Abstract

We introduce an application of thermionic emission in a PMT photocathode. Because of the nonlinear dependence of thermionic emission on absorbed laser power, a conventional PMT is found to produce a virtual pinhole effect that rejects unfocused light at least as strongly as a physical pinhole. This virtual pinhole effect is exploited in a scanning transmission confocal microscope equipped with a cw laser source. Because the area of the PMT photocathode is large, signal descanning is not required and thermionic detection acts as a self-aligned pinhole. Our technique of thermionic-detection autoconfocal microscopy is further implemented with graded-field contrast to obtain enhanced phase-gradient sensitivity in unlabeled samples, such as rat hippocampal brain slices.

© 2008 Optical Society of America

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References

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  1. T. Wilson and C. J. R. Sheppard, Theory and Practice of Scanning Optical Microscopy (Academic, 1984).
  2. T. Pons and J. Mertz, J. Opt. Soc. Am. B 21, 1486 (2004).
    [CrossRef]
  3. K. K. Chu, R. Yi, and J. Mertz, Opt. Express 15, 2476 (2007).
    [CrossRef] [PubMed]
  4. W. Denk, J. H. Strickler, and W. W. Webb, Science 248, 73 (1990).
    [CrossRef] [PubMed]
  5. P. B. Coates, J. Phys. D 5, 1489 (1972).
    [CrossRef]
  6. W. Schottky, Annalen der Physik 362, 541 (1918).
    [CrossRef]
  7. K. K. Chu, D. Lim, and J. Mertz, Opt. Lett. 32, 2846 (2007).
    [CrossRef] [PubMed]

2007 (2)

2004 (1)

1990 (1)

W. Denk, J. H. Strickler, and W. W. Webb, Science 248, 73 (1990).
[CrossRef] [PubMed]

1972 (1)

P. B. Coates, J. Phys. D 5, 1489 (1972).
[CrossRef]

1918 (1)

W. Schottky, Annalen der Physik 362, 541 (1918).
[CrossRef]

Chu, K. K.

Coates, P. B.

P. B. Coates, J. Phys. D 5, 1489 (1972).
[CrossRef]

Denk, W.

W. Denk, J. H. Strickler, and W. W. Webb, Science 248, 73 (1990).
[CrossRef] [PubMed]

Lim, D.

Mertz, J.

Pons, T.

Schottky, W.

W. Schottky, Annalen der Physik 362, 541 (1918).
[CrossRef]

Sheppard, C. J. R.

T. Wilson and C. J. R. Sheppard, Theory and Practice of Scanning Optical Microscopy (Academic, 1984).

Strickler, J. H.

W. Denk, J. H. Strickler, and W. W. Webb, Science 248, 73 (1990).
[CrossRef] [PubMed]

Webb, W. W.

W. Denk, J. H. Strickler, and W. W. Webb, Science 248, 73 (1990).
[CrossRef] [PubMed]

Wilson, T.

T. Wilson and C. J. R. Sheppard, Theory and Practice of Scanning Optical Microscopy (Academic, 1984).

Yi, R.

Annalen der Physik (1)

W. Schottky, Annalen der Physik 362, 541 (1918).
[CrossRef]

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

J. Phys. D (1)

P. B. Coates, J. Phys. D 5, 1489 (1972).
[CrossRef]

Opt. Express (1)

Opt. Lett. (1)

Science (1)

W. Denk, J. H. Strickler, and W. W. Webb, Science 248, 73 (1990).
[CrossRef] [PubMed]

Other (1)

T. Wilson and C. J. R. Sheppard, Theory and Practice of Scanning Optical Microscopy (Academic, 1984).

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Figures (4)

Fig. 1
Fig. 1

A laser beam ( 1064 nm ) is focused onto a bialkali PMT photocathode to generate thermionic emission. Peak counts (triangles) or the mean of the output (squares) are plotted against laser power. Solid curves indicate fits to the Richardson equation and to a quadratic, respectively.

Fig. 2
Fig. 2

TD-ACM schematic. A focused laser beam is scanned across a sample. Light transmitted through the sample is collected and focused onto the photocathode of a PMT. The resulting thermionic current is amplified by the PMT and then fed into a peak counting circuit to obtain J, or integrated to obtain the mean.

Fig. 3
Fig. 3

Virtual pinhole effect. Unfocused-light rejection capability of TD is demonstrated by sliding the PMT along z through the focal plane ( z = 0 ) of a focused beam of confocal parameter about 1 mm . The output of the PMT is either integrated to find the mean (dotted curve), or analyzed by a peak counting algorithm (solid cuve) to find J. The FWHM of the curve obtained by integrating is roughly the same as that obtained using a photodiode equipped with a physical pinhole ( 10 μ m diameter). The FWHM obtained by peak counting is significantly narrower.

Fig. 4
Fig. 4

Graded-field-contrast-enhanced image of a rat hippocampus slice ( 400 μ m thick) acquired with (a) TD-ACM and (b) a linear detector. Both images were acquired 70 μ m below the tissue surface. (c)–(e), TD-ACM images of same sample taken at depths 30, 60 and 150 μ m . Acquisition time was 2.8 s with an illumination power incident on the sample of about 25 mW . All images were subjected to 4 × averaging and a gamma of 0.75. Panels (a) and (b) are 380 × 350   pixels .

Equations (1)

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J = A T 2 exp ( W eff k B T ) ,

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