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In this work, the effect of atomic defects created by gallium ion irradiation on the optical properties of single-layer molybdenum disulfide is studied by means of micro-photoluminescence measurements. The induced defects give rise to an additional emission band located at about 170 meV below the free exciton. The micro-photoluminescence intensity of this defect-related emission band is found to be proportional to the defect density. The large spectral width suggests the presence of binding sites with different binding energies available for excitons that remain optically active up to 230 K.

Laser-thinning of 2D materials such as MoS2 is a promising approach for a local reduction of the number of multilayers down to a monolayer. For a precise control of the thinning process real-time m...

We report on the formation of Tamm plasmon (TP) modes in the visible spectral region at the interface between a Ag film and a ZnSe-based distributed Bragg reflector (DBR). The tuning of the TP eigenenergies within the stop-band in dependence on the DBR top layer thickness is investigated by micro-reflectivity measurements. The experimental findings are compared with calculations using the transfer matrix method. The latter calculations show that strong coupling can be achieved between quantum well (QW) excitons and the TP modes. In addition, the Rabi-splitting energy can be enhanced in a simple TP structure by increasing the number of QWs inside the top layer of the DBR. When a metal layer is deposited on a cold MC sample, micro-reflectivity spectra show a blue shift of the cavity resonance. The TP modes are likewise observed in the same MC sample when reducing the top layer thickness of the DBR. This shift of cavity resonance and formation of TP mode are in good agreement with calculations. These results are rather promising in order to realize a spatial confinement of the polaritons by utilizing metal strips. Such channeling of polaritons opens the way to practical demonstration of exciton-polariton integrated circuits. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The structure and morphology of uncapped and capped InGaN quantum dots formed by spinodal decomposition was studied by AFM, SEM, XRD, and EXAFS. As result of the spinodal decomposition, the uncapped samples show a meander structure with low Indium content which is strained to the GaN template, and large, relaxed Indium-rich islands. The thin meander structure is responsible for the quantum dot emission. A subsequently deposited low-temperature GaN cap layer forms small and nearly unstrained islands on top of the meander structure which is a sharp interface between the GaN template and the cap layer. For an InGaN cap layer deposited with similar growth parameters, a similar morphology but lower crystalline quality was observed. After deposition of a second GaN cap at a slightly higher temperature, the surface of the quantum dot structure is smooth. The large In-rich islands observed for the uncapped samples are relaxed, have a relatively low crystalline quality and a broad size distribution. They are still visible after capping with a low-temperature InGaN or GaN cap at 700 °C but dissolve after deposition of the second cap layer. The low crystalline quality of the large islands does not influence the quantum dot emission but is expected to increase the number of defects in the cap layer. This might reduce the performance of complex devices based on the stacking of several functional units.

Microcavity structures which consist of a cavity formed by two distributed Bragg reflectors can be used to study light-matter interaction in the weak and strong coupling regime. In this contribution microcavities with one or three ZnSe quantum wells embedded into the cavity are presented. In contrast to the sample with the single quantum well, strong light-matter coupling can be observed for that one with three quantum wells. The sample possesses an energy splitting of about 19meV between the upper and lower polariton branch observed by reflectivity measurements. In addition, the transition from strong to weak coupling and the starting of the photonic lasing is demonstrated by excitation density dependent measurements. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present evidence for the existence of a hybrid state of Tamm plasmons and microcavity exciton polaritons in a II-VI material based microcavity sample covered with an Ag metal layer. The bare cavity mode shows a characteristic anticrossing with the Tamm-plasmon mode, when microreflectivity measurements are performed for different detunings between the Tamm plasmon and the cavity mode. When the Tamm-plasmon mode is in resonance with the cavity polariton four hybrid eigenstates are observed due to the coupling of the cavity-photon mode, the Tamm-plasmon mode, and the heavy- and light-hole excitons. If the bare Tamm-plasmon mode is tuned, these resonances will exhibit three anticrossings. Experimental results are in good agreement with calculations based on the transfer matrix method as well as on the coupled-oscillators model. The lowest hybrid eigenstate is observed to be red shifted by about 13 meV with respect to the lower cavity polariton state when the Tamm plasmon is resonantly coupled with the cavity polariton. This spectral shift which is caused by the metal layer can be used to create a trapping potential channel for the polaritons. Such channels can guide the polariton propagation similar to one-dimensional polariton wires.

