Attosecond pulses and quantum dots: exploring the physics behind the Nobel Prize

In May 2023, the World Health Organization declared COVID-19 to have ended as a global health emergency.

Vaccines against the virus have been introduced at an unprecedented rate. According to publicly available data as of August 2023, more than 13.5 billion doses of the vaccine have been administered, and more than 70% of the global population has received at least one dose.

Now, two scientists from Pfizer-BioNTech and Moderna's COVID-19 mRNA vaccine development have won the Nobel Prize in Medicine - and they are the first recipients this year.

Hungarian scientist Katalin Kariko and her American colleague Drew Weissman met in 1998 while waiting in line at a photocopier and began working together.

In 2005, they overcame a major obstacle to the use of messenger ribonucleic acid (mRNA) technology by developing "nucleoside base modifications" that prevent the immune system from launching an inflammatory attack on lab-made mRNA.

Weisman said, "We couldn't get people to notice that RNA was something interesting. Almost everyone has given up."

Ultrashort pulses of light open the door to the world of electronics

From left to right: Pierre Agostini, Ferenc Kraus and Anne L’Huillier

Pierre Agostini, Ferenc Krausz and Anne L'Huillier have used light to illuminate the movement of electrons inside atoms and molecules in a way that was previously impossible: the trio have created ultrashort pulses of light that can deliver snapshots of changes inside atoms, providing a tool to help detect disease molecules in blood samples.

To understand their results, Ferenc Kraus said that typically, physicists use femtosecond pulses of light to track the motion of atoms in molecules, and sources that emit even shorter bursts of X-rays are now being developed to study and control the motion of electrons inside atoms.

When a femtosecond laser illuminates neon atoms in this metal tube, it generates ultrafast X-ray pulses

Many amateur photographers are frustrated when they take pictures of fast-moving objects and find that the pictures are blurred beyond recognition. The most likely reason is that the exposure time is not short enough to freeze the motion. In contrast, modern ultra high-speed cameras can take up to a million images per second, capturing motion that is normally imperceptible to the human eye. By projecting the photos onto a screen in sequence, the action can be replayed in slow motion. While these techniques are perfect for studying macroscopic objects, how can we possibly track the motion of atoms and electrons?

The motivation for tracking the motion of atoms and electrons comes from many areas of science and technology. The ability to observe chemical or biochemical processes is a prerequisite for guiding reactions, while a deeper understanding of the dynamics of electrons and holes in semiconductor structures is essential for speeding up electronic devices. At a more fundamental level, tracking the movement of electrons in atoms is essential if we want to understand what is happening inside excited atoms and utilize these processes in applications such as X-ray lasers.

So how short must the exposure time be in order to capture the dynamics of atoms and electrons? Surprisingly, the answer stems from classical physics and is determined by the mass of the atom and the Coulomb forces around the atom. For example, the motion of the nucleus of an atom in a molecule or lattice structure can be tracked with a probe that lasts 10-100 femtoseconds (1 fs = 10-15 seconds). At the same time, electrons have less mass, so their motion is quite agile. In fact, the time scale of the motion of bound electrons in excited atoms and molecules is 10-1000 arcseconds (1 arcsecond = 10-18 seconds), hence the need for attosecond probes.

The exposure time required to freeze-frame an atom is more than a million times shorter than the exposure time provided by the fastest high-speed cameras. In the last decade, this huge gap has been bridged by the invention of the laser: the laser produces a flash of light that lasts only a few femtoseconds.

Physicists can take precise snapshots of the evolution of atomic systems by using ultrashort flashes of light to trigger the dynamics and illuminate the system. The method involves splitting each laser pulse with a partial transmission mirror and delaying the lower-energy "probe" pulse relative to the higher-energy excitation or "pump" pulse. In this way, a powerful femtosecond laser pulse can initiate the same microscopic process at millions of molecules or sites in the lattice. The weaker part of the pulse (or a replica of the frequency shift) can then be probed for its dynamics at a later time by measuring changes in the optical properties of the system, such as absorption.

By using a series of femtosecond pulses and increasing the delay between successive pump and probe pulses, we can replay atomic or molecular dynamics in slow motion-a method known as time-resolved or pump-probe spectroscopy, where the temporal resolution is limited only by the duration of the pump and probe pulses.

