Recent advances in optimizing the efficiency and lifetime of far-UVC LEDs with emission wavelengths below 240 nm are presented. The design of the semiconductor heterostructure is considered as well as the chip layout. Cross-comparisons are used to draw general conclusions about degradation mechanisms in UV LEDs and to identify development strategies to minimize them. Furthermore, it is discussed which chip packaging is particularly suitable for a combination of far-UVC LEDs with spectral filters. Finally, far-UVC irradiation systems for skin-friendly irradiation of the human body are presented and their performance is illustrated with selected medical and biological data.
Far-UVC LEDs are interesting for applications such as skin-tolerant inactivation of multiresistant pathogens and gas sensing. We present the development of 233 nm AlGaN-based far-UVC LEDs with an emission power of 3 mW at 200 mA and L50 lifetime of more than 1000 h, after burn-in. Additionally, the design of a far-UVC LED-based irradiation system, with a spectral filter which supresses emission >240 nm, to study the inactivation of bacteria and skin compatibility of the radiation will be presented. The system can be used to homogeneously irradiate a target area of 70 mm diameter with a mean irradiance of 0.4 mW/cm².
In systems of coupled III-V semiconductor nanostructures of mixed dimensionality, the different classes like quantum wells, quantum dots, and sub{monolayers form new mixed states when they are combined. To address the complexity of such systems over a wide energy range, we present a white-light approach for multidimensional coherent spectroscopy. As a proof of principle we determine the homogeneous linewidth of InAs quantum dots in an inhomogeneously broadened ensemble.
Quantum state tomography and the reconstruction of the photon number distribution are techniques to extract the properties of a light field from measurements of its mean and fluctuations. These techniques are particularly useful when dealing with macroscopic or mesoscopic systems, where a description limited to the second order autocorrelation soon becomes inadequate. In particular, the emission of nonclassical light is expected from mesoscopic quantum dot systems strongly coupled to a cavity or in systems with large optical nonlinearities. We analyze the emission of a quantum dot-semiconductor optical amplifier system by quantifying the modifications of a femtosecond laser pulse propagating through the device. Using a balanced detection scheme in a self-heterodyning setup, we achieve precise measurements of the quadrature components and their fluctuations at the quantum noise limit1. We resolve the photon number distribution and the thermal-to-coherent evolution in the photon statistics of the emission. The interferometric detection achieves a high sensitivity in the few photon limit. From our data, we can also reconstruct the second order autocorrelation function with higher precision and time resolution compared with classical Hanbury Brown-Twiss experiments.
The investigation of surfaces and thin films is of particular interest in current research as it provides a basis for a multiplicity of applications such as waveguides, sensors, solar cells and optoelectronics. The losses of light emitting structures, here CdSe nano-platelets, can be reduced by harmonizing the orientation of the transition dipoles with the optical mode that the light is coupled to. The electronic structure of the emitting nanoparticle can be optimized via its shape and the density of states strongly depends on the dielectric function of the environment which can be tuned to modify the emission characteristics.
Stefan Meister, Aws Al-Saadi, Bülent Franke, Shaimaa Mahdi, Miroslaw Szczambura, Berndt Kuhlow, Ulrike Woggon, Lars Zimmermann, Harald Richter, David Stolarek, Sigurd Schrader, Hans Eichler
We have designed, fabricated and investigated one-dimensional (1D) micro-cavities in Silicon-on-Insulator (SOI)
waveguides. The single mode waveguides are fabricated in a 220 nm silicon device layer. The 1D micro-cavities in
Fabry-Perot structure consist of two Bragg-mirror regions formed by a sinusoidal modulation of the waveguide width.
The mirror regions are separated by a sub-micron spacer.
The SOI photonic structures are produced in a CMOS environment using 248 nm DUV lithography. The waveguides as
well as the width modulated mirror regions are designed using a single mask and are fabricated in a shallow trench
process.
The transmission spectra of these width modulated micro-cavities with different mirror reflectivities and cavity lengths
are investigated. Q-factors up to 855 could be observed at 1550 nm wavelength with low insertion loss of 1.9 dB.
The width modulated micro-cavities, including the mirror regions, have lengths of less than 20 microns and widths of
maximum 450 nm. These small foot-print cavities act as band pass filters and can be used as resonators for laser or
electro-optic modulation of light.
