Abstract
Current time-resolution-limited dynamic measurements clearly show the need for improved techniques to access processes on the sub-10-femtosecond timescale. To access this regime, we have designed and constructed a state-of-the-art time-resolved magneto-optic Kerr effect apparatus, based on a new dual-color scheme, for the measurement of ultrafast demagnetization and precessional dynamics in magnetic materials. This system can operate well below the current temporal ranges reported in the literature, which typically lie in the region of around 50 fs and above. We have used a dual-colour scheme, based on ultra broadband hollow-core fibre and chirped mirror pulse compression techniques, to obtain unprecedented sub-8-fs pump and probe pulse durations at the sample plane. To demonstrate the capabilities of this system for ultrafast demagnetization and precessional dynamics studies, we have performed measurements in a ferrimagnetic GdFeCo thin film. Our study has shown that the magnetization shows a sudden drop within the first picosecond after the pump pulse, a fast recovery (remagnetization) within a few picoseconds, followed by a clear oscillation or precession during a slower magnetization recovery. Moreover, we have experimentally confirmed for the first time that a sub-10-fs pulse is able to efficiently excite a magnetic system such as GdFeCo.
Highlights
Based on the premise that the photo-excitation of a magnetic system with ultrashort laser pulses significantly alters the thermodynamic equilibrium among the constituent degrees of freedom of the system, a variety of dynamical processes have been observed on an extremely fast timescale, which are inaccessible via thermal equilibrium transitions[1,2,3,4,5,6,7,8]
Rapid advances in ultrafast optical methods have allowed the temporal resolution to be gradually improved, the most recent measurements have been routinely performed with a temporal resolution that is typically limited to few tens of fs (e.g., 50 fs in reference 5)
The need to achieve shorter temporal resolutions has been recently asserted by Bossini et al.[12] where it was demonstrated that a successful excitation of high-frequency magnons demands laser pulses shorter than 50 fs
Summary
Our pump-probe system (see Fig. 2) is based on a commercial titanium:sapphire laser amplifier (Femtolasers Compact Pro CE-phase) delivering sub-30-fs laser pulses (approximately 40 nm bandwidth centered at 800 nm). An optical chopper operating at 500 Hz and synchronized with the laser pulse train provides the reference for the detection electronics, which include a boxcar integrator and a lock-in amplifier coupled to an analog-to-digital converter In both pump and probe beams, we place lenses with 200 and 100 mm focal length which produce focused spots with diameters of about 350 and 180 μm, respectively. In order to generate intense radiation, with pulse energies of hundreds of microjoules and durations in the sub-4-fs regime, as envisaged for versatile high resolution pump-probe experiments, a high-energy pulse compression technique based on pulses initially created by CPA, is employed This technique is composed of two steps: i) nonlinear spectral broadening and ii) broadband dispersion compensation[13,27,28] (Fig. 3). These demonstrate that dual-colour ultrashort pump and probe pulses with sub[10] fs duration have been successfully obtained at the plane of the sample, which ensures an ultra-high temporal resolution
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