Function Of Excitation Photon Energy Research Articles

Hot-carrier dynamics in InAs/AlAsSb multiple-quantum wells

A type-II InAs/AlAs_{0.16}Sb_{0.84} multiple-quantum well sample is investigated for the photoexcited carrier dynamics as a function of excitation photon energy and lattice temperature. Time-resolved measurements are performed using a near-infrared pump pulse, with photon energies near to and above the band gap, probed with a terahertz probe pulse. The transient terahertz absorption is characterized by a multi-rise, multi-decay function that captures long-lived decay times and a metastable state for an excess-photon energy of >100 meV. For sufficient excess-photon energy, excitation of the metastable state is followed by a transition to the long-lived states. Excitation dependence of the long-lived states map onto a nearly-direct band gap (E{_g}) density of states with an Urbach tail below E{_g}. As temperature increases, the long-lived decay times increase <E{_g}, due to the increased phonon interaction of the unintentional defect states, and by phonon stabilization of the hot carriers >E{_g}. Additionally, Auger (and/or trap-assisted Auger) scattering above the onset of the plateau may also contribute to longer hot-carrier lifetimes. Meanwhile, the initial decay component shows strong dependence on excitation energy and temperature, reflecting the complicated initial transfer of energy between valence-band and defect states, indicating methods to further prolong hot carriers for technological applications.

Resonant photoluminescence studies of carrier localisation in c-plane InGaN/GaN quantum well structures

In this paper we report on changes in the form of the low temperature (12 K) photoluminescence spectra of an InGaN/GaN quantum well structure as a function of excitation photon energy. As the photon energy is progressively reduced we observe at a critical energy a change in the form of the spectra from one which is determined by the occupation of the complete distribution of hole localisation centres to one which is determined by the resonant excitation of specific localisation sites. This change is governed by an effective mobility edge whereby the photo-excited holes remain localised at their initial energy and are prevented from scattering to other localisation sites. This assignment is confirmed by the results of atomistic tight binding calculations which show that the wave function overlap of the lowest lying localised holes with other hole states is low compared with the overlap of higher lying hole states with other higher lying hole states.

Quantum Dot Thin-Films as Rugged, High-Performance Photocathodes.

Typical use of colloidal quantum dots (QDs) as bright, tunable phosphors in real applications relies on engineering of their surfaces to suppress the loss of excited carriers to surface trap states or to the surrounding medium. Here, we explore the utility of QDs in an application that actually exploits their propensity toward photoionization, namely within efficient and robust photocathodes for use in next-generation electron guns. In order to establish the relevance of QD films as photocathodes, we evaluate the efficiency of electron photoemission of films of a variety of compositions in a typical electron gun configuration. By quantifying photocurrent as a function of excitation photon energy, excitation intensity and pulse duration, we establish the role of hot electrons in photoemission within the multiphoton excitation regime. We also demonstrate the effect of QD structure and film deposition methods on efficiency, which suggests numerous pathways for further enhancements. Finally, we show that QD photocathodes offer superior efficiencies relative to standard copper cathodes and are robust against degradation under ambient conditions.

Photostimulated detection of defect formation in BaF2 under irradiation of synchrotron radiation

AbstractWe have investigated the defect formation processes of BaF2 single crystals under vacuum ultraviolet radiation with the use of combination spectroscopy of synchrotron radiation (SR) and laser. The defect formation efficiency was measured as a function of excitation photon energy of SR by means of a laser‐induced luminescence method. It was found that the efficiency depends significantly on the excitation photon energy. We show that the dependence can be explained in terms of the diffusive motion of hot conduction electrons generated by the photoexcitation. The relationship between the defect formation and the energy relaxation of core excitons is also discussed.

(Invited) Photoluminescence Excitation Spectroscopy of Si Nanocrystals in SiO2

We present results on time-resolved spectroscopy and quantum yield of photolu-minescence from Si nanocrystals (NCs) embedded in a SiO2-matrix. The materials are prepared by co-sputtering. Transmission electron microscopy reveals formation of Si NC with diameters in the 3-5 nm range. We identify 3 different emission bands, corresponding to radiative recombination of excitons, hot carriers and electron-hole pairs trapped at oxygen-related defects, and give their spectral and temporal characteristics. We evaluate external quantum yield (EQY) of these recombination channels and conclude that the overall emission is determined by the excitonic channel, with the other contribution not exceeding 1‰. Subsequently, we investigate EQY of the excitonic emission as a function of excitation photon energy and find a significant increase for shorter wavelengths. This is explained in terms in exciton multiplication taking place for sufficiently large energies of the incoming photons.

