Resonant Excitation Conditions Research Articles

On carrier spillover in c- and m-plane InGaN light emitting diodes

The internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) in InGaN light-emitting diodes (LEDs) emitting at 400 nm with and without electron blocking layers (EBLs) on c-plane GaN and m-plane GaN were investigated in order to shed some light on any effect of polarization charge induced field on efficiency killer carrier spillover. Without an EBL the EQE values suffered considerably (by 80%) for both orientations, which is clearly attributable to carrier spillover. Substantial carrier spillover in both polarities, therefore, suggests that the polarization charge is not the major factor in efficiency degradation observed, particularly at high injection levels. Furthermore, the m-plane variety with EBL did not show any discernable efficiency degradation up to a maximum current density of 2250 A cm−2 employed while that on c-plane showed a reduction by ∼40%. In addition, IQE of m-plane LED structure determined from excitation power dependent photoluminescence was ∼80% compared to 50% in c-plane LEDs under resonant and moderate excitation condition. This too is indicative of the superiority of m-plane LED structures, most probably due to relatively larger optical matrix elements for m-plane orientation.

Dynamics of the magnetic-field-induced optical alignment of triplet bound excitons in GaSe under resonant excitation conditions

The dynamics of the transverse-magnetic-field-induced linear polarization of the radiation of triplet bound excitons in uniaxial crystals has been studied by time-resolved spectroscopy on an example of GaSe under resonant excitation by linearly polarized light. The linear polarization of the radiation arises due to the difference between the behaviors of its π- and σ-components in a magnetic field. It has been found that the magnetic-field dependences of the π- and σ-component intensities are functions of the pump light polarization and vary considerably with the lifetime t of the excited states. It has been revealed that the resonant excitation of excitons by light polarized with E∥B and E ⊥ B is accompanied by the magnetic-field-induced alignment of their dipole moments along the vector E of the pump light. In the case of excitation by light with E ⊥ B, the optical alignment of excitons is observed only in the initial period of their life, and the linear polarization of exciton radiation changes sign at a certain t>t0. A theoretical description of these effects is proposed.

Angular anisotropy of the RCE X-rays under planar channeling as manifestation of geometric properties of the in-crystal electric field

Planar channeled ion under resonant coherent excitation conditions experiences an action of the oscillating electric field arising in the ion rest frame. We show how the alignment of the angular momentum of coherently excited ions and, hence, the angular anisotropy of their characteristic X-ray radiation are connected with the geometrical and symmetry properties of this field. The consideration is based on two examples of ( k , l ) = ( 2 , - 1 ) and (1, 3) resonances with 423 MeV/u Fe 24 + ions in ( 2 2 ¯ 0 ) planar channel of Si crystal, corresponding to different symmetries of the resonant field. In both cases the resonant electric field is elliptically polarized. A choice of an appropriate coordinate frame allows us to show the connection between the geometrical properties of the resonant field and the X-ray angular distributions especially clear. To illustrate, we calculate angular distributions of X-rays for individual ionic trajectories using density matrix formalism and then consider formation of the angular distributions not resolved by ion trajectory. Comparison with recent experimental data is done.

Radiation force mediated by exciton of a carbon nanotube

AbstractResonant radiation force exerted on a single‐walled carbon nanotube is theoretically studied. Even under weak laser irradiation at room temperature, the acceleration of a nanotube caused by a radiation force becomes significantly large in the resonant excitation conditions of an exciton.The spectral peak frequencies of the acceleration are sensitive to the diameter of nanotubes and to the light polarization. These features are quite useful for nondestructive remote sorting and collection of nanotubes with desired structure and alignment. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Time dependence of a level anticrossing signal in exciton emission from a GaSe crystal under resonant excitation conditions

The time dependence of the Zeeman-sublevel anticrossing signal in triplet-bound exciton emission from gallium selenide has been investigated under resonant excitation conditions. It has been found that the shape of the anticrossing signal varies during the lifetime of the excited state. The time dependence of the anticrossing signal in the case of the resonant excitation of exciton states differs significantly from the corresponding dependence observed in the case of the band-to-band excitation of luminescence. A theoretical description is proposed for the observed effects.

