This PDF file contains the front matter associated with SPIE Proceedings Volume 12023, including the Title Page, Copyright information, Table of Contents and Conference Committee list.

Liquid Crystals are birefringent materials, which address many applications such as visualization with Liquid Crystal Display (LCD) or beam shaping with Liquid Crystal on Silicon devices (LCoS). Recently, several research teams proposed using liquid crystals in photonics devices applied to new kinds of projection displays. Augmented Reality (AR) is one of the domains, which could benefit from these developments, thanks to the necessity to create active and transparent optical function. In this contribution, we present recent works at CEA Leti to develop a switchable photonic extraction grating adapted to a specific near-eye device. Two different technics are detailed and studied with FDTD simulations. We also show first experimental characterization of an impregnated diffraction grating used in a free space optical set-up

There is a need for a tunable lens technology in AR/VR head mounted displays to solve the accommodation-convergence mismatch issue. Liquid crystal lens technology is one of the promising approaches to achieve a simple, low cost and compact design. However, it is challenging to make a lens with a 5 cm aperture with the required variable power range of 0 D to 2.50 D, with an acceptable switching speed. In the device considered here, we use multiple cell design with segmented phase profile to achieve a tunable focus ability with switching speed under 500 ms. To simplify the electronics of the device, inner-ring resistor network have been used between concentric electrodes. Previously, the effect of the electrode gaps has been studied and the improvement with floating electrodes demonstrated for lower power and smaller aperture (2 cm) LC lenses. In this paper, the effect of the fringing electric fields associated with the discrete electrodes are explored through modeling and imaging characterization, and an improved fabrication process are detailed.

Using the principle of photo-alignment it has become possible to pattern the orientation of liquid crystal on a substrate. With this approach liquid crystal gratings and lenses with high optical quality have been realized. Recently we have demonstrated that the period of the photo-alignment can be reduced below one micrometer. Chiral liquid crystal that is deposited on this photo-alignment layer will tilt its helical axis and circularly polarized light is diffracted over a certain angle with high yield. The resulting device behaves as a volume hologram with interesting properties: high reflectivity in a certain wavelength range, tunable diffraction angle and low scattering.

Photoalignment on azo dye films is a simple and efficient way to achieve anisotropic ordering of molecules, which is promising to achieve ultra-thin high-performance dichroic polarizers. Besides, the photoalignment technique shows a unique advantage on patterned alignment applications, and enables the realization of advanced optical elements such as liquid crystal (LC) Dammann grating, LC Fresnel lens, holographic polarizers and bifocal optical-vortex lens.

A possible driving torque investigation of an SSD (Smectic Single Domain) liquid crystal panels has been investigated. In addition to the previous investigations, a prove light wavelength dependence of dynamic retardation switching behaviors were clarified both by means of optical light throughput profile and polarity switching behavior. The result shows that SSD liquid crystal molecular directors switching is always perpendicular to the applied electric filed, which strongly suggests quadrupole moment origin driving torque.

Switchable optical elements incorporating polymer structure and a liquid crystal material offer devices with a voltage controlled phase difference at low cost. The polymer structure may be fabricated using established replication techniques. Earlier work has demonstrated such devices with good optical quality and high diffraction efficiencies. Such a material allows the director to be actively driven either towards homeotropic or planar alignment depending upon the frequency of the drive waveform. For this investigation devices were fabricated using a dual frequency liquid crystal with a polymer structure. Use of a liquid crystal material with a dielectric anisotropy inversion has the advantage in this case that the liquid crystal orientation is less dependent upon surface forces for switching effects. Liquid crystal reorientation and therefore optical switching is field driven into different orientations. Measurement s of the intensity of the transmitted diffraction orders as a function of time were carried out as means to compare the liquid crystal reorientation times for devices with different gratings. The same devices were driven in two ways; changing driving frequency so switching was always field driven or by changing applied voltage so that switching was partly determined by surface forces. It would be expected to observe faster optical switching due to an applied field; this was not always observed to be the case, this may be due to differences in the initial state between field dominated and surface force dominated initial conditions. Work to understand this more fully is in progress.

We demonstrate a method combining direct ink writing of a chiral nematic liquid crystal oligomer ink with a photolithography step in a procedure that gives the user great flexibility in the design of striking polymer optics. The printable chiral nematic ink is first made by oligomerizing a reactive nematic liquid crystal monomer with a reactive chiral dopant. This ink can be readily printed using direct ink writing and quickly forms the cholesteric phase after deposition. Increasing the lateral nozzle speed forces the helical chiral nematic alignment into a slanted configuration, which has deviating optical properties compared to conventional planar chiral nematic liquid crystal reflectors—when inspected from the surface normal, these slanted photonics do not display circular polarization dependence, a feature characteristic for chiral nematics. The direction of this slant is determined by the printing direction, and thus highly customizable. For instance, when printing in a single direction, the peak reflected wavelength λmax is only seen at ca. 50° angle of incidence. Writing a back-and-forth pattern turns the object into an “inverse cholesteric” in two dimensions, where the longest reflected wavelength is seen from 50° in either direction. More intricate print path designs lead to more complex appearances. With a two-step photo-crosslinking procedure using masks, two-dimensional designs can be imprinted in direct ink written coatings, resulting in highly complex but programmable reflection patterns.

