Excitonic Component Research Articles

Relaxation kinetics for temperature and degeneracy of microcavity polaritons

We apply a semi-classical Boltzmann kinetics for a gas of laser-pulse excited microcavity polaritons taking into account their mutual interaction and their interaction with acoustic phonons. Fitting the temporally evolving polariton distribution above the ground state with a Bose–Einstein distribution, we find the evolution of the temperature and the degeneracy parameter, i.e. the ratio of the chemical potential to the thermal energy. Studying the relaxation in particular for GaAs microcavities we compare our results with recent measurements by Deng et al. In agreement with the experiment we find that the lattice temperature can be reached and that the degeneracy of the condensed gas holds up to 60–80 ps provided a detuning of the cavity mode is applied which increases the exciton component of the lower-branch polaritons and thus their scattering rates.

Polariton spin beats in semiconductor quantum well microcavities

Abstract Time-resolved Kerr (Faraday) rotation experiments allow for the observation of polariton spin beats in both InGaAs and CdMnTe quantum well (QW) microcavities. The existence of these beats is an unambiguous manifestation of the coherent energy exchange between exciton and photon components of polariton states created by a circularly polarized and spectrally wide femtosecond laser pulse. The polariton states are also shown to be split into a linearly polarized doublet. This splitting is responsible for the polarization transfer between linearly and circularly polarized states. In a highest-quality sample, the resulting spin dynamics could be detected.

Magnetophotoluminescence study of GaxIn1−xP quantum wells with CuPt-type long-range ordering

Magnetophotoluminescence (PL) measurements are performed on GaxIn1−xP∕AlGaInP quantum wells with CuPt-type long-range ordering. Curve-fitting and second-order derivative operations are conducted, respectively, on the PL spectra, and two different PL components are identified with an energetic separation of about 5∼25meV. Obvious diamagnetic shift can be observed only for the PL component with higher energy, indicating the excitonic feature of the related PL transition. The in-(001)-plane exciton reduced effective mass and binding energy are evaluated based solely on the excitonic PL component, by a quasi-two-dimensional exciton effective mass approximation. The results suggest that the ordering competes with compressive strain, and enlarges the exciton reduced effective mass and binding energy. The enlargement is discussed and a comparison with previous reports is given.

The radiative recombination coefficient and the internal quantum yield of electroluminescence in silicon

The results of the analysis of variations in the radiative recombination coefficient with varying doping level and concentration of excess electron-hole pairs are reported. It is shown that, along with the effect of narrowing of the band gap calculated in the many-electron approximation, the effect of screening of the Coulomb interaction responsible for the decrease in the excition binding energy should be taken into account. Both effects produce similar trends and decrease the radiative recombination coefficient with increasing levels of doping or injection. The contributions of excitonic radiative recombination and band-to-band radiative recombination to the total radiative recombination coefficient are separated from each other. It is shown that, in the region of room temperature, both contributions are comparable, while at liquid-nitrogen temperature, the excitonic component dominates over the band-to-band component. The results obtained by refined calculations of the limiting value of the internal quantum yield of electroluminescence for the silicon diodes and p-i-n structures are presented. It is shown that the internal quantum yield of electroluminescence can be as high as 14%. However, this values sharply decreases with increasing surface recombination rate and decreasing lifetime of excess charge carriers in the bulk.

Dynamics of the Emission Spectrum of a Single LH2 Complex: Interplay of Slow and Fast Nuclear Motions

We have studied the relationship between the realizations of static disorder and the emission spectra observed for a single LH2 complex. We show that the experimentally observed spectral fluctuations reflect realizations of the disorder in the B850 ring associated with different degrees of exciton delocalization and different effective coupling of the excitons to phonon modes. The main spectral features cannot be explained using models with correlated disorder associated with elliptical deformations of the ring. A quantitative explanation of the measured single-molecule spectra is obtained using the modified Redfield theory and a model of the B850 ring with uncorrelated disorder of the site energies. The positions and spectral shapes of the main exciton components in this model are determined by the disorder-induced shift of exciton eigenvalues in combination with phonon-induced effects (i.e., reorganization shift and broadening, that increase in proportion to the inverse delocalization length of the exciton state). Being dependent on the realization of the disorder, these factors produce different forms of the emission profile. In addition, the different degree of delocalization and effective couplings to phonons determines a different type of excitation dynamics for each of these realizations. We demonstrate that experimentally observed quasistable conformational states are characterized by excitation energy transfer regimes varying from a coherent wavelike motion of a delocalized exciton (with a 100-fs pass over half of the ring) to a hopping-type motion of the wavepacket (with a 350-fs jump between separated groups of 3–4 molecules) and self-trapped excitations that do not move from their localization site.

