15. The Universe of ultrafast processes¶
Observation and explanation of the microscopic universe require new scientific theories and modern technological devices for measuring ultrafast processes.
Attosecond science has fundamentally shifted our perspective from analyzing the static energy levels of the quantum world to observing, measuring and manipulating evolving ultrafast processes by “filming” electron motion in real time. What was once deemed unobservable has been transformed into a tangible and dynamic field of exploration.
The ability to measure means to be able to understand and control. Attosecond science has provided a direct observational window into the previously inaccessible world of electron dynamics and unlocked the study of the fundamental processes that govern the behavior of matter. While the 20th century was defined by our understanding of the atom, the 21st century will be shaped by our mastery over the electron.
Objetives and content of sections.¶
15.1. Description of Motion at Different Timescales.
Describe the duration in seconds of some physical phenomena and provide examples of physical systems where motions are considered in terms of different timescales.
Definitions of time units.\ Attosecond as a new timescale.\ Clocks for measuring time.\ Lasers as devices for measuring time.
15.2. Attosecond Science and the Study of Electron Dynamics.
Explain the contributions made by the laureates of the 2023 Physics Nobel Prize Anne L’Huillier, Pierre Agostini, and Ferenc Krausz.
The discovery was made by Anne L´Huillier.\ The birth of experimental attosecond science.\ Resolving the photoelectric effect delay.
Section 15.3. Applications of Attosecond Science.
Consider some practical applications in fields such as the following:
Fundamental Phenomena and Electronic Dynamics\ Materials Science and Electronics\ Chemistry and Medical Diagnostics
| Table 15.1. Applications of attosecond sciences. | |||
|---|---|---|---|
| Domain | Application Examples | Core Problem It Solves | Potential Impact |
| Fundamental Physics |
|
How do electrons behave during the most basic quantum processes? | Directly tests foundational quantum mechanics and answers century-old scientific questions. |
| Chemistry |
|
How do electrons rearrange to break and form chemical bonds? | Enables a deeper understanding of reactions and the future potential to control them with light. |
| Materials and Electronics |
|
How do electrons move through materials that are key for technology? | Paves the way for petahertz electronics (millions of times faster than current tech) and more efficient solar cells. |
| Medical Diagnostics |
|
How can we identify specific molecules in biological samples quickly and non-invasively? | Offers the promise of new diagnostic techniques for early disease detection. |
15.1. Description of Motions at Different Timescales.¶
In the choreography of the universe, space and time serve as the fundamental axes upon which all phenomena are described. We will trace time evolution in Physics beginning with the standards derived from celestial rhythms, advancing through the quartz and atomic revolutions, and then exploring the use of lasers as quantum chronometers.
Definitions of time units.
To precisely quantify time durations, it is common to use the second (s) as a basic unit of time. In the International System of Units (SI) the duration of the second corresponds to 9,192,631,770 vibrations of the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom. Humans relate to time in two primary contexts: first, as a sequential narrative that distinguishes the past, present, and future; and second, as the measurable duration between the beginning and end of an event.
(image credit: https://www.examples.com/physics/units-of-time.html)
Time is counted in the first context as a narrative in terms of multiples of many seconds, corresponding to movements of astronomical objects: the Earth´s orbital period around the sun defines a year with a duration of 315,576,600 s, the Moon´s orbital period around the Earth defines a month and corresponds to 12,629,800 s, and the time it takes the Earth to rotate on its own axis, relative to the Sun, as equal to a day of 86,400 s. A year contains 12 months of durations of 28, 29, 30 or 31 days; a day contains 24 hours, one hour 60 minutes and one minute 60 seconds. Time is counted in the second context as a duration in terms of submultiples of many fractions of seconds. Next Table 15.1 indicates in the first column different units of time, including name, scientific shorthand notation and equivalence in seconds; the second column describes a characteristic physical phenomenon measured in such unit, including an estimated value.
| Table 15.2. Physical phenomena described with different time units | |
|---|---|
| Unit | Phenomenon |
| Second (1 s) | The duration of a human heartbeat. |
| Decisecond (1 ds = 10-1 s) | The length of a single blink of an eye: (1 – 4) s. |
| Centisecond (1 cs = 10-2 s) | The human reflex response to visual stimuli: (10 – 20) cs. |
| Millisecond (1 ms = 10-3 s) | The time for a neuron to fire one impulse and stop: 1 ms. |
| Microsecond (1 μs = 10-6 s) | The lifetime of a muon: 2.2 μs. |
| Nanosecond (1 ns = 10-9 s) | The time light takes to travel 30 cm: 1 ns. |
| Picosecond (1 ps = 10-12 s) | The mean lifetime of a bottom quark: 1 ps. |
| Femtosecond (1 fs = 10-15 s) | The period of vibration of a hydrogen molecule: 7.58 fs. |
| Attosecond (1 as = 10-18 s) | The shortest electron laser pulse: 53 as. |
| Zeptosecond (1 zs = 10-21 s) | A photon travers a hydrogen molecule: 247 zs. |
| Yoctosecond (1 ys = 10-24 s) | The mean lifetime of a Higgs boson: 156 ys. |
| Rontosecond (1 rs = 10-27 s) | The mean lifetime of W and Z bosons: 300 rs. |
| Planck time (tp = 5.39 x 10-44 s) | The briefest physically meaningful span of time. |
Let us consider first physical phenomena at the extreme of cosmic times and explain the meaning of Planck time. It is the time it takes light to travel one Planck length in vacuum \(t_P = \sqrt{([\hbar G]/c^5)}\), where ℏ is the is the reduced Planck constant and represents quantum mechanics, G is the gravitational is the gravitational constant and represents gravitation, and the speed of light $\(c = \frac{l_P}{t_P}\)$ represents relativity. Planck length \(l_P = \sqrt{([\hbar G]/c^3)}\) ~ 1.62 x 10−35 \(m\) marks the boundary where these three theories collide. Distances smaller than one Planck length cannot be meaningfully defined with current physics.
