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Optics Letters

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  • Editor: Anthony J. Campillo
  • Vol. 32, Iss. 3 — Feb. 1, 2007
  • pp: 259–261
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4Pi microscopy with linear fluorescence excitation

Marion C. Lang, Johann Engelhardt, and Stefan W. Hell  »View Author Affiliations


Optics Letters, Vol. 32, Issue 3, pp. 259-261 (2007)
http://dx.doi.org/10.1364/OL.32.000259


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Abstract

Practical 4Pi microscopy has so far exclusively relied on multiphoton excitation of fluorescence, because the nonlinear suppression of contributions from higher-order sidelobes was mandatory for unambiguous axial superresolution. We show that novel lenses of 74 ° semiaperture angle enable biological 4Pi microscopy with regular one-photon fluorescence excitation, thus increasing the signal and reducing system complexity and cost. An axial resolution of 95 nm , corresponding to a more than fourfold improvement over confocal microscopy, is verified in the imaging of microtubules in mammalian cells.

© 2007 Optical Society of America

Multiphoton excitation greatly helps in reducing the sidelobe artifacts in two ways[3

3. H. Gugel, J. Bewersdorf, S. Jakobs, J. Engelhardt, R. Storz, and S. W. Hell, Biophys. J. 87, 4146 (2004). [CrossRef] [PubMed]

, 8

8. S. W. Hell and E. H. K. Stelzer, Opt. Commun. 93, 277 (1992). [CrossRef]

]: First, the higher-order dependence of the excitation on the illumination intensity decreases the fluorescence from the lower sidelobes with respect to that from the main diffraction maximum. Second, for a 4Pi configuration of type C, i.e., simultaneous coherent addition of the illumination and fluorescence wavefronts, the disparity between the multiphoton excitation and fluorescence wavelength causes the lobes of the illumination and detection PSF to peak at different points in space. Since the effective PSF is a product of the two, the lobes cancel out each other, whereas the main peak is reinforced. However, limitations of the multiphoton strategy are that it usually requires expensive mode-locked lasers and that it is usually accompanied by stronger bleaching. Thus, the use of one-photon excitation in 4Pi microscopy would not only reduce complexity and cost but also lead to brighter images. Moreover, the shorter excitation wavelength used for one-photon excitation should provide a narrower main maximum of the effective 4Pi PSF, not only along the z axis but also in the focal plane.

Several strategies have been proposed to render 4Pi microscopy feasible with one-photon excitation, such as the suppression of the fluorescence from the lobes by stimulated emission,[9

9. S. W. Hell, in Topics in Fluorescence Spectroscopy, J. R. Lakowicz, ed. (Plenum, 1997), p. 361.

] and the use of dedicated pupil filters.[10

10. M. Martínez-Corral, Micron 34, 319 (2003). [CrossRef] [PubMed]

, 11

11. M. Martínez-Corral, A. Pons, and M. T. Caballero, J. Opt. Soc. Am. A 19, 1532 (2002). [CrossRef]

] Unfortunately, none of them can be readily integrated into existing systems. In this Letter, we solve the problem by building on the very premises of the concept, that is, on the increase of the solid aperture angle of the system. As a result, we demonstrate the feasibility of 4Pi fluorescence microscopy with one-photon excitation, allowing reliable 3D imaging in cells with an axial resolution of 95nm.

As invoked by the name, 4Pi microscopy and also I5M rely on large-angle spherical wavefronts. So far the largest semiaperture angle α available in (flat-field) well-corrected immersion lenses was 68°. Recent developments in lens manufacturing have allowed a significantly larger angle, α=74° (Objective HCX PL APO CS 100/1.46 Oil, Leica Microsystems, Mannheim). The calculation of the PSF shows that the increase in angle by 6° suppresses the lobes by 7% in the intensity PSF of two opposing lenses (Fig. 1 ). A confocalized one-photon-excitation 4Pi microscope of type C features an effective PSF that is given by heff(r)=hexc4Pihdet4Pi=Eexc(r)+M̂Eexc(M̂r)2Edet(r)+M̂Edet(M̂r)2p(r), where Eexc and Edet are the focal fields for illumination and detection, respectively. M̂ is a transformation matrix accounting for the counterpropagation of the spherical wavefronts, and p(r) denotes the pinhole function.[7

