60th Annual Report on Research 2015

The scientific objective of this project is to experimentally study nonlinear-optics and wave-dynamics in soft-condensed-matter systems. A series of experiments have been performed to explore the different nonlinear-optical-responses in various colloidal suspensions with focusing type of nonlinearity. We also have investigated novel defocusing nonlinear medium due to optical-thermal effects. We focused on the demonstration of non-diffracting beams and self-induced transparency in focusing nonlinear systems as well as dark-soliton formation and interaction and optical shock wave formation and interaction in defocusing nonlinear systems.

In particular, the PI’s team and their collaborators have created two types of soft-matter systems 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 in the colloid suspensions. 2) We have realized stable dielectric suspensions with the negative polarizability, and observed a four-fold enhancement of transmission of self-trapped libght 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. 5) We have investigated the phase transition between saturatable and non-saturatable nonlinearity in colloidal systems, by studying samples with different mixing-ratio of positive and negative polarizabilities.

Fig. 1. Schematic illustrations (a-c) & experimental snap shots (d-f) of light-particle interactions in colloidal suspensions.  In all these cases, index along the beam path increases and the beam can self-focus. (a,d) Positive-Polarizability (PP) particles are attracted. (b,e) Repulsion of Negative-Polarizability (NP) particles (c,f) Collective motion of PP and NP particles, when both species are present.

Fig. 2. Observation of stable optical self-trapping in a colloidal suspension with negative polarizabilities. a) Input; b) linear output profile at low power; c) nonlinear output at high power; d) side view photograph of such a self-trapped sable soliton over a distance of 5 mm.

Fig. 3. a) Normalized nonlinear response of a mixed-polarizability suspension.  The transition between saturable NP and supercritical PP behaviors is indicated by the Kerr plane (gray). The mixing ratio is defined to be 0 for pure NP systems and 1 for pure PP systems. b) Measured output beam size as a function of the input power. c) Measured intensity-dependent transmission for different suspensions with changing mixing ratio. 

These findings may lead to 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.

In addition, we 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 dispersions due to thermal effects. The measured 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 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 optical spatial shock wave in these defocusing media and the threshold conditions. 5) The observation of the shock wave propagation and interaction in these media.

Fig. 4. Quantifying the defocusing nonlinearity of the m-cresol/nylon solutions with different concentrations. (a) The output beam diameter as a function of input power. (b) The measured absolute values of the Kerr coefficient increases rapidly as a function of nylon concentrations.

Fig. 5. Formation of a dark soliton observed in m-cresol/nylon solution. a) Input: a dark stripe. b) Linear output beam profile at power less than 1 mW. c) Nonlinear output beam profile after 10 mm of propagation at 50mW power.

Fig. 6. a) Interference showing the two separate 180-degree phase-jumps.  b) The two dark stripes at the input, separated by a distance of 100mm. c) Linear output at power less than 1mW. d-f) Output patterns taken at different powers of 1.0, 2.0 and 3.0 W respectively.  g) Plot of the output stripe separation demonstrates increasing dark soliton attractions with power.

These results also bring about many possibilities of using these optical-thermal solutions as extraordinary nonlinear optical materials for studying nonlinear wave dynamics, including vortex dynamics and modulation instability.

This project has also promoted education and research for students at SFSU, one of the nationally recognized minority institutions. Many undergraduate and M. S. degree students received systematic training in nonlinear optics, laser operation, laser safety, sample preparation and data analysis. Some students, including undergraduates, have co-authored journal papers, as well as delivered research talks on this project in leading peer-reviewed conference in the field, Conference on Lasers and Electro-Optics. These precious experiences significantly enhanced their chances to be admitted into decent Ph. D. programs or to find industrial jobs. The PI also adapted experimental methods and techniques developed through the proposed project into her laser optics course.

Overall, we have generated plenty of publishable results during the funded period, with active participation of students. Those students supported by this grant have made remarkable contribution to the project. One paper has been published in Physical Review Letters, one in Optics Letters, and another in Optics Material Express. More recent results on the optical-shock-wave study are to be submitted. We greatly appreciate the ACS PRF fund.