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Register a new userTime-domain Brillouin scattering for the determination of laser-induced temperature gradients in liquids
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We present an optical technique based on ultrafast photoacoustics to precisely determine the local temperature distribution profile in liquid samples in contact with a laser heated optical transducer. This ultrafast pump-probe experiment uses time-domain Brillouin scattering (TDBS) to locally determine the light scattering frequency shift. As the temperature influences the Brillouin scattering frequency, the TDBS signal probes the local laser-induced temperature distribution in the liquid. We demonstrate the relevance and the sensitivity of this technique for the measurement of the absolute laser-induced temperature gradient of a glass forming liquid prototype, glycerol, at different laser pump powers - i.e. different steady state background temperatures. Complementarily, our experiments illustrate how this TDBS technique can be applied to measure thermal diffusion in complex multilayer systems in contact to a surrounding liquid.
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Time-domain Brillouin scattering uses ultrashort laser pulses to generate coherent acoustic pulses of picoseconds duration in a solid sample and to follow their propagation in order to image material inhomogeneities with sub-optical depth resolution. The width of the acoustic pulse limits the spatial resolution of the technique along the direction of the pulse propagation to less than several tens of nanometres. Thus, the time-domain Brillouin scattering outperforms axial resolution of the classical frequency-domain Brillouin scattering microscopy, which uses continuous lasers and thermal phonons and which spatial resolution is controlled by light focusing. The technique benefits from the application of the coherent acoustic phonons, and its application has exciting perspectives for the nanoscale imaging in biomedical and material sciences. In this study, we report on the application of the time-domain Brillouin scattering to the 3D imaging of a polycrystal of water ice containing two high-pressure phases. The imaging, accomplished via a simultaneous detection of quasi-longitudinal and quasi-shear waves, provided the opportunity to identify the phase for individual grains and evaluate their crystallographic orientation. Monitoring the propagation of the acoustic waves in two neighbouring grains simultaneously provided an additional mean for the localisation of the grain boundaries.
We demonstrate the use of the micro-Brillouin light scattering (micro-BLS) technique as a local temperature sensor for magnons in a Permalloy thin film and phonons in the glass substrate. A systematic shift in the frequencies of two thermally excited perpendicular standing spin wave modes as the film is uniformly heated allows us to achieve a temperature resolution better than 2.5 K. We demonstrate that the micro-BLS spectra can be used to measure the local temperatures of phonons and magnons across a thermal gradient. Such local temperature sensors are useful for investigating spin caloritronic and thermal transport phenomena in general.
Brillouin light scattering spectroscopy from so-called standing spin waves in thin magnetic films is often used to determine the magnetic exchange constant. The data analysis of the experimentally determined spin-wave modes requires an unambiguous assignment to the correct spin wave mode orders. Often additional investigations are needed to guarantee correct assignment. This is particularly important in the case of Heusler compounds where values of the exchange constant vary substantially between different compounds. As a showcase, we report on the determination of the exchange constant (exchange stiffness constant) in Co$_2$MnSi, which is found to be $A=2.35pm0.1$ $mu$erg/cm ($D=575pm20$ meV AA$^2$), a value comparable to the value of the exchange constant of Co.
A simple theory is developed for an interpretation of the time-domain Brillouin scattering experiments where the coherent acoustic pulse and the probe light pulse beams are propagating at an angle to each other. The directivity pattern of their acousto-optic interaction in case of heterodyne detection of the acoustically scattered probe light (in nearly backward direction to the probe light) is predicted. The theory reveals the dependences of carrier frequency and duration of acoustically induced wave packets in the transient reflectivity signals, on the widths of light and sound beams, and on the angle of their relative propagation (interaction angle). It also describes the transient dynamics of these wave packets when the probe light and the coherent acoustic pulses are incident on material interfaces (inter-grain boundaries) and Brillouin scattering by incident acoustic field is transformed into Brillouin scattering by the reflected and transmitted (refracted) acoustic fields. In general, these transformations are accompanied by the modifications of the interaction angles between the coherent acoustic pulses and probe light beams. The sensitivities of the carrier frequencies and wave packet amplitudes in the reflected/transmitted beams to the angle of the beams incidence on the interface are evaluated and compared. The theory confirms the expected possibility of strong and dominant reduction in the time-domain Brillouin scattering amplitude following the reflection/transmission processes for large interaction angles.
Although ultrafast manipulation of magnetism holds great promise for new physical phenomena and applications, targeting specific states is held back by our limited understanding of how magnetic correlations evolve on ultrafast timescales. Using ultrafast resonant inelastic x-ray scattering we demonstrate that femtosecond laser pulses can excite transient magnons at large wavevectors in gapped antiferromagnets, and that they persist for several picoseconds which is opposite to what is observed in nearly gapless magnets. Our work suggests that materials with isotropic magnetic interactions are preferred to achieve rapid manipulation of magnetism.
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