Jianhong Wu

Fabrication of blazed grating by native substrate grating mask

Abstract. The blazed grating is one of the key elements in a spectrometer. Hence we have adopted a new method of its fabrication. First, the combination of ion beam etching (IBE) and reactive IBE is used to fabricate a native substrate grating mask to replace the traditional photoresist grating mask. This is because the former mask allows for more accurate control of the profile than the latter. Then tilted IBE is used to etch the native substrate grating to ensure the precise control of the blazed angle and antiblazed angle. Therefore, an optimal blazed grating profile is successfully fabricated. Two types of blazed gratings with the same period of 833 nm have been fabricated: one with the blaze angle of 11 deg and the antiblaze angle of 72 deg; and one with the blaze angle of 20 deg and the antiblaze angle of 68.5 deg, respectively.

The convex grating with high efficiency for Hyperspectral Remote Sensing

The convex grating is one of the key elements in Offner imaging spectrometers. In this paper the diffraction efficiency of the convex grating is investigated by using rigorous coupled-wave analysis (RCWA), which indicates that within the visible and near infrared broadband, the first-order diffraction efficiency can be over 40% through controlling the blaze angle of blazed convex grating. The blazed convex grating with the period of 6.17μm in the center, the blaze angle about 2.9 degree, the anti-blaze angle about 21 degree, and the ruled area – a convex substrate with its radius 36.31mm and aperture 23.6mm has been fabricated by holographic lithography - scan ion beam etching. Experimental measurements show that the first-order diffraction efficiency is more than 30%, and the first-order diffraction efficiency at blazed wavelength can reach 62%.

The fabrication of the holographic dual-blazed grating

A holographic dual-blazed grating with the period of 833 nm at ultraviolet-visible-near infrared was designed. To achieve higher and uniform diffraction efficiency, the grating profile was optimized by using rigorous coupled-wave analysis. The results show: when the two blaze angles of the dual-blazed grating, one within the range from 10 degree to 12 degree and the other one within the range from 26 degree to 32 degree respectively, the first-order diffraction efficiency is more than 30 percent within the wavelength from 0.25μm to 1μm. The holographic dual-blazed grating, one half with the blaze angle of 11 degree and the anti-blaze angle of 72 degree; and the other half with the blaze angle of 29 degree and the anti-blaze angle of 56 degree respectively, have been fabricated by improved holographic ion beam etching.

A method of fabricating blazed grating by homogenous grating mask

In this paper, a new approach has been proposed to fabricate holographic blazed grating. Firstly, a rectangular or trapezoidal grating is fabricated by the combination of photoresist ashing and reactive ion beam etching, and then blazed grating can be achieved by etching rectangular or trapezoidal grating. With this approach homogenous mask profile can be precisely controlled. The result we get by simulating the evolution of rectangular mask for ion beam milling with advanced segment motion algorithm which turns out to be similar with the experimental results. The blazed grating with a line density of 1200 lp/mm is fabricated by above method, and then the blaze angle and anti-blaze angle are measured by Scanning Electron Microscope (SEM). Obtained result shows that blazed grating has 7.1 degree blaze angle and about 22.5 degree anti-blaze angle.

Fabrication of the metal wire-grid polarizer in visible light

The polarization characteristics of the metal wire-grid polarizer are investigated by Finite Difference Time Domain method. To achieve a high extinction ratio and transmissivity in visible light, the metal wire-grid profile was optimized. The simulation results show that the period should be less than 200 nm. When the period, the glass substrate etching depth and the Al groove depth of metal wire-grid were at 180nm, 20nm and 120nm respectively, the extinction ratio could reach the maximum 40 dB; the transmissivity could reach the maximum 85%. Herein, considering the high reflection of metal aluminum in the manufacturing process, we propose using the copper as anti-reflection coating. Finally, a prototype of the metal wire-grid polarizer has been fabricated by holographic-immersion lithography and ion beam etching.

