Increasing Excitation Density Research Articles

Defect recombination induced by density-activated carrier diffusion in nonpolar InGaN quantum wells

We report on the observation of carrier-diffusion-induced defect emission at high excitation density in a-plane InGaN single quantum wells. When increasing excitation density in a relatively high regime, we observed the emergence of defect-related emission together with a significant efficiency reduction of bandedge emission. The experimental results can be well explained with the density-activated carrier diffusion from localized states to defect states. Such a scenario of density-activated defect recombination, as confirmed by the dependences of photoluminescence on the excitation photon energy and temperature, is a plausible origin of efficiency droop in a-plane InGaN quantum-well light-emitting diodes.

Competition between second harmonic generation and two-photon-induced luminescence in single, double and multiple ZnO nanorods

The nonlinear optical properties of single, double and multiple ZnO nanorods (NRs) were investigated by using a focused femtosecond (fs) laser beam. The excitation wavelength of the fs laser was intentionally chosen to be 754 nm at which the energy of two photons is slightly larger than that of the exciton ground state but smaller than the bandgap energy of ZnO. Second harmonic generation (SHG) or/and two-photon-induced luminescence (TPL) were observed and their dependences on excitation density were examined. For single ZnO NRs, only SHG was observed even at the highest excitation density we used in the experiments. The situation was changed when the joint point of two ZnO NRs perpendicular to each other was excited. In this case, TPL could be detected at low excitation densities and it increased rapidly with increasing excitation density. At the highest excitation density of ~15 MW/cm(2), the intensity of the TPL became comparable to that of the SHG. For an ensemble of ZnO NRs packed closely, a rapid increase of TPL with a slope of more than 7.0 and a gradual saturation of SHG with a slope of ~0.34 were found at high excitation densities. Consequently, the nonlinear response spectrum was eventually dominated by the TPL at high excitation densities and the SHG appeared to be very weak. We interpret this phenomenon by considering both the difference in electric field distribution and the effect of heat accumulation. It is suggested that the electric field enhancement in double and multiple NRs plays a crucial role in determining the nonlinear response of the NRs. In addition, the reduction in the bandgap energy induced by the heat accumulation effect also leads to the significant change in nonlinear response. This explanation is supported by the calculation of the electric field distribution using the discrete dipole approximation method and the simulation of temperature rise in different ZnO NRs based on the finite element method.

Linear polarization dependence of microwave-induced magnetoresistance oscillations in high-mobility two-dimensional systems

CdSe/CdS nanocrystal tetrapods are interesting building blocks for excitonic circuits, where the flow of excitation energy is gated by an external stimulus. The physical morphology of the nanoparticle, along with the electronic structure, which favors electron delocalization between the two semiconductors, suggests that all orientations of a particle relative to an external electric field will allow for excitons to be dissociated, stored, and released at a later time. While this approach, in principle, works, and fluorescence quenching of over 95$%$ can be achieved electrically, we find that discrete trap states within the CdS are required to dissociate and store the exciton. These states are rapidly filled up with increasing excitation density, leading to a dramatic reduction in quenching efficiency. Charge separation is not instantaneous on the CdS excitonic antennae in which light absorption occurs, but arises from the relaxed exciton following hole localization in the core. Consequently, whereas strong electromodulation of the core exciton is observed, the core multiexciton and the CdS arm exciton are not affected by an external electric field.

Electro-optical trap for dipolar excitons

An electro-optical trap for spatially indirect dipolar excitons is implemented in a GaAs/AlAs Schottky diode with a 400-A-wide GaAs single quantum well. In the presence of bias voltage applied to the gate, the trap for excitons appears upon annular illumination of the structure by a continuous-wave or pulsed laser generating hot electron-hole pairs in the quantum well. A barrier for excitons collected inside the illuminated ring appears due to screening of the applied electric field by nonequilibrium charge carriers within the excitation region. Excitons are collected inside the ring due to the ambipolar drift of carriers and the dipole-dipole repulsion of excitons in the optically pumped region. For dipolar excitons thus accumulated in the middle of the ring-shaped electro-optical trap, significant narrowing of the luminescence line is observed with increasing excitation density, which indicates the collective behavior of the excitons.

