Ursula Hedenig

Silicon photonics in the mid-infrared: Waveguide absorption sensors

We present a silicon (Si) based infrared (IR) absorption sensor for fluids which is suitable for integration into a miniaturized sensor system. The sensor is designed to operate in the mid infrared (MIR) region ranging in the wavelengths λ=5 μm to λ=6 μm. We present the design, the modeling and the optical characterization of the used materials. The sensor is designed as a single-mode Si waveguide (WG) on low refractive index Si3N4/SiOx/ Si3N4 membrane. The single-mode requirement for the WG is needed to avoid losses due to imperfections on the WG walls causing redistribution of the carried energy among the different modes. The waveguide interacts with the fluid sample by means of the evanescent field which extends into the sample. This sensor configuration is not only compatible to the Si technology, but can also be realized on a single chip. In addition, the principle of operation is not limited to a single wavelength: by changing the waveguide dimensions, it can be applied to a broad spectral range. Thus, by its dimensions, performance and Si-compatibility, the sensor is expected to overcome previously published devices1 [1].

A photonic silicon waveguide gas sensor using evanescent-wave absorption

We present an evanescent-wave absorption sensor for gas detection using a silicon slab waveguide fabricated in MEMS technology. We demonstrate the feasibility for measuring CO2 concentrations quantitatively with the used sensor structure. Furthermore, spectroscopic measurements show the capability of the sensor to resolve the rotational-vibrational structure of the CO2 absorption band.

High sensitivity liquid sensing by optimized slot photonic crystal ring resonator

In this work we present a design to enhance absorption of infrared light by a fluid analyte being in contact with a slot photonic crystal ring resonator (slot-PCRR). For this purpose, we propose a new PCRR design facilitating higher interaction between guided mode and analyte. These types of PCRRs are based on two-dimensional photonic crystals, which consist of an array of holes in a silicon slab being arranged in a hexagonal lattice. The holes will be filled with liquid analyte. A slot is embedded in this hexagonal ring cavity to create a slot-PCRR. The strong confinement of light in the low index region, occupied by the analyte, is the key advantage of the slot- PCRR. We also calculate the relative intensity change in the transmission spectrum due to the absorption in the analyte. The maximum change obtained is given by a mode which has most of the electromagnetic field energy in the region the region filled with the analyte. Furthermore, this mode is well separated from neighboring bands, which has the advantage that impinging light with specified frequency is less likely to spuriously couple to other modes with the same frequency, which would decrease the amount of energy coupled to desired mode. The slot-PCRR yields a higher relative change due to absorption compared to the PCRR without a slot. In this work, the radii of six rods at the outer PhC were tuned to enhance the quality factor of slot-PCRR. Using these optimum values of radii, the Q-factor rises up to 80000.

Single-platform Si photonic components for mid-infrared sensing and chemical imaging

Basic challenges for mid-infrared (MIR) Si photonics are developing of appropriate sources and detectors, detection sensitivity, size minimization and downscaling to a single-platform, spectral tunability. We address such challenges via proper design, modeling and material choice for a series of photonic structures. Our research is done in three steps: modeling, fabrication, characterization. The modeling starts with ellipsometry investigation of Si, Si3N4 and SiOx, to estimate the materials’ complex dielectric function ε =ε r + i ×ε i in MIR. The technique showed Si and SiN optical transparency in the range λ=4.5-6.5 μm, and negligible absorption for SiOx, which makes it appropriate for MIR photonics (Figure 1). Figure 2 demonstrates the device concept: MIR source emits electromagnetic field, which is coupled to/from a Siwaveguide (WG) via grating couplers. The WG performs as interaction medium between the propagating field and fluid atop the WG. It results in field attenuation, measured at the output, due to partial absorption by the fluid. To achieve efficient device performance, size, spectral tuning and evaluation of the attenuation, the structures were investigated by means of 3D photonic simulations. The structures were fabricated via the 200-mm-wafer-CMOS technology in Infineon involving deep-UV lithography and Bosch etching. PhC structures were fabricated as holes in a Si-slab with SiOx-filling to avoid residuals from the fluid into the holes, which modifies the photonic band gap and device sensitivity. Figure 3 shows SEM images of the structures. Our paper discusses the design, material characterization, single-platform integration of the source, WG and detector and first experiments with recently fabricated prototypes.

Mid-infrared rib waveguide absorption sensors based on Si

We present a silicon (Si) based infrared (IR) absorption sensor which is suitable for integration into a miniaturized sensor system. The sensor is designed to operate in the wavelength region around λ=5 μm. We particularly discuss the design, the modeling and the optical characterization of the used materials. The sensor operates as a singlemode Si waveguide (WG) on low refractive index Si3N4 membrane. The single-mode requirement for the WG is needed to avoid losses due to imperfections on the WG walls causing redistribution of the carried energy among the different modes. The waveguide interacts with the sample by means of the evanescent field which extends into the sample. This sensor configuration is not only compatible to the Si technology, but can also be realized on a single chip. In addition, the principle of operation is not limited to a single wavelength: by changing the waveguide dimensions, it can be applied to a broad spectral range. Thus, by its dimensions, performance and Si-compatibility, the sensor is expected to overcome previously published device concepts. The single-mode requirements lead to WG dimensions of 2 μm width x 600 nm height for an operation at λ=5 μm, which are verified by 3D simulations. For those parameters, the WG will support one transverse electric (TE) mode and one transverse magnetic (TM) mode. Efficient guidance is only obtained for the fundamental TE and TM modes. As an example, it is shown that mode TE1 is a non-guided mode. The experimentally obtained WG dimensions are 605 nm height and 2 μm width. In our paper we discuss issues with the design, the material characterization and first experimental results obtained with the recently fabricated prototypes.