Photonic Crystal Templates Research Articles

Nanoimprinting lithography of a two-layer phase mask for three-dimensional photonicstructure holographic fabrications via single exposure

We report a combined holographic and nanoimprinting lithography technique toproduce three-dimensional woodpile photonic crystal templates through onlyone single exposure. The interference lithography process uses an integratablediffractive optical element for large throughout 3D pattern manufacturing. Thediffractive optical element consists of two layers of phase grating separated by anintermediate layer, fabricated by repeated nanoimprinting lithography, followed by anSU8 photoresist bonding technique. Grating periods, relative orientation, diffraction angle, andefficiency, as well as layer to layer phase delay, are well designed during manufacturing. Bythermally optimizing the thickness of the intermediate layer, this paper demonstrates thefabrication of interconnected 3D photonic structures with arbitrary symmetry through asingle laser exposure. The two-layer phase mask approach enables a CMOS-compatiblemonolithic integration of 3D photonic structures with other integrated optical elements andwaveguides.

Photonic crystals with defect structures fabricated through a combination of holographic lithography and two-photon lithography

This paper presents the capability of direct laser writing of complex defect structures in holographically formed three-dimensional photonic crystals in dipentaerythritol penta/hexaacrylate (DPHPA) monomers mixed with photoinitiators. The three-dimensional photonic crystal template was fabricated through prism-based holographic lithography. Defect structures are fabricated through the two-photon polymerization excited by a femtosecond laser. The strengths of two optical lithographic techniques are combined with holographic lithography providing a rapid and large area microfabrication and two-photon lithography providing flexibility in fabrication of defect structures. The optical fabrication process is simplified in the negative tone DPHPA without prebake and postexposure bake as is required of SU-8 while maintaining a capability for constructing photonic structures with small features.

INVESTIGATIONS AND MIMICRY OF THE OPTICAL PROPERTIES OF BUTTERFLY WINGS

Structural color in Nature has been observed in plants, insects and birds, and has led to a strong interest in these phenomena and a desire to understand the mechanisms responsible. Of particular interest are the optical properties of butterflies. In this paper, we review three investigations inspired by the unique optical properties exhibited in a variety of butterfly wings. In the first investigation, conformal atomic layer depositions (ALDs) were used to exploit biologically defined 2D photonic crystal (PC) templates of Papilio blumei with the purpose of increasing the understanding of the optical effects of naturally formed dielectric architectures, and of exploring any novel optical effects. In the second study, it was demonstrated that faithful mimicry of Papilio palinurus can be achieved by physical fabrication methods through using breath figures to provide templates and ALD routines to enable optical properties. Finally, knowledge of the optical structure properties of the Princeps nireus butterfly has resulted in bioinspired designs to enhanced scintillator designs for radiation detection.

Holographic realization of hexagonal two dimensional photonic crystal structures with elliptical geometry

A complete investigation of holographic photonic crystal structures has been conducted. From both theoretical and experimental results, profiles of resultant patterns under different process conditions can be estimated and controlled. The use of antireflection layers is crucial for realizing submicron photonic crystals with good uniformity over a large area. We successfully realize submicron-scale photonic crystal templates on silicon substrates with an aspect ratio of 2.5 and good quality by a laser holography technique. The samples are highly uniform in an area of >2×2 cm2 and present good reproducibility. A lift-off process is performed to transfer inversed pillar patterns into a chromium hard mask for the following dry etching into silicon substrates. A single-step deep reactive ion etching with controlled mixture of Ar/SF6/C4F8 gases is used to directly transfer pillar patterns into silicon. Transferred patterns with a high aspect ratio and vertical sidewalls (no scalloping) are demonstrated over a large area.

