Fig. 3

Membrane lasers as security features on banknotes. a Barcode-like narrow-band lasing emission from a combination of different second-order gratings with periods ranging from 340 to 360 nm. b Photograph of a £5 banknote with a series of membrane lasers transferred on the transparent window of the banknote. Two second-order distributed feedback (DFB) gratings are indicated by black arrows. Scale bar, 5 mm. c Photograph of same banknote when bent parallel to the grating grooves. d Input−output characteristics of a mixed-order DFB grating membrane laser on a banknote. e Lasing spectra of a banknote with a second-order membrane laser after repeated bending (radius of curvature, ~ 8 mm). Black dashes mark the measured peak wavelength after each bending cycle. f Lasing spectra acquired from a banknote containing a membrane with three different gratings over a time span of 200 days. For clarity, the spectra are offset vertically