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arXiv:1005.1576 · Optics Express

Twin-photon confocal microscopy

David S. Simon, Alexander V. Sergienko

Published Oct 5, 2010• 12 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1364/oe.18.022147

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Novel imaging techniques utilizing nondegenerate, correlated photon pairs sparked intense interest during the last couple of years among scientists of the quantum optics community and beyond. It is a key property of such “ghost imaging” or “quantum interference” methods that they use those photons of the correlated pairs for imaging that never interacted with the sample, allowing detection in a spectral range different from that of the illumination of the object. Extensive applications of these techniques in spectroscopy and microscopy are envisioned, however, their limited spatial resolution to date has not yet supported real-life microscopic investigations of tiny biological objects. Here we report a modification of the method based on quantum interference by using a seeding laser and confocal scanning, that allows the improvement of the resolution of imaging with undetected photons by more than an order of magnitude, and we also present examples of application in the microscopy of biological samples.
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.
Higher-order correlations of the radiation field improve resolution in stellar interferometers, as in the Hanbury-Brown–Twiss effect. It is also possible to improve microscopic resolution beyond the Rayleigh limit by using quantum light fields composed of entangled photons. Focusing on two photons, we distinguish two types of entanglement: frequency entanglement, where the photons in different paths are correlated in frequency, and path entanglement, where the correlation between paths is in photon number. Two paradigms of quantum microscopy are discussed: spectral microscopy, where path- and frequency-entangled photons produced in cascade decay of two atoms make possible sub-natural linewidth resolution of atomic levels, and spatial microscopy, where path-entangled photons emitted by an atomic array produce sub-wavelength diffraction resolution as compared to an equivalent classical grating. These scenarios require two-photon correlation or coincidence measurements. The connection between the two paradigms, and the two types of entanglement, highlights the link between the temporal and spatial aspects of quantum interferometry.