Surface plasmons, as a local electromagnetic field mode generated or stimulated at the interface between common metal and dielectric, can be used to greatly break through the optical diffraction limit and also localize the electric-field and then light energy in a sub-wavelength scale. It is already a research hotspot in recent years. As shown, several patterned metal nano-array can be utilized to produce relatively strong surface plasmon resonance, so as to achieve a nano-scaled localized light field on the surface of the functioned metal structure. In this paper, silicon dioxide materials are used as the substrate, and the common gold materials are fabricated into a metal film, and then the sub-wavelength metal nano-tip arrays with several morphology such as the cone-shaped, the triangular-pyramid-shaped, and the quadrangular-pyramid-shaped, are designed respectively. The functioned metal nano-structures are symmetric and asymmetric coating mode. The electric field distribution characteristics of the structure under the internal excitation mode of the incident light with vertical incidence are analyzed. The simulations show that the local field enhancement can be clearly observed at the nano-tip of the cone-shaped in the asymmetric case, but the symmetry is not. Analysis shows that the destructive interference occurs when the surface plasmons are excited by a linearly polarized light on both sides of a conical structure propagate to its top, so failure to produce focusing effect. Therefore, to the case of symmetrical film through adjusting the incident angle of light, different incident angles will affect the enhancement of the local field at the tip of the cone.
As an effective method for collecting light field information and further extending the depth of field, a new imaging technology based on key electrically controlled liquid-crystal microlens array (EC-LCMLA), has been proposed. Compared with common lenses with defined surface profile, the liquid-crystal microlenses can be used to regulate the focal length only through applying different signal voltages to achieve focus tuning or even swing on the observation plane. Generally, the traditional autofocus operations are no longer suitable to EC-LCMLA because the controlling orders for LC structures should be generated through image process. So, an autofocus method, which is used to dynamically adjust the focal length of each imaging unit in the EC-MLA, is proposed for controlled LCMLA in this paper. The method is used to extract the light field information from low-quality image, so as to obtain the key focusing distance of the plane observed by each imaging unit, and then calculate the focal length of the EC-LCMLA without additional sensors. The signal voltage of each liquid-crystal microlens can be adjusted by the driving control unit, which implements an automatic focusing of the LCMLA. The active autofocus therefore is achieved and then all the imaging units in an optimal working state. Based on theoretical analysis and the focusing algorithm constructed by us, the imaging experiments are carried out so as to show a higher performance and then image quality and focusing efficiency of LCMLA. The novel autofocus method highlights a construction of a new kind of plenoptic camera with stronger performances.
In this paper, we propose a novel dual-function infrared liquid-crystal device (DF-ILCD), which can simultaneously perform both tunable focusing and filtering functions through applying alternating current (AC) voltage signals. The key functional micro-structure of the DF-ILCD includes: two paralleled 1-mm-thick ZnSe substrates with 20-nm-thick aluminum (Al) films over their inner surfaces and a periodic repetition microholes over both substrates. The Al films act as both high-reflection films and conductive films. The conventional UV-photolithography and wet-etching process are used to fabricate an arrayed micro-hole with a diameter of 120μm and a period of 336μm over each substrate. The micro-cavity formed between ZnSe substrates has a typical depth of ~12 μm, which is fully filled by a nematic liquid-crystal (LC) materials. Experiments demonstrate that the proposed device exhibit both filtering function based on Fabry–Perot (FP) effect and electrically controlled tunable focusing function generated by the micro-hole electrodes, which is very promising for realizing both the light-field imaging and spectral imaging in the infrared wavelength range.
The Fabry-Perot interferometer (FP) can be used as a kind of filter for obtaining spectral information of targets in several wavelength ranges such as in the visible or infrared regions. So far, the spectral imaging devices based on FP effect mainly include the electrically controlled liquid-crystal filtering structures and the micro-electro-mechanical filtering architectures (MEMS). MEMS are generally micro-structures that integrate micro-sensors for converting incident microbeams into arrayed electronic signals and micro-actuators. The MEMS-FP filter constructed by combining the MEMS and FP functions, can be further integrated into a chip-level imaging spectrometer to achieve spectral imaging operation. The design of distributed Bragg reflectors (DBRs) is an important part to obtain a high transmittance for MEMS-FP structure. Different number of layers of optical film is calculated and compared in this paper and the transmittance can reach 82% and the FWHM is ~ 1nm in the infrared region of 3-5um. Angle of incidence is also considered and the simulation result shows poor robustness. We propose that two liquid-crystal microlens arrays can be mounted on FP arrays to get a high filling-factor and a more flexible range of incident angles.
The Fabry-Perot interferometer (FP) can be used as a kind of filter for obtaining spectral information of targets in several wavelength ranges such as in the visible or infrared regions. Micro-electro-mechanical systems (MEMS) are generally micro-structures that integrate micro-sensors for converting incident microbeams into arrayed electronic signals and micro-actuators. The MEMS-FP filter constructed by combining the MEMS and FP functions, can be further integrated into a chip-level imaging spectrometer to achieve spectral imaging operation. In our design, the MEMS-FP filter is also mounted a liquid-crystal microlens array with a high filling-factor. The key micro-bridges of each MEMS-FP unit are modeled and simulated in this paper. We have designed two types of supporting structures and simulated them with the simulation software COMSOL Multiphysics 5.2. The key factors include tuning range, filling-factor, and parallelism of the bridges. After calculating and analyzing, we found that the tuning range can be optimized by changing the thickness of the micro-bridge and the arm width of the cantilever beam. The filling-factor is already increased by geometry design. The parallelism of the bridge in the two micro-structures differs greatly, which is related to the shape of the bridge itself. According to the simulations, a tuning range of 160 nm has been achieved in the visible and near-infrared wavelength range, with a maximum filling-factor of more than ~80%.
Generally optical micro-nano-antenna can be used to modulate lightwaves in the sub-wavelength scale, which is a hot and difficult research issue. Patterned metal nano-antenna array can be utilized to stimulate intense surface plasmon polaritons (SPPs), so as to realize sub-wavelength focusing by breaking through diffractive limit, and thus remarkably improving THz imaging efficiency. In this article, firstly, based on SPPs, the Drude dispersion model for metallic film is analyzed, and the dispersion relations and excitation modes of the SPPs are discussed, and the numerical analysis methods of the metallic micro-nano-antenna are also presented including a time-domain finite integral method and a frequency-domain finite element method. According to related literature, the key optical micro-nano-antenna unit is modeled, and a metasurface formed by etching a gold thin film on a silicon substrate is designed. Through regulating parameters including the number and size of the openings and the line width, the SPP excitation in THz band is studied. Using finite element and adaptive mesh division method, the common electromagnetic properties such as transmission intensity and electric field distribution are simulated and analyzed. The simulations show that the optical micro-nano-antenna element can resonantly induct terahertz wave, and demonstrate a resonant electric-field at the aperture gap, which will move towards high frequencies end as increasing the gap size or line width, so as to lay a concrete foundation for continuously fabrication THz-SPPs devices.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.