Abstract

Polarization microscopes are widely used to image the magnetic domains of a magneto-optical disk and to characterize the birefringence of the disk substrate. For high-resolution imaging, unfortunately, the coupling of the polarization rotation from the Kerr signal, the effect of Fresnel’s reflection coefficients, and the substrate birefringence severely deteriorate the image contrast obtained from conventional observations. Here we present the technique of differential polarization microscopy, which replaces the analyzer with a Wollaston prism, for providing better image contrast. Images of a magnetic pattern obtained with both conventional and differential methods are observed for objective lenses that have different numerical apertures and magneto-optical disks with and without a birefringent substrate. The computer simulations and experimental results show that the use of this differential method improves the image contrast and provides excellent tolerance for defects of the optical system.

© 1999 Optical Society of America

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References

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  1. S. Inoué, R. Oldenbourg, “Microscopes,” Handbook of Optics, 2nd ed. (McGraw-Hill, New York, 1995), Vol. II, Chap. 17.
  2. J. R. Benford, H. E. Rosenberger, “Microscopes,” in Applied Optics and Optical Engineering, R. Kingslake, ed. (Academic, New York, 1967), Vol. IV, pp. 70–76.
  3. Y. C. Hsieh, M. Mansuripur, “Image contrast in polarization microscopy of magneto-optical disk data-storage media through birefringent plastic substrates,” Appl. Opt. 36, 4839–4852 (1997).
    [CrossRef] [PubMed]
  4. M. Born, E. Wolf, Principles of Optics, 6th ed. (Pergamon, Oxford, UK, 1980).
  5. H. Kubota, S. Inoue, “Diffraction images in the polarizing microscope,” J. Opt. Soc. Am. 49, 191–198 (1959).
    [CrossRef] [PubMed]
  6. J. R. Benford, “Microscope objectives,” in Applied Optics and Optical Engineering, R. Kingslake, ed. (Academic, New York, 1965), Vol. III.
  7. W. Mickols, I. Tinoco, J. E. Katz, M. F. Maestre, C. Bustamante, “Imaging differential polarization microscope with electronic readout,” Rev. Sci. Instrum. 56, 2228–2236 (1985).
    [CrossRef]
  8. T. Numata, K. Wakabayashi, S. Inokuchi, “Contrast reversal for domain image enhancement with garnet film rotator in a polarizing microscope,” J. Appl. Phys. 70, 7055–7059 (1991).
    [CrossRef]
  9. D. J. Rawlins, Light Microscopy (Bios Scientific, Oxford, UK, 1992), Chap. 4.
  10. T. C. Bristow, K. Arackellian, “Surface roughness measurements using a Nomarski type scanning instrument,” in Metrology: Figure and Finish, B. E. Traux, ed., Proc. SPIE749, 114–118 (1987).
  11. M. Mansuripur, “Certain computational aspects of vector diffraction problems,” J. Opt. Soc. Am. A 6, 786–805 (1989).
    [CrossRef]
  12. T. D. Goodman, M. Mansuripur, “Subtle effects of the substrate in optical disk data storage systems,” Appl. Opt. 35, 6747–6753 (1996).
    [CrossRef] [PubMed]

1997 (1)

1996 (1)

1991 (1)

T. Numata, K. Wakabayashi, S. Inokuchi, “Contrast reversal for domain image enhancement with garnet film rotator in a polarizing microscope,” J. Appl. Phys. 70, 7055–7059 (1991).
[CrossRef]

1989 (1)

1985 (1)

W. Mickols, I. Tinoco, J. E. Katz, M. F. Maestre, C. Bustamante, “Imaging differential polarization microscope with electronic readout,” Rev. Sci. Instrum. 56, 2228–2236 (1985).
[CrossRef]

1959 (1)

Arackellian, K.

T. C. Bristow, K. Arackellian, “Surface roughness measurements using a Nomarski type scanning instrument,” in Metrology: Figure and Finish, B. E. Traux, ed., Proc. SPIE749, 114–118 (1987).

Benford, J. R.

J. R. Benford, H. E. Rosenberger, “Microscopes,” in Applied Optics and Optical Engineering, R. Kingslake, ed. (Academic, New York, 1967), Vol. IV, pp. 70–76.

J. R. Benford, “Microscope objectives,” in Applied Optics and Optical Engineering, R. Kingslake, ed. (Academic, New York, 1965), Vol. III.

Born, M.

M. Born, E. Wolf, Principles of Optics, 6th ed. (Pergamon, Oxford, UK, 1980).

Bristow, T. C.

T. C. Bristow, K. Arackellian, “Surface roughness measurements using a Nomarski type scanning instrument,” in Metrology: Figure and Finish, B. E. Traux, ed., Proc. SPIE749, 114–118 (1987).

Bustamante, C.

W. Mickols, I. Tinoco, J. E. Katz, M. F. Maestre, C. Bustamante, “Imaging differential polarization microscope with electronic readout,” Rev. Sci. Instrum. 56, 2228–2236 (1985).
[CrossRef]

Goodman, T. D.

