The laboratory facilities

The Femto-VINIR project will launch a new experimental research direction at DTU Fotonik, namely ultra-fast femtosecond cascaded nonlinear processes. The research covers frequency conversion processes where a "cascade" of nonlinear processes occur. This might be because the conversion process is not phase matched or because several frequency conversion processes are phase matched at the same time. These processes are very weak except for large intensities, which is why very energetic femtosecond pulses are needed to study them. These can currently be made by commercial state-of-the-art solid-state laser systems available at DTU Fotonik.

The most challenging part of the Femto-VINIR project is to carry out the experiments. These are cutting-edge experiments using some of the most advanced optical equipment in the world, and the aim is to generate some of the shortest pulses ever observed. DTU Fotonik has the experimental expertise and the facilities to carry out these experiments.

 

Spitfire regenerative amplifier and TOPAS OPA

 Commercial infrared solid state femtosecond laser amplifier anno 2008 located in the labs of DTU Fotonik. The laser system generates high-energy ultra-short femtosecond pulses. An infrared diode-pumped solid-state laser pumps a femtosecond oscillator containing a titanium sapphire crystal as a laser medium. From the oscillator emerges 80 million infrared (λ=800 nm) pulses per second that are 35 fs short. Since these pulses have a low energy (around 1 nJ, or 1 billionth of a Joule) they are sent into an amplifier, also containing a titanium sapphire crystal. The amplifier is pumped by a green (λ=527 nm) pulsed solid-state laser, and every millisecond a pulse from the oscillator is selected (the rest are rejected and lost!) and is amplified more than a million times in energy. At the exit of the amplifier the infrared (λ=800 nm) pulse has an energy of 3.5 mJ and a duration of 35 fs. After the amplifier the pulses can be frequency converted in an optical parametric amplifier so we can generate pulsed light of different colours than λ=800 nm (the near-UV, visible and near-infrared can be generated).

Specifically, DTU Fotonik bought in 2008 a state-of-the-art commercial high-power femtosecond laser system, and this laser is the cornerstone in the experiments in the femtosecond research done at DTU Fotonik. The laser delivers each second 1,000 ultra-short 35 fs pulses at a near-infrared wavelength of 800 nm and each pulse has an energy of 3.5 mJ. This might not seem so much, but when the pulses are as short as 35 fs the laser radiation can reach incredibly high intensities (>1020 W/m2). The laser system is based on solid-state technology, and uses titanium sapphire crystals as gain medium. The high-energy pulses are generated in a regenerative amplification setup.

 

This laser system is very expensive and is consequently only found in the most advanced laboratories. To put it into perspective, it is the most powerful laser system in Denmark, and costs around 300,000 €. It is quite common to own one or two of this kind of laser system in leading photonics departments around the world, but due to its high cost it is usually shared by many researchers.

 

At DTU Fotonik a parallel setup means that the Femto-VINIR research group has full-time access to the laser, so the experiments are not held up by scheduling problems between many users as is often seen in other groups. Having this laser at DTU Fotonik is a huge asset to the ultra-fast femtosecond research carried out there.

 

The laser is installed in a large laboratory where up to half a dozen experiments can use the laser quasi-simultaneously. Several cutting-edge experiments are performed, e.g. for generating single-cycle terahertz (long-wavelength) radiation, ultra-fast pump-probe spectroscopy as well as the research on ultra-fast pulse compression connected with the Femto-VINIR project and the current project proposal. The main beam from the laser system is currently split in two and then transported to separate optical tables in the lab. For our experiment we have installed a commercial optical parametrical amplifier so we can convert the 800 nm pulses to any wavelength we desire between 500 nm and 2,000 nm (spanning the visible and near-mid—infrared). This shared but parallel use of the laser facility ensures a high degree of collaboration, synergy and cross-disciplinary interaction.