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.     

Design and fabrication of optical filters with very large stopband (≈500 nm) and small passband (1 nm) in silicon-on-insulator

In this paper, we report on the design, fabrication and characterization of a broadband photonic crystal filter. Modeling with a genetic algorithm (GA) was used to investigate the effect of changing the number of periods and thickness ratios of a photonic crystal filter structure with two alternating materials. Theoretical optimized parameters were obtained as a function of wavelength for a photonic crystal filter with a very broad filter bandwidth as well as a very narrow transmission window. We used the determined optimum parameters at a wavelength of 1550 nm to fabricate the structure using e-beam lithography and inductively coupled plasma (ICP) etching. Experimental results show that the structure indeed has a very narrow transmission window and a low loss of just 4 dB. Hence, this structure can be regarded as a high precision filter for optical communication and photonic integrated chip technologies.     

Single layer silicon photonic crystal slab

We present here the fabrication and characterization of single layer silicon photonic crystal mirror on a silicon-on-insulator wafer. By a combination of electron beam lithography, fast atom beam etching with deep reactive ion etching, silicon photonic crystal slabs are achieved on 260 nm freestanding silicon membrane and sandwiched with air on the top and bottom. Their high refractive index contrasts enable photonic crystal slabs function as dielectric mirrors for externally incident light. The optical performances of fabricated photonic crystal slabs can be tuned by varying the width of separation grooves or the air-hole size, which represents a significant advantage of offering various approaches for optical response control.     

Low-loss amorphous silicon-on-insulator technology for photonic integrated circuitry

We report the fabrication of low-loss amorphous silicon photonic wires deposited by plasma enhanced chemical vapor deposition. Single mode photonic wires were fabricated by 193 nm optical lithography and dry etching. Propagation loss measurements show a loss of 3.46 dB/cm for photonic wires and 1.34 dB/cm for ridge waveguides.     

Loss reduction in silicon nanophotonic waveguide micro-bends through etch profile improvement

Single mode silicon photonic wire waveguides allow low-loss sharp micro-bends, which enables compact photonic devices and circuits. The circuit compactness is achieved at the cost of loss induced by micro-bends, which can seriously affect the device performance. The bend loss strongly depends on the bend radius, polarization, waveguide dimension and profile. In this paper, we present the effect of waveguide profile on the bend loss. We present waveguide profile improvement with optimized etch chemistry and the role of etch chemistry in adapting the etch profile of silicon is investigated. We experimentally demonstrate that by making the waveguide sidewalls vertical, the bend loss can be reduced up to 25% without affecting the propagation loss of the photonic wires. The bend loss of a 2 μm bend has been reduced from 0.039dB/90° bend to 0.028dB/90° bend by changing the sidewall angle from 81° to 90°, respectively. The propagation loss of 2.7 ± 0.1dB/cm and 3 ± 0.09dB/cm was observed for sloped and vertical photonic wires respectively was obtained.     

Correlation Length Measurement of Sidewall Roughness of Dry Etched Facet and Its Effect on Reflectivity

Dry etching is an important tool to fabricate various semiconductor photonic devices. The roughness of etched sidewalls should be avoided as it reduces the scattering loss. We present a spatial frequency analysis of the sidewall roughness of dry etched facets processed by reactive ion etching. A characteristic parameter corresponding to a correlation length is estimated to be ∼0.5 μm. In addition, its effect on reflectivities of etched reflectors is discussed.     

