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We have experimentally observed the spin polarization process of single electrons in InP/InGaP quantum dots by time-resolved Kerr rotation measurements. It is found that the inversion of the spin polarization direction occurs with the variation of the intensity of the optical pulse. The spin coherence lifetime abruptly changes on the occurrence of the inversion. We have reproduced the inversion in numerical simulations using the density operator of the electron–trion four-level system, assuming a small deviation of the optical pulse from circular polarization. The change of the spin lifetime is attributed to the qualitative change of the four-level system in the electric polarization state.

The indistinguishability of photons emitted from a nitrogen luminescence center in GaAs is investigated by two-photon interference under nonresonant optical excitation. A clear dip is observed in a parallel polarization configuration for consecutively emitted two photons with a 2-ns time interval. The indistinguishability is approximately 0.24, and is found to be independent of the time interval between 2 ns and 4 ns. These results suggest the existence of a very fast dephasing mechanism within 2 ns.

TitleCarrier relaxation dynamics in InP quantum dots studied byartificial control of nonradiative lossesAuthor(s)Ignatiev, Ivan V.; Kozin, Igor E.; Nair, Selvakumar V.; Ren,Hong-Wen; Sugou, Shigeo; Masumoto, YasuakiCitation Physical review B, 61(23): 15633-15636Issue Date 2000-06Text version publisherURL http://hdl.handle.net/2241/98298DOI 10.1103/PhysRevB.61.15633Right ©2000 The American Physical Society

An intense anti-Stokes photoluminescence is observed in a structure with InP quantum dots in the presence of a direct electric current and cw optical pumping below the lowest electron hole transition in the quantum dots. A simple model is proposed for the explanation of the observed phenomenon. Its essential point is the existence of deep level defects around the QDs.

ERATO Single Quantum Dot Project, Japan Science and Technology Corporation, 5-9-9 Tokodai, Tsukuba 300-2635, Japanand Institute of Physics, University of Tsukuba, Tsukuba 305-8571, Japan~Received 29 December 1998!We report the observation of inverse exciton series (M-emission series! in the optical decay of an excitonicmolecule to the exciton excited states (n52s,3s, and 4s). A theory is developed which relates the inverseexciton series to the molecule wave function using the bipolariton model. The components of the excitonexcited states n52, 3, and 4 in the biexciton wave function in CuCl are determined from the relative intensitiesof M

Strong optical anisotropy is observed in the photoluminescence (PL) bands of both the InP self-assembled quantum dots and the ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}$ matrix. From the linearly polarized PL spectra measured under weak excitation, we found that large size quantum dots show strong anisotropy. The luminescence from a single quantum dot observed by the micro-PL technique revealed a doublet fine structure of the exciton levels that obey the linear polarization selection rule. The observed fine structure is shown to arise from an interplay of the electron-hole exchange interaction and the asymmetric crystal structure of the ${\mathrm{I}\mathrm{n}\mathrm{P}/\mathrm{G}\mathrm{a}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}$ system.

A coherent emission of excitons in semiconductor multiple quantum wells was investigated by using transient Brewster reflection spectroscopy. The method reduced nonresonant background reflection greatly and allowed us to observe the coherent dynamics of the excitons clearly. The coherent signal showed not only the quantum beats of the heavy- and light-hole excitons but also the oscillatory modulation structures that resulted from the radiative coupling of exciton polaritons. In the linear regime, these observations were compared with results calculated from the transfer-matrix method involving multiple-reflection effects, and good agreement was obtained. As the excitation density of the incident pulses was raised, the decay rate of the coherent signal increased due to the exciton-exciton scattering.

application/pdf, Biexciton and triexciton states in CuCl quantum dots were studied by means of time-resolved size-selective pump-and-probe technique. A clear induced absorption band is observed on the high-energy side of the excitation photon energy. The new induced absorption is assigned to the transition from the exciton ground state to one of the weakly correlated exciton pair states which were theoretically predicted to exist and to play an important role in nonlinear optical processes. Its pump energy dependence and temporal evolution strongly support this assignment. Under high-density or two-color excitation condition, a triexciton state in quantum dots is observed for the first time.

