Generation of Intense LWIR fields via cascaded SRS

Most lasers for industrial and scientific applications do not emit a continuous beam of light (like a common red laser pointer), but they rather emit flashes of light, named pulses and similar to those of a camera, at regular time intervals. For most applications, the relevant characteristics of these pulses are their energy, duration and wavelength, i.e. their colour. Actually, different wavelengths (within the visible range) are perceived by our eyes as different colours. Many applications require pulses as short and energetic as possible, but the generation of such pulses is a technological challenge and it can be achieved only at few specific wavelengths, typically in the near-infrared. From everyday experience, we know that materials respond differently to colours, absorbing some and reflecting others for instance. The same concept applies to molecules, atoms and electrons, and for this reason it is interesting to investigate their behaviour using laser pulses with different wavelengths. There are many laser sources emitting over a wide range of wavelengths but, as mentioned before, only very specific ones fulfil the requirements of pulse energy and duration. For this reason, over the years several techniques were established to change the wavelength of the pulses after the laser source. These techniques allows to cover a wide, but still limited, range of wavelengths and have typically low input-output energy efficiency (~10-30%). Also, it is not possible to access directly the wavelength range in the immediate vicinity of the input (0 to ~30% shift). When the target wavelength is out of range (for example, when starting from the near-infrared of a typical laser and targeting mid-infrared), it is possible to apply them in cascade, i.e. a second stage performs the conversion taking as input the output of the first stage. Cascaded conversions thus extend the available range, but at the price of the overall energy efficiency. Our group has recently developed a new method to perform laser wavelength conversion. As compared to the other existing techniques, it has less flexibility but way higher energy efficiency (>60%), especially in the vicinity of the input, unlike the other techniques. Most importantly, these characteristics make it particularly suitable for its use as the first stage in the cascaded conversion scheme in combination with the already established techniques, which can be used in the second stage. In this project, we aim to generate laser pulses in the mid-infrared with short pulse duration and higher energy thanks to the improved input-output efficiency and to more performing lasers developed by our group. We will then use these pulses not only to investigate the response of electrons, atoms and molecules but also for specific applications which mostly benefit from mid- infrared pulses, such as the emission of X-rays from solid surfaces.

Laser sources emit light at specific wavelengths-or "colours," as we perceive them when they fall within the visible spectrum. However, for many scientific and technological applications, it would be extremely useful to have access to laser light that spans a much broader and more continuous range of colours. As we know from everyday experience, different materials can react very differently depending on the colour of light they are exposed to. There are several ways to generate laser light in various colours, but most of these solutions face limitations-especially when combined with other demands such as high power, very short light pulses, or large energy per pulse. Some methods exist that can transform laser light into nearly any desired colour within a certain range. However, these methods are not perfectly efficient-some energy is inevitably lost in the process. And while the spectrum of achievable colours can be wide, it is not limitless. Extending this range further often requires repeating the colour transformation process, which leads to increasingly significant energy losses. This creates a key challenge: even if the right colour is technically reached, it might not be available with enough power to be useful. Our project is motivated by exactly this problem. We developed a new approach to transforming laser light that is simpler and more efficient than established methods-though, like everything, it comes with trade-offs. By combining this new technique with existing ones, we were able to expand the range of usable laser light while maintaining better energy performance. This enables us to build the light sources we need not only to explore and investigate the world at the nanoscale, but also to control it. For example, being able to finely adjust the "colour" of laser light is a key factor in steering the motion of electrons-an essential step toward achieving more precise control over physical processes at the smallest scales.