Hongshi Sang

An electrically tunable plenoptic camera using a liquid crystal microlens array

Plenoptic cameras generally employ a microlens array positioned between the main lens and the image sensor to capture the three-dimensional target radiation in the visible range. Because the focal length of common refractive or diffractive microlenses is fixed, the depth of field (DOF) is limited so as to restrict their imaging capability. In this paper, we propose a new plenoptic camera using a liquid crystal microlens array (LCMLA) with electrically tunable focal length. The developed LCMLA is fabricated by traditional photolithography and standard microelectronic techniques, and then, its focusing performance is experimentally presented. The fabricated LCMLA is directly integrated with an image sensor to construct a prototyped LCMLA-based plenoptic camera for acquiring raw radiation of targets. Our experiments demonstrate that the focused region of the LCMLA-based plenoptic camera can be shifted efficiently through electrically tuning the LCMLA used, which is equivalent to the extension of the DOF.

Ommatidia structure based on double layers of liquid crystal microlens array

We present a new liquid crystal (LC) microlens structure having ommatidia function, which consists of the overlapped patterned electrode layers. Each electrode layer has a circular electrode array with a different size. Two electrode layers are deposited on the surface of a glass substrate and insulated by thin SiO₂ coating, which act as controlling electrodes. A plane electrode layer deposited on the surface of another glass substrate acts as the base electrode. Two glass substrates are made into a LC cell filled with nematic LC material. When a voltage signal is applied to the controlling electrode and base electrode, each circular unit of the array can focus along the optical axis and has good focusing character, and it behaves as a common microconvex lens. The whole circular array achieves ommatidia optical characters because of many microconvex lenses imaging to the objects simultaneously. Two circular arrays can both image to the objects and have different fields of view, which have extraordinary ommatidia effects. The common optical properties of the LC microlens are also demonstrated experimentally.

An Arrayed Liquid Crystal Fabry–Perot Infrared Filter for Electrically Tunable Spectral Imaging Detection

An arrayed electrically tunable infrared (IR) filter based on the key structure of liquid crystal Fabry-Perot (LC-FP) working in the wavelength range from 2.5 to 5 μm is designed and fabricated successfully. According to the electrically controlled birefringence characteristics of nematic LC molecules, the refractive index of LC materials filled into a prefabricated microcavity can be adjusted by the spatial electric field stimulated between the top and bottom aluminum (Al) electrodes. As a crucial component of the filter, the Al film with a typical thickness of ~30 nm acts as the electrode as well as the reflective mirror. The particular functions, including key spectral selection and spectral adjustment, can be realized by the developed LC-FP filter driven electrically. Our experiments show that the maximum transmittance of the transmission peaks is ~24%, and the transmission spectrum can be shifted remarkably through applying different voltage signals with a root mean square value range from 0 to ~21.7 Vrms. The experimental results are consistent with the simulation according to the model constructed by us. As a 2 × 2 or four-channel IR filter, the top electrode of the device is composed of four same sub-electrodes. Each channel in the device is powered separately and synchronously to select desired transmission spectrum, which means that the device can be used to obtain spectral sub-images in different spectral bands in one shot.

Dual-mode photosensitive arrays based on the integration of liquid crystal microlenses and CMOS sensors for obtaining the intensity images and wavefronts of objects.

In this paper, we present a kind of dual-mode photosensitive arrays (DMPAs) constructed by hybrid integration a liquid crystal microlens array (LCMLA) driven electrically and a CMOS sensor array, which can be used to measure both the conventional intensity images and corresponding wavefronts of objects. We utilize liquid crystal materials to shape the microlens array with the electrically tunable focal length. Through switching the voltage signal on and off, the wavefronts and the intensity images can be acquired through the DMPAs, sequentially. We use white light to obtain the objects wavefronts for avoiding losing important wavefront information. We separate the white light wavefronts with a large number of spectral components and then experimentally compare them with single spectral wavefronts of typical red, green and blue lasers, respectively. Then we mix the red, green and blue wavefronts to a composite wavefront containing more optical information of the object.

Oscillation characteristics in terahertz transmission through a dipole-based metamaterial

A dual-film metamaterial is proposed that presents oscillation characteristics during terahertz transmission. An n-type gallium arsenide (n-GaAs) film with a free electron density of 4.7 × 1017 cm−3 is grown on a semi-insulating GaAs wafer. Then, a metallic film, which is fabricated on the n-GaAs film, is patterned into an arrayed single-gap microstructure with narrowed edges using traditional ultraviolet photolithography methods. The metal and n-GaAs films form a Schottky junction. The transmission frequency spectrum of the dual-film metamaterial contains an obvious fluctuation with a frequency interval of f = 0.15 THz in the 0.1–1 THz range, and the experimental results show that the frequency region of the locally intensive oscillatory signal essentially agrees with that of the characteristic transmission spectrum of the dual-film metamaterial in the 0.1–1 THz range. The terahertz characteristic transmission spectrum of the fabricated metamaterial is measured at the central frequency of ∼0.515 THz, and...

