Dependency of high-order harmonics to polarisation properties of many-cycle driving fields

Abstract

Light-matter interactions can be used to probe both light and matter as they yield information on a system's state. On the ultrafast timescale, it is possible to employ these interactions to probe fast-occurring phenomena.
In particular, one of the few tools for ultrafast spectroscopy are coherent attosecond XUV pulses: light pulses in the extreme ultraviolet range (XUV) whose typical duration is a few hundred attoseconds. These pulses are generated by a non-linear light-matter interaction process called high-order harmonic generation (HHG). It can even be possible to control the properties of generated harmonics by controlling those of the electric field that generated them. However, their dependency to properties of the electric field such as its polarisation state or its symmetries needs to be investigated to gain greater control over the properties of attosecond XUV pulses.

This thesis examines the impact of driving HHG with elliptically polarised or two-colour laser fields. Simulations and experimental work focused on studying HHG spectra using Argon when varying laser parameters, including the driving laser field's intensity and polarisation state. Simulations used the time-dependent Schrödinger equation with a single Argon atom and assumed a single-active electron; experiments used a user-friendly source of harmonics based on a solid-state femtosecond infrared laser emitting many-cycle pulses.

For the harmonic order closest in energy to Argon's first ionisation energy, simulations and experiments produced harmonic yields displaying a local maximum for certain driving fields ellipticities, with maximum yields found for slightly elliptically polarised fields instead of linearly polarised ones. Further simulations show that a linearly or circularly polarised two-colour field can generate sets of harmonics among all orders instead of only odd ones, or weakly generate orders multiple of 3 with elliptical polarisations, respectively. Harmonics are further enhanced or suppressed depending on the relative intensity and the phase delay between the field's colour components.

These results add to existing knowledge on how to select, suppress, or enhance harmonics in Argon-based spectra by shaping the laser field driving HHG. Broader knowledge would ultimately allow appropriate harmonic selection for spectroscopic aims. Envisioned uses include following the temporal evolution of chiral systems using attosecond XUV pulses.

Popular Abstract

What kind of music can atoms play? Although they have no instruments, atoms – bricks of matter that build the world – can strongly interact with laser light and generate brief light flashes that can be as highly harmonic as music. Understanding how both lights are related through their interactions with atoms can help us follow events that happen too fast to be seen.

Firstly, light can be described as a wave, just like sound or vibrations on a string. An atom has a core to which electrons are bound; these are tiny orchestra players gathered around the core-conductor. Sometimes, electrons can however escape and travel away from the atom if an external light strong enough to disturb it – such as laser light – is directed on it. Imagine musicians being led by a conductor to play a music piece that only goes up and down in waves; some musicians may prefer to play something else instead (a concerto solo for instance) and will take a leave from the orchestra.

When these free electrons return to the atom, they will have gained too much energy from the laser light. Just as musicians must give up their concerto and play the conductor's wave music if they want to be back in the orchestra, electrons emit excess energy as light when they return to the atom. Every time the laser light is repeating its wave pattern (every time the conductor waves up and down its arm), this emission happens twice (one time up, one down). If this pattern is repeated very fast, the emitted light is emitted as brief pulses, whose duration is several attoseconds (1 attosecond = 1 billionth of a billionth of a second). These light pulses are far more harmonic than wave-shaped music who would contain a single tone.

Light has also a property called polarisation, telling us which shape it takes when vibrating. For instance, a circular polarisation results in corkscrew-shaped vibrations since vibrations rotates along a circle. Of particular interest in this thesis was to investigate how laser light polarisation changes the light's shape and in turn the harmony of light pulses from atoms. By simulating the strong interactions between an atom and laser light, and by using a real-life attosecond pulse source, we saw for instance that harmony within pulses can sometimes be tuned to be stronger when light is polarised as an ellipsis.

In the future, this work will hopefully help us composing our own attosecond pulses, light flashes that allow us to take pictures of events almost as short as them – electrons moving around atoms, for instance – and to better understand how our world is built as whole by seeing more of it. So the next time you think about atoms, think of how the tiniest of musicians can play a much larger part in the universe – and thank them for the music.

Popular Abstract (Swedish)

Hur låter musiken som spelas av atomer? Även om de inte kan sjunga så kan atomer – de små byggstenarna som bygger världen – interagera starkt med laserljus och då skapa kortvariga ljusblixtar som är lika harmoniska som musik. Att förstå hur laserljus och ljusblixtar är relaterade är genom deras interaktioner med atomer. Detta kommer att hjälpa oss att följa aktiviteten som händer så snabbt att den inte syns direkt.

Ljus kan beskrivas som en våg, precis som vibrationer på en sträng. En atom har en kärna till vilken elektroner – små körmedlemmar samlades runt dirigenten – vilket runt de är bundna. Emellertid kan elektroner lossna och rör sig bort från atomen om ett yttre ljus med högt intensitet rubbar atomen – exempelvis laserljus. Detta visar sig som att en kör leds av en dirigenten att sjunga en sång som endast går upp och ner som ett vågmönster. Vissa körer vill hellre sjunga jazz och därför kommer de att lämna kören.

När de fria elektronerna drivs tillbaka till atomen så har de fått ett överskott av energi från laserljuset. När elektronerna återvänder så släpper de ut överskottet som ljus – som exempel kan du tänka på en körmedlem som borde överge jazz och sjunga vad dirigenten vill så att hen kan återkomma till kören. När laserljuset upprepar sitt vågmönster (när dirigenten för sin arm upp och ner) gör atomen av med överskottet två gånger: en gång på toppen av vågen och en till på botten. Om laserljus mönstret upprepas väldigt snabbt så återfaller ljus som korta pulsar; deras varaktighet är flera attosekunder (1 attosekund = en miljarddel av en miljarddel av en sekund). De här ljuspulsarna är mycket mer harmoniska än ”vågmusik”, dvs musik med mer än bara en ton.

Ljus har också en egenskap som heter polarisering som berättar vilken form det tar genom att vibrera. Cirkulär polarisering resulterar till exempel i korkskruvformade vibrationer eftersom de roterar längs en cirkel. Något som vi lade ett speciellt intresse i min avhandling var att undersöka hur laserljusets polarisering ändrar ljusets form och i sin tur harmonin av atomers pulsar. Genom att simulera starka interaktionerna mellan en atom och laserljus samt att använda en attosekundljuskälla, har vi bland annat sett hur harmonin inom pulsar ibland kan stärkas när laserljuset polariseras som ellips.

I framtiden kommer det här arbetet förhoppningsvis att hjälpa oss att komponera våra egna attosekundpulsar, de ljusblixtar som låter oss att ta bilder av händelser nästan lika korta som blixtar. De här bilderna kan därför förbättra vårt kunskap om hur världen bör byggas genom att vi får möjlighet att se mer av den. Så nästa gång du tänker på atomer, tänk på hur de minsta körsångarna har en viktigare stämma i universum än vad vi trodde var möjligt– låt oss tacka dem för alla sånger.

• author: Ramboazanaka, Timothé
• supervisor: Johan Mauritsson; Emma Rose Simpson; Neven Ibrakovic
• organization: Atomic Physics; Department of Physics
• course: FYSM60 20181
• year: 2018
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Atomic physics, attosecond pulses, high-order harmonic generation, polarisation, polarisation state, electron wave packet trajectory, three-step model, time-dependent schrödinger equation, TDSE, two-colour, selection rules, symmetry, conservation laws
• publication/series: Lund Reports on Atomic Physics
• report number: 547
• language: English
• id: 8951248
• date added to LUP: 2018-06-21 14:50:35
• date last changed: 2018-06-21 14:50:35