Planar waveguide device with nano-sized filter

A planar waveguide device(PWD) for interacting with a fluid (FLD) is disclosed, the planar waveguide device (PWD) comprising -a waveguide layer (WGL)for supporting optical confinement, -a coupling arrangement (CPA) for in-coupling and out-coupling of light into and from the waveguide layer (WGL), -a fluid zone (FZN) for accommodating the fluid (FLD), -a filter layer (FTL) arranged between the fluid zone (FZN) and the waveguide layer (WGL) in an interaction region (IAR) of the waveguide layer (WGL), wherein the filter layer (FTL) comprises filter openings (FOP) arranged to allow the fluid (FLD) to interact with an evanescent field of light guided by the waveguide layer (WGL), wherein the filter openings (FOP) are adapted to prevent particles (PAR) larger than a predefined size from interacting with said evanescent field, wherein the filter openings (FOP) are arranged as line openings having their longitudinal direction in parallel with the direction of propagation (DOP) of light guided by the waveguide layer (WGL). Also, use of the planar waveguide device(PWD)for detecting blood hemolysis and a method of interacting light with a fluid is disclosed.