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2.
Nano Lett ; 13(8): 3539-45, 2013 Aug 14.
Article in English | MEDLINE | ID: mdl-23822764

ABSTRACT

In this study, we investigate for the first time morphological and compositional changes of silicon quantum dot (SiQD) light-emitting diodes (SiLEDs) upon device operation. By means of advanced transmission electron microscopy (TEM) analysis including energy filtered TEM (EFTEM) and energy dispersive X-ray (EDX) spectroscopy, we observe drastic morphological changes and degradation for SiLEDs operated under high applied voltage ultimately leading to device failure. However, SiLEDs built from size-separated SiQDs operating under normal conditions show no morphological and compositional changes and the biexponential loss in electroluminescence seems to be correlated to chemical and physical degradation of the SiQDs. By contrast, we found that, for SiLEDs fabricated from polydisperse SiQDs, device degradation is more pronounced with three main modes of failure contributing to the reduced overall lifetime compared to those prepared from size-separated SiQDs. With this newfound knowledge, it is possible to devise ways to increase the lifetimes of SiLEDs.

3.
Opt Express ; 21(1): 594-604, 2013 Jan 14.
Article in English | MEDLINE | ID: mdl-23388952

ABSTRACT

Fabrication of small nanoantennas with high aspect ratios via electron beam lithography is at the current technical limit of nanofabrication and hence significant deviations from the intended shape of small nanobars occur. Via numerical simulations, we investigate the influence of geometrical variations of gap nanoantennas, having dimensions on the order of only a few tens of nanometers. We show that those deviations have a significant influence on the performance of such nanoantennas. In particular, their resonance wavelength as well as the magnitude of absorption and scattering cross section and the electric field distribution in the near field is strongly altered. Our findings are thus of importance for applications based on near field as well as those based on far field interactions with nanoantennas and have to be carefully and individually considered in both cases.

4.
J Microsc ; 243(1): 31-9, 2011 Jul.
Article in English | MEDLINE | ID: mdl-21155995

ABSTRACT

High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) at low energies (≤30 keV) was used to study quantitatively electron scattering in amorphous carbon and carbon-based materials. Experimental HAADF STEM intensities from samples with well-known composition and thickness are compared with results of Monte Carlo simulations and semiempirical equations describing multiple electron scattering. A well-defined relationship is found between the maximum HAADF STEM intensity and sample thickness which is exploited (a) to derive a quantitative description for the mean quadratic scattering angle and (b) to calculate the transmitted HAADF STEM intensity as a function of the relevant materials parameters and electron energy. The formalism can be also applied to determine TEM sample thicknesses by minimizing the contrast of the sample as a function of the electron energy.

5.
Opt Express ; 18(5): 4939-47, 2010 Mar 01.
Article in English | MEDLINE | ID: mdl-20389505

ABSTRACT

The photo-Dember effect is a source of impulsive THz emission following femtosecond pulsed optical excitation. This emission results from the ultrafast spatial separation of electron-hole pairs in strong carrier gradients due to their different diffusion coefficients. The associated time dependent polarization is oriented perpendicular to the excited surface which is inaptly for efficient out coupling of THz radiation. We propose a scheme for generating strong carrier gradients parallel to the excited surface. The resulting photo-Dember currents are oriented in the same direction and emit THz radiation into the favorable direction perpendicular to the surface. This effect is demonstrated for GaAs and In(0.53)Ga(0.47)As. Surprisingly the photo-Dember THz emitters provide higher bandwidth than photoconductive emitters. Multiplexing of phase coherent photo-Dember currents by periodically tailoring the photoexcited spatial carrier distribution gives rise to a strongly enhanced THz emission, which reaches electric field amplitudes comparable to a high-efficiency externally biased photoconductive emitter.

6.
Opt Express ; 18(6): 5890-5, 2010 Mar 15.
Article in English | MEDLINE | ID: mdl-20389606

ABSTRACT

We modified the optical properties of organic semiconductor distributed feedback lasers by introducing a high refractive index layer consisting of tantalum pentoxide between the substrate and the active material layer. A thin film of tris-(8-hydroxyquinoline) aluminium doped with the laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylamino-styryl)-4H-pyran was used as the active layer. By varying the intermediate layer thickness we could change the effective refractive index of the guided laser mode and thus the laser wavelength. With this technique we were able to tune the laser emission range between 613 nm and 667 nm. For high index layer thicknesses higher than 40 nm the laser operated on the TE(1)-mode rather than the fundamental TE(0)-mode.


