Supercontinuum Generation in Condensed Matter

Abstract

Supercontinuum generation, the production of intense ultrafast broadband "white light" pulses, arises from the propagation of intense picosecond or shorter laser pulses through condensed or gaseous media. Various processes are responsible for continuum generation. These are called self-, induced-, and cross-phase modulations and four-photon parametric generation. Whenever an intense laser pulse propagates through a medium, it changes the refractive index, which in turn changes the phase, amplitude, and frequency of the incident laser pulse. A phase change can cause a frequency sweep within the pulse envelope. This process has been called self-phase modulation (SPM) (Alfano and Shapiro, 1970a). Nondegenerate four-photon parametric generation (FPPG) usually occurs simultaneously with the SPM process (Alfano and Shapiro, 1970a). Photons at the laser frequency parametrically generate photons to be emitted at Stokes and anti-Stokes frequencies in an angular pattern due to the required phase-matching condition. When a coherent vibrational mode is excited by a laser, stimulated Raman scattering (SRS) occurs. SRS is an important process that competes and couples with SPM. The interference between SRS and SPM causes a change in the emission spectrum resulting in stimulated Raman scattering cross-phase modulation (SRS-XPM) (Gersten et al., 1980). A process similar to SRS-XPM occurs when an intense laser pulse propagates through a medium possessing a large second-order χ2 and third-order χ 3 susceptibility. Both second harmonic generation (SHG) and SPM occur and can be coupled together. The interference between SHG and SPM alters the emission spectrum and is called second harmonic generation cross-phase modulation (SHG-XPM) (Alfano et al., 1987). A process closely related to XPM, called induced phase modulation (IPM) (Alfano, 1986), occurs when a weak pulse at a different frequency propagates through a disrupted medium whose index of refraction is changed by an intense laser pulse. The phase of the weak optical field can be modulated by the time variation of the index of refraction originating from the primary intense pulse.