This communication presents some recent advances in the use of liquid-crystal on silicon (LCoS) spatial light modulators (SLM) to filter and control the broadband light spectrum of a supercontinuum (SC) laser source. An optical system is presented that first disperses the input laser and the spectrally separated components are projected onto the LCoS-SLM, where the state of polarization of each wavelength is separately modulated. This polarization control is based on the dynamic control of the spectral retardance function of the LCoS device. Then, recombining the reflected modulated spectral components results in an output laser source where the spectrum can be controlled dynamically from a computer. A new system is presented that covers the visible (VIS) and near the near infrared (NIR) bands. The system incorporates two branches to independently control the VIS and NIR spectral content, thus providing a programmable control of the SC laser source spectrum from 450 nm to 1600 nm. This new ability for controlling at will the wide spectra of the SC laser sources can be extremely useful for biological imaging applications.

Since the discovery of dye lasers, constant efforts are made towards developing optical gain materials with improved performance. Choosing an optical gain material for application in high-performance cholesteric liquid crystal micro-lasers requires a dedicated screening method. We employ Amplified Spontaneous Emission (ASE), and time-dependent fluorescence under pulsed excitation to study the optical gain parameter and photo-stability of laser dyes pyrromethene 567, pyrromethene 580, pyrromethene 597, pyrromethene 650, DCM, and Nile Red, dissolved in a nematic liquid crystal 5CB. Optical gain of CdSe/ZnS coreshell quantum dots dispersed in dodecene and CdSe/CdS coreshell quantum rods, dispersed in toluene was also measured and found to be much lower compared to the gain of fluorescent dyes. Pump energy and polarization dependence of ASE from dye molecules incorporated in the planar-aligned liquid crystal matrix is addressed. We determine the optical gain coefficient for different laser dyes by using the variable laser stripe illumination method using the small-signal gain model. The polarization dependence of ASE is determined in various geometries. We depict photostability from the half-life of emission intensity decay over a large number (10⁶-10⁷) of repeated laser pulses for fluorescent dyes, quantum rods and quantum dots. We find that quantum rods and quantum dots are much more stable compared to fluorescent dyes. Based on the photophysical characterization, we select pyrromethene 567 and pyrromethene 597 as efficient laser dyes for nematic liquid crystal micro-lasers.

Crosslinked Liquid Crystal Elastomers (CrLCEs) are at the forefront of the development of optoelectronics and photonics. Over the years, a materials toolset has been developed ranging from bulk synthesised nematic monomers to dedicated smectic and chiral monomers with advanced applications such as polarisation optics, iridescent coatings, and photo responsive mechanics. CrLCEs are often prepared through a two-step approach. First, conventional reactive liquid crystal monomers are chain-extended into (short) oligomer chains which are then crosslinked to form the elastomer network. The advantage of this approach is that it gives access to a wide range of processing methods, including conventional techniques suited for liquid crystal monomers, but also emerging processing techniques such as 3D- or roll-to-roll printing. Adding chiral molecules to a nematic oligomer forms the chiral nematic or cholesteric liquid crystal phase, well-known for its characteristic helicoidal ordering of the calamitic molecules. Currently we are interested in the effect of the position of the chiral component in relation to the elastomer network: either as part of the oligomer main chain or as a pendant group to it. In this contribution we found that the dopant position may influence the way in which the cholesteric material aligns. The main chain dopant follows behaviour previously reported in similar systems, while the side chain dopant adds more process-related complexity to the optical properties. With this work we aim to add a new design consideration to the already versatile platform of CrLCEs.

Today, liquid crystals (LC) and LC polymers (LCP) are the basis of display technologies and widely use in research laboratories for creation of new generation optical elements. In the above-mentioned technologies LC cells are used as a check sample. Having rich experience in the field, we are well acquainted with devices used for LC cells making and investigation. In particular, cell filling is mainly carried out in non-standard conditions using the technical capabilities of laboratories, and to study time and electro-optical characteristics standard generators and oscillographs are used. In this paper our developed devices for LC cells making and investigation are presented. Currently, research laboratories work with high-quality, expensive LC materials, and their use without losses is an urgent task. In addition, when filling LC cells, it is necessary to strictly control technological parameters and visually observe the process. In our developed “LC Vacuum Filler” device all these tasks are solved in a complex manner. Another device, “LC Driver” is designed to generate signals of various shapes, to control the operation of LC devices and to register phase change under influence of control signals. The following technical opportunities are available: selection and configuration of various waveforms, registration of control voltage amplitude and signal from output of photodiode, smooth change of control voltage within the given range by the selected rate, registration of phase shift vs. the applied voltage. To ensure the required temperature conditions, the device is equipped with a temperature stabilization unit based on Peltier elements.