Transmissive Liquid Crystal Research Articles

Numerical study of transmissive liquid crystal Pancharatnam–Berry gratings with small periods

Advances in patterning molecular orientations in liquid crystals have made possible the fabrications of various high-quality flat optical elements, ranging from gratings to beam shaping devices. These devices are based on modulating the Pancharatnam–Berry (PB) phase. It is well known that the diffraction efficiency for liquid crystal PB optical elements decreases for large diffraction angles. Here we study this issue using numerical simulations based on finite-difference time-domain algorithms and show that the optical efficiency formula commonly used for liquid crystal gratings deviates significantly from numerical results and is inaccurate for small grating periods because of its assumption that light beams do not diffract inside the liquid crystal films. We show that, for large diffraction angles, the polarization of the diffracted beam is generally elliptically polarized, and we further show that, by using liquid crystal materials with larger birefringence Δ n ∼ 0.4 , the diffraction efficiency for a grating period of 0.8 µm can go above 90%, which can be used for various flat optical elements with numerical aperture ∼ 0.7 .

Improving two-photon excitation microscopy for sharper and faster biological imaging.

Two-photon excitation laser scanning microscopy (TPLSM) has provided many insights into the life sciences, especially for thick biological specimens, because of its superior penetration depth and less invasiveness owing to the near-infrared wavelength of its excitation laser light. This paper introduces our four kinds of studies to improve TPLSM by utilizing several optical technologies as follows: (1) A high numerical aperture objective lens significantly deteriorates the focal spot size in deeper regions of specimens. Thus, approaches to adaptive optics were proposed to compensate for optical aberrations for deeper and sharper intravital brain imaging. (2) TPLSM spatial resolution has been improved by applying super-resolution microscopic techniques. We also developed a compact stimulated emission depletion (STED) TPLSM that utilizes electrically controllable components, transmissive liquid crystal devices, and laser diode-based light sources. The spatial resolution of the developed system was five times higher than conventional TPLSM. (3) Most TPLSM systems adopt moving mirrors for single-point laser beam scanning, resulting in the temporal resolution caused by the limited physical speed of these mirrors. For high-speed TPLSM imaging, a confocal spinning-disk scanner and newly-developed high-peak-power laser light sources enabled approximately 200 foci scanning. (4) Several researchers have proposed various volumetric imaging technologies. However, most technologies require large-scale and complicated optical setups based on deep expertise for microscopic technologies, resulting in a high threshold for biologists. Recently, an easy-to-use light-needle-creating device was proposed for conventional TPLSM systems to achieve one-touch volumetric imaging.

3D Switchable Diffractive Optical Elements Fabricated with Two‐Photon Polymerization

AbstractDirect laser writing is demonstrated by two‐photon polymerization of multi‐element diffractive optical components that can be switched on and off with an applied voltage. By exploiting the 3D capabilities of the laser microfabrication technique, multiple diffractive optical elements are written into a single liquid crystal (LC) layer. The switching behavior of the diffractive optical elements is controlled by simply changing the write‐voltage of the anisotropic polymer structures during fabrication. Initially, 2D diffraction gratings are written at different depths within the LC layer. Each element is then activated by applying a voltage of sufficient amplitude that causes the second diffractive optical element to become inactive. This is then followed by a demonstration of multi‐element computer generated holograms that are written at different depths within the LC layer. By altering the magnitude of the applied voltage, different images/patterns can be observed in the replay field using a simple electrode configuration. These compact and transmissive LC optical components could excel in applications where a degree of switchability is required but a highly pixelated fully programmable device is excessive.

Transmissive Single-Pixel Microscopic Imaging through Scattering Media.

Microscopic imaging is of great significance for medical diagnosis. However, due to the strong scattering and absorption of tissue, the implementation of non-invasive microscopic imaging is very difficult. Traditional single-pixel microscopes, based on reflective optical systems, provide an alternative solution for scattering media imaging. Here, the single-pixel microscope with transmissive liquid crystal modulation is proposed. The microscopic ability of the proposed microscope is calibrated. The multi-spectral microscopic imaging of the object is demonstrated. The transmissive imaging of the object behind the scattering media is analyzed. The proposed prototype of the transmissive single-pixel microscope is expected to be applied in microscopic imaging through scattering media and medical imaging.

Efficiency of diffraction gratings based on liquid crystals

Wehave recorded geometric-phase diffraction gratingusing the method of holography on a thin layer of dichroic dye. A transmission liquid crystal cell hasbeen assembled inorder to obtain electrically-controlled diffraction in visible range. Also, using software for electrooptical effects modeling wehavecalculated the diffraction efficiency (DE) of the LC-based phase grating as a function of the voltage amplitude at the electrodes,es-timatedtheDEvariation as a function of LCcell thickness, LC materialbirefringenceand the radiation wavelength. The experimental data qualitatively matched with calcu-lation results. 1-st order DE of16% (at 532 nm) has been obtainedforsingle-sided phase grating LC. Furthermore, we have observed the decrease ofDEwith the increase of radiation wavelength.

