4 research outputs found
A quantum model of charge capture and release onto/from deep traps
The rapid development of optical technologies and applications revealed the
critical role of point defects affecting device performance. One of the
powerful tools to study influence of defects on charge capture and
recombination processes is thermoluminescence. The popular models behind
thermoluminescence and carrier capture processes are semi-classic though. They
offer good qualitative description, but implicitly exclude quantum nature of
the accompanying parameters, such as frequency factors and capture cross
sections. As a consequence, results obtained for a specific host material
cannot be successfully extrapolated to other materials. Thus, the main purpose
of our work is to introduce a reliable analytical model that describes
non-radiative capture and release of electrons from/to the conduction band
(CB). The proposed model is governed by Bose-Einstein statistics (for phonon
occupation) and Fermi's golden rule (for resonant charge transfer between the
trap and the CB). The constructed model offers a physical interpretation of the
capture coefficients and frequency factors, and seamlessly includes the Coulomb
neutral/attractive nature of traps. It connects the frequency factor to the
overlap of wavefunctions of the delocalized CB and trap states, and suggests a
strong dependence on the density of charge distribution, i.e. the
ionicity/covalency of the chemical bonds within the host. Separation of the
resonance condition from the accumulation/dissipation of phonons on the site
leads to the conclusion that the capture cross-section does not necessarily
depend on the trap depth. The model is verified by comparison to reported
experimental data, showing good agreement. As such, the model generates
reliable information about trap states whose exact nature is not completely
understood and allows to do materials research in a more systematic way
QCL active region overheat in pulsed mode: effects of non-equilibrium heat dissipation on laser performance
Quantum cascade lasers are of high interest in the scientific community due
to unique applications utilizing the emission in mid-IR range. The possible
designs of QCL are quite limited and require careful engineering to overcome
some crucial disadvantages. One of them is an active region (ARn) overheat,
that significantly affects the laser characteristics in the pulsed operation
mode. In this work we consider the effects related to the non-equilibrium
temperature distribution, when thermal resistance formalism is irrelevant. We
employ the heat equation and discuss the possible limitations and structural
features stemming from the chemical composition of the AR. We show that the
presence of alloys in the ARn structure fundamentally limits the heat
dissipation in pulsed and CW regimes due to their low thermal conductivity.
Also the QCL post-growths affects the thermal properties of a device only in
(near)CW mode while it is absolutely invaluable in the pulsed mod
Simultaneous creation of multiple vortex-antivortex pairs in momentum space in photonic lattices
Engineering of the orbital angular momentum (OAM) of light due to interaction with photonic lattices reveals rich physics and motivates potential applications. We report the experimental creation of regularly distributed quantized vortex arrays in momentum space by probing the honeycomb and hexagonal photonic lattices with a single focused Gaussian beam. For the honeycomb lattice, the vortices are associated with Dirac points. However, we show that the resulting spatial patterns of vortices are strongly defined by the symmetry of the wave packet evolving in the photonic lattices and not by their topological properties. Our findings reveal the underlying physics by connecting the symmetry and OAM conversion and provide a simple and efficient method to create regularly distributed multiple vortices from unstructured light. © The Authors.11Nsciescopu
Active Region Overheating in Pulsed Quantum Cascade Lasers: Effects of Nonequilibrium Heat Dissipation on Laser Performance
Mid IR Quantum cascade lasers are of high interest for the scientific community due to their unique applications. However, the QCL designs require careful engineering to overcome some crucial disadvantages. One of them is active region (ARn) overheating, which significantly affects laser characteristics, even in the pulsed mode. In this work, we consider the effects related to the nonequilibrium temperature distribution when thermal resistance formalism is irrelevant. We employ the heat equation and discuss the possible limitations and structural features stemming from the chemical composition of the ARn. We show that the presence of solid solutions in the ARn structure fundamentally limits the heat dissipation in pulsed and CW regimes due to their low thermal conductivity compared with binary compounds. Also, the QCL postgrowths affect the thermal properties of a device closer to CW mode, while it is by far less important in the short-pulsed mode