Quasi-phase-matched gratings printed by all-optical poling in polymer films

The all-optical poling technique permits polar orientation of molecules. For efficient poling of thin films the relative phases, amplitudes, and polarizations of the two interfering beams must be controlled. We present an original stable one-arm interferometer that is specific to the recording of two-color interference. It relies on the index dispersion of optical glasses. This interference technique permits true real-time nonperturbative monitoring of the polar orientation process and easy all-optical poling of thin-film materials without the need for phase control. This new configuration opens the door to the realization of customized phase-matched wave-guided frequency-conversion devices for the near infrared.     

偶氮聚合物薄膜的全光极化研究

报告了分散红共聚物膜HMMM-DR1,HMMM-DR19和偶氮侧链聚合物膜PCN6和PCN2的合成和制备, 并以HMMM-DR1和PCN6为代表比较研究了这两种具有不同吸收性质材料的全光极化特性,研究了他们的实时极化和弛豫过程,对他们的全光极化性质作了最基本的表征.研究了倍频光吸收对薄膜光极化效率的影响,讨论了偶氮聚合物材料光诱导二阶非线性极化率的效率和倍频光透射率之间的折衷关系,这对实用化的全光器件的研制是至关重要的.实验确证了在PCN6薄膜中实现了准相位匹配.对厚膜中光诱导二阶非线性极化率的弛豫抑制效应作出解释.     

Orientation of azobenzene molecules in polymer films induced by all-optical poling

A model of the alignment of azobenzene molecules in polymer film induced by all-optical poling is proposed and verified by experiment. We found that when the writing beams of frequencies ω and 2ω are both linearly polarized with their polarization directions parallel to each other, azobenzene molecules tend to reorient to the direction perpendicular to the writing beams polarization. At the end of the writing process, more molecules orient to the direction perpendicular to the writing beams polarization than those which orient to the parallel direction. The alignment of molecules parallel or perpendicular to the polarization of the writing beams is characteristic of polarity or no polarity, respectively. The alignment of molecules along the polarization of writing beams results in the second order nonlinearity in the polymer film. According to the model, a new method to improve the optical poling efficiency is put forward.     

Real-time monitoring of all-optical poling of azo-dye polymer thin film

Simultaneous writing and probing of photoinduced second-order susceptibility chi((2)) in an azo-dye polymer thin film is demonstrated. The method is based on the fact that tensor properties of chi((2)) provide different planes of polarization for seeding and signal second harmonics. Our technique allows the chi((2)) value to be monitored in real time, completely eliminating the distortion of optical poling that can occur in traditional probing configurations. The possibility of attaining high values of chi((2)) in a DR1-PMMA guest?host system under low seeding intensities is shown.     

Carrier-envelope phase measurement by all-optical poling with a polar side-chain polymer

Dependence of all-optical poling efficiency on carrier-envelope phase (CEP) could be measured using photoisomerization of dye molecules which are covalently bound to a polymer main chain and have large difference in static dipole moment between the ground state and excited state. Increased chromophore density leads to an order of magnitude reduction in signal-detection time from a dye doped polymer. Analysis of all-optical poling experiments with CEP changes clearly showed the presence of polarization restoring force to zero polarization. This enables resetting of SH activity in the all-optical poling process to be used for fast response loop of CEP stabilization. Phenomenological model could explain well the difference in the growth-and-decay dynamics of poling between sample of dye doped in polymer studied previously and that grafted to a polymer main chain used in the present paper.     

Stable frequency doubling by all-optical poling in dye-doped polymer optical fibers

We present experimental studies of all-optical poling (AOP) in large-core dye-doped poly(methyl methacrylate) plastic optical fibers. We show that the confinement of light within the fiber core permits us to reach the upper limit of second harmonic generation achievable by AOP using repeated trans-cis isomerization. Quantification of the output power indicates self-seeding that can counteract relaxation of the orientation during readout of the induced nonlinearity.     

Optically inscribed surface-relief zone plates in azo polymer films

We describe a simple and cost-effective holographic method for the fabrication of surface-relief zone plates.The zone plate is inscribed by interference between the first- and second-order diffracted waves from an ionetched Fresnel zone plate. The inscribed surface-relief zone plates are observed by atomic force microscope (AFM). The formation process of the surface grating and the mass diffusion in azo polymer are analyzed.     

Method of improving optical poling efficiency in polymer films

The alignment of dye molecules in optically poled polymer films is modeled. We point out that when writing beams (fundamental and second harmonic) are linearly polarized with their polarization directions parallel, the majority of the dye molecules in a poled film are axially oriented perpendicular to the writing-beam polarization, whereas the minority are polar oriented parallel to the writing-beam polarization. Only the latter, however, contribute to the second-order nonlinearity of the film. This model is verified by experiments. Based on the model, a new method of improving optical poling efficiency is put forward.     

Identification of the alignment of azobenzene molecules induced by all-optical poling in polymer film

The alignment of azobenzene molecules in DR19-PUI film induced by all-optical poling is identified. When the writing beams of frequencies ω and 2ω are both linearly polarized with their polarization directions parallel to each other, azobenzene molecules tend to reorient to perpendicular direction (i.e. direction perpendicular to the polarization of writing beams). At the end of the writing process more molecules orient in perpendicular direction than those in parallel direction (i.e. direction parallel to the polarization of writing beams). The alignment of molecules in parallel or perpendicular direction is, respectively, characteristic of noncentrosymmetry or centrosymmetry. It is the alignment of molecules in parallel direction that results in the second-order nonlinearity in the poled film.     

