We study the generation of microwave electronic signals by pumping a (311) GaAs Schottky diode with compressive and shear acoustic phonons, generated by the femtosecond optical excitation of an Al film transducer and mode conversion at the Al-GaAs interface. They propagate through the substrate and arrive at the Schottky device on the opposite surface, where they induce a microwave electronic signal. The arrival time, the amplitude, and the polarity of the signals depend on the phonon mode. A theoretical analysis is made of the polarity of the experimental signals. This analysis includes the piezoelectric and deformation potential mechanisms of electron-phonon interaction in a Schottky contact and shows that the piezoelectric mechanism is dominant for both transverse and longitudinal modes with frequencies below 250 and 70 GHz, respectively.

Picosecond duration compressive and shear phonon wave packets injected into (311) GaAs slabs transform after propagation through ∼1 mm into chirped acoustic pulses with a frequency increasing in time due to phonon dispersion. By probing the temporal optical response to coherent phonons in a near surface layer of the GaAs slab, we show that phonon chirping opens a transformational route for high-sensitivity terahertz and subterahertz phonon spectroscopy. Temporal gating of the chirped phonon pulse allows the selection of a narrow band phonon spectrum with a central frequency up to 0.4 THz for longitudinal and 0.2 THz for transverse phonons. DOI: 10.1103/PhysRevLett.119.255502 Terahertz (THz) and sub-THz phonon spectroscopy has become established as a powerful tool for probing and nanoscopy of various solid objects and nanostructures. The advances achieved in this field are described in a number of reviews [1][2][3]. An important technique for THz phonon spectroscopy is based on picosecond ultrasonics developed in the 1980s by Thomsen et al. [4] and described in detail in the recent review by Matsuda et al. [5]. Depending on the methods of phonon generation and detection, it is possible to operate with broadband [4,5] or close to monochromatic phononic wave packets [2,6,7]. It is also possible to control phonon polarization, longitudinal or transverse, associated with compressive or shear elastic perturbations, respectively [8][9][10][11][12][13]. Information about the phonon spectrum is obtained in most of the experiments by performing a fast Fourier transform (FFT) of the optical reflectivity signal measured in the temporal domain with femtosecond resolution.It is a challenge to increase the sensitivity of THz phonon spectroscopy making it a more reliable instrument for studying nano-objects. This task is beyond standard technical solutions like asynchronous optical sampling (ASOPS) [14] and requires transformational physical approaches. An appealing approach, which has not yet been realized in THz phonon spectroscopy, would be to exploit chirped phonon pulses. If similar improvements could be realized as in the fields of microwaves [15] and optics [16], it would significantly improve the signal-tonoise ratio and, in the long term, open prospectives for phonon frequency, phase, and polarization control. This could be used in data processing, interference, and quantum computing, as has been proposed for chirped optical pulses [16].In the present work, we generate sub-THz and THz upchirped shear and compressive acoustic wave packets and demonstrate the potential of the technique in phonon spectroscopy by obtaining the spectrum of the photoelastic signal from probe optical pulses. The basis of the chirp effect is phonon dispersion. Because of the curving of the acoustic dispersion, 2πf ¼ cq − γq 3 (in the long-wave approximation f and q are the phonon frequency and wave vector, γ is the dispersion parameter, and c is the sound velocity) high-frequency acoustic phonons propagate slower than low-frequency ones. A...