 Filter structures that are analogous to the waffle @-@ iron filter are used in photonics but operate at a much higher frequency and are much smaller than those used in electronics . Like the waffle @-@ iron , these structures have good suppression of unwanted transmission modes . A filter operating in the 0 @.@ 1 to 4 @.@ 0 THz band has been built using parallel @-@ plate waveguide ( PPWG ) technology with 50 dB of rejection in the stopband . The filter was constructed of two plates of highly polished aluminium spaced 100 μm apart . The teeth consisted of gold @-@ sputtered aluminium cylinders on a silicon die . In this design it is not convenient to provide a gap through the centre of the teeth as is done in the microwave version . Instead , the air gap is provided between the top of the teeth and one of the PPWG plates . 
