Abstract

Using polarized-light microscopy, we have investigated the magnetic domains of perpendicularly magnetized media under several different conditions, including direct observation of the thin-film magnetic layer and observations through the glass or plastic substrates on which the magnetic film was deposited. The results show that the image contrast is reduced with an increasing numerical aperture of the objective lens. They also indicate that the polarization rotation caused by differences between the reflectivity–transmissivity of the p and s components of polarization deteriorate the magnetic image contrast. Furthermore, by comparing the image quality using the same objective lens on samples having different substrates, we found that the images obtained through plastic substrates are worse than those obtained through glass substrates. Birefringence of the plastic substrate is shown to be responsible for the additional degradation of the image contrast.

© 1997 Optical Society of America

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References

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  1. A. Takahashi, M. Mieda, Y. Murakami, K. Ohta, H. Yamaoka, “Influence of birefringence on the signal quality of magnetooptic disks using polycarbonate substrates,” Appl. Opt. 27, 2863–2866 (1988).
    [CrossRef] [PubMed]
  2. I. Prikryl, “Effects of disk birefringence on a differential magneto-optic readout,” Appl. Opt. 31, 1853–1862 (1992).
    [CrossRef] [PubMed]
  3. H. Kubota, S. Inoue, “Diffraction images in the polarizing microscope,” J. Opt. Soc. Am. 49, 191–198 (1959).
    [CrossRef] [PubMed]
  4. A. B. Marchant, Optical Recording (Addison-Wesley, Reading, Mass., 1990).
  5. G. Bouwhuis, J. Braat, A. Huijser, J. Pasman, G. van Rosmalen, K. Schouhamer Immink, Principles of Optical Disk Systems (Hilger, Boston, Mass., 1985).
  6. M. Mansuripur, The Physical Principles of Magneto-Optical Recording (Cambridge U. Press, Cambridge, 1995).
    [CrossRef]
  7. M. Born, E. Wolf, Principles of Optics, 6th ed. (Pergamon, Oxford, U.K., 1980).
  8. M. Mansuripur, “Certain computational aspects of vector diffraction problems,” J. Opt. Soc. Am. A 6, 786–805 (1989).
    [CrossRef]

1992

1989

1988

1959

Born, M.

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

Bouwhuis, G.

G. Bouwhuis, J. Braat, A. Huijser, J. Pasman, G. van Rosmalen, K. Schouhamer Immink, Principles of Optical Disk Systems (Hilger, Boston, Mass., 1985).

Braat, J.

G. Bouwhuis, J. Braat, A. Huijser, J. Pasman, G. van Rosmalen, K. Schouhamer Immink, Principles of Optical Disk Systems (Hilger, Boston, Mass., 1985).

Huijser, A.

G. Bouwhuis, J. Braat, A. Huijser, J. Pasman, G. van Rosmalen, K. Schouhamer Immink, Principles of Optical Disk Systems (Hilger, Boston, Mass., 1985).

Inoue, S.

Kubota, H.

Mansuripur, M.

M. Mansuripur, “Certain computational aspects of vector diffraction problems,” J. Opt. Soc. Am. A 6, 786–805 (1989).
[CrossRef]

M. Mansuripur, The Physical Principles of Magneto-Optical Recording (Cambridge U. Press, Cambridge, 1995).
[CrossRef]

Marchant, A. B.

A. B. Marchant, Optical Recording (Addison-Wesley, Reading, Mass., 1990).

Mieda, M.

Murakami, Y.

Ohta, K.

Pasman, J.

G. Bouwhuis, J. Braat, A. Huijser, J. Pasman, G. van Rosmalen, K. Schouhamer Immink, Principles of Optical Disk Systems (Hilger, Boston, Mass., 1985).

Prikryl, I.

Schouhamer Immink, K.

G. Bouwhuis, J. Braat, A. Huijser, J. Pasman, G. van Rosmalen, K. Schouhamer Immink, Principles of Optical Disk Systems (Hilger, Boston, Mass., 1985).

Takahashi, A.

van Rosmalen, G.

G. Bouwhuis, J. Braat, A. Huijser, J. Pasman, G. van Rosmalen, K. Schouhamer Immink, Principles of Optical Disk Systems (Hilger, Boston, Mass., 1985).

Wolf, E.

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

Yamaoka, H.

Appl. Opt.

J. Opt. Soc. Am.

J. Opt. Soc. Am. A

Other

A. B. Marchant, Optical Recording (Addison-Wesley, Reading, Mass., 1990).

G. Bouwhuis, J. Braat, A. Huijser, J. Pasman, G. van Rosmalen, K. Schouhamer Immink, Principles of Optical Disk Systems (Hilger, Boston, Mass., 1985).