Photonic nanostructures for advanced light trapping in silicon solar cells: the impact of etching on the material electronic quality

Dry plasma etching, commonly used by the Photonics community as the etching technique for the fabrication of photonic nanostructures, could be a source of device performance limitations when used in the frame of silicon photovoltaics. So far, the lack of silicon solar cells with state‐of‐the‐art efficiencies utilizing nanophotonic concepts shows how challenging their integration is, owing to the trade‐off between optical and electrical properties. In this study we show that dry plasma etching results in the degradation of the silicon material quality due to (i) a high density of dangling bonds and (ii) the presence of sub‐surface defects, resulting in high surface recombination velocities and low minority carrier lifetimes. On the contrary, wet chemical anisotropic etching used as an alternative, leads to the formation of inverted nanopyramids that result in low surface recombination velocity and low density of dangling bonds. The proposed inverted nanopyramids could enable high efficiency photonic assisted solar cells by offering the potential to achieve higher short‐circuit current without degrading the open circuit voltage. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Formation of three‐dimensional GaAs microstructures by combination of wet and metal‐assisted chemical etching

Previously, plasma‐enhanced dry etching has been used to generate three‐dimensional GaAs semiconductor structures, however, dry etching induces surface damages that degrade optical properties. Here, we demonstrate the fabrication method forming various types of GaAs microstructures through the combination etching process using the wet‐chemical solution. In this method, a gold (Au)‐pattern is employed as an etching mask to facilitate not only the typical wet etching but also the metal‐assisted chemical etching (MacEtch). High‐aspect‐ratio, tapered GaAs micropillars are produced by using [HF]:[H₂O₂]:[EtOH] as an etching solution, and their taper angle can be tuned by changing the molar ratio of the etching solution. In addition, GaAs microholes are formed when UV light is illuminated during the etching process. Since the wet etching process is free of the surface damage compared to the dry etching process, the GaAs microstructures demonstrated to be well formed here are promising for the applications of III–V optoelectronic devices such as solar cells, laser diodes, and photonic crystal devices. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Robust free space board-to-board optical interconnect with closed loop MEMS tracking

We present a free-space optical interconnect system capable of dynamic closed-loop optical alignment using a microlens scanner with a proportional integral and derivative controller. Electrostatic microlens scanners based on combdrive actuators are designed and characterized with vertical cavity surface emitting lasers (VCSELs) for adaptive optical beam tracking in the midst of mechanical vibration noise. The microlens scanners are fabricated on silicon-on-insulator wafers with a bulk micromachining process using deep reactive ion etching. We demonstrate dynamic optical beam positioning with a 700 Hz bandwidth and a maximum noise reduction of approximately 40 dB. Eye diagrams with a 1 Gb/s modulation rate are presented to demonstrate the improved optical link in the presence of mechanical noise.     

Stiffness properties of porous silicon nanowires fabricated by electrochemical and laser-induced etching

Nanowires with dimensions of few nanometers were formed on the whole etched surface. The optical analysis of silicon nanostructures was studied. Blue shift luminescence was observed at 660 nm for PS produced by electrochemical etching, and at 629 nm for laser-induced etching. PS produced a blue shift at 622 nm using both etching procedures simultaneously. X-ray diffraction (XRD) was used to investigate the crystallites size of PS as well as to provide an estimate of the degree of crystallinty of the etched sample. Refractive index, optical dielectric constant, bulk modulus and elasticity are calculated to investigate the optical and stiffness properties of PS nanowires, respectively. The elastic constants and the short-range force constants of PS are investigated.     

a-Si:H based two-dimensional photonic crystals

We describe the fabrication processes of silicon-based two-dimensional photonic crystals (2D-PCs) with a photonic band gap in the near-IR range. The procedures involve electron beam lithography followed by an anisotropic etching step of hydrogenated amorphous silicon thin films deposited by plasma enhanced chemical vapor deposition. Micrometric and submicrometric arrays of cylindrical holes are transferred using a poly-methylmethacrylate resist layer as a mask. A careful comparison between standard parallel plate reactive ion etching and inductively coupled plasma etching techniques is performed, aimed at obtaining periodic structures with high aspect ratio and good profile sharpness.     

