Infiltrated Photonic Crystal Fibers for Sensing Applications

Abstract

Photonic crystal fibers (PCFs) are a special class of optical fibers with a periodic arrangement of microstructured holes located in the fiber's cladding. Light confinement is achieved by means of either index-guiding, or the photonic bandgap effect in a low-index core. Ever since PCFs were first demonstrated in 1995, their special characteristics, such as potentially high birefringence, very small or high nonlinearity, low propagation losses, and controllable dispersion parameters, have rendered them unique for many applications, such as sensors, high-power pulse transmission, and biomedical studies. When the holes of PCFs are filled with solids, liquids or gases, unprecedented opportunities for applications emerge. These include, but are not limited in, supercontinuum generation, propulsion of atoms through a hollow fiber core, fiber-loaded Bose⁻Einstein condensates, as well as enhanced sensing and measurement devices. For this reason, infiltrated PCF have been the focus of intensive research in recent years. In this review, the fundamentals and fabrication of PCF infiltrated with different materials are discussed. In addition, potential applications of infiltrated PCF sensors are reviewed, identifying the challenges and limitations to scale up and commercialize this novel technology.

Keywords: liquid crystals; optical fiber sensors; optofluidics; photonic crystal fibers; plasmonic sensors.

Figure 1

An assortment of optical (OM) and scanning electron (SEM) micrographs of PCF structures. (A) SEM of an endlessly single-mode solid core PCF; (B) far-field optical pattern produced by the fiber in (A) when excited by red and green laser light; (C) SEM of a recent birefringent PCF; (D) SEM of a small (800 nm) core PCF with ultrahigh nonlinearity and a zero chromatic dispersion at a wavelength of 560 nm; (E) SEM of the first photonic bandgap PCF, its core formed by an additional air hole in a graphite lattice of air holes; (F) near-field OM of the six-leaved blue mode that appears when the fiber shown in (E) is excited by white light; (G) SEM of a hollow-core photonic bandgap fiber; (H) near-field OM of a red mode in hollow-core PCF (white light is launched into the core); (I) OM of a hollow-core PCF with a Kagome cladding lattice, guiding white light, reprinted with permission from [13].

Figure 2

Guiding mechanisms in a PCF: (a) index-guiding. (b) bandgap-guiding through the photonic bandgap effect.

Figure 3

(a) SEM image of a micro-channel fabricated in a hollow-core photonic bandgao fiber: arrows indicate damage caused by laser “scoring” (inset shows channel and “scoring” lines on uncoated fiber surface, prior to cleaving); (b) optical microscope image showing the cross section of a microchannel fabricated in a SCF, reprinted with permission from [46].

Figure 4

(a) Schematic flowchart on selective filling of photonic crystal fibers. The images (b,c) are optical microscope images of the fiber cross sections at the cleave positions I and II. The light regions correspond to the holes filled with polymer, reprinted with permission from [52].

Figure 5

The spliced-fiber pressure-filling technique. (a) a gold wire is inserted into a silica capillary; (b) the wire is pushed into the capillary using a tungsten wire and the capillary end cleaved off; (c) capillary with wire is spliced to a silica fiber with hollow channels; (d) the spliced section is heated to the melting point of gold and high pressure argon gas is applied; (e) the filled structure. Reprinted with permission from [56].

Figure 6

(a) schematic of a metal-dielectric preform, drawn into a metamaterial via heating; (b) SEM micrograph of a fabricated 590 µm indium-filled PMMA fiber cross-section, reprinted with permission from [65].

Figure 7

Metal coating-based plasmonic PCF sensors. (a) gold coated in the second ring ($d_c=0.45\Lambda$, $d_1=0.6\Lambda$, $d_2=0.8\Lambda$, $\Lambda =2$ µm and gold layer thickness equal to 40 nm); (b) (i) selectively silver deposited core ($d_c=0.8\Lambda$, $d_1=0.6\Lambda$, $d_2=0.8\Lambda$, $\Lambda =2$ µm and silver layer thickness equal to 40 nm), (ii) field distribution with phase matching phenomena; and (iii) phase matching phenomena shifted with varying analyte RI. (c) Selectively gold-coated with liquid-filled core ($d_c=0.8\Lambda$, $d_1=0.5\Lambda$, $d_2=0.8\Lambda$, $\Lambda =2$ µm and gold layer thickness, $t=40$ nm); (d) multiple holes coated with gold-TiO₂ layer ($r_c=3.5$ µm, $r=6$ µm, $\Lambda =13$ µm, gold layer thickness equal to 30 nm and TiO₂ layer thickness equal to 75 nm); (e) liquid and silver nanowire filled temperature sensor; (f) hollow-core filled with liquid and silver nanowires and (g) silver-wire filled HC-PBF, reprinted with permission from [200].