Categories: Basic Science

Nobel prize in Physics was won by Pierre Agostini, Ferenc Krausz and Anne L’Huillier. Their area of specialisation and affiliation are given as below.

1-Pierre Agostini
French experimental physicist and Emeritusprofessor at The Ohio State University, Columbus, USA. He is especially known for the observation of above-threshold ionization and the invention of the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) technique for characterization of attosecond light pulses.

2-Ferenc Krausz
Max Planck Institute of Quantum Optics, Garching and Ludwig-Maximilians-Universität München, Germany. His research team has generated and measured the first attosecondlight pulse and used it for capturing electrons‘ motion inside atoms, marking the birth of attophysics.

3-Anne L’Huillier

Lund University, Sweden. She leads an attosecond physics group which studies the movements of electrons in real time, which is used to understand the chemical reactions on the atomic level. Her experimental and theoretical research are credited with laying the foundation for the field of attochemistry.

In order to get a feel of time scales at which physical processes take place, let us do it by use of some examples of physical processes at different time scales. 

Example of physical processes at Milliseconds (ms) time scale are Blinking of an eye, Reaction time of a human, Common computer processing tasks takes 

Physical processes at Microseconds (µs) are Flash photography, Duration of a single CPU cycle, Many electronic signal processing tasks

Physical processes at Nanoseconds (ns) are Speed of light in a few feet of cable, Operation time of typical transistors in a computer, Some high-speed electronic communication

Physical processes at Picoseconds (ps) are 

Time taken for light to travel a hair’s breadth, Operation time of high-speed transistors, Some laser interactions with matter

Physical processes at Femtoseconds (fs) are Time for light to travel the diameter of a small molecule, Duration of certain chemical reactions, Atomic and molecular processes

Physical processes at Attoseconds (as) are Time scale of electron motion in an atom, Some ultrafast nuclear reactions

The lifetime of a typical excited state of an atom can vary widely depending on the specific electronic configuration of the atom, the energy of the excited state, and the elements involved. In general, lifetimes of excited states range from fractions of a nanosecond (10^-9 seconds) to milliseconds (10^-3 seconds) or even longer.

For example, some excited states in highly unstable or short-lived isotopes might have lifetimes measured in femtoseconds (10^-15 seconds) or picoseconds (10^-12 seconds). On the other hand, in more stable atoms, especially those with “forbidden” transitions (which are transitions that have a very low probability of occurring), excited states can last much longer.

It’s worth noting that precise measurements of excited state lifetimes are critical in various areas of atomic and molecular physics, including spectroscopy, laser physics, and quantum optics.

Metastable states are crucial in the operation of lasers. In a laser, atoms or molecules are raised to a higher energy state, typically through an external energy source. These excited particles then transition to a metastable state, which has a longer lifespan compared to other excited states. This longer lifetime allows for a buildup of population in the metastable state.

When a particle in the metastable state eventually returns to its ground state, it releases a photon. This process, called stimulated emission, is what generates the coherent and intense light characteristic of lasers. The presence of metastable states ensures a sufficient population in the excited state, which in turn leads to a significant output of laser light.

A couple of examples of elements or ions that have metastable states important for laser operation:

1. Neon (Ne) gas lasers: Neon atoms can be excited to a metastable state (2p^53s^1) by an electric discharge. When they return to their ground state, they emit red light at 632.8 nanometers, which is commonly used in applications like barcode scanners and holography.

2. Helium-neon (He-Ne) lasers: These are a type of gas laser that uses a mixture of helium and neon. Helium is used to pump the system, and neon provides the metastable state necessary for laser action. The transition from the metastable state (2s^2 2p^5) to the ground state (2s^2 2p^6) results in the emission of red light at 632.8 nanometers.

There are many different types of lasers that utilize various elements and molecules with specific metastable states to produce laser light for different applications.

To go further it is important to appreciate that how are extremely short light pulses of attosecond generated.

Attosecond pulses are generated through a process called high harmonic generation (HHG) in intense laser fields. The process consist of 

A high-intensity laser beam is focused onto a target material (often a noble gas like argon or helium).

The intense laser field ionizes the atoms in the target. This means it strips electrons from the atoms, creating a plasma.

The freed electrons are then accelerated back towards their parent ions due to the oscillating electric field of the laser. When they recombine with the ions, they release energy in the form of high-energy photons.

This process produces a broad spectrum of high harmonics, which include X-ray and ultraviolet wavelengths, along with attosecond pulses in the extreme ultraviolet (XUV) range.

Specialized optics are used to select and filter out the specific attosecond pulses from the generated spectrum.

The actual process involves complex quantum mechanical phenomena and advanced optical techniques.

An interesting question that comes to students minds when studying Modern Physics is when in a photoelectric effect an electron is emitted after absorbing a photon in a metal, can the time it takes for photoelectric process to take place, be measured. 

In the photoelectric effect, an electron is emitted from a material after absorbing a photon. The time it takes for this process to occur is incredibly short, on the order of femtoseconds (10^-15 seconds) or even shorter.

There are techniques capable of measuring these ultrafast processes. One of the primary methods is called pump-probe spectroscopy:

This technique involves two laser pulses.

 The first pulse (the “pump”) excites the material, providing the energy needed for the photoelectric effect to occur.

   – The second pulse (the “probe”) is time-delayed and used to measure the resulting changes in the material, including the emission of electrons.

   – By varying the time delay between the pump and probe pulses, scientists can effectively “freeze-frame” the ultrafast processes and study their dynamics.

This technique has been crucial in understanding a wide range of ultrafast phenomena, including the photoelectric effect.

To be continued.