The birth of a quasiparticle in Si observed in time-frequency space

The concept of quasiparticles in solid-state physics is an extremely powerful way to describe complex many-body phenomena in terms of single particle excitations. Introducing a simple particle such as electron, e, hole, h, or a phonon, p, deforms a many-body system through interaction with other particles. We say the added particle is dressed or renormalized by a self-energy cloud that describes the response of the many-body system forming a new entity, the quasiparticle. With ultrafast laser techniques we can impulsively generate bare particles and observe their dressing by the many-body interactions, that is quasiparticle formation, on the time and energy scales governed by the Heisenberg uncertainty principle. Here we present the coherent response of Si to excitation with a 10 femtosecond (10-14 s) laser pulse. The optical pulse interacts with the sample via the complex second-order nonlinear susceptibility to generate a force on the lattice driving coherent phonon excitation. Transforming the transient reflectivity signal into frequency-time space by wavelet transform reveals interference effects leading to the coherent phonon generation and subsequent dressing of the phonon by electron-hole, e-h, pair excitations.