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The polarization dynamics of zero-dimensional exciton emission is studied for CdSe/ZnSe quantum dots going systematically from strictly resonant to nonresonant excitation. Independent on the excess energy, no transient decrease of the polarization is found for ground state exciton emission. However, the absolute value of the polarization degree exhibits a strong nonmonotonic dependence on the excitation energy. We demonstrate that the initial loss of polarization after nonresonant excitation can be minimized if the exciton excess energy corresponds to an integer multiple of LO-phonon energies.

We demonstrate the generation of triggered single photons at a predetermined and well defined energy and two-photon emission with slightly different emission energies using the radiative recombination of individual nitrogen-bound excitons in ZnSe:N.

We demonstrate the ability to monitor the magnetization of a single CdSe/ZnMnSe magnetic quantum dot by means of nano-optical spectroscopy. The average magnetic moment as well as statistical magnetic fluctuations are extracted from our data.

The influence of ion implantation on the interdiffusion of Cd and Mg in CdTe/CdMgTe single quantum wells was studied. The samples were implanted with 150 keV Ar+ ions and subsequently annealed for 1 min by rapid thermal annealing at temperatures between 400 and 480 °C. Ion implantation and annealing lead to an intermixing of barrier and well material, causing a blue shift of the characteristic emission spectrum. The dependence of the energy shift on the ion dose and the annealing temperature was studied. For an ion dose of 1×1014 cm−2 and an annealing temperature of 400 °C a blueshift of 80 meV compared to a nonimplanted reference sample indicates a significant enhancement of the interdiffusion of Cd and Mg by the implantation.

In this article lasing action of (CdMg)Te quantum well laser structures is reported. The separate confinement quantum well laser structures were grown by molecular beam epitaxy. Stimulated emission has been observed at 77 K and at room temperature. The structures emitted at wavelengths of around 650 nm at room temperature. A (CdMg)Te/CdTe superlattice has been used for the active layer as well as for the electrical confinement layer. Due to this an efficient radiative recombination even at room temperature could be obtained. The distribution of the intensity of the light across the waveguide has been calculated.

Thermally induced interdiffusion in CdTe/CdMnTe quantum wells was investigated by photoluminescence spectroscopy. The single quantum well structures were grown by molecular beam epitaxy annealed by rapid thermal annealing for 1 min at temperatures between 380 and 520 °C. A blue shift close to the barrier energy was observed indicating an almost perfect interdiffusion between the well and the barrier material. We derive an activation energy of 2.8 eV for the interdiffusion process from a Fickian diffusion model applied to our experiments.

We have investigated the optical response of a red emitting CdTe/CdMgTe laser structure after optical excitation. For the first time, stimulated emission has been observed in this material system at room temperature, demonstrating the potential for the development of CdTe/CdMgTe laser structures in the visible spectral range. An analysis of our experimental data yields a threshold carrier density of about 3.5/spl middot/10/sup 12/ cm/sup /spl minus/2/ and a net gain coefficient of about 95 cm/sup /spl minus/1/ at T=300 K. >

Polarization dependence of 1.55 /spl mu/m wavelength GaInAsP/InP compressively-strained (CS) quantum-wire laser structures assisted by a quantum-film was measured and compared with quantum-film laser structures fabricated on the same wafer. As a result, better anisotropic polarization properties of the quantum-wire structure were obtained. PL intensity of an electric field perpendicular to the quantum-wire (Q-Wire) was about 60% of that parallel to the Q-Wire. In addition, we fabricated Q-Wire lasers with a wire width of about 25 nm. As a result, a blue shift /spl Delta/E/spl sim/10 meV and a threshold current density J/sub th/=34 A/cm/sup 2/ at 90 K were obtained.

We demonstrate three-dimensional optical confinement in CdTe-based microcavities. While the vertical confinement is determined by epitaxially grown CdMnTe/CdMgTe Bragg mirrors, the lateral confinement is achieved by defining pillar structures with electron beam lithography and reactive ion etching. The optical confinement is shown by a size-dependent energy splitting between the fundamental and higher photonic modes.

We demonstrate the generation of triggered single photons at a predetermined and well defined energy using the radiative recombination of single nitrogen-bound excitons in a semiconductor. The nitrogen atoms are embedded in a ZnSe quantum well structure and were excited by nonresonant optical pumping (82 MHz) at low temperature (4 K). We find resolution-limited photoluminescence lines ($280\text{ }\ensuremath{\mu}\mathrm{e}\mathrm{V}$) which display photon antibunching under continuous optical pumping. Our results also suggest that single nitrogen-bound excitons are well suited for cavity quantum electrodynamics experiments.