7. M. Nagorni and S. W. Hell, J. Opt. Soc. Am. A 18, 36 (2001). [CrossRef]

]

Figure 1 displays the numerical results for the z response of the system Iz(z)=dxdyheff(r), i.e., the normalized signal generated by an ultrathin horizontal plane, for NA=nsinα equal to 1.40 and 1.46 oil immersion, corresponding to α of 68° and 74°, respectively, along with NA=1.20 water immersion. The refractive indices are n=1.51 (oil) and n=1.33 (water), whereas the excitation and fluorescence wavelengths were assumed to be 488 and 605nm, respectively. The Fourier transformation of Iz(z) yields the optical transfer function (OTF) along the inverse optical axis kz. High lobes in the spatial domain imply an attenuation of the OTF at the critical spatial frequency kc corresponding to the distance between the max ima. The effect of the latter can be removed only if OTF(kz) is not swamped by noise.

Figure 1 shows that even the small increase in aperture angle by 6° decreases the sidelobes by 7%, thus elevating OTF(kz) by 7%. Since the effect of the lobes can be removed if they are <50% of the main maximum, the data of Fig. 1 suggest that one-photon 4Pi microscopy should be possible with standard lenses. Unfortunately, the sidelobes observed in practice are generally 10%15% higher than the theoretical values, which is probably due to residual aberrations introduced by the instrument or by the sample. The larger solid angle of a NA=1.46 oil immersion lens is now expected to render the 4Pi approach more robust.

To put this expectation to the test, we built a 4Pi module in the type C configuration attached to a beam scanning confocal microscope[3

3. H. Gugel, J. Bewersdorf, S. Jakobs, J. Engelhardt, R. Storz, and S. W. Hell, Biophys. J. 87, 4146 (2004). [CrossRef] [PubMed]

] (Leica Microsystems, Mannheim, Germany). A pair of movable glass wedges placed in each arm of the 4Pi module enabled the achromatic adjustment of constructive interference for both the excitation and the fluorescence wavelengths. In such a 4Pi (type C) system, disparities between the excitation and the fluorescence wavelengths (Stokes shift) can be selected to reduce the sidelobes encountered in heff(r), because the sidelobes of hexc4Pi(z) and hdeg4Pi(z) are located at different positions along the z axis.

First, we measured Iz(z) by using a monolayer of quantum dots (Qdot 605 streptavidin conjugate, Quantum Dot Corporation) one-photon excited at λexc=488nm and emitting at λdet=605nm. The resulting z response (Fig. 2 ) agrees well with the theoretical results based on a vectorial calculation.[12

12. B. Richards and E. Wolf, Proc. R. Soc. London, Ser. A 253, 358 (1959). [CrossRef]

] However, more important is the fact that the axial sidelobes of 36% are substantially below the 50% value of the main maximum. Moreover, the axial full width at half-maximum (FWHM) is 95nm, which signifies a more than fourfold improvement compared with confocal microscopy at the same wavelength. Additionally, the lateral resolution is slightly improved as compared with the two-photon excitation mode as demonstrated in the measurement shown in Fig. 3 . This finding is not surprising, since two-photon excitation of the dye requires an excitation wavelength (here 820nm) that is almost doubled.

