Unclonable human-invisible machine vision markers leveraging the omnidirectional chiral Bragg diffraction of cholesteric spherical reflectors

Abstract

The seemingly simple step of molding a cholesteric liquid crystal into spherical shape, yielding a Cholesteric Spherical Reflector (CSR), has profound optical consequences that open a range of opportunities for potentially transformative technologies. The chiral Bragg diffraction resulting from the helical self-assembly of cholesterics becomes omnidirectional in CSRs. This turns them into selective retroreflectors that are exceptionally easy to distinguish—regardless of background—by simple and low-cost machine vision, while at the same time they can be made largely imperceptible to human vision. This allows them to be distributed in human-populated environments, laid out in the form of QR-code-like markers that help robots and Augmented Reality (AR) devices to operate reliably, and to identify items in their surroundings. At the scale of individual CSRs, unpredictable features within each marker turn them into Physical Unclonable Functions (PUFs), of great value for secure authentication. Via the machines reading them, CSR markers can thus act as trustworthy yet unobtrusive links between the physical world (buildings, vehicles, packaging,…) and its digital twin computer representation. This opens opportunities to address pressing challenges in logistics and supply chain management, recycling and the circular economy, sustainable construction of the built environment, and many other fields of individual, societal and commercial importance.

Heads or tails: investigating the effects of amphiphile features on the distortion of chiral nematic liquid crystal droplets

“…15 By using CLC materials with a pitch on the same order as the wavelength of visible light, this response can manifest itself in the form of strongly reflected colors that can be seen by the unaided eye. [16][17][18][19] The mechanism of how the response is generated depends on what is being sensed: with gas sensing, often one sees a phase transition in the LC due to a depression of the clearing point temperature at which the liquid crystal phase becomes lost. 9,13,16,20 On the other hand, amphiphile sensing typically looks at a change in the alignment and orientation of the LC induced by the adsorption of a surface-active molecule at the interface.…”

“…When chiral nematic (or cholesteric) liquid crystals (CLCs) -which have an additional helical modulation along with the long-range orientational order -are used, different textures can emerge 15 . By using CLC materials with a pitch on the same order as the wavelength of visible light, this response can manifest itself in the form of strongly reflected colors that can be seen by the unaided eye [16][17][18][19] . The mechanism of how the response is generated depends on what is being sensed: with gas sensing, often one sees a phase transition in the LC due to a depression of the clearing point temperature at which the liquid crystal phase becomes lost 9,13,16,20 .…”

Pixelating Structural Color with Cholesteric Spherical Reflectors

“…The first problem is much reduced if the structurally colored material is produced not in the form of flat films-as is usually the case-but rather in the shape of spheres, with radial orientation of the symmetry axis. [24] This is achieved particularly easily using cholesteric liquid crystals (CLCs), [19,22] since these longrange ordered and birefringent liquids can be processed into spherical droplets just like with any other liquid, [25][26][27][28][29][30][31][32] for instance using microfluidic or other emulsification techniques, and since they spontaneously organize with a helical organization of the optic axis that modulates the refractive indices with a period p that is easily adjusted from %100 nm to several tens of microns by varying the CLC mixture composition. The helical structure gives rise to Bragg diffraction that is selective not only in wavelength but also in polarization: only the circularly polarized component of light with the same handedness as the cholesteric helix is Bragg-diffracted.…”

Impact of mesogenic aromaticity and cyano termination on the alignment and stability of liquid crystal shells

“…17,18 They also hold potential for various applications, in particular when using elastomeric, 19,20 photoresponsive 21 or cholesteric liquid crystals. [22][23][24][25] Applied work generally requires the use of specific liquid crystal molecules (mesogens) that may be different from the typical research mesogens, to introduce new functionalities such as polymerizability, [26][27][28][29][30] or to tune key properties like refractive indices, birefringence, phase transition temperatures or helix pitch of cholesteric shells. 25,30 This need exposes a great knowledge gap in the field, because almost all studies of liquid crystal shells have been made with a single class of mesogens, namely the popular cyanobiphenyl-based n(O)CB family, where n is the length of the single alkyl (CB)/alkoxy (OCB) chain, most often with n = 5 or n = 8, see Table 1.…”

“…[22][23][24][25] Applied work generally requires the use of specific liquid crystal molecules (mesogens) that may be different from the typical research mesogens, to introduce new functionalities such as polymerizability, [26][27][28][29][30] or to tune key properties like refractive indices, birefringence, phase transition temperatures or helix pitch of cholesteric shells. 25,30 This need exposes a great knowledge gap in the field, because almost all studies of liquid crystal shells have been made with a single class of mesogens, namely the popular cyanobiphenyl-based n(O)CB family, where n is the length of the single alkyl (CB)/alkoxy (OCB) chain, most often with n = 5 or n = 8, see Table 1. Mesogens typically consist of two distinctive units with antagonistic features: 31 (1) a rigid core comprising at least two cyclic hydrocarbons, often but not necessarily aromatic, sometimes carrying lateral substituents for tuning certain physical or chemical properties; (2) flexible terminal chains, usually alkyl or alkoxy chains, linked to the outside of the core.…”