Statistical fluctuations of the magnetization are measured on the nanometer scale. As the experimental monitor we use the characteristic photoluminescence signal of a single electron-hole pair confined in one magnetic semiconductor quantum dot, which sensitively depends on the alignment of the magnetic ion spins. Quantitative access to statistical magnetic fluctuations is obtained by analyzing the linewidth broadening of the single dot emission. Our all-optical technique allows us to address a magnetic moment of only $\ensuremath{\approx}100{\ensuremath{\mu}}_{B}$ and to resolve statistical changes on the order of a few ${\ensuremath{\mu}}_{B}$.

We investigated thermally induced interdiffusion in CdTe/CdMnTe quantum wells by luminescence spectroscopy. CdTe/CdMnTe quantum wells grown by molecular beam epitaxy were annealed by rapid thermal annealing for 1 min at temperatures between 380 and 550°C. We observed simple diffusion behaviour in this temperature region, offering the possibility of shifting the luminescence spectrum up the barrier energy. Our results are discussed using an interdiffusion model.

The dynamical response of a paramagnetic spin system to the exchange field of quasi-zero-dimensional electron-hole pairs in semiconductor quantum dots is investigated by time-resolved spectroscopy. The spin response time is extracted from the transient spectral shift of the photoluminescence signal caused by the dynamical spin alignment of magnetic ions incorporated in the crystal matrix. The formation of this ferromagnetically aligned spin complex is demonstrated to be surprisingly stable as compared to bulk systems, even at elevated temperatures and high external magnetic fields.

We have studied single pairs of self-assembled and strain-induced quantum dots by means of highly spatially resolved photoluminescence and photoluminescence excitation spectroscopy. A spectrally narrow resonance in the excitation spectrum of a single self-assembled quantum dot at the emission energy of the corresponding strain-induced quantum dot indicates an efficient interdot transfer of a single zero-dimensional exciton.

The dependence of the biexciton binding energy and of the exciton--LO-phonon scattering rate on the wire width is investigated in wet-etched ZnSe quantum wires by temperature-dependent transient four-wave mixing. We observe an increase of the biexciton binding energy with decreasing wire width, reaching an enhancement of about 30% in the smallest wire structure. In addition, we find a decrease of the exciton--LO-phonon scattering rate with decreasing wire size, which is discussed with consideration of the reduced polarity of the exciton wave function and the modified density of final states in narrow wire structures.

Single epitaxially grown CdSe/ZnSe quantum dots have been studied by using photoluminescence spectroscopy with a high spatial resolution. The lifting of the spin degeneracy due to exchange interaction results in a splitting of the exciton ground state, strongly dependent on the symmetry of the quantum dot. By applying a magnetic field in Faraday geometry, the energy splitting as well as the polarization properties of the exciton transition can be varied and remarkably, even at high magnetic field a spin coherence time of about 3 ns is found, which exceeds the recombination lifetime of single excitons significantly. As the biexciton state is a spin singlet, both its fine structure splitting as well as its degree of polarization are shown to be controlled by the final state of recombination, the single exciton state. Besides the discrete energy splitting of optical transitons in single quantum dots, we observe a rather statistical, but strongly correlated energy shift was well as a correlated on-off switching behavior of the exciton and the biexciton emission on a typical time constant of seconds. These effects are related to the influence of charge carriers in the nanoenvironment of the dot and to thermal or Auger-driven carrier escape into trap states in the vicinity of the dot, respectively.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Using time‐resolved photoluminescence (PL) spectroscopy with high spatial (0.1 μm) and temporal (50 ps) resolution, we have investigated exciton scattering in a set of strained InxGa1−xAs/GaAs quantum wells as a function of well width Lz and In content x. Summarizing the results, a strong correlation between the exciton mobility and the emission linewidth (LW) is found: High mobilities correspond to small luminescence LWs, indicating reduced exciton scattering. From the dependence of the excitonic mobility on temperature and In content x, we conclude, that interface roughness scattering and alloy scattering, respectively, are the dominant processes controlling the mobility as well as the PL LW at low temperatures.

Focused ion beam implantation was used to modulate characteristic optical properties of II-VI heterostructures on a lateral nm-scale. Three different applications will be discussed, including basic physics as well as optoelectronic applications for the visible spectral range. In waveguides based on ZnSe heterostructures, both gain-coupled distributed feedback operation and quasi-phase matched second harmonic generation are investigated by modulating laterally the optical gain and the second order susceptibility χ(2), respectively. In CdTe/CdMnTe quantum wells, bandgap modulation is demonstrated by ion implantation and subsequent rapid thermal annealing, revealing a large potential for realizing semimagnetic nanostructures by selective interdiffusion.