Subject(s)
Lasers, Semiconductor , Organic Chemicals/chemistry , Refractometry/instrumentation , Equipment Design , Equipment Failure Analysis
7.
Opt Lett ; 31(11): 1669-71, 2006 Jun 01.
Article in English | MEDLINE | ID: mdl-16688256

ABSTRACT

We report the first realization of a cw solid-state dye laser. The laser medium consists of a laser dye (Rhodamine 6G) dissolved in a photopolymer. The UV-cured solution is sandwiched between two DVD substrates. The resonator design was derived from a conventional liquid solvent dye laser geometry. The laser radiation can be tuned from 565 to 615 nm by using a birefringent filter. A pump power of 2 W leads to a cw output power of more than 20 mW.

8.
Opt Lett ; 31(8): 1157-9, 2006 Apr 15.
Article in English | MEDLINE | ID: mdl-16625935

ABSTRACT

We have realized a 76 MHz white-light differential transmission spectroscopy system. The technique employs a Ti:sapphire laser oscillator and a tapered fiber to generate a white-light continuum spanning almost the full visible to near-infrared spectral range. Using acousto-optical modulation and subsequent lock-in detection, transient relative transmission changes as small as 10(-5) are detected. The method is applied to study the ultrafast gain dynamics of the active layer of a vertical-external-cavity surface-emitting laser based on a multiple-quantum-well structure.

9.
Phys Rev Lett ; 91(26 Pt 1): 267403, 2003 Dec 31.
Article in English | MEDLINE | ID: mdl-14754089

ABSTRACT

Using low temperature single molecule spectroscopy on rigid-rod conjugated polymers we are able to identify homogeneously broadened, strongly polarized emission from individual chromophore units on a single chain. Gated fluorescence spectroscopy allows real time imaging of intramolecular energy transfer as the chain behaves as a series of weakly interacting chromophores. Energy transfer is controlled by the chromophoric spectral linewidth, which depends on temperature. Linewidths exceeding intramolecular disorder lead to incoherent chromophore coupling and collective fluorescence phenomena.

10.
Berl Munch Tierarztl Wochenschr ; 115(5-6): 179-85, 2002.
Article in English | MEDLINE | ID: mdl-12058591

ABSTRACT

The recent oral immunisation trials in wild boar against classical swine fever (CSF) in Germany are described and evaluated in summary. After the first field study in Lower Saxony from 1993-1995 further immunisation trials started in Mecklenburg-Western Pomerania, Brandenburg, Lower Saxony, Baden-Württemberg and Saxony-Anhalt. The immunisation strategies and the size of the vaccination zones were different in the individual federal states. In principle, the bait vaccine based on the CSF virus strain "C" were laid out by hand. Later also the aerial distribution was carried out in selected areas of Mecklenburg-Western Pomerania. The application of baits by plane was introduced at the beginning of the immunisation measures in Saxony-Anhalt apart from the manual distribution. Up to now, the field trials show that the oral immunisation can be an additional tool for CSF control by increasing of herd immunity and reduction of the CSFV prevalence. However, the immunisation was not sufficient enough for young boars in the most field studies. Based on the evaluation of the immunisation experiments an improved immunisation procedure is recommended.


Subject(s)
Classical Swine Fever Virus/immunology , Classical Swine Fever/prevention & control , Vaccination/veterinary , Viral Vaccines/administration & dosage , Administration, Oral , Animals , Animals, Wild , Antibodies, Viral/blood , Classical Swine Fever/epidemiology , Germany/epidemiology , Seroepidemiologic Studies , Swine
11.
Phys Rev Lett ; 88(14): 147401, 2002 Apr 08.
Article in English | MEDLINE | ID: mdl-11955173

ABSTRACT

Photocurrent experiments using two femtosecond laser pulses are performed on a photodiode using a ladder-type conjugated polymer as the active layer. With a photon energy of 3.1 eV the first pulse excites singlet excitons. A time-delayed second pulse with a photon energy of 2.49 eV leads to a decrease of the photocurrent by exciton depletion due to stimulated emission. S1 excitons being dissociated during their entire lifetime are identified as the only relevant channel for charge carrier generation. Intrachain polaron pairs are also formed on an ultrafast time scale with a yield of approximately 10%. They can be efficiently dissociated by reexcitation with photons of an energy of 1.9 eV.

12.
Opt Lett ; 26(9): 593-5, 2001 May 01.
Article in English | MEDLINE | ID: mdl-18040393

ABSTRACT

Optically pumped organic semiconductor lasers are fabricated by evaporation of a thin film of tris(8-hydroxyquinoline) aluminum (Alq(3)) molecularly doped with a laser dye on top of a polyester substrate with an embossed grating structure. We achieve low-threshold, longitudinally monomode distributed-feedback laser operation. By varying the film thickness of the organic semiconductor film, we can tune the wavelength of the surface-emitting laser over 44 nm. The low laser threshold allows the use of a very compact all-solid-state pump laser ( approximately 10 cm long). This concept opens up a way to obtain inexpensive lasers that are tunable over the whole visible range.

19.
Phys Rev Lett ; 70(24): 3820-3823, 1993 Jun 14.
Article in English | MEDLINE | ID: mdl-10053970
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