Shortening the light pulses

What is the Nobel Prize 2023 for Physics About?

The winners

The 2023 Nobel Prize in Physics has been awarded to three physicists: Pierre Agostini, Ferenc Krausz, and Anne L’Huillier, in recognition of their collaborative work in developing experimental methods to generate attosecond pulses of light. In this article, we will explore the fundamental concepts behind attosecond pulses, the experimental techniques employed by these scientists, and the significance of their achievements.

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What is ‘Atto’?

Let’s begin with the notion of attosecond pulses of light. The prefix ‘atto’ corresponds to 10^-18, representing an incredibly small unit of time. To put this in perspective, the ratio of an attosecond to the duration of a heartbeat pulse (approximately 1 second) is equivalent to the ratio of that 1 second to the age of the universe since the Big Bang!

How are attosecond pulses produced?

The method used by the trio of physicists in the 1990s to generate such ultra-short pulses is called High Harmonic Generation (HHG). This method begins with laser pulses.

Ionisation and recombination

• An intense laser pulse produces short bursts of pulses that typically is 10^-15 seconds (aka femtoseconds). The laser pulse is directed at noble gas targets, such as helium or argon, which are maintained at low pressures to facilitate ionization.

Why low pressure ? you may ask. For HHG, the laser pulse is required to ionize the inert gas atoms. At low pressures, the distance between gas atoms is relatively large that implises the laser pulse will be able to effectively interact with individual atoms. High pressure will make the atoms come close together and though the laser pulse may interact with multiple atoms simultaneously, this will result in the efficiency of ionization of the gas atoms.

• During the ionization process, electrons within the atom absorb energy from the laser pulse, becoming unbound from a specific atom or molecule. These free electrons are subjected to the laser’s electric field, which accelerates them, imparting kinetic energy and causing them to move away from the atom. But, the opposite might also happen.
• Laser light behaves as a sinusoidal wave, oscillating and changing direction. Consequently, the energy gain for an electron varies depending on the electron’s ionization state and the phase of the laser’s oscillation.
• For instance, if an electron is ionized at the peak intensity of the laser, it will accelerate away from the atom. As it moves, it may encounter a point where the laser’s field strength decreases. This prompts the electron to release the excess energy it gained and return to an atom, a phenomenon known as recombination.
• The energy released during recombination takes the form of a photon, with an energy corresponding to the difference between the ionized state’s energy and the recombination energy.

Generation of Harmonics

• Now, here’s where things might get interesting. The released photon has a high frequency/ energy that is an integral multiple of the original laser frequency/energy. In physics, we call it ‘harmonics’. Due to the presence of many electrons in the gas, numerous harmonic energies are released, determined by factors such as laser intensity, ionization potential, focus, laser-gas molecule interaction, atomic resonances, and more.

It is also important to note that the interaction between the laser and the emission of electrons here is nonlinear. In other words, the intensity of the incoming laser beam does not have a direct linear relationship with the output. Doubling the input does not guarantee a doubling of the output. Instead, in this case, when the intensity of the input light exceeds a certain threshold, nonlinear effects come into play, leading to harmonic generation.

Filtering for Attosecond pulses

• Returning to the released harmonic energies, the high-energy photons can span from the extreme ultraviolet range to the X-ray range, creating a mixture of harmonics. To generate an attosecond pulse, a setup comprising multiple mirrors, beam splitters, lenses, and other optical components, known as a ‘streaking setup,’ is employed. The primary objective is to introduce a time delay by adjusting the lengths of various optical elements. This process selectively affects only the attosecond pulses while leaving other portions of the harmonics unaffected, achieving isolation. During detection, only the attosecond pulses stand out in the entire spectrum.

And that is how, we obtain attosecond pulses of light! But why is this such a big deal?

What is the significance of this experiment?

You can imagine attosecond pulses as a slow motion camera! These pulses are capable of capturing fast-motion events. This capability enables us to potentially comprehend the ultrafast motion of electrons within atoms and molecules, unravel the processes of bond formation and bond breaking in various chemical reactions, explore quantum mechanical phenomena such as electron tunneling, and advance fields like information processing in quantum computing.

References and Additional

[embed]Announcement of the 2023 Nobel Prize in Physics The Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics 2023 to Pierre Agostini, Ferenc Krausz, and…www.youtube.com

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[embed]High-order harmonic generation in rare gases with a 1-ps 1053-nm laser We present experimental results of harmonic generation using a 1-ps…journals.aps.org