecond time scale. In contrast, excitation of the inactive branch monomeric bacteriochlorophyll (BB) and the high exciton component of P (P+) resulted in electron transfer only after relaxation to P*. The multiple pathways for primary electron transfer in the bacterial RC are discussed with regard to the mechanism of charge separation in the RC of photosystem II from higher plants.

Optical response of excitonic polaritons in photonic crystals

Nonlocal investigations are presented for exciton-photon coupling in photonic crystals consisting of two kinds of alternating slabs (CuCl/NaCl), for which excitons exist only in one (CuCl) of the two slabs. Studies are carried out for several typical combinations of period and slab thickness. The lower branch of the excitonic polariton for this system is found to split into many small bands separated by small band gaps (polariton gaps). This phenomenon is explained as the band splitting caused by the coherent interference of polaritonic waves in periodic systems. At the same time, the group velocity of light is greatly reduced in the presence of the excitons. The present nonlocal study demonstrates a double exciton-photon coupling, in which the upper branch of the polariton couples again with the size-quantized exciton states. A long-wavelength approximation is also presented along with a discussion of its validity for simplifying the nonlocal theory. The absorbance and reflectance spectra computed using the transfer matrices exactly reproduce the above small bands for the same systems. An examination of the coupling scheme among excitons, photons, and the structural periodicity indicates that the former two couple with each other more strongly than the other combinations of them. The exciton component of polariton, which is localized in each slab in darkness, could be construed as being delocalized with the assistance of the photon.

Energy Transfers in the B808–866 Antenna from the Green Bacterium Chloroflexus aurantiacus

Energy transfers within the B808–866 BChl a antenna in chlorosome-membrane complexes from the green photosynthetic bacterium Chloroflexus aurantiacus were studied in two-color pump-probe experiments at room temperature. The steady-state spectroscopy and protein sequence of the B808–866 complex are reminiscent of well-studied LH2 antennas from purple bacteria. B808→B866 energy transfers occur with ∼2 ps kinetics; this is slower by a factor of ∼2 than B800→B850 energy transfers in LH2 complexes from Rhodopseudomonas acidophila or Rhodobacter sphaeroides. Anisotropy studies show no evidence for intra-B808 energy transfers before the B808→B866 step; intra-B866 processes are reflected in 350–550 fs anisotropy decays. Two-color anisotropies under 808 nm excitation suggest the presence of a B808→B866 channel arising either from direct laser excitation of upper B866 exciton components that overlap the B808 absorption band or from excitation of B866 vibronic bands in nontotally symmetric modes.

Identification of the upper exciton component of the B850 bacteriochlorophylls of the LH2 antenna complex, using a B800-free mutant of Rhodobacter sphaeroides.

In this paper, we report the circular dichroism (CD) spectra of two types of LH2-only mutants of Rhodobacter sphaeroides. In the first, only the wild type LH2 is present, while i the second, the B800 binding site of LH2 has been either destabilized or removed. For the first time, we have identified a band in the CD spectrum of LH2, located at approximately 780 nm, that can be ascribed to the high exciton component of the B850 band. The experimental spectra have been modeled by theoretical calculations. On this basis, the average interaction strength between the monomers in the B850 ring can be estimated to be approximately 300 cm-1. In addition, we suggest that in LH2 of Rb. sphaeroides the angles made by the Qy transitions of the B850 BChls with respect to the plane of the ring are slightly different from those calculated from the crystal structure of the Rhodopseudomonas acidophila LH2 complex.

Quantum-size effects on the exciton excited states of CuCl nanocrystals

We have investigated infrared transient absorption at 77 K, which appears under size-selective excitation of I S excitons in CuCl nanocrystals embedded in Nad matrices. The transient absorption is mainly ascribed to the optical transition between the 1S to 2P exciton states in CuCl nanocrystals. The peak energy of the transient absorption band increases by about 200 meV with the decrease in the crystal size from more than 10 nm down to 1.4 nm in effective radii, while the width of the band increases by more than 100 meV. These facts indicate that the quantum size effect on the exciton excited state is much deviated from the exciton confinement regime and the hydrogen-like P exciton components are widely distributed among the various excited confinement states split by the quantum size effect.

Identification of the multicomponent nature of E 0 photoreflectance spectra from moderately doped GaAs substrates

The paper considers methods used to identify the multicomponent character of photoreflectance (PR) spectra in the vicinity of the E0 transition measured at room temperature from moderately doped GaAs substrates. It is shown that that if an E0 PR spectrum contains an excitonic component, its presence can be established from both a mathematical and phase analysis, as well as from measurements performed at different laser excitation densities.

Temperature Dependence of Excitation Transfer in LH2 of Rhodobacter sphaeroides

Using two-color pump−probe femtosecond spectroscopy, the temperature dependence of the energy transfer rate within the peripheral light-harvesting antenna (LH2) of the photosynthetic bacterium Rhodobacter sphaeroides has been measured. The energy transfer time from B800 to B850 is determined to be 0.7, 1.2, and 1.5 ps at 300, 77, and 4.2 K, respectively. These data, combined with earlier results, have been analyzed with regard to the crystal structure and spectroscopic properties of the purple bacterial LH2 complex. We conclude that the transfer within B800 occurs mainly via the incoherent Förster hopping mechanism. For B800 to B850 transfer, estimates based on the Förster formula yield considerably slower transfer times than experimentally observed, suggesting that an additional mechanism may be involved in enhancing the transfer rate. We suggest two possibilities: transfer via the upper excitonic component of B850 band and/or transfer mediated by a carotenoid molecule.

