Linear and Nonlinear Optical Properties of Photochromic Molecules and Materials.

A series of amorphous azobenzene-containing polymers were cast as thin films and shown to produce both reversible volume diffraction gratings and high-efficiency surface gratings by laser irradiation at an absorbing wavelength. The latter process involves localized mass transport of the polymer chains to a high degree, as atomic force microscopy reveals surface profile depths near that of the original film thickness. A mechanism for this phenomenon is proposed which involves pressure gradients as a driving force, present due to different photochemical behaviors of the azo chromophores at different regions of the interference pattern. The phase addition of the two beams in the interference pattern leads to regions of high trans-cis-trans isomerization by the absorbing azo groups, bordered by regions of low isomerization. As the geometrical isomerization requires free volume in excess of that available in the cast films, the photochemical reaction in these areas produces a laser-induced internal pressure ab...

Mechanism of Optically Inscribed High-Efficiency Diffraction Gratings in Azo Polymer Films

We review the third-order nonlinear performance of pseudo-stilbene type azobenzenes with an eye to application in ultrafast optical signal processing. We discuss mechanisms responsible for the nonlinear response of the azobenzenes. By aggregating experimental data and theoretical trends reported in the literature, we identify five characteristic regions of optical nonlinear response. Analyzed with respect to Stegeman figures of merit, pseudo-stilbene type azobenzenes show promise for ultrafast optical signal processing in two spectral regions, one lying between the main and two-photon absorption resonances, and the other for wavelengths longer than the two-photon absorption resonance. © 2001 Kluwer Academic Publishers

Azobenzenes for photonic network applications: Third-order nonlinear optical properties

A liquid crystal with an azobenzene moiety as a photoactive mesogenic unit is investigated in photoinduced birefringence. Measurements reveal that it possesses large magnitude of photoinduced birefringence (Δn∼10−2) and the characteristic of long-term optical storage. The resulting polarization information can be erased either by heating the sample to a temperature much higher than the glass transition temperature, or by optical method with a circularly polarized light. The mechanism of long-term optical storage is discussed in this letter.

Photoinduced birefringence and reversible optical storage in liquid-crystalline azobenzene side-chain polymers

Transient biphotonic optical storage in the guest−host system 4-amino-4‘-nitroazobenzene/polycarbonate is accomplished by excitation of the azo dye molecules at 488 nm with an argon ion laser and subsequent writing at 633 nm with two interfering HeNe laser beams. The mechanism of the observed refractive index change is investigated by detailed kinetic studies. It is based on the formation of cis isomers of the azo dye by the argon ion laser. These are subsequently transformed back to the trans isomers by fast photoinduced cis−trans isomerization where the interference of the writing beams is constructive and by slow thermal cis−trans isomerization where the interference of the HeNe beams is destructive, respectively. The cis concentration produced by the exciting light is thus modulated with the HeNe beams, resulting in a modulation of the refractive index.

Biphoton-Induced Refractive Index Change in 4-Amino-4‘-nitroazobenzene/Polycarbonate

Biphoton holograms are recorded by a He–Ne laser in Methyl Orange and Ethyl Orange dyes on exposure to 514.5-nm argon-ion laser, and the dynamic of holographic recording is reported. The recording utilizes an unusual photochemistry that is due to either to a triplet–triplet transition from a long-lived triplet lower state of the cis configuration or a transition back to the trans state.

Biphoton holographic storage in Methyl Orange and Ethyl Orange dyes

Biphoton holographic storage in Methyl Orange and Ethyl Orange dyes: erratum

Phase conjugation in methyl orange doped polyvinyl alcohol film by DFWM based on excited state absorption

Degenerate Four-Wave Mixing (DFWM) based on excited-state absorption in azo-dye doped polymer films is firstly reported. Under a pre-exciting argon laser at 514.5 nm, DFWM is performed using a 632.8 nm HeNe laser which is absorbed only slightly when the molecules are in the ground state. The dynamic behavior of the DFWM signal due to excited-state absorption is also treated theoretically.

Degenerate four-wave mixing based on excited-state absorption in azo-dye-doped polymer films

The Raman-enhanced nondegenerate four-wave mixing (RENFWM) in benzene and methylbenzene was studied for five polarization geometries. One geometry gave suppression of the nonresonant background. A comparison between the experimental results and a theoretical curve has been made. From this, the transverse relaxation time of the dominant Raman modes in each liquid was obtained.

Li Han

Papers

Biphoton holographic storage in Methyl Orange and Ethyl Orange dyes

Biphoton holographic storage in Methyl Orange and Ethyl Orange dyes: erratum

Phase conjugation in methyl orange doped polyvinyl alcohol film by DFWM based on excited state absorption

Degenerate four-wave mixing based on excited-state absorption in azo-dye-doped polymer films

Raman-enhanced nondegenerate four-wave mixing