In 1990, Wilson Sibert of the University of St. Andrews, Scotland, invented the self-locking mode laser, which physicists can now use to generate conventional pulses shorter than 10 fs; early mode-locked lasers developed in the 1970s were difficult to build and operate. In contrast, self-mode locking occurs naturally in carefully designed laser cavities that employ solid-state amplifiers such as titanium-doped sapphire, a laser medium developed in the 1980s by Peter Moulton of MIT. Meanwhile, Robert Szipocs and Ferenc Kraus at the Institute of Solid State Physics and Optics in Budapest designed specialized "chirped" mirrors in 1993 that could further compress the pulse.

Using these techniques, Ursula Keller at ETH Zurich, Franz Kartner at MIT and their colleagues developed oscillators that produce pulses with a duration of only 5 fs and a wavelength of 800 nm. In other words, these pulses contain only two cycles of the laser field.

This means that the wavelength of light limits the temporal resolution of observing atoms and also limits the spatial resolution. Interestingly, physicists have an easier time observing moving atoms than stationary ones. While observing stationary atoms with visible light is impossible due to the diffraction limit, femtosecond pulses are able to follow moving atoms in real time, which move slowly compared to the speed of light, with displacements of only 0.01 nanometers.

The formation and breaking of chemical bonds between atoms in a molecule is one of the most important microscopic processes that affect our lives. Ultrafast laser pulses allow physicists and chemists to follow these femtosecond processes by tracking the modal displacements of modal atoms; however, in complex systems, atomic modal motions can be determined more accurately by studying core electrons close to the nucleus, but this requires X-ray wavelengths.

Ultrafast laser pulses also allow us to measure the frequency of chemical bond stretching and contraction. However, molecular vibrations can be more easily measured in the frequency domain using existing techniques, for example by measuring the absorbance of infrared radiation as a function of frequency. So what are the advantages of ultrafast lasers?

Unlike frequency-domain techniques, ultrafast lasers can obtain the relative phases of vibrational modes, which is essential for reconstructing the dynamics of molecular structures. For example, these time-domain measurements can determine which chemical bond breaks first and the course of a chemical reaction. In addition, femtosecond pulses can even control chemical reactions, as demonstrated by Gustav Gerber and Thomas Baumert at the University of Würzburg in Germany.

Since picosecond and femtosecond lasers have been widely used, they have been used to study the dynamics of charge carriers in semiconductors. Physicists have gained new insights into the fundamental phenomena that limit the speed of integrated circuits, allowing them to expand the limits of high-speed electronics. Ultrashort light pulses can be used to explore the frontiers of electronics because they are far faster than the fastest existing electronic devices.

In addition, the terahertz electric fields generated by ultrashort light pulses can induce and control currents in integrated circuits on time scales that are beyond the reach of electronic instruments. This capability would help increase the speed of computers by a factor of a thousand compared to the current state of the art. Even if terahertz electronics becomes a reality, femtosecond laser pulses will still be fast enough to advance semiconductor technology.

Beyond the Femtosecond Barrier

In principle, simply adding waves oscillating at the angular frequency ω0 + NΔω produces a series of optical pulses shorter than λ/c, where Δω is a fixed displacement with respect to the laser fundamental, ω0 = 2πc/λ, and N is an integer; the result is a series of intense spikes spaced at intervals of 1/Δv = 2π/Δω.

The duration of these spiky pulses is inversely proportional to the frequency shift Av and the number of waves added. This means that to generate sub-femtosecond pulses using a finite number of waves, Δv must be similar to the frequency of visible light. Conceptually, this technique is closely related to the mode-locking method typically used to generate femtosecond pulses in laser resonators. However, the significant difference between the two is that the Δv used to generate the sub-femtosecond pulse must be several orders of magnitude larger than the frequency spacing between neighboring modes of the laser resonator. In fact, Δv must be so large that no laser can amplify all of these frequency-shifted waves. The only way to generate these waves is to use nonlinear optical techniques that are not part of the femtosecond laser oscillator itself.

