Polymer photonic band-gaps fabricated by nanoimprint lithography

We report on the fabrication and characterization of photonic band-gaps structures by nanoimprint lithography in a dye-doped polymer. Photonic band calculations show that photonic crystal slabs composed of a triangular array of polymer pillars could exhibit photonic band-gaps for the magnetic-like modes. The resulting structures show that the nanoimprint lithography process is well-suited to fabricate in a single-step process, these challenging photonic structures opening perspectives to realize integrated photonic band-gap circuits.     

全息制作不同晶面取向光子晶体模板

用单向光入射和单次曝光的激光全息技术制作了类金刚石结构, 此类金刚石结构有宽完全带隙和低介质折射率要求等优点. 在同一种光束配置下，引入恰当的匹配棱镜，用激光全息技术制作了大面积的类金刚石光学晶格不同晶面取向的亚微米周期结构. 满足如负折射效应等研究中所需的特殊晶面取向要求. 关键词： 激光全息技术 光学晶格 光子晶体     

Fabrication of alignment layer free flexible liquid crystal cells using thermal nanoimprint lithography

In this article, thermal nanoimprint lithography (thermal-NIL) has been utilized to transfer the microgroove pattern onto the plastic substrate. Without coating alignment polymer, the microgrooves on the flexible substrates can align liquid crystals (LCs) directly. The flexible LC cell is shown to maintain comparable electro-optical properties while bending. The plastic film and the alignment layer integrated into an alignment substrate could effectively prevent cracks of the additional polymer alignment layer during the bending process. This method is applicable to the roll-to-roll process to increase the production efficiency.     

Fabrication of molecular-electronic circuits by nanoimprint lithography at low temperatures and pressures

We have utilized a modified version of thermal nanoimprint lithography to fabricate a rewritable, nonvolatile, molecular memory device with a density of 6.4 Gbit/cm². It has the advantages of a relatively low operating temperature of (70 °C) and pressure of (<500 psi or 4.5 MPa), both of which are critical to preserving the integrity of the molecular layer. The architecture of the circuit was based on an 8×8 crossbar structure, with an active molecular layer sandwiched between the top and bottom electrodes. A liftoff process was utilized to produce the top and bottom electrodes made of Pt/Ti bilayers. The active molecular layer was deposited by the Languir–Blodgett technique. We utilized a new class of nanoimprint resist formulated by dissolving a polymer in its monomer. The formulation we used, was poly(benzyl methacrylate), dissolved in benzyl methacrylate with t-butyl peroxy 2-ethylhexanoate added as a self-initiator (8:90:2 by weight). The new resist allowed us to achieve Pt/Ti lines of 40 nm in width and 130 nm in pitch. PACS 86.65.+h; 81.16.Nd; 81.16.Rf     

Rhombohedral photonic crystals by triple-exposure interference lithography: Complete photonic band gap

The band structure of 3D photonic crystal that could be synthesized by means of interference lithography with triple-exposure two-beam interference technique has been investigated. As a result of the geometry optimization the optimal conditions for maximal band gaps with different refractive index contrast have been obtained. The refractive index threshold for gap opening equaled to 2.14 has been predicted for this method of photonic crystals synthesis. This value is close to the refractive index thresholds of the best known structures. The continuous transition between simple cubic, face centered cubic and bulk centered cubic symmetries has been considered.     

纳米压印技术制备表面光子晶体LED的研究

利用表面光子晶体能大幅提高发光二极管(LED)的外量子效率, 但如何制备大面积的纳米光子晶体是该研究方向的主要难点之一. 本文基于纳米压印技术在氮化镓基发光二极管(GaN-LED)表面制作孔状二维光子晶体. 通过以金属和聚合物双层掩膜干法刻蚀法, 得到了很好的光子晶体图形转移效果. 最终在LED的p-GaN层表面获得了大面积光子晶体, 周期为450 nm, 纳米孔直径为240 nm. 器件测试结果显示, 有表面光子晶体的LED比没有光子晶体的LED, 光致发光强度峰值提高到了7.2倍.     

Fabrication of ITO diffraction grating structure for infrared plasmonics by thermal nanoimprint lithography

Optical Review - Indium tin oxide (ITO) diffraction grating was fabricated on a polyimide (PI) film by using a combination of thermal nanoimprint lithography (NIL) process and radio frequency (RF)...     

Fabrication of photonic crystals in ZnS-doped glass

Three-dimensional photonic crystals were fabricated in a ZnS-doped silicate glass by use of a femtosecond pulsed laser. Woodpile structures consisting of 36 layers were produced by focusing with a 100x (1.35-numerical-aperture) objective. Attenuations of approximately 3-8 dB arising from a photonic bandgap were observed in the visible and the near-infrared regions.     

Scalable interference lithography alignment for fabrication of three-dimensional photonic crystals

Holographic lithography is an ideal technique for fabricating three-dimensional photonic crystals. However, a critical stage in the fabrication is the minute alignment of the layers with one another. We present a simple moirelike alignment technique with better than 20-nm translation resolution and 45-murad rotation resolution. This technique can easily be extended to other situations when low-cost, high-precision alignment is needed.     

