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Dipartimento di Fisica - Politecnico di Milano

Two-photon polymerization by femtosecond lasers

Fabrication of 3D micro/nanoscale structures is achieved by Two-Photon Polymerization (TPP) under femtosecond laser irradiation. An ultrashort infrared laser pulse is tightly focused in a UV photopolymerizable transparent material. Due to the high intensity in the focus, a two-photon absorption process is induced triggering the photochemical polymerization reaction with resolution down to 100 nm. By moving the focal spot inside the sample, complex patterns of polymerized material can be realized. After the irradiation process, a development step removes the unexposed regions and leaves free-standing 3D microstructures.

The fabrication setup for 2PP encompasses the ultrafast laser, which is a home-built mode-locked Ti:Sapphire laser with 87-MHz repetition rate, 40-fs pulse width, 800-nm central wavelength and 400-mW average power. The writing beam is directed through a power attenuator, a beam expander and a high speed shutter (LS6T2-NL-100, Uniblitz), and is then focused onto the sample using a 1.4 numerical aperture objective (Plan-APOCHROMAT, 100× oil immersion, Carl Zeiss AG). The sample is mounted on a three-axis piezoelectric motion stage (P-611.3 NanoCube, Physik Instrumente) with nanometer resolution and 100 µm × 100 µm × 100 µm travel range. The irradiation for two photon polymerization (2PP) is performed in continuous mode at a constant velocity. The beam is overlapped with a 633 nm Helium Neon laser and a CCD camera is used to observe the back-reflections from the sample in order to achieve fine alignment of the writing beam to the desired area and depth.

The negative 2PP resist is a hybrid organic-inorganic sol-gel(SZ2080) developed in recent years for this technology.  Its low shrinkage properties make it suitable for applications sensitive to structural deformations, since additional design steps are not needed to compensate for distortions. The three dimensional geometry of the laser formed structure is normally observed under scanning electron microscopy (SEM) to fully appreciate the high resolution power of the technology.