A simple stack of ordinary white paper can be used as a high-quality polarization waveplate for the terahertz spectral region, according to researchers in Germany and the USA (Opt. Express 19, 24884–24889; 2011).

Benedikt Scherger and co-workers from Philipps Univerisität Marburg, University of Arizona and the University of Colorado cut standard office paper (120 μm thick and with a weight of 80 g m−2) into strips and stacked them with alternating air gaps formed by spacers. They then used a clamp or rubber bands to hold the resulting stack of 150–200 paper–air pairs in position.

Credit: © 2011 OSA

Characterization of the stack's properties in the terahertz region revealed that it was strongly birefringent, with refractive indices of 1.295 and 1.149 for p- and s-polarized light, respectively, at a frequency of 0.244 THz. This allowed the stack to rotate the polarization of incident terahertz light and thus function as a waveplate.

By placing the paper stack between traditional wire grid polarizers, the researchers were able to make transfer function measurements of the paper waveplate's performance. Data show that transmission contrast ratios as high as 40 dB are possible, suggesting that the waveplate can rotate a linear polarization state as well as output a pure polarization state, with negligible depolarizing.

The behaviour of the stack was not perfect: it exhibited polarization-dependent loss, with s- and p-polarizations experiencing different levels of attention. For frequencies of around 0.25 THz, however, the overall transmission losses due to absorption were less than 5 dB.

“These waveplates have the advantage of being extremely cheap and easy to fabricate,” comment the authors of the study. “They show excellent performance at their design frequency.”

The researchers are now thinking of constructing paper achromatic waveplates for operation over wider bandwidths based on the use of multiple plates of varying thickness.