Resonant hyper-Raman-scattering spectroscopy was applied for studies of the exciton\char21{}LO-phonon interaction in spherical CuCl nanocrystals of different sizes embedded in a glass matrix. Both the LO and 2LO phonon bands have shown a prominent resonance with the lowest energy $1S$ confined exciton state. The ratio of the integral intensities of the LO and 2LO bands \ensuremath{\rho} was found to increase with increase of the incident photon energy, or decrease of the nanocrystal radius. The Huang-Rhys factor $S$ has been calculated as a function of the nanocrystal size from the experimentally measured values of \ensuremath{\rho} on the offset harmonic-oscillator model of electron-vibrational coupling. It has been found that $S$ increases monotonically from 0.22 to 0.7, with the nanocrystal radius decreasing from 3.6 to 1.6 nm.

We analyze luminescence spectra resulting from a resonant size-selective two-photon excitation of the low-energy confined ${\mathrm{Z}}_{3}$-exciton states in CuCl spherical nanocrystals. Excitation spectra of the two-photon-excited luminescence (TPL) bands and size dependences of band energies show that the TPL bands arise due to annihilation of the exciton in the lowest-energy state excited both directly and through the higher-energy confined states. It is shown that the oserved features of the TPL fine structure can be unambiguously interpreted in terms of longitudinal (L) and transverse (T) confined exciton states and resonant enhancement of phonon-assisted luminescence when the energy gap between 1S and 1P states of the T exciton is close to the LO-phonon energy.

Dynamics of excitons and excitonic molecules in CuCl nanocrystals were investigated by means of both picosecond time-resolved luminescence and femtosecond transient absorption. Stochastic as well as rate-equation treatments are made to explain the temporal changes of both luminescence and absorption. In the stochastic treatment, a Monte Carlo simulation is made for ${10}^{5}$ nanocrystals with the initial condition that the exciton number distribution follows the Poisson distribution. The stochastic treatment satisfactorily explains all the experimental results, while the rate-equation treatment does not. Success of the stochastic treatment shows the unique property of the dynamics of excitons and excitonic molecules in quantum dots arising from discreteness of the exciton number. \textcopyright{} 1996 The American Physical Society.

A quantum size effect of nanometer-size CuI microcrystallites (nanocrystals) embedded in glass was investigated and was found to follow the strong-confinement model. Persistent spectral hole burning and thermally annealing hole filling phenomena were observed in samples (CuI nanocrystals in glass). Unusual luminescence behavior (luminescence elongation followed by the increase of light exposure) was also observed. These observed phenomena are explained by the photoionization model of nanocrystals.

Persistent spectral hole-burning phenomenon in CuBr quantum dots embedded in glass was observed. The spectral hole and the sideband holes were made together with the redshift of the absorption spectrum in proportion to the logarithm of the laser fluence and were preserved for more than 8 h at 2 K. They were erasable by the temperature rise or by the light exposure. Electron trapping at the surface of the dots and hole tunneling into traps in the glass host is the most probable origin of the persistent spectral hole-burning phenomenon in CuBr quantum dots.

application/pdf, We have observed a coherent oscillation of zone-folded acoustic phonons in GaAs-AlAs superlattices for the first time by means of a femtosecond time-resolved pump-probe technique. The oscillatory component in the time-resolved reflection signal corresponds to the upper branch (B2 symmetry) of the first-order doublet mode. Carriers were selectively excited in well layers and the B2-symmetry phonon mode was selectively generated. The dephasing time of the B2 mode was measured to be 70 ps from our experiments.

Population dynamics of excitons and biexcitons in nanometer-sized CuCl microcrystallites (nanocrystals) was studied with a time-resolved luminescence measurement. A recurring process between excitons and biexcitons was clearly observed as a function of the second decay component of the biexciton luminescence and with an increase of the exciton luminescence. They become prominent with an increase in the excitation intensity. The biexciton luminescence is broadened asymmetrically to higher energy with an increase in the biexciton density. These temporal as well as spectral characteristics of biexciton luminescence are inherent to CuCl nanocrystals but are not observed in a CuCl bulk crystal, which shows that biexcitons are much more efficiently formed from excitons in a CuCl nanocrystal than in a CuCl bulk crystal.

application/pdf, We have investigated optical nonlinearity in beryllium-doped low-temperature (LT) molecular-beam-epitaxy-grown GaAs/AlAs multiple quantum wells (MQWs). The response time of the nonlinearity is reduced by Be doping in the MQW. While the undoped LT MQW shows a 0.7–0.9 ps response, the response time of the Be-doped LT MQW is as short as 0.25 ps. The saturation density of the Be-doped MQW is almost the same as that of the undoped MQW, and is smaller than that of bulk GaAs. These results demonstrate that the Be-doped LT MQW exhibits a faster response than the undoped LT MQW, and a faster response as well as larger nonlinearity than LT bulk GaAs.

