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Register a new userTheory of time-domain Brillouin scattering for probe light and acoustic beams propagating at an arbitrary relative angle: Application to acousto-optic interaction near material interfaces
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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.
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Time-domain Brillouin scattering is an opto-acousto-optical probe technique for the evaluation of the transparent materials. Ultrashort pump laser pulses via optoacoustic conversion launch in the sample picosecond coherent acoustic pulses. The time-delayed ultrashort probe laser pulses monitor the propagation of the coherent acoustic pulses via photo-elastic effect, which induces light scattering. A photodetector collects acoustically scattered light and the probe light reflected by the sample structure for the heterodyning. The scattered probe light carriers the information on the acoustical, optical and acousto-optical parameters of the material in the current position of the coherent acoustic pulse. Thus, among other applications, the time-domain Brillouin scattering is a technique for three-dimensional imaging. Sharp focusing of the coherent acoustic pulses and probe laser pulses could increase lateral spatial resolution of imaging, but could potentially diminish the depth of imaging. However, the theoretical analysis presented in this manuscript contra-intuitively demonstrates that the depth and spectral resolution of the time-domain Brillouin scattering imaging, with collinearly propagating paraxial sound and light beams, do not depend at all on the focusing/diffraction of sound. The variations of the amplitude of the time-domain Brillouin scattering signal are only due to the variations of the probe light amplitude caused by light focusing/diffraction. Although the amplitude of the acoustically scattered light is proportional to the product of the local acoustical and probe light field amplitudes the temporal dynamics of the time-domain Brillouin scattering signal amplitude is independent of the dynamics of the coherent acoustic pulse amplitude.
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.
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.
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.
We present a laser beam shaping method using acousto-optic deflection of light and discuss its application to dipole trapping of ultracold atoms. By driving the acousto-optic deflector with multiple frequencies, we generate an array of overlapping diffraction-limited beams that combine to form an arbitrary-shaped smooth and continuous trapping potential. Confinement of atoms in a flat-bottomed potential formed by a laser beam with uniform intensity over its central region confers numerous advantages over the harmonic confinement intrinsic to Gaussian beam dipole traps and many other trapping schemes.We demonstrate the versatility of this beam shaping method by generating potentials with large flat-topped regions as well as intensity patterns compensating for residual external potentials to create a uniform background to which the trapping potential of experimental interest can be added.
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