The recombination dynamics of vapor-liquid-solid grown GaAs-nanowires with an axial p-n heterojunction is investigated by spatially and time-resolved photoluminescence spectroscopy. By scanning across the doping transition of single p-n and n-p doped nanowires, respectively, the particular influence of surface losses in differently doped areas is studied. We found a significantly reduced non-radiative recombination for the n-doped region compared to the p-doped one, which can be attributed to suppressed surface losses because of the characteristic band bending at the surface.

We report on an experimental evidence of a significantly different dynamic nuclear polarization (DNP) for localized and itinerant electrons in n-GaAs. Optically injected spin-polarized electrons are used to generate dynamic nuclear polarization via electron-nucleus hyperfine interaction. Using time resolved Kerr rotation measurements for probing the transient Overhauser field, the DNP time constants for itinerant and localized electrons are extracted to be 10 min and less than 1 min, respectively. This is attributed to a rapid DNP occurring in the vicinity of the donors followed by a delayed nuclear spin polarization in between the donor sites.

Single photon emission from an epitaxially grown quantum dot at room temperature is presented. CdSe/ZnSSe quantum dots are embedded into MgS barriers, providing dominant radiative recombination up to 300 K. Under continuous wave optical excitation, the autocorrelation function g(2)(t) exhibits a sharp dip at (t = 0) with g(2)(0) = 0.16 ± 0.15 at T = 300 K, revealing excellent suppression of multiphoton emission even at room temperature.

Optically detected nuclear magnetic resonance (NMR) with micrometer resolution is demonstrated in n-GaAs using an on-chip microcoil. To trace the Overhauser field, the electron Larmor frequency is monitored via time-resolved magneto-optical Kerr rotation. Sweeping the frequency of the rf magnetic field induced by an on-chip microscale current loop, nuclear spin depolarization is achieved for each isotope species. The experimental data indicate an impact of a local quadrupole field, most likely caused by ionized donors, on the amplitude and linewidth of the NMR spectrum. By applying rf pulse sequences, the Rabi oscillation of A75s nuclear spins is obtained with an effective dephasing time of ∼200 μs.

We report on the coherent dynamics of excitonic wave packets observed in transient four-wave mixing experiments on strained InGaAs/GaAs quantum-well structures. The excitonic noneigenstates are created by using laser pulses shorter than the electron round trip time within the exciton orbital. A characteristic temporal modulation of the diffracted signal is observed, which resembles the coherent dynamics of an electron wave packet in a hydrogen atom expected for the case of a simultaneous excitation of all bound and the lowest continuum states.

We report on ultrafast control of the charge state of a single InGaAs quantum dot in a charge-tunable p-i-n diode structure. Focused ion beam etching is employed to decrease the capacitance of the device to enable radio frequency operation. A time-resolved photoluminescence technique is demonstrated that allows monitoring the charge state with a time-resolution which is limited only by the radiative lifetime of the charged and neutral exciton, respectively. Experimental data show that the charge state can be manipulated on time scales shorter than the radiative lifetime of approximately 1.4 ns.

We have fabricated micrometer-sized single-turn coils on top of charged CdSe/ZnSe quantum dot heterostructures by lithographical techniques. Current injection creates magnetic fields in the some 10 mT range, strong enough to modulate the hyperfine interaction. The very low coil inductance allows for generation of fast field transients. We demonstrate local control of the resident electron spin as well as read-out of the nuclear spin state on the 10 ns time scale by electrical current pulses.

We demonstrate the potential of Kelvin probe force microscopy for simultaneously probing the topography and the work function of individual nanowires. Our technique allows us to visualize both the material and the doping contrast in single GaAs-based nanowires without the need to electrically contact the nanowires. In a GaAs/GaP heterostructure nanowire, a core-shell structure is found. This is attributed to a thermally activated radial overgrowth of GaAs, while in the GaP region the vertical nanowire growth dominates. In partially p-doped GaAs nanowires the doping transitions can be localized and the width of the depletion layer is estimated.

We report on room-temperature electroluminescence from an all-inorganic light emitting device based on spin-coated ZnO nanoparticles. A tight submicron thick layer was fabricated on a fluorine doped tin oxide glass as a substrate using commercially available ZnO nanoparticles from the gas phase. After the evaporation of the top Al electrode, a diodelike I-V characteristic was obtained. An emission peak at around 390 nm and a broad defect-related electroluminescence in the visible range were observed at voltages below 10 V and ambient air conditions.

