Institut Fresnel

Focusing a light beam with a lens produces a spot that is strongly elongated along the optical axis, because the illumination is not uniformly spherical but comes from only one side of the focal point. This anisotropy has plagued all optical instruments for more than 100 years. In the domain of data storage, it is limiting strongly the density of optical storage media (DVD, Blu-Ray).

In a recent article [1], we describe a new and surprisingly simple idea to focus light into a spherical spot using a single lens. Using time reversal concept, we show theoretically and experimentally that isotropic focusing can be achieved by placing a mirror after the focal point and engineering the incident beam.

The principle of the method is explained in the movie below.

This video can be downloaded here (14Mb)

This approach, that we call ISO focusing (for Isotropic Single Objective) [2], has been applied to confocal microscopy and brings about a dramatic improvement of the axial resolution [3]. It has been recently extended to two-photon excitation microscopy [4]. It should open the way to exciting applications, in particular in three-dimensional imaging or data storage (higher density of layers in DVD or Blu-Ray)

Figure 1 : Left : Conventional focusing produces an elongated spot. Right : ISO focusing produces a spherical spot.

Figure 2 : Improvement of the axial resolution. The XZ section of the 3D Point spread function of a confocal microscope (left), is elongated along the Z direction. In ISO microscopy, it is spherical, with sidelobes, like in 4pi microscopy. Measurements (right) are in good agreement with theroretical predictions (middle).

References

[1] E. Mudry, E. Le Moal, P. Ferrand, P.C. Chaumet, A. Sentenac, "Isotropic diffraction-limited focusing using a single objective lens", Phys. Rev. Lett. 105, 203903 (2010) - PDF
[2] Patent pending
[3] E. Le Moal, E. Mudry, P. C. Chaumet, P. Ferrand, and A. Sentenac, "Isotropic Single Objective (ISO) microscopy : Theory and Experiment", J. Opt. Soc. Amer. A Vol. 28, pp.1586 (2011) - PDF
[4] E. Le Moal, E. Mudry, P. C. Chaumet, P. Ferrand, and A. Sentenac, "Two-photon fluorescence isotropic-single-objective microscopy", Opt. Lett. 37, 85 (2012) - PDF

Press coverage

 "Mirror trick could boost Blu-ray discs" in NewScientist, 17 nov 2010
 Highlighted as publication of the week by the Cell Observatory facility at Leiden University, NL

People involved

ISO microscopy is a joint project between the two teams of the Fresnel institute : SEMO for time-reversal theory, and MOSAIC for the design of the experiment
 Patrick Ferrand
(MOSAIC)
 Anne Sentenac (SEMO)
 Eric Le Moal (MOSAIC/SEMO)
 Emeric Mudry (SEMO)

Contact

Dr. Patrick Ferrand, E-mail