59th Annual Report on Research 2014

The scientific objective of this project is to experimentally study nonlinear optics and optical wave dynamics in soft condensed matter systems. In the past year, continuing our previous success, a series of experiments have been performed to investigate a novel type of defocusing nonlinear medium made of m-cresol/nylon solutions, as well as to explore the different nonlinear optical responses in various colloidal suspensions. We focused on the demonstration of dark-soliton formation and interaction in defocusing-type nonlinear systems, as well as non-diffracting beams and self-induced transparency in focusing-type nonlinear systems.

In particular, the P.I. and her research group have discovered a new type of thermal nonlinear media (m-cresol/nylon solutions) that exhibits a giant tunable self-defocusing nonlinearity. The PI’s team designed and performed successful experiments in the following aspects. 1) The demonstration of enormous and tunable self-defocusing nonlinearity in nylon/m-cresol solutions due to thermal effects. The measured effective “Kerr coefficient” in such thermal nonlinear solutions is orders of magnitude higher than that of previously known thermal materials. The strength of the nonlinearity can be easily controlled by varying the nylon concentration in the solutions. 2) The demonstration of stable non-diffracting dark spatial soliton in these isotropic nonlocal nonlinear defocusing media at milli-Watt power levels. 3) The observation of the strongest effect of dark-soliton attraction ever reported in thermal defocusing media. 4) The generation of spatial shock wave in these defocusing media. Our results bring about many possibilities of using these media as extraordinary nonlinear optical materials for studying nonlinear wave dynamics, including vortex dynamics and modulation instability.

On the other hand, together with their collaborators, the PI’s team has created two types of colloidal suspensions with tunable focusing type of optical nonlinearities - the dielectric and metallic colloidal suspensions. Successful experiments in the following aspects have been performed: 1) In both the dielectric and metallic colloidal systems, we can alter at will the nonlinear light-matter interactions in order to overcome the effects of diffraction and form spacial bright solitons. 2) We have realized stable dielectric suspensions with negative polarizabilities, and observed a four-fold enhancement of transmission of self-trapped light through such scattering media. We demonstrated saturatable and stable nonlinear responses in colloidal suspensions with negative polarizabilities and unstable and non-saturatable responses in colloidal suspensions with positive polarizabilities. 3) We also demonstrated phase-controlled attractive or repulsive actions between two non-diffracting beams in colloidal suspensions with negative polarizabilities. 4) We have synthesized colloidal suspensions of metallic nano-particles and demonstrated nonlinear self-trapping of light beams and their robust soliton-like propagation over distances up to 25 diffraction lengths, which in turn allows for deep penetration of long needles of light through dissipative colloidal media. Our findings may bring about solutions to overcome large scattering loss in various soft-matter systems and to pave the avenue for engineering them with tunable nonlinearities, promising for various applications including optical trapping and manipulation as well as initiation of chemical reactions.

Another objective is to promote education and research of students at San Francisco State University, one of the nationally recognized minority institutions, in the fields of physical sciences and engineering. These objectives have also been fulfilled very well. Many undergraduate and graduate students received training in this project and two students are finishing their master’s degree thesis on this proposed project in fall 2014.

We are happy to report that, during the second project year, we have made significant progress in this funded project. One paper has been published in Physical Review Letters, and another one is published in Optics Material Express. Some results have also been presented on leading peer-reviewed conference in the field, Conference on Lasers and Electro-Optics. Equally important is that several students supported by this grant have made remarkable contribution to the project. Overall, we have made significant progress in this project, with active participation of undergraduate and graduate students.

In the coming year, we will continue in these successful studies and focus on the following aspects:

1) Observing and understanding the behaviors of various non-conventional beams (i. e. Vortex beams, Airy beams) in these new nonlinear soft-matter systems in order to control their propagations and interactions.

2) Observing and explaining the phase transition between saturatable and non-saturatable nonlinearity in colloidal systems with mixed polarizabilities.