Generation and detection of gigahertz acoustic phonons and their interactions with charge carriers in nanomembranes
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Date
2024-07-17
Authors
Σύμπουρα, Κλεονίκη
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Abstract
The mechanical and thermal properties of materials are strongly connected with the properties of hypersonic, gigahertz (GHz) acoustic phonons, which are the carriers of heat and sound. Thus, multiple applications, such as wireless communication devices, ultrasonic sensors and heat management in electronic devices, require an in-depth knowledge of the mechanisms for generation, propagation, and attenuation of hypersonic phonons. One of the central properties related to propagation is the phonon group velocity, which can be controlled by tuning the phonon dispersion relation. The ability to tune the phonon dispersion relationships (band structure) is the main goal of Phononics. This objective is fulfilled with the development of phononic crystals (PnCs). PnCs are elastic metamaterials with phononic band gaps that are used to control the propagation of acoustic waves. In this way, PnCs can be used to control the transport of infrasound, sound, hypersound, or heat depending on their operational frequency range (Hz to kHz, GHz and THz, respectively). In the context of hypersound management, PnCs are used to mediate signal processing with microwaves for wireless communications and various nanomechanical sensors. These applications take advantage of the unique properties of hypersonic phonons, such as their ability to propagate through solids with low attenuation and their high spatial resolution. Most importantly, it is possible to convert electromagnetic radiation (microwaves) to acoustic phonons in the GHz range using piezoelectrics, meaning materials that convert electric fields into pressure. PnCs with band gaps can then be used to guide the acoustic signals or block out external noise. Then, these acoustic signals can be filtered or measured, which is how multiple operations are executed on smartphones. In the context of heat management, PnCs can be used for heat removal, energy harvesting and controlled transport of heat, for example, one-way heat transport.3 Hence, by manipulating the propagation of phonons with PnCs, it is possible to modify the thermal conductivity of materials and other thermal properties, e.g., thermoelectric efficiency. In more detail, selectively blocking certain phonon frequencies can reduce thermal conductivity and develop better insulating materials to reduce heat transfer in specific components of electronic devices. Usually, the PnCs used for controlling heat transport must operate in the terahertz (THz) range. All of the above make clear the need to develop nanophononics and PnCs that operate in the GHz to THz range. One of the most important tools for the characterization of nanophononic materials and PnCs is Brillouin Light Scattering (BLS). This technique is particularly useful for studying phonons because it allows for the measurement of both the frequency and the wavevector (or momentum) of phonons, which are then giving the phonon band structure. Moreover, it is possible to extract the phonon lifetime from the width of the spectroscopic peaks or with direct time-resolved BLS measurements. However, as described above, most applications require PnCs that operate above 10 GHz (post 5G networks) and even in the THz range for thermal management. These higher frequencies are achieved with the use of sub-micrometer feature size PnCs. However, the small mass of the samples can lead to poor signal-to-noise ratio for BLS characterization. Moreover, the higher phonon frequencies translate into low phonon occupation and poor BLS signal. In addition, the used materials are often semiconducting, meaning that they appear semi-opaque and absorb light. This leads to an additional decrease in the BLS signal-to-noise ratio. Finally, the acoustic phonons can interact with various other microscopic excitations like electric charges, excitons, and plasmons (charge oscillations). To overcome all these obstacles, we need to develop methods to enhance the BLS signal. A recently developed method for enhancing the BLS signal is called pumped-Brillouin Light Scattering (pumped-BLS). While conventional BLS is based on spontaneous light scattering by thermal phonons, pumped-BLS is carried out in non-thermal conditions. In pumped-BLS, the GHz phonons are pumped with femtosecond laser pulses and probed with inelastic light scattering. The photoexcited GHz acoustic phonons population displays a hundredfold enhancement compared to thermal equilibrium for pure silicon nanomembranes. For gold- silicon nanomembranes, the enhancement of BLS can be four orders-of-magnitude larger than in thermal equilibrium. During this work, we have advanced the pumped-BLS setup to demonstrate interference between phonons. The femtosecond laser pulses are split into two beams, and one of them is delayed in time using a 50/50 beamsplitter and a delay stage. Depending on their relative phase, these two beams excite coherent, GHz acoustic phonons that show constructive or destructive interference. These results pave the way for studying interesting phenomena like time- refraction and time-diffraction for acoustic signals in the GHz range. Finally, we could observe a new, exotic phenomenon called light diffraction in the time domain. In this phenomenon, electromagnetic waves are scattered by two-time perturbations (as happens in space with two slits) and show maxima and minima in the frequency domain.
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Keywords
Phonons, Nanomembranes, Acoustics