arXiv subjects

M. Willatzen

Publications and source records attributed to M. Willatzen.

4 recordsLinked to original sources

arXiv2013

Extraordinary absorption of sound in porous lamella-crystals

We present the design of a structured material supporting complete absorption of sound with a broadband response and functional for any direction of incident radiation. The structure which is fabricated out of porous lamellas is arranged into a low-density crystal and backed by a reflecting support. Experimental measurements show that strong all-angle sound absorption with almost zero reflectance takes place for a frequency range exceeding two octaves. We demonstrate that lowering the crystal filling fraction increases the wave interaction time and is responsible for the enhancement of intrinsic material dissipation, making the system more absorptive with less material.

cond-mat.mtrl-sci↗

arXiv2010

Analysis of optical properties of strained semiconductor quantum dots for electromagnetically induced transparency

Using multiband k*p theory we study the size and geometry dependence on the slow light properties of conical semiconductor quantum dots. We find the V-type scheme for electromagnetically induced transparency (EIT) to be most favorable, and identify an optimal height and size for efficient EIT operation. In case of the ladder scheme, the existence of additional dipole allowed intraband transitions along with an almost equidistant energy level spacing adds additional decay pathways, which significantly impairs the EIT effect. We further study the influence of strain and band mixing comparing four different k*p band structure models. In addition to the separation of the heavy and light holes due to the biaxial strain component, we observe a general reduction in the transition strengths due to energy crossings in the valence bands caused by strain and band mixing effects. We furthermore find a non-trivial quantum dot size dependence of the dipole moments directly related to the biaxial strain component. Due to the separation of the heavy and light holes the optical transition strengths between the lower conduction and upper most valence-band states computed using one-band model and eight-band model show general qualitative agreement, with exceptions relevant for EIT operation.

cond-mat.mes-hall↗

arXiv2005

Electronic structure of wurtzite quantum dots with cylindrical symmetry

This paper presents a six-band k.p theory for wurtzite semiconductor nanostructures with cylindrical symmetry. Our work extends the formulation of Vahala and Sercel [Physical Review Letters 65, 239 (1990)] to the Rashba-Sheka-Pikus Hamiltonian for wurtzite semiconductors, without the need for the axial approximation. Results comparing our formulation for studying the electronic structure of wurzite quantum dots with the conventional formulation are given.

cond-mat.mes-hall↗

arXiv2005

Dynamics of a nanowire superlattice in an ac electric field

With a one-band envelope function theory, we investigate the dynamics of a finite nanowire superlattice driven by an ac electric field by solving numerically the time-dependent Schroedinger equation. We find that for an ac electric field resonant with two energy levels located in two different minibands, the coherent dynamics in nanowire superlattices is much more complex as compared to the standard two-level description. Depending on the energy levels involved in the transitions, the coherent oscillations exhibit different patterns. A signature of barrier-well inversion phenomenon in nanowire superlattices is also obtained.

cond-mat.mes-hall↗