Gigantic Third-order Optical Nonlinearity in One-dimensional Mott Insulators

Third-order optical nonlinearity has been studied in one-dimensional Mott insulators, Sr 2 CuO 3 , Ca 2 CuO 3 , and halogen-bridged Ni compounds [Ni(chxn) 2 X]Y 2 (X=Br,Cl; Y=Br,Cl,(NO 3 )), by electroreflectance (ER) spectroscopy. Im X (3) (-ω;0,0,ω) (ER) is evaluated to be X (3) 10 -8 to 10 -5 esu, which is significantly larger than those of conjugated polymers, 10 -10 ∼10 -8 esu. An analysis of the ER spectral shape reveals that the strong enhancement of nonlinear optical response originates from the large transition dipole moments between nearly degenerate odd and even states.