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Optoelectronics & Communications

Process improvements in DNA-based photonic-device fabrication

A hybrid structure of DNA-cetyltrimethylammonium chloride and poly(methyl methacrylate) exhibits excellent processability in combination with wet etching.
17 December 2010, SPIE Newsroom. DOI: 10.1117/2.1201011.003222

Recent research1–10 on DNA-lipid complexes has highlighted various attractive features of electro-optic or opto-electronic devices such as optical memory, switches, modulators, sensors, LEDs, and lasers. For instance, the finding that the fluorescence intensity of light-emitting devices (in particular) can be significantly enhanced by employing DNA as a host material promises vastly improved performance, including reduced threshold and increased efficiency as compared to devices based on conventional polymer host materials. Several results9–11 show great promise for realization of DNA-based lasers or amplifiers,12 although the potential of DNA use has not yet been demonstrated. Further studies are still needed, especially regarding realistic device structures, to show the advantages of DNA-based photonic devices.

Many photonic devices employ a waveguide structure to confine the propagation of light or, sometimes, electrons and holes. Typically, such structures are formed using dry- or wet-etching processes. DNA-lipid complexes are highly absorbent under high-humidity conditions, which causes difficulties during device fabrication because water and/or organic-solvent treatment is required. This leads to some degradation in both processability and device performance. To overcome this issue, the incorporation of a hybrid structure of DNA lipid with other moisture-resistant polymers is a promising approach for improvement of moisture resistance and processability. We introduced poly(methyl methacrylate) (PMMA) for fabrication of the hybrid structure because the hybrid film exhibited both good moisture resistivity and excellent transparency.13 We used single-chain trimethylammonium (CTMA)-type lipids to synthesize DNA-lipid complexes2,3,8 and 4-[4-(dibutylamino) stylyl]-1-methyl pyridinium iomide (DBASMPI)—one of the hemicyanine-dye derivatives known as nonlinear optical materials—to prepare dye-doped DNA-CTMA.8,10

Figure 1 shows typical amplified-spontaneous-emission (ASE) spectra and the spectral width (full width at half maximum) of our DBASMPI-doped DNA-CTMA-PMMA hybrid film. We used the second-harmonic-generating light pulse of a neodymium-doped yttrium aluminum garnet laser at 532nm as excitation source. The laser pulse was fed onto the DNA-CTMA-PMMA film's surface by a cylindrical lens, while the exciting light power was monitored by a calibrated photodetector. Output light from the sample was recorded by an optical-spectrum analyzer. We observed spectral narrowing due to ASE when increasing the excitation energy with a threshold energy around 5mJ/cm2. This indicates that use of a PMMA/DNA-CTMA mixture does not lead to any degradation in the optical characteristics of dye-doped DNA-CTMA.

Figure 1. (a) Amplified-spontaneous-emission spectra for different excitation energies (in mJ/cm2) and (b) spectral width of 4-[4-(dibutylamino) stylyl]-1-methyl pyridinium iomide (DBASMPI)-doped DNA-cetyltrimethylammonium chloride (CTMA)-poly(methyl methacrylate) (PMMA) hybrid films. FWHM: Full width at half maximum.

To investigate its moisture resistivity, we placed the DNA-CTMA-PMMA hybrid film in different humidity conditions for 48 hours and measured the changes in fluorescence intensity. (The films were first stored for 24 hours under relative humidity of 60–70% to stabilize the initial performance.) Figure 2 shows the changes in fluorescence intensity of DNA-CTMA-PMMA films with a DNA-CTMA:PMMA weight ratio of 10:1. The fluorescence intensity was rather stable for almost all conditions tested, although after 48 hours of exposure to 100% relative humidity, the data showed degradation for one of the two samples. This was due to a small, swelling-induced air gap, which caused the film to partially separate from the glass substrate. This result suggests that we have to be careful to avoid immersion of the film into too much water during fabrication.

Figure 2. Fluorescence intensity (in arbitrary units, a.u.) of two DNA-CTMA-PMMA films under various humidity conditions, as indicated in the legend (DBASMPI:DNA-CTMA molar ratio = 20:1, DNA-CTMA:PMMA weight ratio = 10:1).

Figure 3 shows an example of a test pattern formed on a silicon wafer with waveguide widths of 5, 10, 15, and 20μm. We employed chemical etching using organic solvent and achieved good uniformity for these straight waveguide patterns. For splitting waveguide patterns, splitters with 10μm-wide waveguides could be formed smoothly (see Figure 4), but for waveguide widths smaller than 5μm, some residue remained between the two splitting waveguide arms.

Figure 3. Straight waveguide pattern (5, 10, 15, and 20μm wide).

Figure 4. Splitter pattern with 10μm-wide waveguides.

In summary, waveguides fabricated based on dye-doped DNA-CTMA films exhibited good uniformity and processability after introduction of a DNA-CTMA-PMMA hybrid structure. However, splitter waveguides with relatively narrow widths showed some residue between the splitting waveguides after wet etching. We will next further optimize a number of process parameters, such as the curing temperature of the DNA and/or photoresist as well as the etching conditions, to achieve formation of fine patterns.

The authors are indebted to Nobuaki Omata, Yoichi Takahashi, and Hiroharu Ikeda for fruitful discussions and comments throughout the course of this work. We also thank Sho Kato, Ryota Ueda, and Takashi Tajima for their help in preparing DNA lipids and for enthusiastic discussions. This work was partially supported by the Strategic Base Technology Project of the Japanese Ministry of Economy, Trade, and Industry, and by the Innovation Research Fund of the Ogasawara Science and Technology Foundation.

Junichi Yoshida, Yutaka Kawabe
Chitose Institute of Science and Technology
Chitose-shi, Japan

Junichi Yoshida joined the Nippon Telegraph and Telephone (NTT) Electrical Communication Laboratories in 1973. In 1999, he was appointed professor at the Chitose Institute of Science and Technology. He is a senior member of the IEEE, a fellow of the Institute of Electronics, Information, and Communication Engineers of Japan, and a member of both SPIE and the Japan Society of Applied Physics.

Naoya Ogata
Ogata Research Laboratory Ltd.
Chitose-shi, Japan