Kinetics of Stop-Flow Atomic Layer Deposition for High Aspect Ratio Template Filling through Photonic Band Gap Measurements

Atomic layer deposition (ALD) is shown as a unique method to produce high aspect ratio (AR) nanostructures through conformal filling and replication of high AR templates. The stop-flow process is often used as an alternative to the conventional continuous flow process to obtain high step coverage. However, there is a need for understanding the deposition kinetics and optimizing the deposition process to fabricate defect-free nanostructures. In this Article, TiO2 ALD in high AR self-assembled opal photonic crystal templates was performed in stop-flow fill−hold−purge process in comparison with continuous flow pulse−purge process. Photonic band gap properties of opal templates were characterized and compared with simulated band diagrams for quantitative investigation of filling kinetics and the effect of shrinking pore size on filling uniformity. Γ−L bands in the transmittance spectra of ALD-infiltrated opals accurately represented the depth profile of the depositions without the need for expensive sample preparation techniques and characterization tools. It was found that the stop-flow process attains higher Knudsen flow rates of precursor gases, thereby achieving homogeneous and complete filling at considerably lower cycle time.

Optimization of Emulsion Polymerization for Submicron-Sized Polymer Colloids towards Tunable Synthetic Opals

Submicron-sized polymeric colloidal particles can self assemble into 3-dimensional (3D) opal structure which is a useful template for photonic crystal. Narrowly dispersed polymer microspheres can be synthesized by emulsion polymerization in water using water-soluble radical initiator. In this report, we demonstrate a facile and reproducible emulsion polymerization method to prepare various polymeric microspheres within 200 - 400 nm size ranges which can be utilized as colloidal photonic crystal template. By controlling the amount of monomer and surfactant, monodisperse polymer colloids of polystyrene (PS) and acrylates with various sizes were successfully prepared without complicated synthetic procedures. Such polymer colloids self-assembled into 3D opal structure exhibiting bright colors by reflection of visible light. The colloidal particles and the resulting opal structures were rigorously characterized, and the wavelength of the structural color from the colloidal crystal was confirmed to have quantitative relationship with the size of constituting colloidal particles as predicted by Bragg equation. The tunability of the structural color was achieved not only by varying the particle size but also by infiltration of the colloidal crystal with liquids having different refractive indices.

Fabrication of desired three-dimensional structures by holographic assembly technique

We demonstrate a novel technique to fabricate desired three-dimensional (3D) periodic structures by holographically assembling multiple one-dimensional (1D) or multiple two-dimensional (2D) structures. Thanks to the high-absorption effect of the used material, we fabricated for each time, by employing a two-beam interference technique, a 1D or a 2D structure with very limited film thickness. By using the same sample and repeating the same fabrication process, i.e., (i) spin coating, (ii) exposure, and (iii) post-exposure bake, we created, layer-by-layer, a 3D structure as desired, without the limitation of the number of layers. This technique allows rapid fabrication of very large and thick 3D photonic crystal template with variable period, flexible design, low cost, and possible introduction of arbitrary defects inside a 3D structure.

Blue phases as templates for 3D colloidal photonic crystals

Blue phases as templates for 3D colloidal photonic crystals

Flexible fabrication of three-dimensional optical-domain photonic crystals using a combination of single-laser-exposure diffractive-optics lithography and template inversion

We demonstrate inversion of three-dimensional photonic crystal templates fabricated in a large area with diffractive-optics lithography. A custom-designed two-dimensional diffractive optical element was used to generate highly uniform, bicontinuous, three-dimensional photonic crystal templates in a single-laser exposure. Chemical vapor deposition successfully infiltrated the thick periodic polymer structure to deposit amorphous silica and thereby define an all-silica inverted photonic crystal after polymer removal, as confirmed by focused ion-beam milling and energy-dispersive x-ray spectroscopy. The diffractive-optics lithography permitted a large number of uniform layers to form that manifested in the recording of a strong -28 dB stopband in the telecom band.

