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Nobel Prize in Physics 2023: an unprecedented image of the infinitely small

Stefan Haessler
Stefan Haessler
CNRS Research Fellow at Laboratoire d'Optique Appliquée (CNRS, ENSTA, École Polytechnique)
Key takeaways
  • The 2023 Nobel Prize in Physics was awarded to Pierre Agostini, Ferenc Krausz and Anne L’Huillier for their work on attosecond laser pulses.
  • These lasers pulses make it possible to observe the dynamics of electrons in matter and open the door to a wide range of studies.
  • The generation of attosecond pulses is mainly based on the method of colliding electrons with their atoms, developed by Anne L’Huillier.
  • This ability to take a snapshot of the infinitely small is useful in many fields, such as biology, to gain a better understanding of the damage caused to DNA by certain types of radiation.
  • In the future, scientists hope to develop even shorter pulses to observe protons and neutrons in atomic nuclei.

The 2023 Nobel Prize in Physics was award­ed for the devel­op­ment of exper­i­men­tal meth­ods for gen­er­at­ing attosec­ond laser puls­es (10–18 sec­onds), flash­es with a speed of around one bil­lionth of a bil­lionth of a sec­ond. They are use­ful for study­ing the dynam­ics of elec­trons in mat­ter. Three sci­en­tists received awards: Pierre Agos­ti­ni, Fer­enc Krausz and Anne L’Huillier.

Why generate attosecond laser pulses?

Laser puls­es are like cam­era flash­es that allow us to freeze and observe the move­ment of mat­ter. The faster the flash, the faster the dynam­ics we can observe. Pre­vi­ous­ly, we were able to gen­er­ate laser puls­es in the fem­tosec­ond range (10-15 sec­onds). This is the speed at which atom­ic nuclei move dur­ing chem­i­cal reac­tions. By get­ting down to the attosec­ond scale, we are now able to observe the rearrange­ments of the elec­trons them­selves: this process is extreme­ly rapid – it was even con­sid­ered instan­ta­neous in many the­o­ret­i­cal models!

Why is it important to observe electron dynamics?

Elec­tron rearrange­ments take place dur­ing crit­i­cal stages in the trans­for­ma­tion of atoms, mol­e­cules and mate­ri­als. Under­stand­ing these dynam­ics is a fun­da­men­tal issue in physics and chem­istry. Attosec­ond lasers have opened the way to unprece­dent­ed exper­i­men­tal obser­va­tion of nature. Many ques­tions can be addressed: how does the elec­tron cloud reor­gan­ise after a rapid dis­tur­bance, and how long does it take? How does this influ­ence the move­ment of the nucle­us? Can we con­trol and steer the rearrange­ment of electrons?

Why is it so difficult to characterise the dynamics of electrons?

Quan­tum mechan­ics pro­vides us with the equa­tions that describe the behav­iour of mat­ter, includ­ing elec­trons. But solv­ing these equa­tions accu­rate­ly requires enor­mous com­put­ing pow­er. And that’s even for a very sim­ple atom like heli­um, made up of a nucle­us and two elec­trons… So, we’re a long way from mov­ing on to more com­plex atoms, or even molecules!

A few reminders about physics

What are the objects around us made of? Let’s delve into the infi­nite­ly small by tak­ing the exam­ple of water. Water is made up of mol­e­cules, the basic struc­tures of mat­ter. A glass of pure water con­tains a large quan­ti­ty of H2O mol­e­cules. The mol­e­cules them­selves are made up of atoms: 2 hydro­gen atoms and 1 oxy­gen atom in our exam­ple. On an even small­er scale, atoms are made up of a nucle­us around which elec­trons revolve. Elec­trons are essen­tial to the bonds in mol­e­cules and are involved in chem­i­cal reac­tions. They are also involved in physics, con­tribut­ing to con­duc­tiv­i­ty, mag­net­ism, elec­tro­mag­net­ic radi­a­tion, etc.

The only solu­tion is to use approx­i­ma­tions to sim­pli­fy the cal­cu­la­tions. These approx­i­ma­tions also pro­vide sim­pli­fied men­tal mod­els for think­ing about this com­plex physics. It is there­fore essen­tial that they are accu­rate. This is where attosec­ond puls­es come in: they pro­vide extreme­ly pre­cise exper­i­men­tal mea­sure­ments, which are invalu­able for estab­lish­ing and val­i­dat­ing these approximations.

What scientific discoveries have attosecond pulses led to?