Solvent, Temperature, and Excitonic Effects in the Optical Spectra of Pseudoisocyanine Monomer and J-Aggregates

The absorption and fluorescence spectra of 1,1‘-diethyl-2,2‘-cyanine (pseudoisocyanine, PIC) aggregates have been studied between 8 and 293 K in water/glycerol glass containing 2−4 M of alkali halogenides. In this system the J-aggregates have a single sharp band and there is practically no contamination with the monomeric dye, dimers, or H-aggregates. This allowed us to better resolve the high-energy portion of the spectrum and to assign the middle 535 nm band to the upper exciton transition. The excitonic splitting at 8 K is the same for both the blue and the red forms of aggregates (1270 ± 10 cm-1). The average energy of exciton components (18195 ± 15 cm-1 for the blue form) was found to be very close to the 0−0 energy of the first strong site of PIC monomer (18223 cm-1) embedded in a 9-aza-PIC iodide matrix, which is transparent above 500 nm [Marchetti, A. P.; Scozzafava, M. Chem. Phys. Lett. 1976, 41, 87]. The 0−0 frequency of the nonsolvated PIC monomer cation (ν00 = 19716 ± 40 cm-1 or 507.2 ± 1 nm) was obtained from the solvent shift measurements at room temperature. The absorption bandwidths and shifts of both the PIC cation in poly(methyl methacrylate) matrix and the aggregates were recorded in the temperature range between 8 and 300 K. The thermal shift of band maxima was analysed in terms of the change in dispersive shift and excitonic splitting as a result of the expansion of the matrix and a pure thermal or phonon-induced contribution. The thermal shift and broadening behavior of molecular and excitonic transitions reveals large differences in the mechanism and strength of the coupling to low-frequency vibrations.

Excitonic Interactions between the Reaction Center and Antennae in Purple Photosynthetic Bacteria

The interaction between the light-harvesting antenna (LH1) and the reaction center (RC) in the photosynthetic purple bacteria Rhodobacter sphaeroides R26 and strain 2.4.1 was studied with low-temperature (1.5 K) absorbance-detected magnetic resonance (ADMR) experiments combined with computer simulations. The triplet-minus-singlet (T−S) spectra of chromatophores of Rb. sphaeroides R26 and strain 2.4.1 show an extra positive component at approximately 884 and 887 nm, respectively, and an extra negative component at 897 and 901 nm, respectively, when compared to RCs of Rb. sphaeroides R26 and strain RCO1. Computer simulations using a simple model of the RC encircled by the antenna of LH1 show that the dipole strength and energies of the second-lowest exciton component of the antenna and the RC change when the coupling term between the RC and antenna is included in the Hamiltonian of the system. Thus the difference in the shape of the ADMR-detected T−S spectra of isolated RCs (or strain RCO1 chromatophores) and chromatophores is plausibly caused by a change in the excitonic interaction between the RC and antenna.

Optical spectroscopy of MBE grown [formula omitted] quantum wells at various acceptor doping levels

An experimental study of the optical properties of acceptor-center doped quantum wells (QWs) is presented. We have studied the effects of acceptor doping at concentration levels varying from 10 16 up to 10 19 cm −3 using steady-state photoluminescence (PL) and photoluminescence excitation (PLE). The quenching of excitons has been investigated by comparing the results obtained for n-and p-type bulk and QW structures doped both in the well and in the barrier. At a doping concentration higher than 5 × 10 16 cm −3, an additional excitonic component is observed on the low energy side of the principal neutral acceptor bound exciton (BE). The second feature is associated with BEs formed by excitons bound at interacting acceptors. Additional information is obtained from temperature, excitation intensity dependence and time resolved measurements. The effect of hydrogen passivation of the same samples is also studied.

Carrier-ion exchange interactions in crystals Cd 1− xCo xTe

Magnetoreflectivity investigations of the Cd 1− x Co x Te crystals with x = 0.004 were carried out in the region of 1 s exciton in a magnetic field up to 3.5 T and temperature of 2 K. A giant spin splitting (GSS) of exciton transitions in σ + and σ − polarizations, as well as two π transitions, was observed. A strong asymmetry between the average energies of the σ and π exciton components has been found. The model we propose is based on the expanded carrier-ion exchange Hamiltonian which in addition to the usual Heisenberg-like term with the exchange constant β includes also a biquadratic term with exchange constant β 1. Estimates of the GSS of the exciton components agree with the experimental data if we assign ( x′ x )N 0β = −1.8 eV and β 1 β = 0.26 ; x′ is the active content of magnetic ion and N 0 is the cation concentration. Previous experimental data obtained in different laboratories can also be described by this model with the same β 1 β ratio. The nature of the biquadratic term as well as the role of spinorbital interaction in its generation are discussed.

Band alignment in ZnSe/Zn1−x−yCdxMnySe quantum-well structures

We present a magneto-optical study of ZnSe/Zn{sub 1{minus}x{minus}y}Cd{sub x}Mn{sub y}Se quantum-well structures in which a suitable choice of the Cd composition leads to a system that is type I at zero magnetic field. When a magnetic field is applied perpendicular to the layers of the structure, the band edges split in such a way as to make the upper {bold {sigma}}{sub {minus}} (1/2, t 3/2) exciton transition type II, while the ground state {bold {sigma}}{sub +} ({minus}1/2, {minus}3/2) exciton component remains type I at all field values. This alignment reduces the probability for carrier relaxation from the higher-energy exciton component and opens the possibility of hole-spin population inversion via optical pumping. {copyright} {ital 1997} {ital The American Physical Society}