We demonstrate maskless lithography fabrication of nanolayered heterostructured hybrid plasmonic waveguides. This includes the measured optical properties of pulsed magnetron sputtered 15 nm films of aluminum oxide and aluminum nitride. Hybrid plasmonic waveguides, where the modes highest intensity is largely confined to the thin aluminum oxide layer, were constructed by maskless lithography using an aperture-type near-field scanning optical microscope.
Temperature effects in terahertz (THz) step well quantum cascade (QC) structures are investigated. Step well QC
structures with diagonal optical transitions that use fast intrawell electron-longitudinal optical (LO) phonon scattering for
depopulation are considered. A density matrix method is used to model the electron transport coherence and is
incorporated into the Monte Carlo simulations of these structures. A phenomenological dephasing time is also included.
The influence of the lattice temperature on the population inversion is modeled and the effects due to gain spectral
broadening are also considered. Optical gain greater than typical waveguide resonator thresholds are estimated out to
T ~ 200 K.
A density matrix and tight binding model along with a Monte Carlo approach are used to model electron transport in one
and two-well terahertz (THz) step well quantum cascade (QC) structures. Two new structures were analyzed, a multi-step
one-well structure and a principally two-well structure. Both of these structures use a diagonal optical transition for
improved upper to lower lasing state lifetime ratio and feature a step well injector to provide near unity injection
efficiency due to the spatial separation of the wavefunctions. Fast intrawell electron-longitudinal optical (LO)-phonon
scattering is used to depopulate the lower lasing state which does not require the use of resonant tunneling. Density
matrix Monte Carlo simulations are used to analyze these structures in order to investigate these properties. In these
simulations scattering mechanisms including LO-phonon, electron-electron, impurity, and interface roughness scattering
are treated semiclassically, while also contributing to dephasing scattering. A phenomenological dephasing time is also
included to investigate the influence of dephasing on the electron transport within these structures. Subband populations,
electron temperatures, optical gain, and current density are extracted from the simulations. The analysis indicates that it
is necessary to include incoherent transport dephasing in order to provide realistic estimates of the transport process
because the transport is primarily dominated by transitions between weakly coupled states. In addition, this analysis
shows these simplified step well structures are capable of yielding high optical gain ~ 80 cm-1 while at the same time
expected to have relatively low threshold current densities |e|j ~ 380 A/cm2.
In this paper, electron transport properties of terahertz (THz) step well quantum cascade laser structures are analyzed.
These types of structures can allow for the radiative and LO-phonon transitions to be placed within the same well.
Under such an arrangement there are three main energy levels, where the transition from the upper state to the middle
state is at the THz radiative spacing and the transition from the middle state to the lower state is at or near the LOphonon
energy (~ 36 meV in GaAs). The middle state (upper phonon or lower lasing state) is a single energy state,
contrasting to previous LO-phonon based quantum cascade laser (QCL) designs that have doublet states. By having
vertical radiative and LO-phonon transitions within the same well, it is possible for these types of structures to yield high
oscillator strengths, which can lead to increased gain in the active region provided the upper state lifetime and injection
efficiency are maintained. The step in the well allows for high injection efficiency due to the spatial separation of the
wavefunctions. Monte Carlo simulations are used to analyze the structure in order to investigate these properties.
Subband populations, electron temperatures, gain, and current density are extracted from the simulations. Comparisons
are made to other existing conventional square well LO-phonon based QCLs. Our analysis indicates that these types of
structures should be comparable to other design approaches and that step well injectors can be used to increase the
injection efficiency for THz QCLs.
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