Hsieh, Y. C.

Inokuchi, S.

T. Numata, K. Wakabayashi, S. Inokuchi, “Contrast reversal for domain image enhancement with garnet film rotator in a polarizing microscope,” J. Appl. Phys. 70, 7055–7059 (1991).
[CrossRef]

Inoue, S.

Inoué, S.

S. Inoué, R. Oldenbourg, “Microscopes,” Handbook of Optics, 2nd ed. (McGraw-Hill, New York, 1995), Vol. II, Chap. 17.

Katz, J. E.

W. Mickols, I. Tinoco, J. E. Katz, M. F. Maestre, C. Bustamante, “Imaging differential polarization microscope with electronic readout,” Rev. Sci. Instrum. 56, 2228–2236 (1985).
[CrossRef]

Kubota, H.

Maestre, M. F.

W. Mickols, I. Tinoco, J. E. Katz, M. F. Maestre, C. Bustamante, “Imaging differential polarization microscope with electronic readout,” Rev. Sci. Instrum. 56, 2228–2236 (1985).
[CrossRef]

Mansuripur, M.

Mickols, W.

W. Mickols, I. Tinoco, J. E. Katz, M. F. Maestre, C. Bustamante, “Imaging differential polarization microscope with electronic readout,” Rev. Sci. Instrum. 56, 2228–2236 (1985).
[CrossRef]

Numata, T.

T. Numata, K. Wakabayashi, S. Inokuchi, “Contrast reversal for domain image enhancement with garnet film rotator in a polarizing microscope,” J. Appl. Phys. 70, 7055–7059 (1991).
[CrossRef]

Oldenbourg, R.

S. Inoué, R. Oldenbourg, “Microscopes,” Handbook of Optics, 2nd ed. (McGraw-Hill, New York, 1995), Vol. II, Chap. 17.

Rawlins, D. J.

D. J. Rawlins, Light Microscopy (Bios Scientific, Oxford, UK, 1992), Chap. 4.

Rosenberger, H. E.

J. R. Benford, H. E. Rosenberger, “Microscopes,” in Applied Optics and Optical Engineering, R. Kingslake, ed. (Academic, New York, 1967), Vol. IV, pp. 70–76.

Tinoco, I.

W. Mickols, I. Tinoco, J. E. Katz, M. F. Maestre, C. Bustamante, “Imaging differential polarization microscope with electronic readout,” Rev. Sci. Instrum. 56, 2228–2236 (1985).
[CrossRef]

Wakabayashi, K.

T. Numata, K. Wakabayashi, S. Inokuchi, “Contrast reversal for domain image enhancement with garnet film rotator in a polarizing microscope,” J. Appl. Phys. 70, 7055–7059 (1991).
[CrossRef]

Wolf, E.

M. Born, E. Wolf, Principles of Optics, 6th ed. (Pergamon, Oxford, UK, 1980).

Appl. Opt. (2)

J. Appl. Phys. (1)

T. Numata, K. Wakabayashi, S. Inokuchi, “Contrast reversal for domain image enhancement with garnet film rotator in a polarizing microscope,” J. Appl. Phys. 70, 7055–7059 (1991).
[CrossRef]

J. Opt. Soc. Am. (1)

J. Opt. Soc. Am. A (1)

Rev. Sci. Instrum. (1)

W. Mickols, I. Tinoco, J. E. Katz, M. F. Maestre, C. Bustamante, “Imaging differential polarization microscope with electronic readout,” Rev. Sci. Instrum. 56, 2228–2236 (1985).
[CrossRef]

Other (6)

J. R. Benford, “Microscope objectives,” in Applied Optics and Optical Engineering, R. Kingslake, ed. (Academic, New York, 1965), Vol. III.

D. J. Rawlins, Light Microscopy (Bios Scientific, Oxford, UK, 1992), Chap. 4.

T. C. Bristow, K. Arackellian, “Surface roughness measurements using a Nomarski type scanning instrument,” in Metrology: Figure and Finish, B. E. Traux, ed., Proc. SPIE749, 114–118 (1987).

M. Born, E. Wolf, Principles of Optics, 6th ed. (Pergamon, Oxford, UK, 1980).

S. Inoué, R. Oldenbourg, “Microscopes,” Handbook of Optics, 2nd ed. (McGraw-Hill, New York, 1995), Vol. II, Chap. 17.

J. R. Benford, H. E. Rosenberger, “Microscopes,” in Applied Optics and Optical Engineering, R. Kingslake, ed. (Academic, New York, 1967), Vol. IV, pp. 70–76.

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Figures (15)

Fig. 1
Fig. 1

Schematic diagram of a conventional polarization microscope. The spatially incoherent light source is linearly polarized and imaged onto the entrance pupil of the objective lens. The reflected light returns through the objective and, after passage through the analyzer, arrives at the image plane. The analyzer is in a rotatable mount, and its transmission axis is adjusted to yield maximum image contrast. If the analyzer is replaced with a Wollaston prism, two images will appear, side by side, on the camera’s CCD plate. The computer downloads both images simultaneously and subtracts one from the other to produce a differential image.

