Fabrication Of Photonic Crystal Structures Research Articles

Design and fabrication of photonic crystal structures by single pulse laser interference lithography

Photonic crystal (PhC) structures formed by periodic surface nanostructuring have emerged as pivotal elements for controlling light-matter interactions. One important application is reducing losses due to the high surface reflectivity of semiconductor optoelectronic devices, such as enhancing light absorption in photovoltaic cells or improving light extraction in light-emitting diodes (LEDs). Although various methods for fabricating such structures have been documented, the utilization of single pulse laser interference lithography (LIL) using commercial photoresist and its subsequent effective use as an etch mask has not been previously reported. Rapid exposure of photoresists with single nanosecond pulses offers benefits for high throughput patterning and reduces the requirement for a stable optical platform. We have successfully employed single pulse LIL to fabricate antireflective PhC structures on GaAs substrates using a commercial photoresist. Exposure is performed with single 7 ns 355 nm pulses of relatively low energy (<10 mJ). High-quality nanohole arrays of pitch of approximately 365 nm are fabricated and depths up to 400 nm have been etched using inductively coupled plasma (ICP) through the exposed photoresist mask. Reflectivity analyses confirmed that these structures reduce the average reflectance of the GaAs to below 5 % across the 450 nm to 700 nm visible wavelength range. The fabrication of PhC structures using this approach has potential for low-cost wafer-level patterning to provide improved light extraction in LEDs and enhanced light trapping in solar cells.

Fabrication of Photonic Crystal Structures on GaAs by Single Pulse Laser Interference Lithography

Fabrication of Photonic Crystal Structures on GaAs by Single Pulse Laser Interference Lithography

Fabrication of photonic crystal structures by tertiary-butyl arsine-based metal–organic vapor-phase epitaxy for photonic crystal lasers

The fabrication of air/semiconductor two-dimensional photonic crystal structures by air-hole-retained crystal regrowth using tertiary-butyl arsine-based metal–organic vapor-phase epitaxy for GaAs-based photonic crystal lasers is investigated. Photonic crystal air holes with filling factors of 10–13%, depths of ∼280 nm, and widths of 120–150 nm are successfully embedded. The embedded air holes exhibit characteristic shapes due to the anisotropy of crystal growth. Furthermore, a low lasing threshold of ∼0.5 kA/cm2 is achieved with the fabricated structures.

GaN nanostructuring for the fabrication of thin membranes and emerging applications

We present a review of technological methods developed in recent years for the purpose of gallium nitride nanostructuring, with the main focus on fabrication of thin GaN membranes. In particular, we report on traditional methods of wet etching undercutting for membrane manufacturing, technologies applied for the fabrication of photonic crystal structures based on GaN nanomembranes, double side processing, and surface charge lithography. Prospects of membrane applications in photonic devices, sensors, and microoptoelectromechanical and nanoelectromechanical systems are discussed, taking into account the advantageous piezoelectric, optical, and mechanical properties of GaN and related III-V nitride materials.

Synthesis of ZnO submicron spheres by a two-stage solution method

This paper presents a two-stage method to synthesize the mono-dispersed ZnO spheres. The size and uniformity of ZnO spheres can be controlled in the sub-micrometer range. This makes the method applicable to the fabrication of photonic crystal structures. The effects of different reaction parameters such as reaction temperatures, reaction times, and water concentrations on the formation of ZnO spheres were investigated. On the basis of the experimental results, we discuss and propose a possible mechanism to elucidate the formation of ZnO submicron spheres. Furthermore, the room temperature photoluminescence (PL) measurements of the products were carried out to investigate their optical properties. The PL results showed that the ZnO submicron spheres exhibited a strong ultraviolet emission and a broadband green emission in the visible region.

Fabrication of photonic crystal structures on flexible organic light-emitting diodes using nanoimprint

Nanoimprint lithography (NIL) was used to create a film with photonic crystal structures in the organic light-emitting diode (OLED) component. By utilizing various stamps, such as with photonic crystal structures and with grating structures in polymethylmethacrylate (PMMA) were studied. The better formability belongs to a stamp of photonic crystal patterns and molecular weight of PMMA in 350K and 996+15K in double layers. A photonic crystal with nanostructures in a period of 400nm was fabricated as a photonic crystal film and was integrated into an organic light emitting diode (OLED). The experiments show that the OLED without photonic crystal films behaves 2.11cd/m^2 for luminous intensity, 2.336cd/A for lightening efficiency (@hL), and 0.475lm/W for lightening power (@hP), under the driving voltage of 10V and current of 0.009012A; the OLED with the photonic crystal films behaves 250cd/m^2 for luminous intensity and 2.774cd/A for @hL, and 0.871lm/W for @hP, under the same driving voltage and same current, which shows that the latter performs well.