At scales near \(t_P\) quantum fluctuations of spacetime become important, there is no well-defined causal order, and “before” and “after” lose meaning. Below Planck time, the concept of time itself may lose physical meaning. It marks the scale where quantum gravity effects become dominant, gravity disrupts quantum measurements, motion cannot be defined and cause and effect blur; all of this indicates where new physics is required. Planck time defines the earliest meaningful moment in the universe’s history. It is the shortest “tick” the universe can meaningfully have, not because time stops, but because our concepts of time stop working.
Planck epoch refers to the first \(t_P\) after the big bang. During this epoch, spacetime itself is believed to have been highly quantum and fluctuating. No existing theory can reliably describe physics earlier than this time and our current physical theories no longer make reliable predictions. Before this epoch there is no classical spacetime and no known physical
laws are useful; furthermore, quantum gravity dominates. At the scale of Planck length quantum fluctuations of spacetime become so strong that classical geometry breaks down.
Attosecond as a new timescale.
There is a quantitative connection between the attosecond timescale (1 as = 10-18 s) and the age of the universe which is estimated to be 13.8 x 109 years = 4.35 x 1018s. This numerical equivalence does not imply that measuring in attoseconds directly influences descriptions of cosmic dynamics; it is a conceptual circumstance that places ultrafast physics in the whole context of time evolution.
Atoms and nuclei are large and heavy, their motions are measured in femtoseconds, while electrons are light and agile; they move so fast that their changes are blurred on the femtosecond scale; they are measured in attoseconds, which are one thousand shorter. Attosecond timescale defines the limit necessary to study the dynamics of matter at its most intimate level, establishing a bridge between the quantum world and the macrocosm. This time window has made it possible to take images or "film" the internal processes of atoms and molecules in real time, as well as the exchanges of energy between light and matter. At this timescale, it becomes possible to create attoecond science as a combination of ultrafast laser technology, quantum mechanics, and nonlinear optics serves to explore the microscopic world with very high temporal resolution.
Different timescales can be defined by indicating comparative magnitudes (cosmic scale), physical necessities (atomic scale) or technological impacts (scientific scale). These timescales cover an enormous gap between the infinitesimally small (the attosecond) and the immensely large (the age of the universe). Electronic movements initiate essential processes that create and sustain life and are behind the exchange of energy between light and matter. Changes in the positions and energies of electrons occur at speeds in the order of tenths of an attosecond or in hundreds of attoseconds. Nowadays attosecond images can describe sharp details of the movement of electrons, overcoming those blurred descriptions obtained from femtosecond images.
Clocks for measuring time
Timekeeping requires to find a periodic event that repeats at a constant rate and to establish a basic unit of time interval like the second. To define a timescale as a system of timekeeping representing an unambiguous description to order events, we must proceed in two steps: to measure a selected time unit like the second and afterwards to count the number of elapsed seconds corresponding to the specific time interval in consideration. These numbers represent time measurements usually made by devices like clocks.
A clock is a device that measures and displays duration, elapsed time or intervals of time; it is a timekeeper. Clocks display time in two main different ways: analog clocks consist in a set of moving arrows or hands that turn around a flat circular surface with imprinted numbers and marks; they indicate positions corresponding to hours, minutes and sometimes seconds. Digital clocks display numeric representations of time. A timepiece is a clock that does not strike the hours audibly, a watch is a clock carried on one´s person, a carillon clock plays music simultaneously with the harmonized motion of figures or toys, and a chronometer is a very complicated clock working with extraordinarily accuracy, efficiency and precision, useful for measuring duration of events. Each clock has a resonator, a device that produces a well-controlled periodic event that consumes energy to ensure continuity and regularity. The resonator and the required energy source are the main components of an oscillator. A clock contains also a counter of the number of oscillations and a converter indicating the number of seconds corresponding to the number of oscillations. Afterwards, this outcome of the counting of the clock needs to be visually displayed or recorded. The timekeeping element in every clock is a physical object that vibrates (oscillates) at a particular frequency: the number of times the oscillation is repeated in each period of duration. The fundamental principle behind timekeeping is always the same: consistent and equal intervals of time measurements. Originally, mechanical clocks were intended for administrative purposes, for instance signaling or notification of the timing of services and public events.
Modern times have been characterized by important advances in digital clocks as well as in atomic clocks. Atomic clocks did not ground the definition of time in a mechanical or astronomical process, but in a fundamental, unchanging property of the universe at the atomic scale. The International System of Units (SI) now defines the second based on the quantum behavior of the caesium-133 atom. The second corresponds to 9,192,631,770 vibrations of the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom.
(image credit: https://upload.wikimedia.org/wikipedia/commons/0/0c/Atomic_clocks.jpg)
Lasers as devices for measuring time.
The following sequence describes the basic five steps operation process in lasers: (1) pumping an energy source excites atoms in a gain medium; (2) this creates a population inversion, where more atoms are in an excited state than in the ground state; (3) an incoming photon triggers stimulated emission, causing an excited atom to release an identical photon; (4) many of those produced photons bounce between mirrors in an optical cavity, causing exponential amplification; and (5) a fraction of the amplified light escapes through a partially reflective mirror as the final laser output.
Two techniques are frequently used to measure time with lasers: Time of Flight ranging and Pump–Probe Timing