共振域矩形介质光栅的逆向设计

采用模态法研究了共振域光栅分束器及闪耀光栅等光学器件的设计原理及逆向设计方法,给出了-1级非偏振闪耀光栅的典型设计示例。研究表明,自准直角入射时,光束在光栅内传输将激发产生离散模,通过调节离散模透过光栅传输时的累积相位差,可对衍射光进行调控,实现偏振(非偏振)分束、偏振(非偏振)闪耀等功能。采用严格耦合波法对基于模态法逆向设计的-1级非偏振石英闪耀光栅的几何参数进行验证,计算结果显示两者非常吻合,证明了该方法的正确性。     

共振域矩形介质光栅的逆向设计

采用模态法研究了共振域光栅分束器及闪耀光栅等光学器件的设计原理及逆向设计方法,给出了-1级非偏振闪耀光栅的典型设计示例。研究表明,自准直角入射时,光束在光栅内传输将激发产生离散模,通过调节离散模透过光栅传输时的累积相位差,可对衍射光进行调控,实现偏振(非偏振)分束、偏振(非偏振)闪耀等功能。采用严格耦合波法对基于模态法逆向设计的-1级非偏振石英闪耀光栅的几何参数进行验证,计算结果显示两者非常吻合,证明了该方法的正确性。     

Diffraction efficiency increase by corona discharge in photoinduced surface-relief gratings on an azo polymer film

The diffraction efficiency of a surface-relief grating (SRG) was dramatically increased by corona discharge. We fabricated this SRG by applying surface modulation to azo polymer films, using laser interference fringes. The electric charge was deposited upon the SRG by a corona discharge in an oven. The first-order diffraction efficiency measured before and after corona discharge above the glass-transition temperature increased from ~2% to ~40% in a SRG written with a circularly polarized beam. The relief depth of the SRG as measured with an atomic-force microscope also increased, from ~130 nm to ~450 nm. The increase in diffraction efficiency was independent of the writing polarization.     

Permanent light-induced polar orientation via all-optical poling and photothermal cross-linking in a polymer thin film

An epoxy-based crosslinkable polymer functionalized with nonlinear optical (NLO) photochromic dye known as Red Acid Magly was polarly oriented via thermally-assisted all-optical poling to enhance the photostability of the induced order. During the writing (seeding) process the film was cross-linked by heating with a hot-plate and irradiated simultaneously by the coherent superposition of the 1.06 μm fundamental and 0.532 μm second harmonic beams of a nanosecond pulsed Nd:YAG laser. We demonstrate that crosslinking between the polymer matrix and the pendant chromophore groups induced during the thermally-assisted all-optical poling procedure results in a stable light-induced second order susceptibility.     

表面具有光栅结构的sol-gel薄膜制备及衍射效率的研究

介绍了一种在 sol- gel薄膜表面形成光栅结构的方法。用扫描电子显微镜 (SEM)表征了薄膜的表面形貌 ,用原子力显微镜 (AFM)测量了光栅结构的深度。测试了这种结构表面的衍射特性 ,并对结果进行了分析     

Improvement of all-optical switching effect based on azobenzene-containing polymer films

A method for improving the quality of all-optical switching is presented. Pumping poly(methyl methacrylate) (PMMA) films doped with azobenzene chromophore Disperse Red 1 (DR1) with two beams (a linearly and a circularly polarized), all-optical switching effects with very low background and high stability are demonstrated. High frequency switching, which is hard to be realized under pump of a single beam, is obtained. The circularly polarized pump beam can erase the photoinduced birefringence and quicken the relaxation process, thus lowering the background and improving the stability and the extinction ratio of the switching signal. PACS 42.70.Jk; 42.25.Lc; 82.30.Qt; 78.20.Fm; 77.80.Fm     

Investigation of charge effects on poling and stability for corona-poled polymer films

The optimal corona-poling temperature of polymer films was accurately determined by measuring the temperature dependence of the in situ second-harmonic (SH) intensity profile under the applied poling electric field. The in situ SH intensity profile was first measured by probing both the surface voltages and the poling currents induced by surface/space charges for the corona-poled polymer films. Moreover, charge effects on the stability of the chromophore orientation were first studied by using the thermally stimulated discharge-current technique. PACS 72.20.Jv; 42.65.Ky; 73.61.Ph; 81.40.Tv     

Real-time monitoring of all-optical poling by two-beam second-harmonic generation

We demonstrate a simple method for monitoring all-optical poling in real time. The poling pattern created by the writing beam at the fundamental frequency and the seed beam at the second-harmonic frequency is reconstructed by two-beam second-harmonic generation because of the writing beam and an additional probe beam at the fundamental frequency. When the probe beam is sufficiently weak, it does not distort the poling process. The method provides a significantly stronger signal than monitoring based on a probe beam alone.     

Femtosecond laser induced index and relief gratings in polymer films

A true single-step process suitable for fabrication of micro-periodic structure in polymer films by two photon initiated photopolymerization and laser ablation is presented. By the right choice of the irradiation energy, the irradiated zone is modified or ablated in the 1.44-μm-thick film. The mechanism of grating generation and the potential application of the gratings in integrated optics are discussed.     

Optical poling in a crosslinkable polymer system

Received: 30 September 1998 / Published online: 24 February 1999     

Second-harmonic generation in resonant waveguide gratings incorporating ionic self-assembled monolayer polymer films

Experimental results on resonantly excited second-harmonic generation (SHG) in a periodic ionically self-assembled monolayer (ISAM) film are reported. A double-layer guided-mode resonance filter (GMRF) structure is coated with 40 bilayers of pyrlium-based chi(2) ISAM thin film and excited with the fundamental of a Nd:YAG laser. Enhanced second-harmonic conversion in the ISAM film is achieved because of the local field enhancement associated with the fundamental resonating leaky mode. This method of SHG is particularly promising, as the ISAM films under investigation exhibit anomalous dispersion that may be applied for phase matching to improve nonlinear conversion efficiency.