M. Mansuripur, The Physical Principles of Magneto-Optical Recording (Cambridge U. Press, Cambridge, 1995).
[CrossRef]

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

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Figures (16)

Fig. 1
Fig. 1

Schematic diagram showing the experimental setup. A variable-diameter pinhole is imaged to the front focal plane of an objective lens to yield uniform illumination over the sample. With the help of crossed polarizers, the magnetic domains on the sample are rendered visible on the TV monitor. Between the objective lens and the sample, one can insert a glass or plastic plate to investigate the effects of the substrate.

Fig. 2
Fig. 2

Images of a bare magnetic sample by use of microscope objectives with different NA’s: Values of the NA are (a) 0.4, (b) 0.6, (c) 0.8, and (d) 0.95. At high NA’s the image contrast is substantially reduced.

Fig. 3
Fig. 3

Images obtained from a bare magnetic sample with a 1.2-mm-thick glass or plastic plate inserted between the sample and the objective lens: (a) with a glass plate and value of NA = 0.4, (b) with a plastic plate and value of NA = 0.4, (c) with a glass plate and value of NA = 0.8, and (d) with a plastic plate and value of NA = 0.8. The objective lens with NA = 0.8 is corrected for a 1.2-mm cover-plate thickness. It is seen that with the glass plate the image remains of good quality, whereas with the plastic plate the contrast is poor.

Fig. 4
Fig. 4

Intensity distributions IX and IY at the exit pupil of a NA = 0.8 objective lens. The sample is a MO disk with a 1.2-mm-thick glass or plastic substrate: (a) IX for a disk with a glass substrate, (b) IY for a disk with a glass substrate, (c) IX for a disk with a plastic substrate, and (d) IY for a disk with a plastic substrate. The polarization rotation caused by the plastic substrate is much stronger than that caused by the glass substrate.

Fig. 5
Fig. 5

Images of a phase object obtained with objective lenses of different NA’s. The phase object is an etched glass surface, covered with 2 µm × 2 µm pits on a regular mesh; each pit has a depth of 20 nm. (a) NA = 0.8 and a bare sample. (b) NA = 0.95 and a bare sample. (c) NA = 0.8 with a glass plate inserted. (d) NA = 0.8 with a plastic plate inserted. Notice that the objective lens used for (a) is different from that used for (c) and (d). The former is corrected for the 1.2-mm cover-plate thickness. Also notice that the magnifications of these two objectives are different even though they have the same NA (=0.8). Apparently the inserted plate does not degrade the image quality.

Fig. 6
Fig. 6

Schematic diagram showing the coordinate system at the entrance pupil of the objective lens. In our theoretical analysis, each ray at the entrance pupil is decomposed into its p and s components of polarization. The E field emerging from the exit pupil is obtained by recombination of these components after they are independently reflected from the sample and passed through the optical system.

Fig. 7
Fig. 7

Quadrilayer structure used in the computer simulations. This structure has a glass substrate coated with an aluminum layer. A dielectric layer separates the aluminum layer from the MO film. The magnetic medium is coated with another dielectric layer, and the medium of incidence is air (ninc = 1). The incident beam is a monochromatic plane wave with a wavelength of 780 nm.

Fig. 8
Fig. 8

Results of numerical calculations for the magnetic quadrilayer shown in Fig. 7: (a) Dependence of the amplitude reflectivities on the incident angle. (b) Dependence of the phase on the various reflection coefficients at the incident angle. (c) Difference between the amplitudes and phases of the p and s reflection coefficients as a function of the incident angle. (d) Dependence of the effective rotation angles ΘA and ΘF on the NA of the objective lens. (e) Dependence of the image contrast on the NA of the objective lens. The continuous curve is obtained from the theoretical analysis based on the concept of geometrical optical rays. The points indicated by solid squares were obtained from computer simulations by use of the program diffract.

Fig. 9
Fig. 9

Effective rotation angle ΘA caused by the Fresnel transmission coefficients plotted as a function of the NA of the objective lens.

Fig. 10
Fig. 10

Effective rotation angle ΘF caused by the 1.2-mm-thick substrate’s vertical birefringence plotted as a function of the NA of the objective lens. It is seen that, as the value of the NA tends toward 0.8, the rotation angle becomes as large as 34°, which explains why the image shown in Fig. 3(d) has no contrast at all.

Fig. 11
Fig. 11

Normalized MO readout signal as a function of the NA of the objective lens in the presence of the 1.2-mm-thick plastic substrate. At NA = 0.8 the signal is down to approximately 0.45.