In general, two nonlinear optical phenomena can be utilized to generate waves that are coherent with the phase of the incident laser beam but at a higher frequency: excited Raman scattering and high harmonic generation. Both processes are carried out in a gas in order to minimize UV dispersion and absorption at shorter wavelengths.

Raman Sidebands and Higher Harmonic

Raman scattering occurs when light passes through a gas molecule. Light can excite vibrational or rotational energy levels in the molecule, which modulates the laser radiation. Several research groups, including Alexi Sokolov and Stephen Harris at Stanford University, have demonstrated the potential for sub-femtosecond pulse trains from excited Raman scattering. If the relative phases of the Raman modes are properly tuned, only a small fraction of this spectrum (including blue, violet, and ultraviolet lines) is needed to produce a series of sub-femtosecond pulses spaced at 1/Δv ≈ 11 fs.

Using higher-order harmonic generation techniques to shift the laser frequency to the extreme ultraviolet, even "roaring pulses" can be generated. This phenomenon occurs when a high-power beam of linearly polarized femtosecond laser pulses passes through an atomic gas. The strong electric field of the light pulse rips electrons from the atom and accelerates them. When the light field reverses in the next half cycle, the electrons crash back into the ions and release energy as X-ray photons. This process is repeated in each half cycle of the laser field, resulting in discrete lines in the extreme ultraviolet and X-ray spectra. These "higher harmonics" are twice as frequent as the frequency of the driving laser field, and they combine to produce a series of attosecond pulses.

A collaboration of French and Dutch physicists, led by Harm Geert Muller at the FOM Institute of Atomic and Molecular Physics in Amsterdam, did not obtain conclusive experimental evidence for the existence of such a string of pulses until 2001, when it was shown that a 40 fs pump pulse emitted by a titanium sapphire laser produced a series of 250 as pulses.

Unfortunately, both excited Raman scattering and high harmonic generation have their drawbacks in pump-probe spectroscopy. The time interval between pulses is so short, 11 fs in the Stanford experiment and 1.3 fs in the Amsterdam-Paris study, that the physical, chemical, or biological process under study does not have time to fully decay and return to its initial state before the next pulse arrives. Thus, the high repetition rate of the pulses imposes unacceptably severe limitations on the processes that can be studied.

As Paul Corkum of the Steacie Institute for Molecular Science in Ottawa, Canada, pointed out in his seminal proposal for attosecond optics, it is clear that pump-probe spectroscopy can only benefit from these ultrafast sources if individual pulses are selected from a train of pulses. The most promising way to isolate such pulses is to drive the molecules that produce the Raman lines or the atoms that emit the higher harmonics for such a short time that a single pulse is naturally produced during the generation process - this avoids the arduous task of selecting a single pulse from a multi terahertz train.

One way to solve this problem is to pass an intense 20 fs pulse through a gas-filled hollow waveguide, a device invented in 1995 by Orazio Svelto and Sandro De Silvestri of the Politecnico di Milano in Italy. The emerging pulse lasts less than 10 fs and has a peak intensity of more than 100 gigawatts. Pulses with these properties opened the way for the generation of sub-femtosecond single pulses.

A successful first step in this direction was taken in 2001 using Raman scattering and high harmonic generation techniques. Georg Korn of the Max Born Institute in Berlin and his collaborators passed a 15 fs pulse of violet light through a pre-excited sulfur fluoride molecule with a vibrational frequency of 23 THz. The result was a continuous sideband, resulting in a single pulse with a wavelength of 400 nm and a duration of only 4 fs.

In another experiment, Ferenc Krauss's group, in collaboration with researchers in Germany and Ottawa, produced individual soft X-ray pulses with a wavelength of 14 nm. These isolated X-ray pulses were collimated laser-like beams generated by high harmonics in a neon atomic gas that was exposed to an intense pulse containing several laser field cycles, with an upper limit of about 2 fs on the measured pulse duration.

An attosecond pulse in operation

The attosecond polar pulse: Toward atomic physics

Physicists are now close to controlling the motion of electrons on time scales much shorter than the period of visible light oscillations.

