Entangled-Photon Fluorescence Microscopy

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9800300 Teich The proposed research pertains to the development of a new form of microscopy: entangled-photon fluorescence microscopy. The recent surge in the development of fluorescence microscopy based on two-photon excitation using classical (or laser) light has been driven by the principal advantages of this technique over single-photon excitation: a pair of low-energy photons can deposit as much energy as a single ultraviolet photon thereby exciting a fluorescent molecule within a sample with greater penetration depth, better resolution, and less risk of damage upon absorption along the optical path. However, in order to obtain two-photon absorption with a classical light source such as a laser, a very large photon-flux density is necessary to place two photons within a small enough volume and within a small enough time window to effect the absorption. In this case, a femtosecond-pulsed high-power laser is used directly &- the source of light, which can produce undesired phototoxicity and photobleaching. On the other hand, entangled light generated by the process of spontaneous parametric downconversion in a nonlinear crystal comprises intrinsically paired photons within a very short time window and small volume. Far smaller photon-flux densities can be used and the absorption of entangled-photon pairs is directly proportional to the photon-flux density, whereas for ordinary two-photon absorption it is quadratic in the photon-flux density. The sum of the photon energies in each pair is a constant and equal to the energy of the downconverted pump photon, whereas the sum energy is far broader for photons from a femtosecond laser. These features lead to unique advantages with the use of nonclassical entangled light when compared to classical light sources like a laser. Simply put, at low photon-flux densities entangled light is a more efficient source of two-photon excitation than a conventional classical laser source. The use of a weak entangled-light source is therefore expec ted to cause less damage than that caused by a direct laser source while providing the same amount of fluorescent light for detection. An experimental investigation of the feasibility of entangled-photon fluorescence microscopy is proposed. Entangled light for fluorophore excitation will be generated using nonlinear parametric downconversion. A basic study of the two-photon absorption rate will be carried out using measurements of the induced fluorescence.