Understanding Attosecond Pulses and High Harmonic Generation

Attosecond Pulses of Light

Attosecond pulses of light are extremely short bursts of light, each lasting only an attosecond (10^-18 seconds). These pulses are generated through a process called high harmonic generation (HHG).

In this process, incoming photons from a laser hit atoms in a gas (usually noble gases), liberating some of their electrons. The freed electrons fly away, loop back, and reconnect with their home atoms, generating attosecond bursts of light. These bursts combine to form an attosecond laser pulse.

This technique is similar to how pressing on a guitar string produces a note that’s higher in pitch. Shining laser light through certain materials changes the nature of the light, shifting it to higher energies and shorter pulses than a laser can reach on its own.

The ability to generate these incredibly short pulses of light has revolutionized research into ultrafast processes. For example, it has allowed scientists to observe the motion of electrons in atoms, which was previously too fast to be captured by traditional methods. This has opened up new avenues for understanding fundamental processes in physics and chemistry.

High Harmonic Generation (HHG)

High harmonic generation (HHG) is a non-linear optical process that generates high-frequency photons from low-frequency lasers. It requires a high power laser source and a target (gas, plasma, solid or liquid sample) that is illuminated by an intense laser pulse. The target will emit the high harmonics of the laser frequency, which can be above the fifth harmonic.

When a very intense light pulse is focused into a gas (usually at reduced pressure), strong nonlinear interactions can lead to the generation of very high odd harmonics of the optical frequency of the pulse, i.e., to an extreme form of nonlinear frequency conversion. This typically occurs at optical intensities of the order of 10^14 W/cm^2 or higher.

HHG has applications in spectroscopy, attosecond science and so on. Due to the coherent nature of the process, high-harmonics generation is a prerequisite for attosecond physics.