Rik Harbers

SU-8 for real three-dimensional subdiffraction-limit two-photon microfabrication

We report the inherent utility of two-photon-absorption (TPA) in the fabrication of real three-dimensional (3D) structures with subdiffraction-limit resolution, based on SU-8 as the threshold polymer media. We exploit the nonlinear velocity dependence of TPA photopolymerization as the shutter mechanism for disruptive 3D lithography. We show that low numerical aperture optics can be used for the rapid microfabrication of ultrahigh-aspect ratio photoplastic pillars, planes, and cage structures.

Integrated all-optical switch in a cross-waveguide geometry

An all-optical switch for future computer optical interconnect systems based on an optical cavity with a high quality factor and a nonlinear material is computionally investigated in two dimensional with a finite-difference time domain method. The signal and control bus are perpendicular to each other and can couple into a high-Q cavity consisting of a nonlinear material. It is designed in such a way that the control bus switches the signal bus on and off. Owing to the nonlinearity in the cavity, the resonance is shifted in frequency when increasing the power in the control bus so that the signal can pass through the resonator. The high Q of the cavity maximizes the interaction with the nonlinear material, and the symmetry of the cavity mode is designed in such a way that the cross talk between the signal bus and the control bus is minimized.

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.

Control of Fano line shapes by means of photonic crystal structures in a dye-doped polymer

The emission of a two-dimensional organic photonic crystal structure is investigated. A photon emitted by a molecule in this structure can take two different coupled pathways. It can either be emitted directly into the surroundings or first be transferred to a discrete state of the photonic crystal structure from where it is then emitted. This unique coupling of the pathways results in a particular asymmetric spectral line shape referred to as Fano resonance. By studying the Fano line shape as a function of the quality factor, the authors can gain insights into the coupling between the pathways within the photonic crystal structure.

Enhancement of the mode coupling in photonic-crystal-based organic lasers

The mode coupling of organic photonic crystal lasers is enhanced by using a photonic crystal that consists of a thin layer of a high-index material. The high-index material increases the index contrast, the confinement in the waveguide and thus the mode coupling. Using such a photonic crystal gives rise to new design criteria. We investigate these criteria, and employ them in the design of an organic photonic crystal laser. Calculations of the coupling constant of the organic laser show that using high-index materials results in much higher coupling constants and thus smaller devices.

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.

Efficient coupling into and out of high-Q resonators

The temporal-coupled-mode theory is directly applied to the design of devices that feature a resonator with a high quality factor. For the temporal-coupled-mode theory we calculate the decay rate of the resonator to determine the transmission properties of the device. The analysis using the decay rates requires little computational effort, and therefore the optimum device properties can be determined quickly. Two examples, a wavelength filter and a resonator crossing, are presented to illustrate the use of the analysis.

Photonic integration for high density and multi-functionality in the InP material system

Monolithic photonic integration offers unsurpassed perspectives for higher functional density, new functions, high per-formance, and reduced cost for the telecommunication. Advanced local material growth techniques and the emerging photonic crystal (PhC) technology are enabling concepts towards high-density photonic integration, unprecedented per-formance, multi-functionality, and ultimately optical systems-on-a-chip. In this paper, we present our achievements in photonic integration applied to the fabrication of InP-based mode-locked laser diodes capable of generating optical pulses with sub-ps duration using the heterogeneous growth of a new uni-traveling carrier ultrafast absorber. The results are compared to simulations performed using a distributed model including intra-cavity reflections at the sections inter-faces and hybrid mode-locking. We also discuss our work on InP-based photonic crystals (PhCs) for dense photonic integration. A combination of two-dimensional modeling for functional optimization and three-dimensional simulation for real-world verification is used. The fabricated structures feature more than 3.5μm deep holes as well as excellent pattern-transfer accuracy using electron-beam lithography and advanced proximity-effects correction. Passive devices such as waveguides, 60° bends and power splitters are characterized by means of the end-fire technique. The devices are also investigated using scanning-near field optical microscopy. The PhC activity is extended to the investigation of TM bandgaps for all-optical switches relying on intersubband transitions at 1.55μm in AlAsSb/InGaAs quantum wells.

Enhanced feedback and experimental band mapping of organic photonic-crystal lasers

Photonic-crystal lasers whose feedback is enhanced by a thin layer of titanium dioxide (TiO2) have been experimentally investigated. The use of TiO2 increases the index contrast in the photonic crystal as well as the confinement in the waveguide, resulting in larger feedback of the lasing modes. This allows lower thresholds and much smaller devices. Vertically emitting laser devices, using the methyl-substituted ladder-type poly(p-phenylene) (MeLPPP) as gain material, have been fabricated according to optimized parameters, and the measured dispersion relation of the structures matches excellently 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.

Lasing in interferometrically structured organic materials

We present feedback structures for an organic-photonic-crystal laser created by directly structuring the gain medium, consisting of UVII-HS resist doped with a mixture of the laser dyes Coumarin and DCM. The photonic-crystal structure is realized by means of interference lithography. This approach greatly reduces the number of steps required to realize the feedback structure of an organic laser. The dispersion relations obtained expermimentally agree very well with the computer simulated band diagram.