⁄ was recorded as a function of field strength for five different NR concentrations (Figure ). In each case, the mag-nitude of ∆𝐴 𝐴 ! ⁄ was found to increase with field strength, and saturate at approximately 15 V/µm, showing excellent agreement with previous studies. However, the limiting optical response exhibits a strong dependence on NR concentration: ∆𝐴 𝐴 ! ⁄ saturates at more negative values for samples with higher NR concentration, suggesting an overall improvement in ordering. We believe that the trend in ∆𝐴 𝐴 ! ⁄ can be attributed to inter-particle interactions that facilitate alignment, in particular, the spontaneous self-assembly of colloidal NRs into rafts or bundles, an effect that has been observed in similar colloidal systems. Increasing the NR concentration likely enhances the free-energy driving force for the formation of aggregates, resulting in an augmented electro-optic response. It is important to note that the field-induced optical response of NRs depends not only on their ordering in solution, but also on the ensemble optical anisotropy of species in the colloidal dispersion. For example, given the same degree of ordering, NRs with a more ideal dipolar absorption and emission pattern would produce a larger shift in absorbance compared to NRs with a more isotropic absorption and emission pattern. Thus, quantitatively analyzing the optical anisotropy of the luminescent species is crucial for accurate determination of the overall degree of order.