Application of broadband, time-resolved magneto-optics to problems in ultrafast photophysics

Abstract

Ever faster computers require ever more efficient data storage technologies. A major bottleneck for faster data storage in magnetic media is the speed at which magnetisation can be manipulated. To circumvent this, a better understanding of magnetisation and magnetic materials on ultrashort timescales (10⁻¹⁵ − 10⁻⁹ s) is required. Pump-probe spectroscopy allows us to measure the response of systems to an optical stimulus over such times. When combined with magneto-optics, this provides a valuable method for investigating ultrafast magnetisation dynamics. A promising group of materials for future technologies is that of 2D materials, where the reduced dimensionality can cause them to have different properties to their 3D counterparts. Cᵣ₂Gₑ₂Tₑ₆ (CGT) is a recently discovered 2D ferromagnetic semiconductor which is a potential system for studying and understanding magnetism on ultrashort timescales. Transient ellipticity and transmittance spectra of a small, thin, single crystal of CGT were measured at 20 K , well below the Curie temperature of ∼63 K. Strong wavelength dependence was observed in the transient ellipticity spectrum, meaning that the ellipticity change was proportional to not only magnetisation change. The spectrum could be decomposed into the contributions arising from photoexcited electrons and demagnetisation, both of which showed very different dynamics.

Magnetic fields in a medium influence the polarisation of transmitted light through magneto-optic phenomena. Most studies of ultrafast magnetism use only single wavelengths to probe samples, which can miss the vital information that broadband spectra can provide. I have therefore built a broadband, ultrafast spectrometer that can measure transient polarisation change with a 350–750 nm, 200 fs supercontinuum probe. Ellipticity change is measured through the change in transmittance of a circularly polarised probe pulse when an applied magnetic field is reversed. Similarly, the transient ellipticity change is found from the change in transient transmittance. Rotation can be measured in a similar manner. To compare performance relative to other published instruments and validate its correct operation, the spectrometer was tested on various samples, from ferromagnetic films to paramagnetic ions in solution.

Porphyrins are organic, naturally occurring compounds which show rich ellipticity and absorbance spectra so their photophysics are well studied. Zinc tetraphenylporphyrin, a member of this family of compounds, shows a strong ellipticity spectrum at room temperature and undergoes intersystem crossing from singlet to triplet states upon photoexcitation. Ultrafast electronic changes may therefore result in transient changes to the ellipticity, which could be measured with the above setup and compared to the transient absorbance. Broadband ellipticity spectra of the complex in solution were measured and the transient depopulation of the ground state was observed through a signal with the same shape as the ground state ellipticity spectrum. Furthermore, transient ellipticity spectra whose dynamics corresponded to an excited singlet state could be observed.