Nonradiative Relaxation Channels Research Articles

Femtosecond Spectroscopy of Carrier Relaxation Dynamics in Type II CdSe/CdTe Tetrapod Heteronanostructures

Branched nanocrystal heterostructures synthesized from CdSe and CdTe exhibit a type II band structure alignment that induces separation of charge upon photoexcitation and localizes carriers to different regions of the tetrahedral geometry. The dynamics of carrier relaxation examined with femtosecond pump-probe spectroscopy showed heterostructures having rise times and biexponential decays longer than those of nanorods with similar dimensions. This is attributed to weaker interactions with surface states and nonradiative relaxation channels afforded by the type II alignment.

Vacuum ultraviolet excitation of the 1.9-eV emission band related to nonbridging oxygen hole centers in silica

We investigated the red-luminescence band at 1.9 eV in a $\ensuremath{\gamma}$ ray irradiated synthetic wet silica, under ultraviolet and vacuum-ultraviolet excitations in the temperature range 17--300 K. The excitation profile detected at $T=17\mathrm{K}$ evidences a broad band peaked at 6.4 eV, with full width at half maximum of 1.7 eV, besides the well-known component at 4.8 eV. On increasing the temperature up to $T=300\mathrm{K},$ for both ultraviolet and vacuum-ultraviolet excitations, the red luminescence decreases by a factor 10. These results give experimental support to the assignment of the vacuum-ultraviolet absorption transition to a nonbridging oxygen hole center, proposed in a recent paper [T. Suzuki et al., Phys. Rev. Lett. 90, 186404 (2003)]. Moreover, the thermal quenching of the red-luminescence evidences the activation of a nonradiative relaxation channel inhibiting the radiative emission from the excited state.

Photocreated carrier dynamics in isolated carbon nanotubes

We report the first combined photoluminescence and pump–probe measurements on single wall carbon nanotubes isolated in micelles. The bandgap photoluminescence is observed at room temperature and is concomittent with a drastic change in the photocreated carrier dynamics as compared to the one observed in usual macroscopic samples. Probing the fundamental bandgap leads to a non-exponential dynamics which is related to a lifetime distribution within the macroscopic nanotube population. Probing the second van Hove transition leads to a strong increase of the excitation lifetime which demonstrates that some non-radiative relaxation channels are quenched for isolated nanotubes. These observations constitute an important step in the understanding of the existence of photoluminescence in isolated nanotubes.

Magnetic field in-plane quantization and tuning of population inversion in a THz superlattice quantum cascade laser

The operation of a 4.4 THz, GaAs-based quantum cascade laser is investigated in a 0--7.5 T magnetic field applied parallel to the growth axis. Pronounced oscillations are found in the threshold current and output power as the magnetic field is increased. At 4.3 T the threshold current density is drastically reduced to $160{\mathrm{A}/\mathrm{c}\mathrm{m}}^{2}$ from the $290{\mathrm{A}/\mathrm{c}\mathrm{m}}^{2}$ measured at 0 T. We attribute these results to a modulation of the lifetime of the upper state of the laser transition owing to the combined effects of inter-Landau level resonances and a progressive quenching of nonradiative relaxation channels due to the additional in-plane quantization. The latter is also responsible for a dramatic decrease in the conductivity, with a reduction in the number of carriers injected into the upper state of the laser transition at fixed voltage. This process limits the maximum output power and eventually prevents lasing action at high magnetic fields.

Size-dependent electronic transition rates in cubic nanocrystalline europium doped yttria

In cubic nanocrystalline as well as polycrystalline Y 2O 3:Eu 3+ prepared by combustion, the radiative transition rate of 5D 1– 7F 1 and the nonradiative relaxation rate of 5D 1– 5D 0 were obtained by measuring the temperature-dependence of emission intensity and lifetime. The results indicate that the radiative and nonradiative rates both increase with decreasing particle size. The increase of radiative transition rate was attributed to the lower symmetry surrounding the Eu 3+ ions, while the increase of nonradiative relaxation rate to the additional nonradiative relaxation channels caused by surface defects.

Nonradiative relaxation of photoexcited O01 centers in glassy SiO2

The processes involved in the excited-state relaxation of hole O 1 0 centers at nonbridging oxygen atoms in glassy SiO2 were studied using luminescence, optical absorption, and photoelectron emission spectroscopy. An additional nonradiative relaxation channel, in addition to the intracenter quenching of the 1.9-eV luminescence band, was established to become operative at temperatures above 370 K. This effect manifests itself in experiments as a negative deviation of the temperature-dependent luminescence intensity from the well-known Mott law and is identified as thermally activated external quenching with an energy barrier of 0.46 eV. Nonradiative transitions initiate, within the external quenching temperature interval, the migration of excitation energy, followed by the creation of free electrons. In the final stages, this relaxation process becomes manifest in the form of spectral sensitization of electron photoemission, which is excited in the hole O 1 0 -center absorption band.