Fig. 2
Fig. 2

Various distributions of the reflected light at the exit pupil of the objective, when a single, monochromatic point source is used to illuminate the sample. The intensity plots in (a) and (b) correspond, respectively, to the components of the polarization parallel and perpendicular to the polarizer’s transmission axis. The polarization rotation angle ρ is depicted in (c), and the polarization ellipticity η is shown in (d). The gray scale of plots (c) and (d) depicts positive values of ρ and η as bright and negative values as dark.

Fig. 3
Fig. 3

Pattern of magnetic domains on a perpendicularly magnetized sample. The magnetic material rotates the polarization of a linearly polarized beam at normal incidence by ±0.5°. The domains are chosen to represent a wide range of sizes and shapes; the smallest domain appearing in the center is 1λ0 in diameter.

Fig. 4
Fig. 4

Images of the sample of Fig. 3 in a polarization microscope with a 50×, 0.4-NA objective lens. (a) Conventional image obtained with the analyzer set 0.5° from extinction. (b) Differential image obtained with the Wollaston prism.

Fig. 5
Fig. 5

Images of the sample of Fig. 3 in a polarization microscope with a 50×, 0.8-NA objective lens. (a) Conventional image obtained with the analyzer set +1.5° from extinction. (b) Same as (a) but the analyzer is set -1.5° from extinction to reverse the contrast. (c) Differential image. (d) Same as (c) but with the order of subtraction reversed.

Fig. 6
Fig. 6

Distribution of incident intensity at the plane of the sample corresponding to a single point source defocused by 10λ0 through a 0.8-NA objective. The incident beam entering the lens is linearly polarized along the x axis. Top to bottom: intensity distributions corresponding to polarization components along the x, y, and z axes.

Fig. 7
Fig. 7

Images of the sample of Fig. 3, coated with a birefringent layer and placed in a microscope with a 50×, 0.8-NA objective. (a) Conventional image obtained with the analyzer set optimally at 5° from extinction. (b) Differential image. (c) Same as (b) but with the order of subtraction reversed.

Fig. 8
Fig. 8

Images of the magnetic pattern on a bare (i.e., front-surface) MO disk. The domains are approximately 2 µm in diameter, and the microscope has a 0.4-NA objective lens. (a) Conventional image obtained with the polarizer set +10° from extinction. (b) Conventional image obtained with the polarizer set -6° from extinction. (c) Differential image. (d) Differential image with a reversed contrast.

Fig. 9
Fig. 9

Images of the magnetic pattern on a bare MO disk viewed with a 0.6-NA objective lens. (a) Conventional image obtained with the polarizer set +10° from extinction. (b) Conventional image obtained with the polarizer set -7° from extinction. (c) Differential image. (d) Differential image with a reversed contrast. The impact of nonuniform illumination is visible in the conventional image but not in the differential image.

Fig. 10
Fig. 10

Images of the magnetic pattern on a bare MO disk viewed with a 0.8-NA objective lens. (a) Conventional image obtained with the polarizer set +12° from extinction. (b) Conventional image obtained with the polarizer set -8° from extinction. (c) Differential image. (d) Differential image with a reversed contrast.

Fig. 11
Fig. 11

Images of the magnetic pattern on a MO disk with a polycarbonate substrate. The domains are approximately 2 µm in diameter, and the 0.4-NA objective lens is corrected for the 1.2-mm-thick substrate. (a) Conventional image obtained with the polarizer set +7° from extinction. (b) Conventional image obtained with the polarizer set +7° from extinction. (b) Conventional image obtained with the polarizer set -11° from extinction. (c) Differential image. (d) Differential image with a reversed contrast. The conventional images suffer more from the effects of birefringence than do the differential images.

Fig. 12
Fig. 12

Images of the magnetic pattern on a MO disk with a polycarbonate substrate. The 0.6-NA objective has been corrected for the substrate. (a) Conventional image obtained with the polarizer set +19° from extinction. (b) Conventional image obtained with the polarizer set -20° from extinction. (c) Differential image. (d) Differential image with a reversed contrast.

Fig. 13
Fig. 13

Images of the magnetic pattern on a MO disk with a polycarbonate substrate. The 0.8-NA objective lens has been corrected for the substrate. (a) Conventional image obtained with the polarizer set +20° from extinction. (b) Conventional image obtained with the polarizer set -20° from extinction. (c) Differential image. (d) Differential image with a reversed contrast. The contrast of the conventional images suffers severely from birefringence.

Fig. 14
Fig. 14

Schematic diagram of a simplified conoscopic microscope. Double passage of the focused beam through the birefringent crystal causes varying degrees of polarization rotation over the beam’s cross section. The crossed analyzer converts these rotations into an intensity pattern.

Fig. 15
Fig. 15

(a) Intensity and (b) logarithmic intensity distributions at the observation plane in the system of Fig. 14 corresponding to a biaxially birefringent crystal.

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