Recently, many kinds of biological materials attract attention not only for bio- and medical applications but also for electronic and/or photonic devices as new renewable resources. Biomaterials such as DNA are basically insulating and transparent in original forms not being adequate for such industrial application. The drawbacks of DNA was overcome by complexation with other organic molecules in various fashions thanks to its highly complex and organized structure. DNA, only soluble in water, can be coupled to cationic surfactants, transforming it into hydrophobic polymer complex which can be easily dissolved in organic solvents and processed into thin films accompanying other compounds. Previously, we fabricated the bilayer devices composed of PMMA layer containing an azobenzene derivative and a cyanine dye doped DNA complex layer, and demonstrated laser wavelength tuning. Recently, we prepared single layer films composed of DNA-CTMA, azo polymer having push-pull substituents where laser dye were doped later with an immersion method. By choosing adequate combination of laser and azo dyes in order to eliminate or reduce the reabsorption of amplified photons, we have confirmed amplified spontaneous emission from the system through spectral line narrowing and superlinear dependence of light emission on pump intensity. Lasing and its wavelength tuning for the film were succeeded with a relatively compact optics of 10 cm dimension which is manipulatable by one mechanical knob. For further improvement, development of optimized materials is very important issue. We have synthesized an azocarbazole molecule with a cationic alkyl chain being able to couple to anionic sites of DNA. The results on photoinduced birefringence of the material will be described at the conference.
In order to realize a simple and small sized tunable laser with biomaterial based thin films, azo molecule Disperse Red 1 (DR1) was incorporated into DNA-surfactant complex with several fashions. Large photoinduced response with was observed under cw green laser excitation. With introducing a laser dye into blend films composed of DR1-copolymer and DNA complex by immersing them into dye solutions, we succeeded in the demonstration of laser oscillation with low pumping energy and quick tuning with a simple mechanical operation. For improvement of the film quality, a new surfactant including azocarbazole dye was synthesized, showing comparable laser induced birefringence.
With an aim to fabricate wavelength tunable thin laser films, monolithic DNA complex layers including azo and laser dyes were prepared. Azo dyes, DO3 and DR1, were incorporated into the complex layer as guests dispersed in the host. A laser dye, Rhodamine 6G was added to the DO3 doped film by immersion method, demonstrating amplified spontaneous emission (ASE) from the single layered film under the excitation at 532nm. Threshold energy density was determined to be 2mJ/cm2 which was the twice of the counterpart without DO3. It was also found that photo-response of the azos in DNA complex was very weak via experiments of photoinduced birefringence and photoinduced transparency. Instead, a blend of the complex and a PMMA based copolymer with DR1 side chain showed strong photo-isomerization response, which will be applicable to dye doping for laser action.
Azobenzene derivatives in or bound to polymer show photoinduced birefringence and dichroism under polar light excitation through trans-cis isomerization and following reorientation of the molecules, sometimes being succeeded by macroscopic deformation as surface reliefs. In general, the transition process begins with the angular hole burning due to selective isomerization of the molecules aligned parallel to the light field, being followed by directionally random molecular reorientation in relaxation. As most of preceding studies were made with optical Kerr effect and four wave mixing, signal intensities reflected the difference between two index components and two regions, respectively, making it difficult to discriminate the temporal evolution of respective optical constant components.
In this study, to elaborate the elementary processes, simple material system as DR1 doped PMMA was employed and in situ measurement of absorption spectrum and its polarization dependence were made under and after the excitation with linearly polarized light. The results indicated that the absorbance reduced strongly when the probe light was parallel to the pump, and that in perpendicular direction it also reduced with one-thirds amount of the counterpart. It showed that the angular hole burning process was dominant and reorientation effect made less contribution to the dichroism, contrary to common understanding. Estimation of extinction coefficient modulation caused by both mechanisms was made based on a simple model. Quantum efficiency of photoisomerization, longtime stability, dye concentration dependence, comparison to induced birefringence, and comparison between doped polymer and side chain polymer will be discussed.
Strong fluorescence enhancement is induced for many organic dye systems by the coupling to DNA or DNA-surfactant complex, prompting the development of thin film laser devices. Since fluorescence characteristics of the dyes are very sensitive to the interaction mode, it is worthwhile to elaborate their optical properties when interacting with DNA and its complexes. In this study, absorption and circular dichroic (CD) spectra were examined for solutions dissolving hemicyanines and DNA or its complex, and the details of laser emission were studied with the complex films prepared by different methods. Absorption spectra for the dyes changed remarkably by the interaction with pure DNA, while DNA-complex caused blue shift in fluorescence emission process. Clear Cotton effect in induced CD spectra in the dye absorption band indicated the direct interaction with pure DNA presumably due to groove binding. Meanwhile, relatively weak CD signal from the dyes mixed with DNA-complex suggested indirect interaction mode between them. Laser oscillation properties depended on the film formation method. Recently-developed ‘immersion’ process provided thin film devices which showed strong laser emission under optical pumping with population grating. Initial output intensity was two orders of magnitude greater than that from conventionally prepared films. Low threshold and long durability of the immersion made devices were obtained in three types of hemicyanine dyes, making them promising for future application.
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