In this contribution, we present strong coupling of ZnSe quantum well excitons to Bragg modes resulting in the formation of Bragg polariton eigenstates, characterized by a small effective mass in comparison to a conventional microcavity. We observe an anticrossing of the excitonic and the photonic component in our sample being a clear signature for the strong-coupling regime. The anticrossing is investigated by changing the detuning between the excitonic components and the Bragg mode. We find anticrossings between the first Bragg mode and the heavy- as well as light-hole exciton, respectively, resulting in three polariton branches. The observed Bragg-polariton branches are in good agreement with theoretical calculations. The strong indication for the existence of strong coupling is traceable up to a temperature of 200 K, with a Rabi-splitting energy of 24 meV and 13 meV for the Bragg mode with the heavy- and light-hole exciton, respectively. These findings demonstrate the advantages of this sample configu...

Strong coupling is demonstrated in monolithic ZnSe-based microcavities. Under nonresonant excitation the polariton dispersion has been investigated in dependence on the photon-exciton detuning for different excitation densities at low temperatures. For zero detuning indications of a polariton lasing threshold are observed like a k-space and energy dispersion narrowing of the lower polariton branch with increasing excitation density. Furthermore, it is observed that this effect is hampered for measurements at negative detunings as a result of the less effective polariton relaxation to the ground state. Latter results in the formation of a discrete polariton distribution at finite k values as known for the polariton bottleneck. In order to investigate the influence of a three-dimensional optical confinement on the polariton relaxation, pillar structured microcavities were fabricated. The formation of discrete polariton states in the k-space distribution is observed. Furthermore, indications for a softening of the k-conservation arising from the structural confinement are found leading to a more effective polariton relaxation. This process would be beneficial for the realization of efficient polariton lasing processes.

Phone: þ49-421-21862217, Fax: 49-421-2184581Highly ordered GaN nanorods were grown self-organized andwithoutcatalystonr-planesapphirebymolecularbeamepitaxywhile the AlN nucleation sites for the nanorods were providedby a nitridation process using a metal-organic vapor-phaseepitaxy system. The growth window for the nanorod formationwas analysed in detail and turned out to be very sensitive withrespect to the growth temperature. The nanorods are symmetri-cally tilted with an inclination angle of 628 between thesubstrate and the nanorods. The mirror axis is the c-direction ofthe compact GaN layer surrounding the roots of the nano-columns.Methodsforthecontrolofthenanoroddensityandthesuppressionofonenanorodgrowthdirectionarepresented.Theresults indicate a diffusion based growth mechanism. Trans-mission electron microscopy studies and high resolution X-raydiffraction (HRXRD) polar plots reveal the epitaxial relation-ship between substrate, compact GaN layer and nanorods. Thenanorods grow in c-direction and the side facets are m-planes.Transmission electron microscopic and optical analysis of thenanorods reveal the good structural and optical properties,respectively. A full width at half maximum (FWHM) of1.2meVofthedonor-boundexcitonemissionwasmeasuredforboth the ensemble and single free-standing nanorods. Success-ful n- and Mg-doping of nanorods was verified by a strongincrease of the micro-photoluminescence intensity of therespective donor-bound and acceptor-bound exciton emissionpeak in comparison to an undoped sample.The background shows a secondary electron microscope image(angle of 108between sample surface and electron beam) of thenanorods. Superimposed to the SEM image, a typical micro-photoluminescence spectrum of the excitonic emission of anensemble at 25K is presented. The donor-bound, acceptor-bound and free exciton lines are labelled with D

