Detection Of Acoustic Phonons Research Articles

Generation and Control of Coherent Terahertz Phonons in Silicon Metasurfaces

AbstractSemiconductor nanostructures are used in many applications, from electronics to nanophotonics to cite just a few, and most of these applications are based on silicon or silicon‐related technologies. With the miniaturization of these nanoscale devices, heat dissipation becomes a limitation to their efficiency, robustness, and reliability. Hence, managing thermal effects at nanoscale is attracting a growing interest and a promising approach relies on the control of acoustic phonons that are responsible of heat transfers. However, these phonons in silicon have high frequencies in the terahertz range which makes them hard to control and detect, thus limiting basic investigation and practical implementations. In this work, employing coherent control of femtosecond laser pulses and inelastic scattering spectroscopy, it is reported on the coherent generation, the control, and the detection of acoustic phonons in silicon metasurfaces in which light–matter interactions are enhanced.

Excitation and detection of acoustic phonons in nanoscale systems.

Phonons play a key role in the physical properties of materials, and have long been a topic of study in physics. While the effects of phonons had historically been considered to be a hindrance, modern research has shown that phonons can be exploited due to their ability to couple to other excitations and consequently affect the thermal, dielectric, and electronic properties of solid state systems, greatly motivating the engineering of phononic structures. Advances in nanofabrication have allowed for structuring and phonon confinement even down to the nanoscale, drastically changing material properties. Despite developments in fabricating such nanoscale devices, the proper manipulation and characterization of phonons continues to be challenging. However, a fundamental understanding of these processes could enable the realization of key applications in diverse fields such as topological phononics, information technologies, sensing, and quantum electrodynamics, especially when integrated with existing electronic and photonic devices. Here, we highlight seven of the available methods for the excitation and detection of acoustic phonons and vibrations in solid materials, as well as advantages, disadvantages, and additional considerations related to their application. We then provide perspectives towards open challenges in nanophononics and how the additional understanding granted by these techniques could serve to enable the next generation of phononic technological applications.

Coherent generation and detection of acoustic phonons in topological nanocavities

Inspired by concepts developed for fermionic systems in the framework of condensed matter physics, topology and topological states are recently being explored also in bosonic systems. Recently, some of these concepts have been successfully applied to acoustic phonons in nanoscale multilayered systems. The reported demonstration of confined topological phononic modes was based on Raman scattering spectroscopy [M. Esmann et al., Phys. Rev. B 97, 155422 (2018)], yet the resolution did not suffice to determine lifetimes and to identify other acoustic modes in the system. Here, we use time-resolved pump-probe measurements using an asynchronous optical sampling (ASOPS) technique to overcome these resolution limitations. By means of one-dimensional GaAs/AlAs distributed Bragg reflectors (DBRs) used as building blocks, we engineer high frequency (∼200 GHz) topological acoustic interface states. We are able to clearly distinguish confined topological states from stationary band edge modes. The generation/detection scheme reflects the symmetry of the modes directly through the selection rules, evidencing the topological nature of the measured confined state. These experiments enable a new tool in the study of the more complex topology-driven phonon dynamics such as phonon nonlinearities and optomechanical systems with simultaneous confinement of light and sound.

Terahertz acoustic phonon detection from a compact surface layer of spherical nanoparticles powder mixture of aluminum, alumina and multi-walled carbon nanotube

We present terahertz spectroscopy study on spherical nanoparticles powder mixture of aluminum, alumina, and MWCNTs induced by surface mechanical attrition treatment (SMAT) of aluminum substrates. Surface alloying of AL, Al2O3 0.95% and MWCNTs 0.05% powder mixture was produced during SMAT process, where a compact surface layer of about 200 μm due to ball bombardment was produced from the mixture. Al2O3 alumina powder played a significant role in MWCNTs distribution on surface, those were held in deformation surface cites of micro-cavities due to SMAT process of Al. The benefits are the effects on resulted optical properties of the surface studied at the terahertz frequency range due to electrical isolation confinement effects and electronic resonance disturbances exerted on Al electronic resonance at the same range of frequencies. THz acoustic phonon around 0.53–0.6THz (17–20cm−1) were observed at ambient conditions for the spherical nanoparticles powder mixture of Al, Al2O3 and MWCNTs. These results suggested that the presence of Al2O3 and MWCNTs during SMAT process leads to the optically detection of such acoustic phonon in the THz frequency range.

Coherent control of sub-terahertz confined acoustic nanowaves: Theory and experiments

Recently it has been theoretically proposed that phonons could be used to store and process information. We report coherent control experiments on acoustic-phonon nanocavities in the sub-THz range and discuss the feasibility of using this system as a phononic memory. The nanocavities provide localized and monochromatic waves, two essential characteristics to work as memories and actuators. A writing-erasing-reading procedure based on the coherent generation, control, and detection of acoustic phonons using ultrashort light pulses is presented. The results are in agreement with numerical simulations using an implementation of a photoelastic model. Our results open the way to the practical realization of complex nano-optomechanical devices.