Photoluminescence of “dark” excitons in CdMnTe quantum well, embedded in a microcavity

A diluted magnetic semiconductor (DMS) quantum well (QW) microcavity operating in the limit of the strong coupling regime is studied by magnetoptical experiments. The interest of DMS QW relies on the possibility to vary the excitonic resonance over a wide range of energies by applying an external magnetic field, typically about 30 meV for 5 T in our sample. In particular, the anticrossing between the QW exciton and the cavity mode can be tuned by the external field. We observe the anticrossing and formation of exciton polaritons in magneto-reflectivity experiments. In contrast, magneto-luminescence exhibits purely excitonic character. Under resonant excitation conditions an additional emission line is observed at the energy of the dark exciton. The creation of dark excitons is made possible due to heavy hole–light hole mixing in the QW. The emission at this energy could be due to a combined spin flip of an electron and a bright exciton recombination.

Anomalous reflection and excitation of surface waves in metamaterials

We consider reflection of electromagnetic waves from layered structures with various dielectric and magnetic properties, including metamaterials. Assuming periodic variations in the permittivity, we find that the reflection is in general anomalous. In particular, we note that the specular reflection vanishes and that the incident energy is totally reflected in the backward direction, when the conditions for resonant excitation of leaking surface waves are fulfilled.

Resonant excitation photoluminescence studies of InGaN∕GaN single quantum well structures

The optical properties of InGaN∕GaN quantum well structures, with indium fractions of 0.15 and 0.25, have been studied under resonant excitation conditions. The low-temperature (T=6K) photoluminescence spectra revealed a broad recombination peak that the authors have attributed to the acoustic-phonon assisted emission from a distribution of localized states, excited via an acoustic-phonon assisted absorption process. Comparing these results with theoretical calculations, where the authors consider the deformation potential coupling of the separately localized electron/hole pairs to an effectively continuous distribution of acoustic phonons, gives a value of approximately 2.5Å for the in-plane localization length scale.

Resonant photoluminescence excitation studies of InGaN/GaN single quantum wells

AbstractThe optical properties of InGaN/GaN single quantum well structures, with indium fractions of 0.15 and 0.25, have been studied under resonant excitation conditions. The low temperature (T = 6 K) photoluminescence spectra show a broad recombination feature that we attribute to acoustic phonon accompanied absorption and emission from a distribution of localized states. Also we observe in the sample with the indium fraction of 0.15 a sharp line that we attribute to LO phonon accompanied recombination of excitons in the resonantly excited localized states. Comparing the form of the spectra with model calculations, where we consider the deformation potential coupling of localized excitons to an effectively continuous distribution of acoustic phonons, enables us to show that the in‐plane localization length scale of the excitons may be as small as 2.5 Å. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Contribution of two-photon and excited state absorption in ‘axial-bonding’ type hybrid porphyrin arrays under resonant electronic excitation

Two-photon absorption (2PA) and excited state absorption (ESA) properties of ‘axial-bonding’ type hybrid porphyrin arrays are investigated in the picosecond regime under single-photon resonant excitation condition. A crossover from reverse saturable absorption (RSA) to saturable absorption (SA) and back to RSA is observed with the increase of excitation intensity. In the corresponding intensity ranges, third- and fifth-order phase conjugate signals from degenerate four wave mixing are observed. These observations are explained using rate equations for population densities in a three-level energy scheme. At higher intensities, 2PA is found to be dominant contributor to nonlinear absorption compared to ESA.

Photoluminescence from impurity codoped and compensated Si nanocrystals

Photoluminescence (PL) properties of B and P codoped and compensated Si nanocrystals were studied. The compensation of carriers in nanocrystals was confirmed by the annihilation of confined-carrier optical absorption in the infrared region. In the PL spectra obtained under the resonant excitation condition, the codoped samples did not exhibit structures related to momentum-conserving phonons, which were clearly observed for pure Si nanocrystals. The result strongly suggests that in impurity codoped Si nanocrystals, nonphonon quasidirect optical transition is the dominant recombination path for electron-hole pairs, and thus impurity codoping is a possible approach to further improving PL efficiency of Si nanocrystals.

Excitation and propagation of surface plasmon polaritons on the gold covered conical tip

The phenomenon of resonant excitation of surface plasmon polaritons (SPP) at the gold covered conical tip of an optical fiber is considered. The SPP mode is excited by the energy of the waveguide mode. The SPP resonant excitation condition is realized at the presence of an additional transparent dielectric coating of certain thickness on the metal surface. During the experiment, we detected an output radiation from the tip when the tip came out of the liquid (glycerin). The peaks of output optical intensity are obtained. This is explained by the resonant excitation of SPP due to the formation of a liquid layer in the form of meniscus surrounding the tip. This mechanism of SPP excitation in the region of the tip of the optical waveguide opens up new possibilities for increasing the resolution of the nanometric scanning optical microscope.

