  This article offers an extensive survey of results obtained using hybrid
photonic crystal fibers (PCFs) which constitute one of the most active research
fields in contemporary fiber optics. The ability to integrate novel and
functional materials in solid- and hollow-core PCFs through various
post-processing methods has enabled new directions towards understanding
fundamental linear and nonlinear phenomena as well as novel application
aspects, within the fields of optoelectronics, material and laser science,
remote sensing and spectroscopy. Here the recent progress in the field of
hybrid PCFs is reviewed from scientific and technological perspectives,
focusing on how different fluids, solids and gases can significantly extend the
functionality of PCFs. In the first part of this review we discuss the most
important efforts by research groups around the globe to develop tunable linear
and nonlinear fiber-optic devices using PCFs infiltrated with various liquids,
glasses, semiconductors and metals. The second part is concentrated on the most
recent and state-of-the-art advances in the field of gas-filled hollow-core
PCFs. Extreme ultrafast gas-based nonlinear optics towards light generation in
the extreme wavelength regions of vacuum ultraviolet (VUV), pulse propagation
and compression dynamics in both atomic and molecular gases, and novel soliton
- plasma interactions are reviewed. A discussion of future prospects and
directions is also included.
