An implantable product such as an article, device, or system can include analyte and non-analyte containers in parts that can be operated as optical cavities. The product can also include fluidic components such as filter assemblies that control transfer of objects that affect or shift spectrum features or characteristics such as by shifting transmission mode peaks or reflection mode valleys, shifting phase, reducing maxima or contrast, or increasing intermediate intensity width such as full width half maximum (FWHM). Analyte, e.g. glucose molecules, can be predominantly included in a set of objects that transfer more rapidly into the analyte container than other objects, and can have a negligible or zero rate of transfer into the non-analyte container; objects that transfer more rapidly into the non-analyte container can include objects smaller than the analyte or molecules of a set of selected types, including, e.g., sodium chloride. Output light from the containers accordingly includes information about analyte.