Investigation of quantum dots within electrically excited vertical and external cavity surface emitting lasers for the realization of a coherent and compact dual wavelength laser – IDYLIC

The IDYLIC project implies several scientific challenges, which have been clearly identified, and have motivated the structuration of the profect with the following tasks : - The task 1 is dedicated to the project material optimization. This task aims to optimize several important material issues to be considered in the final device. QD structural properties will be defined in terms of dimensions and in plane density, as function of homogeneous / inhomogeneous linewidth interplay and modelling predictions. QD gain will be optimized for VECSEL operation. Wafer fusion of GaAs based DBR and QD on InP(113)B reliabilities will be tested, and wafer fused VECSEL processing will be upgraded to accommodate both materials requirements The task 2 aims in developing optical and electrically pumped VECSEL using wafer fusion approach and its optimization for bi-frequency operation. In order to evaluate the difference between QD and QW active region behaviour in such a VECSEL, in this task it will be fabricated VECSELs based on QD (growth at FOTON) and QW (growth at EPFL). - The task 3 is devoted to advanced characterization of both optically and electrically pumped QD-VECSEL. Their amplitude noise and phase noise properties will be explored and compared to that of QW VECSELs. A peculiar attention will be paid to possible dual frequency operation of QD structures. To this aim a deep investigation of simultaneity/bistability behavior as a function of the frequency difference between the two modes will be carried out. The final goal of this task is the first demonstration of electrically pumped shot noise limited dual-frequency laser. -The task 4 is dedicated to improve QD bi-frequency modelization. In paritcular, expeirmental data coming from task 1 and 3 will be integrated withihn the model, and also a more complete 2D model will be developped to take into account carriers and temperature spatial inhomogeinities.

This first period of the project has been dedicated to the starting point of most studies and experimental set-up to be installed. The most important achievements are the following : - a Spectral Hole Burning set-up has been created. It enables the measurement of the homogeneous linewidth, from 10 to 400 K, with carrier densities close to laser regime. - An optically excited VeCSEL set-up fo bi-frequency generation within mono-axis configuration has been created. This set-up integrates selective loss actuators on each laser arms, enabling the investigation of simultaneity/bistability of the two modes, and the direct measurement of coupling constant. - an electrically excited VeCSEL set-up has been developped - The fabrication process of electrically excited VeCSEL has been optimized. Laser operation has been demonstrated at room temperature on 30-50 µm in diameter devices, with good performances (Is<10 mA, Pmax > 2mW). - InP(311)B substrates have been tested in order to fix the technological compatibility with processing requierements, in particular with wafer fusion and burried tunnel junction (BTJ). BTJ based LEDs have been successfully realized, and a quantum well based optically excited VeCSEL is under fabrication. - QD growth has been investigated. QD densities from 10^10 up to 2.10^11 cm-2 have been obtained, with good optical properties. The objective of these first tests, is to fix the ideal QD density, to get enough gain and reduced homogeneous linewidth and coupling, for the future bi-frequency operation. - At last, 0 D simulation has already shown the great potentials of QD for the dual frequency operation, this model is under investigation to integrate real experimental data. Going further, this model has been optimized (mainly when considering simulation time), in order to develop a more realistic 2 D model (integrating spatial inhomogeinities of temperatuire and carriers).

At midterm of the project, most of scientific tasks have started according to the project planning, and should able to demonstrate shortly important results. In particular, next steps of the project will be : - homogeneous linewidth measurement as function of QD density. These measurements will able to complete 0D simulation, and will help to design bi-frequency VeCSEL. - QD gain measurements will be carried according to length dependant laser experiments, as a first step prior to QD VeCSEL fabrication - Optically pumped QD VeCSEL will be realized and tested. These first experiments are expected to evidence class A QD VeCSEL operation, and obviously to demonstrate dual frequency lasing and its range of operation (stability, coupling constants and frequency spacing, temperature ..). With the proposed 2D model, these will enable to increase the understanding of such new laser, and to be able to go further in desgin optimzation. These results will consist one of the most scientific breakthrough of the project. - Electrically pumped VeCSEL with quantum wells (and next QDs) will be realized and tested, in order to evidence awaited improvements on low frequency phase and intensity noise, and its impact on dual frequency lasing and beatnotes. Finally, partners of the project would like to underline two recent major advances in the scientific community, which clearly brings evidences on IDYLIC objectives reliability: - Fedorova et al (PTL 2017) has recently demonstrated beatnotes from a QD bi-frequency edge emitting laser, ranging from 0.28 up to 34 THz, showing experimental evidences of the reliability of QD to reach dual frequency operation in a frequency span similar to IDYLIC objectives. - Two partners of the project have recently demonstrated class A operation of a quantum dashes based VeCSEL, giving some insights that QD based VeCSEL should also operate in a class A laser regime.

