Generic picture of the emission properties of III-nitride polariton laser diodes: Steady state and current modulation response

The main emission characteristics of electrically driven polariton lasers based on planar GaN microcavities with embedded InGaN quantum wells are studied theoretically. The polariton emission dependence on pump current density is first modeled using a set of semiclassical Boltzmann equations for the exciton polaritons that are coupled to the rate equation describing the electron-hole plasma population. Two experimentally relevant pumping geometries are considered, namely the direct injection of electrons and holes into the strongly coupled microcavity region and intracavity optical pumping via an embedded light-emitting diode. The theoretical framework allows the determination of the minimum threshold current density J(thr),(min) as a function of lattice temperature and exciton-cavity photon detuning for the two pumping schemes. A J(thr, min) value of 5 and 6 A cm(-2) is derived for the direct injection scheme and for the intracavity optical pumping one, respectively, at room temperature at the optimum detuning. Then an approximate quasianalytical model is introduced to derive solutions for both the steady-state and high-speed current modulation. This analysis makes it possible to show that the exciton population, which acts as a reservoir for the stimulated relaxation process, gets clamped once the condensation threshold is crossed, a behavior analogous to what happens in conventional laser diodes with the carrier density above threshold. Finally, the modulation transfer function is calculated for both pumping geometries and the corresponding cutoff frequency is determined.

InGaN alloys and heterostructures

Marlene Glauser

The III-nitride semiconductor material system - (InAlGa)N - is of highest interest for optoelectronic applications due to its direct bandgap, tunable from the ultraviolet to the infrared spectral range. The most well-known are white light-emitting diodes, which are presently revolutionizing the general lighting market, and 405 nm laser diodes, the key elements of the Blu-ray Disc[TM] technology. Furthermore, sophisticated devices like electrically-driven verticalcavity surface-emitting lasers, which are microcavity based lasers, have been demonstrated recently, paving the way for electrically-driven low threshold room temperature polariton lasers. In 1996 Imamoglu and coworkers predicted the possibility of achieving such a low threshold coherent light source from a polariton condensate in a suitable microcavity. The eigenmodes of such a microcavity are exciton-polaritons, admixed particles resulting from the strong coupling between a photonic mode and an excitonic resonance. Thanks to their very light effective mass at the center of the Brillouin zone (105 times lighter than that of a free electron) and efficient relaxation mechanism a phase transition toward an exciton-polariton condensate might occur at elevated temperatures (observed up to 340 K in GaN-based microcavities). The goal of the present study is to provide a detailed theoretical analysis of the main emission features of electrically-driven polariton lasers based on planar GaN microcavities with embedded InGaN/GaN multiple quantum wells and to further derive the stringent requirements necessary for their experimental implementation. The complete polariton phase diagram is established for two experimentally relevant pumping geometries. Furthermore, the steady-state, the high-speed current modulation response, the relative intensity noise, and the Schawlow- Townes linewidth of those two geometries are derived. The resulting general expressions can be applied to any inorganic semiconductor polariton laser diodes. Then the building blocks of such microcavities are experimentally analyzed separately, i.e., the bottom III-nitride based distributed Bragg reflector, the active medium, and the top dielectric Bragg reflector. An innovative optical characterization method allows to study the effect of the substrate on lattice-matched InAlN/GaN Bragg reflectors. The best optical quality is obtained for such Bragg reflectors when grown on high quality free-standing GaN substrates. Furthermore, particular attention is paid to the excitonic localization via simulations and optical characterizations. For this purpose, a photoreflectance setup allowing the determination of the Stokes shift in the InGaN alloys grown on free-standing GaN substrates either as thick layers or as heterostructures (quantum wells) was carefully designed to operate from cryogenic to room temperature. The stacking of several In0.1Ga0.9N/GaN quantum wells results in a detrimental increase in the Stokes shift and thus the absorption linewidth, which is incompatible with the requirements of the strong coupling regime. Various possible reasons such as inhomogeneous built-in field distribution among the quantum wells and their partial strain relaxation are then identified, hence explaining why it is presently not possible to achieve the strong coupling regime with such an active region embedded in a semi-hybrid microcavity. Alternative solutions for the microcavity design to achieve the strong coupling regime with the InGaN alloy are discussed, supported by results obtained from transfer matrix simulations. The inhomogeneous absorption linewidth of thick InGaN layers is deduced from transmission measurements and for low In content layers (x < 12%) it is successfully reproduced by a model based on a stochastic distribution of indium atoms at the atomic scale. It is finally proposed that gain dilution in green laser diodes should not be much higher than in blue laser diodes provided that the In content within the QWs is homogeneous and abrupt interfaces are present.

EPFL2014

A novel class of coherent light emitters: polariton lasers

Raphaël Butté, Nicolas Grandjean

The specificities of polariton lasers-a new generation of coherent light-emitting devices where the relaxation of the bosonic quasiparticles responsible for the light emission, the polaritons, is stimulated whereas the photon emission process issued from the radiative decay of those polaritons is purely spontaneous-are reviewed. It is shown that the practical realization of such devices able to operate at room temperature would most likely rely on wide bandgap semiconductors (inorganic or organic ones) exhibiting highly stable excitons/polaritons. Using III-nitrides, the most advanced of the candidates to date, an electrically driven multiple quantum well polariton laser would display a threshold current density down to similar to 100 A cm(-2), a value more than one order of magnitude lower than that of state of the art GaN-based edge-emitting laser diodes. For the sake of comparison, the main features of optically pumped polariton lasers available that differ from those of their conventional counterparts, namely vertical cavity surface-emitting lasers, are also discussed.

2011

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.