Linsen Chen

Color filter based on a two-dimensional submicrometer metal grating

A color filter incorporating a two-dimensional (2D) submicrometer metal grating is proposed. The device is designed by utilizing rigorous coupled-wave analysis (RCWA) and consists of four parts: a polymethyl methacrylate (PMMA) substrate, a dielectric film of high refractive index, a submicrometer grating, and a dielectric overlay of low refractive index. Its performance is enhanced effectively by applying a dielectric film of high refractive index. With a dielectric film height of 30 nm, a grating depth of 70 nm, and an overlay height of zero, three different devices are designed with the following structural parameters: a period of 420 nm for the red, a period of 350 nm for the green, and a period of 260 nm for the blue. For the red filter, the center wavelength is 645 nm and the peak transmission is 72%; for the green one, the center wavelength is 546 nm and the peak transmission is 75%; and for the blue one, the center wavelength is 455 nm and the peak transmission is 71%. The calculated result shows that its peak transmission efficiency increased more than 14% compared to the previous color filters incorporating a grating.

Diffraction characteristics of a submicrometer grating for a light guide plate.

The diffraction transmission characteristics of submicrometer gratings (SMGs) designed for coupling tricolor light out of a light guide plate (LGP) are discussed. Three discrete SMGs are designed for three special wavelengths: red (700 nm), green (546.1 nm), and blue (435.8 nm). The propagation direction of the output tricolor light is perpendicular to the surface of the LGP and can pass through the corresponding pixels of liquid crystal. Calculated by the rigorous coupled-wave theory, the first-order transmission efficiency as a function of grating depth is found to be approximate to a sinusoidlike curve and can be utilized to obtain uniform illumination. The theoretical maximum transmission is as much as 44%. The performance of the LGP composed of SMGs is also demonstrated by our experiments. Compared with other types of LGP, the present device has the advantage of flexible control of illumination angle and wavelengths. Additionally, lossy and costly color filters are unnecessary in the proposed configuration.

Efficiency improvement for a polarizing color filter based on submicron metal grating

A polarizing color filter, combining the function of polarizer and color filter, is proposed and theoretically investigated. The proposed color filter comprises of a metal grating and a dielectric layer on a glass substrate. The influence of the geometrical parameters of dielectric layer on the transmission efficiency are discussed in detail by rigorous coupled-wave analysis (RCWA). The result shows that a dielectric layer of high equivalent refractive index can enhance its performance effectively. A optimum tricolor filter with more than 74.1% broadband transmission and a polarization extinction ratio of 8.39dB is obtained.For TE-polarized light, it is reflected and can be recycled in the backlight units to increase the total energy utility. The numerical result shows that the peak transmission efficiency can increase 21.5% by using the proposed devices.

Wide-Angle Near-Perfect Absorber Based on Sub-Wavelength Dielectric Grating Covered by Continuous Thin Aluminum Film

We design and numerically investigate an optical absorber consisting of the sub-wavelength dielectric grating covered by continuous thin aluminum film. In this absorber, the aluminum film act as an efficient absorbing material because of the enhanced electric field in the air nano-grooves, and the absorption spect+rum can be manipulated by Fabry-Perot cavity mode resonance. According to the spectrum manipulation mechanism, the wavelength of absorption peak can be tuned by changing the heights and widths of the air nano-grooves. More importantly, the high absorption is very robust to the incident angle around the designed wavelength. From the nanofabrication point of view, the light absorber can be fabricated more easily without the need for ion or electrochemical etching of metal and it is easy to be integrated into complex photonic devices.

Both improvements of the light extraction efficiency and scattered angle of GaN-LED using sub-micron Fresnel lens array

With the demanding requirements for light source, light emitting diodes (LED) attracts more and more attention because of its inherent advantages such as low power consumption, high reliability and longevity. However, there are two disadvantages for LED, one is the low light extraction efficiency resulting from the total internal reflection, and the other is the relative large scattered angle. In order to improve the light extraction efficiency and collimate the out-coupling light, a sub-micron Fresnel lens array is introduced and investigated in this paper. The focal length of the proposed Fresnel lens is 3μm and the minimum width of the outmost ring is about 150nm. To calculate and analyze the light extraction efficiency and the scattered angle of LED with such Fresnel lens array structure, we optimize the parameters of the Fresnel lens, such as the depth of the Fresnel lens array structure and the thickness of the p-type gallium nitride layer by using the finite difference time domain method (FDTD). By comparing the discussed patterned GaN-based LED with that traditional flat LEDs, it can be found that significant enhancement factor of the light extraction efficiency, which is improved by 3.5 times, can be obtained and the scattered angle at half maximum can be decreased 50° from 60° with this novel Fresnel lens structure. It will be expected that the proposed sub-micron structure can find wide applications in LEDs industry.

