Isotropic sub-100nm direct laser writing using spherical reflector-enabled 4Pi excitation at 405nm wavelength

We demonstrate a direct laser writing setup combining 405 nm multi-photon lithography with 4Pi excitation enabled by a spherical reflector (SR) refocussing the transmitted excitation. The SR provides a simplified implementation of the 4Pi geometry, avoiding the need for an additional objective and its interferometrically stabilised excitation beam path, while also recycling the beam power. The reflected beam position is measured by imaging the reflected beam and is controlled by a feedback loop to 10nm in all three dimensions. Using this instrument, the fabrication of sinusoidally modulated nanowires and helicoids with sub-100nm near-isotropic cross-section is demonstrated.