Design and analysis of coherent OCDM en/decoder based on photonic crystal

The design and performance analysis of a new coherent optical en/decoder based on photonic crystal (PhC) for optical code -division -multiple (OCDM) are presented in this paper. In this scheme, the optical pulse phase and time delay can be flexibly controlled by photonic crystal phase shifter and time delayer by using the appropriate design of fabrication. According to the PhC transmission matrix theorem, combination calculation of the impurity and normal period layers is applied, and performances of the PhC-based optical en/decoder are also analyzed. The reflection, transmission, time delay characteristic and optical spectrum of pulse en/decoded are studied for the waves tuned in the photonic band-gap by numerical calculation. Theoretical analysis and numerical results indicate that the optical pulse is achieved to properly phase modulation and time delay, and an auto-correlation of about 8 dB ration and cross-correlation is gained, which demonstrates the applicability of true pulse phase modulation in a number of applications.     

Experimental study of an elliptical-core photonic bandgap fiber with thin core wall and high aspect ratio and its birefringence characteristics

We fabricate an elliptical hollow-core photonic bandgap fiber (EC-PBGF) with a core-wall thickness of 65 nm and an aspect ratio of 1.4. The core is expanded by applying positive pressure in the hollow-core region during the fiber-drawing process. Both the reduction of core-wall thickness and the increase of aspect ratio result in the enhancement of optical properties in terms of a low attenuation loss and group birefringence. The minimum loss and group birefringence of the fabricated EC-PBGF2 are measured to be 0.1 dB/m and 0.012, respectively.     

Optimization of inductively coupled plasma deep etching of GaN and etching damage analysis

Inductively coupled plasma (ICP) etching of GaN with an etching depth up to 4 μm is systemically studied by varying ICP power, RF power and chamber pressure, respectively, which results in etch rates ranging from ∼370 nm/min to 900 nm/min. The surface morphology and damages of the etched surface are characterized by optical microscope, scanning electron microscope, atomic force microscopy, cathodoluminescence mapping and photoluminescence (PL) spectroscopy. Sub-micrometer-scale hexagonal pits and pillars originating from part of the structural defects within the original GaN layer are observed on the etched surface. The density of these surface features varies with etching conditions. Considerable reduction of PL band-edge emission from the etched GaN surface indicates that high-density non-radiative recombination centers are created by ICP etching. The density of these non-radiative recombination centers is found largely dependent on the degree of physical bombardments, which is a strong function of the RF power applied. Finally, a low-surface-damage etch recipe with high ICP power, low RF power, high chamber pressure is suggested.     

Three-dimensional photonic crystals as a cage for light

We review recent developments in three-dimensional photonic crystals. State of the art fabrication methods, such as layer-by-layer micromachining, self-assembly and various etching, lithographic and holographic techniques are discussed. We present an overview of optical studies of photonic band gap formation, such as reflectivity, transmission and time-resolved pulse propagation experiments. The fundamental issues associated with disorder and absorption are also considered. Progress towards the ultimate goal of full spontaneous emission control is reviewed. Finally, remaining open questions are summarized. To cite this article: A.F. Koenderink et al., C. R. Physique 3 (2002) 67–77     

Optical properties of one-dimensional photonic crystals fabricated by photo-electrochemical etching of silicon

The optical properties of one-dimensional photonic crystals (1D PCs), fabricated by electrochemical etching of periodic wall arrays on n-type (100) Si substrates, are investigated in this study. Several 1D PCs were fabricated with lattice periods varying from 4 to 7 μm and with trench depths in the range 160–210 μm. In-plane reflection spectra of the photonic structures at different depths were registered over a wide spectral range of 1.5–15 μm using Fourier Transform Infra-Red (FTIR) micro-spectroscopy. Some of the features observed in the reflection spectra of the structures investigated are believed to be as a result of interface roughness. A corrugated side-wall surface, an artifact of the fabrication technique, results in the degradation of optical reflection characteristics, principally mainly in the near IR spectral range, and the emergence of optical anisotropy. As a result of the periodicity, modulation of the reflection spectra, that is, the difference between the maxima and minima of the interference fringes, reached a value of 95% in the mid-infrared. The optical properties of the structures investigated indicate that they show promise for microphotonics applications.     