Effect of PMMA impregnation on the fluorescence quantum yield of sol–gel glasses doped with quinine sulfate

Abstract The fluorescence quantum yield of quinine sulfate in sol–gel and PMMA impregnated glasses is measured. The observed quantum yield improvement in the sol–gel matrix, compared to ethanol, is interpreted as a reduction of non-radiative relaxation channels by isolation of the molecules by the cage of the glass. PMMA impregnated sol–gel glasses show an extra improvement of the fluorescence yield, which is interpreted as a reduction of the free space and the rigid fixation of the molecules to the matrix.

III-Nitride Unipolar Light Emitting Devices

Unipolar light emitting device based on III-nitride superlattices for the generation of visible light is suggested. The main idea of the unipolar light emitting device (U-LED) is to create the analogue of an n–p junction between two n-type superlattices with a shallow and a deep subband. The superlattice with the shallow subband acts as an effective n-type semiconductor, whereas the superlattice with deep subband plays the role of an effective p-type semiconductor. The radiation arises due to the electron transitions from the shallow subband superlattice into the deep subband superlattice. The conduction band off-set between AlN and InN is about 3 eV and this value can be reduced using the alloys of AlGaN. This allows one to get electron transitions between two superlattices based on these alloys with energies in the range of 0 to 3 eV, which covers the visible and the infrared range of the spectra. The quantum efficiency of these transitions could be enhanced by inserting between two superlattices some optically active layer with two quantum states, which can be a specially designed quantum well, impurity layer or quantum dot layer. The active layer can be pumped directly electrically through the superlattice subbands. The efficiency of the device is limited by the non-radiative energy relaxation channel related to phonon emission and is rather low for the simple U-LED without an active layer or an U-LED with a single quantum well active layer. A significant increase in the efficiency can be achieved with the use of the active layer doped with deep acceptors. In this case the optical transitions take place from the quantum well subband of the active layer into the deep acceptor impurity band. The transition metals Fe and Ni are considered as possible deep acceptors for the active layer in GaInN/AlN superlattices.

Intrinsic defect-related blue-violet and ultraviolet photoluminescence from Si+-implanted fused silica

Photoluminescence (PL) and electron-spin resonance have been used to study intrinsic defects in fused silica during ion implantation and annealing procedures designed to form and H-passivate Si crystallites. Under 250 nm (5 eV) photon excitation, the unimplanted silica has PL bands at 390 (3.2 eV) and 288 nm (4.3 eV). Implantation with 400 keV Si ions creates paramagnetic defects but reduces both the 3.2 and 4.3 eV emissions. Implantation to doses ⩾2×1017Si cm−2 produces an additional weak emission band at 466 nm (2.7 eV). Annealing at 1000 °C and hydrogenation at 500 °C affect both the absolute and relative intensities of the 390 and 288 nm emissions, and this is discussed with reference to known defects in the Si-crystallite/silica system. However, the emissions remain weak compared to those in unimplanted silica even though annealing removes the paramagnetic defects produced by implantation and the weak 466 nm emission observed for high doses. Since no other emission is evident at lower energies, it is concluded that implantation either alters or destroys the defect configurations responsible for the 390 and 288 nm emissions or that it creates diamagnetic defects which offer competing nonradiative relaxation channels.

Non-radiative relaxation of Yb 3+ in highly porous γ-Al 2O 3

Sol–gel grown aluminum oxide is a highly porous material composed of a network of nano-crystalline γ-Al 2O 3 particles. The sol–gel process allows the incorporation of trivalent rare-earth ions into the crystal structure. These ions can be used to study, via optical techniques, the structure and dynamics of the nanoparticles in this material. Here we report on optical studies of Yb 3+-doped γ-Al 2O 3. We find non-exponential decay curves for the 2 F 5/2 to 2 F 7/2 Yb 3+ fluorescence and a strong temperature dependence of the decay rate between 4 and 300 K. We show that this behavior is not caused by confinement but is indicative of non-radiative relaxation channels. The non-radiative relaxation channel is provided by OH molecules at the surface of the γ-Al 2O 3 nanoparticles, demonstrating the importance of surface effects in nanocrystalline materials.

