Hacking detection based on the elastic properties of liquid crystals in different phases

We present a security device that can detect and block hacking using the characteristics of liquid crystals. This device is based on a liquid crystal cell consisting of a uniformly aligned layer and a photo-alignment layer. To inscribe a pattern, the device is illuminated when the liquid crystal is in the smectic phase. The resulting image is invisible after light irradiation. Heating to the nematic phase improves this alignment and reveals the recorded pattern. Returning to the smectic phase distorts the pattern. Because the pattern is not shown without heating and the trace of the pattern does not disappear once viewed, it is possible to detect whether data has been hacked. The device is easy to fabricate, cost-effective, and sensitive to outside intrusion.


Supplement 2
From the experimental data in Fig. 6(b), the anchoring coefficient on each substrate was estimated. In the experiment, the substrates of PI and P3 are rubbed uniformly at first. After cell fabrication in parallel orientation, photo-alignment rotates the director from rubbing direction to perpendicular to the rubbing. So the rubbing and photo-alignment processes of P3 should be considered.
So, firstly we considered the anchoring coefficient of rubbed and photo-aligned P3 substrate. We checked with the twist cell with perpendicularly rubbed PI and P3. The cell showed deviation angle from the rubbing or photo-aligned direction. W PI_rub and W P3_rub are the anchoring coefficients of the lower (PI) and upper (P3) substrates due to the rubbing and W P3_pho is the anchoring coefficient of P3 due to the photo-alignment. With consideration of W PI_rub >> W P3_rub and W P3_pho , we could estimate the anchoring coefficient of rubbed P3 substrate is obtained with the minimization of free energy consisted of twist deformation and anchoring at both substrates. is the rubbing or photo-aligned direction on the P3 and it is 2 here.
∆ is the deviation angle of director from the easy axis on the P3 substrate. Since there is only twist deformation in the NP, the free energy F per unit area can be expressed as follows [2].
In the above equation, _1 and _2 are easy axis direction of bottom and upper substrates respectively. In this experiment, _1 = 0 and _2 = π/2. θ 1 and θ 2 are director directions in NP on the bottom and upper substrates respectively. The angles deviated from the easy axis on the surface are satisfied the relation |0 -1 | ≪ | /2 -2 | due to the difference in anchoring coefficient in the experiment. And it can be set to 1~0 . And as the rubbed P3 surface is modified by the another photo-alignment, we consider that the anchoring coefficients of rubbed and photo-aligned are changed exponentially as function of irradiated energy [3]. So we put as Here 3_ _ and 3_ ℎ _ are the maximum anchoring coefficients on P3 by rubbing and photo-alignment. And is the energy of irradiated light and is the characteristic energy of anchoring coefficient response. Then, we obtain the 2 = as function of irradiation energy. We fitted the angle adjusting and we obtained = 56 J/cm 2 . In Fig.   6(b), we plotted the fitted 2 as function of irradiated energy. There are limitations in the above calculation: We assumed the anchoring coefficient changes exponentially with irradiated energy independent of the processes. We considered that coefficients of decay of rubbing and growth of photo-alignment are the same even the different background of the alignment.
In the SP, twist of the director is not allowed in the cell, and the directors in each substrate are lined up in an average direction that minimizes the free energy. Actually there is striped pattern and the average direction seems to also reflect the defects. If the direction is θ s ,