Superlattice Photocathode for High Brightness and Long NEA-surface lifetime

We have suggested using a superlattice semiconductor instead of a bulk semiconductor as an NEA-photocathode for small emittance and high quantum yield because the mini-band energy width and high joint density of states at the bottom of the conduction band can bring small photoelectron's momentum spread and high quantum yield at excitation energy of the band gap. We developed the test sample of the AlGaAs-GaAs superlattice semiconductor designed by using Klonig-Penny-Bastard model. The obtained quantum yield spectrum from the AlGaAs-GaAs superlattice semiconductor after NEA-surface activation was a step function of excitation photon energy most likely caused by the quantum confinement effect.

Efficiency of spin injection in novel InAs quantum dot structures: exciton vs. free carrier injection

Unambiguous experimental evidence for a significant difference in efficiency of excitonic vs. free carrier spin injection is provided in novel laterally arranged self-assembled InAs/GaAs quantum dot structures, from optical orientation and tunable laser spectroscopy. A lower efficiency of exciton spin injection as compared to free carrier spin injection from wetting layers into QDs results in a distinct feature in luminescence polarization of the QDs as a function of excitation photon energy. It is shown that this difference is not related to carrier density and state-filling effects arising from the difference in optical absorption efficiency between the excitons and free carriers. Rather, it is a genuine property for exciton spin injection that suffers stronger spin relaxation due to Coulomb exchange interaction.

Manifestation of Auger Resonant Raman Effect on Double-Spectator Type Auger Transitions in the Ne [1s2p](3P)3p2 1P Resonant Double Excitation Region

We have measured the relative intensities of double-spectator type resonant Auger (RA) transitions leading to the Ne + [2 p 3 ]3 p 2 final ionic states from the [1 s 2 p ]( 3 P)3 p 2 1 P doubly excited state of Ne, where square brackets indicate hole states, as a function of excitation photon energy. The intensity curve measured across the double excitation resonance, which locates at 902.45 eV, was analyzed based on the theory of radiationless resonant Raman scattering. We present, for the first time, the results which have manifested the characteristic behaviour of the Auger resonant Raman effect on the double-spectator type RA transition associated with the [1 s 2 p ]3 p 2 multiply-excited intermediate state. The resonance energy and natural width of the doubly excited state determined by the Fano profile fitting to the absorption spectrum have been validated through the measurement with constant-ionic-state scan for these double-spectator type RA decay channels.

Photoluminescence excitation spectroscopy of semiconductor microcavities

Photoluminescence excitation (PLE) experiments in a ${\mathrm{Al}}_{0.25}{\mathrm{Ga}}_{0.75}\mathrm{As}$ microcavity, with GaAs quantum wells embedded in it, have revealed strong exciton-cavity coupling. By monitoring the low-temperature $(T=10\mathrm{K})$ photoluminescence (PL) from the GaAs substrate on which the microcavity is grown, we have observed a double peak of the PL intensity as a function of excitation photon energy in the 1.60--1.66 eV region. Based on the angular dependence of the separation between the peaks, we explain the two features in terms of optical absorption involving the quantum-well exciton and the cavity mode. The minimum separation between them (5 meV at an angle of about 25\ifmmode^\circ\else\textdegree\fi{}) corresponds to the maximum exciton-cavity coupling, as confirmed by reflectivity measurements performed under similar conditions. Our results show that PLE can complement or even substitute for other techniques that probe the optical properties of microcavities.

Generation and Transport of Photocarriers in Head-Tail Poly(3-Alkylthiophene) Films

ABSTRACTPhotovoltaic effects in Schottky Junction cells composed with Au/head-tail poly(3-hexylthiophene) (HT-P3HT)/Al have been studied as a function of excitation photon energy and temperature. It has been found that photocarriers are effectively generated at the interface of HT-P3HT/Al from the excitation spectra of photocurrents (photoaction spectra), which were obtained upon illumination to the Au or Al sides. The photoaction spectra also suggest that the photocarrier generation occurs at around 2.3 eV and 3.5 eV, which correspond to the peak of φ-φ* absorption and the photon energy with lower optical density region, respectively. The mechanisms of photocarrier generation are discussed taking the role of the Schottky junction into consideration.

