Investigated Bi nanowires: (a) SEM image of freestanding nanowires, (magnification of ), (b) SEM image (view of nanowires from top, ), and (c) TEM image ().
(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 . 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.
Transient signals of scattered light from Bi nanowires at a relatively low pump fluence of : (a) the temporal response is shown. For curve A the resolution, determined by the translational speed of the delay line, was set to , and in this case acoustic phonons were detected. For curve B (inset) the resolution was set to ; 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 and .
The variations in the optical phonon frequency from experimental data and simulations for and . The lines connecting experimental points are a guide to the eye.
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