Design, fabrication and characterization of a distributed Bragg reflector for reducing the étendue of a wavelength converting system

In this work, the design, fabrication and characterization are reported for a distributed Bragg reflector (DBR) filter with a specific wavelength and angular dependency, which aims to improve the light collection from a wavelength-converter-based light source into a smaller angle than the full angle Lambertian emission. The desired design is obtained by optimizing the transmission characteristics of a multi-layer structure. Titania (TiO2) and silica (SiO2) are used as high and low refractive index materials, respectively. The deposition is made by electron beam evaporation without substrate heating, followed by a post-annealing procedure. The optical properties of the evaporated layers are analyzed by ellipsometer and spectrometer measurements. The angular and wavelength dependency of the fabricated DBR is in good agreement with simulations for the designed structure. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen

Liquid Crystals on Ferroelectric Thin Films

Barium titanate (BTO) and lead zirconate titanate (PZT) are two of the most common ferroelectric materials used in applications. These two materials offer excellent dielectric, piezo-electric, electro-optic and pyro-electric properties. The excellent electro-optic properties of our PZT and BTO deposited thin films may lead to cheap and versatile ultra-fast electro-optic modulators on existing photonic platforms [1], such as the Si or the SiN nanophotonic platform. In this work however, we exploit the extremely high dielectric permittivity of PZT (in the order of 500 to 1000). The permittivity is quasi independent of the underlying substrate material (glass, glass + ITO, glass + Pt, Si, etc.). Liquid crystals exhibit electro-optic effects that are an order of magnitude larger compared to PZT, which makes them ideal materials for use in beam steering applications of focus tunable lenses. In these applications the liquid crystal imposes a spatially varying optical path length to light passing through the liquid crystal layer. By working with a number of separately addressable electrodes the optical path length variation can be accurately controlled. Using multi-electrode designs for example, tunable lenses with high optical quality have been demonstrated. One major problem of multi-electrode designs is the appearance of fringe fields which leads to unwanted behavior of the liquid crystal and may eventually lead to the formation of disclination lines which reduces the optical performance drastically. Using a PZT thin film, we demonstrate that the fringe fields are eliminated and that designs with fewer separately addressable electrodes are necessary. Tunable lenses with a liquid crystal layer integrated on top of a PZT layer are demonstrated [2]. Next to the experimental demonstration we provide numerical simulations of the effect of the high permittivity layer on the liquid crystal. [1] J.P. George, et al. Lanthanide-Assisted Deposition of Strongly Electro-optic PZT Thin Films on Silicon: Toward Integrated Active Nanophotonic Devices. ACS Appl. Mater. Inter. 7 13350-9 (2015) [2] O. Willekens, et al., Ferroelectric thin films with liquid crystal for gradient index applications, Optics Express (submitted

High dielectric constant materials for low power liquid crystal tunable lenses

We have demonstrated the advantageous effect of adding a layer of lead zirconate titanate (PZT) to a vertical field switching (VFS) liquid crystal lens. Simulations show that because of the high dielectric constant of PZT, the voltage profile and hence the director tilt become more smoothly varying, similar to the effect of a potential divider. We fabricated a tunable focal length liquid crystal lens with PZT and compared its performance to that of one without the PZT layer

PZT-based transmissive liquid crystal lens approach

We report the design and fabrication of an electro-optical tunable liquid-crystal-based lens and analysis of its performance. The lens obtains its GRIN profile from multi-electrode addressing using a layer with high dielectric constant to extend and smoothen out the horizontal electric field between the large interelectrode distances

Bridging the electrode gap with ferroelectric thin films

Close to the edges of electrodes, strong electric fields are present which often give rise to unwanted effects in liquid crystal behavior. In liquid crystal microdisplays for projector applications, the gap between electrodes should be as small as possible and the liquid crystal orientation should vary sharply from one pixel to the other in order to achieve the highest possible resolution. In other applications however, a smooth variation in the liquid crystal orientation is desirable and fringe field effects are highly unwanted. Electrode based tunable lenses or beam steering devices are examples in which fringe fields should be suppressed as much as possible. A common way to alleviate the fringe field problem and to smoothen out the spatial variation of liquid crystal orientation is to deposit weakly conductive layers on top of the highly conductive electrodes. In such devices there is a trade-off between high electrical power consumption (high conductivity) and reduced smoothing effect (low conductivity). Also, it is technologically not always straightforward to obtain layers with the desired sheet conductivity. In this work, we demonstrate a new technique exploiting the extremely high dielectric permittivity of ferroelectric thin films based on lead zirconate titanate (PbZrxTi1-xO3, PZT). The high dielectric permittivity of the layer leads to similar effects as the conductivity of a weak conductor. The smoothening and spreading of the electrical field lines between two electrodes is clearly demonstrated in figure 1. At the same time we also prove the excellent reduction of fringe fields near electrode edges. Using multi-electrode designs with a PZT top layer, tunable lenses with high optical quality are demonstrated [1]. Also one-dimensional beam steering is demonstrated. In both cases a reference device was fabricated without PZT layer. The benefit of using a PZT layer is obvious when comparing the PZT and non-PZT devices. [1] O. Willekens, J.P. George, K. Neyts & J. Beeckman, Ferroelectric thin films with liquid crystal for gradient index applications, Optics Express 24, 8088-8096 (2016

Lanthanide-assisted deposition of strongly electro-optic PZT thin films on silicon: toward integrated active nanophotonic devices

The electro-optical properties of lead zirconate titanate (PZT) thin films depend strongly on the quality and crystallographic orientation of the thin films. We demonstrate a novel method to grow highly textured PZT thin films on silicon using the chemical solution deposition (CSD) process. We report the use of ultrathin (5–15 nm) lanthanide (La, Pr, Nd, Sm) based intermediate layers for obtaining preferentially (100) oriented PZT thin films. X-ray diffraction measurements indicate preferentially oriented intermediate Ln2O2CO3 layers providing an excellent lattice match with the PZT thin films grown on top. The XRD and scanning electron microscopy measurements reveal that the annealed layers are dense, uniform, crack-free and highly oriented (>99.8%) without apparent defects or secondary phases. The EDX and HRTEM characterization confirm that the template layers act as an efficient diffusion barrier and form a sharp interface between the substrate and the PZT. The electrical measurements indicate a dielectric constant of ∼650, low dielectric loss of ∼0.02, coercive field of 70 kV/cm, remnant polarization of 25 μC/cm2, and large breakdown electric field of 1000 kV/cm. Finally, the effective electro-optic coefficients of the films are estimated with a spectroscopic ellipsometer measurement, considering the electric field induced variations in the phase reflectance ratio. The electro-optic measurements reveal excellent linear effective pockels coefficients of 110 to 240 pm/V, which makes the CSD deposited PZT thin film an ideal candidate for Si-based active integrated nanophotonic devices

Improvement of liquid crystal tunable lenses with weakly conductive layers using multifrequency driving

A common technique to realize the gradient electric field profile that is required in liquid crystal tunable lenses is the use of a weakly conductive layer. Thanks to this layer, an applied voltage with a certain frequency allows us to obtain a refractive index profile that is required for the lens operation. Due to the limited degrees of freedom, however, it is not possible to avoid aberrations in a weakly conductive layer-based tunable lens for a continuously tunable focal length. In this work, we discuss the use of additional higher frequency components in the voltage signal to reduce the lens aberrations drastically. (C) 2020 Optical Society of Americ