The design of a near-field scanning optical microscope (NSOM) is described. Applications to the study of magneto-optic media and domain detection are discussed

Froehlich, Fred F.

Milster, Tom D.

English

NASA Technical Reports Server

N94-14995
A Near-field Scanning Optical Microscope for Analysis of Magneto-optic Media
Fred F. Froehlich and Tom D. Milster
Optical Sciences Center
University of Arizona
Tucson, AZ 85721
602-621-8263
Abstract
The design of a near-field scanning optical microscope (NSOM) is described.
the study of magneto-optic media and domain detectionare discussed.
Applications to
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A Near-field Scanning Optical Microscope for Analysis of Magneto-optic Media
Fred F. Froehlich and Tom D. Milster
Optical Sciences Center
University of Arizona
Tucson, AZ 85721
602-621-8263
The ability to achieve subwavelength optical resolution for microscopy through the use
of a mechanically scanned near-field optical probe has recently been reported._ This technique
provides a means of exceeding the far-field diffraction limit associated with the use of an
objective lens in conventional optical microscopy, and is known as near-field scanning optical
microscopy (NSOM). The near-field probe may be realized by forming a subwavelength-sized
aperture in an opaque screen. Such a probe may be implemented in practice by tapering an
optical fiber down to the desired diameter and coating its exterior (except for the very tip) with
aluminum. The spatial resolution achieved is dependent on the aperture diameter and the tip-to-
sample separation, and the resolution is approximately equal to the aperture diameter for
separations less than one aperture radius.
NSOM is capable of providing imagery with polarization contrast and is, therefore, a
potentially useful tool for the study and analysis of magneto-optic materials. NSOM also
provides a means of performing very high-density MO data storage, and domains as small as 60
nm have been successfully written and detected. 2 High-resolution lithography on a submicron
scale has also been demonstrated. 3
Our NSOM instrument has been designed for the examination of MO thin film samples
and is illustrated in Figure 1. The instrument consists of three optical subsystems: a
conventional visual polarization microscope, a servo subsystem for maintaining the fiber probe
position relative to the sample, and a characterization subsystem for obtaining the MO signal.
The sample is mounted on a piezo-driven x-y flexure stage, and characterization is performed
in transmission. The probe tip may be coarsely prcaligned to sample features, such as domain
structures, with the visual subsystem. The visual polarization microscope operates in reflection
and consists of the lamp, polarizer, beamsplitter, objective, prism, analyzer, and eyepiece. The
servo subsystem regulates the separation (typically less than 20 nm) of the tip and sample with
a hybrid shear atomic force/optical technique. A small dither is applied to the fiber tip (parallel
to the sample surface), and light from the HeNe laser is scattered from the tip and imaged to the
pinhole. The dither produces a modulated signal on the PMT. Atomic forces between the tip
and sample perturb the amplitude and phase of the dither, which can be detected and used as a
feedback signal to regulate tip position. The characterization subsystem operates at blue
wavelengths and generates an MO signal in transmission. Light from the Ar* laser is launched
into the fiber probe with the aid of waveplates to produce any arbitrary state of incident
polarization. The beamsplitter and prism are removed from the visual subsystem, and
polarization-sensitive optics detect the signal light as the sample is scanned to create an image.
164
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Figure 1. NSOM system layout for detection of MO domain structures.
A typical differential detection scheme is shown consisting of waveplates, a polarizing
beamsplitter, two PMTs, and a difference amplifier, but other schemes are possible as well.
The samples used in this study are Co/Pt multilayers prepared by sputtering. The design
is 13 bilayers of 3.5 J_ Co and 8 APt, yielding a total MO film thickness of 149.5 J_, on a
Pyrex substrate. No cover layer is used because the probe tip must be in proximity to the MO
layer surface to obtain the resolution afforded by the near-field interaction. Domains were
prewritten on the samples by conventional far-field means using an Olympus polarization
microscope fitted with a focused laser diode source and an electromagnet.
NSOM may be applied to a variety of studies of MO materials and data storage
problems. The high spatial resolution of NSOM (down to 12 nm) can be utilized to study
domain wall jaggedness. Depolarization effects due to disk grooves and surface roughness can
be examined on a subwavelength scale. The interaction of the near-field probe and MO domain
structures can be characterized with various detection schemes. Domain-sensitive methods such
as differential detection with linearly polarized light, and edge-sensitive methods using split
detectors and circularly polarized light are two techniques of particular interest.
References
1. E. Bet.zig and J. K. Trautman, "Near-field optics: microscopy, spectroscopy, and surface
modification beyond the diffraction limit," Science, vol. 257, pp. 189-195, I0 July 1992, and
references contained therein.
2. E. Betzig, et al, "Near-field magneto-optics and high density data storage," Appl. Phys. Lett.,
61 (2), 13 July 1992.
3. F. Froehlich, T. Milster, and R. Uber, "High-resolution optical lithography with a near-field
scanning subwavelength aperture," Proc. of the SPIE, vol. 1751, July 1992.
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A near-field scanning optical microscope for analysis of magneto-optic media

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