The birth of a quasiparticle in silicon observed in time–frequency space

The concept of quasiparticles in solid-state physics is an extremely powerful tool for describing complex many-body phenomena in terms of single-particle excitations1. Introducing a simple particle, such as an electron, hole or phonon, deforms a many-body system through its interactions with other particles. In this way, the added particle is ‘dressed’ or ‘renormalized’ by a self-energy cloud that describes the response of the many-body system, so forming a new entity—the quasiparticle. Using ultrafast laser techniques, it is possible to impulsively generate bare particles and observe their subsequent dressing by the many-body interactions (that is, quasiparticle formation) on the time and energy scales governed by the Heisenberg uncertainty principle2. Here we describe the coherent response of silicon to excitation with a 10-femtosecond (10-14 s) laser pulse. The optical pulse interacts with the sample by way of 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 reveals interference effects leading to the coherent phonon generation and subsequent dressing of the phonon by electron–hole pair excitations.