Quantitative 3D optical birefringence for testing ferroelectricity in peptide nanotubes

Self-assembled peptide nanotubes (PNT) are a unique nanoscale object, presenting strong piezo activity. Being a third-rank tensor property, the crystal chemistry of piezoelectric materials is to consider the impact of symmetry on such a property. Of the potentially 20 point groups considered piezoelectric, 10 are polar, such that they have a vector direction in the material that is non-symmetrical, such that spontaneous polarization occurs. Since this is typically a function of temperature, these materials are pyroelectric. Finally, Ferroelectric materials are a subset of pyroelectric materials, in which the spontaneous polarization can be reoriented between crystallographically-defined directions by a realizable electric field.

Another feature of anisotropic crystals, within this pyroelectric group, is the emergence of birefringence. Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. Birefringence can be determined through cross-linearly polarized reflections.

Although direction of light can also affect birefringence, changes in refractive index present an interesting study, when considering testing the ferroelectricity of PNTs. Refractive index is an optical property of a material, determined by a crystals electronic structure. If changes occur to the electronic structure, then the refractive index also changes. This means changes in spontaneous polarization can quantitatively affect the refractive index of a crystal, changes which could be induced by a realizable electric field.

Here we demonstrate the measurement of cross linear polarized light reflection from the surface of PNTs using a custom-built three dimensional optical birefringence apparatus. With this apparatus and under varying electric fields, we tested for ferroelectricity in peptide nanotubes by quantitatively tracking changes in birefringence.