Ultrafast Physics and Laser Metrology

Attosecond Physics and Terawatt few-cycle pulse generation

Frequency comb lasers made it possible to control the electromagnetic waveforms of pulses with high precision. This has led to the birth of a new field of 'attosecond physics' were the aim is to investigate the fastest processes in physics and biology. One attosecond is equal to 10-18 seconds. The main target of atto-research is electron movement between atoms and within atoms. Such processes can take place on a timescale of 100 attoseconds. Especially the movement of electrons between atoms plays a vital role in the initiation of structural changes in complex molecules upon excitation with a laser. Tracking electron movement could allow to understand the structural changes (which typically take much longer than 1 fs) in molecules by investigating what actually drives it at the fastest possible timescale. In order to have a chance to see such things, one needs light pulses shorter than 1 femtosecond (1 fs = 10-15). That is even shorter than the period of one oscillation of an infrared photon!. Wave-transp-harm

We have two main projects in this field: the first one is the development of a few-cycle Terawatt (TW = 1012 W) laser system based on parametric amplification, for generating attosecond pulses. The system we developed in the past few years is quite unique in the world, and is explained in more detail below. The second project just got started and is part of the NWO VICI grant we recently acquired: the idea is to use attosecond pulses from harmonic generation to track hyperfast electron movement in complex molecular ions. These ions will be stored and sympathetically laser cooled in a linear Paul trap. Many different techniques will be used to influence e.g. the internal state of the molecules via laser excitation and coherent control with shaped femtosecond pulses. We already have a project with calcium ions in a trap, and generated high harmonics for the XUV metrology projects.

The way to generate attosecond flashes of light is based on high-harmonic generation (HHG). This process requires extreme laser intensities (>1012W/cm2) from a high power laser. Ultrafast lasers are very well suited for this, especially if the laser beam is focused in the medium (typically argon gas) in which the harmonics are generated.

HHG-principle 

The picture above shows schematically what happens to the atoms that are in the laser focus. Part A shows the situation when there is no light field, so the electron is bound in a Coulomb potential. In B the electromagnetic (EM) wave of the focused laser beam distorts the Coulomb potential of the electrons, lowering the barrier on one side. Quantummechanically a partial wavepacket of the electron tunnels out and is accelerated away from the 'parent ion'. After half an oscillation the EM field switches sign (part C), so that the electron wavepacket is accelerated back. It can then recombine (D) with the ion and emit the energy it acquired in the process in the form of an x-ray photon. This process repeats itself many times over the duration of the laser pulse, which leads to a series of attosecond pulses. The x-ray pulses are significantly shorter (sub-femtosecond) than the period of the original EM wave. In order to generate well controlled, or even single attosecond pulses, a phase-stable high power laser source is needed. And the shorter the pulse is, the better. Frequency comb lasers can generate sub-5fs pulses with full phase control, and are therefore used (after amplification) to generate attosecond pulses.

The source for phase-controlled TW pulses we use to generate X-rays and XUV photons is based on parametric amplification in a nonlinear crystal:

OPA_schematic

A so-called pump beam provides 532 nm photons which are split in a crystal (BBO) in two parts: a 'signal' beam and an 'idler' beam. If you apply a beam at the signal beam wavelength (around 800 nm, from the phase-stable frequency comb oscillator laser) it gets amplified without much phase distortion and with a high bandwidth. The actual implementation is much more involved and is shown below, including the stretching and compressor sections:

NOPCPA_setup 

The system can amplifiy pulses to a level of 30 mJ with less than 1 percent fluorecence, and at a repetition rate of 30 Hz. After compression we got 7.6 fs pulses and more than 2 TW of peak power. In the picture below you can see the spectral phase and and width of the generated TW pulses. The green curve is without 'shaping', and the red one in the left picture is with phase shaping to compress the pulses from ~30 fs to 7.6 fs.

NOPCPA_pulse_and_spectrum_phase 

We have published several papers on the subject of parametric amplification, and were actually the first to demonstrate true TW ampliciation of few-cycle laser pulses:

High-power parametric amplification of 11.8-fs laser pulses with carrier-envelope phase control
R. Th Zinkstok, S. Witte, W. Hogervorst, K.S.E. Eikema
Optics Letters 30, page 78-80 (2005).

Generation of few-cycle terawatt light pulses using optical parametric chirped pulse amplification
S. Witte, R. Th Zinkstok, W. Hogervorst, and K.S.E. Eikema
Optics Express 13, 4903 (2005).

A source of 2 terawatt, 2.7 cycle laser pulses based on noncollinear optical parametric chirped pulse amplification
S. Witte, R. Th Zinkstok, W. Hogervorst, W. Ubachs, and K.S.E. Eikema
Optics Express 14, 8168 (2006).

More recently we have focused our attention to the phase stability of the generated pulses, and demonstrated for the first time that TW peak power laser pulses with stable carrier-envelope phase are possible:

Generation of few-cycle terawatt light pulses using optical parametric chirped pulse amplification
A. Renault, D.Z. Kandula, S. Witte, A.L. Wolf, R. Th Zinkstok, W. Hogervorst, K.S.E. Eikema
Optics Letters 32, 2363 (2007).

The latest result

We just demonstrated amplification of two subsequent pulses from a frequency comb laser, and obtained a rms phase stability between the pulses of ~20 mrad. You can read about it in our most recent publication:

Ultrafast double-pulse parametric amplification for precision Ramsey metrology
D.Z. Kandula, A. Renault, Ch. Gohle, A.L. Wolf, S. Witte, W. Hogervorst, W. Ubachs, K.S.E. Eikema
Optics Express 16, 7071 (2008).


In case you wonder what this is good for: it will be used for direct frequency comb excitation (Ramsey excitation) of helium in the XUV.

Below you can see a picture of the 2 amplifier stages in our non-collinear chirped pulse amplifier. In the top-left is the first stage (passed twice), on the right the power amplification stage. What you see is the high-peak power (>200 mJ, 60 ps) pump laser beams at 532 nm. The glass tube on the right is kept vacuum and is located between two lenses that are used for 'relay imaging'. The vacuum prevents breakdown of the air in the focus between the lenses, otherwise the optics would be damaged due to beam distortion quickly ...

OPA_picture2

These projects are sponsored by:

NWO-logo FOM-logo Laserlab-Europe-logoATLAS-logo

 Questions? Contact: K.S.E.Eikema@vu.nl