Semiquantitative theory of electronic Raman scattering from medium-size quantum dots

A consistent semiquantitative theoretical analysis of electronic Raman scattering from many-electron quantum dots under resonance excitation conditions has been performed. The theory is based on random-phase-approximation-like wave functions, with the Coulomb interactions treated exactly, and hole valence-band mixing accounted for within the Kohn-Luttinger Hamiltonian framework. The widths of intermediate and final states in the scattering process, although treated phenomenologically, play a significant role in the calculations, particularly for well above band gap excitation. The calculated polarized and unpolarized Raman spectra reveal a great complexity of features and details when the incident light energy is swept from below, through, and above the quantum dot band gap. Incoming and outgoing resonances dramatically modify the Raman intensities of the single particle, charge density, and spin density excitations. The theoretical results are presented in detail and discussed with regard to experimental observations.

Conversion between Three- and Two-Dimensional Optical Waves in Attenuated Total Reflection Kretschmann Configuration with Nanostructured Langmuir-Blodgett Films

Surface plasmon (SP) emission light has been investigated for rhodamine-B (RB) dye Langmuir-Blodgett (LB) films on silver (Ag) thin film in the attenuated total reflection (ATR) Kretschmann configuration by means of reverse irradiation of a laser beam. Emission light through the prism was clearly observed for the configuration of prism/Ag/LB films corresponding to the resonant excitation conditions for SPs using the reverse irradiation. The intensities of the SP emission light increased with the thickness of the RB LB films, but nonlinear reductions of the SP emission light and photoluminescence (PL) were observed due to nonradiative energy transfer at the very small separations between the RB and Ag films. It is thought that the SPs, that is, two-dimensional optical waves, are excited by PL of the RB dyes on the Ag film and that this phenomenon is very useful for applications utilizing dye molecules on metal thin films.

Giant enhancement of polariton relaxation in semiconductor microcavities by polariton-free carrier interaction: Experimental evidence and theory

Very large changes in the population distributions of microcavity polaritons are reported under conditions of simultaneous resonant and nonresonant excitation. The strong relaxation bottleneck, which results in weak emission from the lower polariton branch for resonant excitation alone, is fully suppressed if additional nonresonant excitation is employed. By comparison with theory, the massive enhancement of the polariton relaxation is shown to arise from scattering of polaritons by free carriers, present in quantum wells due to residual doping, which are then heated by the nonresonant excitation.

Efficient polyatomic interference reduction in plasma-source mass spectrometry via collision induced dissociation

Evidence is provided that illustrates quadrupole ion traps can be used to selectively attenuate strongly bound diatomic ions occurring at the same nominal mass as an analyte ion of interest. Dissociation rates for TaO+ (D0 ∼ 750 kJ mol−1) are found to be at least an order of magnitude larger than the loss rate of Au+ due to scattering under “slow heating” resonance excitation conditions at qz = 0.67 and using neon as the bath gas. This rate difference is sufficient for the selective removal of this strongly-bound diatomic ion over the loss of the Au+ at the same mass-to-charge ratio. Other examples of quadrupole ion trap CID for the selective reduction of common plasma-generated species are also evaluated by examining the dissociation of GdO+ in the presence of Yb+, and Cu2+ in the presence of Te+. In each case, a different method of applying the excitation signals is presented, and the attenuation rates for the diatomic species due to CID are substantially larger than scattering losses for the bare metal ions. Evidence is also presented that demonstrates CID can be accomplished in concert with a slow mass analysis scan, thereby providing a means of (1) eliminating polyatomic ions (formed in the plasma or reaction cell) over an extended mass range, (2) recovering metal ion signal from the metal-containing polyatomic ions, and (3) minimizing deleterious secondary reactions of product ions.

