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This Year's Physics Nobel: Playing with the fastest Camera in the World

Writer's picture: Mishkat BhattacharyaMishkat Bhattacharya

This post covers the Nobel prize in physics awarded this year to Pierre Agostini, Ferenc Krausz and Anne L'Huillier“for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter”.


Background


In the early days of quantum mechanics it became clear that electrons inside atoms could occupy only certain discrete - quantized - energies (unlike, for example a stone tied to a string, which can be rotated at any energy). The electrons could be sent - excited- from one energy to another by using photons of the appropriate energy.


For a long time, in atomic physics laboratories, lamps were used to supply these photons. The basic idea was to apply a current through a gas contained in a glass jar. The electrons in this current would collide with electrons contained in the atoms in the gas and excite them. The excited electrons would then de-excite to a state of lower energy by emitting light. This light would escape through the jar and could be used for exciting other atoms.


Then, in 1960, Ted Maiman invented the laser. The laser eventually became the preferred source of photons for exciting electrons in atoms and molecules. This was because the photons coming out of a laser (unlike those from the sun or from a lamp) travelled along the same direction in space (like in a laser pointer), were of a well defined frequency (pure in color), and this color could be tuned very precisely.


Pulsed lasers


If a laser emits light without interruption, it is said to be in a continuous wave (CW in the jargon) mode of operation. When it turns on and off, it is said to be pulsed. Pulses can be useful, for example, since they concentrate the laser energy into short but very intense bursts, as opposed to the constant, but lower, level of energy in CW mode. The difference between the two is a little bit like that between sprinting and distance running.


An example of the usefulness of pulsed operation is laser ablation, a technique for using light to melt or remove material from a surface. This is useful for metal machining, semiconductor lithography (i.e. making computer chips), inertial confinement fusion, treatment of atrial fibrillation (irregular heartbeat), tattoo removal, etc.


How short can the laser pulses be made? If we operate a laser in CW mode, but block and unblock the light, we can effectively get pulsed operation. Similar effects can be obtained by switching on and off the power supply to the laser. Techniques such as these can create pulses as short as nanoseconds (10^-9s) i.e. a billionth of a second in duration.


More sophisticated techniques, such as mode-locking (the addition of CW modes of different frequencies), can create picosecond (10^-12s - a million-millionth of a second) or femtosecond (10^-15s - a million-billionth of a second) long pulses.


The Nobel work


The Nobel prize this year was given for the development of techniques which produce attosecond (10^-18s - a billion-billionth of a second!) long pulses. To understand how this was done, we need to realize that shorter-in-time pulses can be made by adding more waves with different frequencies (some of you may be familiar with this Fourier principle).


The technique for generating attosecond pulses is to send femtosecond pulses into a gas. If these pulses are weak, then the same frequencies come out of the gas as are sent in. And the output pulse length is still femtoseconds. (For those who are familiar, this is the case of linear response - the output light field is simply proportional to the input field).


But if the input femtosecond pulses are intense, extra frequencies show up in the output. This is the case of nonlinear response - the output field also has contributions from the square, the cube, etc. of the input field. This leads to output frequencies double, triple, etc. of the input - a process generally called high-harmonic generation.


The presence of these extra frequencies in the output pulse shortens it from femtoseconds to attoseconds. What a cool method.


A camera for electrons


Another cool thing about pulsed lasers is that we can think of them as strobe lights (usually seen at parties!) for electrons in atoms. To see (pun intended) how this works let us remind ourselves that an electron in the hydrogen atom orbits the nucleus in about 150 attoseconds.


So if we shine a pulse of adequate photon energy we can remove (ionize) the electron from the atom. The probability of ionization is lower when the electron is closer to the nucleus, since it feels a stronger attraction.


Now imagine shining a stream of pulses, each a few attoseconds long, on the hydrogen atom. If we do not detect ionization, that means the electron was close to the nucleus. If we detect ionization it means the electron was far away. In this way attosecond pulses allow us to 'see' what the electron is up to.


Since how electrons move plays a central role in atomic and molecular physics and chemistry and in solid state physics, attosecond pulses are a very useful investigative tool. There are also possible applications to areas such as fast optical switching and data transfer.


Two notes before I end:


i) For similar work - using femtosecond pulses to 'track' atoms, and follow chemical reactions in real time - Ahmed Zewail was awarded the Nobel prize in Chemistry in 1999.

ii) The pulse detection techniques have amusing names:


FROG (Frequency-Resolved Optical Grating)

SPIDER (spectral phase interferometry for direct electric-field reconstruction), and

RABBITT (reconstruction of attosecond beating by interference of two-photon transitions),


for example. Looks like the optical physicists are having their fun.

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