In conclusion, the advent of objective lenses with ultrahigh semiaperture angles enables 4Pi fluorescence microscopy of biological samples with linear excitation. Cutting implementation costs, this advancement should facilitate a wider application of this superresolving system in biology, especially in light of the fact that our results have been obtained with a commercially available beam scanner. The three-point deconvolution applied in this study just removed the effect of the sidelobes but did not elevate the higher spatial frequencies in the images. The latter can be accomplished by applying a linear[13

13. A. N. Tikhonov and V. Y. Arsenin, Solutions of Ill-Posed Problems (Wiley, 1977).

] or a nonlinear[14

14. W. H. Richardson, J. Opt. Soc. Am. 62, 55 (1972). [CrossRef]

] image deconvolution, in which case the effective axial resolution should be better than 70nm. Finally, we note that the use of higher aperture lenses is likely to significantly improve the optical performance of the I5M as a wide-field 4Pi alternative.[5

5. M. G. L. Gustafsson, D. A. Agard, and J. W. Sedat, J. Microsc. 195, 10 (1999). [CrossRef] [PubMed]

, 6

6. J. Bewersdorf, R. Schmidt, and S. W. Hell, J. Microsc. 222, 105 (2006). [CrossRef] [PubMed]

]

S. Hell’s e-mail address is shell@gwdg.de.

Fig. 1 Axial resolution in 4Pi confocal fluorescence microscopy of type C using one-photon excitation with high-angle immersion lenses: NA 1.20 water, 1.40 oil, and 1.46 oil, as characterized by [(a), left] the axial responses Iz(z) to an ultrathin plane, and [(a), right] the optical transfer function along the inverse optic axis, OTF(kz). (b) Relative height of the primary axial lobes Ilobes of Iz(z), and of OTF(kc) on the semiaperture angle α. An increase of α by 6° (from 68° to 74°) leads to a 7% decrease of the first-order maximum in the PSF and by the same token to an increase of the OTF at the critical frequency kc by 7%. Calculations are for excitation at 488nm and detection at 605nm; the pinhole diameter corresponds to half of that of the Airy disk.
Fig. 2 Measured z response to a fluorescent layer of Qdots (excitation, 488nm; emission, 605nm). The inset in (b) displays the 4Pi confocal xz image of the layer. The measured FWHM of the z response is 95nm. The corresponding OTF, shown in (b), is contiguous and well above zero; at the critical frequency we have OTF(kz)>12%.
Fig. 3 Side-by-side comparison of focal plane (xy) images of 100nm diameter yellow–green fluorescent beads recorded by (a) one-photon (λexc=488nm) and two-photon (λexc=820nm) excitation, respectively. The profiles are taken from the same bead. The comparison indicates a slightly improved lateral resolution in the one-photon excitation case.
Fig. 4 4Pi microscopy at 488nm one-photon excitation. Axial (xz) images show microtubules in a mammalian cell stained with (d)–(f) DY-485XL (emission 560nm) and (a)–(c) Alexa 488 (emission 519nm). (b), (e) Raw data including the effect of the lobes; (a), (d) standard confocal recording; (c), (f) 4Pi image after lobe removal by three-point deconvolution. (h) The intensity profile through a microtubule exhibits a FWHM of 413nm in the confocal mode, whereas in the 4Pi case the corresponding value is 95nm. The Alexa 488 recordings demonstrate (g) that one-photon excitation 4Pi microscopy is possible at low Stokes shifts, thus facilitating dual-color 4Pi imaging with linear excitation.
1.

S. Hell and E. H. K. Stelzer, J. Opt. Soc. Am. A 9, 2159 (1992). [CrossRef]

2.

S. W. Hell, “Double-scanning confocal microscope,” EuropeanPatent0491289 (1990).

3.

H. Gugel, J. Bewersdorf, S. Jakobs, J. Engelhardt, R. Storz, and S. W. Hell, Biophys. J. 87, 4146 (2004). [CrossRef] [PubMed]

4.

M. G. L. Gustafsson, D. A. Agard, and J. W. Sedat, in Proc. SPIE 2412, 147 (1995). [CrossRef]

5.

M. G. L. Gustafsson, D. A. Agard, and J. W. Sedat, J. Microsc. 195, 10 (1999). [CrossRef] [PubMed]

6.

