The present invention provides a method of manufacturing a structured substrate, the method comprising: providing a working substrate (522) having a side surface (524) and an array of reaction cavities (526), each of the reaction cavities (526) having an opening along the side surface (524) and extending a depth from the corresponding opening into the working substrate (522), the reaction cavities (526) coinciding with an array plane (525); and directing a deposition stream (536) onto the working substrate (522) at a non-orthogonal angle (544) with respect to the array plane (525), the deposition stream (536) including a plasmon resonant material, wherein the working substrate (522) forms a shadow area (558) and an incident area (560) in each reaction cavity (526) relative to a path of the deposition stream (536) such that the plasmon resonant material of the deposition stream (536) is blocked by the side surface (524) from being deposited onto the shadow area (558) and is permitted to pass through the opening and form along the incident area (560).