Photon-hopping conduction and collectively induced transparency in a photonic band gap.

We study the dynamics of photon-hopping conduction between impurity atoms in a photonic band gap via resonance dipole-dipole interaction (RDDI) in the low-excitation density limit. This is done both in the absence and in the presence of a localized dielectric defect mode. The random impurity atom positions are modeled by means of a Gaussian random distribution of RDDI's with variance J and atomic line fluctuations with variance \ensuremath{\delta}. By numerical calculation, we demonstrate the occurrence of a number of collective quantum features of photon-hopping conduction. These include collective enhancement of the energy-transfer rate and collectively induced transparency within the dielectric cavity mode. These effects are shown to depend sensitively on the variances J and \ensuremath{\delta}. Our results suggest that RDDI mediated hopping conduction may protect atomic excitation energy from dissipation through nonradiative relaxation channels.

Optical spectroscopy of Eu3+-doped silica gels

Abstract Laser techniques have been applied to study the spectroscopy of Eu3+ in silica gels. Site selection spectroscopy, resonant fluorescence line narrowing and lifetime measurement show a continuous evolution of the system as a function of the thermal treatments which produce the gel-to-glass transition. Two different environments for the Eu3+ ion, a liquid-like and a dry environment, have been observed during the initial steps of the dehydration process. At heat treatments below 250°C the non-radiative relaxation channels due to coupling with the O-H vibrational modes are very efficient. As the densification continues, clustering of Eu3+ induces strong energy transfer also at a relatively low concentration of the rare-earth ions. Site selection by laser excitation is indeed efficient only for non-densified samples. Dynamic interactions are site dependent as shown by the temperature dependence of the homogeneous linewidth taken at different energies within the 5D0↔7F0 line.

Optothermal detection of nonradiative relaxation channels in electronically excited molecules

Optothermal detection has been used to observe nonradiative relaxation channels in aniline, p-bromoaniline, and trans-stilbene. p-Bromoaniline has no detectable fluorescence due to a heavy atom effect which increases the rate of intersystem crossing to the triplet state. An optothermal spectrum of p-bromoaniline was observed with the origin at 32 625 cm−1. For trans-stilbene, the differences between the laser excitation spectrum and the optothermal spectrum of the S1 state clearly show the onset of isomerization at ∼1250 cm−1 above the origin. Absolute quantum yields of fluorescence, Franck–Condon factors, nonradiative rates, and radiative rates have been obtained for a series of vibronic transitions. For low energy vibrational states, there is good agreement between the current study and previous work. For vibrational energies above the barrier of isomerization, predicted quantum yields do not agree with our experimental results.

Electronic coupling and conformational barrier crossing of 9,9′-bifluorenyl studied in a supersonic jet

Fluorescence excitation, dispersed fluorescence, and picosecond time-resolved fluorescence spectroscopies have been used to study the dimeric molecule 9,9′-bifluorenyl, isolated under supersonic jet conditions. The excitation spectrum reveals a splitting in several principal resonances of the S0→S1 excitation spectrum, which can be attributed to electronic coupling between the two fluorenyl moieties. The splitting, which for different vibronic transitions correlates with Franck–Condon factors, is consistent with an exciton model that includes higher electronic states. The splitting is reasonably well reproduced by a multipole interaction potential. Calculations have verified that the electron exchange coupling is small. Furthermore, the relative intensities of the resonances allow an estimate of the equilibrium geometry, suggesting that the molecular long axes have an angular displacement of 60°–70°, consistent with the geometry found by x-ray diffraction. A most interesting feature of this species is that it is weakly fluorescent in fluid solution, which has been attributed to activated conformational barrier crossing of the excited molecule. In contrast, the fluorescence quantum efficiency of the isolated molecule can be close to unity, the lifetime ranging from 18 to 20 ns at vibrational energies <1400 cm−1. Above this region, the fluorescence decay time decreases steadily, to ≊2 ns by 2550 cm−1, indicating the onset of a nonradiative relaxation channel. Since the molecule was also seen to exhibit vibrational relaxation at low energies (i.e., ≤400 cm−1), the relaxation dynamics observed above 1400 cm−1 reflect the existence of a conformational potential energy barrier in the isolated molecule.

A time-resolved fluorescence study of matrix-isolated Ag 2

The nanosecond lifetimes of the A, B and C states of Ag 2 in Ar, Kr and Xe matrices were determined by time-resolved emission spectroscopy. From an analysis of the rise and decay times after pulsed optical excitation, the non-radiative relaxation channel between the B and A states was quantitatively established.