Water Waveguides

It is perhaps useful to mentally picture the microwaves to travel through the waveguide like a water stream through a pipe. In reality, however, the transport is an electric phenomenon that occurs in a very thin layer of the waveguide s inside. The thickness of this layer is characterized by the skin depth parameter, 8, which depends on the used material and the frequency. For example, for the material copper and a frequency of 10 GHz the skin depth is 8 0.66 pm. While at the surface the amplitude of the electric field of the wave is maximal, at a depth of 8 the E is reduced by a factor e 1 0.37, and at a depth of a few 8 becomes negligibly small. Transmission of microwaves through a waveguide is essentially a surface phenomenon. 

On their way from the source to the resonator the intensity of the microwaves must be attenuable for two reasons (1) full power may be too much for the sample leading to saturation (treated in Chapter 4) or (2) it may be impossible to critically couple the cavity at full incident power (e.g., because the sample contains too much water). Therefore, the main waveguide contains an attenuator, usually of the dissipative, rotary-vane type. Dissipative means that the eliminated power is converted to heat and is not reflected as radiation to the source. Rotary vane means that it contains a section of circular waveguide, in which a flat piece of material is located that can be rotated over an angle 0, where 0 = 0 means no attenuation and 0 = 90° causes full attenuation. The amount of attenuation is expressed in decibels, a non-SI,... 

C. Mouvel, R.D. Harris, C. Maciag, B.J. Luff, J.S. Wolknson, J. Piehler, A. Brecht, G. Gauglitaz, R. Abuknesha, and G. Isamil, Determination of simazine in water samples by waveguide surface plasmon resonance. Anal. Chim. Acta 338, 109-117 (1997). 

Fig. 8.25 E field distribution (TE polarization) of (a) a conventional polystyrene microring with waveguide width of 2.4 pm and height of 2 pm, and (b) an asymmetric slot polystyrene microring with entire waveguide width of 2.4 pm including a slot of 200 nm. Waveguides in both cases sit on silicon dioxide and are embedded in water...

Guided mode calculations were also carried out to compare the sensor response of several waveguide systems. In these simulations a model molecular monolayer is represented by a 2-nm thick layer with a refractive index of n 1.5. The optical properties of this model layer are typical of a dense layer of organic molecules on a substrate1 41, and are a reasonable approximation for a streptavidin protein layer bound to a biotinylated surface, the experimental model system we use to characterize our sensors. The ambient upper cladding was assumed to be water with a refractive index of n 1.32. For all examples, the lower cladding was assumed to be Si02 with an index of n 1.44. In the simulations, the effective index of... 

A typical, experimentally obtained, output spectrum of a waveguide with five resonators of differing sizes is shown in Fig. 16.5a. In this first case, all the five resonators had water as the surrounding medium. As can be seen, each resonator contributes a sharp dip to the output spectrum of the device. We observe that each ID resonator possesses a large -factor varying from 1,500 to 3,000 and a full... 

Fig. 16.5 Response to refractive index interrogation of a single NOSA waveguide, (a) Output spectrum for a NOSA where one of the five resonators is fluidically targeted, first with water and then with a CaCl2 solution. The resonance of the targeted resonator shifts toward the red end of the spectrum due to the higher refractive index of the CaCl2 solution, (b) Experimental data (with error bars indicating inter device variability) showing the redshifts for various refractive index solutions. The solid line is the theoretically predicted redshift from FDTD simulations. The experimental data is in excellent agreement with the theory. Reprinted from Ref. 37 with permission. 2008 Optical Society of America...

It is well known that the low refractive index of water (n = 1.33) causes problems for the construction of optical waveguides that guide light efficiently through... 

Zhang, J. Z. and Chi, J. (2002). Automated analysis of nanomolar concentrations of phosphate in natural waters with liquid waveguide. Environmental Science and Technology 36 1048-1053. 

An evanescent wave fiber optic immunosensor has been used for the detection of ricin in river water.(131) A tapered fiber optic waveguide with covalently bound anti-ricin IgG is used in a sandwich format, with tetramethylrhodamine-labeled antibody as tracer. In a two-step format, ricin in the sample is bound to the fiber first, and then the fiber is exposed to the tracer antibody. Sensitivity is 1 ng/ml for the two-step assay. The one-step assay, in which the fiber optic probe contacts the sample and labeled antibody simultaneously is less sensitive, but more convenient. 

Figure 22. Calculated waveguide effective index shift due to the presence of a 4 nm thick layer with refractive index n = 1.5 on top of the waveguide core. This layer is intended to emulate the the adsorption of a layer of large organic molecules. The index shift is shown for SOI and glass waveguides of varying core layer thickness. The uppermost layer is assumed to be water (n = 1.3). The inset shows an expanded view of the SOI data.

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