Fabrication of photonic crystals for applications in the visible range by Nanoimprint Lithography

The integration of photonic crystals into optical circuits is a decisive factor for further development of photonic crystal applications. The feasibility of these applications depends on fabrication technologies suitable for mass production. In this work, we used Nanoimprint Lithography (NIL) for the fabrication of photonic crystal structures for applications in the visible range. The photonic crystals were integrated into waveguides in order to characterize the created system. The waveguides have dimensions of up to 50 μm whereas the holes in the photonic crystals have dimensions of 80 nm. Due to parameter optimization photonic crystal structures and the corresponding waveguides could be replicated with high accuracy. For the fabrication of the photonic crystal structures a Si substrate with an oxide and a nitride layer was used. A poly-methyl-methacrylate (PMMA) layer was spincoated onto this substrate. A stamp containing the negative structures was fabricated using Electron Beam Lithography (EBL). This stamp was used for imprinting the structures into the PMMA layer. The structures were than transferred into the nitride layer using reactive ion etching (RIE). The underlying oxide layer was used as a sacrificial layer to achieve a nitride membrane. The fabricated structures were characterized by measuring the transmission spectra. The results were compared favorably to a simulation and a photonic band gap (PBG) in the range of 670 nm to 780 nm has been observed.

Fabrication of photonic crystal structure on indium tin oxide electrode of GaN‐based light‐emitting diodes

AbstractWafer‐scale UV nanoimprint and dry etching processes were used to fabricate photonic crystal patterns on an indium tin oxide (ITO) top electrode of GaN‐based green light‐emitting diodes (LEDs). Photonic crystal patterns with pitches ranging from 600 to 900 nm were transferred to the entire 2‐inch LED wafer. Plasma damage in the GaN crystal of the LED device was avoided because the GaN was covered by ITO layer and was not exposed to the etching plasma during the ITO dry etching process. Consequently, the electroluminescence intensity of the patterned LED devices was enhanced up to 25% at 20 mA of driving current, compared to the nonpatterned LED device, while the forward voltage was similar.

UV enhanced substrate conformal imprint lithography (UV-SCIL) technique for photonic crystals patterning in LED manufacturing

The global LED (light emitting diode) market reached 5 billion dollors in 2008 and will be driven towards 9 billion dollors by 2011 [1]. The current applications are dominated by portable device backlighting, e.g. cell phones, PDAs, GPS, laptop etc. In order to open the general lighting market doors the luminous efficiency needs to be improved significantly. Photonic crystal (PhC) structures in LEDs have been demonstrated to enhance light extraction efficiency on the wafer level by researchers [2]. However, there is still a great challenge to fabricate PhC structures on LED wafers cost-effectively. Nanoimprint lithography (NIL) [3] has attracted considerable attentions in this field due to its high resolution, high throughput and low cost of ownership (CoO). However, the current NIL techniques with rigid stamps rely strongly on the substrate flatness and the production atmosphere. Those factors hinder the integration of NIL into high volume production lines. UV-NIL with flexible stamps [4], e.g. PDMS stamps, allows the large-area imprint in a single step and is less-sensitive to the production atmosphere. However, the resolution is normally limited due to stamp distortion caused by imprint pressure. A novel NIL technique developed by Philips Research and Süss MicroTec, substrate conformal imprint lithography (SCIL), bridges the gap between UV-NIL with rigid stamp for best resolution and soft stamp for large-area patterning. Based on a cost-effective upgrade on Süss mask aligner, the capability can be enhanced to nanoimprint with resolution of down to sub-10 nm on an up to 6 inch area without affecting the established conventional optical lithographic processes on the machine. Benefit from the exposure unit on the mask aligners, the SCIL process is now extended with UV-curing option, which can help to improve the throughput dramatically. In this paper, the fabrication of photonic crystal structures with SCIL technique on Süss MA6 mask aligner is demonstrated. In addition, the industrialization considerations of UV-SCIL process in high volume manufacturing are briefly discussed.

Three-dimensional microfabrication of materials by femtosecond lasers for photonics applications

Femtosecond laser fabrication of three-dimensional structures for photonics applications is reviewed. Fabrication of photonic crystal structures by direct laser writing and holographic recording by multiple beam interference techniques are discussed. The physical mechanisms associated with structure formation and postfabrication are described. The advantages and limitations of various femtosecond laser microfabrication techniques for the preparation of photonic crystals and elements of microelectromechanical and micro-optofluidic systems are discussed.

