R. Wüest

Detailed analysis of the influence of an inductively coupled plasma reactive-ion etching process on the hole depth and shape of photonic crystals in InP∕InGaAsP

The authors report on the fabrication of photonic crystals in the InP∕InGaAsP∕InP material system for applications at telecommunication wavelengths. To achieve low optical loss, the photonic crystal holes must demonstrate smooth sidewalls and should be simultaneously deep and cylindrical. The authors present the etching process of these structures based on a Cl2∕Ar∕N2 chemistry with an inductively coupled plasma reactive-ion etching system. A systematic analysis is provided on the dependency of the hole sidewall roughness, depth, and shape on the process parameters such as etching power, pressure, and chemical composition of the plasma. They found that a low plasma excitation power and a low physical etching are beneficial for achieving deep holes, whereas for the nitrogen content in the plasma, a delicate balance needs to be found. Nitrogen has a negative impact on the hole shape and surface roughness but is capable of preventing underetching below the mask by passivation of the sidewalls. With the autho...

Optimization of a 60° waveguide bend in InP-based 2D planar photonic crystals

We present a novel design for a W1 (one missing row of holes) waveguide 60° bend implemented in a substrate-type InP/InGaAsP/InP planar photonic crystal based on a triangular array of air holes. The bend has been designed to provide high transmission over a large bandwidth. The investigated design improvement relies only on displacing holes while avoiding changing individual holes diameter in the interest of better process control (homogenous hole depth). Two-dimensional (2D) finite-element simulations were used to increase the relative transmission bandwidth from 18% to 40% of the photonic bandgap for unoptimized and optimized 60° bends, respectively. The 2D results were verified by means of rigorous three-dimensional (3D) finite-difference time-domain (FDTD) simulations. We show that excellent agreement between 2D and 3D simulations can be obtained, provided a small effective-index shift of −0.024(−0.74%) and an imaginary loss parameter (ϵ″=0.014) is introduced in the 2D simulations. To demonstrate the applicability of our improved design, the bend was fabricated and measured using the endfire technique. A bending loss of 3 dB is obtained for the optimized W1 waveguide bend compared to more than 8 dB in the unoptimized case.

Ultrafast carrier dynamics in InP photonic crystals

Ultrafast time-resolved reflectivity investigations are performed on InP-based photonic crystals with a wide range of structural parameters. It is found that the structure plays a critical role in determining the recombination dynamics of the photo-injected charge carriers. For sufficiently large etched sidewall area densities the carrier lifetime is decreased to a level below 100 ps.

Fabrication of a hard mask for InP based photonic crystals: Increasing the plasma-etch selectivity of poly(methyl methacrylate) versus SiO2 and SiNx

We introduce cyclic reactive ion etching processes for SiO2 and SiNx hard masks where the fluorine-based etch steps are interleaved with N2 flushing steps in order to improve the selectivity to electron-beam resists. For SiO2 etching an etch-step duration of 30s resulted in a doubled selectivity of almost 4:1 between SiO2 and poly(methyl methacrylate) (PMMA) due to a reduced thermal load. We established the pattern transfer from a 200nm thick PMMA resist into a 600nm thick SiO2 layer for 200nm diameter holes. For SiNx etching we demonstrate improved sidewall verticality, an enhanced etch rate, and suppressed redeposition of etch byproducts for a cyclic process. With the use of an additional 30nm titanium intermediate layer we show an excellent overall selectivity between SiNx and PMMA of almost 5:1. This process is applied to the fabrication of planar photonic-crystal devices with 3.5μm deep holes in an InP based slab waveguide with an initial PMMA layer thickness of 220nm.

Spontaneous emission of a nanoscopic emitter in a strongly scattering disordered medium

Fluorescence lifetimes of nitrogen-vacancy color centers in individual diamond nanocrystals were measured at the interface between a glass substrate and a strongly scattering medium. Comparison of the results with values recorded from the same nanocrystals at the glass-air interface revealed fluctuations of fluorescence lifetimes in the scattering medium. After discussing a range of possible systematic effects, we attribute the observed lengthening of the lifetimes to the reduction of the local density of states. Our approach is very promising for exploring the strong threedimensional localization of light directly on the microscopic scale.

