Enhancement of the nonlinear optical absorption of the E7 liquid crystal at the nematic-isotropic transition

We present an experimental study of the nonlinear optical absorption of the eutectic mixture E7 at the nematic-isotropic phase transition by the Z-scan technique, under a continuos wave excitation at 532 nm. In the nematic region, the effective nonlinear optical coefficient \beta for an extraordinary beam is negative and for an ordinary beam is positive, being null in the isotropic phase. The parameter S_{NL} defined in terms of the nonlinear absorption coefficients in a similar way than the optical order parameter from the linear dichoic ratio behavies like an order parameter with a critical exponent 0.22+-0.05, in good agreement with the tri-critical hipothesis of the N-I transition.Comment: 11 pages, 6 figures, to be published in the vol 42 of the Brazilian Jurnal of physic

Current noise in resonance tunnel diodes based on InGaAlAs heterostructures

Noise in resonance tunnel diodes based on InGaAlAs structures is studied at two frequencies. A shot noise caused by the current flowing through two barriers of the heterostructure is identified. It was found that if the voltage across the structure is within the first ascending section of the current–voltage characteristics, the shot noise is suppressed with the suppression ratio G ~ 1/15. In the second ascending section of the current– voltage characteristic, the shot noise coincides with the shot noise of electrons passing through one barrier ( G = 1) by the order of magnitude. The specific features of current flow affecting the behavior of noise in the resonance tunnel structures are discussed