Surface waves excitation

This report presents the results of investigations aimed at the creation of the surface wave transducer for the automated control. The basic attention is drawn to the analysis of the position of the front meniscus of the contact liquid when the surface waves excite through the slot gap and to the development of system for acoustic contact creation. 

For exciting the surface waves the traditional method of transforming of the longitudinal wave by the plastic wedge is used. The scheme of surface waves excitation is shown in fig. 1. In particular, it is ascertained that the intensity of the excitation of the surface wave is determined by the position of the extreme point of the exit of the acoustic beam relatively to the front meniscus of the contact liquid. The investigations have shown, that under the... 

Fig. 10.8. Dispersion relationships for surface waves excited in a half space of 42% Ni-Fe alloy with a layer of gold of thickness d, and with water loading k = v/ca is the propagation number along the specimen surface (Tsukahara et al. 1989a).

Somekh, M. G. (1987). Consequences of resonant surface-wave excitation on contrast in reflection scanning acoustic microscope. IEEE Proc. 134,290-300. [122]... 

In wide sectors of industry there is a growing need of inspection methods which go without liquid coupling media. The excitation of bulk and surface waves by means of air-coupled ultrasonic probes is therefore an attractive tool for NDE. This is tme e.g. for the rapid scanning of large composite structures in the aerospace industry [1]. In other cases, the use of liquid couplants is prohibitive like the thickness measurement of powder layers. 

The use of the surface ultrasonic waves seems to be convenient for these purposes. However, this method has not found wide practical application. Peculiarities of excitation, propagation and registration of surface waves created before these time great difficulties for their application in automatic systems of duality testing. It is connected with the fact that the surface waves are weakened by soil on the surface itself In addition, the methods of testing by the surface waves do not yield to automation due to the difficulties of creation of the acoustic contact. In particular, a flow of contact liquid out of the zone of an acoustic line, presence of immersion liquid, availability of chink interval leads to the adsorption and reflection of waves on tlie front meniscus of a contact layer. The liquid for the acoustic contact must be located only in the places of contact, otherwise the influence on the amplitude will be uncontrolled. This phenomenon distorts the results of testing procedure. 

Figure C 1.5.6. Single Ag nanoparticles imaged with evanescent-wave excitation. (A) Unfiltered photograph showing scattered laser light (514.5 nm) from Ag particles immobilized on a polylysine-coated surface. (B) Bandpass filtered (540-580 nm) photograph taken from a blank Ag colloid sample incubated witli 1 mM NaCl and...

The spectra from strong oscillators have special features which are different from those from metallic and dielectric substrates. Different structures in tanf and A are observed on a metallic substrate, dependent on the thickness of the film (Fig. 4.65). For very thin films up to approximately 100 nm the Berreman effect is found near the position of n = k and n < 1 with a shift to higher wavenumbers in relation to the oscillator frequency. This effect decreases with increasing thickness (d > approx. 100 nm) and is replaced by excitation of a surface wave at the boundary of the dielectric film and metal. The oscillator frequency (TO mode) can now also be observed. On metallic substrates for thin films (d < approx. 2 pm) only the 2-component of the electric field is relevant. With thin films on a dielectric substrate the oscillator frequency and the Berreman effect are always observed simultaneously, because in these circumstances all three components of the electric field are possible (Fig. 4.66). 

Fig. 4.65. Different spectral features of tanf for a strong model oscillator at 1000 cm" on a metal substrate. The TO mode (1000 cm" ), Berreman effect (1050 cm" ), and excitation ofa surface wave (1090 cm" ) are seen for different 1150 thicknesses - 1, 5, 10, 50,100, 500, and 1000 nm.

While planar optical sensors exist in various forms, the focus of this chapter has been on planar waveguide-based platforms that employ evanescent wave effects as the basis for sensing. The advantages of evanescent wave interrogation of thin film optical sensors have been discussed for both optical absorption and fluorescence-based sensors. These include the ability to increase device sensitivity without adversely affecting response time in the case of absorption-based platforms and the surface-specific excitation of fluorescence for optical biosensors, the latter being made possible by the tuneable nature of the evanescent field penetration depth. 

It will become very clear by the end of this book that in a great deal of acoustic microscopy of materials the contrast is dominated by Rayleigh waves excited in the surface of the specimen (Briggs 1985). A summary of the properties of Rayleigh waves will be given in Table 6.2, and their role in the contrast will be introduced in 7.2.1. What all that means in terms of acoustic pictures will be... 

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