We demonstrate a method to electrically manipulate the spin polarization in a semiconductor on a micrometer length scale and a submicrosecond time scale. A variable magnetic field induced by a microscale current loop magnetizes the Mn2+ ions in a CdMnTe/CdMgTe diluted magnetic semiconductor quantum well, and via sp-d exchange interaction polarizes photogenerated electron-hole pairs. A maximum spin polarization degree of ±8.5% is obtained at 4.2 K without external magnetic field. The induced carrier spin polarization and the thermal heating of Mn2+ spins are resolved spatially and temporally by microphotoluminescence measurements.

Voltage controlled electrical charging of a single InAs quantum dot by a spin polarized electron from a ZnMnSe spin aligner is presented, and the spin information is read out optically. We demonstrate that the characteristic single quantum dot photoluminescence polarization pattern of the negative trion and the neutral exciton lines directly reflects the spin state of the electrically injected electron.

We present the ability to manipulate spin states in a semiconductor on a sub-micrometre length scale via the magnetic fringe fields of microstructured magnets. Fe/Tb multilayers with remanent out-of-plane magnetization induce a remanent, vertical magnetization in an underlying diluted magnetic semiconductor (DMS), which in turn results in an efficient spin polarization of optically excited charge carriers via sp–d exchange interaction. By optically switching the magnetization of the ferromagnet, the DMS magnetization can be manipulated and the limits of a dynamical interaction between the spin states in the ferromagnet and the magnetic semiconductor are discussed. Moreover, magnetic ion spins of the DMS initially aligned along the sample surface are tipped by optically generated, spin polarized holes, which leads to a coherent spin precession around the total magnetic field. We show that the fringe field of in-plane magnetized Co wires can be utilized to locally modify the coherent dynamics of the semiconductor magnetization and demonstrate how the field inhomogeneity influences the coherent response of the Mn spin ensemble in the DMS.

Facet temperature changes in broad-area red-emitting high-power AlGaInP lasers are analyzed by means of micro-Raman spectroscopy. Measurements as a function of injection current demonstrate that the temperature at the laser output facet rises linearly with optical output power. Temperature profile measurements across the laser stripe show a strong correlation between near field intensity, facet temperature, and catastrophic optical damage (COD). Additionally, temperature-power analyses reveal that a critical facet temperature is needed to induce COD. The consistent results produced by complementary measurement techniques indicate that absorption of stimulated photons at the laser facet is the major source of facet heating.

The authors report on room temperature photoluminescence from single CdSe quantum dots. The quantum dots, realized by self-organized epitaxial growth, are embedded in ZnSSe∕MgS barriers. The integrated intensity of the emission drops by less than a factor of 3 between 4K and room temperature. Microphotoluminescence with a spatial resolution of 200nm exhibits single dot emission that remains visible up to 300K. The linewidth of the single dot emission increases thereby from 340μeVto25meV at room temperature, which the authors attribute to the interaction of excitons with optical phonons.

The authors report on efficient spin injection from a n-Zn0.96Mn0.04Se layer into an InAs single quantum dot (SQD) in a p-i-n diode structure using magnetomicrophotoluminescence experiments. A circular polarization degree of up to 60% at B=5T is obtained from the InAs single dot emission while no polarized light is observed in a structure with no electrical doping of any of its layers. A pronounced dependence of the spin injection efficiency on the external bias is found resulting in a strong decrease of the SQD circular polarization degree when the biasing is applied in the forward direction. The experiments emphasize the impact of excess energy and current flow on the spin injection efficiency in a SQD.

The authors demonstrate the usage of magnetic fringe fields from nanoscale ferromagnets to locally control the spin degree of freedom in a semiconductor. Fringe fields stemming from Fe∕Tb multilayer ferromagnets induce a local, remanent out-of-plane magnetization in a ZnCdMnSe dilute magnetic semiconductor quantum well, which in turn aligns the spin of photogenerated carriers via sp-d exchange interaction. The authors achieve a local exciton spin polarization of up to ±12% at 4K without the need of an external magnetic field. The spin polarization can be controlled in sign and magnitude via the magnetization of the ferromagnet and is observable up to T=80K.

Contactless electroreflectance (CER) spectroscopy has been applied to study optical transitions in InAs∕In0.53Ga0.23Al0.24As quantum dashes (QDashes) grown on an InP substrate by molecular beam epitaxy. CER resonances related to optical transitions in all relevant parts of the structure, i.e., InAs coverage, In0.53Ga0.23Al0.24As barriers, and the cap layer, have been clearly observed at room temperature. The signal, which is associated with light absorption in the InAs coverage, has been carefully analyzed, and the optical transitions in the wetting layer (WL) quantum well (QW) and QDashes have been identified in CER spectra. It has been shown that measurements of WL transitions and analysis of their energies allow us to determine the band gap discontinuity for the QDash/QDash-barrier interface. It has been found that the conduction band offset for the InAs∕In0.53Ga0.23Al0.24As interface is close to ∼70%. Moreover, it has been observed that the intensity of the WL transition varies with the cap layer whic...

