Three-dimensional optical addressing by ultra low one-photon absorption microscopy

Abstract

The concept of three-dimensional (3D) optical addressing based on low one-photon absorption (LOPA) microscopy is theoretically and experimentally studied in detail. The numerical calculation results show that the intensity distribution of focused light beam strongly depends on the absorption of studied materials and on the numerical aperture of employed objective lenses. Obviously, in the case of LOPA based miscroscopy, with a significant low linear absorption and tight focusing conditions, the light intensity is not affected by absorption and the focused beam shape and intensity remain almost the same everywhere inside the absorbing material. This allows to use LOPA microscopy to perform a highly resolved focusing spot in three dimensions as achieved by the two-photon absorption (TPA) technique. The theoretical calculations were then experimentally verified by focusing different light sources with different objective lenses into a Rhodamine 6G (Rh6G) solution. The experimental results show that it is impossible to realize a deep focusing inside the Rh6G solution when using a laser wavelength at 532 nm (high absorption), even if the light beam is tightly focused. In contrast, by using a light source (coherent or incoherent), emitting a wavelength located in the low absorption range of the Rh6G solution, for example, at 633 nm, the light beam can be focused deeply inside the solution, similar to the result obtained by using a pulsed laser at 1064 nm (TPA). The LOPA based microscopy presents therefore a great advantage over conventional OPA and TPA methods. Indeed, since it operates on the basis of a one-photon absorption mechanism, it does not require an expensive pulsed light source, but only a simple setup and a low-cost, low power continuous laser source, as in the case of standard OPA. It, however, allows to deal with submicrometric 3D imaging and 3D fabrication, as well as 3D data storage, with similar performances as those obtained with TPA microscopy.