Low Excitation Densities Research Articles

Temperature dependence of biexciton luminescence in cubic ZnS single crystals

Temperature dependence of the excitonic and biexcitonic emissions in cubic ZnS single crystals has been studied by photoluminescence technique. We observed the very well-resolved excitonic and biexcitonic emissions from cubic ZnS single crystals with very low excitation density, in the tens-of-milliwatts-per-square-centimetre range. The kinetic energy contribution to the shift with temperature of the biexcitonic emission was determined showing a characteristic difference with the normal shift caused by the temperature-narrowed band gap.

Formation of extended states in disordered two-dimensional In0.4Ga0.6As/GaAs(311)B quantum dot superlattices

Formation of extended states or minibands in two-dimensional (2D) In0.4Ga0.6As/GaAs(311)B quantum dot superlattices (QDSLs) is directly demonstrated in time-resolved photoluminescence measurements. At a low excitation density of 1 W/cm2, photoluminescence transients with ∼15 ps rise time and ∼25 ps decay time are observed. Both rise and decay times are found to increase with increasing excitation density. The excitons in 2D QDSLs exhibit different relaxation and recombination behaviors as compared to those in quantum wells and quantum dots. A physical model treating 2D QDSLs as disordered systems containing localized and extended states can successfully interpret all of the experimental observations.

Temporal evolution of resonant Raman-scattering in ZnCdSe quantum dots

We investigated ZnCdSe/ZnSe quantum-dot structures which include planar and coherently strained three-dimensional islands with different sizes. Optical excitation of these islands well below the ZnSe band gap leads to a resonant enhancement of the Zn0.7Cd0.3Se longitudinal-optical (LO) phonon-scattering efficiency and makes the 2LO and 3LO multiphonon emission observable. Resonant excitation with a power density of about 1.3 MW/cm2 using a micro-Raman setup results in an exponential decrease of the 1LO, 2LO, and 3LO intensity with irradiation time. This decay behavior is not observed for pure ZnSe crystals and can be avoided for the ZnCdSe/ZnSe structures using much lower excitation densities. The decrease in intensity is accompanied by a shift of the LO mode to higher frequencies resulting from a lower cadmium concentration in the alloy. From these experimental findings, we conclude that resonant excitation at a certain power density leads to cadmium out-diffusion from the planar quantum dots, which shifts the resonance away from the excitation energy.

Lasing from excited states in self-assembled InP/GaInP quantum islands

Aiming at laser emission in the visible part of the spectrum, we have investigated the growth of self-assembled InP quantum dots on GaInP by low-pressure metal-organic vapour phase epitaxy using the Stranski–Krastanow growth mode. These InP islands show a photoluminescence emission energy of 1.72 eV (90 K) at low excitation densities. Lasing was observed on optically pumped laser structures with a single layer of InP dots, GaInP waveguide material and AlInP cladding layers above an excitation density of 1.3 kW cm −2 at 90 K. The detected laser line is located at 1.8 eV, about 80 meV higher than the ground-state transition energy. We attribute this behaviour to lasing from excited states in agreement with power-dependent photoluminescence experiments and photoluminescence excitation measurements, and gain measurements using the variable stripe length method. These laser structures show stimulated emission under optical pumping up to room temperature above excitation densities of 30 kW cm −2. Furthermore, injection lasers with Si- and Zn-doped (Al 0.5Ga 0.5)InP cladding layers have been realised where lasing occurred above a threshold current density of 290 A cm −2 with an emission line at 1.78–1.82 eV (90 K).

Linear and non-linear behavior of cavity polaritons

We report on the linear and non-linear emission of cavity-polaritons under resonant excitation. At low excitation density, in addition to polariton photoluminescence, strong Rayleigh scattering is observed. At higher excitation densities, a sudden transition to a highly emissive state is observed, accompanied by spatial patterning. We attribute such phenomena to a combination of nonlinear cavity-polariton relaxation mechanism and nonlinear response of the cavity, leading to transverse pattern formation. A careful analysis of near- and far-field emission patterns with spatial filtering as well as reflectivity shows that an inhomogeneous situation develops, the center of the excited region undergoing a strong to weak coupling transition while the periphery is still in strong coupling. Despite the complex non-linear behavior we observe no signatures of Bose–Einstein condensation or Boser action.

Observation of homogeneous optical spectrum in CdSe quantum dots using femtosecond accumulated photon echo

We performed heterodyne-detected accumulated photon echo experiments in CdSe quantum dots. The excitonic dephasing time T 2 was directly observed under very low excitation density of 10 −3 photons per dot. The echo signal consists of two exponential decays: the fast decay component is due to the confined acoustic phonon sideband, while the slow one to the zero–phonon line. The sharp zero–phonon line width increased linearly with the temperature rise, and its linear temperature coefficient was proportional to 1/ R 2. This feature agrees well with the theory based on the interaction between confined excitons and acoustic phonons through deformation potential. The 1/ R-dependent homogeneous broadening at the low temperature suggests the surface scattering.

