Thermal colour map of 4D smart microtags made by Two-Photon Lithography in cholesteric reactive mesogens

The helicoidal architecture of cholesteric reactive mesogens allows for smart 4D responsive microdevices that reflect selectively the wavelength and polarisation of the incident light. Two-photon lithography is an effective 4D microprinting technology which also enables fine tuning of the Bragg reflection, as a function of the energy delivered during the manufacturing and the polymerisation degree. When CRMs objects undergo temperature variation, thermal expansion causes the elongation of the effective helix pitch and the consequent red-shift of the photonic band gap. A thermal characterisation of the optical response of CRMs microcylinders is presented, showing a tunable red-shift from 2 to 37 nm in the 30–150°C range. The red-shift is enhanced for tall structures, with high polymerisation degree. This characterisation is exploited to add a security label to a 4D cholesteric QR code.