Nitride-based quantum dots have many attractive optical properties for the realization of quantum dot (QD) based devices which will be presented in this contribution. We will analyze the basic characteristics of single InGaN QDs and their electroluminescence (EL) as well as the optical properties of QD stacks and their gain spectra. The single QDs are characterized by the high temperature stability of their emission up to 150 K in PL and EL and even up to room temperature for stacked QD samples. Furthermore, the polarization of individual QD emission lines was analyzed giving an insight into their geometrical shape. Time-resolved microphotoluminescence (µ-PL) measurements on the excitonic and biexcitonic transition of a single QD as well as on the influence of piezoelectric fields on them and on their binding energy were performed. Further, we present an analysis of electrically driven luminescence from single InGaN QDs embedded into a light emitting diode structure showing single sharp emission lines in the green spectral region with a high temperature stability up to 150 K. Furthermore, for the possible integration within optical devices in the future the threshold power density as well as the modal gain were investigated for samples with stacked InGaN QD layers. They show a modal gain per QD being comparable to that of II–VI and III-As compounds. These results give a good insight into the basic optical properties of InGaN QD based devices and showing their high potential for electrically driven single photon emitters as well as for bright, low-threshold InGaN QD based light emitting devices in the near future.

The structure and chemistry of clean GaN surfaces and InGaN thin films and nanostructures grown by metal organic vapour pressure epitaxy (MOVPE) has been studied by means of X-ray photoemission spectroscopy, low-energy electron diffraction as well as scanning tunneling microscopy (STM) and transmission electron microscopy. Thermal annealing strongly improves the cleanliness of samples after dry nitrogen transfer and related exposure to residual oxygen. Nitrogen plasma assisted cleaning is shown to successfully further remove carbon contaminations, while Ga deposition with subsequent desorption to is shown to be superior for an enhanced reduction of surface oxygen. Using STM, the surface morphology has been studied in dependence on major growth parameters at various stages of InGaN MOVPE growth. The formation of nano-islands is reported for different growth conditions. By means of micro- photoluminescence measurements, we find samples to show strong photoluminescence from quantum-dot-like structures, however, the corresponding growth front is found to be rather flat throughout InGaN deposition. This leads to the conclusion that the formation of quantum dots does not proceed in a Stranski-Krastanov-like fashion but most likely during over- growth.

We deposited tris-(8-hydroxyquinoline) aluminum (Alq3) films on glass substrates by using the hot-wall method. The Alq3 films were deposited for changing substrate temperature from 50 to 125 °C. With increasing the substrate temperature, anomalous growth patterns with a butterfly-like shape appeared and spread on the surface of the deposited films. The intensity of emission from the anomalous region is stronger than that from the smooth region as found by fluorescence microscope observations and photoluminescence measurements. The observed anomalous growth in the films deposited at high substrate temperatures was identified to be due to α -phase crystallization of Alq3 from the emission spectrum and X-ray diffraction measurements and from comparing those results with α -Alq 3 crystals grown by the vapour phase growth technique.

A customization of the optical properties of pillar microcavities on the desired applications is essential for their future use as quantum-optical devices. Therefore, all-epitaxial cavities with CdSe or InGaN quantum dots embedded in pillar structures with different geometries have been realized by focused-ion-beam etching. The high temperature stability of the emission and the large oscillator strength of quantum dots made of these material systems are combined with small mode volume pillar microcavities to utilize the light-matter interaction. The quality factors of circularly shaped pillar microcavities have been measured and compared to theoretical calculations. As a possibility to achieve polarized light emission, asymmetrically shaped microcavities are presented. Examples of an elliptically shaped pillar as well as of photonic molecules are investigated with respect to their photoluminescence characteristics and polarization. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This research presents a combined experimental-modeling study of lattice rotations and deviatoric strain gradients induced by focused-ion beam (FIB) milling in nitride heterostructures. 3D X-ray polychromatic microdiffraction (PXM) is used to map the local lattice orientation distribution in FIB-structured areas. Results are discussed in connection with microphotoluminescence (μ-PL), fluorescent analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) data. It is demonstrated that FIB-milling causes both direct and indirect damage to the InGaN/GaN layers. In films subjected to direct ion beam impact, a narrow amorphidized top layer is formed. Near the milling area, FIB-induced stress relaxation and formation of complicated 3D strain fields are observed. The resulting lattice orientation changes are found to correlate with a decrease and/or loss of PL intensity, and agree well with finite element simulations of the three-dimensional strain fields near the relaxed trenches. Experimentally, it is found that the lattice surface normal has an in-plane rotation, which only appears in simulations when the GaN-substrate lattice mismatch annihilates the InGaN-substrate mismatch. This behavior further supports the notion that the film/substrate interface is incoherent.

