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Title: Quantum dot based semiconductor disk lasers
Author: Butkus, Mantas
Awarding Body: University of Dundee
Current Institution: University of Dundee
Date of Award: 2012
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Since its first successful demonstration more than five decades ago [1], laser technology experienced a huge leap forward in terms of technological innovations and the understanding of underlying physical principles of operation. There were many efforts made by those in both the scientific and commercial communities who envisioned the potential of lasers. As a result, the laser now is a powerful scientific tool in many disciplines. It is widely used not only in physics, but also in chemistry, biology, medicine, engineering, environmental sciences, arts and their interdisciplinary fields. Moreover, it now has a vast number of applications in industry and everyday life whether it is light and matter interaction, communication and IT, healthcare and many other uses of this light source. By the same time, photonics comprises a market of multi-billion EUR value [2].At every stage of development, different laser parameters were engineered to suit those to specific application with some other parameters usually being sacrificed. Together with this, things like compactness and cost were always an issue to consider. A huge impact to the field of photonics was made by the development of semiconductor based structures that could be used as a light amplifying medium. Semiconductor lasers not only allowed the miniaturization of many devices, but also provided new opportunities for laser scientists due to ability to engineer their bandgap properties and to confine the carriers in different dimensions.The development of vertical external cavity surface emitting lasers (VECSELs), which are also known as optically pumped semiconductor lasers (OPSLs) or semiconductor disk lasers (SDLs) realized an important feature in semiconductor based lasers – high multi- Watt output power was combined with diffraction limited output beam profile.This work is devoted to the development of semiconductor disk lasers based on novel quantum dot (QD) structures. QD structures were embedded in this type of laser recently and allowed a number of advantages compared with the widely used quantum well (QW) structure. These included new spectral region coverage at 1-1.3 µm, enhanced wavelength tuneability and ultrafast carrier dynamics, which potentially will improve mode locked operation. QDs were also used as a base for semiconductor saturable absorbers in modelocking experiments.During the time of these studies, QD SDLs at new spectral regions and record output power were demonstrated. Power scaling up to 6 W was achieved for 1040 nm, 2.25 W for 1180 nm and 1.6 W for 1260 nm devices. Excited state transition in QDs was shown to be more efficient for high power QD SDLs as compared with ground state transition. New spectral regions were covered by QD SDLs using frequency doubling into the visible region with green, orange and red light emission with output powers of 2 W, 2.5 W and 0.34 W respectively. The broad gain bandwidth of the quantum dot material was explored and wavelength tuneability up to 60 nm around 1040 nm, 69 nm around 1180 nm, and 25 nm around 1260 nm was demonstrated.A QD based saturable absorber was used to mode-lock the quantum well SDL, resulting in the first such type of laser with sub-picosecond pulse widths. Pulses with duration of 870 fs at a repetition rate of 896 MHz and wavelength of 1028.5 nm were demonstrated. Pulses were 1.14 times Fourier limited and an average output power of 46 mW was achieved. Finally, quantum well based VECSELs with electrical pumping schemes were tested. The devices were first tested in the cw configuration. Highest output powers up to 60 mW were achieved from such devices. Devices were then tested in mode-locking experiments. Pulsed operation was observed and the measurements indicated 270 ps width pulses with 8 mW average output power at 1.9 GHz repetition rate. All devices operated at 980 nm.This thesis consists of six chapters. In the introductory part of this work, QD based SDLs and their development and applications will be reviewed together with their operational principles. Chapter two will describe the growth, fabrication and preparation of SDL samples. Continuous wave and mode-locked operation results will be presented in chapters three and four. Electrically pumped devices will be presented in chapter five along with experimental results. Conclusions and future prospects will be given at the end of this work. The list of publications which were generated during the studies is included at the beginning of this work.The work presented in thesis was done under the FAST-DOT project. This is a European FP 7 project targeted at the development of compact and low-cost novel quantum dot based laser sources for biophotonic applications.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Lasers ; Quantum dots ; VECSEL ; Semiconductor disk laser