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Optical Devices
Fabrication of new organic optical devices using liquid crystals and polymers is one of great interests of our group.
- Cholesteric liquid crystal devices
In cholesteric liquid crystals (CLCs), molecules are arranged in a cyclic helical structure. If the pitch of this helix is in the optical range, it can be considered as a "1D photonic crystal" like dielectric layers. Generally, nano-size processing/manipulation techniques are required to fabricate such photonic crystals. However, CLCs have a great advantage in manufacturing process, since CLCs can 'spontaneously' form a helix and its pitch is tunable by adding a chemical substance, controlling temperature, or applying electric field.
By optically exciting laser-dye-doped CLCs, spectral narrowing of emission occurs as a result of the optical suppression in the photonic band. This leads to application of CLCs for laser devices. In our laboratory, CLC laser devices are studied, mainly aiming at improvement of their performance. So far, we have succeeded lowering the threshold energy and/or increasing the emission efficiency by introducing a well-solvable polymeric dye, suppressing the structural fluctuation of CLCs by using polymer materials, applying defect modes in hybrid structures of CLC and nematic LC or metal layers（Adv. Mater.） and so on. Besides, laser action tunable in the whole visible region has been achieved by a position-sensitive CLC device. Currently, we are going for realization of continuous lasing and current injecting laser devices based on CLCs.
position sensitive laser device tunable in the whole visible region
Since the photonic effects in CLCs are coupled with the circularly polarized light through the helical handedness, combination of such CLC films can realize optical phenomena never seen before in the conventional photonic crystals. For example, we made unique devices such as, an optical-diode having the non-reciprocal transmission of circular polarized light at the photonic-bandgap regions（Nature Mater.）, and a white-light (RGB) reflector consisting of several single-pitched CLC layers（Nature Mater.）.
- Organic Light Emitting Diodes
We are also trying to improve the light-extraction performance of the conventional organic light emitting diodes (OLEDs). Recently, increased light outcoupling has been demonstrated in OLEDs with a nano-imprinted amorphous fluoropolymer substrate having a low refractive index and a high transmittance over wide wavelength range. Besides, by introducing the above-mentioned CLC film into the OLED device structure, low-loss polarization tuning of emission has been realized.
Nano-imprinted OLED device improving light-extraction efficiency
- Other devices
We are interested in possibilities of novel LC/polymer devices. For instance, a tunable phase grating using a photo-curable liquid crystal has been reported by our group.