Highly efficient doping of boron into high-quality homoepitaxial diamond films

High-quality boron-doped homoepitaxial diamond (100) films were grown using high-power microwave plasma chemical vapor deposition (MPCVD). Results showed that the incorporation efficiency of boron from gas phase was increased by two orders of magnitude compared to those of conventional growth conditions using high-density plasma. Boron-doped crystal thus grown with the low B / C ratio of 50 ppm in the gas phase had reasonably low resistivity of 2 Ω cm at 290 K, whereas Hall mobility of 830 cm 2 V s − 1 at this temperature was higher than those reported previous for such resistivity. The highly boron-doped film showed strong free exciton recombination emissions with a bound exciton component in the cathodoluminescence spectra taken at room temperature, reflecting the considerably high density of the substitutional boron in the film despite its low density of electronic defects. The diamond growth rate in the high-power MPCVD was 3.5 μm h − 1 . The highest room-temperature Hall mobility achieved in this study was 1620 cm 2 V s − 1 . These results indicate that the resultant high-rate growth with high-power MPCVD is advantageous for depositing high-quality and conductive diamond films.

Acoustic and optical phonon scattering of the1Syellow orthoexciton inCu2O

Resonant Raman measurements have been performed on the $1S$ yellow orthoexciton in ${\mathrm{Cu}}_{2}\mathrm{O}$ in high magnetic field. At zero field, the ${\ensuremath{\Gamma}}_{3}^{\ensuremath{-}}$, ${\ensuremath{\Gamma}}_{4}^{\ensuremath{-}}$, and ${\ensuremath{\Gamma}}_{5}^{\ensuremath{-}}$ phonon replicas are visible. In magnetic field we observe a characteristic fine structure in the optical phonon replicas. It is explained by longitudinal and transversal acoustic phonon scattering of orthoexcitons between different Zeeman components. We derive in detail the polarization dependence of the phonon sidebands for these exciton components. For the dipole-allowed ${\ensuremath{\Gamma}}_{4}^{\ensuremath{-}}$ phonon we take the splitting into longitudinal and transverse phonons into account. There is good agreement with the experimental results confirming the importance of a $\stackrel{P\vec}{k}$-dependent exciton mass for understanding relaxation processes.

Photophysics of an electrophosphorescent platinum (II) porphyrin in solid films

We examine electronic processes in platinum (II) octaethyl porphyrin (PtOEP) embedded in an organic solid state matrix and in the form of vacuum-evaporated neat films in conjunction with potential applications of this compound to organic photovoltaic and electrophosphorescent devices. Absorption, photoexcitation, and luminescence spectra indicate the excitonic dimers to be dominant excited states, and their dissociation underlies the charge photogeneration process. Different charge separation distance (1.5 nm and 2.6 nm) in opposite charge carrier pairs preceding dissociation can be distinguished based on the fit of the three-dimensional Onsager theory of geminate recombination to electromodulated luminescence and photoconduction measurements. The near-positive electrode concentrated triplet dimer excitons, produced by strongly 370 nm absorbed light in neat PtOEP films, are efficiently quenched by electron transfer to the metal (Al), generating the positive charge with an efficiency eta+ exceeding 0.15 at high electric fields and dominating the measured photocurrent. Their dissociation efficiency in the bulk, eta- (negatively biased illuminated electrode), is more than one order of magnitude lower than eta+. The dissociation of singlet dimer states dominates the bulk photogeneration process induced by the weakly-absorbed light at 450 nm, with comparable eta+ and eta-. The "hot excited state" underlying the temperature-increasing emission at 540 nm has been attributed to the upper excitonic component Q+ of the first absorption band Q consistent with the exciton concept applied successfully to the interpretation of all dimer-underlain spectroscopic features of PtOEP samples studied.