The lifetime of indirect excitons in AgBr quantum dots was studied at low temperature. Indirect exciton luminescence shows a nonexponential decay. The unique nonexponential decay is well explained by a model that takes into account the discrete number of iodine impurities in a AgBr quantum dot. The obtained radiative lifetime of indirect excitons is 735±50 μs at 2 K and is independent of the radius of microcrystals. The result clearly shows that the L-Γ mixing effect for holes does not work when the radius is larger than 3.5 nm

application/pdf, We measured the size dependence of the energy relaxation time of PbSe quantum dots at room temperature. The radius of the dots was evaluated by small-angle x-ray scattering. We found a monotonic decrease of the relaxation time from 25 to 1 ps with a decrease of the radius r from 2.9 to 1.4 nm. The 1/r3 dependence of the relaxation rate was indicated.

application/pdf, We report an electroluminescent device fabricated by a europium silicate layer on a silicon substrate. The device exhibits uniform-intense white color electroluminescence with an external quantum efficiency about 0.1% at room temperature, a low operating threshold voltage (about 6 V) and a fast response to the modulation signal at the frequency of 1 MHz.

Microdisk and microring lasers of the organic semiconductor material thiophene/phenylene co-oligomer (TPCO) have been fabricated on Si/SiO2 substrates. Laser emission with the whispering gallery modes (WGM) is observed in 0-1 vibration transitions. Comparing with the bulk film parts, a few times reduction of the lasing threshold has been observed in microdisks. Microring lasing threshold is reduced further when the width of the waveguide is less than 1 μm. WGM with waveguide confinement is supposed to contribute to the decrease of the threshold.

Effects of both the dynamic nuclear polarization (DNP) created by circularly polarized light and the fluctuations of average nuclear spin in a quantum dot (QD) on the electron spin orientation are studied for singly negatively charged InP QDs. From the dependence of the negative circular polarization of photoluminescence on the applied longitudinal magnetic field, the hyperfine field B_N of a few mT appearing due to DNP and the effective magnetic field B_f of a few tens of mT arising from nuclear spin fluctuations (NSF) are estimated. A lifetime of about 1 microsecond is estimated for NSF., 3 pages, 2 figures, presented in ICPS-2006

PUBLISHED, The evolution of the optical phonon spectra of colloidal core/shell CdSe/ZnS quantum dots with an increase of the shell thickness from 0.5 to 3.4 monolayers has been studied by resonant Raman spectroscopy. The results obtained suggest that the ZnS shell changes its structure from amorphous to partly crystalline as the thickness increases. Simultaneously, an increase in Raman scattering by surface ~core/shell interface! phonons and the redshift of the corresponding phonon band have been observed and assigned to variations in the shell structure. The disorder present in the shell provides a major contribution to the line shape of the Raman spectra at higher ZnS coverage. A method to control the quality of quantum dots based on Raman spectroscopy is proposed., Enterprise Ireland Science Foundation Ireland

application/pdf, Lasing of CuCl microcrystals embedded in a NaCl single crystal was observed for the first time. The lasing takes place at 77 K in a sample sandwiched between dielectric mirrors under pulsed ultraviolet laser excitation. The lasing transition is that from biexciton to exciton. The lasing is observed up to 108 K. The optical gain of the sample is almost the same as that of a CuCl bulk crystal in spite of the low concentration of CuCl in the NaCl matrix.