J. Bewersdorf, R. Schmidt, and S. W. Hell, J. Microsc. 222, 105 (2006). [CrossRef] [PubMed]

7.

M. Nagorni and S. W. Hell, J. Opt. Soc. Am. A 18, 36 (2001). [CrossRef]

8.

S. W. Hell and E. H. K. Stelzer, Opt. Commun. 93, 277 (1992). [CrossRef]

9.

S. W. Hell, in Topics in Fluorescence Spectroscopy, J. R. Lakowicz, ed. (Plenum, 1997), p. 361.

10.

M. Martínez-Corral, Micron 34, 319 (2003). [CrossRef] [PubMed]

11.

M. Martínez-Corral, A. Pons, and M. T. Caballero, J. Opt. Soc. Am. A 19, 1532 (2002). [CrossRef]

12.

B. Richards and E. Wolf, Proc. R. Soc. London, Ser. A 253, 358 (1959). [CrossRef]

13.

A. N. Tikhonov and V. Y. Arsenin, Solutions of Ill-Posed Problems (Wiley, 1977).

14.

W. H. Richardson, J. Opt. Soc. Am. 62, 55 (1972). [CrossRef]

OCIS Codes
(100.6640) Image processing : Superresolution
(180.1790) Microscopy : Confocal microscopy
(180.2520) Microscopy : Fluorescence microscopy
(180.6900) Microscopy : Three-dimensional microscopy

ToC Category:
Microscopy

History
Original Manuscript: September 1, 2006
Revised Manuscript: October 21, 2006
Manuscript Accepted: October 26, 2006
Published: January 12, 2007

Virtual Issues
Vol. 2, Iss. 3 Virtual Journal for Biomedical Optics

Citation
Marion C. Lang, Johann Engelhardt, and Stefan W. Hell, "4Pi microscopy with linear fluorescence excitation," Opt. Lett. 32, 259-261 (2007)
http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-32-3-259


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References

  1. S. Hell and E. H. K. Stelzer, J. Opt. Soc. Am. A 9, 2159 (1992). [CrossRef]
  2. S. W. Hell, "Double-scanning confocal microscope," European Patent 0491289 (1990).
  3. H. Gugel, J. Bewersdorf, S. Jakobs, J. Engelhardt, R. Storz, and S. W. Hell, Biophys. J. 87, 4146 (2004). [CrossRef] [PubMed]
  4. M. G. L. Gustafsson, D. A. Agard, and J. W. Sedat, in Proc. SPIE 2412, 147 (1995). [CrossRef]
  5. M. G. L. Gustafsson, D. A. Agard, and J. W. Sedat, J. Microsc. 195, 10 (1999). [CrossRef] [PubMed]
  6. J. Bewersdorf, R. Schmidt, and S. W. Hell, J. Microsc. 222, 105 (2006). [CrossRef] [PubMed]
  7. M. Nagorni and S. W. Hell, J. Opt. Soc. Am. A 18, 36 (2001). [CrossRef]
  8. S. W. Hell and E. H. K. Stelzer, Opt. Commun. 93, 277 (1992). [CrossRef]
  9. S. W. Hell, in Topics in Fluorescence Spectroscopy, J.R.Lakowicz, ed. (Plenum, 1997), p. 361.
  10. M. Martínez-Corral, Micron 34, 319 (2003). [CrossRef] [PubMed]
  11. M. Martínez-Corral, A. Pons, and M. T. Caballero, J. Opt. Soc. Am. A 19, 1532 (2002). [CrossRef]
  12. B. Richards and E. Wolf, Proc. R. Soc. London, Ser. A 253, 358 (1959). [CrossRef]
  13. A. N. Tikhonov and V. Y. Arsenin, Solutions of Ill-Posed Problems (Wiley, 1977).
  14. W. H. Richardson, J. Opt. Soc. Am. 62, 55 (1972). [CrossRef]

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