Low-temperature spectroscopy of fully active PSII cores. Comparisons with CP43, CP47, D1/D2/cyt b 559 fragments

Comparisons of absorption spectra of photosystem II (PSII) core complexes with those of isolated CP43, CP47 and D1/D2/cyt b 559 complexes show broadenings and shifts upon disassembly of the PSII core material. Spectra of PSII cores isolated from plants and cyanobacteria reveal marked changes in energies and intensities of the sharp features associated with P680. Low-temperature, illumination-induced electrochromic shifts in PSII cores allow identification of an excitation localized on pheopytin- a (pheo a) in D1. A weak interaction between an exciton component of P680 and the D1 pheo a, both located near 684 nm, is suggested. MCD spectra of 5- and 6-chlorophyll a D1/D2/cytochrome b 559 preparations provide links to photoactive pigments in intact PSII cores.

Coherent spin dynamics of polaritons in semiconductor microcavities

The influence of the strong light-matter coupling in semiconductor III–V microcavities on polariton spin dynamics is studied in the spontaneous emission regime. Under resonant pulsed excitation, we observe a quenching of spin and alignment relaxation when the polariton is photon-like. These behaviours are attributed to the very small value of the long-range electron–hole exchange term of Coulomb interaction within the excitonic component of the quasi-particle and to the weakness of polariton–polariton Coulomb scattering via the inter-exciton short-range exchange interaction. We investigate then the polariton coherent spin dynamics. Coherence decay occurs in two steps: at short time delay, the polariton keeps the phase memory of the excitation pulse. We show that it is thus possible to manipulate the polariton spin and alignment within the optical dephasing time T2 using the temporal coherent control technique. In a second step (t > T2), optical coherence vanishes, but spin coherences survive and spin quantum beats experiments can be performed within the spin relaxation time under transverse magnetic field. We observe an electron–hole spin correlation within the excitonic component of the quasi-particle under resonant excitation; an increase of the absolute value of the electron effective transverse Landé g-factor with the excitonic character of polaritons is evidenced, which is the consequence of the strong coupling regime. The main experimental features are well accounted by a model taking into account only two classes of excitations in the lower polariton branch.

Nonlinear emission due to electron-polariton scattering in a semiconductor microcavity

Electron-polariton scattering is studied by photoluminescence spectroscopy in a GaAs/AlGaAs microcavity with an embedded quantum well containing a variable density electron gas. The photoluminescence spectra are measured as a function of two parameters: (a) The detuning energy between the cavity-confined photon and the heavy exciton and (b) the intensity of the laser that generates the electron gas. The integrated photoluminescence intensity varies nonlinearly with increasing electron density, showing a large enhancement that reaches a factor of 35 over the intensity observed without an electron gas. The spectra are analyzed in the strong coupling regime, by calculating the energy and electron gas density dependence of the electron-polariton scattering rates. The most effective scattering process is the dissociation of the charged exciton component in the polariton states.

Coherent spin states and spin quantum beats in semiconductor microcavities

The present study evidences the influence of the strong light-matter coupling in semiconductor III–V microcavities on polariton spin dynamics at low excitation. Under resonant excitation, we observe a quenching of spin and alignment relaxation when the polariton is photon like. These behaviours are attributed, respectively, to the very small value of the long-range electron–hole exchange term of Coulomb interaction within the excitonic component of the quasi-particle and to the weakness of polariton–polariton Coulomb scattering via the inter-exciton short-range exchange interaction. Spin dynamics are also investigated under transverse magnetic field. We observe an electron–hole spin correlation within the excitonic component of the quasi-particle under resonant excitation, and an increase of the absolute value of the electron effective transverse Landé g-factor with the excitonic character of polaritons, which can be derived from a model taking into account only two classes of excitations in the lower polariton branch.

Spin Quantum Beats in Semiconductor Microcavities in the Strong Coupling Regime

We have observed polariton spin quantum beats under transverse magnetic field and circular polarization excitation (Voigt configuration) by time-resolved photoluminescence spectroscopy. The temporal behaviour of the beats strongly differs depending on whether the excitation is resonant or not with the lower polariton branch. As in bare quantum wells, we show that this is the consequence of the hole spin instability under non-resonant excitation. In both cases, we evidence a dependence on the cavity detuning δ of the electron effective Landé g-factor (geff). This is attributed to strong coupling regime, where the bare electron Landé g-factor is depends on the polariton exciton component. All these characteristics are well described by a simplified model, which takes into account the transfer dynamics between two classes of polaritons.