application/pdf, Dynamics of hopping charge transport in a polymer matrix doped with two different charge-transport molecules was studied by means of time-of-flight (TOF) photoconductivity measurements. In polymers doped with two molecules of little different ionization potential, the Gaussian (near-rectangular) TOF signal was observed over all compositions and the tail of the Gaussian TOF signal is broadened by a wide distribution of hopping time among molecules due to off-diagonal disorder. On the other hand, in polymers doped with two molecules of different ionization potential, the transition from the Gaussian to the dispersive TOF signal was observed and this is mainly caused by diagonal disorder. It is found that the energetic and spatial distribution of hopping sites play a key role in carrier transport in molecularly doped polymers.

application/pdf, Optical transmission spectra of CuCl microcrystals embedded in NaCl crystals were studied around the Z3 exciton resonance by varying the annealing time. With the increase in the annealing time, extinction spectra of the Z3 exciton change anomalously from the absorption-type spectrum to the emission-type spectrum via the dispersion-type spectrum. The Mie theory [Ann. Phys. 25, 377 (1908)] successfully explains the spectral change due to the growth of microcrystals. We report here for the first time an anomalous change of the exciton spectra due to the growth of semiconductor microcrystals.

application/pdf, The photocarrier generation mechanism in metal-free phthalocyanines (H2Pc) was studied by time-of-flight photoconductivity measurements. The quantum efficiencies of photogeneration of holes depend on the crystal structure of H2Pc. The photogeneration efficiencies in X- and tau-H2Pc are high compared with those of alpha- and beta-H2Pc. The lifetimes of the precursor state of free carriers in X- and tau-H2Pc are between those of alpha-H2Pc and beta-H2Pc. Moreover, infrared absorption and Raman spectra show that X- and tau-H2Pc are intermediate states between alpha-H2Pc and beta-H2Pc. It is pointed out that the photogeneration dynamics of H2Pc is controlled by the intermolecular overlap of the pi-electron wavefunctions and the stacking habit of molecules.

application/pdf, We have measured photoluminescence (PL) spectra and time-resolved PL in CdTe quantum dots (QDs) under the longitudinal magnetic field up to 10T. Circular polarization of PL increases with increasing magnetic field, while its linear polarization is absent under linearly polarized excitation. Time-resolved PL measurements clarified that this behavior is caused by the suppression of spin relaxation induced by the longitudinal magnetic field. We believe that this behavior is related to the hyperfine interaction of electron spin with magnetic momenta of lattice nuclei.

application/pdf, We observed a light-hole-to-electron transition of strain-induced quantum dots (SIQDs) in a strongly strained GaAs quantum well (QW). Circularly-polarized luminescence excitation spectroscopy showed a pronounced counter-circularly polarized component below the heavy-hole exciton transition in the QW. This special feature can be explained by the existence of the light-hole-to-electron transition at this energy level. On the other hand, a very high co-circular polarization (=0.8) was obtained near the lowest transition of SIQD.

application/pdf, Periodic oscillation is observed in transient four-wave mixing signal in strain-induced GaAs quantum dots. The oscillation is assigned to biexcitonic beat based on its polarization dependence and its initial phase, and the biexciton binding energy is deduced. Since strain-induced dots are produced by adding lateral confinement to a quantum well, direct comparison of the biexciton binding energy in 2D and in 0D system is possible. The beat period in quantum dots is found to be shorter than that in quantum well, and the enhancement factor by the reduced dimensionality is 1.5. Magnetic field dependence of the biexciton binding energy is also studied. It is independent of the magnetic field up to 8 T, and exciton g-factor in the strain-induced dots is obtained.

application/pdf, Excitonic lasing from single ZnO nanobelts was observed at room temperature, which was due to the exciton-exciton scattering processes appearing under intense light excitation. Morphologies of the nanobelts are rectangular shapes, so that crystalline facets of the nanobelts acted as the lasing cavity mirrors. We demonstrated that mode spacings correspond to cavity lengths of the respective nanobelts, and directly observed the lasing from single ZnO nanobelts by mapping of the luminescence intensity.

The photoluminescence from single InP/InAs/InP core-multishell nanowires (CMNs) was highly linearly polarized in perpendicular to the nanowire long axis. Taking account of dielectric confinement of light field, we found the internal optical dipole squared perpendicular to the nanotube long axis d0x2 is larger than that parallel to the axis d0z2 by two orders of magnitude. With the increase in the thickness of InAs nanotubes, the ratio β = d0x2/d0z2 increases. This strong anisotropy in β comes from nearly perfect wurtzite structure of InP/InAs/InP CMNs and small inclusion of pz-character into the Γ7(B)-hole state.