Dual-mode liquid crystal microlens arrays

Based on our previous works on liquid crystal microlenses driven and adjusted electrically, we present a new type of liquid crystal microlens arrays with dual-mode function (DLCMAs). Currently, the DLCMAs developed by us consist of a top electrode couple constructed by two layers of controlling electrode structure, and a bottom electrode. The top two electrode layers are respectively deposited over both sides of a glass substrate and insulated by a thin SiO2 coating, so as to act as the mode-control-part in the DLCMAs. Another planar electrode layer acting as the base electrode is deposited over the surface of a glass substrate. Two glass substrates with fabricated electrode structure are coupled into a microcavity filled by nematic liquid crystal material. The DLCMAs proposed in this paper present excellent divergence and convergence performances only loading relatively low driving voltage signal. The common optical properties of the micro-optics-structures are given experimentally.

Design and simulation of electrically addressed infrared filtering chip based on cascaded liquid-crystal Fabry-Perot effect for integration application of infrared spectral imaging sensor array

A wavelength tunable optical filter based on cascaded Liquid-Crystal Fabry-Perot (LC-FP) cavity with many working units has been proposed and simulated in this paper. By choosing different material and according geometric parameters, we simulated the structure in the wavelength of medium infrared (IR)(3-5μm) and far IR(8-14μm) with the algorithm of thin film matrix equation and iterative finite-difference. Finally, we give the spectrum of the structure under different driving-voltage. Combing this structure with uncooled infrared focal plane array (IRFPA), the image of many spectral bands can be obtained in one picture frame by applying different driving-voltage on each unit. Compared with other design, this structure has the advantages of wide free spectral range (FSR), compact integration, low cost and high stability.

Dual-Mode Liquid Crystal Microlens Arrays for Chaotic Encryption

Based on our previous works on liquid crystal (LC) microlenses driven electrically, we present a new type of dual-mode liquid crystal microlens arrays (DLCMAs) for chaotic encryption applications. Currently, the DLCMAs developed by us consist of a top electrode couple constructed by two layers of controlling electrode and a bottom planar electrode. Aluminium and Indium-Tin Oxide (ITO) materials are respectively deposited over both sides of a glass substrate for shaping the top electrode couple, which is used to act as a key mode-control-part in the DLCMAs. Another ITO layer is deposited over the surface of another glass substrate for shaping the bottom public electrode. Both glass substrates with fabricated electrode structures are coupled into a microcavity fully filled by a layer of nematic liquid crystal materials. The DLCMAs proposed in this paper present excellent beam divergence and light convergence performances through loading relatively low driving voltage signals. The common optical properties of the devices, leading to a type of optical modulator of chaotic beams or light intensity adjustment devices for chaotic light coupling between functioned components, are demonstrated experimentally.

Adjustment characteristics in terahertz transmission through a split ring resonator-based metamaterial

The artificially structured metamaterials has led to many potential applications in terahertz regime, but the role in adjusting the terahertz transmission still needs to be carefully investigated. Currently, designs with split ring resonator (SRR) based metamaterials can provide a promising approach for understanding the terahertz transmission characteristics. In the experiments, a SRR-based metamaterial is proposed for presenting terahertz transmission characteristics. The substrate of the metamaterial is an n-type gallium arsenide (n-GaAs) film grown over a semiinsulating GaAs wafer. Then, the metallic film, fabricated on n-GaAs, is patterned into an arrayed four-gap microstructure according to traditional ultraviolet photolithography methods. The metal film and n-GaAs film form a Schottky contact. In the experiments, the transmission frequency spectrum of the metamaterial has an obvious fluctuation in the 0.6–1.23 THz and 1.52–2.4 THz range, and the experimental results show that the frequency region of the intensive oscillatory signal essentially agrees with that of the metamaterial characteristic transmission spectrum in the 0.5–2.5 THz range. The terahertz characteristic transmission spectrum of the fabricated metamaterial are measured at the central frequency of ~0.5, ~1.0, ~1.5, ~2.0 and ~2.5 THz, thus the oscillation characteristics can be explained by dipole resonance. The measured time-domain transmission signals and corresponding frequency responses based on the metamaterial agree well with calculated results. Therefore, our research shows a potential application of the transmission adjusting roles in terahertz regime.

Plenoptic camera based on a liquid crystal microlens array

A type of liquid crystal microlens array (LCMLA) with tunable focal length by the voltage signals applied between its top and bottom electrodes, is fabricated and then the common optical focusing characteristics are tested. The relationship between the focal length and the applied voltage signals is given. The LCMLA is integrated with an image sensor and further coupled with a main lens so as to construct a plenoptic camera. Several raw images at different voltage signals applied are acquired and contrasted through the LCMLA-based plenoptic camera constructed by us. Our experiments demonstrate that through utilizing a LCMLA in a plenoptic camera, the focused zone of the LCMLA-based plenoptic camera can be shifted effectively only by changing the voltage signals loaded between the electrodes of the LCMLA, which is equivalent to the extension of the depth of field.