Orientation dependent emission properties of columnar quantum dash laser structures

InAs columnar quantum dash (CQDash) structures on (100) InP have been realized by gas source molecular beam epitaxy for stacking numbers of up to 24. Laser devices show low threshold current densities between 0.73 and 3.5 kA/cm(2), dependent on the CQDash orientation within the cavity. Photoluminescence and electroluminescence measurements confirm a strong relationship between the polarization degree of the emission and the orientation of the CQDashes. Eventually, the polarization of the CQDash emission could be changed from predominantly transverse electric to transverse magnetic by simply altering the dash alignment relative to the light propagation axis.

Polarization properties of semiconductor quantum dot and dash lasers

Philipp Ridha

Self-assembled quantum-dots (QDs) represent a distributed ensemble of zero dimensional structures, each presenting a near-singular density of states. The QD size, shape, areal density and optical properties depend on growth parameters, such as growth temperature, growth rate and amount of InAs deposed. QDs are very suitable for optoelectronic device applications as during the epitaxial QD growth the phase transition relieves the strain elastically without introducing defects. Embedded in the active medium of a semiconductor laser diode QD's unique properties lead to improved and often novel characteristics as compared to bulk or quantum-well (QW) devices. In particular In(Ga)As QD lasers on GaAs substrates are of largest interest as the spectral range of their emission wavelength reaches further into the infrared than that of QW lasers of the same material system. The emission wavelength of strained InGaAs QW lasers is limited to about 1150nm, whereas in In(Ga)As QD lasers an emission wavelength beyond 1.3µm is feasible. Such kind of QD lasers performed – as recently reported – ultralow threshold current densities combined with decreased temperature sensitivity, relatively high modulation bandwith, low chirp, which has been already predicted many years ago. Beside quantum dot's application in semiconductor lasers, it has been suggested to use QDs also for the optical active region in so-called semiconductor optical amplifier (SOA). So far, experimentally it was possible to demonstrate in QD SOAs a much faster gain recovery time due to more effective carrier re-filling from excited states than in quantum well structures. Also a smaller noise figure ratio compared to QWs was predicted in QD SOA, but has not been experimentally demonstrated yet. However, two major constraints related to the limit of the high-frequency modulation of QD lasers and the QD SOA's polarization sensitivity are required to be first resolved, before it is possible to implement both technologies replacing QW devices in optical broad telecommunications devices. The polarization properties of semiconductor quantum dot/-dash lasers and amplifiers have been addressed in this PhD thesis. Within this research work – studying systematically several quantum dot and -dash heterostructure of different material families – two major goals have been identified: (1) characterization of the optical polarization properties of quantum dot/-dash structures embedded in the active region of semiconductor optical amplifiers and (2) realization of polarization-insensitive quantum dot based SOAs. Moreover, this thesis focuses on the understanding of the underlying physics being responsible for the QD's polarization and the dependence of the optical gain on the QD's aspect ratio and compositional material contrast. In terms of the experimental work broad area laser devices have been fabricated by applying different clean room processing methods like photolithography, metal deposition, wet and plasma etching. Through a subsequent device characterization using optoelectronic measurement techniques the polarization characteristics of dots and dashes have been accessed. From broad area laser device characterization – measuring the polarization-resolved edge-emitted electroluminescence (EL) – the polarization properties of standard Stranski-Krastanov (SK), closely- and columnar stacked InAs/GaAs quantum dots as well of closely-stacked InAs/InP quantum dots and columnar-stacked InAs/InP quantum dashes – in total 32 devices – have been evaluated. The comparison of the transversal magnetic (TM) versus -electric (TE) integrated EL signal for above heterostructures provided very promising results in order to achieve polarization-independent QD SOA: Starting from standard SK QDs – emitting dominantly in TE mode – it was possible first to enhance the TM EL signal compared to TE by epitaxial shape-engineering of the dot/dash structure of the QD's aspect ratio and the compositional material contrast in columnar-stacked InAs/GaAs (InAs/InP) QDs (QDashes), and then inverting the polarization direction from dominant TE towards dominant TM. The latter polarization transition has been also confirmed by photovoltage spectroscopy (PVS), and as well from room-temperature TM-lasing demonstration – both carried out by our research partner from the Cardiff University, Wales (UK). Introducing the well-known segment contact method for gain measurements, which is based on the analysis of the amplified spontaneous emission (ASE) generated by cavities of different length, it was possible to access both the TM and TE gain of such columnar-stacked InAs/GaAs quantum dots. The work was motivated and funded by the European Union project "Zero Order Dimension based Industrial components Applied to teleCommunications" (ZODIAC) within the sixth framework program. Very fruitful collaborations in order to exchange scientific ideas/know-how and achieving joint results have been established in a pan-European and multicultural inspired environment between four industrial partners – Alcatel Thales III-V Lab (France), Bookham (GB), Innolume – GmbH (Germany), Nanoplus Nanosystems and Technologies-GmbH (Germany) – and three national research institutes – Laboratory for Photonics and Nanostructures (France), The Tyndall National Institute (Irland), Saint-Petersburg Physics and Technology Centre for Research and Education of the Russian Academy of Science (Russia) – and three Universities: École Polytechnique Fédérale de Lausanne (Switzerland), Politechnika Wroclawska (Poland), Würzburg University (Germany). Besides this European ZODIAC project it was possible to establish with the Cardiff University, Wales (UK) a very constructive collaboration.

