Bernd Witzigmann

Dispersion, Wave Propagation and Efficiency Analysis of Nanowire Solar Cells

We analyze the electromagnetic properties of InP/InAs nanowire solar cells for different geometries. We address both eigenvalue calculations to determine the wave propagation as well as source problems to simulate direct perpendicular illumination by three-dimensional finite element calculations. We demonstrate the validity of a 2D waveguide modal analysis as a method of estimating the results of the computationally far more demanding 3D analysis. The resulting data is employed in a detailed balance analysis in order to determine the optimum set of bandgap energies for a single-junction and dual-junction cell as well as the corresponding efficiency limit. The efficiency of the nanowire design can approach the efficiency of conventional thin-film designs despite the low volume fill-factor.

Electrically Pumped Vertical External Cavity Surface Emitting Lasers Suitable for Passive Modelocking

Modelocked optically pumped vertical external cavity surface emitting lasers (VECSELs) have generated up to 6.4-W average power, which is higher than for any other semiconductor lasers. Electrical pumping of modelocked VECSELs is the next step toward a higher level of integration. With continuous wave (cw) electrically pumped (EP) VECSELs, an average output power of 900 mW has been demonstrated from the undisclosed proprietary novalux extended cavity surface emitting laser (NECSEL) design. In contrast, modelocked NECSELs have only been demonstrated at 40 mW. Recently, we presented a numerical study of EP-VECSELs suitable for modelocked operation; here, we demonstrate the first realization of this design. Power scaling is achieved with a lateral mode size increase. The competing electrical and optical requirements are, on the electrical side, low ohmic resistance, and on the optical side, low optical losses and low dispersion. Additionally, the device needs to operate in a fundamental mode for stable modelocking. We have fabricated and characterized 60 EP-VECSELs with varying dimensions and compared their lasing performance with our numerical simulations. The tradeoff between good beam quality and output power is discussed with an outlook to the modelocking of these EP-VECSELs. Initial EP-VECSEL devices have generated >;100 mW of cw output power.

Absorption loss influence on optical characteristics of multilayer distributed Bragg reflector: wavelength-scale analysis by the method of single expression

Electrodynamical model of a classical distributed Bragg reflector (DBR) consisting of alternating quarter-wave layers of high and low permittivity is considered at the plane wave normal incidence. Reflective characteristics of DBR possessing absorption loss in constituting layers are analysed via correct wavelength-scale boundary problem solution by the method of single expression (MSE). Analysis of optical field and power flow density distributions within the lossy DBR structures explained the peculiarities of their reflective characteristics. Optimal configurations of lossless and lossy DBRs are revealed. Specific DBR structures possessing full transparency at definite number of layers are also analysed.

Electroluminescence from a Quantum-Well LED using NEGF

Nonequilibrium Greens functions (NEGF) are employed to model carrier transport and luminescence in a single-quantum-well light-emitting diode (LED). The sound theoretical formalism allows for a consistent description of coherence loss as well as fundamental scattering mechanisms and reveals details about physical phenomena such as the quantum-confined Stark and Franz-Keldysh effects, tunneling and carrier capture. A comparison to semiclassical results is made and similarities as well as differences are highlighted.

Optical properties of individual GaN nanorods for light emitting diodes: influence of geometry, materials, and facets

We present a systematic analysis of the optical properties of GaN nanorods (NRs) for the application in Light Emitting Diodes (LEDs). Our focus is on NR emitters incorporating active layers in the form of quantum-disc or core-shell geometries. We concentrate on the properties of individual NRs, neglecting any coupling with neighbouring NRs or ensemble effects. The distribution of power among guided and radiative modes as well as Purcell enhancement is discussed in detail in the context of different NR geometries, materials and the presence of interfaces.

Analysis of photonic crystal defect modes by maximal symmetrization and reduction

We analyze in depth the eigenmodes symmetry of the vectorial electromagnetic wave equation with discrete symmetry, using a recently developed maximal symmetrization and reduction scheme leading to an automatic technique which decomposes every mode into its most fundamental internal geometrical components carrying independent symmetries, the ultimately reduced component functions (URCFs). Using URCFs, geometrical properties of photonic crystal defect modes can be analyzed in great details. In particular we analytically identify the kind of modes that display non-vanishing transverse electric or transverse magnetic amplitude at the cavity center in C2v, C3v, C4v, and C6v symmetries, and their degeneracies. We also build a postprocessing tool able to extract and identify URCFs out of the modes whether from experimental or numerical origin. In the latter case it is independent of the eigenmode computation method. In another variant the whole eigenmode computation can be systematically reduced to a minimal domain, without any need for applying specific non-trivial boundary conditions. The approach leads to strong analytical predictions which are illustrated for specific H1 and L3 cavities using the postprocessing tool on full three-dimensional computed modes. It not only constitutes an unprecedented check of the symmetry of the computational results, but it is shown to also deliver a deep geometrical and physical insight into the structure of the modes of photonic bandgap microcavities, which is of direct use for most modern applications in quantum photonics.

Simulation and design of optical gain in In(Al)GaN/GaN short wavelength lasers

In this contribution, microscopic simulation of optical gain in GaN-based short-wavelength lasers is presented. The model is used to perform a design study of different active regions, and to discuss the impact of inhomogeneous broadening, carrier-induced screening of the piezo charges, and well thickness on material gain and laser threshold current. As a reference, the model parameters are calibrated with temperature dependent Hakki-Paoli measurements of spectral gain. Excellent agreement between measurement and simulation is achieved, which gives the design studies a quantitative character.

Full-band Monte Carlo simulation of high-energy carrier transport in single photon avalanche diodes: Computation of breakdown probability, time to avalanche breakdown, and jitter

The high-energy charge transport of electrons and holes in GaAs single photon avalanche diodes with multiplication region widths of 55 nm to 500 nm is investigated by means of the full-band Monte Carlo technique incorporating computationally efficient full-band phonon scattering rates. Compared to previous works, the solution of the Boltzmann transport equation and the incorporation of the full-band structure put the evaluation of the breakdown probability, the time to avalanche breakdown, and the jitter on deeper theoretical grounds. As a main result, the breakdown probability exhibits a steeper rise versus reverse bias for smaller multiplicator sizes. The time to avalanche breakdown and jitter decrease for smaller multiplicator widths.

Computational study of carrier injection in III-nitride core-shell nanowire-LEDs

We report on the computational analysis of a core-shell GaN/InGaN nanowire LED with a capped pyramidal top. The active region consists of a polar multi quantum well (MQW) at the top, a non-polar MQW along the lateral face and a semi-polar one joining them. Differences in the opto-electronic characteristics of the three crystal orientations can be examined, arising from polarization effects as well as the strain-induced bandedge shift. Furthermore the influence of carrier injection efficiency in a nanowire is investigated.

Electromagnetic analysis of polarization and frequency selective tunable optical MEMS

In this work, we present the computation of different integrated optical micro-electro-mechanical systems by applying the finite element method to the electromagnetic vectorial Helmholtz equation. The finite element solver combines state-of-the-art features with sophisticated symmetry handling and the perfectly matched layer approach to simulate open boundaries. The focus of investigation is on a photonic crystal slab for the purpose of a polarization selective filter in an integrated optical sensor system. The transmission characteristics of the filter is modeled with a finite element solver and the results are compared to analytic computations, published results, and the transfer matrix method for selected designs. A final design is found for maximum polarization selectivity at a given operation wavelength.