Limitations of proximity-effect corrections for electron-beam patterning of planar photonic crystals

We investigate the patterning accuracy limits of electron-beam lithography with different proximity-effect correction (PEC) methods applied to the fabrication of planar photonic crystal structures (PPCS). Energy-intensity distribution simulations reveal that conventional energy-equalization PEC techniques present a lower limit of the best attainable hole-radius variation of 1% for a generic PPCS, while a method proposed by Watson (midpoint-equalization PEC) should inherently account for beam broadening and theoretically can reach perfect accuracy. Simulation results are verified experimentally. Additionally, we introduce a new method to determine the beam-broadening parameter . We compare energy-equalization PEC and midpoint-equalization PEC regarding the impact of geometrical key parameters of PPCS on achievable patterning accuracy, and show that proximity effects impose severe limitations on the patterning of structures with large fill ratios and/or small lattice constants. Furthermore, we perform a sensitivity analysis of both PEC methods on the proximity parameters and show that overestimation of the backscatter efficiency can actually improve the lithographic accuracy of the energy-equalization method and mimic the midpoint-equalization PEC method to a certain degree.

InP-based compact photonic crystal directional coupler with large operation range

We present the design, fabrication and measurement of photonic crystal directional couplers in the InP/InGaAsP/InP material system. A comprehensive analysis of the dependence of the coupling length and usable wavelength range on the diameter of the holes next to the waveguides is given. The possibility to trade-off coupling length against usable wavelength range is shown. Designs with coupling lengths as low as 52 lattice constants and with an operation range covering 16% of the bandgap width are fabricated and measured. Good agreement between optimized and measured devices is achieved.

Nano-Optomechanical Characterization and Manipulation of Photonic Crystals

We describe the application of scanning near-field optical microscopy (SNOM) for the high-resolution visualization of light propagation in photonic crystal structures. We also demonstrate that nanoscopic elements such as sharp tips could be used for the mechanical manipulation of the optical properties of photonic crystals. In particular, our theoretical and experimental results show that narrow resonances of a photonic crystal cavity can be tuned without a substantial influence on its quality factor. Furthermore, we discuss the modification of the fluorescence of a nanoscopic emitter as a function of its location close to a photonic crystal

Sidewall roughness measurement inside photonic crystal holes by atomic force microscopy

We present a measurement technique to quantify sidewall roughness inside planar photonic crystal (PhC) holes. Atomic force microscopy is used to scan hole cross-section profiles. By fitting a circle onto each scan line and subtracting this circle from the measurement data, a quantitative value for the deviation from the ideal cylindrical hole shape is extracted. We investigate the sidewall roughness of InP-based PhC holes depending on the nitrogen content of the semiconductor etching plasma. The existence of a trade-off between hole undercut and surface roughness by optimizing the flux of nitrogen during the plasma etching of the PhC holes is confirmed. We further quantify with this technique the influence of the direct-writing of octagons instead of circles by electron-beam lithography on the measured roughness.

Influence of proximity effects in electron-beam lithography on the optical properties of planar photonic-crystal waveguides

To measure the influence of proximity effects in electron-beam lithography on the optical properties of planar photonic crystal (PPC) waveguides we propose a PPC structure called the “PECmeter.” The PECmeter consists of nearly identical PPC waveguides which only differ in the number of rows of holes along the waveguide. The difference in the number of rows does not influence the modal properties directly but changes the diameter of the holes neighboring the waveguide through the proximity effect. The operation principle of the PECmeter is demonstrated using energy-intensity simulations of a W3 waveguide (three missing rows of holes) mini stop band. The principle is confirmed experimentally with structures fabricated in the InP-based material system and measured by the end-fire transmission technique. The results clearly show that the application of proximity-effect correction (PEC) is crucial for the fabrication of PPC waveguides. We demonstrate that when using the midpoint-equalization PEC method a near-to-perfect correction with sub-nm hole-radius uniformity can be achieved. We show the PECmeter to be sensitive enough to detect hole-radius changes as small as ∆R=0.4 nm.