Circular and Magnetic Circular Dichroism Studies of BacteriochlorophyllcAggregates: T-Shaped and Antiparallel Dimers

Circular dichroism (CD) and magnetic circular dichroism (MCD) spectra have been measured on (31R)-bacteriochlorophyll (BChl) c aggregates in organic solvents. BChl c exists as a monomer in acetone with a Qy(0−0) transition at 661 nm. In a mixed solvent system of methanol and dichloromethane (CH3OH/CH2Cl2 = 1/4000), another Qy(0−0) transition appeared at 680 nm with a nondispersion-type CD signal. The ratio of MCD intensity to its dipole strength (B/D) for the Qy(0−0) transition is half the value of its monomer species, indicating a dimerization of BChl c. The apparent inconsistency between the CD and MCD results has been reconciled by assuming the formation of BChl c dimer with a T-shaped conformation. In neat CCl4, a dispersion-type CD signal was observed for the Qy(0−0) exciton components at 680 and 710 nm. MCD spectra of the exciton-type Qy(0−0) components suggest that two BChl c molecules are parallel with their macrocycle planes as judged by the mixing effect between the excited states of Qx(0−0) and...

Structural Asymmetry of Bacterial Reaction Centers: A Qy Resonant Raman Study of the Monomer Bacteriochlorophylls

To distinguish contributions from either of the monomer bacteriochlorophyll cofactors BL and BM, we have recorded Raman spectra of reaction centers (RCs) from Rhodobacter sphaeroides, strain R26.1 at low temperature and at resonance with the Qy electronic band at 800 nm. Spectra excited from ferricyanide-treated RCs at 800 nm involved a single BChl species, that we identify with the BL cofactor. Spectra excited at 810 nm in the same conditions involved participation from a second, additional cofactor, that we identify with BM. The present, selective RR data on BL fully confirm an earlier interpretation of difference, Soret resonant Raman spectra (Robert, B.; Lutz, M. Biochemistry 1988, 27, 5108−5114), according to which a H-bond engaged with water by the keto group of BL should significantly strengthen upon formation of the P•+ radical state of the primary donor. The present data also indicate that the equivalent H-bond engaged by the keto group of BM should strengthen as well, however by about half the enthalpy change of the L-side bond. The absolute wavenumbers, and compared P•+-induced shifts of the stretching modes of the acetyl CO groups of BL and BM are interpreted as indicating (i) that the acetyl carbonyl of BL is located in a higher permittivity environment than that of BM and (ii) that the frequency of the acetyl CO vibrator of BM may accordingly experience a stronger electric field effect from the P•+ radical state than that of BL. A set of differences observed between BL and BM spectra specifically concerns their Mg-sensitive modes. These differences are not resulting from a differential field effect, and indicate a sizable difference of conformations between the MgN4Nε2(His) groups of BL and BM. The macroring core sizes of both BL and BM, however, are close to the size of relaxed, five-coordinated BChl a in solution. Finally, comparisons of RR spectra excited at 790, 800, and 810 nm from RCs chemically poised in the P° or P•+ states revealed a sizable contribution from the neutral state of the primary donor in RR spectra excited at 810 nm only. This confirms that in electronic spectra of reduced RCs the 810 nm shoulder should arise in part from the upper excitonic component, Py+, of the primary donor.

Quantifying electrical spin injection: Component-resolved electroluminescence from spin-polarized light-emitting diodes

The spin-polarized light-emitting diode (spin-LED) is a very effective tool for accurately quantifying electrical spin injection in a model independent manner. We resolve and identify various components which occur in the electroluminescence (EL) spectra of GaAs quantum-well-based spin-LEDs, and examine the circular polarization of each. While a number of components exhibit significant circular polarization, the values do not necessarily reflect the electrical spin injection efficiency. We show that a reliable measure of spin injection efficiency can be obtained only if one takes care to spectroscopically resolve and accurately identify the free exciton or free carrier components of the EL spectrum, and exclude other components.