We present methods to achieve a spectral matching between a quantum dot (QD) emitter and the resonator mode of an airpost pillar microcavity (MC). These techniques are important to optimize the light-matter interaction in order to increase the collection efficiency and the recombination rate of the emitter leading to an improvement of the properties of, for example, a single photon source. The investigations were performed on different MC structures containing either ZnCdSSe quantum wells or CdSe QDs, respectively, as their active region. The spectral tuning of the QD emission was realized by temperature variation and by laser annealing. The spectral positions of the resonator modes were adjusted by tailoring the diameter of the airpost pillars. Hence, permanent and non-permanent spectral tuning methods will be presented.

We present an analysis of the optical properties of single InGaN quantum dots (QDs) grown by MOVPE. The samples were structured into mesas by focused-ion-beam etching and investigated by micro-photoluminescence measurements. The QDs are characterized by the high temperature stability of their emission up to 150 K. Furthermore, the polarization of individual QD emission lines was analyzed giving an insight into their geometrical shape. Time-resolved microphotoluminescence measurements on the excitonic and biexcitonic transition of a single quantum dot yields a radiative recombination lifetime of 2.06 ns for the exciton. The data can be fitted by a simple model for cascaded emission confirming the expected refilling of the excitonic state by biexcitonic recombination. In addition, the influence of piezoelectric fields on the exciton and biexciton emission and on their binding energy in single QDs was investigated. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we review basic optical properties of II–VI-based monolithic microcavities with quantum wells or quantum dots embedded. The epitaxial growth of high-reflectivity distributed Bragg reflectors for the blue–green spectral range is discussed since they are a key component for micropillar cavities. The pillar structured samples reveal three-dimensionally confined optical modes with high quality factors and small mode volumes making them appropriate to investigate cavity quantum electrodynamic effects. The II–VI material system is very promising for the utilization of this effect for technological applications due to the large exciton oscillator strength and carrier confinement. The theoretical methods needed to describe light–matter interaction in such systems is reviewed. For the quantum well based cavities single-mode stimulated emission at near room temperature is realized. Furthermore, the relatively large cavity polariton stability makes it possible to observe polariton condensation in this kind of samples. For the quantum dot based microcavities a strong enhancement of the emission intensity for quantum dots being in spectral resonance with the cavity modes is observed. To realize the necessary spectral matching a target fine tuning can be achieved by temperature variation or by post fabrication focused ion beam milling. A pronounced enhancement of the spontaneous emission rate of quantum dots coupled to the discrete optical modes of the cavities is demonstrated by the occurrence of the Purcell effect. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present research achievements that advance the development of quantum dot based low threshold vertical cavity surface emitting lasers operating at elevated temperatures. We report on the optical properties of monolithic ZnSe-based pillar microcavities. Measurements were performed on different microcavity structures containing either ZnCdSSe quantum wells or CdSe quantum dots, respectively, as their active region. For the quantum dot based structures, the implementation of additional MgS/ZnSSe barriers yield temperature stability of the quantum dot emission up to room temperature. Microphotoluminescence experiments show an enhanced collection efficiency for the quantum dot emission in spectral resonance with an optical microcavity mode. High quality factors of up to Q = 7860 were achieved for quantum dot structures, while for quantum well devices Q = 3500 was reached as being analyzed by microphotoluminescence and micro-reflectivity spectroscopy. For the quantum well structures, single mode stimulated emission at 510 nm is shown at 280 K under pulsed optical excitation. Threshold reduction with decreasing pillar diameter is observed in accordance to numerical calculations based on microscopic laser theory. The results are promising for the realization of efficient quantum dot based low threshold vertical cavity surface emitting laser devices operating at elevated temperatures in the near future. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present photoluminescence measurements on single InGaN quantum dots (QDs) grown by metalorganic vapor phase epitaxy, and on monolithicly grown GaN-based quantum well airpost pillar microcavities. The observed sharp emission lines of the quantum dots are characterized by excitation density dependent measurements. The photoluminescence of individual quantum dots can easily be detected for temperatures up to 150 K. The micro-photoluminescence measurements on microcavities reveal three-dimensional confined optical modes which are not seen in the luminescence of the simply planar cavity. The realization of rather temperature stable QDs as well as of nitride based microcavity samples are promising with respect to the intended implementation of QD layers into microcavities. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A two-step growth method for creating InGaN quantum dots (QDs) was developed by using a combination of an InxGa1–xN nucleation layer (NL) without island structures and an InyGa1–yN formation layer (FL) with an indium content lower than that of the InxGa1–xN NL. The realization of QDs was confirmed by micro-photoluminescence (µ-PL) measurements only for the sample with both the InxGa1–xN NL and the InyGa1–yN FL. The spectral position of the QD ensemble recombination was controlled mainly by the deposition time of the InxGa1–xN NL. Green (~520 nm) and amber (~600 nm) LEDs with the QD layers grown by the two-step growth method as the active region were also fabricated and compared with that having InGaN QW layers, reported previously.

