Ultrabroadband coherence-domain imaging using parametric downconversion and superconducting single-photon detectors at 1064 nm.

Coherence-domain imaging systems can be operated in a single-photon-counting mode, offering low detector noise; this in turn leads to increased sensitivity for weak light sources and weakly reflecting samples. We have demonstrated that excellent axial resolution can be obtained in a photon-counting coherence-domain imaging (CDI) system that uses light generated via spontaneous parametric downconversion (SPDC) in a chirped periodically poled stoichiometric lithium tantalate (chirped-PPSLT) structure, in conjunction with a niobium nitride superconducting single-photon detector (SSPD). The bandwidth of the light generated via SPDC, as well as the bandwidth over which the SSPD is sensitive, can extend over a wavelength region that stretches from 700 to 1500 nm. This ultrabroad wavelength band offers a near-ideal combination of deep penetration and ultrahigh axial resolution for the imaging of biological tissue. The generation of SPDC light of adjustable bandwidth in the vicinity of 1064 nm, via the use of chirped-PPSLT structures, had not been previously achieved. To demonstrate the usefulness of this technique, we construct images for a hierarchy of samples of increasing complexity: a mirror, a nitrocellulose membrane, and a biological sample comprising onion-skin cells.

Submicron axial resolution in an ultrabroadband two-photon interferometer using superconducting single-photon detectors.

We generate ultrabroadband biphotons via the process of spontaneous parametric down-conversion in a quasi-phase-matched nonlinear grating that has a linearly chirped poling period. Using these biphotons in conjunction with superconducting single-photon detectors (SSPDs), we measure the narrowest Hong-Ou-Mandel dip to date in a two-photon interferometer, having a full width at half maximum (FWHM) of approximately 5.7 fsec. This FWHM corresponds to a quantum optical coherence tomography (QOCT) axial resolution of 0.85 ?m. Our results indicate that a high flux of nonoverlapping biphotons may be generated, as required in many applications of nonclassical light.

Photon-counting optical coherence-domain reflectometry using superconducting single-photon detectors.

We consider the use of single-photon counting detectors in coherence-domain imaging. Detectors operated in this mode exhibit reduced noise, which leads to increased sensitivity for weak light sources and weakly reflecting samples. In particular, we experimentally demonstrate the possibility of using superconducting single-photon detectors (SSPDs) for optical coherence-domain reflectometry (OCDR). These detectors are sensitive over the full spectral range that is useful for carrying out such imaging in biological samples. With counting rates as high as 100 MHz, SSPDs also offer a high rate of data acquisition if the light flux is sufficient.

Ultrabroadband biphotons generated via chirped quasi-phase-matched optical parametric down-conversion.

We generate ultrabroadband biphotons via the process of spontaneous parametric down-conversion (SPDC) in quasi-phase-matched nonlinear gratings that have a linearly chirped wave vector. By using these ultrabroadband biphotons (300-nm bandwidth), we measure the narrowest Hong-Ou-Mandel dip to date, having a full width at half maximum of 7.1 fs. This enables the generation of a high flux of nonoverlapping biphotons with ultrabroad bandwidth, thereby promoting the use of SPDC light in many nonclassical applications.

Broadband light generation by noncollinear parametric downconversion.

Broadband light generation is demonstrated by noncollinear spontaneous parametric downconversion with a cw pump laser. By use of a suitable noncollinear phase-matching geometry and a tightly focused pump beam, downconverted signals that feature a bell-shaped spectral distribution with a bandwidth approaching 200 nm are obtained. As an application of the generated broadband light, submicrometer axial resolution in an optical coherence tomography scheme is demonstrated; a free-space resolution down to 0.8 microm was achieved.

Demonstration of dispersion-canceled quantum-optical coherence tomography.

We present an experimental demonstration of quantum-optical coherence tomography. The technique makes use of an entangled twin-photon light source to carry out axial optical sectioning. It is compared to conventional optical coherence tomography. The immunity of the quantum version to dispersion, as well as a factor of 2 enhancement in resolution, is experimentally demonstrated.