Silk Foam Terahertz Waveguides

Fabrication and characterization in the THz frequency range of silk foams and silk foam-based waveguides using lyophilisation and casting techniques are reported. The lack of biocompatible and biofriendly waveguides for low-loss, low-dispersion guidance of THz waves motivates the work for applications in remote and stand-off sensing in biomedical and agro-alimentary industries. Silk foams produced are 94% porous. Optical characterization is carried out using THz time-domain spectroscopy. The cutback measurements of foam samples show that the foam refractive index is close to that of air (n(foam)(r) approximate to 1.06). Silk foam losses scale quadratically with frequency (alpha(foam) [cm(-1)] approximate to 0.65+ 2.75. (v[THz](2)), being one order of magnitude smaller than those of solid silk. As an example of a basic guided wave device, fabrication and optical characterization of 10 cm-long, 5 mm-diameter step-index THz fibers having silk foam in the core and air in the cladding is demonstrated. Cutback measurements confirm that in the mid-THz spectral range, step-index fibers operate effectively in a single mode regime. Effective refractive index and propagation loss at frequencies higher that 0.2 THz are close to that of a silk foam from which the fiber core is made. At the same time, at these frequencies, modal group velocity dispersion is smaller than 0.2ps/(THz.cm).