We have associated the temporal resolution given by the femtosecond laser pulses with a spatial resolution given by a time resolved magneto-optical microscope in a confocal geometry. It is used in a pump–probe configuration to perform the imaging and the study of the magnetization dynamics in ferromagnetic nanostructures with submicrometer sizes.
Our experimental approach allows a spatial lateral resolution of 300 nm and a temporal resolution of 150 fs. The polarization analysis of the probe beam provides fine details regarding the magnetization dynamics in confined structures.
A sketch of the magneto-optical microscope is depicted in the attached figure. The pump laser pulses are provided by a Titanium:Sapphire regenerative amplifier operating at a repetition rate of 5kHz. The pump wavelength is set at 793 nm, while the probe pulses (396 nm) are obtained by frequency doubling the fundamental wavelength of the amplifier in a beta barium borate (BBO) crystal. Two telescopes in the path of the pump respectively the probe beams insure the finest focusing conditions at the sample place. A microscope objective with NA of 0.65 focuses the two beams onto the sample which is mounted on a xy piezoelectric translation stage, allowing a 2 nm scanning resolution over 80 µm maximum course in each direction.
The theoretical resolution is 300 nm for the probe (λ=396 nm) and 600 nm for the pump (λ=793 nm). The main advantage given by the confocal geometry in our experiments is to discard the diffusion coming from the out of focus regions of the sample or substrate, which would reduce the polarization contrast and would induce a lower signal to noise ratio. The pulses duration of the pump and probe beams are of about 150 fs and 180 fs respectively, slightly higher than at the output of the amplifier, due to the dispersion in the thick objective lens. The reflected probe beam is further collected by a dichroic beam-splitter and refocused on a 20 µm pinhole before being detected by a photomultiplier. Finally, the synchronous detection is realised with a lock-in amplifier for a modulated signal in the range of hundreds of Hz obtained by a mechanical chopper. The polarisation analysis is done either in the crossed polarisers configuration, or in a polarization bridge.
An example which shows the very good resolution obtained with our microscope is shown in the adjoined figure. It represents the magneto-optical image as well as a transverse slice of an individual disk of CoPt3 of 500 nm in diameter, measured in the crossed polarisers configuration for an applied field of 4kOe. The stability and resolution of our ultrafast magneto-optical microscope is highly improved using a novel modulation technique MOPPI (Magneto-Optical Pump-Probe Imaging) proving its versatility in the study of Spin Photonics