Exciton storage in CdSe/CdS tetrapod semiconductor nanocrystals: Electric field effects on exciton and multiexciton states

CdSe/CdS nanocrystal tetrapods are interesting building blocks for excitonic circuits, where the flow of excitation energy is gated by an external stimulus. The physical morphology of the nanoparticle, along with the electronic structure, which favors electron delocalization between the two semiconductors, suggests that all orientations of a particle relative to an external electric field will allow for excitons to be dissociated, stored, and released at a later time. While this approach, in principle, works, and fluorescence quenching of over 95$%$ can be achieved electrically, we find that discrete trap states within the CdS are required to dissociate and store the exciton. These states are rapidly filled up with increasing excitation density, leading to a dramatic reduction in quenching efficiency. Charge separation is not instantaneous on the CdS excitonic antennae in which light absorption occurs, but arises from the relaxed exciton following hole localization in the core. Consequently, whereas strong electromodulation of the core exciton is observed, the core multiexciton and the CdS arm exciton are not affected by an external electric field.

Optical studies of quantum dot‐like emission from localisation centres in InGaN/GaN nanorod array LEDs

AbstractMicrophotoluminescence with one‐photon and two‐photon excitation has been employed to investigate localisation centres found in nanorod array LEDs. Quantum dot‐like features with spectral width less than 1 meV were observed originating from low dimensional carrier confinement within the nanorods. Two‐photon excitation was used to probe the individual localisation centres. The narrow peak PL energy does not shift with increasing excitation density, but rather an increase of discrete new peaks on the high energy side of the spectrum is observed. There is no temperature dependent emission energy shift observed from 4.3 K to 25 K. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical polariton properties in ZnSe-based planar and pillar structured microcavities

Strong coupling is demonstrated in monolithic ZnSe-based microcavities. Under nonresonant excitation the polariton dispersion has been investigated in dependence on the photon-exciton detuning for different excitation densities at low temperatures. For zero detuning indications of a polariton lasing threshold are observed like a k-space and energy dispersion narrowing of the lower polariton branch with increasing excitation density. Furthermore, it is observed that this effect is hampered for measurements at negative detunings as a result of the less effective polariton relaxation to the ground state. Latter results in the formation of a discrete polariton distribution at finite k values as known for the polariton bottleneck. In order to investigate the influence of a three-dimensional optical confinement on the polariton relaxation, pillar structured microcavities were fabricated. The formation of discrete polariton states in the k-space distribution is observed. Furthermore, indications for a softening of the k-conservation arising from the structural confinement are found leading to a more effective polariton relaxation. This process would be beneficial for the realization of efficient polariton lasing processes.

Distribution of visible luminescence centers in hydrogen-doped ZnO

Abstract

Transitions from spontaneous emission to stimulated emission and superfluorescence of biexcitons confined in CuCl quantum dots

Time-resolved photoluminescence spectra of biexcitons in CuCl quantum dots embedded in NaCl matrix have been measured by two configurations of optical Kerr gate method. As for a backward configuration, a transition from spontaneous to stimulated emission is observed with increase in the excitation density. On the other hand, by a configuration collecting signals from the edge of the sample in the direction of the excitation length, an essential change in the time profile of biexciton luminescence from a conventionally exponential to a pulse-shaped according to the increasing excitation density reasonably suggests a shift from spontaneous emission to superfluorescence of biexciton luminescence.

Coexistence of quantum-confined Stark effect and localized states in an (In,Ga)N/GaN nanowire heterostructure

We analyze the emission of single GaN nanowires with (In,Ga)N insertions using both micro-photoluminescence and cathodoluminescence spectroscopy. The emission spectra are dominated by a green luminescence band that is strongly blueshifted with increasing excitation density. In conjunction with finite-element simulations of the structure to obtain the piezoelectric polarization, these results demonstrate that our (In,Ga)N/GaN nanowire heterostructures are subject to the quantum-confined Stark effect. Additional sharp peaks in the spectra, which do not shift with excitation density, are attributed to emission from localized states created by compositional fluctuations in the ternary (In,Ga)N alloy.