Fabrication of photonic crystal structures on light emitting diodes by nanoimprintlithography

Photonic crystal (PC) structures on green light emitting diodes (LED) were successfullyfabricated using nanoimprint lithography. The stamp, with a two-dimensionalpillar structure, was fabricated using laser interference lithography through adouble exposure technique. To achieve PC structures with a precise dimension,thermal treatment of the photoresist was employed during stamp fabrication; thisproved to be effective for the control of the diameter and shape of the hole. Thetwo-dimensional PC structures, with a period of 295 nm, diameter of 180 nm anddepth of 100 nm, on the green LEDs resulted in nine-fold enhancement of thephotoluminescence intensity compared to the as-deposited samples without PC structures.

Fabrication of photonic crystal structures in polymer waveguide material

In this paper we show some aspects of the fabrication of photonic crystal (PC) structures in a polymer waveguide material on different waveguide substrate materials. The photonic structures consist of a slab waveguide perforated by a periodic arrangement of sub-micron high aspect ratio holes with typical hole diameters of 300 nm at periods of 500 nm and a depth of down to 4 μm. The patterns were written by an electron beam writer in an e-beam resist, then transferred into a NiCr-hard mask and etched in the polymer waveguide by means of an electron cyclotron resonance (ECR) high density plasma system. For the waveguide the polymer PMMA-DR1 was used and for the waveguide substrates, materials with different refractive indices (Teflon and an ultra low refractive index “air like” material) were investigated. PCs made in polymer on ultra low “air like” index substrate are easier to fabricate and show higher transmission efficiency and higher Q-factor compared to those made in polymer on Teflon.

Fabrication of photonic crystal waveguides composed of a square lattice of dielectric rods

The use of a negative resist, hydrogen silsesquioxane (HSQ), combined with electron-beam lithography, was found to greatly simplify the fabrication of photonic crystal structures having submicron-sized features. Two methods of fabrication were compared for the creation of photonic crystals; in this example, the structures were composed of a square lattice of dielectric rods. One method utilized positive resist PMMA, whereas another method used a negative resist HSQ. The process sequence using PMMA required a lift-off step and a hard mask in order to convert the hole-patterned PMMA into high-aspect-ratio dielectric rods through reactive ion etching. Alternatively, the electron-beam exposure and development of HSQ resist resulted in the formation of SiO2-like posts which then served as a hard mask for subsequent etches. The use of HSQ eliminated the need for the SiO2 deposition and nickel lift-off, thereby reducing the required number of steps in the process sequence and greatly simplified the fabrication.

Topology optimization and fabrication of photonic crystal structures

Topology optimization is used to design a planar photonic crystal waveguide component resulting in significantly enhanced functionality. Exceptional transmission through a photonic crystal waveguide Z-bend is obtained using this inverse design strategy. The design has been realized in a silicon-on-insulator based photonic crystal waveguide. A large low loss bandwidth of more than 200 nm for the bandgap polarization is experimentally confirmed.

Two-dimensional coupled photonic crystal resonator arrays

We present the design and fabrication of photonic crystal structures exhibiting electromagnetic bands that are flattened in all crystal directions, i.e., whose frequency variation with a wave vector is minimized. Such bands can be used to reduce group velocity of light propagating in an arbitrary crystal direction, which is of importance for the construction of miniaturized tunable optical delay components and low-threshold photonic crystal lasers, and the study of nonlinear optics phenomena.

Fabrication of Photonic Crystal Structures by Femtosecond Laser-Induced Photopolymerization of Organic-Inorganic Film

We report on a fabrication of photonic crystal structures in organic-inorganic hybrid films by a laser interference technique. Films containing the methacrylic group, which is photopolymerable by an adequate laser light, are prepared using a sol-gel technique from a mixture of organosilicate alkoxide and zirconium alkoxide modified by methacrylic acid. For the photopolymerization, the coated films on glass substrate are exposed to the interference light which is arranged with a square lattice in about 1 μm spacing. From microscopic images of microstructures produced by the photopolymerization, the influence of changes in conditions such as pre-bake temperature, photoinitiator and irradiation energy (laser power and duration time) on periodic structure is investigated. Adjusting the conditions, 2D and 3D photonic crystal structures with the micrometer-order period are formed in organo-silicate-zirconate hybrid materials.

Energy flow in photonic crystal waveguides

Theoretical and numerical investigations of energy flow in photonic crystal waveguides made of line defects and branching points are presented. It is shown that vortices of energy flow may occur, and the net energy flow along the line defect is described via the effective propagation velocity. Single-mode and multimode operations are studied, and dispersion relations are computed for different waveguide widths. Both strong positive, strong negative, and zero dispersion are possible. It is shown that geometric parameters such as the nature of the lattice, the line defect orientation, the defect width, and the branching-point geometry have a significant influence on the electrodynamics. These are important issues for the fabrication of photonic crystal structures.