Tunable Organic ENZ Materials with Large Optical Nonlinearity

Epsilon-near-zero (ENZ) materials have shown significant potential for nonlinear optical applications due to their extraordinary enhancement of optical nonlinearity. Our experiments show that poly(3,4-ethylenedioxythiophene) (PEDOT) films can be modified to boost their conductivity and exhibit ENZ wavelengths from mid-infrared to visible regimes. Degenerate optical nonlinearity coefficients of EG-modified PEDOT films with an ENZ wavelength of 1587 nm are enhanced at 1550 nm to be larger than those of most of the inorganic ENZ materials. The small linear index and the large nonlinear index cause a 210% change in transmittance between low and high power intensities. We attribute the ENZ enhancement to the increased absorptance of bipolarons in PEDOT films. The pump–probe technique reveals the transient process of the bipolaron transition. The highly tunable carrier concentration and mobility enable PEDOT to be an extraordinary nonlinear optical material, which leads to new possibilities of PEDOT:PSS in plasmonics, nonlinear optics, and on-chip nanophotonic devices.