Rapid Laser Optical Printing in 3D at a Nanoscale

Three-dimensional (3D) laser structuring ofmaterials is widely used in photopolymer prototyping applications including micro-optical elements, parts of optically actuated micromachines in microfluidics, scaffolds for cell growth, templates for plasmonic andmetamaterials, and photonic crystals (PhCs) [1–7].The technique originated from nonlinear microscopy, providing 3D confined recording inside UV-sensitive polymers [8]. Now it is emerging as the most precise and true 3D printing technology in both scientific and industrial fields [9–12]. In order to achieve the resolution of structuring required for PhC structures operational at the visible spectral range, the feature sizes of 3D structures should be smaller than ∼100 nm in all cross sections.The surface quality has to be even higher in terms of surface roughness, and in some applications the volume fraction of polymer has to be only 30% in a 3D PhC, which are very demanding requirements [13–15]. For this aim, the optical means of light beam delivery as well as the material’s response should be precisely engineered to control the resolution, but without compromising the fabrication throughput in cubicmicrometers per hour (in three dimensions) or square micrometers per hour(in two dimensions). The mask writing by electron-beam lithography (EBL) has a throughput of ∼105μm2h−1 for a current 22-nm node in microelectronics [16]. The most popular materials in the case of 3D laser photopolymerization are acrylateand epoxy-based resins, which were developed decades ago before the era of tabletop femtosecond lasers for one-photon stereolithography. The photopolymers are photosensitized for the wavelength of an eximer laser emitting at 308 nm or the i-line of a Hg light source at 360 nm. The photosensitization and initiation of nonlinear photopolymerization are fundamentally different from the one-photon processes in the case of ultrashort laser pulses at longer wavelengths. The excitation of the electronic subsystem occurs faster than the absorption of energy into ionic subsystem proceeding via electron–ion equilibration, recombination, and thermal diffusion in the case of ultrashort sub-picosecond laser pulses.