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Fig. 1. Investigated Bi nanowires: (a) SEM image of freestanding nanowires, (magnification of 2.6×10³), (b) SEM image (view of nanowires from top, 1.4×10⁴), and (c) TEM image (2×10⁴). First citation in article

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Fig. 2. (Color online) (a) The transmission signal from the Bi nanowires on a glass substrate showing oscillations (curve A) and the signal from a bare substrate (curve B, exhibiting no such oscillations); the laser fluence was F=10  mJ/cm². The inset shows the oscillating contribution to signal A with a fit shown by a dotted line. (b) The spectrum of the oscillating part of signal A, showing a broad peak around 2.2 THz. First citation in article

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Fig. 3. (Color online) Transient signals of scattered light from Bi nanowires at a relatively low pump fluence of F=1.5  mJ/cm²: (a) the temporal response is shown. For curve A the resolution, determined by the translational speed of the delay line, was set to ~1  ps, and in this case acoustic phonons were detected. For curve B (inset) the resolution was set to ~100  fs; this higher temporal resolution revealed the coherent optical phonon oscillations. (b) Spectral analysis of the oscillating portions of signals A and B: two spectral peaks are observed with maxima at f₁=9.5  GHz and f₂=2.8  THz. First citation in article

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Fig. 4. (Color online) The variations in the optical phonon frequency from experimental data and simulations for F=10 and 1.5  mJ/cm². The lines connecting experimental points are a guide to the eye. First citation in article