Strong Emission Enhancement Research Articles

Cathodoluminescence and atomic force microscopy study of n-type doped GaN epilayers

We evaluate influence of n-type doping and of the doping level on structural and optical quality of GaN epilayers. We confirm reported previously strong enhancement of light emission from Si doped samples and discuss possible mechanisms of this enhancement. Kelvin probe measurements indicate smoothing of potential fluctuations in the doped layers.

Photoluminescence investigation of europium-doped alumina, titania and indium sol–gel-derived films in porous anodic alumina

Photoluminescence (PL) properties of Eu-doped xerogel films fabricated on porous anodic alumina (PAA) and flat substrates are analysed with respect to the xerogel content (TiO 2, Al 2O 3, In 2O 3) and excitation wavelength. A strong enhancement of Eu-related emission in the broad UV range of the excitation has been found for the three xerogel systems. However, the enhancement factor is not simple to interpret, it possesses rather a complex nature. We have proposed that the increase of PL intensity is composed of two components. The first component is associated with an improvement of the energy transfer from the host xerogel matrix to lanthanide ions. The second one is evoked by multiple scattering of exciting light in the pore channels. This additional path of the excitation and energy transfer improves essentially the PL of lanthanide ions. Moreover, in periodic matrix of PAA the emission of lanthanide is redistributed by angle contributing into PL enhancement.

A Ratiometric Fluorescent Sensor for AgI with High Selectivity and Sensitivity

A bicyclic cycloadduct 1 bearing a pyrenyl moiety has been synthesized and investigated as a ratiometric fluorescent sensor for AgI. In an aqueous ethanol solution of 1, the presence of silver ion induces the formation of a 1:2 metal-ligand complex, which exhibits a strong intensity enhancement of the pyrene excimer emission at the expense of the emission of monomeric pyrene.

Optical second harmonic generation near a black hole horizon as possible source of experimental information on quantum gravitational effects

Optical second harmonic (SH) generation near a black hole horizon is suggested as a source of information on quantum gravitational effects. Localization phenomena in light scattering from random fluctuations of matter fields and space–time metric near the horizon produce a pronounced peak in the angular distribution of SH light in the direction normal to the horizon. This phenomenon is similar to the strong enhancement of diffuse SH emission from a randomly rough metal surface in the normal direction.

Strong enhancement of spontaneous emission in amorphous-silicon-nitride photonic crystal based coupled-microcavity structures

We investigated photoluminescence (PL) from one-dimensional photonic band gap structures. The photonic crystals, a Fabry–Perot (FP) resonator and a coupled-microcavity (CMC) structure, were fabricated by using alternating hydrogenated amorphous-silicon-nitride and hydrogenated amorphous-silicon-oxide layers. It was observed that these structures strongly modify the PL spectra from optically active amorphous-silicon-nitride thin films. Narrow-band and wide-band PL spectra were achieved in the FP microcavity and the CMC structure, respectively. The angle dependence of PL peak of the FP resonator was also investigated. We also observed that the spontaneous emission increased drastically at the coupled-cavity band edge of the CMC structure due to extremely low group velocity and long photon lifetime. The measurements agree well with the transfer-matrix method results and the prediction of the tight-binding approximation.

Radio-frequency plasma jet generator of singlet delta oxygen with high yield

O 2 (a 1 Δ g ) was generated in a flowing discharge of a radio-frequency (rf) hollow electrode. The radio frequency was 99.9 MHz and the rf power was 200 W. The discharge was done in the gas mixture O2:N2:NO=200:20:10 sccm and then it was chilled reactively by the mixture Ar:NO2=200:10 sccm. The O2(a 1Δg) relative yield of 32% was achieved at the pressure of 0.43 Torr. Usage of the mixtures O2:NO=200:100 sccm and Ar:NO2=100:100 sccm resulted in the O2(a 1Δg) yield of 25% at the pressure of 0.6–0.9 Torr. The effluent was mixed with molecular iodine in a far afterglow region and it was tested in an oxygen–iodine laser. The iodine flow rate was 0.3 mmol/min. A strong enhancement of atomic iodine spontaneous emission at the wavelength of 1315 nm was observed in the optical resonator.

Luminescence, energy transfer and anti-Stokes PL in wide band-gap semimagnetic nanostructures

A strong enhancement of exciton emission accompanied by a line narrowing has been found for (Zn,Cd,Mn)Se/ZnSe QW and QW-disc structures with increasing magnetic field. The line narrowing is caused by a field-induced smoothing of the compositional disorder potential on a microscopic and mesoscopic scale, whereas the intensity increases due to the spin dependence of the energy transfer into the internal Mn 2+(3d 5) states. In specially designed double QW structures a huge anti-Stokes PL has been observed.

Spontaneous Emission in Cylindrical Periodically-Layered Structures

This paper studies the effects of cylindrical dielectric periodically-layered structures on spontaneous emission from an excited dipole line source. The source is modeled as a forced oscillator with radiative damping. The underlying physics lies in how the oscillator can be driven by the reflected field supported by this structure. To match the boundary conditions of EM waves, the transfer matrix method (TMM) is employed for its simplicity and versatility. Both the frequency shift and modified radiative damping rate are formulated analytically. It is shown that due to interference the wave supported by the structures can affect the source emission significantly. Strong enhancement and inhibition of the emission depend on both the layered structure and the frequency. Different compositions of the structures are achieved by tuning the structure parameters such as the reflective index and the width of layers. Furthermore, the results reveal that the strong enhancement and inhibition are sensitive to the layer width. It is found that strong inhibition is stable against source position variation, whereas strong enhancement is quite unstable. Potential applications in cylindrical quantum well lasers and semiconductor devices and perspective for future work are suggested.

Optical and structural studies of the effect of spacers in modulation-doped ZnSe/Zn1−xCdxSe quantum wells

The effect of spacers in modulation-doped Zn1−xCdxSe/ZnSe:Cl multiple quantum wells (MD-MQWs) was investigated by photoluminescence (PL) and time-of-flight secondary-ion-mass spectrometry (TOF-SIMS). A comparison was made between structures with and without spacers as a function of annealing temperature. The diffusion of Cl and Cd was monitored by TOF-SIMS depth profiling and photoluminescence. Although TOF-SIMS does not show any significant diffusion of Cl and Cd in both structures at temperatures up to 385 °C, the PL results indicate the modification of optical properties in the Zn1−xCdxSe/ZnSe:Cl MD-MQWs due to annealing. Up to an annealing temperature of 385 °C, the MD-MQWs with spacers show superior optical quality in the quantum well regions, while quenching of the quantum well band-edge PL and strong enhancement of deep-level emission were observed from the MD-MQWs without spacers. This phenomenon suggests that the radiative deep-level emission may provide more efficient channel for electron–hole recombination with increasing annealing temperature.

ULF-ELF-VLF-HF plasma wave observations in the polar cusp onboard high and low altitude satellites

Satellite Prognoz-8 with high apogee and inclination 65° was operating in 1981. During its operation time a few crossings of the polar cusp were registered. Magion -4 being the subsatellite for Interball-1 (Tail Probe) has a similar orbit. In January, April and May 1996 the registration of the low frequency plasma waves during three crossing of the polar cusp were performed. A comparison of Prognoz-8 and Magion-4 measurements gives the opportunity to identify characteristic modes of plasma waves. There are ion-cyclotron, lower-hybrid and electron cyclotron frequencies registered. In our presentation, the spectra of electric and magnetic field fluctuation and correlation with the plasma and magnetic field data are given. For better understanding the physical processes in the cusp region there are presented HF waves, high energy particle and thermal measurements detected at the low altitude satellite Apex-Magion 3 complex and Coronas. HF wave measurements indicate an enhancement of radiation in the cusp region. The observations done in the day sector at an altitude above foF2 and during substorm indicate emissions in whistler mode beside the strong enhancement of broad band radio emission which cross the local gyrofrequency and spread up to its local electron density.

