Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1997-03-03
1998-11-24
Le, Que
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
25055927, 356382, H01J 314
Patent
active
058411291
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns a device for the optical scanning of objects on a scanning surface and processes for operating the same.
2. Description of the Related Art
The scanning of an optical probe with an aperture smaller than the wavelength of light makes it possible to increase the optical resolution (D. W. Pohl, W. Denk, M. Lanz, Applied Physics Letters 44 (1984) 651). For such a process the resolution is limited by the size of the aperture.
The scanning of an optical probe near an object can be improved by the use of an interaction field between the probe and the object. A process is known for the control of the distance between the probe and surface using evanescent optical surface waves (R. C. Reddick, R. J. Warmack, T. L. Ferrell, Physical Review B 39 (1989) 767). Another process is known where the damping of the modulation frequency (shear force interaction) of the probe is used near the surface (E. Betzig, P. L. Finn, J. S. Weiner, Applied Physics Letter 60 (1992) 2484).
In these and other known processes, the optical probe has been mostly used to illuminate the surface. An increased resolution through a smaller aperture has the consequence that less light is transmitted through the probe and the maximal amount of transmitted light is reduced with smaller aperture which limits the resolution.
An optical process is known through which the Raman sensitivity of adsorbates is increased by the use of a metallic substrate layer and a transparent surface layer (W. S. Bacsa, J. S. Lannin, Applied Physics Letters 61 (1992) 19). This has the effect that an interference maximum of the standing wave, produced through the interference of the incident and reflected light beams, falls on the surface of the transparent layer. This local enhanced light intensity on the surface increases the Raman-signal of adsorbates and ultra-thin layers.
A process is known where standing optical waves near a surface are used to control the movement of an optical probe parallel to the surface (N. Umeda, Y. Hayashi, K. Nagai, A. Takayanagi, Applied Optics, 31 (1992) 4517).
The use of the probe as a light collector for detection has the advantage that even when the amount of transmitted light is limited by the aperture, the maximal intensity of the incident light beam is not limited by the probe aperture. This process is, however, not particularly sensitive to adsorbates and ultra-thin layers due to the fact that the local light intensity is small near opaque surfaces.
SUMMARY OF THE INVENTION
The invention uses the surface of a planar microcavity as a scanning surface. The microcavity can be realized through a transparent layer on top of an opaque layer. Even though the reflectivities of the interfaces of the transparent layer are not very high, the superposition of the reflected waves at the two interfaces is analogous to those in a microcavity. The standing wave is very sensitive to adsorbates and ultra-thin layers when an appropriate thickness is selected for the tansparent layer. Because the standing wave has a minimum at the interface between the opaque and the transparent layer, the optimal thickness of the transparent layer is approximately a multiple of L/4 (L=wavelength of the standing wave) so that an interference maximum falls on the surface. The exact optimal thickness of the transparent layer is given by the penetration depths of the standing wave into the opaque layer and the optical density of the transparent layer. The sensitivity of the standing wave to adsorbates depends also on the optical properties of the reflecting substrate layer and the transparent surface layer. Due to the selected thickness of the transparent surface layer, the light reflected and scattered from the surface interferes destructively with the light reflected from the opaque layer. This has the effect that the light reflected and scattered from the surface has a maximal influence on the standing wave.
Apart of the increased sensitivity of the standing wave to single adsorbates, the loca
REFERENCES:
patent: 4604520 (1986-08-01), Pohl
patent: 4621911 (1986-11-01), Lanni et al.
patent: 5286970 (1994-02-01), Betzig et al.
Applied Optics, vol. 31, No. 22, 1 Aug. 1992, pp. 4515-4518, XP000292096, Norihiro Umeda et al: "Scanning Wiener-Fringe Microscope with an Optical Fiber Tip".
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