Research activities

Nonlinear THz applications

Terahertz–driven magnetization dynamics in ferromagnetic film

The recent development of high-field Terahertz laser sources opens new opportunities for the coherent control of magnetization dynamic at femtoseconds timescale [1]. Our group recently demonstrated that the magnetization in a cobalt thin film can be directly steered by a strong 0.4 Tesla magnetic Terahertz field, which is generated by optical rectification in organic crystals [2]. Contrary to phase transition initiated by femtosecond near-IR laser, in our experiment, the THz pulses drive ultrafast magnetization without significant heat deposition. The THz pulses act in other words as a "cold" stimulus allowing coherent control of the magnetization state. In this new interaction regime, the THz pulse phase and field magnitude characteristics are directly imprinted onto the magnetization response, in absence of any resonant mode. The coupling mechanism injects only minor entropy into the system, thus avoiding the speed limitation caused by the cooling process.
The time-resolved investigations were performed by measurement of the magneto optical Kerr rotation induced on a 50 fs Ti:Sapphire optical probe. The results are presented in the figure below. The evolution shows a magnetization (red curve) which is phase-locked to the THz magnetic field (blue dotted). Unexpectedly, the magnetization evolution linked to the Terahertz magnetic laser field is well described by the semi-empirical Landau-Lifschitz-Gilbert (LLG) equation (black plot). The observed direct steering of magnetization on an ultrafast timescale is expected to play a significant role in the future ultrafast data storage by purely optical means.
Intense Terahertz transients offer moreover intriguing opportunities in other cutting edge experiment such as ultrashort pulse diagnostics or to enhance the soft x-ray photon fluence by high harmonic generation in noble gas.




Reference:
  1. C. Ruchert, C. Vicario and C.P. Hauri, Phys. Rev. Lett. 110, 123902 (2013)
  2. C. Vicario, C. Ruchert, F. Ardana-Lamas, P.M. Derlet, B. Tudu, J.Luning and C.P. Hauri, Nature Photonics 7, 720 (2013)

HHG applications

A table-top soft x-ray laser based on high-harmonic generation is an ideal tool for investigating matter on the sub-femtosecond timescale.
We are aiming to overcome limitations of present HHG sources in (a) photon energy and (b) photon flux by using mid-infrared laser technology and exploring phase matching schemes. These parameters are crucial for our three main applications.
  • Investigation of ultrafast magnetization dynamics at the M absorption edge
  • Attosecond physics in the water window spectral region (285eV-420 eV)
  • Free Electron Laser seeding in the soft x-ray region (<10 nm)

Ultrafast magnetization dynamics

Our table-top HHG source delivers high photon flux between 20-160 eV and is suited for exploring transient dynamics in magnetic materials on a femtosecond timescale via the transverse magneto-optic Kerr effect (T-MOKE). T-MOKE in the EUV is sensitive to the magnetization vector of the material through a change in reflectivity which depends on the projection of the magnetization normal to the scattering plane. We use a few-femtosecond extreme ultraviolet pulse from the HHG source to extract magnetization dynamics induced by an infrared pump pulse. The HHG-based XUV source offers excellent conditions for element-selective investigations, as the HHG spectrum covers simultaneously the M2,3-absorption edges of many ferroelectric materials and compounds. The temporal resolution of HHG combined with elemental specificity will help to answer important fundamental and technological relevant questions in ultrafast magnetism. It is for example still unclear how the transfer of angular momentum during the laser interaction takes place. These and other phenomena we explore with MOKE in the near IR and EUV and x-ray magnetic circular dichroism (XMCD).

Seeding of a free electron laser (FEL)

FELs are capable provide intense, femtosecond x-ray pulses down to 0.1 nm wavelength. The x-ray radiation is produced by a relativistic electron bunch travelling through undulator sections. The underlying principle to produce intense x-ray pulses is self-amplified stimulated emission (SASE), which leads to microbunching of the electron beam and to narrowband emission of x-rays.
SASE FELs suffer from a poor temporal coherence since the lasing process starts from noise. In principle, the temporal coherence can be improved by superposing an external temporal coherent seed beam to the electron beam. The challenge is that the noise floor increases at shorter FEL wavelengths and it gets difficult to overcome the power level by an external coherent seed. A promising candidate for a coherent seed beam is radiation from high-order harmonic generation (HHG). Our group is actively pushing the development of high-power HHG sources towards wavelengths as short as 1 nm (~1 keV photon energy). The goal is to realize a soft x-ray source strong enough to seed the future soft x-ray branch of SwissFEL, which lase between 1-7 nm. To reach this objective we study advanced phase-matching schemes, explore novel HHG schemes and combine those with cutting edge laser technology.