Magued B. Nasr

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.

Quantum-optical coherence tomography with dispersion cancellation

We propose a technique, called quantum-optical coherence tomography (QOCT), for carrying out tomographic measurements with dispersion-cancelled resolution. The technique can also be used to extract the frequency-dependent refractive index of the medium. QOCT makes use of a two-photon interferometer in which a swept delay permits a coincidence interferogram to be traced. The technique bears a resemblance to classical optical coherence tomography (OCT). However, it makes use of a nonclassical entangled twin-photon light source that permits measurements to be made at depths greater than those accessible via OCT, which suffers from the deleterious effects of sample dispersion. Aside from the dispersion cancellation, QOCT offers higher sensitivity than OCT as well as an enhancement of resolution by a factor of two for the same source bandwidth. QOCT and OCT are compared using an idealized sample.

Dispersion-cancelled and dispersion-sensitive quantum optical coherence tomography

Quantum optical coherence tomography (QOCT) makes use of an entangled twin-photon light source to carry out axial optical sectioning. We have probed the longitudinal structure of a sample comprising multiple surfaces in a dispersion-cancelled manner while simultaneously measuring the group-velocity dispersion of the interstitial media between the reflecting surfaces. The results of the QOCT experiments are compared with those obtained from conventional optical coherence tomography (OCT).

Biphoton focusing for two-photon excitation

We study two-photon excitation using biphotons generated via the process of spontaneous parametric down conversion in a nonlinear crystal. We show that the focusing of these biphotons yields an excitation distribution that is the same as the distribution of one-photon excitation at the pump wavelength. We also demonstrate that biphoton excitation in the image region yields a distribution whose axial width is approximately that of the crystal thickness and whose transverse width is that of the pump at the input to the crystal.

Quantum optical coherence tomography of a biological sample

A number of nonclassical (quantum) sources of light have come to the fore in recent years [1], but few practical applications have emerged. One such application is quantum optical coherence tomography (QOCT) [2, 3], a fourth-order interferometric optical-sectioning scheme that makes use of frequency-entangled photon pairs generated via spontaneous optical parametric down-conversion (SPDC). A particular merit of QOCT is that it is inherently immune to group-velocity dispersion (GVD) by virtue of the frequency entanglement of the photon pairs [4]-[6]. Conventional optical coherence tomography (OCT) [7], in contrast, is a second-order interferometric scheme that provides high-resolution axial sectioning by employing ultra-broadband light. Unfortunately, however, this leads to GVD, which degrades resolution. Here we present the first experimental QOCT images of a biological sample: an onion-skin tissue coated with gold nanoparticles. Three-dimensional images are displayed in the form of transverse sections at different depths. The results reveal that QOCT is a viable biological imaging technique.

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.

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.

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.

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.

Generating ultra-broadband biphotons via chirped QPM down-conversion

Nonlinear Optics: Nonlinear optical response has enhanced our ability to produce coherent light throughout the optical spectral region.