Time-resolved Kerr rotation in Voigt geometry is used to study the coherent evolution of spin states in moderately doped n-GaAs crystals with picosecond excitation. Due to the small spectral width of the exciting laser pulses, selective excitation of different electronic states is feasible, which allows one to extract both the dependence of the g factor and the spin dephasing time on energy. The authors are able to observe coherent spin oscillations in bulk n-GaAs at room temperature and deduce a g factor of g=−0.316±0.004 at 300K and a magnetic field dependent room temperature spin dephasing time of, e.g., T2*=110ps at B=1.2T.

The authors present a detailed investigation of defects generated during catastrophic optical damage (COD) in high-power 650nm AlGaInP lasers using microphotoluminescence (μ-PL) mapping, focused ion beam (FIB) microscopy, and deep-etching techniques. High-resolution μ-PL images demonstrated that during COD, nonradiative dark line defects (DLDs) originate from the front mirror of the laser and propagate in several branches into the laser perpendicular to the output facet. Furthermore, FIB microscopy identified the epitaxial layers affected by COD, revealing that DLDs are confined to the active region. In addition, deep etching confirmed that these defects have a noncrystalline nature.

The authors report on quantitative investigations of the voltage drop across the heterostructure layer sequence of an operating AlGaInP light-emitting diode via Kelvin probe force microscopy for different external biases between −2.0 and +1.86V. In the low voltage regime, most of the voltage drops in the active layer. For bias voltages above +1.5V, however, they found an additional voltage drop on the p side of the device, which reduces the power efficiency of the light-emitting diode.

Vertically stacked self-organized InGaAs∕GaAs quantum dots have been investigated by chemically sensitive scanning transmission electron microscopy combined with electron energy loss spectroscopy. While the probability of a laterally correlated growth is strongly enhanced with decreasing spacer layer thickness, no measurable variation of both the size and the average In alloy concentration of the quantum dots within the stacks is found, making the structures ideally suited for potential applications in quantum information processing.

We analyze the electrical injection of spin-polarized electrons into a (GaIn)As∕GaAs light-emitting diode. Using an Fe∕Tb multilayer structure with perpendicular magnetic anisotropy and a reverse-biased Schottky contact, we demonstrate spin injection even in remanence between 90 and 260K. The maximum degree of circular polarization of the emitted light is 0.75% at 90K.

The ac-voltage sensitivity of a needle sensor used in a scanning force microscope has been investigated. The voltage sensitivity varies depending if the needle sensor is used as an active or passive device. Using it as an active device, we achieve a voltage sensitivity down to 100μV if the frequency and phase of the excitation voltage of the needle sensor is matched to the voltage of the device under test.

Self-organized InAs quantum dashes grown on In0.53Ga0.23Al0.24As∕InP have been investigated by chemically sensitive scanning transmission electron microscopy. The quantum dashes, which consist of pure InAs, exhibit a triangular cross section. Most important, the quantum dash size depends linearly on the nominal InAs layer thickness and can be varied by a factor of 3 without changing the height∕width ratio. Thus, the emission wavelength can be controlled between 1.37 and 1.9μm without modifying shape and composition of the quantum dashes by adjusting a single growth parameter.

The authors present for the first time DBR-emission in an electrically pumped CdZnSe/ZnSSe/MgZnSSe laser at room temperature. Longitudinal singlemode emission is observed. By changing the period of the Bragg reflector, the emission peak can be tuned over >40 meV. The threshold current strongly depends on the reflector period with a minimum value of /spl sim/160 mA.

A selective dry etching technique for II–VI semiconductors based on ZnSe has been developed by using thermally assisted electron cyclotron resonance etching with a gas mixture of Ar, Cl2, and BCl3. While the etching process is found to be almost nonselective between ZnSe and MgZnSSe at low ( 220 °C) substrate temperatures, a strong selectivity is obtained in the intermediate temperature range (e.g., 12:1 for Tsub=180 °C). Due to the ultrahigh vacuum (UHV) design of the etching chamber, a complete in situ fabrication of buried II–VI nanostructures should be accessible.

Applying the concept of lateral coupling, green emitting DFB laser diodes based on ZnSe have been fabricated and analysed at room temperature under pulsed current injection. The devices are characterised by longitudinal single-mode emission with a typical threshold current of /spl sim/25 mA.