Radiation from Apertures

Consider a plane polarized wave incident normally on a metal film of finite thickness. The incident energy is transmitted across the film, reflected back, or absorbed in the film (see Fig. 41). Considering that the plane wave is infinite in extent, each of the three energy contributions is infinite. Let the field distribution in the presence of the film be termed the incident field. Let us now etch an aperture of finite cross section in this film. Let the difference of the field after and before etching the aperture be termed the scattered field. We follow an analysis similar to the case of the antennas. The aperture is the source of the scattered field. Owing to the loss in the metal, this scattered field decays inside the metal with increasing lateral distance from the aperture. Since the metal film is infinite in its plane, the hypothetical surface used in defining the equivalent currents has to wrap around the metal at infinity. We define the surface (see Fig. 42) to be SI — S2 — S3 on one side, and S4 — S5 — S6 on the other. The expression for the power flux, (17), is applicable here. However, the 

The scatterer (aperture) is finite in extent. Moreover, owing to the loss in the metal, fields decay in the film away from the aperture hence, /g, /aa — /af and /c are finite quantities. The definition of the scattering cross section is analogous to the antenna case. However, in the definition of the absorption cross section, we replace /a with /aa — /af. In the case of the antenna, the term was stated to be directly proportional to the radiation intensity in the forward direction. For the aperture, a Green s function analysis similar to the antenna case indicates that the contribution to Iq from the surface SI — S2 — S3 is proportional to the radiation intensity in the forward direction. Similarly, the contribution from the surface S4 — S5 — S6 is proportional to the radiation intensity in the backward direction. Thus, /c is a linear combination of the radiation intensity in the forward and backward directions. 

To calculate the radiation pattern of the apertures using FDTD, we need to define the equivalent currents on the hypothetical surface. To overcome the difficulty of dealing with an infinite surface, we choose the closed surface to be S2 — S7 — S5 — S8. The assumption is 

We apply the concepts discussed in the last few sections to the case of a C aperture in aluminum. The thickness of the alumimun film is chosen to be 100 nm. The dimensions of the C aperture are as follows aperture length 155nm, aperture width 70 run, tongue width 25 nm, and gap width 25 nm. The incident field is X-polarized. The XZ plane is a mirror symmetry plane for the C aperture. The surrounding dielectric is assumed to be ifee space. The normalized scattering and absorption cross sections as a function of wavelength are shown in Fig. 43. 

The near-field intensity is calculated at a point, in the gap, 5 nm beyond the transmission side of the aperture. The cross sections are normalized with respect to the physical area of the aperture. For comparison with the near-field intensity, we normalize the cross sections such that the peak cross section is unity. To distinguish this from the area normalization, we call this the magnitude normalization. The cross sections and the near-field intensity are shown in Fig. 44. 

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Radiation from an aperture lamp is emitted in tight "fan" configuration. For. scanning a sheet of printed paper, it is ideal since the emission is concentrated in a relatively narrow band. Hie phosphor most used for this application is Mg2GKi204 Mn. More recently, some of the terbium activated phosphors like (Mg.CelAlnOigrTb , LaP04 Ce Tb , and Y2SiOg Ce Tb have been employed because the major part of their emission is concentrated in the 543 nm line. 

Fig. 4.5 Basic diagram of a FT-Raman spectrometer. S, sample NF, notch filter for rejecting non-lasing radiation from laser RF, Rayleigh filter for rejecting radiation at laser frequency Ap, aperture wheel A, analyser I, interferometer.

Radiation from the other leg of the interferometer was focused onto a 2S-pm diameter pinhole which acted as an aperture stop at the face of a standard Dumont 6911 type S-1 photomultiplier. This provided a relatively accurate method for superimposing the two beams [7.24], This is critical since the double-quantum response is inversely proportional to the illuminated area A. The beams were adjusted to achieve maximum output from the 6911 photomultiplier tube, a procedure which was often difficult and required a great deal of care. 

The field radiated from a finite array is shown quantitatively in Fig. 2.11, bottom. Close to the array the field looks approximately as for the infinite array except for some ripples. These are merely caused by the same type of surface waves encountered in Chapter 1 and discussed in more detail in Chapters 4 and 5. They typically occur every time a general aperture is finite rather than infinite. 

A well-defined molecular beam strictly defines the source area and angular range of molecules and restricts the amount of background vapor that reaches the ionizer. Furthermore, by using a small field aperture one can make the source area of the molecular beam smaller than the cross-sectional area of the cell orifice. This definition of the beam effectively removes the effect of the shape of the orifice on the fiux distribution of the molecular beam and makes KEMS measurements independent of orifice shape. This effect is analogous to the requirements of sampling the radiation from fuUy within the blackbody when temperature is measured with a pyrometer. 

Radiated emission (RE) The potential EMI that radiates from escape coupling paths such as cables, leaky apertures, or inadequately shielded housings. 

In the case of an aperture such as that shown in Fig. 13.1(e), the primary currents are flowing on the wave-guide and horn surfaces and are too difficult to find. In this case, the standard practice is to introduce what are known as equivalent currents. For the rectangular aperture at the mouth of the horn, equivalent electric and magnetic currents can be found from the equivalence principle so that the radiation from the equivalents is the same as from the primary currents. In any event, the current must be known before proceeding with the analysis of an antenna. 

The Container. The sample area, in the broadest sense, consists of all the foreoptics, i.e., those between the source and the entrance slit, and the sample wells. The foreoptics usually comprise the source, a mirror or system of mirrors to focus the diverging radiation from the source onto the entrance slit, and an intemipter (chopper) which alternately sends the sample and reference beam radiation to the entrance slit. The function of the interrupter is to allow the detector to measure the radiation of the two beams and thus get a comparison between them. The sample can be placed at any point in the beam for convenience, however, it is usually placed near a focal point in order to keep the aperture of the cell as small as possible. 

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