Photonic Crystal Fiber
Bio-chemical Sensor
Applications
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Chemical, environmental, and biomedical sensors
High-power beam delivery and nonlinear opticsEndlessly single-mode fibers
Extreme dispersion characteristics
Sub-wavelength optics
Particle trapping
Bent waveguides

OptiMode Software Features
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OptiMode Includes a vector finite element method(VFEM) mode solver which is fast and accurate
relative to other mode solving algorithms.
Triangular mesh can be adapted to accurately
approximate the fine features of the geometry,refractive index profile, and electromagnetic fields.
Built-in VBScript capabilities accelerate the designand optimization of complex waveguide profiles.
Exploiting the symmetric boundary conditions
reduces the simulation domain, and modes of certain
symmetry can be readily targeted.
Supports lossy, dispersive and anisotropic materials
in full vector formulation.
Uniaxial perfectly matched layer (UPML) boundary
condition enables identification of leaky modes.
Supports accurate and spurious-solution proof higher
order hybrid vector/nodal elements.
Specific modes can be targeted through a userspecified complex modal index estimate.
Arbitrary bent waveguides can be accurately
analyzed using transformation optics.

spanning multiple orders of magnitude in variation, e.g. subwavelength metal films and tightly confined surface plasmonic modes
in a PCF with wavelength-scale mode and air-hole radii having
cladding dimensions of 100 times of the wavelength. These
properties require a mode solver that can both efficiently andaccurately approximate the geometry and the electromagnetic fields
over the entire simulation domain.

Simulation Description
The interaction between core-confined and plasmonic modes inhybrid PCF-plasmonic waveguides can enhance the sensitivity of thebio-chemical and environmental sensors. Strong dependence of thedispersion characteristics of the plasmonic mode to the dielectric
profile at the metal interface affects the phase matching between themodes and hence the loss incurred by the core mode. In the strong
coupling regime, anti-resonant crossing manifested as a kink or an sshaped transition in the dispersion relation in the vicinity of the phasematched point may occur [1]. Besides the variation of the propagation
loss, the resultant abrupt phase shift of the transmitted light enhances
the sensitivity in polarimetric and phase-sensitive interrogation
methods.
The surface plasmon resonance assisted D-shaped PCF proposed
as a refractive index sensor in [2] and shown in Fig. 1 is simulatedusing OptiMode software. The D-shaped silica (n=1.45) PCF has acenter air-hole and cladding diameters of 0.4μm and 1.6μm,respectively. All holes are arranged in a 2μm-pitch hexagonal lattice.

Overview
Photonic crystal fibers (PCF) are attractive for chemical,
biomedical, and environmental sensing applications. The
freedom in designing the microstructure geometry
provides a unique platform for realizing the desired modal
dispersion, birefringence, confinement and multiplicity
characteristics. Of significant importance to thesefeatures, chemical, biological and inorganic materials canbe introduced to the PCF by selective hole infiltration ordeposition techniques. Novel PCF sensor designs often
encompass structural and electromagnetic field featuresyx

Figure 1: Simulation domain with the FEM mesh superimposed. (Lower inset)
schematic of the PCF. (Top inset) a zoomed view of the mesh at the metal-dielectricinterfaces and a hole boundary.

A 40nm-thick gold layer is deposited on the flat-polishedcladding surface at a height of 0.9μm from the center ofthe fiber and an aqueous analyte with refractive index ofna=1.33 to1.34 is introduced on the metal film.
The X/Y-polarized modes can be selectively simulated in
OptiMode by imposing the symmetric magnetic (electric)
boundary condition along the y-axis which also reduces
the simulation domain by a factor of 2. As shown in Fig. 1,
the triangular mesh is configured to precisely represent
the sub-wavelength gold layer and the hole boundaries.
This mesh enables efficient and accurate modeling ofboth the surface plasmon and the core modes.
VFEM solver in OptiMode software is employed to
calculate the complex effective indices and the field
profiles of the X- and Y-polarized modes of thewaveguide. The X-polarized core mode, incapable ofexciting the plasmon polaritons, retains the low-loss
propagation, dispersion and field profile characteristics ofan isolated guided mode as shown in Fig. 2 and 3,
respectively. In contrast, strongly coupled Y-polarizedcore and surface plasmonic modes form symmetric (antisymmetric) super-modes with anti-crossing signature atthe phase-matched wavelength, shown as the upper(lower) dispersion bands in Fig. 2. As evident from thefield profiles of Fig. 3 and at shorter wavelengths, theupper (lower) band represents the surface plasmonic(core) mode and vice versa at longer wavelengths. The
field plots in Fig. 4 further illustrates the symmetric (antisymmetric) nature of dispersion bands and the strong
coupling of the modes at the phase- matched wavelength
where the loss of the core-like Y-polarized mode peaks.
The PCF refractive index bio-sensor can be utilized invarious interrogation schemes; namely, tracking the losspeak wavelength with 𝑆𝜆 = Δ𝜆𝑙𝑜𝑠𝑠−𝑝𝑒𝑎𝑘 ⁄Δ𝑛𝑎 ~3300 [𝑛𝑚𝑅𝐼𝑈],

wavelength, or utilizing the phase difference between the twopolarizations of the core mode defined in normalized form as Δ𝜙 =2𝜋𝜆𝑦−𝑝𝑜𝑙𝑅𝑒(𝑛𝑒𝑓𝑓𝑥−𝑝𝑜𝑙𝑟𝑎𝑑− 𝑛𝑒𝑓𝑓 ) [𝜇𝑚], as depicted in Fig. 5 (a)-(c), respectively.

(a)

(b)

(c)

(d)

(e)

(f)(g)(h)Figure 3: Electric-field intensity profile of, (a)-(f) y-polarized and (g)-(h) x-polarizedmodes from left to right at 560nm, 664nm and 750nm wavelength, respectively. Top(middle) row corresponds to the upper (lower) dispersion band in Fig. 2.

Figure 4: Electric field profile of the Y-polarized modes, real(Ey), along the y-axis at
specified wavelengths (fields normalized to unit power and assuming na=1.33).

monitoring the power at a fixed wavelength with peaksensitivity of 𝑆𝑎 = (Δ𝛼 ⁄Δ𝑛𝑎 )/𝛼𝑛𝑎=1.33 ~135 [1𝑅𝐼𝑈] at 700nm

Figure 5: (a) Loss spectra and (b) the amplitude sensitivity of the Y-polarized coremode. (c) Phase difference between the y-polarized and X-polarized core modes.

References

Figure 2: Complex modal indices of the D-shaped PCF calculated in
OptiMode when analyte refractive index is 1.33.

[1] H. Ditlbacher, N. Galler, D. Koller, A. Hohenau, A. Leitner, F. Aussenegg, and J.
Krenn, “Coupling dielectric waveguide modes to surface plasmon polaritons,”
Opt. Express 16, 10455-10464 (2008).
[2] N. Luan, R. Wang, W. Lv, and J. Yao, “Surface plasmon resonance sensor basedon D-shaped microstructured optical fiber with hollow core,” Opt. Express 23,8576-8582 (2015).