MESA+ University of Twente
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Dimitri Geskus (promotion date: 16 November 2011)

Channel waveguide lasers and amplifiers in single-crystalline ytterbium-doped potassium double tungstates

Promotion date: 16 November 2011

Promotor: Prof. dr. Markus Pollnau

Co-promotor: Dr. Kerstin Wörhoff

In the coming years, many applications in photonics will take advantage of miniaturization by on-chip integration of optical components. In most applications, high-performance integrated lasers are required, having properties such as: large output power, excellent beam quality, broad wavelength tunability and/or the ability to generate ultrashort pulses.

Rare earth (RE) doped potassium yttrium double tungstates (KYW) are recognized to be excellent candidates for fulfilling these roles in bulk configurations.

The aim of this work was to develop microchip optically active devices in KYW, to exploit its favorable laser-related properties. Therefore thin films of crystalline co-doped KYW were grown onto substrates, where the material properties like: the lattice constants, refractive index and laser related properties of the grown layers, could be designed by heavily co-doping of the grown layers. The advantageous laser properties of the heavily co-doped thin films were confirmed by demonstration of highly efficient (>80%) laser operation.

Etching of channel structures into these crystalline planar waveguides resulted in channel waveguide lasers demonstrating large output power (650 mW), broad tuning range (980 - 1045 nm) and low quantum defect (0.7%) laser operation. In addition, focused-ion-beam nanostructured photonic cavities were fabricated, leading to laser emission from on-chip laser devices.

Moreover, extremely high optical gain has been demonstrated in a channel waveguide by designing a specific composition of the material, reaching similar optical gain as observed in state-of-the-art semiconductor waveguide optical amplifiers.

Which were the high-lights of your thesis project?

The demand of microchip optical circuitry started in the field of the optical telecommunication, where intercontinental fibre networks started to scale down to the fibre of the home, hence requiring smaller optical switches.

In addition novel applications for integrated optics were developed, such as optical sensing. Laser sources are required to drive these passive optical circuits.

I worked on the fabrication of optically active channel waveguides, using the crystalline material KYW, in order to develop on-chip laser devices and optical amplifiers.

The novelty of the obtained results are the fabrication of smooth channel waveguide structures in KYW and the achieved optical gain. The optical gain in KYW originates from the RE doping. Since the doping is regarded as impurity no large gain per unit length is commonly expected. However due to the crystalline character of KYW we managed to increase the doping concentration by two orders of magnitude. Thereby the observed optical gain is also improved by two orders of magnitude, making it comparable to the gain achieved in semiconductor waveguide optical amplifiers.

The used KYW can open new applications in many areas, for example in creating near-infra red short pulses. These short pulse sources are for example used in eye surgery, as the pulses have a high energy output in a very short time period, therefore not leading to burning wounds.

The developed technology like crystal growth and micro structuring of the crystalline material in combination with the available laser laboratory, was very important in this thesis project. New research projects have been started to further develop micro-chip devices in KYW for real applications.

Did you manage to have some nice publications?

I had two publications in Optics Express and one in Laser Physics Letters. Two additional publications are pending.

How did you develop as a researcher and scientist during your thesis period?

I did follow an unusual track towards my promotion, starting at engineering education and then university. My expertise has always been in the experimental field. In this thesis project, I made the link between experimentally obtained results and theoretical modelling. By communicating a lot with colleagues and experts, and by performing clever simulations, I reached a level of understanding the physics behind the project that is necessary to build new, challenging experiments.

Did you feel a member of the Mesa+ community?

In using the Mesa+ cleanroom facilities and making use of the expertise of the cleanroom technicians, I felt like a Mesa+ member very much so. The technicians do have years and years of experience and they are always willing to help you as a scientist, even if you want to perform experiments that are slightly strange and not always in line with the materials commonly used.

If one finds the right expert, it is well possible to find the right recipe for your experiment within a short range of time, whereas for you as a newcomer it would take months before reaching the same results, or even longer than that.

What, in your opinion, is important for Mesa+ to stay successful in future years?

The experimental spirit in using the cleanroom facilities is the most important, I believe. The experts have an open scientific mind, understanding the special demands of the researchers. In my opinion, it is important for students and other researchers to learn how to find the right persons for advice and help with the experiments, using their social skills to the full.

However, the most skilled professional technicians are always the busiest ones, as their success attracts extra work, as a matter of principle.

What are your future plans?

At the moment I am working for two spinoff companies of the University of Twente: Lionix and Xio Photonics. I help them assembling various novel optical chips, where no chip is alike.

We work at the front end of optical chips, where we expand and push the limits every day. I like that kind of work, where I can combine my experimental expertise with a lot of creativity. I guess in later years I would like to work in a research department as part of a company. Now, I would love to work for some years abroad, before finding a permanent job.