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In the past, probing of scattered light proved to be a powerful tool for studies of condensed matter,¹ in particular Raman spectra of optical phonons in Bi were measured.² More recently, excitation of acoustic³ and optical phonon^4,5,6 modes in films of different materials, including Bi,^7,8 with short laser pulses was performed. Such pulses excite all active vibrations with frequencies contained in the pulse envelope.⁹ The laser light produces electronic excitations, which are coupled to the lattice vibrations via deformation potential³ and stimulated Raman scattering.⁹ After action of a short laser pulse the equilibrium positions of the nuclei are displaced, and they start oscillating around these new equilibrium positions. Thus the lattice distortions give rise to coherent acoustic and optical phonons. The excitation of the lattice oscillations of optical phonons was explained via the displacive mechanism,¹⁰ which was shown to be related to a particular manifestation of the impulsive stimulated Raman scattering mechanism in absorbing materials.^11,12 A significant interest presents the application of the diagnostics with short laser pulses to nanosystems, such as nanoparticles of various shapes.^13,14,15,16 In nanomaterials high frequency acoustic modes appear due to confinement of coherent acoustic phonons. Acoustic oscillations reflect the elastic properties, shape, and dimensions of the nanostructure, while optical phonon frequencies depend on the rigidity and the effective mass of the atoms involved in the optical phonon oscillation.

We investigated optical transient responses from samples of Bi nanowires with a femtosecond pump-probe technique. Bi nanowires are of special interest because of their exceptional thermoelectric properties.¹⁷ In nanosystems the surface contribution to the free energy is essential, so that their properties can be size dependent and different from bulk materials.¹⁸ The structure of the electronic density of states and the carrier transport properties of a nanowire, which can be considered as a quasi-one-dimensional quantum system, is also modified compared to the bulk material due to a confinement effect in the lateral directions.¹⁸ The Bi crystalline lattice (A7, 3m point group symmetry) presents a distorted simple-cubic structure with two atoms per a primitive cell and is especially sensitive to electronic excitations, shifting the equilibrium lattice position.¹⁹ A short laser pulse excites Raman active modes near the center of the Brillouin zone; for Bi these are the totally symmetric breathing A_1g and the doubly degenerate E_g mode.²⁰ Note that in femtosecond pump-probe experiments with bulk Bi samples the magnitude of the A_1g mode was dominant, so that for the observation of the E_g mode an increase in the fluence, a manipulation of the pump/probe polarizations, or lowering the temperature was necessary.^8,21,22 For bulk Te and Bi softening of the lattice and reduction in the optical phonon frequency were observed at high excitation levels.^6,8 We demonstrate detection of both acoustic and optical phonon modes in Bi nanowires excited by femtosecond laser pulses and also consider major factors affecting the dynamics of the optical phonons and changes of their frequency at high excitation levels.