A laser pulse strikes a TiSe₂ crystal, and the crystal responds with a whole orchestra of vibrations. Some sound louder, others softer, but until now it was unclear why each mode has its own character. Now, spectroscopy with probe-wavelength resolution has made it possible to hear each note separately.
The experiment was performed at 4.8 K, only slightly above the boiling point of liquid helium. The researchers used a titanium-sapphire laser with a pulse duration of 35 fs and a repetition rate of 80 MHz. The reflected probe was passed through a monochromator with a resolution of less than 7 nm, which made it possible to vary the probe energy near an important optical transition at 1.5 eV.
The non-oscillatory part of the signal changes sign at an energy of 1.52 eV: positive on the low-energy side and negative on the high-energy side. This is explained by a red shift of the optical transition energy at the M point of the Brillouin zone, caused by photoexcited carriers and lattice heating. The relaxation times were 608 fs and 6.0 ps, consistent with carrier lifetimes measured by tr-ARPES.
Four modes were identified in the oscillatory response: the A₁g optical phonon at 6.2 THz and three CDW-related modes: an amplitude mode (AM) at 3.5 THz and two zone-folded phonon modes (ZFP1 at 3.4 THz and ZFP2 at 5.2 THz). The amplitudes of the CDW modes reach a maximum near 1.48 eV, whereas the amplitude of the A₁g phonon is suppressed in this region and increases at the edges of the spectrum. This indicates a different coupling strength to the optical transition at the M point.
The A₁g optical phonon modulates the transition energy at the M point through the deformation potential, which leads to a derivative-like shape of the spectral dependence. The CDW-related modes appear to be more strongly coupled to transitions sensitive to the CDW order parameter rather than to the transition at the M point. This opens a path to selective optical control of individual vibrational modes in layered materials and deepens our understanding of the microscopic details of ultrafast processes in CDW systems.
The temperature of 4.8 K at which the measurements were performed is only slightly above the boiling point of liquid helium (4.2 K). At this temperature, the amplitude of coherent phonons is maximal and the CDW state is stable, which makes it possible to resolve even closely spaced modes that differ by only 0.1 THz.