The need of atmospheric information with a higher spatial and temporal resolution drives the development of small satellites and satellite constellations to complement satellite flagship missions. Since optical systems are a main contributor to the satellite size, these are the prime candidate for their miniaturization. We present here a novel optical system where the complete spectrometer part of the optical system is compressed in one flat optical element. The element consists of an array of photonic crystals which is directly placed on a detector. The photonic crystals act as optical filters with a tunable spectral transmission response. From the integrated optical signals per filter and the atmosphere model, greenhouse gas concentrations are obtained using computational inversion. We present in this article the instrument concept, the manufacturing and measurement of the photonic crystals, methods for the filter array optimization, and discuss the predicted retrieval performance for the detection of methane and carbon dioxide.

Recently novel types of imaging spectrometers have been successfully demonstrated, where photonic crystals filters are placed onto a detector. For focused beams, it is however unclear how numerical aperture (NA), aperture shape and aberrations affect the transmission of these filters and there is currently no established and computational efficient method to assess this. In this work we present an angular spectrum method to reconstitute focused beam fields and transmissions based on plane wave simulations. Once these plane wave simulations have been performed, the transmission of arbitrary pupils, aberrations and NA can be directly computed. We investigate the accuracy of this method by comparing the synthesized transmission and Poynting vectors with a fully simulated focused Gaussian beam. These results show that an angular sampling between 0.5° and 0.8° is required to achieve an accurate result depending on the mesh sampling of the simulation. Lastly, we demonstrate how this method can be used to investigate the effect of the pupil shape and aberrations on the transmission profiles. This analysis enables the identification of spectral regions which are sensitive to the instrument design but also regions which appear robust. We believe that the method described in this work can provide scientists and optical engineers a tool to perform detailed performance studies on these promising new instruments.

As global climate change poses one of the most important challenges this century, there is an increasing demand to monitor trace gases with high spatial and temporal resolution. To accomplish this these instruments need to be compact to have constellations of satellites or be able to be equipped on High Altitude Platform Systems. Recently, we introduced a novel highly compact spectrometer instrument concept for trace gas retrieval, making use of photonic crystal filters instead of traditional diffraction based optics. Here the photonic crystal filter transmission profiles are specifically tailored to retrieve the desired trace gases. A particular challenge for this new type of instrument is the design optimization. In addition to traditional performance trade-offs, such as SNR (F-number) and system complexity, the photonic crystal transmission functions also depend on the F-number of the system. Since it is extremely computationally demanding to compute the transmission of many filters for different F-numbers (brute-force optimization approach), a more tailored method is needed. In this work we present a method how to perform a trade-off regarding F-number and trace gas retrieval performance by pre-selecting filters and analyze the performance for these filters as function of F-number. These results show, for the first time, that the sensitivity of the photonic crystal filters to the F-number of the system can substantially degrade the performance compared to only SNR based optimization. It appears that the pre-selected filters, for which the analysis is performed, are particular sensitive the the F-number of the system. This indicates the need for a method to identify robust photonic crystal transmission profiles.

We study straylight of metalenses both by systematically adding controlled manufacturing errors as well as numerically. For the experimental realisation, we nanofabricate amorphous silicon (a-Si) nanopillars on a silicon nitride (SiN) membrane via electron beam lithography. For the numerical comparison employ a Finite-Difference in Time-Domain solver.