Biexcitonic superfluorescence from CuCl quantum dots under resonant two‐photon excitation

AbstractTime‐resolved photoluminescence spectra of biexcitons confined in CuCl quantum dots embedded in a NaCl matrix have been measured by an optical Kerr gate method. Ultrashort pulsed emission was observed for resonant two‐photon excitation of the biexcitons. The peak intensity of this pulsed emission depended on the fourth power of the excitation density, which indicates that the superfluorescence originates from coherently coupled excited quantum dots. The photon energy of this pulsed emission increased with increasing excitation density. This implies that the superfluorescence efficiently depends on the dot size because biexcitons are directly generated by resonant two‐photon excitation for an ensemble of quantum dots with a size distribution (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The UV-laser induced heating effect on photoluminescence from ZnO nanocrystals deposited on different substrates

Photoluminescence (PL) from ZnO nanocrystals deposited on silicon, sapphire and glass substrates was measured at different temperatures and UV-laser excitation densities. Our studies show that the PL properties of the nanocrystals are a strong function of the ultraviolet laser excitation density and the thermophysical properties of the substrates used. Even at low temperatures (10–30 K), high-power laser illumination can heat the top layer of nanocrystals up to 100 K. The heating effect induced by laser illumination results in a saturation behaviour of the PL intensity with increasing excitation density. It is determined that the degree of the saturation depends on the substrates' thermophysical properties.

Photoluminescence properties of various CVD-grown ZnO nanostructures

We have studied systematically room-temperature photoluminescence (PL) properties of many nanostructured ZnO samples grown by chemical vapour deposition (CVD). Their PL spectra consist of two emissions peaked in the ultraviolet (UV) and green regions. The relative intensity of these emissions depends on the excitation energy density, size and morphology of ZnO nanostructures. Based on the excitation-density dependence of the integrated intensity ratio of UV-to-green emission, we could classify PL spectra of ZnO nanostructures into three groups characteristic of size and morphology. Our study also reveals that with increasing excitation density, the UV-peak position shifts slightly towards longer wavelengths while the green emission around 514–520 nm is almost unchanged. This green-luminescence emission is dominant when the nanostructure sizes range from 20 to 200 nm, which is related to a large surface-to-volume ratio.

Spectral behavior of the emission around 3.31 eV (A-line) from ZnO nanocrystals

The emission at around 3.31 eV (A-line) from three types of ZnO nanocrystals with different particle sizes (10–1000 nm) was studied. The photoluminescence (PL) measurements were performed under different excitation densities and at different temperatures. The A-line emission exhibited a strong dependence on temperature and excitation power density. With increasing excitation density and temperature overlapping of the closely spaced first longitudinal optical (LO) phonon replica of free excitons by the A-line emission was observed.

Type-I interband transition in undoped ZnSe/BeTe type-II quantum wells under high excitation density

A spatially direct photoluminescence (PL) spectrum associated with type-I interband transition of a ZnSe layer in undoped ZnSe/BeTe type-II quantum structures was investigated by varying the photo-excitation density. For a sample with a narrower ZnSe layer both PL intensity and linewidth of the trion show a superlinear increase with increasing excitation density in comparison to that with a wider ZnSe layer. The results are explained by the effective increase of the electron concentration and an enhanced dephasing rate of the trion resulting from the electron–trion scattering. The effective increase of the electron concentration in the ZnSe layer is considered to be originating from both the greater transfer rate of the hole into the BeTe layer due to the narrower ZnSe layer and the efficient spatially indirect transition PL of the complex states through the interface. With decreasing excitation density, no indication of any shift in peak energy density was observed indicating that the studied structures are of excellent quality.

Excitation density dependence of luminescence spectrum of electron-hole plasma in diamond

Time-resolved band edge luminescence spectrum in IIa diamond has been measured with the 5th harmonics of a pulsed YAG laser (5.82 eV) and an ICCD image intensifier of 5 ns gate width at 290 K. The time-resolved luminescence spectrum is decomposed into three components of free exciton (FE), excitonic complex (EC) and electron-hole plasma (EHP). The decay times of the FE and EC luminescence are 45 and 27 ns, respectively and that of the EHP luminescence has been seen to be shorter than the gate width, 5 ns. The low energy onset of the EHP luminescence spectrum has been observed to decrease with increasing excitation density and attains the onset of the electron-hole drop luminescence spectrum at the excitation density of 0.6 J/cm 2, at which the electron-hole pair density is 1.2 × 10 20 cm − 3 . Furthermore, the excitation density dependences of the FE, EC and EHP luminescence intensities are explained with the percolation theory.