Longitudinal-optical-phonon-assisted energy relaxation in self-assembled CdS quantum dots embedded in ZnSe

The energy relaxation processes of CdS self-assembled quantum dots (QDs) embedded in ZnSe were investigated. Longitudinal-optical (LO)-phonon resonant structures were discerned in the photoluminescence spectra under the CdS selective excitation, especially under nearly resonant excitation conditions. Strong energy selectivity among the Gaussian distributed energy levels of the QDs are observed in energy relaxation processes, in which excitons are dominantly injected into the QDs whose ground state energies with respect to the excitation energy are equal to the multi-LO-phonon energy of ZnSe. Detection-energy dependent photoluminescence excitation (PLE) measurements also suggest that the coupling strength between the excitons and LO phonons is strongly size-dependent, i.e., coupling is strongly enhanced in smaller QDs. In addition, type-II band alignment of the CdS/ZnSe heterostructure is experimentally confirmed directly by PLE measurements.

High-occupancy effects and stimulation phenomena in semiconductor microcavities

This paper describes recent work on high-occupancy effects in semiconductor microcavities, with emphasis on the variety of new physics and the potential for applications that has been demonstrated recently. It is shown that the ability to manipulate both exciton and photon properties, and how they interact together to form strongly coupled exciton-photon coupled modes, exciton polaritons, leads to a number of very interesting phenomena, which are either difficult or impossible to achieve in bulk semiconductors or quantum wells. The very low polariton density of states enables state occupancies greater than one to be easily achieved, and hence stimulation phenomena to be realized under conditions of resonant excitation. The particular form of the lower polariton dispersion curve in microcavities allows energy and momentum conserving polariton-polariton scattering under resonant excitation. Stimulated scattering of the bosonic quasi-particles occurs to the emitting state at the center of the Brillouin zone, and to a companion state at high wave vector. The stimulation phenomena lead to the formation of highly occupied states with macroscopic coherence in two specific regions of k space. The results are contrasted with phenomena that occur under conditions of nonresonant excitation. Prospects to achieve "polariton lasing" under nonresonant excitation, and high-gain, room-temperature ultrafast amplifiers and low-threshold optical parametric oscillator under resonant excitation conditions are discussed.

Stereochemical effects from doubly-charged iron clusters for the structural elucidation of diastereomeric monosaccharides using ESI/IT-MS

Several approaches may be used to contribute to the structural elucidation of carbohydrates. Distinction of diastereomeric monosaccharides is often a tedious task by mass spectrometry, and ultimately the monosaccharide stereochemistry must be determined. Diastereomers of glucose, mannose, galactose and talose stereochemically differ at the C (2) and C (4) positions and previous work described an efficient system used to differentiate these four monosaccharides easily. The approach was based on ESI mass spectrometry and scrutinized the behavior of a sugar molecule towards the iron(II) chloride and the resulting effects in the gas phase. The produced cationized monomeric [MFe IICl] + and dimeric [M 2Fe IICl] + ions as singly-charged species were studied by sequential MS/MS experiments. When submitted to low resonant excitation conditions these selected ions undergo fragmentation within a kinetic control (i.e., stereochemical effects on the rate constants of competitive unimolecular processes). Enhancement of stereochemical effects was displayed in the CID spectra of the cationized [M 2Fe IICl] + dimeric ions. The present work first reports the influence of the sample preparation on the produced cationized species using the electrospray process. Formation of doubly-charged cluster M n Fe 2+ ions when the sugar molecule (M) is introduced in excess with the iron(II) chloride was observed. Secondly, behavior of these cluster ions under resonant excitation conditions reveals an increase of stereochemical effects compared with that observed in the case of singly-charged monomeric [MFe IICl] + and dimeric [M 2Fe IICl] + ions previously studied [Eur J Mass Spectrom 6 (2000) 421; Eur J Mass Spectrom 7 (2001) 331]. The dissociation pathways of doubly-charged complexes appeared to depend upon the n number of monosaccharides which constitute the formal ligand of the iron(II). M n Fe 2+ cluster ions are useful to efficiently determine the stereochemistry of the investigated monosaccharides and represent an advantageous alternative for the structural elucidation of their respective stereochemistry.

Carrier Recombination in InAs/GaAs Self-Assembled Quantum Dots under Resonant Excitation Conditions

Resonant photoluminescence experiments have been performed on self-assembled quantum dots. The emission bands measured in the investigated samples can be deconvoluted in several Gaussian components, which could be related to different size families of dots. The experimental results reveal the importance of GaAs phonons for the carrier relaxation of excess energy, specially for the sample with low dot density. With increasing temperature, electrons from smaller dots can be activated to reach the wetting layer electron states, from which they can be trapped at larger dots (below 100 K) or escape to the GaAs barriers (above 100 K).