Ultrafast Laser System Interrogates Magnetic and Structural Properties of Perovskites and Van der Waals Crystals

The Challenge

Advanced 2D materials such as complex oxide heterostructures and so-called van der Waals (vdW) crystals — multilayer sandwiches of 2D materials — offer the promise of improved functionality for magnetic memory, solar cells, and other photonic applications. A group led by Professor Andrea Caviglia at the Kavli Institute of Nanoscience, Delft University of Technology in the Netherlands is performing important fundamental research into the ultrafast dynamics of several classes of these materials. He states, “One of our goals is to understand how transient structural perturbations impact magnetic properties on the sub-picosecond timescale. This requires a method of generating pristine samples of the various layers and then a means to excite low-frequency phonon modes and observe the results.”

The Solution

Producing the sample films has been made possible by a Coherent LPX Pro excimer laser at 248 nm for pulsed laser deposition (PLD), that allows the fabrication of stoichiometric films of complex materials.

Interrogating the ultrafast dynamics is technically challenging because their pump-probe method requires tunable light into the midinfrared to act as the pump pulses that selectively excite specific phonon vibrations. The pump-induced dynamics are then measured by tracking the intensity I and the rotation of the polarization plane θ of time-delayed copropagating near-infrared probe pulses at a photon energy of 1.55 eV, i.e., 800 nm. Caviglia explains, “We need to generate pulses in the mid-infrared with relatively high intensity, corresponding to electric fields at MV/cm and we need high pulse to pulse stability. We get these pulses by using a Coherent Astrella amplifier to pump a pair of tunable optical parametric amplifiers (OPAs) and combining their outputs for difference frequency generation (DFG) in a GaSe crystal. Altogether, that’s a lot of non-linear processes going on, so the output stability of the amplifier is a big key to our success.”

The Result

A recent publication by the Caviglia group illustrates the success of their pump-probe approach. In this study, they demonstrated control of magnetic anisotropy in the two-dimensional van der Waals antiferromagnet NiPS3 by exciting magnon modes. Some of the data runs here spanned hours, enabled by the long-term stability of the Astrella amplifier and their DFG setup.

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“The LPX excimer laser enables our fabrication of stoichiometric films. The output stability of the Astrella amplifier is a big key to our successful pump-probe method.”

— Andrea Caviglia, Kavli Institute of Nanoscience, Delft University of Technology, Netherlands

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