EPFL2008

GaAs nanowires: from doping to plasmonic hybrid devices

Alberto Casadei

Semiconductor nanowires (NWs) are filamentary crystals with the diameter ranging from few tens up to few hundreds of nanometers. In the last 20 years, they have been intensively studied for the prospects that their unique quasi-one dimensional shape offers to both fundamental and applied science. More recently particular attention has been dedicated to use NWs as building blocks for nano-electronic devices. In this thesis we investigate the electro-optical properties of NWs in order to put some light on the mechanisms governing the electrical transport and the light coupling between NWs and metal nanostructures. We investigate Be and C doping in GaAs NWs synthesized by Molecular Beam Epitaxy (MBE). We obtain a doping control over a large range of densities and we identify a new in situ incorporation path. Since strong surface impurity scattering in III-V materials degrade the electronic performances, we grew NWs passivated with an AlGaAs layer and we investigate their properties. The NW passivation allows for the increase of the electron mean free path by a factor of almost 10. In addition, we designed AlGaAs/GaAs modulation doping NWs. The modulation doping structure allows for the to enhancement of the NW electron mobility revealing excellent properties for the realization of nano-electronic devices. We calculate the electron distribution in the modulation doped NWs and we observe a six-fold symmetry with six 1D electron channels when the carrier concentration is high, while for low concentrations, electrons are delocalized in the GaAs NW core. Thanks to their special interaction with light, semiconductor NWs have opened new avenues in photonics, quantum optics and solar energy harvesting. Here, we design a new system composed of a NW and an array of nanoantennas. Initially, we successfully demonstrated the plasmonic coupling between NWs and nanoantennas, observing an electric field enhancement in the NW as a function of the nanoantenna's gap distance. This finding represented an initial step toward the development of coupled nano-structures for the realization of a new generation of solar cells, detectors and non linear optical devices. Near field coupling was also used between a NW and Yagi-Uda antennas to obtain directional emission. In particular the precise tuning of the Yagi-Uda dimensions and positions leads a strong variation of the NW emission, being able to change this from backward to forward. One of the major challenges for NWs full technological deployment has been their strong polarization dependence in light absorption and emission. Here, we demonstrate that a hybrid structure formed by GaAs NWs with a highly dense array of bow-tie antennas is able to modify the polarization response of a NW. As a result, the increase in light absorption for transverse polarized light changes the NW polarization response, including the inversion of the polarization response. We calculated that the absorption of transverse polarized light can be enhanced up to 15 times. We also fabricate several electrical devices proving our calculated predictions.

EPFL2016

Polarization response of nanowires a la carte

Esther Alarcon Llado, Francesca Amaduzzi, Alberto Casadei, Anna Fontcuberta i Morral, Martin Heiss, Eleonora Russo

Thanks to their special interaction with light, semiconductor nanowires have opened new avenues in photonics, quantum optics and solar energy harvesting. One of the major challenges for their full technological deployment has been their strong polarization dependence in light absorption and emission. In the past, metal nanostructures have been shown to have the ability to modify and enhance the light response of nanoscale objects. Here we demonstrate that a hybrid structure formed by GaAs nanowires with a highly dense array of bow-tie antennas is able to modify the polarization response of a nanowire. As a result, the increase in light absorption for transverse polarized light changes the nanowire polarization response, including the polarization response inversion. This work will open a new path towards the widespread implementation of nanowires applications such as in photodetection, solar energy harvesting and light emission.