There is a long history of using light to change the shape of a material. More than a decade ago, our group proposed and demonstrated that the length of an optical fiber should change due to a guided mode in analogy to the refractive index change due to the Optical Kerr Effect. The mechanisms that we postulated as being responsible included photothermal heating and photoisomerization. In the present studies, we report on a polymer optical fiber cantilever, which is excited by launching a light beam off-axis into the fiber. In measurements of the degree of bending as a function of time after the light beam is turned on or turned off, we find that there are two distinct time responses, each of different magnitude. We show that the dynamics of photobending is consistent with coupling between the photothermal heating and photoisomerization mechanisms. More interestingly, we find that a collective release of stress must be invoked to describe the observations. We propose new kinetic models of the phenomena, and show that they are consistent with the data.
We report holographic recording in photosensitive polymer optical fibers by guided beams. The fibers are made of poly(methyl methacrylate) doped with Disperse Red 1as photosensitive element. Holographic recordings are performed with parallel- and orthogonally-polarized writing beams. Recording of Fourier-transform images in these fibers is also demonstrated. The recording mechanism is photoinduced reorientation of Disperse Red 1 molecules.
Degenerate four-wave mixing phase conjugation is observed in azo-dye-doped polymer optical fibers under low-power, continuous-wave laser irradiation. A maximum phase conjugate efficiency of 1% has been obtained under a power less than 3 mW for each beam inside the fiber. Phase conjugation is observed for both parallel- and orthogonally-polarized probe and pump beams. The polarization and intensity profile are verified to be preserved in the conjugate signal. The predominant phase conjugation signal is attributed to photoinduced isomerization and reorientation of azo-dye molecules.
The principle of mode-cut optical limiting in fibers is reviewed briefly, and a calculation method based on angular spectrum analysis is proposed. Experiments that show high efficiency holographic grating generation and self defocusing in disperse-red-1 (DR1) doped poly(methyl methacrylate) (DR1/PMMA) bulk material suggest that it is a good candidate to be used as a core material in polymer fibers to achieve mode-cut optical limiting. Such fibers are fabricated in our lab and its optical limiting effect is reported.
We report on a procedure to fabricate a specific micro-structured fiber with metal wires array running down its length. The goal is to develop an unique photonic bandgap lattices with enhanced optical nonlinearity. Mechanism of enhanced nonlinearity is briefly discussed. FDTD simulation on single-wire fiber is presented.
Azo-dye-doped polymers have a large intensity dependent refractive index with at least two mechanisms that are characterized by their response times and by the sign of amplitudes. τ<sub>1</sub> is in a range of 700<i>ms</i> to 900<i>ms</i> and τ<sub>2</sub> in a range of 50<i>s</i> to 60<i>s</i>. To charcterize the dynamics of the real and imaginary part of this response, we use a T-scan technique(using open and closed apertures), where the intesity dependent focusing/defocusing processes of the material is studied as a function of time.
Optical phase conjugation (PC) by non-resonant degenerate four-wave mixing (DFWM) in thick media of poly(methyl methacrylate) (PMMA) with doped disperse red 1 (DR1) is reported. With vertically polarized counterpropagating pump waves, PC reflectivities of 43% and 37% were achieved respectively for a horizontally and vertically polarized probe wave, which is more than 50 times higher than the value reported on resonance. Reflectivities over 30% were achieved over a wide range of intensity for both polarization configurations. Photoinduced modulation of ordering of the DR1 chromophore is the main mechanism of the PC wave generation. Other mechanisms involved in the configuration of all vertical polarization waves are also examined. Influence of the squeezing process in making volume samples on the PC wave efficiency is significant.