Sometimes working with laser pulses that are just femtoseconds in duration can make the job much harder! With very short laser pulses, all of the energy in the pulse is squeezed into a very short burst, this means that the peak power in the laser pulse can be very high. Pulses from the Light Conversion Orhpeus One HE Optical Parametric Amplifier (OPA) are approximately 190 fs in duration, and have a time-averaged power of just 0.65 W. However, at the peak of each laser pulse, just for an infinitesimal length of time, the power reaches around 500 MW. To put that into perspective a power station delivering 1 MW of electricity would be able to power about 650 average homes! Even though the laser power is only this high for an instant this can cause damage and strange effects inside materials that the laser beam passes through.
For the LEAF-2D project researchers at the University of Southampton, Optoelectronics Research Centre particularly want to avoid “non-linear effects”. An example of this type of phenomena could be a material that allows a dim light to pass straight through but completely blocks a much brighter light. To reduce this type of non-linear effect researchers have designed and built the pulse stretcher shown below. Each laser pulse is made up of a spread of wavelengths (colours) and each of these propagates through glass at a different speed. This effect (called dispersion) means that after passing through almost 1 m of glass the laser pulses are stretched from 190 fs to over 1000 fs (or 1 picosecond) in duration, reducing the peak power to below 100 MW. It is hoped that this will help the team to achieve their goal of transferring 2D materials onto a range of substrates using Laser Induced Backwards Transfer (LIBT).