# Application of Photo-Induced Chirality in Cover Authentication
## 2 Materials and Methods  
Thin-film deposition: To demonstrate the application of chirality in covert authentication labels, pure Ge\({}_{2}\)Sb\({}_{2}\)Te\({}_{5}\) (GST) amorphous film 55 \(\pm\) 3 nm thick (as measured by depth profilometry) was deposited by magnetron radio frequency (RF) sputtering using a commercially sourced GST sputter target (Kurt J. Lesker Company, Clairton, PA, USA) in an argon atmosphere on a 25 mm diameter 2 mm thick LiF disk substrate attached to a rotating stage. The substrate was optically transparent for wavelengths between 180 nmand 600 nm, making it suitable for optical measurements at the maximum of the observed chiral signal at about 530 nm. Label writing: A label consisting of a predesigned pattern of spots was written by inducing phase transitions in the as-deposited amorphous GST film using 8 ns full width at half maximum pulses with a 10 Hz repetition rate of left- and right-circularly polarized light and linearly polarized light generated using an Nd:YAG laser at a wavelength of 532 nm (second harmonic). In the writing setup, a Glan-Taylor prism was used first as a linear polarizer to prepare 100% plane-polarized light. This light was then incident on a quarter-waveplate with the plane of polarization at a 45\({}^{\circ}\) angle with respect to the fast or slow axes of the quarter-waveplate. The laser beam was switched between left- and right-handed circularly polarized states by rotating the quarter-waveplate by 90\({}^{\circ}\). To use the laser beam with linearly polarized light, the quarter-waveplate was removed from the beam path. For writing, the GST film sample was mounted on a movable stage with the film facing the beam and was irradiated by a focused beam of approximately 450 \(\upmu\)m diameter with a fluence of 15.5 mJ/cm\({}^{2}\). Spots with three different polarization states, l-CPL, r-CPL and linear-PL, of the incident beam were written in a barcode pattern. The exposure for each spot was 15 s, which produced the largest CD signal for circularly polarized light. The label design and written labels are shown in Figure 1.  
Synchrotron circular dichroism (SRCD) spectroscopy and imaging: The individual spots on the sample generated using light of opposite chirality and the original GST film were examined by recording the CD spectra in transmission mode. The spectra were acquired using a collimated and focused synchrotron beam of about 100 \(\upmu\)m in diameter in a 200 nm to 600 nm wavelength range with 5 nm intervals. For each spot, scans were  
Figure 1: A demonstration label containing covert code within a visible barcode. (**a**) The schematic label design, with the color representing the polarization of the laser beam used to write the spot. Blue—right-circular polarization; red—left-circular polarization; green—linear polarization. (**b**) The label in a sample of GST film on the LiF substrate disk. (**c**) Enlarged barcode observed with unpolarized white light. recorded with an integration time of 0.5 s for each data point. The resulting spectra are shown in Figure 2.  
To reveal the encoded information, at a single wavelength of 520 nm, at which the CD response has the largest magnitude, an SRCD image was measured in transmission mode from the label area by scanning in a 43 \(\times\) 26 grid with a 200 \(\upmu\)m step [11]. All measurements were performed at the circular dichroism beamline B23 at Diamond Light Source. Standalone label reader: For the proposed label technology to be practical, reading should not require the use of synchrotron radiation. Here, we demonstrate a standalone label reader, as illustrated in Figure 3.  
The reader consists of a green LED (-520 nm) as a light source, collimating optics to illuminate an area of approximately 1 cm\({}^{2}\), a linear polarizer and a rotatable quarter-waveplate, a sample holder and a camera (12 bit, 1000 \(\times\) 1000 pixels, 0.49 cm\({}^{2}\) area sensor).