Laser pulses with extremely high energy play an important role in research today - applications range from atomic physics to the investigation of the atmosphere. However, many applications require laser beams with longer wavelengths than those that can be generated with conventional solid-state lasers. For a long time now, researchers have therefore been working on special tricks that can be used to increase the wavelength of a laser beam.
A team from TU Wien, with support from scientists from Canada and Russia, has now pursued a completely new approach. The laser beam is sent through a narrow tube filled with nitrogen gas. When a photon of the laser beam comes into contact with a nitrogen molecule, it can lose a small amount of energy, thereby increasing the wavelength. Thus, the wavelength of the laser can be adjusted by many small light scattering events without losing a large part of the light intensity. This light can then be used to generate short, extremely powerful laser pulses. The new method has now been presented in the trade journal "Optica".
Different wavelengths for creating a pulse
An ordinary laser pointer, which shines continuously, has only one very specific wavelength. However, this is different with short laser pulses. "You can only generate a short pulse if you superimpose different wavelengths in exactly the right way," explains Paolo Carpeggiani from the Photonics Institute at TU Wien. "That's why we're looking for methods to change the laser light so that it consists of many different wavelengths. The micrometer range is particularly advantageous – those are wavelengths that are significantly longer than those that can be generated with conventional sapphire lasers, for example.”
Special tricks for adjusting the wavelengths of a laser beam in this range have already been tested – the so-called "Optical Parametric Amplifiers". “They are very flexible in terms of the wavelength spectrum they can produce, but everything comes at a cost”, says Paolo Carpeggiani. “They also have decisive disadvantages: They are extremely complicated and a large part of the laser energy is lost in the process."
Molecules as energy absorbers
The Viennese team therefore relied on an effect well known in atomic physics - the so-called Raman scattering. A photon is scattered at a molecule and a small part of its energy is transferred to the molecule. When the photon leaves the molecule, it therefore has a slightly longer wavelength than before.
A hollow glass fiber with a diameter of one millimeter and a length of more than five meters was used. The fiber was filled with nitrogen and the laser beam was sent through. "By changing the pressure of the nitrogen gas inside the fiber, we can determine how often scattering processes occur inside. This allows us to control how much the wavelength of the laser light should increase," explains Paolo Carpeggiani.
Because not all photons lose exactly the same amount of energy on their way through the fiber, the laser beam is made up of many different wavelengths – and this is exactly the kind of laser light needed for generating ultra-short and extremely high-energy pulses.
“For this method to work, we had to choose very special kinds of Ytterbium lasers. Actually, the one we used is a prototype developed by our group. Also, creating a very long and perfectly straight hollow fiber was a technological challenge”, says Paolo Carpeggiani.
Promising new tool
"The basic idea is simple, but initially it was anything but obvious, that this method would actually work" says Paolo Carpeggiani. "In fact, our experimental results were much better than could initially be explained with simple theoretical models. It was only when a detailed three-dimensional simulation model was developed in Moscow that it became clear why the method is so successful".
The new fiber optic method now provides a new tool for generating laser pulses in a wavelength range that was previously difficult to access - and at an extremely high energy. “There are countless laser applications which require tunable wavelengths at high laser intensities.. Therefore, our method can be highly useful for many different lines of research”, says Paolo Carpeggiani. The laser pulses can be used to create X-rays for highly advanced measurements in atomic physics, or they can even be used as chemical sensors measuring trace amounts of gases in the air.
P.A. Carpeggiani et al., Extreme Raman red shift: ultrafast multimode nonlinear space-time dynamics, pulse compression, and broadly tunable frequency conversion, Optica 7, 10 (2020)., opens an external URL in a new window
Paolo Carpeggiani, PhD