Effects of Carrier Localization on the Optical Characteristics of MOCVD-Grown InGaN/GaN Heterostructures

We have studied both the spontaneous and stimulated emission (SE) properties as a function of excitation photon energy for InGaN/GaN multiple quantum wells (MQWs). A significant redshift of the SE peak with decreasing excitation photon energy was observed as the excitation photon energy was tuned below a certain photon energy (“mobility edge”) for the InGaN/GaN MQWs, with similar behavior observed for the spontaneous emission. The relative position of the mobility edge with respect to the absorption edge and the spontaneous and stimulated emission peak positions indicates the emission originates from carriers localized by extremely large potential fluctuations in the InGaN active layers of the MQWs. Therefore, carrier localization in the InGaN active regions explains the observed spontaneous and stimulated emission behaviors of these materials.

Energy selective optically pumped stimulated emission from InGaN/GaN multiple quantum wells

Optically pumped stimulated emission (SE) from InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition has been systematically studied as a function of excitation photon energy (Eexc) to further understand the origin of SE in these structures. Optically pumped SE was observed for excitation photon energies well below that of the absorption edge of the MQWs, indicating the states responsible for the soft absorption edge in these structures can efficiently couple carriers with the gain region. “Mobility edge”-type behavior in the SE peak was observed as Eexc was varied. The effective mobility edge measured in these SE experiments lies ∼110 meV above the main spontaneous emission peak and ∼62 meV above the SE peak. Tuning the excitation energy below the mobility edge was found to be accompanied by a drastic increase in the SE threshold due to a decrease in the effective absorption cross section. The experimental results indicate that the SE peak observed here has the same microscopic origin as the spontaneous emission peak, i.e., radiative recombination of localized states.

Anti-Stokes luminescence in nitrogen doped GaAs1−xPxalloys

In indirect band gap alloys, the Nx levels of the excitons bound to nitrogen have been resonantly excited. The detection of the photoluminescence (PL) at energies greater than the excitation energy (Anti-Stokes Luminescence (ASL)) shows a new PL band which appears at temperatures lower than 40 K. Its mean energy 2.205 eV is nearly independent of the alloy composition. We investigate ASL as a function of excitation photon energy, excitation power and temperature. Moreover ASL has been time resolved.

In-plane transport of excitons in quantum well structures

We have investigated the in-plane transport of photoexcited carriers and excitons in Al 0.3Ga 0.7As/GaAs quantum well structures as a function of excitation photon energy and temperature at low carrier density (10 8 cm -2). We have discovered a strong correlation between the diffusivity and the excitation photon energy; the exciton diffusivity becomes quenched as the excitation photon energy decreases closer to the heavy-hole exciton state. We have also found significant enhancement of diffusivity when a light-hole exciton state is resonantly excited. We attribute the enhancement to the difference between exciton scattering and free carrier scattering.

Relaxation of optically excited F centers

The relaxation of optically excited F centers was studied by resonant secondary radiation (RSR), which occurs successively as Raman scattering, hot luminescence and ordinary luminescence. The RSR and its linear polarization were measured as a function of excitation photon energy mainly at 77 K, for F centers in seven alkali halides. Dynamic processes, including these optical processes were analyzed in terms of a semi-classical model based on the Franck-Condon principle in an A 1g anharmonic adiabatic potential derived from the Born-Mayer potential. It is proposed that, abruptly after the optical excitation, the F electron expands its orbital radius and changes its nature including decoupling from the T 2g Jahn-Teller mode. This model works particularly for KCl, RbCl and NaCl. The relaxation of F centers in KBr and KI is different from that in KCl, RbCl and NaCl. The difference is related to the characteristic differences in lattice dynamics, namely predominant coupling with localized modes and the T 2g frequency near that of the LO phonons. The relaxation of type II F A centers is attributed to two steps, one F-like and the other Li +-dominated.

Picosecond Relaxation Processes of High Density Electron–Hole Plasma in CdSe

AbstractPicosecond time‐resolved spectra of the spontaneous luminescence from high density electron‐hole plasma in CdSe are measured at 4.2 K with changing excitation photon energy in the vicinity of the bandgap energy by using tunable picosecond pulses. Spectra observed are analyzed, and the electron‐hole concentration and effective carrier temperature of the plasma are obtained as a function of excitation photon energy as well as of the delay time. It is found that both the electron‐hole concentration and effective carrier temperature decrease remarkably for the first 150 ps after pulse excitation, and that thereafter the concentration and temperature remain almost unchanged at ≈3 × 1017 cm−3 and ≈20 K, respectively, until to the very late stage down to the 800 ps delay time. It is concluded that the effective carrier temperature does not fall below ≈20 K in spite of decreasing the excess energy given to the carriers at the instance of pulse excitation by means of changing the excitation photon energy. It is sure that there exists a process which makes the plasma hot. The phonon‐assisted Auger process is suggested as such a process.