Once amplified high-intensity pulses with stabilized phases are available, strong-field processes (such as light-field ionization or high-harmonic generation) will allow scientists to directly measure the carrier-envelope phase. With this parameter, coupled with the use of standard techniques to measure amplitude and frequency variations in the laser field, it would be possible to determine and control the evolution of the electromagnetic field in a light wave, and even synthesize arbitrary optical waveforms containing several cycles.

Such waveforms allow scientists to control the motion of electrons after they have been detached from atoms or molecules. Immediate consequences include reliable generation and control of attosecond pulses at extreme ultraviolet and soft X-ray wavelengths through high harmonic generation. At higher intensities, attosecond electron pulses have the potential to accelerate to energies of several mega-electron volts.

Because these X-ray and electron pulses are synchronized with the laser field with attosecond precision, scientists will be able to take snapshots of the different stages of atomic, molecular, and plasma dynamics. These processes will be triggered and guided by light waves controlled by the strong field and probed by the X-ray pulses, allowing us to control and reconstruct the electron motion on the atomic length scale and with attosecond resolution. In this way, for example, the trajectory of an electron wave packet can be measured from the instant it is ejected from an atom and can be customized for different applications. These applications might include the development of more efficient, shorter-wavelength X-ray lasers, or a source of ultrafast, single-energy relativistic electron pulses for next-generation particle gas pedals.

The attosecond X-ray pulses will also pave the way for observing the movement of electrons inside atoms. For example, we hope to be able to track the motion of electrons within one femtosecond of their release from a bound state in the presence of a strong laser field; and also the relaxation of the remaining bound electrons to a new equilibrium state with attosecond resolution.

Once we are able to generate higher energy attosecond pulses, we will be able to use them as pumps and probes in pump-probe spectroscopy. In this way, we can selectively remove electrons from the inner shell with a strong attosecond X-ray pulse and probe the filling of the inner shell vacancies with a weaker time-delayed replica of the same pulse with an accuracy better than 1 fs. Is this necessary? Why not just rely on relaxation times derived from linewidth measurements? The answer is simple: the effect of ultrashort strong fields on the dynamics of bound electrons - which is important for applications such as X-ray lasers.

Ultrashort flashes allow us to take snapshots of microscopic particles and reconstruct their motion. State-of-the-art femtosecond lasers emit pulses that are short enough to resolve the motion of atoms in molecules and solids, as well as the dynamics of charge carriers in semiconductors. At the same time, specially tailored femtosecond pulses can control these processes and will be widely used in chemistry, biochemistry and electronics.

Krauss and his colleagues focused on the challenge of isolating ultrashort pulses, a requirement for researchers to probe subatomic processes. In just under a decade, the field has evolved to the point where Krauss and his team were able to quantify the time scale of the photoelectric effect of electrons in neon atoms. Using a technique called attosecond streaking, the researchers combined 200-arcsecond pulses with infrared pulses: the ionization time of the 2p subshell was found to be about 21 arcseconds longer than that of the 2s subshell. Six years ago, Le Huillier and colleagues used the RABBIT technique to explain the deviation of Krauss's results from theory.

More recently, the experimentalists created laser pulses that are only a few tens of arsec long.Matthias Kling, director of science, research and development at LCLS, says that attosecond science was the main motivation for building an upgraded Linac Coherent Light Source at SLAC-a facility that employs the Krauss-developed attosecond streak technology.

This year, Agostini, Krauss and Le Huillier will share the 11 million Swedish kronor (about $1 million) prize money equally. Le Huillier is the fifth woman to win the physics prize and the first to share the prize money equally with all the winners. Marie Curie (1903), Maria Goeppert Mayer (1963), Donna Strickland (2018) and Andrea Ghezzi (2020) each shared half of the prize money equally with a male counterpart, with the other half going to a man.

The emergence of these tools will herald a new era of experimental physics. In response, Eva Olsson of the Nobel Prize in Physics selection committee said, "The ability to generate attosecond pulses of light opens the door to tiny, extremely tiny time scales and to the world of electronics."

The three 2023 Nobel Prize winners in Chemistry are also being recognized for their pioneering work on tiny scales - nanotechnology that creates tiny particles, whose properties are determined by quantum phenomena.

