On October 3, three scientists, Anne L’Huillier, Pierre Agostini, and Ferenc Krausz, were awarded the 2023 Nobel Prize in Physics for their groundbreaking work in generating attosecond pulses of light to study electron dynamics in matter. But what exactly is an attosecond? It is an incredibly short unit of time, one quintillionth of a second, or 10^-18 seconds. At this scale, the properties of electrons change, making it crucial to study them in order to truly understand their behavior.
This is where attosecond science comes in. Also known as attosecond physics or attophysics, attosecond science focuses on producing extremely short light pulses to investigate ultrafast processes. To put this into perspective, consider a hummingbird’s wings, which beat 80 times per second. Each wingbeat lasts only 1/80th of a second. However, the human eye can typically perceive up to 60 frames per second, making it impossible to capture a single wingbeat in action. Instead, the motion of the wings would appear as a blur.
In order to capture such fast processes, one solution is to use a digital camera that captures light with a sensor, allowing for precise timing. To capture a single wingbeat, the camera would need to have its aperture open for exactly 1/80th of a second. Another option, particularly when studying electrons, is to keep the aperture open at all times and release a light pulse lasting 1/80th of a second towards the wing, capturing its reflection. This latter method is more suitable for attosecond science, where electron “wingbeats” occur at a few hundred attoseconds.
So, how do scientists produce these attosecond pulses of light? The underlying concepts come from wave mechanics. In 1988, Anne L’Huillier and her colleagues in Paris discovered that when they passed an infrared light beam through a noble gas, the gas emitted light with frequencies that were multiples of the beam’s frequency. This phenomenon, called high-harmonic generation, resulted in emitted waves known as overtones. They also observed that as they increased the frequency of the original beam, the intensity of the emitted light dropped sharply, remained constant for a range, and then dropped again.
By 1994, researchers had unraveled the reasons behind these effects. They found that a beam of light consists of oscillating electric and magnetic fields. This oscillation imparts energy to electrons, allowing them to come loose from atoms, and then they recombine, releasing excess energy in the form of light. Quantum mechanics equations were used to describe this process and explained why the intensity of the re-emitted light plateaued at certain frequencies.
To produce an attosecond pulse, the infrared beam is directed at noble gas atoms, resulting in multiple overtones. When the peaks of these overtones merge, they undergo constructive interference and produce a larger peak. However, when the peak of one overtone merges with the trough of another, they undergo destructive interference and cancel each other out. By carefully combining numerous overtones, physicists can tailor setups to produce attosecond light pulses, which last a few hundred attoseconds due to constructive interference, and then stop due to destructive interference. These pulses are only produced within a specific frequency range known as the plateau range.
But how do scientists measure the duration of an attosecond pulse? One technique is called RABBIT, which was developed by Pierre Agostini and his colleagues in Paris in 1994. In this method, both the attosecond pulse and another longer pulse are shined on noble gas atoms. Through the photoelectric effect, electrons are kicked out from the atoms by the photons in the two pulses. By analyzing the data of these electrons and atoms, physicists can extract mathematical information about the properties of the pulse, including its duration.
It wasn’t until 2001 that Agostini’s team and Ferenc Krausz’s team in Vienna were able to produce verified attosecond pulses in a train-like sequence, consisting of a pulse followed by a gap, repeating. The pulse duration in one case was 250 attoseconds, and in the other, Krausz’s group achieved a pulse duration of 650 attoseconds. The Krausz group also developed a filtering technique to isolate a single pulse, comparable to a bullet of light. From these initial achievements, scientists continued to refine these techniques, and by 2017, they were able to produce pulses as short as 43 attoseconds.
So, what are the applications of attophysics? While the devices used to generate attosecond pulses are currently expensive, require skill to operate, and are bulky, miniaturization has been a significant technological advancement in recent years. Someday, we may have pocket-sized devices to study electrons. The quest for miniaturization has been instrumental in various fields of science. Microbiology revealed the existence of bacteria, femtochemistry enabled precise manipulation of chemical reactions, and attophysics allowed scientists to investigate the properties of electrons.
As an example of the potential applications, in 2010, Ferenc Krausz’s team used attophysics to discover a 21-attosecond delay in electrons leaving two slightly different energy levels in a neon atom due to the photoelectric effect. This delay has implications for solar power, as the photoelectric effect is central to its operation. By refining our theoretical understanding of this effect through studies like Krausz’s, we could make significant advancements in harnessing solar energy.
Attophysics extends its influence to various fields, be it physics, chemistry, or biology, wherever electrons play a crucial role. While the cost and complexity of attosecond pulse generation may be hurdles, the understanding, techniques, and potential for future miniaturization provide an exciting opportunity for researchers to dive deeper into the ultrafast world of electrons and pave the way for groundbreaking discoveries.