| Abstract |
The possibility of having low-threshold, inversion-less lasers, making
use of the macroscopic occupation, of the low density of states, at the
bottom of the lower polariton branch, has intensified polariton research
in the last two decades. State of the art devices based on this admixed
quasiparticle have already been realized using GaAs and CdTe active layers,
although the accomplishment of room temperature lasers has been
limited by their relatively weak exciton binding energy. The high exciton
binding energy and oscillator strength, as well as the advantageous relaxation
dynamics of wide bandgap semiconductors, such as GaN, are well
suited for room temperature polariton operation. The up to date demonstrations
of GaN based polariton lasers have used as the active layer bulk
GaN, GaN quantum wells (QW’s), and GaN nanowires. In the latter
approach, individual nanowires are positioned in a microcavity showing
remarkable polariton characteristics, but questions remain on the scalability
of the approach, as well as on how to turn these nanowire-based
structures into real electrically-injected devices. The former two cases
are technologically viable, but are currently limited by the relatively poor
quality of the active region, due to the structural disorder introduced by
the bottom GaN based Distributed Bragg Reflector (DBR) mirrors.
In this thesis, a very straightforward processing technique is used to
etch away an InGaN sacrificial layer, using photo-electrochemical (PEC)
etching, creating ultra-smooth membranes containing GaN/AlGaN QW’s,
which are then embedded between high quality dielectric DBR mirrors,
on which polaritonic studies are performed. The GaN membrane or the
xi
active region is carefully engineered, ensuring superior optical properties,
both prior to and after etching. At room temperature, the QW emission is
state of the art, with a linewidth of ~ 28meV, and a corresponding lifetime
of ~ 275ps. The PEC lateral etching parameters are optimised in such
a way, that the rms roughness of the membranes, measured by Atomic
Force Microscopy (AFM), is as small as 0.65nm, very close to epitaxial
quality. A temperature dependent study on the full-microcavity structure,
unveils the strong coupling regime, exhibiting a robust Rabi splitting
as large as 64meV at room temperature. The non-linear properties
are examined, under non-resonant quasi-continuous excitation, with polariton
lasing demonstrated at an ultra-low, average threshold of ~ 4.5W
/ cm2 (~ 594μJ / cm2), the lowest ever reported for a 2D GaN based
system, accompanied by a spectacular condensation pattern in k-space.
The latter is attributed to a site-specific polariton trapping mechanism,
where polaritons accumulate in discrete levels within the trapping potential,
helping to escalate the polariton density locally. This, along with the
high optical quality of the all-dielectric microcavity (Q-factor ~ 1770), explains
the obtained ultra-low threshold. It should be noted that the use
of ultra-smooth GaN membranes in microcavities is fully compatible with
the realisation of electrically injected GaN polariton devices.
In the direction of obtaining even more robust polaritonic devices, the
basic optical properties of high quality, strain free, GaN nanowires are
studied. However, to make the most out of this novel system, the absorption
coefficients are extracted from as-grown GaN nanowires, on silicon<
111> substrates, developing an all-optical method, analysing merely
the reflectivity spectra, which is demonstrated for the first time. It should
be noted that the absorption coefficients (directly proportional to oscilxii
lator strengths) corresponding to the excitons, provide a glance into the
appropriateness of the respective GaN nanowire system, as optimal candidates
for hefty polaritonics. However, the nanowires studied here, failed
to shown an enhancement of absorption, which can be mainly attributed
to the nanowire dimensions. The new method demonstrated here, can be
extended to any family of nanowires, provided they are grown on a substrate
having considerable difference in permittivity with the nanowire-air
matrix
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