We investigate the coherent cavity-field coupling in chains of polystyrene microspheres doped with CdSe nanocrystals. The coupled cavity emission is studied by imaging spectroscopy and polarization-sensitive mode mapping. The spatial dependence and polarization nature of coupled and uncoupled cavity states are discussed for applications as building blocks in coupled resonator optical waveguides. When coherent photon states are formed in the coupled cavities, the symmetric field distribution of a single microsphere evolves into a directional emission pattern with strongest modifications along the axes of the coupled cavity system. For weak coherent cavity coupling, the coupled modes show almost vanishing field intensity along the axis. In case of weak coupling still cavity modes can be observed for which the single sphere Q-factor is maintained which is promising for applications in coupled resonator optical waveguides.
The dephasing time in semiconductor quantum dots and quantum-dot molecules is measured using a sensitive four-wave mixing heterodyne technique. We find a dephasing time of several hundred picoseconds at low temperature in the ground-state transition of strongly-confined InGaAs quantum dots, approaching the radiative-lifetime limit. Between 7 K and 100 K the polarization decay has two distinct components resulting in a non-Lorentzian lineshape with a zero-phonon line and a broad band from elastic exciton-acoustic phonon interactions. On a series of InAs/GaAs quantum-dot molecules having different interdot barrier thicknesses a systematic dependence of the dephasing dynamics on the barrier thickness is observed. The results show how the quantum mechanical coupling of the electronic wavefunctions in the molecules affects both the exciton radiative lifetime and the exciton-acoustic phonon interaction.
We present temperature-dependent measurements of the dephasing time in the ground-state transition of strongly-confined InGaAs quantum dots, using a highly sensitive four-wave mixing technique. At low temperature we measure a dephasing time of several hundred picoseconds. Between 7 and 100 K the polarization decay has two distinct components resulting in a non-Lorentzian lineshape with a sharp zero-phonon line and a broad band from elastic exciton-acoustic phonon interactions. We also explore the dephasing time beyond the one exciton occupation, by electrically injecting carriers. Electrical injection into the barrier region results in a dominantly pure dephasing of the excitonic ground-state transition. Once the injected carriers have filled the electronic ground state, additional filling of the excited states creates multiexcitons that show a fast dephasing due to population relaxation.
We present a model for gain in a quasi zero-dimensional quantum confined semiconductor system. Due to a multitude of one-electron-hole pair and two-electron-hole pair transitions, the gain region is broad, quasi-continuous and stretches below the absorption edge. Femtosecond experiments in the gain region of strongly confined CdSe quantum dots confirm our theoretical predictions.
For the first time we report spectral hole-burning with the width as small as (Gamma) equals 10 meV for semiconductor quantum dots of the size much smaller than the excitonic Bohr radius of the bulk material. The narrow nonlinear resonances found are typical of the structures prepared at the nucleation and normal growth precipitation stages. The saturation intensity on the order of <EQ 102 kW/cm2 registered for high-quality CdSe-doped glass is close to that for excitonic nonlinearity of CdSe monocrystalline films. The structures grown at the coalescence stage show only broad-band nonlinear absorption with a saturation intensity much higher than the proper bulk material.
The authors report on optical investigation of CdSe-microcrystallites (MCs) embedded in glass and grown diffusion controlled to create MCs near the stage of nucleation having small radii (approximately equals 1.6 nm) and a symmetric size distribution. Room temperature measurements of (i) absorption bleaching and its kinetics at resonant MC-excitation using frequency doubled single ps-pulses from YAG-laser, and (ii) spectral hole burning at different delays in the ps-time scale, before and after a surface passivation of the samples by hydrogenation to remove MC- glass interface states responsible for the carrier transfer into the matrix, have been performed. After passivation, higher bleaching values, a slower recovery, and a blue shift of the DTS signal are obtained. A kinetic model including three levels and a fast carrier capture into the glass matrix is discussed to explain the observed nonlinear optical behavior.
We report on optimum conditions for the diffusion-controlled growth of C4Se-microcrystallites (MC's) near the nucleation stagehaving a very small and symmetric size distributiolL Temperature dependent linear absorption measurements and room-ternperature ps-hole burning spectroscopy have been performed for material chaiacterization and for the determination of MC-parameters. The influence of MC-glass interface states on the nonlinear optical behaviour is studied investigating the bleaching behaviour and its kinetics at surface passivated (hydrogenated) and at non-passivated samples.
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