Diffractive lens

All the energy-filter systems referred to above are incorporated into the electron-microscopic column, usually between the diffraction lens and the following intermedium lens. They are, therefore, known as in-column filters, which are also commer-... 

Electron diffraction data collection involves the required microscope optical alignment and special attention to the sample position, sample thickness and rotation, diffraction CL and diffraction lens focusing, and the detector used to record electron diffraction patterns. The CL is determined by the projection lenses in combination with the objective lens (the first lens after the specimen). To use the calibrated CL, the sample position and the objective lens setting must be the same setting between the calibration and experiment. 

Diffraction lens Intermediate lens Projector lens... 

Fig. 95 SEM micrograph of diffractive lens structures ablated into polyimide (left, lens diameter 2.5 mm) and array of lens structures (using a step and repeat mode, lens dimensions 900x900 pm). REPRINTED WITH PERMISSION OF [Ref. 322], COPYRIGHT (2001) Elsevier Science...

Abstract. The basic features of crystal diffraction and their application to the construction of a crystal diffraction lens for focusing energetic gamma rays are described using examples from the work perfonned at the Argonne National Laboratory. Both on-axis and off-axis performance are discussed. The review includes the use of normal crystals bent crystals, and crystals with variable crystal-plane spacing to develop both condenser-type lenses and point-to-point imaging lenses. 

This work was supported in part by die DOE contract No. W31-109-38-ENG. The gennanium crystal lens was designed and built with funds supplied by the DOE Office of Nonproliferation and National Security for the work performed at ANL foe die project entitled Crystal Diffraction Lens For Long-Range Passive Detection Of Fissile Material . 

The scientific potential of nuclear gamma-ray astronomy is outlined in section 2. In sections 3 instruments for spectroscopy in the low and medium gamma-ray channel are presented modulating aperture systems, Compton telescopes and diffraction lens telescopes. The three techniques actually reflect our current perception of light itself - they are based on the principles of geometrical optics, quantum optics and wave optics, respectively. 

Ultimately however, the concept should be put to use in space where longer exposures and steady pointing would result in outstanding sensitivities. A tunable crystal diffraction lens (von Ballmoos et al. 1995) can observe any identified source at any selected line-energy in the cardinal range... 

A space borne telescope using an adaptative crystal diffraction lens will consist of three modules the lens module, the detector module, and a boom. Optimally the lens module is located directly on the spacecraft, while the detector module is perched on the boom. The characteristics of a possible space borne gamma-ray lens telescope are summarized in Table 1 - an artists view of the concept is shown in Fig 1. 

As a first step in the project schedule, a ground-based prototype telescope has been achieved. It consists of a diffraction lens focusing at finite distances (provided by ANL, Chicago) and a 3x3 Ge array detector (provided by the CESR, Toulouse). 

In this paper, we present some of the experiments achieved and the measured performance of the system. The results obtained validate the diffraction-lens-based telescope concept and open interesting perspectives for the development of the balloon-telescope model. 

FIGURE 12 (a) A refractive spherical lens and analogous diffractive lenses, (b) Diffractive-lens focusing via ray tracing. 

The expression for the focal length shows that the diffractive lens has an infinite number of focal points that correspond to the diffracted orders m, and that it is highly dispersive (i.e., / depends on X). The diffraction efficiency can approach 100% for a(X) = 1. In contrast, the peak efficiency of a diffractive lens with a multilevel profile is given by... 

FIGURE 13 Applications of subwavelength gratings (a) antireflection surface, (b) retardation plate, (c) polarizing beam splitter, (d) diffractive lens, (e) blazed grating, and (f) wavelength-selective reflector (filter). 

FIGURE 17 Construction of subwavelength distributed-index devices, (a) Refractive surface (center zone of diffractive lens), (b) Analogous structure realized by binary fill-factor modulated subwavelength gratings. 

Figure 5.12a depicts an example of the measured interferograms. Five interferograms were taken with a phase shift between each interferogram and a wrapped phase map could be generated. The unwrapped phase map representing the actual optical path difference profile generated by the diffractive lens appears in Figure 5.12b. A good spherical wave was obtained with very few higher order aberrations as indicated by an rms wavefront error of only 5.0889A,.

Imaging using two-diopter electroactive diffractive lens with model eye. The object is placed at reading distance ( 30 cm), (a) The image is severely out of focus in the model eye when the diffractive lens is off. (b) When the diffractive lens is activated, the object is imaged clearly. (Source Li, G.Q., P. Valley, M.S. Giridhar et al. 2006. Applied Physics Letters, 89(14). With permission.)... 

G. Q. Li, P. Valley, M. S. Giridhar, D. L. Mafhrne, G. Meredith, J. N. Haddock, B. Kippelen, and N. Peyghambarian, "Large-aperture switchable thin diffractive lens with interleaved electrode patterns," Applied Physics Letters, vol. 89, p. 141120, Oct 2006. 

The objective lens is the most important lens for a transmission electron microscope. The condenser lenses are the next most important, the combination of these condenser lenses forms different electron beam to illuminate TEM specimens. The first lens of the projector system (also called the intermediate or diffraction lens) is the third most important one. All the other lenses can be ignored, even though they magnify images. Since the objective lens is the most important, the center of the objective is acmally the optical axis, every other lenses must be aligned with it. TEM alignment starts from top to the bottom. 

The effective area can be increased either by increasing the FZP radius or by decreasing the photodetector-active area. The maximum diameter of the diffractive lens is limited by the properties of the photomask and by the dimensions of the housing. The minimum diameter of the idealized detector is equal to the diameter of the focus. In the above case, when the focal length is 10 mm, and the FZP diameter calculated according to (2.2) is 5 mm (50 zones), as shown in Fig. 2.18, the optimum detector diameter is about 20 pm. Such a detector coupled with the binary amplitude FZP theoretically receives about 2,000 times more radiation than the same device without a concentrator. For a detector with an active area diameter of 250 pm, the effective improvement achieved by the same diffractive lens is 40 times. 

The principle of the selected-area diffraction technique is illustrated in Figure 10. The diffraction pattern is obtained on the viewing screen of a conventional electron microscope by adjusting the diffraction lens. The pattern is focused from the back-focal plane of the objective lens onto the object plane for the projector lens, which then... 

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