A novel two-step growth method for creating InGaN quantum dots (QDs) was developed by using a combination of an InxGa1–xN nucleation layer with a platelet structure and an InyGa1–yN formation layer with an indium content lower than that of the InxGa1–xN nucleation layer. The realized QDs were investigated by micro-photoluminescence measurements. We observed sharp emission lines at 4 K with a spectral width down to the spectral resolution limit of the experimental setup of 0.17 meV. This growth concept is discussed in comparison with conventional growth methods. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present photoluminescence measurements on single InGaN quantum dots grown by metalorganic vapor phase epitaxy. The spatially and spectrally resolved luminescence properties of the single quantum dots were measured using low-temperature micro-photoluminescence spectroscopy. The observed sharp emission lines of the quantum dots were characterized by excitation density dependent measurements. The photoluminescence of individual quantum dots can easily be detected for temperatures up to 150 K. In addition, temporal fluctuations of the emission peak positions of single quantum dots are attributed to spectral diffusion processes. Indications for recombination processes from excited states are observed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The emission properties of CdSe/ZnSe quantum dots embedded in high-Q all-epitaxial ZnSe-based pillar microcavities are studied. Temperature variable experiments show a strong enhancement of the detected photoluminescence signal of single quantum dots when tuned in resonance with modes of the cavity. The Purcell effect is reflected in a pronounced enhancement of the spontaneous emission rate of quantum dots coupling to the fundamental modes of the cavities. The dependence of the enhancement on the pillar diameter is discussed in detail. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present photoluminescence measurements on single InGaN quantum dots grown by metalorganic vapor phase epitaxy. The spatially and spectrally resolved luminescence properties of the single quantum dots were measured using low-temperature micro-photoluminescence spectroscopy. The observed sharp emission lines of the quantum dots were characterized by excitation density dependent measurements. They can easily be observed at temperatures up to 150 K.

We report on the successful fabrication and optical investigation of monolithicly grown GaN based vertical-cavity surface-emitting laser structures featuring quality factors of 250. Short-period AlN/InGaN superlattices are used for the low-index layers of the distributed Bragg reflectors. Pillar microcavities are realized by focused-ion-beam etching. Micro-photoluminescence measurements reveal the longitudinal and transversal mode spectra of the cavities in good agreement with theoretical calculations based on a vectorial transfer matrix method.

Monolithic II-VI pillar microcavities made of ZnCdSSe/ZnSSe A cavities and Bragg mirrors composed of ZnSSe and MgS/ZnCdSe supperlattices have been fabricated by molecular-beam epitaxy and focused-ion-beam etching. Discrete optical modes of the pillar microcavities are studied in photoluminescence measurements. The measured data show full agreement with calculations based on an extended transfer-matrix method. A strong enhancement in the detected photoluminescence signal is obtained for single emitters in resonance with modes of the cavity.