Optical alignment tolerances in double-side irradiated self-written waveguide-induced fiber arrays packages

In this paper, we present our experimental study on the optical alignment tolerance between the couplings of single-mode fibers (SMFs) connected with a double-side irradiation-induced self-written waveguide (SWW). The study firstly focuses on the coupling of two SMFs and then on the two fiber arrays (FAs) for parallel optical communication. The SWW was formed in dye-dispersed epoxy materials by the photopolymerization technique. Rhodamine 6G dye was dispersed in epoxy, which is commonly used in the photonic packaging industry as a bonding adhesive. Using double-side irradiated SWW, we found the alignment tolerance for such optical interconnect to relax significantly. All the formed SWWs were evaluated in terms of optical loss. In our study, up to 4 µm misalignment tolerance was allowed for only 1 dB loss penalty. In addition, the optical interconnect formed by this technique was also able to tolerate up to ± 10 µm lateral shift with only 1 dB extra loss. The wavelength-dependent loss (from 1520 to 1610 nm) and polarization-dependent loss were less than 0.4 dB. The double-side irradiated SWW-induced couplings between two FAs also provided low optical loss. They were found to be less sensitive to temperature changes, and no significant distortion in the digital signal transmission test was observed. We believe that the findings are useful and applicable to other dye-dispersed epoxy material systems for relaxing the alignment tolerance of the optical interconnects in various photonic packaging situations.     

Extraordinary wavelength dependence of self-collimation effect in photonic crystal with low structural symmetry

We present the optical properties of a new type of photonic crystal (PC) named star-shaped PC (STAR-PC) with anomalous equi-frequency contours. Intentionally introducing low-symmetry in the primitive unit cell gives rise to progressively tilting flat contours, which are observed in the fifth band of the transverse magnetic mode. Due to the intrinsic dispersive feature of the proposed PCs, i.e. tilted self-collimation, the incident signal with different wavelengths can be successfully separated in a spatial domain without introducing any corrugations or complexities inside the structure. We show numerical investigations of wavelength selective characteristic of the proposed PC structure in both time and frequency domains. The STAR-PC approach can be considered a good candidate for the wavelength division applications in the design of compact photonic integrated circuits. For the purpose of wavelength separation implementations, the proposed structure may operate within the wavelength interval of 1484.5–1621.5 nm with a broad bandwidth of 8.82%. The corresponding inter-channel crosstalk value is as low as ?19 dB and the calculated transmission efficiency is above 97%.     

Butt-joint regrowth method by MOCVD for integration of evanescent wave coupled photodetector and multi-quantum well semiconductor optical amplifier

We have realized integration of evanescent wave coupled photodetector (ECPD) and multi-quantum well (MQW) semiconductor optical amplifier (SOA) on MOCVD platform by investigating butt-joint regrowth method of thick InP/InGaAsP waveguides to deep etched SOA mesas. The combination of inductively coupled plasma etching and wet chemical etching technique has been studied to define the final mesa shape before regrowth. By comparing the etching profiles of different non-selective etchants, we have obtained a controllable non-reentrant mesa shape with smooth sidewall by applying one step 2HBr:2H₃PO₄:K₂Cr₂O₇ wet etching. A high growth temperature of 680 ℃ is found helpful to enhance planar regrowth. By comparing the growth morphologies and simulating optical transmission along different directions, we determined that waveguides should travel across the regrowth interface along the [110] direction. The relation between growth rate and mask design has been extensively studied and the result can provide an important guidance for future mask design and vertical alignment between the active and passive cores. ECPD-SOA integrated device has been successfully achieved by this method without further regrowth steps and provided a responsivity of 7.8 A/W. The butt-joint interface insertion loss is estimated to be 1.05 dB/interface.