Heterodyne-detected accumulated photon echo in CdSe quantum dots

The homogeneous optical spectra of CdSe quantum dots were investigated under very low excitation density of 10 −3 photons per dot by using the heterodyne-detected accumulated photon echo. The photon echo decay is composed of two exponential decays in femto- and picosecond time domains. The fast decay component comes from the confined acoustic phonon sideband, while the slow one comes from the zero-phonon line. The linewidth of the zero-phonon line at 0 K is inversely proportional to the radius of the dots and ranges from 0.1 to 0.2 meV, suggesting the dominant contribution of the surface scattering. It increases linearly with the temperature rise. The temperature broadening linear coefficient is inversely proportional to the square of the radius of the dots consistence with the theory based on the deformation potential coupling to the confined acoustic phonon modes.

Femtosecond mid-infrared study of electron dynamics in InAs/InAlAs quantum dots

We present pump-mid-infrared-probe measurements in InAs/InAlAs self-organized quantum dots allowing to isolate the electron dynamics. By tuning the probe to an intraband transition in the conduction band at 13 μm, we measured a characteristic time of 3 ps for the electron capture from the barrier to the ground state of the dots at low excitation densities. At higher densities, this capture time decreases to 1.5 ps indicating the contribution of Auger-like processes.

Piezoelectric polarization in the radiative centers of GaInN/GaN quantum wells and devices

We identify, quantify, and correlate the polarization dipole across the well of device-typical piezoelectric GaInN/GaN heterostructures with the luminescence properties of the well. This quantity reflects in the asymmetry of the barrier height on either side of the well. By a detailed comparison of photoreflection, electroreflection, low and high excitation density photoluminescence we find that a very similar splitting occurs in the emission characteristics of the well. We therefore conclude that the electronic band structure within the wells is also to a very large extent controlled by the quantity of the polarization dipole in such polarization heterostructures.

Investigation of post-growth Cu-diffusion in In-rich CuInSe2 films

In-rich CuInSe2 films have been submitted to post-growth Cu diffusion. The photoluminescence investigation carried out shows a transition from the commonly observed broad band around 0.94 eV to a much sharper peak around 0.985 eV under low excitation density. This recombination appears at a slightly but definitely larger energy than the recombination usually reported around 0.96 eV. The excitation power dependence leads us to ascribing the peak observed here to a donor–acceptor pair band, as evidenced by the 2.5 meV/decade shift of the peak-energy and the temperature-dependence of the photoluminescence signal. From the Arrhenius plot of the luminescence intensity an activation energy of 40 meV is extracted. When these data are analyzed in the frame of the most recent theoretical predictions of Cu- and In-related defects a possible candidate for the center appears to be [CuIn–Cui]. However CuSe could equally be involved.

Two-dimensional exciton dynamics and gain formation processes inInxGa1−xNmultiple quantum wells

We investigated exciton dynamics in ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ multiple quantum wells at 2 K and room temperature by means of time-resolved photoluminescence and pump-and-probe measurements. Under low excitation density, the temporal change of the spontaneous emission indicated slow dynamical features of the two-dimensional exciton localization, while, above a stimulation threshold density, the decay time of the emission was shortened to be less than $\ensuremath{\sim}30 \mathrm{ps}$ due to the stimulated emission process. Further, the time-resolved pump-and-probe measurement revealed a fast relaxation of photoexcited delocalized electron-hole $(e\ensuremath{-}h)$ pairs into localized states. Above stimulation threshold, localized states were saturated, and $e\ensuremath{-}h$ pairs at delocalized states were observed. Two-dimensional $e\ensuremath{-}h$ pairs at delocalized states relaxed into localized states and excitons were formed, from which the optical gain was formed in terms of the stimulated emission process.

Giant anti-Stokes photoluminescence from semimagnetic heterostructures

A giant anti-Stokes photoluminescence is observed at low excitation densities on specially designed semimagnetic II--VI asymmetric double quantum well structures by applying an external magnetic field. The signal is due to a two-step absorption process, mediated by a long-living spatially indirect exciton, created via rapid electron tunneling from the primarily excited direct exciton state.

Carrier energy relaxation by means of Auger processes in InAs/GaAs self-assembled quantum dots

Carrier relaxation processes are investigated in self-assembled InAs/GaAs quantum dots using time-resolved photoluminescence spectroscopy. The quantum-dot photoluminescence rise time has been measured as functions of carrier excitation density and excitation wavelengths. The measured relaxation time is about 32 ps at low excitation density and decreases by 1 over the excitation density from about 3 W/cm2, under nonresonant laser excitation. The threshold of this density-dependent regime occurs at a slightly higher density as the excitation wavelength increases and it disappears when the photon pumping energy is below the wetting layer barrier energy. These results clearly establish the regime where Auger processes become the dominant carrier relaxation mechanism in these self-assembled quantum dots.