Five beam holographic lithography for simultaneous fabrication of three dimensional photonic crystal templates and line defects using phase tunable diffractive optical element

This paper demonstrates an approach for laser holographic patterning of three-dimensional photonic lattice structures using a single diffractive optical element. The diffractive optical element is fabricated by recording gratings in a photosensitive polymer using a two-beam interference method and has four diffraction gratings oriented with four-fold symmetry around a central opening. Four first-order diffracted beams from the gratings and one non-diffracted central beam overlap and form a three-dimensional interference pattern. The phase of one side beam is delayed by inserting a thin piece of microscope glass slide into the beam. By rotating the glass slide, thus tuning the phase of the side beam, the five beam interference pattern changes from face-center tetragonal symmetry into diamond-like lattice symmetry with an optimal bandgap. Three-dimensional photonic crystal templates are produced in a photoresist and show the phase tuning effect for bandgap optimization. Furthermore, by integrating an amplitude mask in the central opening, line defects are produced within the photonic crystal template. This paper presents the first experimental demonstration on the holographic fabrication approach of three-dimensional photonic crystal templates with functional defects by a single laser exposure using a single optical element.

Replicated photonic crystals by atomic layer deposition within holographically defined polymer templates

We report the replication of holographically defined photonic crystals using multistage atomic layer deposition. Low- and high-temperature atomic layer depositions were combined with selective etching to deposit and remove multiple conformal thin films within three-dimensional polymer templates. Using intermediate Al2O3 inverse replicas, temperature-sensitive SU-8 photonic crystal templates were faithfully replicated with TiO2 and GaP, greatly increasing the dielectric contrasts of the photonic crystals. Optical measurements are in good agreement with the calculated band structures.

Phase tunable holographic fabrication for three-dimensional photonic crystal templates by using a single optical element

This paper demonstrates a phase tunable holographic fabrication of three-dimensional photonic lattice structures using a single optical element. A top-cut four-side prism is employed to generate five-beam three-dimensional interference patterns. A silica glass slide is inserted into the optical path to adjust the phase of one interfering beam relative to other four beams. The phase control of the interfering laser beam renders the lattice of the interference pattern from a face-center tetragonal symmetry into a high contrast, interconnecting diamondlike symmetry. This method provides a flexible approach to fabricating three-dimensional photonic lattices with improved photonic band structures.

Self-assembled SiO_2 photonic crystal infiltrated by Ormosil:Eu(DBM)_3phen phosphor and its enhanced photoluminescence

Luminescence films were prepared by infiltration of the tris(dibenzoylmethane) mono(1, 10-phenanthroline) europium incorporated ormosil into colloidal SiO(2) photonic crystal templates. Because a stopband of the template was not overlapped with the PL excitation and emission bands, the stopband did not suppress the PL intensity. The PL intensity of the infiltrated film into the template was about 13.1 times higher than that of the plane film prepared without the template. Three major terms, which are the mass term, the scattering term, and the crystallinity term, were considered as factors that improve the PL intensity. The relative ratio of the effects of the mass term : the scattering term : the crystallinity term was 2.1 : 2.8 : 2.2.

Complex three-dimensional conformal surfaces formed by atomic layer deposition: computation and experimental verification

Atomic layer deposition in complex three-dimensional porous materials is useful for manipulating properties such as pore size, pore connectivity, density, and dielectric constant. In order to calculate a material's properties it is necessary to determine the material distribution. A generally applicable algorithm for determining the material distribution and pore connectivity is presented. Calculations using the algorithm compared favorably with experimental results for the important case of infilling dielectric material into three-dimensional photonic crystal templates.

Holographic fabrication of photonic crystals using multidimensional phase masks

This paper reports the experimental approaches to the fabrication of two-layer integrated phase masks and the fabrication of photonic crystal templates using the phase mask based on holographic lithography technique. The photonic crystal template is formed by exposing photoresist mixtures to five-beam interference patterns generated through the phase mask. The fabricated phase mask consists of two layers of orthogonally oriented gratings produced in a liquid crystal and photoresist mixture. A polymerization-induced phase separation preserves the grating structure during the exposure. The vertical spatial separation between two layers of gratings produces a phase difference among diffracted laser beams, which enables the holographic fabrication of diamondlike photonic crystal structures. The fabricated photonic crystal structure is consistent with simulations based on the five-beam interference. The two-layer phase mask opens up an opportunity of direct printing photonic structures.