Photon energy dependence of valence band photoemission and resonant photoemission of polystyrene

The photon energy dependence of valence band photoemission from polystyrene thin films has been investigated using synchrotron radiation in the range 150–450 eV, covering the carbon K edge. Six valence peaks were found which show an energy dependence that can be qualitatively explained by the relative contributions of 2s and 2p atomic orbitals to the molecular orbital photoemission cross-section. The experimental results are generally in agreement with valence effective Hamiltonian (VEH) calculations in the literature. In the vicinity of the carbon 1s absorption edge, there is a strong enhancement of the valence band emission because of resonant photoemission, reported here in organic polymers for the first time. The relative contribution of spectator and participator decay channels varies among the valence orbitals, in a way that can be associated with the spatial distribution of the benzene-derived molecular orbitals involved in the resonance transition.

The UK FEL project: Status and measurement of optical gain

The aim of the UK FEL project is to study the characteristics of a single-pass FEL over a wide operating range, using a four-section 5 m wiggler yielding a maximum K value of 2.7. The laser is driven by the 165 MeV Kelvin Laboratory linac. It has a design operating wavelength range of 2–20 μm, although higher harmonics are also under investigation. We report direct measurement of optical gain using a cw CO 2 laser between 9 and 11 μm. Simultaneous spectra are obtained of both gain and spontaneous emission as a function of linac energy or wiggler field as the tuning parameter. The linewidth obtained is close to that expected for homogeneous broadening, and the gain is consistent with that predicted theoretically for the current available. Strong enhancement of both spontaneous emission and measured gain is obtained in the presence of a high- Q FEL cavity, as expected by simple considerations of optical confinement. These results imply that despite the shortness of our macropulse (∼ 2.5 μm), it will be possible to diagnose buildup to oscillation from an injected signal.

Effect of Easily Ionized Elements upon the Spatial Emission Properties of Microwave-Induced Plasmas

Spatially resolved measurements of emission from microwave-induced plasmas (MIPs) sustained in helium in a Beenakker resonant cavity and in a surfatron have been compared and contrasted. Particular emphasis has been placed upon the study of a common interference effect in MIP sources, namely the enhancement (or suppression in some instances) of analyte emission intensity by matrices containing an easily ionized element (EIE). The presence of the EIE caused a strong enhancement of analyte emission in the Beenakker cavity system but had a relatively smaller influence upon analyte emission in the surfatron system at higher concentrations of EIE. The difference in behavior between the two systems toward EIEs appears to be attributable to improved sample injection into the plasma in the case of the surfatron.

Europium phosphorescence as a probe of binding to phospholipids

The binding of Eu 3+ to phospholipids was studied by pulsed, time-resolved fluorimetry (phosphorescence). Binding of Eu 3+ results in strong enhancement of light emission measured 0.4–0.9 ms after exciting flashes of 0.5 μs duration. The light emission was largest with acidic phospholipids, proportional to the amount of added Eu 3+ over a wide range, but not linearly related to the amount of phospholipids. The technique could also be adapted to the study of the distances between lanthanide binding sites by studying energy transfer coupling to Tb 3+.

Inverse photoemission from Ni(100) and Ni(111) with oriented oxygen overlayers

Bremsstrahlung isochromat spectra have been recorded at ℏω0=9.7 eV for clean‐ and oxygen‐covered Ni(100) and Ni(111) surfaces. Angle‐integrated and normal incidence angle‐resolved spectra are presented. In angle‐integrated spectra, ordered c(2×2) and p(2×2) oxygen overlayers on Ni(100) and Ni(111), respectively, cause strong emission enhancement just above the Fermi level. This is attributed to adsorbate‐induced antibonding empty electronic states. Energetic position and width of these states is different on the two crystal faces. Normal incidence spectra behave similarly upon oxygen adsorption but are more complicated due to extra emission from strong direct interband transitions.