Theory of coherent generation and detection of THz acoustic phonons using optical microcavities

The coherent generation and detection of acoustic phonons in a superlattice embedded in an optical microcavity is theoretically analyzed. In this optical resonator, femtosecond light pulses can be spatially confined and amplified. We show that the acoustic phonon generation is enhanced as the intensity of the incident electromagnetic field is amplified in resonance with the optical microcavity. The detection process is also enhanced by the optical resonator. In the case of real photoelastic constants the maximum sensitivity occurs when the probe wavelength is tuned to where the derivative of the reflectivity has its maxima, at the optical cavity mode edges. We also analyze the role of the imaginary part of the photoelastic constants of the structure in the generation and detection processes. Finally, we study the enhancement efficiency of the microcavities when the coherent generation and detection are optimized simultaneously; we estimate phonon signals up to six orders of magnitude higher than the ones obtained with the superlattice without optical confinement.

Enhanced optical generation and detection of acoustic nanowaves in microcavities

The enhancement of the ultrafast coherent generation and detection of acoustic phonons in an optical microcavity is experimentally studied. We report pump-probe terahertz ultrasonic experiments in an optical microcavity as a function of laser energy and probe incidence angle. By tuning the laser wavelength to resonate with the microcavity mode at normal incidence and simultaneously varying the incidence angle of the probe beam we achieve a double optical enhancement. Under this condition both the coherent generation and detection of acoustic nanowaves are strongly enhanced. We demonstrate the use of optical microcavities as a promising tool to study acoustic phonons in reduced dimensions with increased sensitivity.

Ultrafast acoustics in the middle UV range: coherent phonons at higher frequencies and in smaller objects

We show that the propagation of coherent acoustic phonons generated by femtosecond optical excitation can be clearly resolved using a probe laser in the middle UV (MUV) range. The MUV probe is easily produced from a high-repetition-rate femtosecond laser and a homemade frequency tripler. We present various experimental results that demonstrate efficient and high frequency detection of acoustic phonons. Thus, we show that the MUV range offers a unique way to reach higher frequencies and probe smaller objects in ultrafast acoustics.

Influence of doping profiles on coherent acoustic phonon detection and generation in semiconductors

The doping profile in different n-doped GaAs homoepitaxial structures grown by molecular beam epitaxy is investigated in the time domain by employing a laser based picosecond ultrasound technique in a contactless and noninvasive way. Experiments based on asynchronous optical sampling employ two femtosecond lasers, which allow us to detect changes in the optical reflectivity over a 1 ns time delay with a signal-to-noise ratio of 107 and 100 fs time resolution in <1 min of acquisition time. We show that the doping profile with doping densities of the order of 1018 cm−3 can be detected with picosecond ultrasound, although there is no difference in the acoustic properties of the doped and undoped region. The detection mechanism is based on a different sensitivity function for a coherent strain pulse in the doped and undoped regions. These results are corroborated by experiments at room temperature and 10 K.

Decoupling of optical generation and detection of acoustic phonons in semiconductor superlattices

We present a study of the generation and detection of terahertz acoustic phonons in GaAs/AlAs superlattices through femtosecond pump and probe optical pulses, where the spectral contributions of the generation and detection processes are decoupled and identified. The processes are spatially separated in different superlattices grown with an intermediate 1-µm-thick GaAs layer. Pumping and probing occur on opposite superlattices. A thickness gradient present on one of the superlattices allows us to identify the features in the spectra resulting either from the generation or from the detection. A Raman scattering characterization of the sample is also presented.

Laser-wavelength dependence of the picosecond ultrasonic response of a NiFe/NiO/Si structure

Ultrafast optical excitation and detection of acoustic phonons has been used to analyze ultrathin films composed of NiFe/NiO/Si which are important for applications in magnetic storage and processing. Results are presented on the wavelength dependence of the ultrasonic response of the thin NiO film and bulk Si. Significant changes are observed between detection using the fundamental and the second harmonic of the femtosecond laser as the probe beam. Beatings between low order longitudinal phonons in the NiO layer are observed and measurements of its refractive index and absorption coefficients are performed.

New method of detection of terahertz acoustic phonons in quantum well structures

The effect of nonequilibrium acoustic phonons upon the luminescence of a quasi-resonantly excited ultrathin CdTe/ZnTe quantum well has been studied. It was shown that changes in the quantum well luminescence spectrum manifest already for a low intensity of phonon generation. We suggest to use this effect for the terahertz acoustic phonon detection in low-dimensional heterostructures.