Design of a Planar Concave Grating in Silicon Nitride for biomedical sensing applications in the visible range

Abstract

The importance of light-matter interaction based laboratory measuring techniques, together with the availability of the advanced fabrication processes in the microelectronics industry, the knowledge in telecommunications in the 1.3 to 1.65 ?m wavelength region, and the advances on the photonics field; has led to a wide variety of approaches of integrated spectrophotometers with potential applications for measurement of diverse samples in the biomedical field. This research has had a special focus on the spectral filters [1-5] and the detectors used on these devices [6,7]. The spectral filtering techniques include the use of ring resonators (RR), Mach-Zehnder interferometers (MZI), diffraction gratings, directional couplers (DC), multimode interference filters (MMIs), arrayed waveguide gratings (AWG) and planar concave gratings (PCG) [8]. Within these options, it has been shown that the so called ?planar spectrographs? (AWGs and PCGs) present higher flexibility and performance in the field of integrated spectroscopy [8].

In the past years, the advantages of PCG demultiplexers in nanophotonic SOI has been demonstrated over the AWG demultiplexers [2], and this has led to an increase research on PCGs with more channels and higher complexity based on the same material system [9-11]. Nevertheless, most of the applications have a focus on the telecommunication wavelength range, which limits the application to the measurement of analytes interacting with this wavelength, and indicates the need to study devices working in shorter wavelengths (visible spectra), as many of the target biological analytes interact with light in this range.

Following this, there is a need to use wave-guiding materials that are transparent in the visible spectra, which is not the case for the Silicon used in the 1.3-1.65 ?m range. Silicon Nitride on the other hand, has good transmission capabilities for visible wavelengths, and has been used in some devices such as RRs [12]. In spite of this, the refractive index of Silicon Nitride (around 1.9) reduces the reflectivity of the interface with Silicon Oxyde (which is required for an adequate PCG performance), and brings a challenge when designing PCG spectral filters in the visible range, as a lower reflectivity will increase significantly the losses in the structure. This challenge needs then to be addressed by specific modifications to the design of the PCG, such as coating the facets of the device, as suggested by S. Janz et al. [13], or other different techniques that will be addressed in this thesis. Besides the design of the PCG in the Silicon Nitride platform, this thesis will also consider the potential biomedical applications of this device, in combination with grating couplers; one of the sample interaction devices currently being studied in the group, that has the potential to focus light out of the chip and receive light after the interaction with the sample.

The importance of light-matter interaction based laboratory measuring techniques, together with the availability of the advanced fabrication processes in the microelectronics industry, the knowledge in telecommunications in the 1.3 to 1.65 ?m wavelength region, and the advances on the photonics field; has led to a wide variety of approaches of integrated spectrophotometers with potential applications for measurement of diverse samples in the biomedical field. This research has had a special focus on the spectral filters [1-5] and the detectors used on these devices [6,7]. The spectral filtering techniques include the use of ring resonators (RR), Mach-Zehnder interferometers (MZI), diffraction gratings, directional couplers (DC), multimode interference filters (MMIs), arrayed waveguide gratings (AWG) and planar concave gratings (PCG) [8]. Within these options, it has been shown that the so called ?planar spectrographs? (AWGs and PCGs) present higher flexibility and performance in the field of integrated spectroscopy [8].

In the past years, the advantages of PCG demultiplexers in nanophotonic SOI has been demonstrated over the AWG demultiplexers [2], and this has led to an increase research on PCGs with more channels and higher complexity based on the same material system [9-11]. Nevertheless, most of the applications have a focus on the telecommunication wavelength range, which limits the application to the measurement of analytes interacting with this wavelength, and indicates the need to study devices working in shorter wavelengths (visible spectra), as many of the target biological analytes interact with light in this range.

Following this, there is a need to use wave-guiding materials that are transparent in the visible spectra, which is not the case for the Silicon used in the 1.3-1.65 ?m range. Silicon Nitride on the other hand, has good transmission capabilities for visible wavelengths, and has been used in some devices such as RRs [12]. In spite of this, the refractive index of Silicon Nitride (around 1.9) reduces the reflectivity of the interface with Silicon Oxyde (which is required for an adequate PCG performance), and brings a challenge when designing PCG spectral filters in the visible range, as a lower reflectivity will increase significantly the losses in the structure. This challenge needs then to be addressed by specific modifications to the design of the PCG, such as coating the facets of the device, as suggested by S. Janz et al. [13], or other different techniques that will be addressed in this thesis. Besides the design of the PCG in the Silicon Nitride platform, this thesis will also consider the potential biomedical applications of this device, in combination with grating couplers; one of the sample interaction devices currently being studied in the group, that has the potential to focus light out of the chip and receive light after the interaction with the sample.