Interferometer for pitch and yaw measurement using LC-screen and four ball lenses

The positioning accuracy of machine-tool tables used for translatory movement and coordinate measuring machine carriages requires improvement. Transmission and blocking of light can be realized by combining a transmissive liquid crystal and a polarizing plate. In this study, this technique was applied to a laser interferometer to realize selective multi-degree-of-freedom motion measurement using one measurement system. A ball lens was adopted as a reflecting mirror, and the rotational-movement amount was measured using a photodetector. In addition, rotational movement with two degrees of freedom was selectively measured using one interferometer, through transmission and blocking of light. It was found that the pitch and yaw rotational movement can be selectively measured using the difference between displacements of the axial directions of two ball lenses for the movement of the goniometer stage with 0.0032° resolution and the rotational stage with 0.004° resolution, respectively, and that the influence of the crosstalk of yaw and pitch is slight when the pitch and yaw rotational movements are simultaneously applied to the stage. Therefore, there is scope for this technique to be used to improve multi-degree-of-freedom motion measurements.

Varifocal Occlusion for Optical See-Through Head-Mounted Displays using a Slide Occlusion Mask.

We propose a varifocal occlusion technique for optical see-through head-mounted displays (OST-HMDs). Occlusion in OST-HMDs is a powerful visual cue that enables depth perception in augmented reality (AR). Without occlusion, virtual objects rendered by an OST-HMD appear semi-transparent and less realistic. A common occlusion technique is to use spatial light modulators (SLMs) to block incoming light rays at each pixel on the SLM selectively. However, most of the existing methods create an occlusion mask only at a single, fixed depth-typically at infinity. With recent advances in varifocal OST-HMDs, such traditional fixed-focus occlusion causes a mismatch in depth between the occlusion mask plane and the virtual object to be occluded, leading to an uncomfortable user experience with blurred occlusion masks. In this paper, we thus propose an OST-HMD system with varifocal occlusion capability: we physically slide a transmissive liquid crystal display (LCD) to optically shift the occlusion plane along the optical path so that the mask appears sharp and aligns to a virtual image at a given depth. Our solution has several benefits over existing varifocal occlusion methods: it is computationally less demanding and, more importantly, it is optically consistent, i.e., when a user loses focus on the corresponding virtual image, the mask again gets blurred consistently as the virtual image does. In the experiment, we build a proof-of-concept varifocal occlusion system implemented with a custom retinal projection display and demonstrate that the system can shift the occlusion plane to depths ranging from 25 cm to infinity.

Advanced easySTED microscopy based on two-photon excitation by electrical modulations of light pulse wavefronts.

We developed a compact stimulated emission depletion (STED) two-photon excitation microscopy that utilized electrically controllable components. Transmissive liquid crystal devices inserted directly in front of the objective lens converted the STED light into an optical vortex while leaving the excitation light unaffected. Light pulses of two different colors, 1.06 and 0.64 μm, were generated by laser diode-based light sources, and the delay between the two pulses was flexibly controlled so as to maximize the fluorescence suppression ratio. In our experiments, the spatial resolution of this system was up to three times higher than that obtained without STED light irradiation, and we successfully visualize the fine microtubule network structures in fixed mammalian cells without causing significant photo-damage.

