Patric Strasser

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.

Enhanced feedback in organic photonic-crystal lasers

The mode coupling of organic lasers is greatly enhanced by a photonic crystal that consists of a thin layer of titanium dioxide (TiO2) with a rectangular lattice of holes. The use of TiO2 increases the index contrast in the photonic crystal as well as the confinement in the waveguide, which results in larger feedback given to the lasing modes. This in turn leads to lower thresholds and much smaller devices. Vertically emitting laser devices have been fabricated according to optimized parameters, and the spectral features measured are in excellent agreement with simulations. The devices feature a three to five times lower threshold than devices whose feedback structure is etched directly into the fused silica substrate.

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.

Optical waveguide structure for an all-optical switch based on intersubband transitions in InGaAs/AlAsSb quantum wells

A vertical slab waveguide design for an all-optical switch based on intersubband transitions in molecular beam epitaxy (MBE)-grown coupled double InGaAs/AlAsSb quantum well (QW) structures is presented. We propose a waveguide with two surrounding high refractive index InGaAsP guiding layers, which confine the optical mode in the low refractive index QW region and thus enable light guiding with low contrast InP cladding layers. We investigate the proposed concept by means of 1D simulations of several waveguide configurations. We confirm its validity by fabricating deeply etched waveguiding structures using either wet- or dry-etching technologies. Optical losses as low as 13.5 dB cm(-1) and 12.8 dB cm(-1) were measured for TM- and TE-polarized light, respectively.

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.

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.

A “standing-wave meter” to measure dispersion and loss of photonic-crystal waveguides

We demonstrate a “standing-wave meter” for measuring dispersion and loss along the length of a planar InP-based photonic-crystal waveguide. Light from a tunable cw laser was coupled into a single line-defect waveguide that terminated inside the crystal structure to form a retroreflector. This structure created a standing wave which was imaged using a scanning near-field optical microscope. By measuring the intensity distribution of the standing wave for a range of optical frequencies, waveguide dispersion and loss were measured with high accuracy. Comparisons of the measurement results with three-dimensional numerical simulations reveal that material dispersion effects as small as 0.8% affect the band structure measurably.

InP-based planar photonic crystal waveguide in honeycomb lattice geometry for TM-polarized light.

We investigate manufacturable substrate-type photonic crystal waveguides relying on honeycomb lattice geometry, which shows large photonic bandgaps for TM-polarized light. To the best of our knowledge, air-hole-based photonic crystal slab waveguides with photonic bandgaps for TM-polarized light are experimentally demonstrated for the first time. The results are analyzed with numerical simulations based on the plane-wave expansion and the finite-difference time-domain method. The transmission spectra are measured, and a minimal propagation loss of 1600 dB/cm of TM modes at lambda=1520 nm for lattice constant a=590 nm is acquired with the cut-back method.

Compact and Integrated TM-Pass Photonic Crystal Waveguide Polarizer in InGaAsP–InP

A novel design of a compact and integrated transverse-magnetic-pass optical waveguide polarizer in InGaAsP-InP for use with a telecom wavelength window is presented. The polarizer is realized by integrating an air-hole-type photonic crystal waveguide in a honeycomb lattice geometry. For 20-μm-long polarizer devices with two different lattice constants a = 350 and 590 nm, the flat bandwidth and the measured polarization extinction ratio are ~ 80 nm, 16 dB and ~ 50 nm, 16 dB, respectively.