Near with small apertures

Near-Fleld Microscopy with Small Apertures... 

A standing wave (SW) microwave linear accelerator consists of a linear array of resonant cavities that are energized by a common source of microwave power. These cavities are nearly isolated by webs with small-diameter apertures, and the high-energy electron beam passes through these apertures. However, they are coupled through intermediate cavities, which stabilize the microwave phase relationship between the accelerating cavities. 

Ultraminiaturized fiber optic sensors under 100 fim have only recently been fabricated. Tan et al [23, 24] developed a submicrometer optical fiber tip by pulling out silica fibers on a micropipette puller using a 25 W CO2 infrared laser as a heat source. Tips as small as 0.1 /xm could be reliably fabricated. After pulling, the tips were sputtered with aluminum in a vacuum chamber. This fabrication technique leaves a very small aperture at the tip, which can then be used as a near-field optical device (discussed in the next section). 

Figure 4.9 Schematic drawing of near-field optical probes. By pulling out an optical fiber and coating with aluminum, an optical probe with a very small aperture (<100 nm) is created. The probe can detect fluorescence or phosphorescence from objects in the near field close to the aperture, such as a thin film or flat surface (a) or from a labeled molecule (b). The probe is relatively insensitive to light in the medium in the far field further from the tip. The excitation light source can be transmitted through the probe or externally.

The experimental techniques for single molecule spectroscopy described in the previous chapters differ mainly in the method employed to reduce the excitation volume of the sample (combined with different fluorescence collection methods). This was achieved in four different ways (i) the laser was focused to a tiny spot on the sample by a lens immersed in liquid helium, (ii) the excitation light was coupled into an optical fiber carrying the sample at its end, (iii) the sample was mounted behind a small aperture (pinhole with typically 5 pm diameter). All these methods reduce the excitation area to a few pm. The near-field technique (iv) allows investigations beyond the classical diffraction limit the tapered tip used had a typical diameter in the order of 50-100 mn. 

As described above, the major problem hindering XPS investigations under real conditions is the strong interaction between the emitted electrons and the gas environment. All methods allowing such measurements are based on the idea of capturing the electrons at distances comparable with their mean free path in the gas atmosphere. This means that emitted electrons with a typical kinetic energy of a few hundred eV have to be detected at a distance of a few millimetres from the sample surface when the total pressure is around 1 mbar. This is accomplished by small apertures near the sample and by differential pumping on the other side of the aperture to decrease the pressure and, thus, the collision rate. A typical set-up with an aperture is sketched in Fig. 19.13. ... 

Most small consumer (e.g. for ceU phones) as well as automotive cameras are built up of small image sensors and compact lenses with a very small aperture and short focal length. Images captured with such cameras usually do not show any defocus blur for larger distances than several centimeters to meters. From this it follows that these images render a nearly uniform sharpness within the entire image. 

A determination of the particle size, which is independent from the distance of the particle to the detector, assumes that the extinction coefficients for the minimum and the maximum aperture angle do not differ significantly. Water droplets in air whose diameters are less than 1000 pm result in nearly identical extinction coefficient when the aperture is less than 0.01"". The smaller the particles, the larger the aperture angles are which lead to significantly different extinction coefficients. Water droplets with diameters less than 100 pm arise in almost identical extinction cross sections when the aperture is less than 0.1°. For other particle systems the qualitative correlation is similar. The determination of particle size that is independent of the position of the particle in the measurement volume requires an optical construction of the SE-Sensor with a correspondingly small aperture angle. 

This technique became recently quite important in context with seismic risk and hazard studies. Since ambient noise mostly consists of surface-wave energy, one can use local dispersion-curve data, measured with temporary, very small aperture arrays, to invert for the near-surface S-velocity structure for further details see, e.g., Schweitzer et al. (2012). 

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