Shaoping Bian

High-efficiency holographic volume index gratings in DR1-dye-doped poly(methyl methacrylate).

Holographic volume index gratings with high diffraction efficiency (greater than 80%) are recorded in a thick sample of Disperse Red 1-doped poly(methyl methacrylate) with red light (647 nm), which is far away from the absorption peak (488 nm) of the material. Measurements of photoinduced birefringence and polarization holography recording confirm that the azo-dye reorientation mechanism is responsible for the grating formation. Energy coupling between the two writing beams is observed even when the incident beams have equal intensity.

High-efficiency optical phase conjugation by degenerate four-wave mixing in volume media of disperse red 1-doped poly(methyl methacrylate)

High-efficiency optical phase conjugation (PC) by degenerate four-wave mixing in stress-processed volume media of disperse red 1 (DR1)-doped poly(methyl methacrylate) in the transparent region (647 nm) 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 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 reorientation of the DR1 chromophore through trans-cis-trans isomerization is the dominant mechanism for the PC wave generation. Other mechanisms involved in the configuration of all vertical polarization waves are also examined. The roles of the polymer matrix and azo-dye photoisomerization in this high-efficiency PC process are also discussed.

Dark spatial solitons in bulk azo-dye-doped polymer using photoinduced molecular reorientation

We report the generation of dark spatial solitons in bulk Disperse Red 1 doped poly(methyl methacrylate) using photoinduced reorientation of azo-dye molecules. Planar solitions are formed when illuminated with a continuous-wave laser at intensities of the order of hundreds of miliwatts per square centimeter. The width of the soliton saturates to a minimum value at high intensity; and when the width of the initial dark notch is reduced, the equilibrium minimum width is unchanged.

Real-time holographic reflection gratings in volume media of azo-dye-doped poly(methyl methacrylate)

Real-time formation of holographic reflection gratings is experimentally demonstrated in thick media of Disperse Red 1- (DR-1) doped poly(methyl methacrylate) at a nonresonant wavelength (647 nm). Our diffraction efficiencies of 30% and 23% are achieved for reflection gratings inscribed by spatial modulation of intensity and polarization, respectively, and are believed to be the highest achieved for a dye-doped polymer. In addition to the recording of amplitude and phase, the polarization state is also recorded and reconstructed.

Phase conjugation by low-power continuous-wave degenerate four-wave mixing in nonlinear optical polymer fibers

We report on the observation of phase conjugation (PC) using low-power, continuous-wave degenerate four-wave mixing in nonlinear polymer multimode fibers. The fibers are made of poly(methyl methacrylate) doped with Disperse Red 1 as the photosensitive chromophore. With a power less than 2.5 mW at wavelength 633 nm for each beam inside the fiber, a maximum PC reflectivity of 1% is obtained. Phase conjugation is observed for both parallel and orthogonally polarized probe and pump beams. Experimental results show that polarization and intensity profile are preserved in the PC wave. The predominant phase conjugation wave is attributed to the formation of sub-refractive-index gratings inside the fibers.

Erasable holographic recording in photosensitive polymer optical fibers

We report what is to our best knowledge the first holographic recording in photosensitive polymer optical fibers by guided beams. The fibers are made of poly(methyl methacrylate) doped with Disperse Red 1 dye. We also demonstrate what we believe is the first recording of Fourier-transform images in these fibers.

Mode-cut optical limiting in polymer fibers with DR1/PMMA cores

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.

Dynamics of intensity dependent refractive index using T-Scan

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. τ1 is in a range of 700ms to 900ms and τ2 in a range of 50s to 60s. 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.

An all-optical polymer fiber cantilever

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.

Photomechanical effects due to photoisomerization in dye-doped polymer fibers

In this paper, optically-induced differential expansion in a dye-doped polymer optical fiber is studied, which can be used to make an optically-controlled cantilever. A kinetic model that includes both the photothermal and photoisomerization mechanisms is developed