February 7, 2018
- Lattice vibrations (phonons) are main carriers of heat in nonmetallic solid. At room temperature, heat-carrying phonons are in the THz frequency range. While phonons from thermal sources are incoherent, we can now use femtosecond laser pulses to excite coherent acoustic phonons at THz frequencies.
- In a recently published paper we report the first experiment in which coherent THz phonons propagate "from point A to point B" over a distance exceeding 2 μm.
- We designed a sample with two InGaN/GaN multiple-quantum-wells (MQW) structures used to suimultaneously generate and detect coherent phonon wavepackets (see Fig. 1).
Fig. 1. (a) Schematic diagram of the sample structure, with arrows indicating the propagation paths of acoustic wavepackets generated by the optical excitation of MQW structures. Numbers indicate subsequent points in time when acoustic wavepackets propagate through MQWs and are detected by the probe beam. (b) STEM image of the top superlattice structure (MQW2) of sample 2.
- Femtosecond optical excitation of MQW structures generates longitudinal acoustic waves at a wavelength equal to the MQW period. The structure shown in Fig. 1(b) with a period of 5.5 nm generated acoustic waves at the 5.5 nm wavelength, which corresponds to a frequency of 1.4 THz.
- Acoustic waves with such a tiny period cannot be detected optically when they propagate in a bulk material. However, when they propagate through an MQW structure with the same period they modulate the effective refractive index of the structure which can be detected using a probe laser pulse. Thus whenever a phonon wavepacket passes through an MQW, we see a "tone-burst" in the signal (see Fig. 2).
Fig. 2. (a) Delayed acoustic arrivals at 1.4 THz corresponding to events 3 and 4 in Fig. 1(a) and (b) the Fourier spectra of the first (solid line) and second (dashed line) wavepackets.
- We obtained lower bound estimates for the longitudinal acoustic phonon lifetimes in GaN at 1-1.4 THz. We have also observed a specular reflection of THz phonon wavepackets from the free surface of GaN (the second tone-burst in Fig. 2(a) is observed after a reflection from the surface).
- This work opens the prospect of experiments with THz phonon wavepackets which hitherto have only been possible in molecular dynamics suimulations (see, e.g., Schelling et al., Appl. Phys. Lett. 80, 2484 (2002); Tian et al., Appl. Phys. Lett. 96, 263113 (2010))
For more details, read the paper: