Figure shows the transient optical response of a 2 μM dispersion of NRs in the presence of a ±7 V/µm AC field. The transmittance of the sample fluctuates in response to the periodic alignment and relaxation via rotational diffusion of NRs. At zero field strength (1), the NRs are randomly oriented in solution, and the transmittance is at a minimum (maximum absorbance). However, as the field strength is gradually increased, the NRs begin to align parallel to the field direction (perpendicular to the polarization of incident laser beam), resulting in an overall decrease in absorbance and consequently an increase in transmittance. At peak field strength (2), the transmittance reaches a maximum, indicative of the system having reached a maximum degree of alignment for that magnitude of applied field. The rapid alignment and relaxation of NRs which occurs at a millisecond time scale can be perceived by the naked eye as a time-averaged increase in optical transmission (see Figure in Supporting Information). The magnitude of the periodic absorbance shift is correlated with the degree of alignment in the NR ensemble: higher degrees of order result in larger shifts in absorbance. The relative change in absorbance (∆𝐴 𝐴 ! ⁄ ) has been proposed as a quantitative measure of ensemble alignment that is independent of external factors such as cell thickness, NR concentration or laser power. Eq 1: