Temporal superoscillation of sub-terahertz coherent acoustic phonons

Brehm, S.; Akimov, A .V.; Campion, R. P.; Kent, A. J.

Abstract

We observe coherent acoustic phonon superoscillations at sub-terahertz frequencies. The superoscillations result from the interference of optically excited coherent longitudinal acoustic phonon modes in a GaAs/AlGaAs superlattice. The superoscillations are seen in local temporal regions and their frequency is up to 60% larger than the highest frequency excited/detected phonon mode. Such superoscillatory phonon pulses may potentially be used for high-resolution acoustic measurement in strong scattering media. Coherent acoustic phonons with frequencies in the 100s of GHz range, which are also known as acoustic nanowaves, are a powerful tool for investigating nano-scale objects with high temporal and spatial resolution. The established technique, known as picosecond acoustics, uses ultrafast (femtosecond) pulsed lasers to optically generate and detect coherent acoustic phonons with frequencies up to ~1 THz [1]. Such acoustic nanowaves can be used to probe the structural and mechanical properties of matter with sub-nm axial resolution and sub-µm transverse resolution [2]. Examples of the applications include studies of various nano-objects [3, 4], interfaces [5], defects in solids [6], soft condensed matter [7], liquids [8] and biological matter (single cells) [9] (for a review see also [10]). Acoustic nanowaves have also been used for ultrafast control of photonic [11], electronic [12], and spintronic [13] devices. Despite the successes of the picosecond acoustics technique, it suffers from a significant practical drawback: the strong attenuation and/or scattering, i.e. short mean free path, of high frequency (>~100s of GHz) phonons in many of the objects of interest. Causes of phonon attenuation and scattering include: propagation through non-ideal interfaces [14], e.g. between a stick-on optoacoustic transducer and the object under investigation; propagation in polycrystalline materials and crystals with a high defect density [15]; viscous damping [16]; natural isotope (point mass defect) scattering [17], which occurs even in perfect crystals. At ambient, room, temperature, the mean free path of THz acoustic phonons in high-quality (semiconductor grade) crystals is only a few µm [18]. The result of these is that the mean free path decreases strongly with increasing phonon frequency with the net effect of imposing a bandwidth limit, restricting the highest frequency of the phonons that can be propagated in the system. This determines the highest phonon frequency that can be used in a picosecond acoustic measurement and hence the best resolution that could normally be obtained. Bandwidth limitations also occur in measurements using electromagnetic waves and the search for ways to overcome these has led to renewed interest in the phenomenon of "superoscillation". Superoscillations occur in a band-limited function due the interference of a series of harmonic waves, Fourier series, with the appropriate amplitudes and phases. They are oscillations found in local temporal regions having rates arbitrarily higher than the highest frequency Fourier component [19]. Superoscillations have relevance in all areas of physics where waves are superposed, e.g. in quantum mechanics [20]. It has also been proposed and demonstrated that superoscillations could be exploited

Citation

Brehm, S., Akimov, A. .., Campion, R. P., & Kent, A. J. (2020). Temporal superoscillation of sub-terahertz coherent acoustic phonons. Physical Review Research, 2(2), Article 023009. https://doi.org/10.1103/PhysRevResearch.2.023009

• Journal Article Type: Article
• Acceptance Date: Feb 28, 2020
• Online Publication Date: Apr 6, 2020
• Publication Date: Apr 6, 2020
• Deposit Date: Mar 5, 2020
• Publicly Available Date: Apr 6, 2020
• Journal: Physical Review Research
• Publisher: American Physical Society
• Peer Reviewed: Peer Reviewed
• Volume: 2
• Issue: 2
• Article Number: 023009
• DOI: https://doi.org/10.1103/PhysRevResearch.2.023009
• Public URL: https://nottingham-repository.worktribe.com/output/4091707
• Publisher URL: https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.2.023009