Understanding Attoseconds

An attosecond is a unit of time equivalent to one quintillionth of a second. To put this into perspective, on a logarithmic scale, an attosecond is to a second what a second is to the age of the universe. In the realm of physics, attoseconds represent the timescale relevant to electron motion, so brief that even atomic nuclei can be treated as static.

Electrons, as fundamental particles, exhibit dynamics in atoms, molecules, and condensed matter that are only beginning to be understood. Phenomena like photoemission, photoionization, and coherent electron dynamics initiate on the attosecond scale.

Ultrafast Imaging Techniques

To investigate attosecond electron dynamics, we require light pulses of sufficient brevity to capture the relevant timescales. Drawing from photography, longer exposure times lead to blurrier images. Consider the following example of a water droplet falling into a bowl; a 1-second exposure fails to show the dynamics, while a 1/100-second exposure reveals clear ripples.

Achieving attosecond pulses necessitated significant advances in both science and engineering.

The Science Behind Attosecond Light Pulses

It’s established in Fourier analysis that the duration of a light pulse is linked to its frequency bandwidth. Shorter pulses demand broader spectral widths. The central wavelength determines the minimum duration achievable. To create a pulse lasting 100 attoseconds, the central wavelength must be approximately 30 nanometers, situated within the extreme ultraviolet (XUV) region of the spectrum.

The broad bandwidth required is achieved through High Harmonic Generation (HHG). Here, strong laser light interacts with atoms, inducing a series of ionization and recombination events that emit photons at very short wavelengths.

The well-known three-step model of HHG involves:

1. The strong laser field alters the atomic potential, allowing the electron to tunnel through.
2. The electron is accelerated back toward the atom due to the laser field.
3. Upon recombining with the ionized atom, the electron emits an XUV photon, conserving energy.

Interestingly, noble gases undergoing strong-field ionization produce a photoemission spectrum featuring a plateau of high harmonics, which are subsequently combined into attosecond pulses through sophisticated experimental setups.

Meet the Laureates

Each laureate received one-third of the Nobel Prize.

Pierre Agostini serves as a Physics Professor at The Ohio State University, having earned his Ph.D. in 1968 from the University of Aix-Marseille, France. In 2001, after over two decades of research in fundamental physics, his laboratory successfully produced a train of pulses lasting under 250 attoseconds.

Ferenc Krausz is the Director of the Max Planck Institute for Quantum Optics and a Physics Professor at LMU Munich, Germany. He obtained his Ph.D. in 1991 from the University of Vienna, Austria, and independently generated an isolated light pulse with a duration of 650 attoseconds.

Anne L'Huillier is a Physics Professor at Lund University, Sweden, where she received her Ph.D. in 1986 from the University Pierre and Marie Curie in Paris, France. Her pioneering work laid the groundwork for generating attosecond pulses through the study of high harmonic generation.

Notably, Anne L'Huillier’s recognition marks the fifth instance of a female physicist receiving the Nobel Prize in Physics, following the legacies of Marie Curie, Maria Goeppert-Meier, Donna Strickland, and Andrea Ghez. It's encouraging to see the Nobel Committee acknowledge the significant contributions of women in foundational research.

Applications of Attosecond Pulses

Beyond being a remarkable scientific milestone, attosecond light pulses serve as vital tools for both fundamental and applied research. I will now outline a typical experimental configuration used in ultrafast dynamics, known as the pump-probe experiment, and showcase some applications of attosecond pulses in the analysis of photoemission and coherent electron dynamics.

Pump-Probe Experiment Overview

A standard experimental setup utilizing attosecond (and femtosecond) light pulses is the pump-probe experiment. In this method, a pump pulse initiates the dynamics in an atom, molecule, or condensed matter sample. After a tunable time delay, a probe pulse captures a snapshot of the sample. By varying the delay, researchers can compile a series of images into an ultrafast movie.

Investigating Photoemission

Using a pump-probe technique with XUV and infrared pulses, Ferenc Krausz's group has measured the time difference in photoemission from two atomic orbitals in neon—the 2s and 2p orbitals. Photoemission involves an electron exiting an atom after being excited by light, a fundamental quantum mechanical effect traditionally seen as instantaneous. Their experiment revealed that photoemission from the neon 2p orbital is approximately 20 attoseconds delayed compared to the 2s orbital.

This discovery has driven further theoretical exploration of photoemission and electron correlation, as the observed delay challenges existing concepts. It also raises questions about the utility of photoelectrons as a timing mechanism in ultrafast experiments.

Coherent Electron Dynamics

Attosecond laser pulses, characterized by their brevity and extensive frequency range, are also coherent. This coherence allows them to induce coherent electron dynamics. By exciting electrons within a molecule using an attosecond pulse, scientists create a quantum superposition of states.

This leads to intriguing dynamics that can be further investigated using appropriate probe pulses. Ultimately, the goal is to manipulate these dynamics by generating specially shaped light pulses, potentially enabling control over chemical reactions directly with light and reducing reliance on costly catalysts and thermal energy.