Lifang Shi

Nanolithography method by using localized surface plasmon mask generated with polydimethylsiloxane soft mold on thin metal film

We propose a photolithographic method to fabricate nanostructures by employing a localized surface plasmon (LSP) mask generated by a soft mold on a thin metal film. The soft mold can be formed by transparent materials, such as polydimethylsiloxane, contacting firmly to the metal film. The pattern edges of the mold, serving as the fine tapers, can be used to excite LSPs and accumulate a large amount of localized energy from the incident light field, providing a modulated optical field in the resist with nanometer feature size. Nanolithographic results with a minimum feature size of 30 nm are demonstrated.

Experimental study of a multiwavelength photon sieve designed by random-area-divided approach

In this paper, a design method for a multiwavelength photon sieve is described based on a random-area-divided approach, where the whole aperture of a multiwavelength imaging photon sieve is divided into multiple discrete spaces corresponding to the number of the selected working wavelengths. The micropinhole distribution in each discrete space can be calculated for the defined wavelength with one fixed focal length in terms of the normal design for photon sieve. A three-wavelength photon sieve was designed and fabricated in the lab, and its imaging properties are analyzed in the experimental optical system with satisfactory results.

Superlens nano-patterning technology based on the distributed Polystyrene spheres

Based on surface plasmon resonant enhancement, a method to realize photolithography beyond diffraction limit by using polystyrene spheres (PSs) self-assembled on silver slab was proposed in this paper. The optimum parameters for PS with different diameters were presented. In order to verify this method, numerical simulation on a typical configuration with 1.5 microm diameter of PS was carried out by using the finite-difference time-domain (FDTD) method, and the minimum feature size of 88 nm beyond diffraction limit at 365 nm working wavelength was obtained.

Effective formation method for an aspherical microlens array based on an aperiodic moving mask during exposure

An aperiodic mask design method for fabricating a microlens array with an aspherical profile is proposed. The nonlinear relationship between exposure doses and lens profile is considered, and the select criteria of quantization interval and fabrication range of the method are given. The mask function of a quadrangle microlens array with a hyperboloid profile used in the infrared was constructed by using this method. The microlens array can be effectively fabricated during a one time exposure process using the mask. Reactive ion etching was carried out to transfer the structure into the substrate of germanium. The measurement results indicate that the roughness is less than 10 nm (pv), and the profile error is less than 40 nm (rms).

Non-iterative phase-only Fourier hologram generation with high image quality

We report a novel and non-iterative method for the generation of phase-only Fourier hologram for image projection. Briefly, target image is first added with a special quadratic phase and then padded with zeros. A complex Fourier hologram is generated via the simple fast Fourier transform. Subsequently, the error diffusion algorithm is applied to convert the complex hologram into a phase-only hologram. The numerical, as well as the optical reconstructed images with the proposed method are of higher visual quality and contain less speckle noise compared to the original random phase method, which add the random phase to the target image and then preserve the phase component of the complex hologram. The influences of quadratic phase and zero-padding on the image quality are also discussed in detail.

The Effects of Profile Errors of Microlens Surfaces on Laser Beam Homogenization

Microlens arrays (MLAs) are key optical components in laser beam homogenization. However, due to imperfect surface profiles resulting from microfabrication, the functionalities of MLAs in beam modulation could be compromised to some extent. In order to address this issue, the effects of surface profile mismatches between ideal and fabricated MLAs on beam homogenization were analyzed. Four types of surface profile errors of MLAs were modeled theoretically and numerical simulations were conducted to quantitatively estimate the effects of these profile errors on beam homogenization. In addition, experiments were conducted to validate the simulation results, revealing that profile errors leading to optical deviations located on the apex of microlenses affected beam homogenization less than deviations located further away from it. This study can provide references for the further applications of MLAs in beam homogenization.

One exposure processing to fabricate spiral phase plate with continuous surface

An economical method for fabricating spiral phase plate (SPP) with continuous surface is proposed in this paper. We use an interval to quantize a three dimensional surface of an SPP into two dimensional bars to form a binary mask. The exposure dose can be precisely distributed through this mask in the exposure process. We discuss the select criterion of the quantization interval and the fabricating processes of SPP in detail. In the results, we present the fabrication of four kinds of high quality SPPs with different topological charges. The morphology analysis and the corresponding optical measurements verify the reliability of our fabrication method.

An artificial compound eye system for large field imaging

With the rapid development of science and technology, optical imaging system has been widely used, and the performance requirements are getting higher and higher such as lighter weight, smaller size, larger field of view and more sensitive to the moving targets. With the advantages of large field of view, high agility and multi-channels, compound eye is more and more concerned by academia and industry. In this work, an artificial spherical compound eye imaging system is proposed, which is formed by several mini cameras to get a large field of view. By analyzing the relationship of the view field between every single camera and the whole system, the geometric arrangement of cameras is studied and the compound eye structure is designed. By using the precision machining technology, the system can be manufactured. To verify the performance of this system, experiments were carried out, where the compound eye was formed by seven mini cameras which were placed centripetally along a spherical surface so that each camera points in a different direction. Pictures taken by these cameras were mosaiced into a complete image with large field of view. The results of the experiments prove the validity of the design method and the fabrication technology. By increasing the number of the cameras, larger view field even panoramic imaging can be realized by using this artificial compound eye.

Resolution and stability analysis of localized surface plasmon lithography on the geometrical parameters of soft mold

We have recently shown that patterns with 30 nm line width and micrometer scale periodicity could be steadily fabricated by employing localized surface plasmons lithography based on a soft mold [Opt. Lett. 35, 13 (2009)]. In this paper, the dependence of the resolution (pattern periodicity), critical dimension, and electric field intensity on the geometrical parameters of the soft mold, such as ridge width, mold periodicity, ridge depth, and slope, have been systematically studied and analyzed. The relevant simulation results by finite-difference time-domain demonstrate that the critical dimension exhibits a perfect stabilization and the value of electric field intensity would be especially large, when the ridge depth is in the range from 100 to 270 nm and the slope angle is below 35°. Importantly, the optimal resolution and critical dimension can reach 100 and 17 nm, respectively, by reasonably designing the corresponding mold periodicity and ridge width, which indicates that the method is particularly suitable for obtaining patterns with high density and is extremely promising for bio-sensing and photonic crystals application.

Center Off-Axis Tandem Microlens Arrays for Beam Homogenization

Tandem microlens arrays are commonly used in many applications for the beam shaping of an arbitrary input intensity distribution into a top hat. Because of the periodic structure of the regular arrays, the output intensity distribution is modulated by equidistantly located sharp intensity peaks, which reduces the homogeneity of the beam. A new concept is proposed in this paper for tandem microlens arrays incorporating a center off-axis microlens array for the generation of an intensity far-field distribution with improved homogeneity under coherent illumination with the envelope of a top hat. The geometric centers of the sub-apertures are randomly offset by the light axis of the sub-lens in the array to break the periodicity. The influences of the various off-axis amounts on the homogeneity are discussed. The center off-axis microlens array has been fabricated, and the corresponding laser beam homogenizing experiment has been carried out. The results verify that a high homogeneity output intensity distribution can be achieved based on the presented method.