Jianwu Ding

Supra-nonlinear photorefractive response of single-walled carbon nanotube- and C60-doped nematic liquid crystal

We have observed an extremely large electro-optically induced photorefractive effect in nematic liquid crystal doped with single-walled carbon nanotubes and fullerene C60. The effective refractive index change coefficient can be as large as 7 cm2/W, which is >1000 times larger than previous observations. We describe the basic mechanisms and conditions necessary for occurrence of such nonlinearities.

Nonlinear-absorbing fiber array for large-dynamic-range optical limiting application against intense short laser pulses

A complete quantitative description of a nonlinear fiber array for optical limiting application against laser pulses in the picosecond–nanosecond regime is presented. We discuss the dynamics of the molecular photonic processes accompanying the propagation of a laser pulse through the fiber core made of materials that possess reverse saturable absorption, two-photon absorption (TPA), and excited-state absorption (ESA), and we detail the optical limiting effectiveness and limitations of these nonlinear absorption processes individually and in concert. In particular, we demonstrate the importance of excited-state population recycling in extending the dynamic range of the limiting action. Experimental results obtained from a particular fiber core material that possesses TPA and ESA show good agreement with theoretical expectations and demonstrate the optical limiting capability of such a nonlinear fiber array.

Recent Studies of Optical Limiting, Image Processing and Near-Infrared Nonlinear Optics with Nematic Liquid Crystals

The mechanisms for recently observed optically induced refractive index changes in nematic liquid crystals, and their use in novel optical limiting and image processing are reviewed, along with an assessment of their potentials for applications. We have also studied the optical nonlinearities of nematic liquid crystals in the near IR communication spectral region [1.55 µm]. Phase modulation of several πs can be generated with mW laser power in micron thick films.

Nonlinear Optical Effects in Nematic Liquid-Crystal Films in the 1.55 µm Spectral Region

We have studied the optical nonlinearities of nematic liquid crystals in the near IR communication spectral region [1.55 µm]. The origins of the refractive index changes are thermal indexing effect and director axis reorientations. Very large refractive index and phase modulation of several πs can be generated with mW laser power in micron thick dye-doped nematic liquid crystal films.

Collective and Individual Molecular Nonlinear Photonics of Liquid Crystals

This review will examine the origins of nonlinear light scattering processes in nematic liquid crystals, and explore various nonlinear photonic processes associated with optically induced director axis reorientation effects. Our theoretical prediction shows that the upper limit of nematic liquid crystal reorientation nonlinearity can be as high as 1000 cm2/W. The supra-nonlinear responses of nematic liquid crystals enable various self-action or electro-optical guiding, mixing, switching and modulation of light with unprecedented low power thresholds. Owing to the broadband birefringence of NLC, we expect to realize optical elements/devices capable of similar multi-functional operations throughout the visible — infrared regime. We also discuss the optical limiting action of isotropic-phase liquid crystal filled nonlinear fiber arrays.

Widely tunable nonlinear liquid crystal-based photonic crystals

We report theoretical and experimental studies of 1-D and 2-d tunable nonlinear photonic crystals made of liquid crystal or liquid crystal infiltrated periodic structures. Theoretical modeling shows that such structures exhibit tunable bandgap, and sugar-prism effect. Experimentally, we have demonstrated the possibility of writing dynamic or permanent [but switchable] index gratings to dye-doped LC films that act as planar waveguides.

Communication-wavelength (1.55 μm) laser-induced refractive index change in nematic liquid crystalline films

We have studied the optical nonlinearities of aligned nematic liquid crystalline films in the near IR communication spectral region (1.55 micrometers ). The measured refractive index coefficients are on the order of 10-3 cm2/W. The origins of the refractive index changes are thermal indexing effect and director axis reorientation. Phase modulation of several (pi) s can be generated with mW-power near IR lasers in micron thick films.

Dynamics of cross-polarization-stimulated orientation scattering in a nematic liquid-crystal film

We have studied the dynamic evolution of stimulated cross-polarization scattering in a nematic liquid-crystal film. The effect is initiated by two-beam coupling between a cw incident polarized laser and its orthogonally polarized multifrequency noise components scattered by fluctuations in the director axis. Complex time- and intensity-dependent oscillations and energy exchanges among the pump and stimulated waves were observed. Imparting an ac modulation to the film further amplified the characteristic oscillations while it suppressed the background noise. The conditions for stimulated laser generation and the observed frequency spectra of the intensity oscillations are in good agreement with a quantitative theory that accounts for the temporal-spatial dependence of the wave mixing process.

Multiphoton and excited-state absorptions of an organic liquid for optical limiting applications

A detailed theoretical study of nonlinear molecular photonic processes accompanying the propagation of short intense laser pulses through a thin organic liquid cell and an organic liquid cored fiber array is presented. A model is proposed to account for the measurements of a recently developed organic liquid, and a comparison with pure two-photon and excited state absorption mechanisms is performed.

All-optical self-induced polarization switching by two-wave mixing in nematic liquid crystals

We have performed quantitative theoretical and experimental studies of all-optical self-induced polarization switching in thin films and waveguides of nematic liquid crystals, and demonstrated the feasibility of such processes over the entire visible-near infrared [0.4 - 1.55 microns] spectrum. We have also studied the detailed dynamics of the nonlinear optical interaction with the nematic crystalline axis, and observed interesting frequency selective beam amplification and stimulated scattering effects.