Space-time wave packets (STWPs) are pulsed beams that propagate invariantly (without diffraction or dispersion) in linear media. The behavior of STWPs in free space is now well established, and recently their propagation invariance was confirmed in both the normal and anomalous group-velocity dispersion (GVD) regimes. However, rich yet-to-be-observed spectral dynamics have been predicted to occur in the presence of normal GVD, which manifests itself in the reorganization of the spatiotemporal spectrum into distinct structures upon tuning the appropriate optical parameters of the STWP. Indeed, broad varieties of distinct field structures are compatible with propagation invariance in normal GVD, which can be classified in a two-dimensional space parametrized by the group velocity of the STWP and its central axial wave number. Tuning these two parameters gives rise to spatiotemporal spectral reorganization into X-shaped, hyperbolic, parabolic, elliptical, or even spatiotemporally separable spectra. Here, we observe the entire span of spectral reorganization for paraxial STWPs in normally dispersive ZnSe at a wavelength $\sim 1\mu \mathrm{m}$ with on-axis pulse widths $\sim 120$ fs. By tuning the STWP group velocity and central axial wave number while maintaining propagation invariance, we observe transitions in the spatiotemporal spectrum and verify the associated structural changes in the spatiotemporal intensity profile. These results lay the foundations for understanding the potential uses of STWPs in phase-matched nonlinear processes in dispersive media.

Close

PrevNext

View figure in article

• Ultrafast optics