Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1996-04-25
1998-04-14
Westin, Edward P.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250216, H01J 314
Patent
active
057395276
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a near-field optical microscope, in particular to a scanning near-field optical microscope (SNOM). Especially, it concerns an aperture scanning optical microscope (a-SNOM).
BACKGROUND OF THE INVENTION
Near-field optical microscopes are well known in the art. A first, more theoretical proposal was made by E. A. Synge, "A Suggested Method for Extending Microscopic Resolution into the Ultra-Microscopic Region" in Phil. Mag. 6 (1928) p. 356-362. Later, J. A. O'Keefe, "Resolving Power of Visible Light" in J. Opt. Soc. Am., Vol. 46, No. 5, May 1956, p. 359, pointed out that the realization of the proposal was rather remote in view of the--at the time--still unresolved difficulty of providing scanning motion over the minute distances involved. Yet the basic idea was further pursued as evidenced by the paper by E. A. Ash and G. Nicholls, "Super-Resolution Aperture Scanning Microscope" in Nature, Vol. 237, No. 5357, Jun. 30, 1972, pp. 510-515.
All of these proposals used a pinhole in a thin membrane as the aperture. The requirement to place the surface to be inspected at a distance from the membrane that is approximately equal to the diameter of the aperture implied the limitation that only objects could be inspected that would have a surface flatness significantly better than an applied optical wavelength.
The first practical realization of a scanning near-field optical microscope was reported by W. D. Pohl in EP-A-0 112 401 (1982). Here, the aperture is implemented as a sharply pointed optically transparent body covered with an opaque layer into which an opening is formed at the apex of the body, the opening having a diameter small compared to the wavelength of the light used.
The name scanning near-field optical microscope (SNOM) denotes a whole variety of near-field devices, and for the sake of information, the following papers are made of reference, all relating to theoretical and/or practical details of the present state of the art of near-field optical microscopy: 1986, pp. 3318-3327. Applied Optics, Vol. 29, No. 26, 10 Sep. 1990, pp. 3734-3740. in: Rev. Sci. Instr., Vol. 61, No. 12, Dec. 1990, pp. 3669-3677.
The classical scanning near-field optical microscope employs a tiny aperture with an entrance pupil diameter that is small with respect to the wavelength of the light used for illuminating the object to be inspected. Within the class of SNOMs, it is therefore known as aperture-scanning near-field microscope (a-SNOM). Directed at the aperture is a laser beam of which a minute part passes to impact the surface of the object. If the object is placed from the aperture at a distance which is also small compared to the wavelength, that is, in the near-field, the light reflected by, or transmitted through, the object can be collected. The transmitted light is collected at an axis perpendicular to the sample surface and opposite of the aperture. In U.S. Pat. No. 5,138,159, the use of a concave mirror having a central bore for the laser beam is described for collecting the reflected light and for focussing it at a detector placed at the opposite side of the sample plane. The detected light is processed to yield an image of the surface investigated. Presently, a-SNOMs reach lateral resolutions of about .lambda./20 with visible light.
The potential of the various SNOM techniques as pad of high density storage devices has been pointed out, for example, in the European patent application EP-A-0 437 170.
Being subject to a periodic modulation probably caused by interference, the detected light intensity of the described a-SNOM is, however, insensitive to variations in the distance between tip and sample. It is therefore difficult to use the measured signal to control the approach and distance of the tip and sample.
The basis of operation of a second type of SNOM, the conventional scanning tunneling optical microscope (STOM), as described in the Courjon et al. reference and also known under the name photon scanning tunneling microscope (PSTM), as described in the above-cited Reddick et al.
REFERENCES:
patent: 5018865 (1991-05-01), Ferrell et al.
patent: 5294790 (1994-03-01), Ohta
patent: 5410151 (1995-04-01), Buckland
patent: 5539197 (1996-07-01), Coujon et al.
De Fornel et al., "An Evanescent Field Optical Microscope," Proc. SPIE: Optical Storage and Scanning Technology, vol. 1139, Apr. 1989, pp. 77-84.
Hecht Bert
Heinzelmann Harald
Novotny Lukas
Pohl Wolfgang
International Business Machines - Corporation
Lee John R.
Strunck Stephen S.
Westin Edward P.
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