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Register a new userContra-Intuitive Features of Time-Domain Brillouin Scattering in Collinear Paraxial Sound and Light Beams
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Vitalyi E. Gusev
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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.
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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.
Recent years witnessed much broader use of Brillouin inelastic light scattering spectroscopy for the investigation of phonons and magnons in novel materials, nanostructures, and devices. Driven by developments in instrumentation and the strong need for accurate knowledge of energies of elemental excitations, the Brillouin - Mandelstam spectroscopy is rapidly becoming an essential technique, complementary to the Raman inelastic light scattering spectroscopy. We provide an overview of recent progress in the Brillouin light scattering technique, focusing on the use of this photonic method for the investigation of confined acoustic phonons, phononic metamaterials, magnon propagation and scattering. The Review emphasizes emerging applications of the Brillouin - Mandelstam spectroscopy for phonon engineered structures and spintronic devices and concludes with a perspective for future directions.
Fundamental and applied concepts concerning the ability of light beams to carry a certain mechanical angular momentum with respect to the propagation axis are reviewed and discussed. Following issues are included: Historical reference; Angular momentum of a paraxial beam and its constituents; Spin angular momentum and paradoxes associated with it; Orbital angular momentum; Circularly-spiral beams: examples and methods of generation; Orbital angular momentum and the intensity moments; Symmetry breakdown and decomposition of the orbital angular momentum; Mechanical models of the vortex light beams; Mechanical action of the beam angular momentum; Rotational Doppler effect, its manifestation in the image rotation; Spectrum of helical harmonics and associated problems; Non-collinear rotational Doppler effect; Properties of a beam forcedly rotating around its own axis. Research prospects and ways of practical utilization of optical beams with angular momentum.
Microwave photonic systems are compelling for their ability to process signals at high frequencies and over extremely wide bandwidths as a basis for next generation communication and radar technologies. However, many applications also require narrow-band $(simtext{MHz})$ filtering operations that are challenging to implement using optical filtering techniques, as this requires reliable integration of ultra-high quality factor $(sim 10^8)$ optical resonators. One way to address this challenge is to utilize long-lived acoustic resonances, taking advantage of their narrow-band frequency response to filter microwave signals. In this paper, we examine new strategies to harness a narrow-band acoustic response within a microwave-photonic signal processing platform through use of light-sound coupling. Our signal processing scheme is based on a recently demonstrated photon-phonon emitter-receiver device, which transfers information between the optical and acoustic domains using Brillouin interactions, and produces narrow-band filtering of a microwave signal. To understand the best way to use this device technology, we study the properties of a microwave-photonic link using this filtering scheme. We theoretically analyze the noise characteristics of this microwave-photonic link, and explore the parameter space for the design and optimization of such systems.
A generalized Wigner-Moyal statistical theory of radiation is used to obtain a general dispersion relation for Stimulated Brillouin Scattering (SBS) driven by a broadband radiation field with arbitrary statistics. The monochromatic limit is recovered from our general result, reproducing the classic monochromatic dispersion relation. The behavior of the growth rate of the instability as a simultaneous function of the bandwidth of the pump wave, the intensity of the incident field and the wave number of the scattered wave is further explored by numerically solving the dispersion relation. Our results show that the growth rate of SBS can be reduced by 1/3 for a bandwidth of 0.3 nm, for typical experimental parameters.
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