Anisotropic Fabry-Perot resonators for anti-counterfeiting applications

The prevalence of counterfeited and pirated goods in modern society means that the demand for novel anti-counterfeiting technologies has become tremendous in recent years. Indeed, the global trade of such items in 2015 was estimated to be worth $960 billion, and a danger to 2.5 million jobs.¹ These activities, therefore, place an enormous drain on the global economy. Under these circumstances, much effort has been made in the development of smart security labels for anti-counterfeiting applications. In contrast to conventional labels, these smart security labels are designed to hide information in normal conditions and reveal it in specific viewing conditions. Such viewing conditions arise under specific viewing angles² and polarization states,³ or upon application of external stimuli (e.g., an electric field, a magnetic field,⁴ or mechanical stress⁵).

In the development of such security labels, a number of strategies—based on colloidal photonic crystals,⁶ fluorescent nanostructures,⁷ and plasmonic nanostructures⁸—have so far been demonstrated. In these approaches, the manufacturing process relies primarily on the precise manipulation of nanostructures. However, this significantly limits the scalability and throughput of the manufacturing for practical applications. Moreover, the authentication process is rather complicated, and it requires high-cost and high-resolution facilities. It remains a challenge, therefore, to develop a new type of smart security label that can be recognized with the naked eye and that can be fabricated with a scalable and high-throughput process.

In this work,⁹ we demonstrate an array of Fabry-Perot (FP) resonators as a novel and practical route toward the production of highly efficient and low-cost security labels. In these arrays, we incorporate a liquid crystal polymer (LCP) layer inside each resonant cavity (RC), as illustrated in Figure 1(a). The optical anisotropy of this LCP layer means that the FP resonators behave as bandpass filters,¹⁰ with different peaks of transmittance for two orthogonal polarizations. To record predefined images, we use a photoalignment process to align the LCP molecules in the FP resonator array along different directions that correspond to the image. Depending on the polarization state of the incident white light, we obtain different images (because of the match/mismatch between the effective refractive indices in different image regions). This unique image selection capability therefore provides an ideal platform for anti-counterfeiting applications.

Figure 1. (a) Schematic illustration of a Fabry-Perot (FP) resonator, which includes a liquid crystal polymer (LCP) layer (with refractive index n_LCP) inside the resonant cavity. The yellow double-ended arrows represent the alignment of the LCP molecules. θ: The angle between the polarization (red arrow) and the direction normal to the LCP alignment. (b) Optical transmittance of the FP resonator for two orthogonal polarizations.

The optical transmittance of our FP resonator for two orthogonal polarizations (one parallel and one perpendicular to the LCP alignment) is shown in Figure 1(b). For these measurements, we used an LCP layer and a photoalignment layer that were 603 and 80nm thick, respectively. The results clearly show that we obtained three resonant peaks (at the 5th, 6th, and 7th orders). In addition, we measured a peak transmittance of about 50%. This is significantly higher than that which is achieved with previous approaches for similar devices. In addition, we find that the peak shift, which results from the difference in refractive index (n_eff) for the two orthogonal polarizations, was about 40nm.

To demonstrate the success of our technique, we constructed an array of our anisotropic elemental FP resonators. We then encoded predefined images onto the array so that only one specific image among them was readable, according to the polarization state of the incident light. To do this—see Figure 2(a)—we used a series of photoalignment processes, in which UV light was polarized along three different directions, to record two images (of ‘SNU’ and ‘MIPD’) on the background of the FP resonator array. In particular, we note that the three polarization states were separated equally by an angle of 60°.

Figure 2. (a) Recording conditions of the polarization states for three different regions (i.e., the background, and the ‘SNU’ and ‘MIPD’ images). Microscope images of the FP resonator observed under (b) unpolarized (unpol.) light, and linearly polarized light with a polarization angle of (c) 150°, (d) 30°, and (e) 0°. In (b)–(e) scale bars represent 3mm.

Our microscope images of the FP resonators indicate that under unpolarized light—see Figure 2(b)—no image appeared because the n_eff of the different regions were identical. In contrast, when we illuminated the resonators with the same polarization of incident light that we used for the image recordings, we successfully observed—see Figure 2(c) and (d)—the encoded images. In addition, at the mid-angle between the two polarization states, the SNU and MIPD images—see Figure 2(e)—appeared simultaneously (because a linear combination of the two polarization states occurs at the mid-angle between them). These results therefore illustrate that our approach provides a simple scheme for selecting a specific image and for differentiating information with the naked eye, and without a complicated design or fabrication method.

In summary, we have demonstrated that an array of FP resonators containing an LCP layer in each RC can be used to achieve image selection according to the polarization state of the incident light. To record different images in the array, we use a series of photoalignment processes to align the LCP molecules in the RCs along different directions, in a massively parallel manner, and over a large area. With our approach, a specific image can only be observed when the input polarization coincides with the polarization state of the recorded image. Our LCP-based FP resonator therefore represents a versatile way of producing low-cost security labels for anti-counterfeiting applications. In our future work we will extend our technology to realize a new type of storage media for multiple holographic images and a platform for visual arts.

This work was partly supported through the 2016 BK21 Plus Program of Korea.