Thread by @RealSci_Nano

Hello again!

I have this itch to swing back to talking about physics after yesterdays more meta thread, much like a pendulum always striving towards the middle position, but constantly overshooting. BTW, pendulums are an excellent representation of how short laser pulses are created!

You have probably heard (or learned) that laser light has a very defined colour (one specific wavelength/frequency), and all photons are in phase with each other, all their waves are perfectly synchronised (coherent).
(Image source: miridiatech.com/news/2014/02/l…)

However, if we only allow one single wavelength, then the laser light is continuous. Only when we add up several slightly different wavelengths (or frequencies) do we get a short and intense pulse, right at the moment when all those different waves happen to align with each other

Before and after, when all the different contributions are not perfectly aligned, their individual troths and peaks partially (or completely) compensate each other.

Obeying the laws of the Fourier Transforms, shorter pulses require a broader spectrum. And ultrashort pulses consequently require a very broad spectrum, which means only very few lasing materials can be used to create these.
[rp-photonics.com/titanium_sapph…]

With Ti:sapphire lasers, the pulse duration can be around 5 - 30 femtoseconds (fs). For the attoclock method, this is fine because one rotation of the field is around 2.7 fs, giving attosecond resolution in the angle.

https://twitter.com/RealSci_Nano/status/1326189399839793154

But for most other methods, we need pulses which are themselves shorter than one femtosecond. We are exploiting an effect which is part of strong-field physics: High Harmonic Generation (HHG), which converts infrared photons of a fs pulse into extreme ultraviolet (XUV) photons.

The key components of HHG are beautifully demonstrated in this video by the Photonics and Laser Applications Group of University of Salamanca, and JILA, University of Colorado

In my own research, I have 1⃣ looked into phase-matching: How coherent are the different XUV photons which are being emitted by all the recombining electrons distributed in the gas cloud?

doi.org/10.3390/app803…

and 2⃣ also played around with: what happens if the laser field is a mix of two very different frequencies creating a fancy pattern? And if additionally, you have nano-particles nearby, creating a spatial dependence in the laser field?
(second image: doi.org/10.1103/PhysRe…)

(yes, this is me actually talking about something nano finally 😉)

The three-fold symmetry is imprinted onto the typical paths that electrons are taking (first image). But with the spatial dependence on top, that symmetry is completely broken (second image), and the energies with which these electrons are recombining are vastly different.

Preliminary results of this investigation are on arXiV: arxiv.org/abs/1909.04645

oh, I forgot the source for the pendulum laser movie!
here: nccr-must.ch/pictures_video…

(thread by @coenneli)