Bipolariton coupling in biexciton optical decay: Degenerate and nondegenerate polariton emissions in CuCl

Radiative decay of biexcitons wave-vector $(\mathbf{K})$ selectively generated in a low-$K$ region $(0<~|\mathbf{K}|\ensuremath{\lesssim}{2k}_{0})$ is studied in CuCl, where ${2k}_{0}$ is the biexciton wave number of degenerate two-photon excitation, and the bipolariton nature of low-$K$ biexcitons is experimentally demonstrated. All the radiative relaxation channels of low-$K$ biexcitons are clarified, which dissociate into two lower-branch polaritons and into upper- and lower-branch polaritons yielding nearly degenerate and nondegenerate pair emission lines, respectively. The three-dimensional scattering geometry allows for simultaneous observation of both degenerate and nondegenerate lines. Nearly degenerate two-polariton emissions have larger intensities than nondegenerate emissions. Completely degenerate emissions from $\mathbf{K}=2{\mathbf{k}}_{0}$ biexcitons have the largest intensity, reflecting singularity of the joint density of polariton states. These features are in remarkable contrast with the resonant two-photon absorption process of biexcitons, which shows resonance enhancement not at the degenerate two-photon energy but at the exciton resonance. In spite of the larger transition probability, nearly degenerate emissions have a negligible contribution to the emission band arising from thermally distributed biexcitons. The dispersion curve of the ${Z}_{1,2}{(Z}_{3})$-exciton lower(upper)-branch polariton is determined from the K dependence of the emission energies. The observed emission lines are all unambiguously assigned and the relative intensities of the simultaneously observed lines are compared with a biexciton radiative decay theory based on the bipolariton model. The energy dependence of the transition probability of the observed biexciton radiative decay channels is quantitatively reproduced by the theory, verifying that the biexciton radiative decay rate is determined by the exciton components in final-state polaritons (Hopfield coefficients) through the exciton-exciton Coulomb interaction. The radiative lifetimes of $K\ensuremath{\simeq}0$ biexcitons are found to be shorter than those of $K\ensuremath{\simeq}{2k}_{0}$ biexcitons as expected from the theory. The lifetimes of low-$K$ biexcitons are strongly affected by the coupling with final two-polariton states, leading to a 20% reduction of the lifetimes compared with the case of no polariton effects.

Hopfield coefficients measured by inverse polariton series

We report observation of the inverse polariton series and a detailed experimental study of the exciton components in polaritons (Hopfield coefficients). Spontaneous emission of excitonic molecules into outgoing polaritons associated with the $i(=1,2,3,4,5)\mathrm{th}$ exciton states in bulk CuCl is detected and analyzed by the bipolariton model. Because the intensities of the emission lines, which form the inverse polariton series, are determined by the exciton components in the final-state polaritons, we are able to measure the Hopfield coefficients for the highly composite outgoing polaritons. Quantum interference in the optical decay of excitonic molecules, due to the multiple exciton components in the polariton and biexciton states, is also demonstrated.

Alkaline denaturation of the light-harvesting complex II from the purple bacterium Ectothiorhodospira sp.: kinetic evidence of the existence of the 780 nm upper exciton component of the B850 bacteriochlorophylls.

The light-harvesting complex II of the purple bacteria has two strong near-infrared electronic absorption bands around 800 (B800) and 850 (B850) nm, arising from the Q(y)() transitions of the bacteriochlorophyll a. In the present work, high concentrations of NaOH were used to study the destabilization of the complex of the Ectothiorhodospira sp. The majority of the bacteriochlorophylls were monomerized within 90 min of treatment. However, the kinetic patterns of the two near-infrared bands were remarkably different. After an instantaneous blue shift from 853 to 828 nm, B850 showed a first-order monomerization with a rate constant of -0.016 min(-1). This instantaneous blue shift was previously attributed to the deprotonation of a lysine and was independent of the monomerization process. The observed native B800 is in fact composed of two bands, one at 796 nm and the other at 780 nm. The band absorbing at 780 nm red shifted also instantaneously to 786-788 nm and then disappeared in a first-order process as B850. The other band absorbing at 796 nm has a two-step process of monomerization; after a rapid conversion a slower first-order process occurred with a rate constant of -0.025 min(-1). The similarity between the kinetic behaviors of B850 and the 780 nm band indicated a strong relationship between these two bands. Our interpretation of the results considers the 780 nm band as the upper exciton component of the B850 bacteriochlorophylls.

Influence of heat treatment on luminescence of semi-insulating undoped GaAs crystals

Variations in the spectra of edge-emission photoluminescence of semi-insulating undoped GaAs crystals as a result of heat treatment for 20–90 min at 900°C were studied. It is shown that heat treatment substantially affects (transforms) the excitonic component of the spectrum. The observed changes in the spectra are related to variations in the impurity composition of the crystals studied. These variations are shown to be caused by heat treatment.