Dynamic retinal vessel analysis using a spatial light modulator for stimulation

Purpose : Dynamic vessel analysis (DVA) with the Retinal Vessel Analyzer (Imedos Systems UG) is the Gold standard to measure alterations in retinal vessel diameter in health and disease. The DVA guidelines provide flicker stimulation of 12.5 Hz in a 30° field of view [1] (fixed full-field). However, to investigate local effects in the regulation of retinal perfusion, e.g. neuro vascular coupling, spatially varying stimuli are required. We introduce and validate a spatial light modulator (SLM) based setup for DVA providing arbitrary stimuli in location and size. Methods : We studied 15 volunteers (8m, 7f, 24.7±1.1 yr) to investigate the repeatability of the novel setup and to compare the new setup with the Gold standard. The relative vaso-dilatation value was determined for both. We measured four primary vessels: one superior temporal artery and vein (STa/STv), one inferior temporal artery and vein (ITa/ITv). All subjects were free of ocular and systemic diseases. A high-power LED (515 nm) and a transmissive liquid crystal SLM were connected to a mydriatic fundus camera. The field of view was set to 30°. The SLM was controlled via standard graphics card and a customized software (60 Hz refresh rate / 15 Hz flicker frequency). Each volunteer was measured three times. One measurement with the Gold standard and two with the new setup, in a random order (10 min pause time). Repeatability and comparison of the two setups were analyzed using the Bland-Altmann method. To value the differences among the group means we performed the t-test. For groups that did not meet the normality assumption we employed the Mann-Whitney U test. Results : The repeatability analysis showed the following mean differences (MD), Limits of Agreement (LoA), both in %, and p-values. STa: MD=-0.78, LoA=-4.35/+2.79, p=0.134. ITa: MD=-0.67, LoA=-3.82/+2.48, p=0.131. STv: MD=-0.69, LoA=-4.30/+2.92, p=0.186. ITv: MD=-0.03, LoA=-3.00/+2.94, p=0.947. Comparing the two different setups the p-values were 0.545 (STa), 0.405 (ITa), 0.05 (STv), and 0.137 (ITv). Conclusions : We realized a working experimental setup and applied a new approach of Dynamic Vessel Analysis using a spatial light modulator. Validation was realized in comparison to the Gold standard. The agreement between the two different setups was predominantly good. Neither the repeatability analysis nor the comparison of the two setups showed significant differences.

Computationally efficient brightness compensation and contrast enhancement for transmissive liquid crystal displays

High-quality displays tend to consume significant power in mobile devices. Currently, transmissive liquid crystal displays are among the most common. They are non-emissive and rely on a backlight behind the display panel. Brightness compensation refers to the signal processing technique to adaptively dim the backlight to reduce the power consumption, while increasing the pixel values to preserve the visual quality of the images or even enhance their contrast. Fast computation of the brightness compensation algorithm is essential for practical use. In this paper, we show that a state-of-the-art brightness compensation algorithm, which requires iterations and is computationally demanding, can in fact be solved with a closed-form solution. We also demonstrate with experimental results that we can achieve approximately an 800-fold speedup, while providing effectively identical images to those obtained by the original method.

Generation of Perfect Optical Vortices by Using a Transmission Liquid Crystal Spatial Light Modulator

We have experimentally created perfect optical vortices by the Fourier transformation of holographic masks with combination of axicons and spiral functions, which are displayed on a transmission liquid crystal spatial light modulator. We showed theoretically that the size of the annular vortex in the Fourier plane is independent of the spiral phase topological charge but it is dependent on the axicon. We also studied numerically and experimentally the free space diffraction of a perfect optical vortex after the Fourier back plane and we found that the size of the intensity pattern of a perfect optical vortex depends on the topological charge and the propagation distance.

Adaptive optics full-field optical coherence tomography.

We describe a simple and compact full-field optical coherence tomography (FFOCT) setup coupled to a transmissive liquid crystal spatial light modulator (LCSLM) to induce or correct aberrations. To reduce the system complexity, strict pupil conjugation was abandoned because low-order aberrations are often dominant. We experimentally confirmed a recent theoretical and experimental demonstration that the image resolution was almost insensitive to aberrations that mostly induce a reduction of the signal level. As a consequence, an image-based algorithm was applied for the optimization process by using the FFOCT image intensity as the metric. Aberration corrections were demonstrated with both an USAF resolution target and biological samples for LCSLM-induced and sample-induced wavefront distortions.

Two-photon excitation STED microscopy by utilizing transmissive liquid crystal devices.

Transmissive liquid crystal devices (tLCDs) enable the modification of optical properties, such as phase, polarization, and laser light intensity, over a wide wavelength region at a high conversion efficiency. By utilizing tLCDs, we developed a new two-photon excitation stimulated emission depletion microscopy technique based on a conventional two-photon microscope. Spatial resolution was improved by compensating for phase shifts distributed in the optical path. Using this technique, we observed the fine structures of microtubule networks in fixed biological cells.

Single-mode lasing from dye-doped holographic polymer-dispersed liquid crystal transmission gratings

We demonstrate single-mode laser operation in dye-doped holographic polymer-dispersed liquid crystal (HPDLC) transmission gratings. The gratings are fabricated in cells made from specifically chosen glass substrates to decrease the refractive index difference between the waveguide core layer and cladding layer. The phase separation degree of liquid crystal after holographic recording is further optimized to confine only the lowest propagation mode in the device. The mode selection mechanism is explained under the framework of the waveguide distributed feedback (DFB) theory. The wavelength of single-mode lasing can be tuned between 620 and 660 nm by varying the grating period. Our results show the HPDLC technique could provide single-mode organic DFB lasers in a tunable, simple, and large-area manner.

