# Application of Photo-Induced Chirality in Cover Authentication
## 3 Results and Discussion  
The results of reading the sample label using both SRCD imaging and the standalone reader are presented in Figure 4.  
The light polarization used to write the covert pattern revealed by the standalone reader is in excellent agreement with the results obtained by SRCD imaging.  
The results in Figure 4 demonstrate that the polarization state of a writing laser beam can be successfully recorded in the suggested application of an authentication label and read not only by SRCD imaging, but also by a relatively simple standalone device, with excellent qualitative agreement.  
Although the direction of the polarization of the spots obtained from the SRCD imaging and the reader agree well, the reader measures an increased range of the ellipticities. This increase in signal may be attributed to considerably higher sampling of the label in the reader (~1,000,000 pixels) compared to the SRCD imaging (~1000 pixels), where it is possible that high-intensity signals are spatially averaged with low-intensity signal areas.  
A Gaussian profile of the writing laser beam generates a ring pattern in the spot (Figure 1c), and there are areas within the ring where the CD signal is strongest. According to the results obtained by the reader, the strongest signal is generated by the darker ring of the written spot.  
The averaging in SRCD can also explain why the CD spectra of individual spots (Figure 2) display a lower response at 530 nm than that observed by the reader and also the SRCD imaging. The individual SRCD spectra were collected from arbitrarily selected positions within each spot, which, due to the lower resolution of the SRCD spectroscopy, only provide an averaged and, therefore, lower signal.  
The observed maximum CD signal is strong, about 400 mdeg, as found by the CD reader, even for a ~450 \(\upmu\)m diameter spot. This suggests that a further reduction in size of the written spot is feasible with consequent future miniaturization of this type of authentication label.  
The prepared label showed no deterioration in signal when retested after being stored under ambient conditions for at least six months. Preliminary experiments also indicated  
Figure 4: The hidden code revealed by measuring the circular dichroism response of the barcode expressed as ellipticity in mdeg. (**a**) Measured by SRCD imaging. (**b**) Recorded by a standalone reader. Blue—the circular dichroism (CD) response induced by right–circularly polarized light; red—the CD response induced by left-circularly polarized light; green—the CD response induced by linearly polarized light or that of the as-deposited amorphous film.  
that the label and the polarization signal remained stable even after short heat treatment at 100 \({}^{\circ}\)C, suggesting longer-term stability.  
The reading device was tested under ambient conditions. Although a more thorough investigation is required to establish the influence of temperature and humidity on the device, we believe that moderately higher temperatures and lower humidity will not affect the performance of the reading device and the label.  
The main challenge in scaling up the proposed technology is expected to be the speed of writing individual labels. We suggest that optimization of the laser power, pulse duration and frequency, number of written areas and automated positioning will be required to manufacture such labels at an industrial scale.