Samples containing CdSe quantum dots (QDs) embedded in MgS barriers have been grown by molecular beam epitaxy. Photoluminescence (PL) investigations at room temperature show a QD ensemble shift between 240 meV and 280 meV due to the enhanced carrier confinement compared to a reference sample without MgS barriers. Temperature dependent micro-PL measurements on single QDs indicate an activation energy of 70 meV for MgS barriers, which is a factor of 2.5 higher compared to ZnS0.06Se0.94 barriers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN-based airpost pillar microcavities are realized by focused-ion beam etching starting from an all-epitaxially grown vertical-cavity surface-emitting laser structure. Pillar diameters below 1 μm are well controllable. The sidewalls are smooth and show a damaged surface layer of a thickness less than 2 nm only. Micro-photoluminescence measurements reveal the longitudinal and transversal mode spectra of the cavities in good agreement with theoretical calculations based on a vectorial transfer-matrix method.

We report on microphotoluminescence-spectroscopy results giving strong evidence for the existence of quantum-dot like localization centers (LCs) in InGaN/GaN heterostructures. The observed low surface density of the LCs allows to address individual emitting centers without lateral structuring of the sample. The optical properties of the LCs with respect to the dependence on excitation density and temperature are studied. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we report on recent results concerning conventional edge emitting laser structures containing either quantum wells or quantum dots as region and vertical-cavity surface-emitting lasers. In the first part a series of four similar laser structures containing quantum wells with an emission wavelength of 520 nm was grown by molecular beam epitaxy in order to perform a systematic study of lifetime improvement. They differed in the alternating implementation of an additional 5 nm thick ZnSSe layer with a high sulfur composition of 25% neighboring the quantum well. A high stability of the CdZnSSe active layer was observed by introducing such a kind of strain compensating layers. Lifetime measurements showed a significant improvement up to one order of magnitude using p- and n- & p-side layers. In the second part electro-optical characteristics of ridge and planar CdSe quantum dot laser diodes were compared. A reduction of the threshold current density by a factor of 4.7 for the ridge structure was obtained. This has to be associated to the reduction of current spreading inside the laser diodes. Furthermore, a significant slower degradation of CdSe quantum dot structures compared to common ZnSe-based QW structures was observed. An operating time over 2700 h in pulsed mode experiments at 50 A/cm2 in LED mode was achieved. In the third part we report on the realization of an optically pumped monolithic vertical-cavity surface-emitting laser operating at a wavelength of 511 nm. The microresonator has a quality factor of 3200 while the threshold excitation power density for the onset of lasing is 22 kW/cm2 at room temperature. Micropillars of different diameter fabricated out of this structure show discrete optical modes due to the three dimensional optical confinement of the optical wave. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the generation of triggered single photons and photon pairs relying on pulsed optical excitation of epitaxially grown self-assembled CdSe quantum dots (QDs). Single-photon emission is studied on individual CdSe/Zn(S,Se) QDs for temperatures up to 200 K. At low temperatures nearly perfect single-photon emission is achieved whereas at higher temperatures an increasing multi-photon emission probability due to spectrally overlapping acoustic phonon sidebands of neighboring QDs is observed. The multi-photon emission probability of a bare QD (background subtracted) is strongly suppressed. Furthermore, the polarization cross correlation of the biexciton-exciton cascade has been investigated in a CdSe/ZnSe QD at low temperatures. A strong polarization correlation of the emitted photon pairs of ∼74% is observed in a linear detection basis.

The role of excitonic excitations in wide-gap II-VI and nitride semiconductors is reviewed with focus on the recent research at Bremen University. More-particle excitations such as trions and biexcitons are studied including their contributions to nonlinearities in the coherent regime and to lasing. Specific questions concerning laser processes in wide-gap materials are addressed.

Systematic gain measurements of CdSe quantum dot stacks were carried out using the variable-stripe-length method at various pump densities and temperatures. The fivefold quantum dot stacks show an effective optical gain at double threshold of the order of 125 cm -1 (150 cm -1 ) for the doped (undoped) sample. A maximum optical gain of 400 cm -1 was achieved at a pump density of 980 kW/cm -2 and a temperature of 10 K. High-resolution transmission electron microscope and micro-photoluminescence measurements were carried out. Temperature depending measurements yield a strong increase of the threshold densities for temperatures above 100 K.

