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Confocal microscopy with a high numerical aperture parabolic mirror

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Abstract

A novel high-resolution stage scanning confocal microscope for fluorescence microscopy and spatially resolved spectroscopy with a high numerical aperture (NA≈1) parabolic mirror objective is investigated. A spatial resolution close to the diffraction limit is achieved. As microscopic fluorescent test objects, dye-loaded zeolite microcrystals (diameter approx. 0.4 µm) and single fluorescent molecules were used. Confocal fluorescence images show a spatial resolution of Δx=0.8·λ both at room temperature and at 1.8 K. Imaging of a quasi-point light source and focusing by the parabolic mirror were investigated experimentally and theoretically. Deviations between the theoretical results for a perfect parabolic mirror and the experimental results can be attributed to small deviations of the mirror profile from an ideal parabola.

©2001 Optical Society of America

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Figures (7)

Fig. 1.
Fig. 1. Layout of the optical path of a confocal microscope with a parabolic mirror objective. A parallel beam generated by the output of an optical single-mode fiber and a beam expander lens is focused onto the sample by a parabolic mirror. The light from the focal region is collected by the same mirror and is focused backwards into the image plane.
Fig. 2.
Fig. 2. (a) Photograph of the mounted parabolic mirror. (b) Arbitrary cross section through one half of the mirror. The diagram shows the deviations Δz(r) of the mirror surface from an ideal parabola. The dotted line indicates the smoothed curve which is used for the calculations.
Fig. 3.
Fig. 3. Images of a quasi-point light source (λ=532 nm): (a) experimental results, (b) simulations with an ideal parabolic mirror, and (c) simulations with a parabolic mirror with phase errors. From top to bottom the light source is shifted away from the optical axis to the right by 0, 7, 14 and 20 µm. Every picture is normalized to its intensity maximum. The length bar corresponds to 5·M·λ (M=magnification, λ=wavelength).
Fig. 4.
Fig. 4. Logarithmic representation of the calculated intensity distribution in the focal region of a parabolic mirror illuminated with circularly polarized light (λ=532 nm). The cross section is oriented along the optical axis (z-direction). The optical axis is located in the center of the illustration. (a) Focal region of an ideal parabolic mirror and (b) focal region of a mirror which produces phase errors due to its imperfect surface profile (see Fig. 3c). In both cases the intensity distribution through the focal spot in x-direction has a FWHM of 0.30 µm. Along the mirror axis the ideal mirror yields a FWHM of 0.54 µm whereas the imperfect mirror yields a FWHM of 0.50 µm for the most intense maximum. The central intensity maximum for the ideal mirror is approximately twice as high as the maximum for the imperfect mirror.
Fig. 5.
Fig. 5. Experimental and calculated intensity distribution in the focal region of the parabolic mirror along the axis. The origin corresponds to the location of the focal spot for an ideal mirror. The width of the intensity distribution along the mirror axis (FWHM) is 0.80 µm in the experiment, 0.53 µm in the calculation for a mirror with phase errors and 0.62 µm for the ideal mirror.
Fig. 6.
Fig. 6. (a) Confocal fluorescence image of dye-loaded zeolite L microcrystals dispersed on a quartz glass surface. The brightest crystallite has an intensity maximum of 1.5·103 counts per 5 ms. (b) Line sections through one of the crystals. The excitation light is circularly polarized. Differences in brightness correspond to the size of the crystals. (c) Scanning electron micrograph of the same section of the sample as in (a).
Fig. 7.
Fig. 7. (a) Confocal fluorescence image of a single terrylene molecule embedded in the Shpol’skii matrix octadecane, immersed in superfluid Helium at 1.8 K. The lines through the fluorescence spot indicate the cross sections in graph (b). A two dimensional Gaussian fit yields a FWHM of 0.46 µm. The excitation light is circularly polarized with a wavelength λ=571.6 nm.

Equations (1)

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Φ ( r ) = e i φ ( r ) = e i 2 π ( 1 + cos Θ ) · Δ z ( r ) λ .
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