Holographic fabrication of diamondlike photonic crystal template using two-dimensional diffractive optical elements

This letter demonstrates holographic fabrication of three-dimensional diamondlike photonic crystal templates in SU8 photoresist using a single diffractive optical element. Five coherent laser beams produced by a two-dimensional phase mask were used to construct face-centered-cubic or tetragonal interference patterns. The superposition of two interference patterns through double exposures yields diamondlike photonic crystal templates in SU8. Photonic bandgap calculation reveals a full bandgap in inverse structures based on the template. The utilization of the two-dimensional phase mask simplifies the fabrication configuration in multiple beam holographic lithography for three-dimensional photonic fabrication.

Templating and Replication of Spiral Photonic Crystals for Silicon Photonics

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper describes femtosecond laser lithography of 3-D photonic crystal templates in commercial photoresist SU-8 and replication of these templates with silicon. Using this approach, silicon-based photonic crystals having 3-D square spiral architecture and exhibiting photonic stop gaps near the 2.5-<formula formulatype="inline"> <tex>$\mu$</tex></formula>m wavelength were fabricated. Possibilities to use a multiple-beam interference technique for two-photon absorption templating of photonic crystals are explored. </para>

Single laser exposure fabrication of diamond-like 3-dimensional photonic crystal microstructures using circularly polarized light

Phase tunable multi-level diffractive optical elements define an attractive approach for single laser exposure fabrication of 3-dimensional photonic crystal microstructures. The significant advantage of these multi-level diffractive optical elements over two-level diffractive optical elements is the flexibility of fabricating a wide range of 3-dimensional periodic structures by manipulating the relative phase of different diffracted beams. Here, phase tuning was applied to demonstrate fabrication of a hybrid 3-dimensional structure intermediate between previously reported diamond-like Woodpile-type structure of tetragonal symmetry and structure having body-centered-tetragonal lattice symmetry. Circularly polarized light was applied for the first time to balance the diffraction order efficiencies and improve the structural uniformity. Design guidelines are presented for generating diamond-like photonic crystal template that possesses complete photonic bandgap when inverted with high refractive index materials.

Fabrication of two-layer integrated phase mask for single-beam and single-exposure fabrication of three-dimensional photonic crystal

In this paper, we report a new design and fabrication of an integrated two-layer phase mask for five-beam holographic fabrication of three-dimensional photonic crystal templates. The phase mask consists of two layers of orthogonally oriented gratings produced in a polymer. The vertical spatial separation between two layers produces a phase shift among diffractive laser beams, which enables the holographic fabrication of inter-connected three-dimensional photonic structures. A three-dimensional photonic crystal template was fabricated using the two-layer phase mask and was consistent with simulations based on the five beam interference. The reported method simplifies the fabrication of photonic crystals and is amendable for massive production and chip-scale integration of three-dimensional photonic structures.

Colorful humidity sensitive photonic crystal hydrogel

A colorful humidity sensitive photonic crystal (PC) hydrogel was facilely fabricated by infiltrating acrylamide (AAm) solution into a P(St–MMA–AA) PC template and subsequently photo-polymerizing. The color of the samples was sensitive to humidity; it could reversibly vary from transparent to violet, blue, cyan, green and red under various humidity conditions, covering the whole visible range. This could be attributed to the humidity sensitivity of the sample's stopband, with a maximum change of 240 nm resulting from the varying humidity. Furthermore, the color response showed good stability under cyclic humidity experiments. As-prepared PAAm–P(St–MMA–AA) PC hydrogel successfully combined the humidity sensitivity of PAAm and structure color of the PC template, which suggested a promising composite material as an economical alternative to traditional humidity sensors, and also provided a new insight into the design and development of novel composite functional materials based on a PC template.