the beginning of the project has been mainly dedicated to the development of the main tasks of the project (modelization, experimental set-up, materials expeirmentations and compatiblities, device processing). Most of those important building blocks are at the moment effective, or will be shortly. First important scientific results are expected in the month to come, which would imply at least 3 or 4 scientific papers single or multi partner. Up to now, IDYLIC related works have been communicated throught one international conference (talk) and one national presentation (poster).

IDYLIC is a research project, dedicated to the development and demonstration of a new concept of an ultra-stable coherent dual frequency laser for microwave photonic applications at telecommunication wavelength. It covers applications from radio-over-fiber communications, delivery and detection of radar signals, up to THz wave signal generation. When most of dual frequency lasers demonstrated so far are based on complex and bulky solid state lasers which require optical excitation, some attempts have been done to propose a compact, electrically excited dual frequency lasers using semiconductors. Nevertheless their fabrication and/or manipulation are still complex mainly limited by the semiconductor active layer itself (quantum well) behaving as a homogeneous medium. This implies that simultaneous dual frequency generation is not feasible within the same volume because of mode competition. As a consequence, complex architectures have been proposed based on phase locked edge emitting lasers (with external electronics or coupled waveguides), or using two different and spatially separated and slightly coupled oscillating modes. In the first case, such laser cavities suffer from a well-known low spectral purity, far from their solid state counterpart, which severely limits the beatnote linewidth of the two frequencies. In the second case, the spatial separation lying on a two axis optical configuration implies complex optical alignment rules to avoid mode competition, and make it very difficult to use electrical injection in such devices.
In IDYLIC project, we propose to combine a semiconductor based active layer behaving as an inhomogeneous medium like solid state material by using quantum dot (QD), and a vertical external cavity surface emitting laser (VECSEL) cavity design ensuring a high output power, class-A laser regime, and a high spectral purity. This unique union will enable a dual frequency emission without spatial separation (single axis configuration) of the two lasing mode which is highly suitable for a better compactness and a simplified pumping scheme, making electrical injection possible. As a preliminary and essential step, first demonstrations of coherent dual frequency emission at 1.5 µm will be carried out with single axis optically pumped QD VECSEL, exploiting QD potentialities to cover frequency difference from 100 GHz up to 4 THz, with very narrow lasing and beatnote linewidth (< 10 kHz). According to state of the art thermal management processing, consisting in wafer fusion technics of GaAs/AlAs mirror and bonding on diamond substrate, device output power in the watt range will be demonstrated. In addition, the second important objective of the project will be the development of an electrically pumped QD-VECSEL. Apart the fact that homogenous electrical injection in such large in diameter lasers (> 50-100 µm) is very challenging, such a pumping scheme is more promising for integration in an optoelectronic system, but above all may enable to decrease low frequency noise These two main IDYLIC challenges are the key points to develop a device with performances without any equivalent at the moment.
To succeed in these ambitious objectives, the IDYLIC project will address material optimizations (QD issues), which will define, in close collaboration with QD modelization, the best QD structural properties for the dual frequency emission. Also state of the art VECSEL processing will be considered and developped, prior to device characterization. At the end of the project, coherent dual frequency lasers developed in IDYLIC project will be tested and evaluated in microwave photonic applications, including the generation of THz radiation for medical imaging and sensing systems.