Fig. 12
Fig. 12

Schematic diagram of the unfolded optical system used in the diffract simulations. The incident X-polarized plane wave truncated by the objective lens forms a diffraction-limited spot at the focal plane. The reflected light is recollimated by the same lens and subsequently passes through the analyzer. The final lens focuses the beam onto a photodetector.

Fig. 13
Fig. 13

Image contrast ratio versus θ, the orientation angle of the analyzer obtained from the diffract simulations with a NA = 0.8 objective lens. The sample used here is the same as that for Fig. 7. The maximum contrast ratio occurs at θ = 85°, which agrees with the value of 84.8° predicted by Eq. (7).

Fig. 14
Fig. 14

Intensity distributions IX and IY obtained from the diffract simulations at the exit pupil of a NA = 0.8 objective lens for samples with and without a 1.2-mm-thick substrate: (a) IX for a bare sample, (b) IY for a bare sample, (c) IX for a sample with a glass substrate, (d) IY for a sample with a glass substrate, (e) IX for a sample with a plastic substrate, and (f) IY for a sample with a plastic substrate. In (d) the dark areas are regions where the Kerr rotation is canceled out by the Fresnel polarization rotation.

Fig. 15
Fig. 15

Structure of the sample used in the computer simulation and designed to show the cancellation of the birefringence effect. The product of t and Δn is 0.6 µm for the plastic substrate and 0.71 µm for the compensating crystal.

Fig. 16
Fig. 16

Intensity distributions IX and IY obtained from the diffract simulations at the exit pupil of a NA = 0.8 objective lens for a disk with a uniaxial crystal plate placed atop its birefringent substrate. The four black corners of IX, which can be observed in Fig. 14(e), have now disappeared. The contrast ratio in this case is computed to be 0.1, up from the value of 0.007 in the absence of the crystal slab.

Tables (1)

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Table 1 Variables and Their Definitions Used in the Theoretical Derivation

Equations (16)

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E0α=Eρρˆ+Eϕϕˆ=Eppˆ+Esŝ=EpEs=E0cos α-sin α.
E1α, ρ=EpEs=tp expiδbEptsEs.
E2α, ρ=rpp expiϕppEp-rsp expiϕspEsrss expiϕssEs+rps expiϕpsEp,
E3α, ρ=EpEs=E0app expi2δb+ϕppcos α+asp expiϕsp+δbsin α-ass expiϕsssin α+asp expiϕsp+δbcos α,
Iθ=Ep cosθ-α+Es sinθ-α2ρdρdα  =πρ02Ep cosθ-α+Es sinθ-α2exit pupil
Iθπρ02=14app2+ass2+cos2θ18app2+ass2+14appass cosδf+2δb+sin2 θasp2+sin θ cos θaspapp cosϕpp-ϕsp+δb+ass cosϕss-ϕsp-δb,
Cθ=Iθ, asp-Iθ, -aspIθ, asp+Iθ, -asp=J1J2 cot θ+J3 tan θJ12J2J31/2
J=2aspapp cosϕpp-ϕsp+δb+ass cosϕss-ϕsp-δb,J2=34app2+ass2+12appasscosδf+2δb,J3=2asp2+14app2+ass2-12appasscosδf+2δb.
Cmax=aspapp cosϕpp-ϕsp+δb+ass cosϕss-ϕsp-δb×app+ass2+12app-ass2-2appass sin22δb+δf/2-1/2×asp2+18app-ass2+12appass sin22δb+δf/2-1/2=aspā cosϕ¯-ϕspcos δ¯-12aspΔasinϕ¯-ϕspsin δ¯asp2ā21/2×1+18Δa2ā2-12appass sin2 δ¯ā2-1/21+18Δa2asp2+12appass sin2 δ¯asp2-1/2,
Cmaxcosϕ¯-ϕsp1+18Δaasp2+12āasp1/2sin2 δ¯1/2cosϕ¯-ϕsp1+ΘA2ΘK2+ΘF2ΘK21/2
ΘA2=18Δaā2,  ΘF2=12sin2 δ¯,  ΘK=aspā.
S1asp-S2asp=cos2θ14app2+ass2+12appass cos2δb+δf+2 sin2 θasp2+sin 2θaspapp cosϕpp-ϕsp+δb+ass cosϕss-ϕsp-δb.
ΔS=S1asp-S2asp-S1-asp-S2-asp=4 sin 2θaspā cosϕ¯-ϕspcos δ¯-12aspΔasinϕ¯-ϕspsin δ¯.
ΔSmax=4aspācosϕ¯-ϕspcosδb+12δf.
tstp=cos2β1-β2.
δb=2πΔn sin2 β2cos β2tλ.

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