Distributed feedback lasing from electrically tunable dye-doped polymer–liquid crystal transmission gratings

In the present work we report low-threshold distributed feedback (DFB) lasing from electrically tunable holographic polymer–liquid crystal transmission gratings of POLIPHEM type doped with pyrromethene 567. Due to their uniform droplet-free micro-morphology, the POLIPHEM gratings possess high diffraction efficiency and excellent optical quality. Second-order lasing with a threshold of ~0.8 µJ/pulse and a bandwidth of ~1 nm was achieved under the excitation of a frequency-doubled Nd:YAG laser operating at 532 nm. The laser emission wavelength was tuned from 572–625 nm by varying the grating period. Application of an electric field switches off or tunes the lasing intensity. An electrically-induced blue-shift of the output laser emission was observed.

Efficient laser emission from organic semiconductor activated holographic polymer dispersed liquid crystal transmission gratings

Performances of organic distributed feedback (DFB) lasers made from holographic polymer dispersed liquid crystal (HPDLC) transmission gratings were greatly improved by replacing common laser dye with organic semiconductor poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV) as the gain medium. The MEH-PPV layer was laminated between the glass substrate and the HPDLC grating layer. Light interaction for feedback is provided by an evanescent wave spread into the grating layer. The device showed single-mode, TE polarized laser emission with a threshold of only 0.17 μJ per pulse (17 μJ cm−2). Slope efficiencies as high as 4.7% and 6.5% were measured for optical pumping of the device with P polarization and S polarization, respectively. Lasing was also coupled by the grating out of the waveguide and into free space. Furthermore, the output laser wavelength was systematically tuned over a range of 43 nm by varying the grating period. The laser thresholds at different wavelengths trace well with the amplified spontaneous emission spectrum of the MEH-PPV film. In terms of the HPDLC technique, our results show there is still plenty of potential to be exploited for fabricating organic DFB lasers.

Optimized Brightness Compensation and Contrast Enhancement for Transmissive Liquid Crystal Displays

An optimized brightness-compensated contrast enhancement (BCCE) algorithm for transmissive liquid crystal displays (LCDs) is proposed in this paper. We first develop a global contrast enhancement scheme to compensate for the reduced brightness when the backlight of an LCD device is dimmed for power reduction. We also derive a distortion model to describe the information loss due to the brightness compensation. Then, we formulate an objective function that consists of the contrast enhancement term and the distortion term. By minimizing the objective function, we maximize the backlight-scaled image contrast, subject to the constraint on the distortion. Simulation results show that the proposed BCCE algorithm provides high-quality images, even when the backlight intensity is reduced by up to 50-70% to save power.

Generation of arbitrary vector beams with cascaded liquid crystal spatial light modulators

A flexible approach is presented to generate vector beams with arbitrary polarization and complex amplitude by means of two cascaded transmissive liquid crystal spatial light modulators (LCSLMs). The configuration of the cascaded LCSLM system and its modulation characteristic are introduced. Theoretical analysis and experimental demonstration prove that the system in combination with a double-pass computer-generated hologram and a black-and-white pattern can generate vector beams with arbitrary polarization and complex amplitude by respectively controlling the complex amplitudes of two orthogonal polarization components of the beams. Using this system, we successfully generate radially polarized vector beams with helical phase distributions and vector Bessel beams with inhomogeneous amplitude distributions in experiments.

In-line FINCH super resolution digital holographic fluorescence microscopy using a high efficiency transmission liquid crystal GRIN lens

We report a new optical arrangement that creates high-efficiency, high-quality Fresnel incoherent correlation holography (FINCH) holograms using polarization sensitive transmission liquid crystal gradient index (TLCGRIN) diffractive lenses. In contrast, current universal practice in the field employs a reflective spatial light modulator (SLM) to separate sample and reference beams. Polarization sensitive TLCGRIN lenses enable a straight optical path, have >90% transmission efficiency, are not pixilated, and are free of many limitations of reflective SLM devices. For each sample point, two spherical beams created by a glass lens in combination with a polarization sensitive TLCGRIN lens interfere and create a hologram and resultant super resolution image.

Transmissive liquid crystal light-valve for near-infrared applications

An optical valve is realized by associating a nematic liquid crystal layer with a Cr-doped gallium arsenide as a photoconductive substrate. The light-valve is shown to efficiently operate in transmission at 1.06 μm optical wavelength. The optical phase shift and refractive index change are measured as a function of the incident light intensity and of the voltage applied. Additionally, the light-valve is shown to act as a self-defocusing medium. Combining transmissive properties and nonlinear features, applications for dynamic holography in the near-infrared region of the spectrum can be envisaged.