Spectral diffusion of type-II excitons in InP/InAs/InP core-multishell nanowires (CMNs) and type-I excitons in an InAs/InP single quantum well (SQW) was studied by means of time-resolved and spectrally resolved photoluminescence. InP/InAs/InP CMNs in hexagonal symmetry are made of six facets and six edges which work as two-dimensional quantum wells and one-dimensional quantum wires, respectively. At 5 K type-II excitons lose their energy in two stages. In the first stage, two-dimensional spectral diffusion takes place in the type-II quantum well region in CMNs similar to spectral diffusion of type-I excitons in the InAs/InP SQW. In the second stage, slower one-dimensional spectral diffusion takes place in the quantum wire region in CMNs. Acoustic-phonon-mediated migration of excitons to lower-energy-localized states leads to the spectral diffusion in two dimensions and one dimension.

Experimental investigation of nuclear spin effects on the electron spin polarization in singly negatively charged InP quantum dots is reported. Pump-probe photoluminescence measurements of electron spin relaxation in the microsecond timescale are used to estimate the time-period $T_{N}$ of the Larmor precession of nuclear spins in the hyperfine field of electrons. We find $T_{N}$ to be $\sim 1$ $\mu$s at $T \approx 5$ K, under the vanishing external magnetic field. From the time-integrated measurements of electron spin polarization as a function of a longitudinally applied magnetic field at $T \approx 5$ K, we find that the Overhauser field appearing due to the dynamic nuclear polarization increases linearly with the excitation power, though its magnitude remains smaller than 10 mT up to the highest excitation power (50 mW) used in these experiments. The effective magnetic field of the frozen fluctuations of nuclear spins is found to be 15 mT, independent of the excitation power.

The spin dephasing relaxation in single-electron-doped InP quantum dots was studied by means of Hanle measurements. When an InP quantum dot is doped with one electron on average, a narrow Lorentzian dip with half-width of $4.6\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$ appeared and was superposed on two Lorentzians with half-widths of 1.54 and $128\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$ in the Hanle curve. The half-widths $1.54\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, $128\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$, and $4.6\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$ are ascribed to spin-dephasing relaxation of holes, electron-hole pairs, and doped electrons consistuting negative trions, respectively. The corresponding spin coherence time of the doped electrons at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$, which is determined by the frozen fluctuation of nuclear spins in the quantum dots. With increase of temperature, the spin-dephasing rate of the doped electrons increases.

Biexcitons confined in parabolic quantum dots are investigated both experimentally and theoretically. A biexcitonic beat is observed by using time-resolved four-wave mixing in strain-induced $\mathrm{GaAs}$ quantum dots formed in the quantum well. The period of the beat is about 1.5 times shorter than that from the quantum well region in the same well. The shortening of the beat period is direct evidence of an increase in the biexciton binding energy in the quantum dots due to the lateral confinement induced by the stressors. Magnetic field dependence of the biexciton binding energy is investigated, and is found to be almost independent of the external magnetic field up to $8\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. A theoretical calculation of the biexciton binding energy is presented to explain the observed magnetic field dependence.

We observed electron spin precession under magnetic field in single-layer quantum dots (QDs) by highly sensitive time-resolved Kerr rotation measurement. The spin lifetime is longer than that for the quantum well (QW). This is a result of the additional spatial confinement of electrons in QDs. Below $60\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the spin lifetime is almost constant, and is 7 times shorter than the carrier lifetime. This suggests that the strong electron-hole exchange interaction dominates over the electron spin lifetime in QDs at low temperature.

The work is an experimental study of optical spin polarization in $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots (QDs) with two resident electrons or holes. A capture of a photogenerated electron-hole pair into such a QD creates a negative or positive tetron (doubly charged exciton). Spin polarization was registered by the circular polarization of the QD photoluminescence (PL). The spin state was found to be radically different in the dots with the opposite sign of the charge. Particularly, under excitation in a GaAs barrier, the polarization of the ground-state PL is negative (relative to the polarization of exciting light) in the negatively charged QDs and positive in the positively charged QDs. With increasing excitation intensity, the negative polarization rises from zero up to a saturation level, while the positive polarization decreases. The negative polarization increases in weak magnetic fields applied in Faraday geometry; however, it is suppressed in strong fields. The positive polarization always increases as a function of magnetic field. We propose a theoretical model that qualitatively explains the experimental results.

