Polymer materials offer unique opportunities in nanophotonics and nanobiosystems since both top-down and bottom-up strategies can be pursued and combined towards the nanoscale. Besides, polymer materials can be, with simple methods, functionalized with nonlinear optical or fluorescent materials (organic, inorganic, or metal). The ensemble can be optically structured in a flexible way to obtain a polymer-based photonic nanostructure (host) containing active materials (guest), which may provide an enhancement of the guest optical response, leading to attractive applications. We have developed two important fabrication techniques, namely interference and one-photon absorption direct laser writing, which present different advantages and allow both to obtain desired micro and nanometric 2D and 3D structures. These polymer-based structures are promising for many potential applications, for example, laser, nonlinear optics, and plasmonics.
We demonstrate a novel and very simple method allowing very easy flexible fabrication of 2D and 3D submicrometric structures. By using a photosensitive polymer (SU8) possessing an ultralow one-photon absorption (LOPA) coefficient at the excition laser wavelength (532 nm) and a high numerical aperture (NA = 1.3, oil immersion) objective lens, various submicrometric structures with feature size as small as 150 nm have been successfully fabricated. We have further investigated the energy accumulation effect in LOPA direct laser writing when the structure lattice constant approaches the diffraction limit. In this case, a proximity correction, i.e., a compensation of the doses between different voxels, was applied, allowing to create uniform and submicrometric structures with a lattice constant as small as 400 nm. As compared to commonly used two-photon absorption microscopy, the LOPA method allows to simplify the experimental setup and also to minimize the photo-damaging or bleaching effect. The idea of using LOPA also opens a new and inexpensive way to optically address 3D structures, namely 3D fluorescence imaging and 3D data storage.
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.
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