Fabrication of large-area 3D photonic crystals using a holographic optical element

A holographic technique for fabricating 3D photonic crystal is presented. The key element in the fabrication system is a holographic optical element (HOE) consisting of three gratings. Used in combination with a mask, the HOE can generate four beams under single illuminating beam, and 3D lattice structures can be formed by the interference of the four beams. Holographic approach is used to make HOE, so large area lattice structures can be fabricated. Numerical simulations indicate that beam intensity ratio of central beam to outer beam is one of the factors that affects the structures fabricated in photoresist, and high diffraction efficiency of the gratings in HOE is favorable when using cw laser with relatively low power as light source. Experimental results show clear 3D lattice structures fabricated using the HOE, verifying the effectiveness of the technique.     

Sub-nanometer precision modification of the optical properties of three-dimensional polymer-based photonic crystals

We present a convenient post-fabrication technique to precisely tune the optical properties of polymer-based three-dimensional photonic crystals with sub-nanometer precision. Conventional air-plasma etching is utilized to modify the filling fraction of direct laser written polymer photonic crystals beyond the wavelength-imposed limits. Tuning of the optical properties is monitored in transmission and reflection spectroscopy. A simple model for the etching process is proposed and found to be in good agreement with the experimental observations.     

Fabrication of three-dimensional photonic crystals with tunable photonic properties by biotemplating

Photonic crystals with tunable D-surface structures for possible high-temperature gas- and temperature-sensing applications were prepared by a biotemplating method. This included infiltrating colored scales of the beetle Entimus imperialis with an organopolysiloxane mixture followed by simultaneous combustion of the template and calcination of the cured organopolysiloxane. A high-yield inorganic silica-based replica of the original structure was obtained, which is capable of withstanding temperatures up to 600 °C. Light- and scanning electron microscopy combined with focused ion beam milling showed a precise replication of the whole scales and their internal D-surface structure. Fourier-transform infrared spectroscopy and X-ray diffraction analysis confirmed the complete curing of the organopolysiloxanes and their transformation into amorphous silica during calcination. The dielectric constant of the manufactured materials determined by Abbé refractometry was ? = 2.3180 and used to perform band structure calculations utilizing the plane wave expansion method. By changing the chain length and degree of crosslinking of the organopolysiloxane precursor mixture, the lattice parameters and filling factors, and therefore the photonic properties of the replicas, could be tuned.     

Fabrication and annealing analysis of three-dimensional photonic crystals

Three-dimensional face-centered-cubic (fcc) photonic crystals (PhCs) are fabricated on quartz substrate using vertical deposition technique, and followed by annealing in a temperature range of 200-700 °C. The monodispersed SiO₂ microspheres with a diameter of 220 nm in colloidal solution are synthesized using tetraethylorthosilicate as a precursor material. The as grown opal structure exhibits a strong photonic band gap (PBG) around 450 nm in the transmission spectrum. We find that the position of PBG peak in the spectrum is relevant to incident angle of light. Moreover, it is very sensitive to annealing temperature. It quickly shifts to short wavelength direction with annealing temperature increasing. The effect results from the decrease in refraction index due to the moisture evaporation in silica microspheres.     

Controllable fabrication of two-dimensional compound photonic crystals by single-exposure holographic lithography

We demonstrate an approach of single-exposure holographic lithography for controllable fabrication of large-scale two-dimensional square and hexagonal compound photonic crystals. In the sublattices, both circular and elliptical micropores on a 100 nm scale can be achieved. Theoretical analysis reveals that the inverse structure of a sample possesses a unique complete photonic bandgap pair in high frequency regions. This method is very robust for the general fabrication of complex periodic microstructures on the optical scale.     

Demolding temperature in thermal nanoimprint lithography

In thermal nanoimprint lithography, temperature is one of the most important process parameters. Temperature is not only important for the flow of resist during molding but also for demolding, the process by which the imprint stamp is removed from the molded resist/substrate. This is because thermal stress and friction and adhesion forces generated at the stamp/resist interface and the mechanical strength of the resist are all dependent on temperature. In this paper, we demonstrate via both experimentation and numerical simulation that an optimal temperature (T _d) leading to minimal deformation of molded resist exists for demolding. The ease of demolding was directly accessed by measuring demolding force at different T _d for a Si stamp/PMMA/Si substrate system of 4-in.-diameter using a mechanical tester. Numerically, the demolding process for a simple two-dimensional model of a Si stamp/poly(methyl methacrylate) (PMMA) resist/Si substrate system was simulated using a finite-element method for different T _d, assuming viscoelasticity of the PMMA resist and temperature dependence of friction coefficients at the stamp/PMMA interface. We found that a temperature leading to the minimum in both the demolding force and the normalized stress vs. T _d curves exists below the glass transition temperature of the PMMA resist, from which the optimal T _d was derived.     

Terahertz modulator using photonic crystals

In this letter, a novel terahertz wave modulator based on a silicon oxide/polyaniline photonic crystal is proposed. The modulation mechanism of the novel modulator is based on a dynamic shift of the photonic band gap by the applied external electric field. Its performances were investigated with the finite-difference time-domain method. The novel modulator has 3 dB modulation bandwidth of 10 kHz, a size as small as 20 mm and its extinction ratio larger than 30 dB at the frequency of 1 THz.