Zn2SiO4/ZnO Core/Shell Coaxial Heterostructure Nanobelts Formed by an Epitaxial Growth

Si-doped ZnO can be synthesized on the surface of the early grown Zn2SiO4 nanostructures and form core/ shell coaxial heterostructure nanobelts with an epitaxial orientation relationship. A parallel interface with a periodicity array of edge dislocations and an inclined interface without dislocations can be formed. The visible green emission is predominant in PL spectra due to carrier localization by high density of deep traps from complexes of impurities and defects. Due to band tail localization induced by composition and defect fluctuation, and high density of free-carriers donated by doping, especially the further dissociation of excitons into free-carriers at high excitation intensity, the near-band-edge emission is dominated by the transition of free-electrons to free-holes, and furthermore, exhibits a significant excitation power-dependent red-shift characteristic. Due to the structure relaxation and the thermalization effects, carrier delocalization takes place in deep traps with increasing excitation density. As a result, the green emission passes through a maximum at 0.25I(0) excitation intensity, and the ratio of the violet to green emission increases monotonously as the excitation laser power density increases. The violet and green emission of ZnO nanostructures can be well tuned by a moderate doping and a variation in the excitation density.

Recombination dynamics of localized excitons in AlGaN‐based quantum wells

AbstractThe effect of internal electric field on exciton localization in AlGaN‐based quantum wells (QWs) has been studied by means of time‐resolved photoluminescence (PL) spectroscopy. The observed time‐dependent PL was analyzed using rate equations, which enabled us to obtain the PL lifetime and the localization time of excitons for three QWs with different well‐layer thickness. The localization time became longer with increasing well‐layer thickness. This result indicated that a quantum confined Stark effect caused the reduction in transition probability of excitons from extended to localized states. In addition, the localization time decreased with increasing excitation density and the three QWs indicated almost the same localization time at higher excitation density. Since the alloy composition was the same for the three QWs, the degree of localization due to alloy disorder did not change so much. Therefore, the screening of the internal electric field resulted in the same localization time. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Observation of many-body Coulomb interaction effects on the photoluminescence spectra of InAs/GaAs quantum dots

InAs quantum dots (QDs) on GaAs (0 0 1) substrates were grown by Molecular Beam Epitaxy (MBE) using two growth temperatures. Photoluminescence (PL) pump power dependence measurements at low temperature were carried out for sample grown at higher temperature (520 °C). With increasing excitation density, the ground-state transition energy is found to decrease by 8 meV, while the excited-state transition energies exhibit resonance behaviour. The redshift of the ground-state emission was related to the band-gap renomalization (BGR) effect whereas the blueshift of the excited-state emissions was assigned to the compensation between filling of fine structure states and BGR effects. Using a quasi-resonant PL measurement, we have shown that the renormalization of the band-gap had to occur in the QD barrier.

Free-exciton luminescence spectrum broadening due to excitonic complex in diamond

The excitation density evolution of a band-edge luminescence spectrum of diamond in the photon energy region 5.14 to 5.40 eV was investigated at 190, 240 and 290 K with the fifth harmonic (5.82 eV) of a pulsed YAG laser in the excitation density range 2.2 to 272 mJ/cm 2. A broadening of a free-exciton luminescence (FE) spectrum with the peak energy ( E peak) of 5.28 eV due to the growth of the low-energy tail was observed with increasing excitation density. From a detailed spectrum analysis, it is found that the broadening originates in excitonic complex (EC) luminescence ( E peak = 5.26 eV) in addition to electron-hole plasma (EHP) luminescence ( E peak = 5.23 eV). The EC luminescence intensity was observed to be proportional to the 1.5 power of the FE luminescence intensity, which is the same manner as in the case of the EC luminescence in crystalline silicon. The excitation density dependence of the EHP luminescence intensity is discussed with percolation theory.