Hollow-core photonic crystal fiber (HC-PCF) has been proved to be a versatile platform for lab-on-a-fiber applications. By filling the fiber with various gases, liquids or solid materials, the light-matter interaction could be greatly enhanced. Many novel optical phenomena that are unattainable in free-space could be easily identified inside the fiber. Such a platform offers a promising route for creating compact, integrable and biocompatible all-fiber multifunctional optical devices. Here, we review our recent progress in developing a novel HC-PCF coined "hollow-core negative curvature fiber" (NCF) that could provide light guidance at spectral ranges covering from UV, visible, NIR to MIR. These NCFs show attributes of low transmission loss, octave-spanning transmission bandwidth, high damage threshold and single modeness. As a proof-of-concept demonstration for lab-on-a-fiber applications, we filled one of the fibers with ethanol (refractive index 1.36) to form a liquid-core anti-resonant fiber. At a low volume of 1 μL, Raman signal from ethanol was observed at a pump power of 2 mW. Such a high performance NCF opens a window for applications in fiber-enhanced spectroscopy, biochemical sensing and nano-plasmonics.
Four-wave mixing (FWM) has been extensively explored in optical fibers and more recently in on-chip silicon-oninsulator (SOI) waveguides. A phase-matched FWM with a pair of degenerate pump photons generating and amplifying signal and idler photons is referred as modulational instability (MI). Following theory of FWM in waveguide arrays, we utilize evanescent couplings between neighboring waveguides to control the phase-matching condition in FWM. In experiments, a set of single-channel SOI nanowaveguides with the waveguide width decreasing from 380nm to 340nm demonstrate that changing the waveguide group velocity dispersion (GVD) at the pump wavelength from being anomalous to being normal makes MI gain gradually disappear. We also perform the same experiment with an array of two 380nm-wide SOI waveguide, and demonstrate that for the large separation of 900nm and 800nm, MI gain is present as for the single waveguide; while for the small separation of 400nm, the MI gain disappears. This transformation of phase-matching in FWM is attributed to the fact that the coupling induced dispersion changes the net GVD of the symmetric supermode from being anomalous for large separation to being normal for small separation. Our observation illustrates that the coupling-induced GVD can compete and exceed in value the GVD of a single SOI nanowaveguide. This creates a new previously unexplored degree of freedom to control FWM on chips.
One-dimensional PhC mirrors are constructed in a single-mode silica slab waveguide with a row of elliptical holes. The
photonic band gap (PBG) of the PhC structure is attained by fast eigen-mode calculations. Being aware that component radiated waves of the PhC mirror are generated at interfaces between different waveguide sections, when propagating guided waves impinge on these interfaces, we point out that the total radiation loss of the PhC mirror is consequence of interferometric interplays of component radiated waves. We visualize this radiation generation process with intuitive pictures. We also estimate total radiation losses of PhC mirrors by using an analytical model. For uniform PhC mirrors, our model explains the oscillations of the total radiation loss with the increase of the period number. The calculated results agree well with the numerical simulations in terms of the oscillation period, the damping speed, the initial phase, and the relative intensity. For non-uniform PhC mirrors, our model finds that the progressively tapered transition from the feeding waveguide to the PhC mirror does not yield the lowest radiation loss. This finding is against to the well known “impedance-matching” picture. The matching of our model with the simulated results certifies the interferometric nature of the radiation generation process in a PhC mirror especially when a low-index waveguide is considered.