Abstract:In this review paper, we report recent progress on Pancharatnam-Berry (PB) phase optical elements, such as lens, grating, and de ector. PB lenses exhibit a fast switching time between two or more focal lengths with large diopter change and aperture size, which is particularly attractive for addressing the accommodation mismatch in head-mounted display devices. On the other hand, PB gratings and de ectors o er a large-angle beam de ection with wide acceptance cone and high e ciency, as compared to conventional volume gratings. Such merits provide great advantages for waveguide-coupling augmented reality headsets. Moreover, the thickness of PB optical elements is only a few micrometers, thus they can be conveniently integrated into modern wearable display systems.

An unprecedented asymmetric acyl-carbamoylation of pendant alkenes tethered on aryl carbamic chlorides with both aliphatic and aromatic aldehydes has been developed via the cooperative catalysis of a chiral nickel-PHOX complex and tetrabutylammonium decatungstate. This reaction represents the first example of merging hydrogen-atom-transfer photochemistry and asymmetric transition metal catalysis in difunctionalization of alkenes. Using this protocol, a variety of oxindoles bearing a challenging quaternary stereogenic center are furnished under mild conditions in highly enantioselective manner.

We report a high performance negative dielectric anisotropy liquid crystal for fringing field switching (n-FFS) display. We compare the electro-optic characteristics of FFS cells using positive and negative LCs. With comparable driving voltage and response time, the n-FFS cell has advantages in higher transmittance, single gamma curve, less cell gap sensitivity and slightly wider viewing angle. LC director deformation distribution is analyzed to explain these performance differences.Index Terms-Fringe field switching, liquid crystal displays (LCD).

Identifying the forces that drive a phase transition is always challenging. The hcp-fcc phase transition that occurs in cobalt at ~700 K has not yet been fully understood, although early theoretical studies have suggested that magnetism plays a main role in the stabilization of the fcc phase at high temperatures. Here, we perform a first principles study of the free energies of these two phases, which we break into contributions arising from the vibration of the lattice, electronic and magnetic systems and volume expansion. Our analysis of the energy of the phases shows that magnetic effects alone cannot drive the fcc-hcp transition in Co and that the largest contribution to the stabilization of the fcc phase comes from the vibration of the ionic lattice. By including all the contributions to the free energy considered here we obtain a theoretical transition temperature of 825 K.Phase transitions are one of the most fundamental phenomena of matter. Understanding the driving forces behind them enables development of new theories, discoveries and tailor-design of new materials. Pressure-induced phase transitions are routinely investigated by means of density functional theory (DFT) based methods 1 . However, it is still a great challenge to describe the mechanisms and forces that control phase transformations induced by a change in temperature.The temperature-induced phase transition in Co is unique among the elements of the periodic table 2 . Right below 700 K, Co undergoes a phase transition from the low temperature hexagonal close-packed (hcp) phase to the high temperature face-centered cubic (fcc) phase 3 . Although, both fcc and hcp phases, are present in the temperature-pressure phase diagram of many elements, for example, rare earths and heavy actinides, only direct transitions between these two phases occur in He, Fe, Co, Tl, Pb and Yb. Moreover, among these few elements, Co is the only one that does not have the body-centered cubic (bcc) phase as well in its phase diagram. This means that standard mechanisms such as the Bain deformation can not be used to describe the phase transition in Co 4 .Magnetism plays an important role in the phase stability of the 3d transition metals [5][6][7][8] . As a matter of fact, the 3d magnetic elements do not follow the crystal structure sequence, hcp → bcc → hcp → fcc from left to right in the periodic table, found in the non-magnetic transition metals. Skriver was able to explain this sequence using a d-band filling argument 9 . Later on, Söderlind et al. extended Skriver's theory to account for the magnetic 3d elements using the fractional filling of both, spin-up and spin-down sub-bands 10 . Following these arguments one can conclude that if Co was not magnetic it would choose the fcc phase as a ground state. Indeed, density functional theory (T = 0 K), shows correctly that the ferromagnetic (FM) hcp is the ground state ( Fig. 1) but if magnetism is ignored the non-magnetic (NM) fcc phase is lower in energy than the NM hcp.The general consensus has been so far that t...

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We report high performance liquid crystal displays (LCDs), including fringe field switching (p-FFS) and in-plane switching (p-IPS), with a small average dielectric constant (ε) but positive dielectric anisotropy material. Our low ε based p-FFS and pIPS LCDs offer several attractive properties, such as high transmittance, low operation voltage, fast response time (even at −20°C), which is particularly desirable for outdoor applications of mobile or wearable display devices, and suppressed flexoelectric effect. Combining these advantages with the inherent outstanding features, such as wide viewing angle, no grayscale inversion, negligible color shift, and pressure resistance, the low ε LC based p-FFS and pIPS are strong contenders for next-generation mobile displays, and high resolution and high frame rate TVs.

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The dynamic response of a polymer-stabilized blue phase liquid crystal (BPLC) is comprised of two distinct processes: Kerr effect-induced local reorientation and electrostriction-induced lattice distortion. A double exponential rise/decay model is proposed to analyze the underlying physical mechanisms. If the electric field is below a critical field (Ec), Kerr effect dominates and the response time is fast. However, when E > Ec electrostriction effect manifests, leading to an increased response time and a noticeable hysteresis. A higher polymer concentration helps suppress electrostriction, but the tradeoff is increased operation voltage. These results provide useful guidelines for future BPLC material and device optimizations.