Negatively charged trions are investigated for ZnSe single quantum wells embedded in ternary barriers. Polarization and excitation dependent photoluminescence measurements were performed in magnetic fields up to 11.8 T. From these measurements we are able to develop an energy-term scheme for the spin-singlet and the spin-triplet state of the trion.

InGaN quantum dots were successfully implemented into fully epitaxially grown nitride-based monolithic microcavities (MCs). The discrete modes of airpost pillar MCs prepared out of the planar sample are shown in microreflectivity as well as in microphotoluminescence. These measurements are compared to theoretical simulations based on a vectorial-transfer matrix method. Quality factors of up to 280 have been achieved and the emission of a single quantum dot was traced up to a temperature of 125 K.

International audience; We report on the realization of monolithic, i.e. fully epitaxial ZnSe-based microcavities with a 1λ-cavity and a 3λ-cavity for the investigation of strong coupling phenomena. A crucial building block is the fabrication of a lattice matched distributed Bragg reflector for the blue spectral region, consisting of quaternary ZnMgSSe layers as the high-index material and a MgS/ZnCdSe superlattice as the low-index material. Strong coupling at low temperatures was achieved with a vacuum Rabi splitting of 19 meV for the 1λ-cavity (at 70K) and of 40 meV for the 3λ-cavity (at 7 K).Microstructures such as micropillars, waveguides, ringresonators, and molecule chains were fabricated using focused ion-beam etching. Discrete optical modes in micropillars were observed for both cavity lengths confirming three-dimensional optical confinement. Q-factors up to 6500 indicate a high optical quality.

We present a comparison of the optical properties of Si and Mg doped GaN nanorods with respect to an undoped sample. The reference sample is characterized by a high crystalline quality as indicated by the spectral width of the dominant emission peaks being comparable for the ensemble and single nanorods as well as by the absence of the yellow photoluminescence. Even after the incorporation of n-dopants this defect related emission band is still not detectable.The p-doped sample shows several modifications of the spectrum. The near band edge emission is dominated by an additional band at 3.41 eV related to partial coalescence of the nanorods, while the excitonic emission is very weak. The DAP band emission is obviously evolved into a double band series with two zero-phonon peaks at 3.26 eV and 3.18 eV indicating the involvement of two acceptor related states, and is further superimposed by a weak yellow luminescence band around 2.5 eV. These results are important as basis for applications of such nanorods as electrically driven semiconductor devices with excellent crystalline quality as well as with very high surface sensitivity, which is useful for solar cells or chemical sensors, for example. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present light emitting diode structures with InGaN quantum dots (QDs) as active material grown by metal-organic vapor-phase epitaxy. The devices reveal a threshold voltage of 3.15 V at room temperature, or 8.8 V at 4 K, respectively. At room temperature, the electroluminescence intensity is still as high as 28% of the intensity at 4 K, the emission covering the spectral region from 2.3 to 2.7 eV at 4 K (2.2-2.6 eV at 300 K). Electrically driven luminescence from spectrally isolated single InGaN QDs emitting in the green spectral region is demonstrated with a thermal stability of up to 150 K. These results are an important step towards applications like electrically driven single photon emitters at elevated temperatures, which are desirable as a basis for communication techniques incorporating plastic optical fibers as well as for modern concepts of free space quantum cryptography.

The polarization- and excitation-intensity-dependent photoluminescence of the negatively charged trion is investigated for ZnSe single quantum wells embedded in ternary and quaternary barriers. The measurements were performed in magnetic fields up to 11.8 T perpendicular to the quantum well. The spin-singlet state of the trion is clearly identified. In contrast to GaAs quantum wells, the increase of the trion binding energy through the magnetic field is found to be negligible, which is explained by the relatively small spatial extent of the trionic wave function in wide-band-gap materials. For magnetic fields beyond 7 T a resonance becomes stabilized that is identified as excited trion spin-triplet state because of its anticorrelation with the trion spin-singlet state behavior for increasing excitation energy.