Coherent effects in resonant quantum well optical emission

The coherence of an excitonic ensemble resonantly excited by an ultrashort pulse is probed by arranging an interaction with a second identical pulse in a time-resolved coherent control experiment. We demonstrate that the decay of the amplitude of coherent control is faster than exciton dephasing for low excitation density. We also study the rise and the decay of the secondary emission from quantum well systems and observe how both of them show a dominant contribution of resonant Rayleigh scattering.

The Structure of NGC 2392

The planetary nebula NGC 2392 possesses an extremely unusual spectrokinematic morphology, characterized by several strange and elusive structures. In this paper, extensive narrowband imaging of the source is used to investigate the shell excitation structure, while [O III] and [S II] images are ratioed to provide fully sampled mapping of electron density and temperature. As a result, we are able to confirm the presence of low densities in the various filamentary structures, an appreciable radial decrease in density in the outer shell, and finally, a probable regime of higher shell densities at the interface of the bright inner shell and lower intensity outer halo. Although temperatures are more uniform, we also find evidence for a significant temperature enhancement close to the inner-outer shell interface, and it is proposed that both anomalies (in density and temperature) arise in a postshock cooling regime. Finally, present imaging and prior spectroscopy are used to construct a revised model of the source, in which it is argued that the outer filamentary structure occupies a disklike regime, congruent with the principal plane of the outer spheroid. Individual filaments arise from low-excitation density contrasts, created through the interaction between the outer shell and a precessing jet.

Excitation Density Dependence of Photoluminescence and Barrier Doping Effect in InxGa1—xN Quantum Wells

The effect of Si-doping in barriers has been investigated in photoluminescence (PL) spectra of 10 period In0.1Ga0.9N/ In0.02Ga0.98N quantum wells with various excitation densities. For the quantum wells with well width of 40 A, the PL peak was blueshifted and the intensity was increased with Si-doping under relatively low excitation density. However, such Si-doping effect was found to be less pronounced with higher excitation density. The blueshift of PL emission with increasing excitation density was larger for larger well widths, which indicates the influence of a piezoelectric potential in our samples. For sufficiently narrow quantum wells, we observed a redshift with increasing excitation density due to the bandgap renormalization effect.

Nonlinearities in emission from the lower polariton branch of semiconductor microcavities

Stimulated scattering of spin-polarized lower branch polaritons ~LP! arising from their bosonic character is reported. The observations are achieved in a microcavity close to zero detuning between the uncoupled exciton and photon modes, under conditions of circularly polarized resonant excitation into the upper polariton branch. The key role played by the bosonic nature of the LP states is demonstrated by measurements of the degree of circular polarization of the emitted light PLP . At low-excitation densities, PLP is of the order of 30% in the whole range of wave vectors k,2.10 cm. However, at higher excitation intensities, PLP increases drastically, up to 80%, for LP’s with k'0, and is accompanied by the appearance of strong nonlinearities in the LP emission intensity whilst P remains unchanged for weakly coupled polaritons with k.10 cm. @S0163-1829~99!51040-0#

Conductivity of ionic crystals when they are irradiated by picosecond electron beams

This paper discusses the pulsed electron conductivity σ of KCl, KBr, and NaCl crystals when they are excited by an electron beam (0.2 MeV, 50 ps) with current densities in the interval j=(30–104) A/cm2. It is shown that the lifetime of the electrons in the conduction band is τ≪100 ps. To explain the experimental σ(j) dependences, a model is proposed that includes electron capture by structural defects and stable radiation defects at low excitation densities and electron capture predominantly by unstable radiation defects generated by the excitation pulse at high excitation densities.

Photoluminescence and Raman characterization of heavily doped Al 0.3Ga 0.7As grown by solid-source molecular beam epitaxy using carbon tetrabromide

Carbon incorporation in Al 0.3Ga 0.7As is systematically studied in solid source molecular beam epitaxy as a function of carbon tetrabromide pressure and substrate temperature. The maximum free carrier concentration attained was 1.5×10 20 cm −3. No effect of substrate temperature on doping density was observed for temperatures in the range 560–700°C. The carbon-induced lattice contraction in AlGaAs determined by X-ray diffraction was found to be proportional to hole density and similar to the contraction in GaAs. Carbon-doped samples exhibited anomalously bright room temperature photoluminescence at low excitation density. The increase in full-width at half-maximum of the 4 K photoluminescence peak and in the TO-to-LO GaAs-like mode intensity ratio in Raman spectroscopy shows a reduction in material quality for the samples grown in the “forbidden temperature range” between 600 and 640°C.

Reduction of photoluminescence quantum yield by interchain interactions in conjugated polymer films

We use new photoluminescence data to conclusively establish that photoexcitation of substituted phenylenevinylene polymer films results in a large quantum yield of nearly nonluminescent interchain excitations at ambient temperatures. These data are acquired in the low excitation density regime where excited state interactions are negligible. This result has significant implications for material design and processing to make polymer light-emitting diodes.