In elec­tron­ics, cur­rent is gov­erned by switch­es con­trolled by elec­tro­mag­net­ic fields. For exam­ple, an elec­tric field is applied to a tran­sis­tor which, depend­ing on whether or not the field is acti­vat­ed, either lets the cur­rent through or blocks it. To explore the speed lim­i­ta­tions of these switch­es, laser puls­es are used instead of tran­sis­tors. Attosec­ond physics has devel­oped the fastest and most pre­cise elec­tro­mag­net­ic fields in exis­tence. Sci­en­tists at Garch­ing in Ger­many and Graz in Aus­tria have used them to test how quick­ly it is pos­si­ble to switch from one mode to anoth­er. The result is that at around one peta­hertz, or one mil­lion giga­hertz, there is an upper lim­it for well-con­trolled opto­elec­tron­ic process­es1.

© Johan Jarnestad/The Roy­al Swedish Acad­e­my of Sciences

Oth­er advances relate to the time tak­en for an elec­tron to leave its atom after absorb­ing a pho­ton. In 2010, Fer­enc Krausz’s team pub­lished exper­i­ments that showed a dif­fer­ence of 20 attosec­onds between the emis­sions from two elec­tron lay­ers of a neon atom2. After sev­en years of sci­en­tif­ic debate, Anne L’Huillier and her team were able to clar­i­fy the ori­gins of this delay as a cor­re­la­tion between neon elec­trons3.

Have attosecond pulses become part of our everyday lives?

No, that’s a long way off. But the sci­en­tif­ic scope for their use is con­stant­ly expand­ing. Chemists have been tak­ing an inter­est in them since 2010. One of their aims is to opti­mise cer­tain chem­i­cal reac­tions. How­ev­er, elec­tron­ic dynam­ics are high­ly com­plex and dif­fi­cult to con­trol, so research is still at a fun­da­men­tal stage. Mol­e­c­u­lar biol­o­gists are using them to observe how and at what speed elec­tric charge migrates along large mol­e­cules after the sud­den removal of an elec­tron. This enables them to bet­ter under­stand the dam­age caused to DNA by cer­tain types of radi­a­tion. The semi­con­duc­tor indus­try is also inter­est­ed in the imag­ing pos­si­bil­i­ties offered by these lasers.

How is it possible to generate such a short laser pulse?

The method most wide­ly used today is that dis­cov­ered by Anne L’Huillier: a laser (in the near infrared or vis­i­ble range) is direct­ed at gas atoms. Under the right con­di­tions, the laser’s elec­tric field pulls on the elec­trons, steer­ing them along tra­jec­to­ries around their atoms and caus­ing them to col­lide with their atoms. These syn­chro­nised col­li­sions between all the atoms gen­er­ate attosec­ond puls­es. We now know that it is also pos­si­ble to use very thin solids, plas­ma mir­rors or even free elec­tron lasers. Tech­ni­cal­ly, it’s not com­pli­cat­ed to gen­er­ate trains of attosec­ond puls­es, but you need to know how to char­ac­terise them. As the attosec­ond sources devel­oped today become more pow­er­ful, they will be able to address new processes.

What is the future of this field?

The sys­tems stud­ied are becom­ing more com­plex: mol­e­cules are larg­er, solids are struc­tured on a nano­met­ric scale… The puls­es will be short­er and short­er, and the zep­tosec­ond fron­tier – thou­sandths of an attosec­ond – will fall. At this scale, it will be pos­si­ble to make sim­i­lar obser­va­tions of pro­tons and neu­trons bound togeth­er in the nuclei of atoms.

Anoth­er poten­tial approach is to con­cen­trate ener­gy over time to unprece­dent­ed pow­ers. Quan­tum the­o­ry shows that with a suf­fi­cient­ly strong elec­tro­mag­net­ic field, it is pos­si­ble to sep­a­rate matter/antimatter pairs from the quan­tum vac­u­um. In oth­er words, light can be trans­formed into mat­ter. How­ev­er, there is cur­rent­ly no instru­ment that can deliv­er the nec­es­sary pow­er. Plas­ma mir­rors, which are of major inter­est to the award-win­ning team, are a promis­ing way4 of com­press­ing the most intense lasers cur­rent­ly avail­able (petawatt) in time and space, such as APOLLON, which is man­aged by the lab­o­ra­to­ry for the use of intense lasers at the Insti­tut Poly­tech­nique de Paris and Sor­bonne Uni­ver­si­ty. The aim is to test fun­da­men­tal the­o­ries in extreme con­di­tions nev­er before encountered.

Anaïs Marechal

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