The polarization-dependent photoluminescence of the negatively charged trion is investigated for ZnSe single quantum wells with varying quantum well thickness between 5 and 20 nm and barrier heights ranging from 30 to 200 meV under application of magnetic fields up to 10 T perpendicular to the quantum well plane. A strong increase of the trion binding energy with decreasing quantum well thickness from about 2.5 to 7 meV is observed. For the trion spin-singlet state the stabilization with increasing magnetic field is found to be negligible. Under high magnetic fields (B>6 T) a shoulder emerges on the low-energy side of the 1 s heavy-hole exciton resonance which is attributed to the excited trion spin-triplet state becoming stabilized in high magnetic fields.

The general characteristics of the ‘photon-avalanche’ up-conversion process are presented. The Pr,Yb system was spectroscopically investigated in Pr;Yb:YLiF 4 and Pr,Yb:YAlO 3 with respect to the realization and understanding of this process. Whereas Pr;Yb:YLiF 4 exhibits an avalanche-type excitation mechanism under infrared pumping, no such behavior is observed for Pr,Yb:YAlO 3 . A time dependent rate equation model was developed to describe these different behaviors using experimentally determined parameters. The absence of an avalanche-type excitation mechanism in Pr,Yb:YAlO 3 is mainly caused by (i) a better spectral overlap between the excited state absorption in the Pr and the ground state absorption of Yb, (ii) higher transfer rates between Yb and Pr and within the Pr, and (iii) lower intrinsic decay rates from the participating excited energy levels ( 3 P 0,1,2 and 1 G 4 ).

We report on the successful integration of a single InGaN quantum dot sheet into a resonant half cavity structure con-sisting of an AlGaN/GaN distributed Bragg reflector and a GaN λ-cavity layer. For reference, small-diameter pillar mi-crocavites fabricated by focused-ion beam etching from an empty planar cavity are studied. These structures exhibit discrete transversal modes and Q factors of 260. The quantum dot active region has been grown by metal-organic vapour-phase epitaxy using a two-step deposition technique. Optical spectra measured by micro-photoluminescence reveal distinct spectrally sharp emission lines around 2.73 eV which can be attributed to the emission of single InGaN quantum dots. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We discuss the optical properties of single InGaN QDs grown by MOVPE. The samples were structured into mesas by focused-ion-beam etching and investigated by micro-photoluminescence measurements. The QDs are characterized by a high temperature stability of their emission up to 150 K and strong localization of the carriers within the quantum dots, indicated by the observation of antibinding biexcitonic complexes. The influences of piezoelectric fields and spectral diffusion effects were investigated. Furthermore, the polarization of individual QD emission lines was analyzed giving an insight into their geometrical shape. The quantum dots were used for the realization of quantum dot stacks. The photoluminescence of these samples can be traced up to room temperature. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural modifications of ZnO single crystals that were created by the irradiation with femtosecond laser pulses at fluences far above the ablation threshold were investigated with micro-Raman spectroscopy. After light-matter interaction on the femtosecond time scale, rapid cooling and the pronounced thermal expansion anisotropy of ZnO are likely to cause residual strains of up to 1.8% and also result in the formation of surface cracks. This process relaxes the strain only partially and a strained surface layer remains. Our findings demonstrate the significant role of thermoelastic effects for the irradiation of solids with intense femtosecond laser pulses.

We report on micro-photoluminescence studies on InGaN/ AlGaN quantum dots grown by metal-organic vapor phase epitaxy. The excitonic emission lines can be observed up to 100 K. Investigations of the quantum-confined Stark effect in dependence on an external electric field show a partial compensation of internal fields and an enhancement of the overlap between electron and hole wave function in the quantum dots. As a consequence, an increase of the PL intensity and a blue shift of the single-quantum-dot emission line is observed. Furthermore, we show how the internal electric field strongly affects the photoluminescence intensity of the emission line up to 40 K. In addition, by investigating the excitonic and biexcitonic states, we demonstrate that the partial compensation of the internal field increases the biexciton binding energy and influences the intensity of its emission.