Directional light-guide devices with continuously variable spatial frequency sub-micron grating structures for autostereoscopic display applications

Autostereoscopic displays are a promising three dimensional display technology for its convenience and compatibility with current display systems which has attracted considerable attention. We describe in detail an autostereoscopic display system with full-parallax using a directional light-guide device with continuously variable spatial frequency sub-micron grating structures. The optimization process of parameters of the multi-direction light-guide is given. A method of implementing sub-micron grating pixels (SMGPs) based on an ultraviolet continuously variable spatial frequency photolithography process has been proposed. The process aims to provide low cost fabrication of variable spatial frequency grating pixels with high efficiency. We fabricate 2 inch backlight plate with nine viewing directions, and the pitch of each diffractive pixel varies between 441 nm and 578 nm. The properties of SMGPs are investigated by the measurement of diffraction efficiency dependence on viewing angle under a collimated 650 nm LED light source at an incidence angle of 60°. The variation of diffraction efficiency with regards to viewing angle is weak. The measured diffraction efficiency is around 6%, which is in good agreement with the simulated value.

Ultra-broadband near perfect absorption of visible light based on one-dimensional metal-dielectric-metal grating for TM polarization

We numerically and analytically report an ultra-broadband near perfect absorber based on one-dimensional metal-dielectric-metal grating at visible light for TM polarization. A unit cell of this design is composed of metal-dielectric-metal grating, where the bottom metallic layer and the upper metallic coating are separated from each other by the intermediate dielectric grating. The absorber exhibits an average absorption of over 90% in the range 400-700nm. Moreover, they remain very high over a wide range of incident angle up to 45°.The electromagnetic field distributions are investigated, which reveals that this broadband absorption behavior is ascribed to the combination of surface plasmon resonance and cavity resonance. Furthermore, impedance calculations were carried out to explain the absorption behavior. The ultra-broadband near-perfect angle-robust absorber can be a good candidate for many fascinating applications, including solar-energy harvesting as well as producing artificial colors on a large scale substrate.

Angular sensitivity for a Fabry-Perot structure incorporating different dielectric materials

Color filters based on different Fabry-Perot structures are investigated extensively, and incident angle dependency is an important characteristic in practical applications. Here, we investigated a color filter incorporating a Fabry-Perot structure, discussing its reflective angular sensitivity related to refractive index of its dielectric material. By finite difference time domain(FDTD) theory, the refractive index of the dielectric material is found to influence the angular sensitivity greatly while the optical thickness keeps constant. The simulated results shows that the higher the dielectric layer’s refractive index is, the more angular insensitive of the reflection will be obtained and a good angular insensitive will achieved when the refractive index is larger than 2.1. Finally, samples with different dielectric layer are fabricated in experiment and the measured results verify influence of the refractive index of dielectric layer on the spectra angular sensitivity, which is helpful for the application of color filter in color display, image sensors and decoration.

A printable color filter based on the micro-cavity incorporating a nano-grating

A printable color filter based on the photonic micro-cavity incorporating a nanostructure is proposed, which consists of a nano-metallic grating, a dielectric layer and aluminum (Al) film. According to the resonance induced by different dielectric depths of the micro-cavity, two dielectric heights for the same resonant wavelength are chosen to form the grating heights relative to the Al film. With the contribution of the cavity resonance and the surface plasmon resonance, the proposed structure performs enhanced broadband filtering characteristics with good angular tolerance up to 48° compared to the one of the micro-cavity as well as the one of the metallic grating. Therefore, reflective filters for RGB colors are designed incorporating the proposed structure. Furthermore, for the proposed structure shows great polarization dependence even at normal incidence, it can also be utilized as an anticounterfeiting certificate.

Glasses-free 3D display based on micro-nano-approach and system

Micro-nano optics and digital dot matrix hologram (DDMH) technique has been combined to code and fabricate glassfree 3D image. Two kinds of true color 3D DDMH have been designed. One of the design releases the fabrication complexity and the other enlarges the view angle of 3D DDMH. Chromatic aberration has been corrected using rainbow hologram technique. A holographic printing system combined the interference and projection lithography technique has been demonstrated. Fresnel lens and large view angle 3D DDMH have been outputted, excellent color performance of 3D image has been realized.