Daniel Pergande

Miniature infrared gas sensors using photonic crystals

We present an optical gas sensor based on the classical nondispersive infrared technique using ultracompact photonic crystal gas cells. The ultracompact device is conceptually based on low group velocities inside a photonic crystal gas cell and low-reflectivity antireflection layers coupling light into the device. Experimentally, an enhancement of the CO2 infrared absorption by a factor of 2.6 to 3.5 as compared to an empty cell, due to slow light inside a 2D silicon photonic crystal gas cell, was observed; this is in excellent agreement with numerical simulations. We show that, theoretically, for an optimal design enhancement factors of up to 60 are possible in the region of slow light. However, the overall transmission of bulk photonic crystals, and thus the performance of the device, is limited by fluctuations of the pore diameter. Numerical estimates suggest that the positional variations and pore diameter fluctuations have to be well below 0.5% to allow for a reasonable transmission of a 1 mm device.

Photonic crystal gas sensors

The bandstructure of photonic crystals offers intriguing possibilities for the manipulation of electromagnetic waves. During the last years, research has mainly focussed on the application of these photonic crystal properties in the telecom area. We suggest utilization of photonic crystals for sensor applications such as qualitative and quantitative gas and liquid analysis. Taking advantage of the low group velocity and certain mode distributions for some k-points in the bandstructure of a photonic crystal should enable the realization of very compact sensor devices. We show different device configurations of a photonic crystal based on macroporous silicon that fulfill the demands to serve as a compact gas sensor.

A Highly Sensitive Refractometric Sensor Based on Cascaded SiN Microring Resonators

We investigate a highly sensitive optical sensor based on two cascaded microring resonators exploiting the Vernier effect. The architecture consists of two microrings with a slight difference in their free spectral ranges. This allows the generation of the Vernier effect for achieving ultra-high sensitivities. The sensor chip was fabricated using a silicon nitride platform and characterized with isopropanol/ethanol mixtures. A sensitivity of 0.95 nm/% was found for isopropanol concentrations in ethanol ranging from 0% to 10%. Furthermore, a collection of measurements was carried out using aqueous sodium chloride (NaCl) in solutions of different concentrations, confirming a high sensitivity of 10.3 nm/% and a bulk refractive index sensitivity of 6,317 nm/RIU. A limit of detection of 3.16 × 10−6 RIU was determined. These preliminary results show the potential features of cascaded silicon nitride microring resonators for real-time and free-label monitoring of biomolecules for a broad range of applications.

Infiltration of individual pores in macroporous silicon photonic crystals

A new and promising approach for the design and fabrication of novel optical devices is the functionalization of individual pores in 2D photonic crystals (PhC). This can be done by infiltrating the pores with polymers or dyes. We present a method to locally infiltrate individual pores. This new technique enables the fabrication of a new class of devices, such as optical switches or multiplexers. For the infiltration of individual pores 2D PhC templates made of macroporous silicon were used. Local addressing of the pores is carried out by using focused ion beam technology. For the infiltration itself the wetting assisted templating process is applied. We will present experimentally the infiltration of different polymers and different optical designs.

Microring resonator arrays for multiparameter biochemical analysis

We have recently demonstrated a particularly economic approach to analyze large arrays of microring resonator (MRR) sensor elements coupled to a single bus waveguide. The sensor elements can be individually functionalized to specifically promote the accumulation of target molecules. The binding of target molecules to the surface of a particular MRR results in an increase of its resonance wavelengths which can be measured with high accuracy. In order to measure the response of the individual MRR from an array to external stimuli, we employ a special frequency modulation scheme in which each MRR is independently modulated and phase sensitive lock-in detection is used to filter the respective frequency component from the superimposed complex transmission spectrum of the bus waveguide. We fabricated test arrays comprising up to 12 MRR coupled to a single bus waveguide. A silicon nitride based material system was chosen to realize the devices. Each element of an array is equipped with a platinum heater electrode for thermo-optical modulation. A tunable laser system was used for optical characterization and a clear readout of the individual MRR resonance frequencies was possible by employing the modulation scheme above. Furthermore, we demonstrated a bulk refractive index sensitivity of 190 nm/RIU for a frequency modulated MRR. With our first results, we point out the large potential for multiplexed label-free detection of diverse bio molecular compounds. Due to the miniaturization of the multisensor arrays the realization of portable sensor systems will be feasible.

Losses and group index dispersion in insulator-on-silicon-on-insulator ridge waveguides

We present polarization-dependent optical transmission properties of a completely symmetric silicon-on-insulator (SOI) microphotonic material system. In contrast to typical SOI based photonic materials, here an insulator-on-silicon-on-insulator (IOSOI) material system has been fabricated. This symmetric structure exhibits average losses between 1510 and 1630 nm of around 0.5 dB/mm for TE and 0.3 dB/mm for TM-polarization. The good transmission for TM-polarization can be explained by the thick insulting cladding layer of 3 microm thickness. Moreover, group index dispersion diagrams are presented and discussed for both polarizations.

Periodic silicon nanostructures for spectroscopic microsensors

Periodic silicon nanostructures can be used for different kinds of gas sensors depending on the analyte concentration. First we present an optical gas sensor based on the classical non-dispersive infrared technique for ppm-concentration using ultra-compact photonic crystal gas cells. It is conceptually based on low group velocities inside a photonic crystal gas cell and anti-reflection layers coupling light into the device. Experimentally, an enhancement of the CO2 infrared absorption by a factor of 2.6 to 3.5 as compared to an empty cell, due to slow light inside a 2D silicon photonic crystal gas cell, was observed; this is in excellent agreement with numerical simulations. In addition we report on silicon nanotip arrays, suitable for gas ionization in ion mobility microspectrometers (micro-IMS) having detection ranges in principle down to the ppt-range. Such instruments allow the detection of explosives, chemical warfare agents, and illicit drugs, e.g., at airports. We describe the fabrication process of large-scale-ordered nanotips with different tip shapes. Both silicon microstructures have been fabricated by photoelectrochemical etching of silicon.

Local infiltration of individual pores with dyes in 2D macroporous silicon photonic crystals

Photonic crystals (PhC) are promising candidates for novel optical components. Passive devices realized with PhC, e.g. complex waveguides, are widely known. However for many applications active devices are required. One possible way to realize such devices is the functionalization of 2D PhC. This can be done by combining 2D PhC with dyes. We present an experimental technique for the infiltration of individual pores which allows the realization of a broad spectrum of different device designs. For the infiltration of individual pores we use 2D PhC templates made of macroporous silicon, electron beam physical vapor deposition, focused ion beam technique, electrochemical deposition and the wetting assisted templating (WASTE)-process [1]-[3].

Silicon-based low-loss photonic crystal waveguides

We present a new high-index-contrast material system to realize ridge waveguides and PhC waveguides made of a thin silicon slab embedded in two silica layers. Hence fully symmetrical structures can be etched and two important conditions for low-loss guiding of light can be matched: The symmetry properties of the material avoid polarization mixing and the high index contrast leads to strong confinement of light. Because of operating completely below the lightcone the PhC waveguides allow theoretically lossless guiding of light.