Qing Tong

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.

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.

Electrically tunable infrared filter based on a cascaded liquid-crystal Fabry-Perot for spectral imaging detection

An electrically tunable infrared (IR) filter based on a key cascaded liquid-crystal Fabry-Perot (C-LC-FP) working in the wavelength range of 3-5 μm is presented. The C-LC-FP is constructed by closely stacking two FP microcavities with different depths of 12 and 15 μm and fully filled by nematic LC materials. Through continuous wavelength selection of both microcavities, radiation with a high transmittance and narrow bandwidth can pass through the filter. According to the electrically controlled birefringence characteristics of nematic LC molecules, the transmission spectrum can be shifted through applying a dual voltage signal over the C-LC-FP. Compared with common LC-FPs with a single microcavity, the C-LC-FP demonstrates better transmittance peak morphology and spectral selection performance. To be more specific, the number and the shifted scope of the IR transmission peak can be decreased and widened, respectively.

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.

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.

Three dimensional measurement with an electrically tunable focused plenoptic camera

A liquid crystal microlens array (LCMLA) with an arrayed microhole pattern electrode based on nematic liquid crystal materials using a fabrication method including traditional UV-photolithography and wet etching is presented. Its focusing performance is measured under different voltage signals applied between the electrodes of the LCMLA. The experimental outcome shows that the focal length of the LCMLA can be tuned easily by only changing the root mean square value of the voltage signal applied. The developed LCMLA is further integrated with a main lens and an imaging sensor to construct a LCMLA-based focused plenoptic camera (LCFPC) prototype. The focused range of the LCFPC can be shifted electrically along the optical axis of the imaging system. The principles and methods for acquiring several key parameters such as three dimensional (3D) depth, positioning, and motion expression are given. The depth resolution is discussed in detail. Experiments are carried out to obtain the static and dynamic 3D information of objects chosen.

Nano-metallic-planar-apex metamaterials

We present the results of numerical simulations and preliminary experiments to investigate the nano-focusing effect of incident light based on the surface plasmon polaritons (SPPs) on the nano-metallic-planar-apex metamaterials (NMPAM). The NMPAM are prepared by Focused Ion Beam lithography (FIB), a nanoscale fabrication tool. The NMPAM can be used to remarkably enhance the strength of the surface evanescent and lead to the excitation of several SPP modes on the metal surface. The interaction of different SPPs result in unique near-field optical properties for imaging and optical storage, so as to focus light into a nano-size point and thus enhance the light power greatly. The energy flow and electromagnetic field distribution is calculated by finite-difference time-domain (FDTD) method. The nano-spot position and intensity is experimentally shown to be controlled by the array of the apex. In our experiments, we fabricate a 10×10 array by FIB, and then the scanning near-field optical microscopy (SNOM) is used to observe the optical power distribution in nano-scale at the air-metal interface in the infrared region. we find that the light can be focus into ~100nm-scale and consequently enhance the light power up to several times than before common focusing method. The principle of nano-focusing based on nano-planar-apex is theoretically explained. The NMPAM can be utilized for coupling with infrared pixels to enhance the incident light converging so as to improve signal to noise ratio of infrared detection.

Imaging application based on an electrically tunable polarization-independent liquid crystal microlens array

Polarization-independent microlens array based on liquid crystal (PI-LCMLA) has been an interesting and important topic in optoelectronic application. In this study, a polarization-independent microlens array using double layered nematic liquid crystals (LC) with orthogonal alignment is proposed and demonstrated. Two orthogonal LC layers are separated by a double-sided indium-tin oxide silica. Further optical experiments and investigations reveals that the PILCMLA can work in polarization and polarization-insensitive mode by operating the driving voltages. The normalized focusing intensity is no polarization dependence on the incident light. Several raw images at different working modes are obtained through by utilizing this novel configuration with low applied voltages. With advantages in high optical efficiency, simple manufacture, electrically tunable focal length, low power consumption, polarization independence and multi operation modes, this device can not only be used for imaging application but also has many potential applications in optical systems.

Arrayed optical switches based on integrated liquid-crystal microlens arrays driven and adjusted electrically

Based on our fundamental work on liquid-crystal microlens arrays (LCMAs) driven and adjusted electrically, a new kind of arrayed optical switch (AOS) constructed by a key LCMA with a special dual-mode function of converging and diverging incident beams according to electrical signals applied over the LCMA is proposed. The LCMA leading to the AOS is constructed by a microcavity with a couple of paralleled electrodes. The top electrodes of the LCMA are fabricated by depositing a layer of indium-tin-oxide (ITO) film and a layer of aluminum film, respectively. The aluminum film is continuously patterned into a circular microhole array, and the ITO film only acts as a planar conductor. Both functioning films are effectively separated by the SiO2 wafer. Another SiO2 wafer is also coated by an ITO film as a planar conductor. The measurements show that the developed AOS can effectively switch on or off beams propagating in arrayed fibers by applying proper voltage signals to them. Compared with other conventional AOSs, the developed AOS demonstrates several merits, including greater integration level, lower cost, and suitability to high-power propagating beams.

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.