Fabrication of the nanoimprint mold with deep-subwavelength structures

To overcome the disadvantages of high-cost, low-efficiency and the difficulty in the realization of the high aspect ratio structure in the fabrication of nanoimprint mold with deep-subwavelength structures by electron beam lithography (EBL), the holographic immersion lithography - ion beam etching is adopted. There are two major challenges of this method: the holographic immersion lithography and the ion beam etching. The former one is to fabricate the photoresist mask with deep-subwavelength structures; and the latter one is to transfer the photoresist mask to the fused silica. Both the two steps have been optimized to achieve the accurate control of the high aspect ratio of the profile. The experiment indicates that titled rotation of the ion beam etching combined with reactive ion beam etching can achieve the accurate control of the high aspect ratio structure. The nanoimprint mold with the period of 180nm and the groove depth of 233nm has been fabricated.

Fabrication of the blazed convex grating by holographic ion beam etching

The blazed convex grating is one of the key elements in Offner imaging spectrometers. In this paper the diffraction characteristics of the blazed convex grating is investigated by using rigorous coupled-wave analysis. The results indicate that within the wavelength from 1 μm to 2.5 μm, the first-order diffraction efficiency can be over 40% through controlling the blazed angle of blazed grating. Furthermore, when the blazed angle is 3.9 degree, the first-order diffraction efficiency is still over 40% with the vertex angles between 120 degree and 150 degree. The blazed convex grating with the spatial frequency of 73 L/mm in the center will be fabricated by holographic lithography-ion beam etching.

Design and fabrication of sub-wavelength metal grating polarizer in visible broadband

In this paper, a sub-wavelength metal grating polarizer at visible region is designed by Finite Difference Time Domain (FDTD) method. We have analyzed the effects of the period, the thickness of metal layer and the duty cycle on the performance of the metal grating polarizer. Simulation results show that in order to increase the metal grating transmissivity and extinction ratio and to reduce the angle sensitivity (±30°), the period should be less than 200 nm; Therefore, the structure with 180nm period and an 120nm Al layer is chosen, and the transmission efficiency and extinction ratio is more than 75% and 25dB respectively. Finally, a prototype of the metal grating has been fabricated by holographic-immersion lithography and ion beam etching.

Fabrication of the nanoimprint template with periodic structures

The holographic lithography - ion beam etching is adopted to fabricate the nanoimprint template with periodic structures. The accurate control of the high aspect ratio of the profile is achieved by the optimization of the holographic lithography and the choice of the appropriate parameters of ion beam etching. There are two major challenging steps of this method: 1) the holographic exposure and development in the fabrication of the photoresist mask and 2) the ion beam etching to transfer the photoresist mask to the fused silica. The experiment indicates that titled rotation of the ion beam etching combined with reactive ion beam etching can achieve the accurate control of the high aspect ratio structure. Two types of nanoimprint template have been fabricated: the period of 250nm and the groove depth of 380nm; the period of 600nm and groove depth of 1400nm, respectively.

Design and fabrication of sub-wavelength metal polarization gratings used in polarization imaging

This paper designs a sub-wavelength metal polarization gratings array composing two orthogonal micro polarization gratings as one unit. Effects of different metal materials and grating profile on the TM and TE polarization transmittance as well as the extinction ratio are analyzed by the Finite Difference Time Domain (FDTD) method. Based on the requirement of the visible light polarization imaging and the resolution of the holographic lithography, we obtain the best parameters for the grating: period is 250nm, Al thickness is 260nm, and duty cycle is 0.4, the transmittance is higher than 45% and extinction ratio is higher than 100. Then, the sub-wavelength metal polarization gratings have been fabricated by two methods: the holographic - ion beam etching - Al oblique deposition or the holographic - reactive ion etch (RIE) - ion beam etching. Preliminary results indicate the polarization information has been obtained. A prototype metal polarization gratings array will be fabricated in late 2011.