Evanescent field Sensors Based on Tantalum Pentoxide Waveguides - A Review

Abstract

Evanescent field sensors based on waveguide surfaces play an important rolewhere high sensitivity is required. Particularly tantalum pentoxide (Ta₂O₅) is a suitablematerial for thin-film waveguides due to its high refractive index and low attenuation.Many label-free biosensor systems such as grating couplers and interferometric sensors aswell as fluorescence-based systems benefit from this waveguide material leading toextremely high sensitivity. Some biosensor systems based on Ta₂O₅ waveguides alreadytook the step into commercialization. This report reviews the various detection systems interms of limit of detection, the applications, and the suitable surface chemistry.

Keywords: Ta2O5.; biosensors; evanescent field; label-free detection.

Figure 1.

Refractive index n and extinction coefficient k of a thin Ta₂O₅ layer on glass (n = 1.52) versus wavelength (light source: Xe arc lamp with corresponding interference filters); measurements performed at an angle of incidence of 50° by means of spectroscopic ellipsometry (EP³, Nanofilm Technologie GmbH, Göttingen, Germany).

Figure 2.

Three-layer planar waveguide system comprising substrate, waveguide and cover layer, with refractive indices n_s, n_w, and n_c, respectively.

Figure 3.

The principle of evanescent field sensing. A waveguide layer on a substrate is guiding a light mode with phase velocity v₁. A change in the effective refractive index and thus a decrease in the phase velocity v₂ < v₁ is caused by either a change in the cover refractive index Δn_c or a change in the adlayer thickness Δt_ad due to molecule adsorption, e.g. binding of an antigen to a capture antibody. From [21].

Figure 4.

Theoretical sensitivities of a waveguide to a) cover refractive index changes and b) surface adlayer changes versus waveguide thickness. Parameters for calculation: n_s = 1.52, n_w = 2.1, n_c = 1.333 and λ = 675 nm. From [21].

Figure 5.

(a) Schematic of the input grating coupler instrument. In an angular scan, the TE₀ and TM₀ modes are successively excited by an s- and a p-polarized laser beam. L = lever arm; MS = micrometer screw; SM = stepping motor; Cu = cuvette; Laser = He-Ne laser (λ = 632.8 nm); FP = Foster prism; M₁ and M₂ = mirrors; P = incident power; P′ = power of incoupled guided mode; α = angle of incidence; D₁ and D₂ = silicon photodetectors. (b) Detailed view of the sensor: C = liquid cover; F = waveguiding film of thickness d_F on glass substrate S. From [34], Copyright Elsevier (1990).

Figure 6.

(a) Schematic of output grating coupler sensor; F = waveguide; S = substrate; C = sample; L_c = cylindrical lens of short focal length f_c (e.g. f_c = 5 mm); L = cylindrical lens of focal length f (e.g. f = 300 mm); α = outcoupling angle; Δα = change in outcoupling angle proportional to the effective index change Δn; PSD = position-sensitive-detector; Δu = displacement of centre of the light spot on the PSD. (b) Schematics of output grating coupler measuring the effective index changes Δn(TE₀) and Δn(TM₀) of the TE₀ and TM₀ modes; M, M′ = mirrors. From [39], Copyright Elsevier (1990).

Figure 7.

Optical arrangement for reflected-mode operation of integrated optical grating coupler. From [41], Copyright Elsevier (1996).

Figure 8.

Schematic of wavelength interrogated optical sensor WIOS. From [46], Copyright Elsevier (2003).

Figure 9.

(a) Cut through dual period sensor chip. (b) Cut through thickness-modulated sensor chip. From [46], Copyright Elsevier (2003).

Figure 10.

Young interferometer as described in [21]. Light from a light source is coupled into the waveguide sensor chip by a grating after being split in two beams by a double slit. The two beams are guided separately down the waveguide, forming sensing and reference path, and coupled out by a second grating. The interference pattern is recorded behind another double slit by a CCD line sensor.

Figure 11.

left: Schematic optical set-up used for first model-assays for ‘volume detection’, interference filters were used for selection of the excitation or emission light and photodiodes or photomultiplier for signal detection; right: comparison of set-ups for volume detection and detection of backcoupled and guided luminescence light, using a second outcoupling grating where different wavelength can be discriminated by the outcoupling angle. From [31], Copyright Elsevier (1997).

Figure 12.

Detection principle of evanescent field fluorescence on planar waveguides (principle of ZeptoREADER): excitation light is coupled into a thin-film waveguide; surface confined fluorescence of bound labeled molecules is detected by a CCD camera. Adapted from [2, 19]

Figure 13.

Schematic of the Waveguide Excitation Fluorescence Microscope (WExFM); photograph of the system on a standard inverted microscope. From [10], Copyright Elsevier (2005).

Figure 14.

Bidentate und monodentate phosphate coordination to tantalum ions. Adapted from [61].