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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Focussing of momentum entangled photon pairs

Daniel Schlenk, Harald Weinfurter

Published Jun 1, 2009• 0 citations
Publisher

Abstract

The resolution of optical scanning microscopes is limited by the spot size of the scanning beam. It has been proposed that momentum entangled photons can be focused more tightly than classical light. We use a confocal microscope to measure the single and two photon distribution in the focus.
DOI
10.1109/cleoe-eqec.2009.5191628

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The resolution of optical scanning microscopes is limited by the spot size of the scanning beam. It has been proposed by Boto1 et al. and Nasr2 et al. that momentum entangled photons can be focused more tightly than classical light. Thus they should enable one to break the di ractive resolution limit. We report experiments measuring the spatial distribution of strong focussed momentum entangled photon pairs at a center wavelength of 812 nm from a spontaneous parametric down conversion source. Using a single gold stripe deposited on a glass surface as test object we observe a two-photon spot size as small as 194 nm, thereby beating the di raction limit even for conventional two-photon microscopy.
All forms of microscopy that have previously been developed rely on classical sources of light, which are relatively noisy by virtue of the photons arriving randomly in time and position. We propose a new technique of two-beam optical microscopy using a nonclassical light source comprising entangled photon pairs generated by the process of nonlinear optical parametric downconversion. Entangled-photon microscopy (EPM) has the potential for providing a substantial improvement over confocal laser scanning fluorescence microscopy (CLSM) as well as over two-photon laser scanning fluorescence microscopy (TPLSM). The latter technique relies on the proportionality of the two-photon absorption rate to the square of the optical photon-flux density, and makes use of a highly focused optical beam to localize the region from which fluorescence is observed. However, because the arrival of pairs of photons is accidental in TPLSM, relatively large values of the photon-flux density are required to achieve photon coincidence and hence absorption. This can lead to undesired phototoxicity and photobleaching of the specimen. EPM, in contrast, relies on the presence of highly correlated photon pairs, which enhances the two-photon absorption rate significantly and enables lower values of the photon-flux density to be used. The process depends linearly, rather than quadratically, on the photon-flux density. Further, the EPM interaction region can be controlled by the use of two beams, providing increased flexibility in the selection of desired regions of a specimen, and leading to enhanced axial and lateral resolution.
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A recently introduced two-channel confocal microscope with correlated detection promises up to 50% improvement in transverse spatial resolution [Simon, Sergienko, Optics Express 18, 9765 (2010)] via the use of photon correlations. Here we achieve similar results in a different manner, introducing a triple-confocal correlated microscope which exploits the correlations present in optical parametric amplifiers. It is based on tight focusing of pump radiation onto a thin sample positioned in front of a nonlinear crystal, followed by coincidence detection of signal and idler photons, each focused onto a pinhole. This approach offers further resolution enhancement in confocal microscopy.
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