On Wednesday, October 4, the Nobel Prize Committee announced the 2023 Prize in Chemistry for Moungi G. Bawendi, Louis E. Brus, and Alexei I. Ekimov for "the discovery and synthesis of quantum dots ".

In the 1980s, Alexei Ekimov achieved the size-dependent quantum effect in stained glass via copper chloride nanoparticles; later, Louis Bruce demonstrated it in particles in suspension, and in 1993, Monge Bavendi improved the production of quantum dots to enable them to be put to practical use.

Their winning papers were:

“A simple model for the ionization potential, electron affinity, and aqueous redox potentials of small semiconductor crystallites”

“Size effects in the excited electronic states of small colloidal CdS crystallites”

Quantum dots are light-emitting nanoparticles made from semiconductor materials that are now used in applications ranging from computer monitors and television screens to LED lights and the latest biological tissue mapping technologies. Researchers predicted back in the 1930s that size-dependent quantum effects should be observed in particles a few nanometers in size. But at the time, it was not yet possible to control the production of such tiny-scale materials. As the 20th century progressed, quantum effects were observed for the first time on the surfaces of thin films and bulk materials - and the 2000 Nobel Prize in Physics was awarded to Zhores Alferov and Herbert Kroemer for their work on such work on semiconductor heterostructures.

Alexei Ekimov first discovered quantum size effects in nanoparticles in the early 1980s while working at the Vavilov State Optical Institute in St. Petersburg. He was conducting experiments with copper chloride doped glass. By varying the temperature and rate of glass formation, Ekimov's team could control the size of the copper chloride crystals that formed in the glass matrix. During X-ray experiments on these materials, he noticed that the wavelength of the copper chloride absorption lines varied with the size of the crystals, which Yekimov attributed to quantum size effects.

At the same time, Louis Bruce was studying colloidal systems of semiconductor materials suspended in liquids.In 1983, Bruce observed the quantum size effect in experiments with cadmium sulfide particles, noting that the optical properties of colloidal CdS changed overnight as the crystals precipitated about three times larger when the crystals dissolved. Spectral analysis showed differences in the absorption behavior of the two sets of crystals, suggesting the influence of quantum size effects.

In 1993, Bavendi developed a method to produce cadmium selenide crystals with near-perfect control over their size. This method involves injecting an organometallic precursor into a hot solvent. This "thermal injection" technique allowed the team to control the nucleation point of the crystals, as the solution rapidly supersaturated to produce quantum dots of nearly uniform size. This discovery opens the door to real-world applications of quantum dots.

Using this and related methods, researchers can now create solutions of rainbow quantum dots that emit and absorb at different wavelengths. Quantum dots are also made using a "top-down" approach: nanometer-thick films are deposited on a surface and then etched or sculpted into nanoscale islands with an electron beam.

Quantum dots are now used to make light-emitting diodes for color monitors and television screens; they are also used in solar cells, where the absorption of light stimulates the transfer of electrons to a conducting material. Quantum dots have transformed biological imaging, as some are non-toxic and can be used to fluorescently label specific biomolecules or structures within living cells.

Quantum dots are now the basis of an industry worth about $4 billion.

Chad Mirkin, a chemist at Northwestern University in Illinois, said, "I think this is a marvelous Nobel Prize. It underscores the core idea of nanotechnology that everything is different when miniaturized; it will open the floodgates for nanotechnology awards and validate the big bet the world made on nanotechnology 25 years ago."

The Royal Swedish Academy of Sciences also said, "Quantum dots are delivering the greatest benefits to humanity." Researchers believe that in the future they could contribute to flexible electronics, miniature sensors, thinner solar cells and encrypted quantum communications - so we're just beginning to explore the potential of these tiny particles.

The versatile Norwegian's work ranges from plays and novels written in a minimalist style to poetry, essays and children's books: more than 1,000 of his plays have been performed, and his works in "New Norwegian" have been translated into 40 different languages.

Anders Olsson, a member of the Swedish Academy, says that Fosse's work "touches on your deepest feelings, anxieties, insecurities, questions of life and death ....... It has a certain universal impact."

Subsequently, on October 6, Iranian gender rights activist Narges Mohammadi was awarded the Nobel Peace Prize.

List of Laureates for 2023