The effect of bulk plasmon-LO-phonon excitations inherent to doped areas of semiconductor heterostructures upon the electronic dynamics of quantum dots spaced apart from the areas is studied. An effective mechanism of intraband carrier relaxation in quantum dots is proposed for such heterostructures. The mechanism involves interaction between the quantum dot carriers and the electric potential induced by bulk plasmon-LO-phonon modes of the doped areas. It is shown that the interaction opens two relaxation windows with the spectral positions and widths controlled, correspondingly, by the free-carrier concentrations and the bulk plasmon-LO-phonon mode dispersion of the doped areas. The relaxation rates related to the mechanism are calculated for quantum dots spaced apart by different distances ${z}_{0}$ from doped substrate (the area) with the different free-carrier concentrations ${n}_{0}.$ The estimations carried out for InAs quantum dots and the GaAs substrate yield the relaxation rates of about ${10}^{8}{\mathrm{s}}^{\ensuremath{-}1}$ for ${n}_{0}{=10}^{18}{\mathrm{cm}}^{\ensuremath{-}3}$ and ${z}_{0}=100\mathrm{nm}.$ The rates increase by two to three orders of magnitude with decreasing the distance down to 20 nm. A manifestation of the mechanism is shown for a system of self-assembled InAs/GaAs quantum dots separated by a distance of 100 nm from an n-doped GaAs substrate by means of the photoluminescence spectroscopy. From the experiment it has been found that ${L}^{\ensuremath{-}}$ and ${L}^{+}$ plasmon-LO-phonon modes of the GaAs substrate are involved in the intraband carrier relaxation in the quantum dots. The data are in good agreement with the theoretical predictions.

The effects of external electric and magnetic fields on InP self-assembled quantum dots (QDs) were investigated by means of single dot spectroscopy. By systematically changing a bias applied to the sample, successive energy shifts of the photoluminescence (PL) peaks from excitons and biexcitons due to the quantum confined Stark effect were clearly observed. The quadratic Stark coefficient was evaluated to be of the order of ${10}^{\ensuremath{-}31}{\mathrm{Fcm}}^{2}.$ The energy separation of the PL peaks arising from the excitons and biexcitons changed with the applied electric field, reflecting a slight difference of the Stark coefficient between the exciton and biexciton states. The existence of permanent dipole moments was also revealed in both the exciton and biexciton states. The spatial separations between the electrons and holes along the growth direction in a QD were estimated to be 7 \AA{} for the exciton state, and 8 \AA{} for the biexciton state. Further, the diamagnetic shift and the Zeeman splitting of the exciton states were clearly observed in a magnetic field. It was found that the diamagnetic coefficient gradually decreases on decreasing the QD size. A simple qualitative model can explain that this result is due to competition between quantum confinement and magnetic confinement.

Spins of resident electrons in charged quantum dots (QD's) act as local magnets inducing the Zeeman splitting of excitons trapped into dots. This is evidenced by the observation of quantum beats in the linearly polarized time-resolved photoluminescence of a biased array of self-assembled InP QD's. An external magnetic field is found to shorten the spin beats' decay time keeping constant the frequency of the beats. A model using the pseudospin formalism allows one to attribute the observed quantum beats to the radiative decay of hot trions having two electrons that occupy different energy levels in a QD.

Radiative decay of biexcitons wave-vector $(\mathbf{K})$ selectively generated in a low-$K$ region $(0l~|\mathbf{K}|\ensuremath{\lesssim}{2k}_{0})$ 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.

application/pdf, Optical dephasing of excitonic polaritons in CuCl is directly measured for the first time in the picosecond time domain by means of time-resolved, nondegenerate four-wave mixing. Dephasing of the ω1 polariton pulse is probed by an interrogation ω2 polariton pulse whose group velocity is faster than that of the ω1 polariton pulse. It is found that the dephasing damping constant Γ / 2 of excitonic polaritons is of order of 0.01 meV and increases as hω1 approaches the exciton resonance energy from below.