Photonic crystal fiber biosensor

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Description

By using fibers in which photonic band gap (PBGs) can be formed, it is possible to (i) increase the efficiency of nonlinear optical interactions, (ii) control the dispersion of guided modes (by shifting the wavelength of zero dispersion toward the visible spectral range) and the effective refractive index of the sheath, and (iii) transmit electromagnetic radiation with high degree of localization. Photonic crystals (PCs) are artificial periodic dielectric media, which can possess PFBs that hinder the propagation of light in a certain wavelength interval. In photonic crystal fibers (PCFs), the PBGs are realized by creating a structured sheath representing a glass-air composite medium with a high reflection coefficient. The period of holes (air channels) in the PCF sheath is a factor that determines the formation of waveguide modes. The structure of a two-dimensional (2D) photonic crystal (2D-PC) consists of glass capillaries with circular or hexagonal cross sections arranged symmetrically or asymmetrically so as to form a periodic 2D lattice around the core that represents a defect in the structure comprised of one or several glass cylinders. The main feature of a PCF is that the light wave energy in this fiber propagates along the core provided that the wavelength and direction of light correspond to the PFB.
PCFs with hollow (air-filled) cores are characterized by a low refractive index of the core as compared to the effective index of the surrounding sheath possessing a certain 2DPC structure. In contrast to usual optical fibers, such PCFs do not employ the total internal reflection (TIR) phenomenon, since the refractive index of the sheath is higher than that of the core. A certain optical mode is guided in the hollow core due to the localizing and directing effect of the PFB. In recent years, biosensors have occupied leading positions among all sensors used for the analytical determination of substrates in medicine. The combination of a high selectivity of enzymes with advantages of the electric and optical response measurement makes it possible to creation of devices characterized by high selectivity and sensitivity, which are applicable to both rapid analysis and monitoring. Capillary biosensors can be manufactured from glass, quartz, silicon, polymer, and other substrates with channels and holes, which can be filled with various working liquids (solutions), in particular, for taking samples and introducing reagents.
A PCF-based sensor represents a transducer of physical quantities, in which an optical fiber serves as a sensitive element and a light-transmitting medium. The sensitive element converts a certain physical factor into a change of the parameters of transmitted, reflected, or scattered radiation. According to the principle of action, fiber sensors can be classified into groups depending on a parameter measured for obtaining information about the given physical interaction intensity, i.e., intensity, phase, polarization state, spectral (or mode) composition.
Presently, PCFs are considered to be promising sensitive elements for fiber transducers of physical quantities. The main advantages of PCFs include stability with respect to the action of electromagnetic fields, high sensitivity, reliability, reproducibility, wide dynamic range of measurements, the possibility of spectral and spatial multiplexing of sensitive elements in one or several fibers, fast response to changes of a measured physical quantity, small sample volume, and small dimensions.

Schemes and typical results

Figure 1 illustrates the effect of modifying the spatial lattice parameters on the transmission spectrum of a PCF with the structure depicted in the inset. The solid curve shows the spectrum of a composite fiber having a hollow central channel (with a diameter of 153 mkm) surrounded by 4mkm capillaries at an outer fiber diameter of 340mkm. This structure is characterized by a 60-nmwide PBG (607–667 nm). Dotted curve refers to a fiber having a hollow central channel with a diameter of 102 mkm, which is surrounded by 2mkm capillaries at an outer fiber diameter of 224 mkm. In this case, the PBG width is 100 nm (414–514 nm).
The influence of a change in the core refractive index on the PFB position was studied using a PCF with the structure presented in the inset in Fig. 1. This PCF had a hollow central channel with a diameter of 114mkm, which is surrounded by 3mkm capillaries at an outer fiber diameter of 250 mkm. The spectrum of this fiber is depicted by dashed curve in Fig. 1; the PBG width is 193 nm (423–616 nm).
The experiments showed that light in this PCF propagates along the walls of capillaries bounding the hollow core. This is confirmed by the small aperture number of the capillaries and by low losses. The near-wall propagation of light implies that the type of the fiber material is not important and the main role is played by the structure, that is, by the diameter, step, and distribution of channels in the sheath (determining the index gradient) ad the central cavity (defect) size and geometry. In the case of a PCF with the capillary wall thickness comparable with the depth of light penetration, the losses will tend to zero. The shorter the light wavelength, the more critical the state of the channel. The PCF was filled with distilled water. Water is a universal solvent characterized by high heat capacity and relatively large (for liquids) thermal conductivity. Water is completely transparent in the visible spectral
range. The PCF sample was immersed in a reservoir with water for 1 min. Under the action of capillary forces, both the microstructural shell and the core were filled with water. Figure 2 shows the transmission spectra of this PCF measured before and after filling the structure with distilled water.

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Publications

• Yu. S. Skibina, Fedotov A.B., Melnikov L.A., Beloglazov V.I. Tuning the photonic band gap of sub 500 nm – pitch holey fibers in the 930 –1030 nm range//Laser Physics.-2000.-Vol.10.No.5.-PP.-723-726.
• Skibina Yu. S., Fedotov A.B., Melnikov L.A., Beloglazov V.I. Holey fibers with 0.4-32-mkm Lattice Constant photonic band gap cladding: fabrication, characterization and nonlinear optical measurements//Laser Physics.-2001.-Vol.11.No.1.-PP.138-145.
• V.I. Beloglazov, M.V. Chainikov, Yu. S. Skibina, V.V. Tuchin, Spectral properties of a soft glass photonic crystal fiber J. X-Ray Science and Technology, Vol.13, No 4, pp. 171 – 177, 2005.
• V.I. Beloglazov, N. Langhoff, V.V. Tuchin, A. Bjeoumikhov, Z. Bjeoumikhova, R. Wedel, N.B. Skibina, Yu. S. Skibina, M.V. Chainikov, Technologies of manufacturing polycapillary optics for x-ray engineering, J. X-Ray Science and Technology, Vol.13, No. 4, pp. 179 – 183, 2005.
• Skibina Y.S., Beloglazov V.I., N.B. Skibina, P. Glas, D. Fischer, et.all, Supercontinuum generation in a two-dimensional photonic kagome crystal, Appl. Phys. B, vol. 81, pp.209-217, 2005
• A chirped photonic crystal fibre, Julia S. Skibina, Rumen Iliew, Jens Bethge, Martin Bock, Dorit Fischer,Valentin I. Beloglasov, Reiner Wedell And Gunter Steinmeyer, Nature Photonics 12 Oct. 2008.
• P. Glas, D. Fischer, G. Steinmeyer, A. Husakou, J. Herrmann, R. Iliew,
N. B. Skibina,V. I. Beloglasov, and Y. S. Skibina, “Supercontinuum generation in a two-dimensional photonic kagome crystal,” Special Issue on Photonic Crystals, Appl. Phys. B 81, 209-17 (2005).

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