This report presents an overview of our study on the optical transmission and thermal light emission properties of sub-wavelength
hole arrays fabricated in a square lattice with 4 μm periodicity. The structures were fabricated in thin
aluminum (Al) films on silicon (Si) substrates using conventional photolithography. The spectra were obtained using a
Fourier transform infrared spectrometer with a port for an external cryostat configured for thermal emission
measurements. The perforated films showed extraordinary transmission bands in the mid-infrared spectral range, which
could be well explained as due to light coupling to surface plasmon-polaritons on the two film interfaces. We fitted the
transmission spectra and calculated the absorption spectra of these structures using a model for the dielectric response
that utilizes an effective plasma frequency determined by the individual holes, as well as several resonant modes
associated with the reciprocal vectors in the lattice structure factor. We found that the thermal emission spectrum from
the perforated films followed the transmission spectrum characteristics, rather than the obtained absorption spectrum; in
apparent contrast to Kirchhoff's law of radiation. We conclude that the perforated films behave as radiation filters,
where the thermal emission radiation is suppressed in the frequency range outside the transmission resonant bands in the
We observed resonantly enhanced (or anomalous transmission) terahertz transmission through two-dimensional (2D) periodic arrays of subwavelength apertures with various periodicities fabricated on metallic organic conducting polymer films of polypyrrole heavily doped with PF6 molecules [PPy(PF6)]. The anomalous transmission spectra are in good agreement with a model involving surface plasmon polariton excitations on the film surfaces. We also found that the resonantly enhanced transmission peaks are broader in the exotic metallic PPy(PF6) films compared to those formed in 2D aperture array in regular metallic films such as silver, indicating that the surface plasmon polaritons on the PPy(PF6) film surfaces have higher attenuation.
Optically pumped laser action has been observed at the edge wavelength of the stop band in a dye-doped flexible freestanding film of photopolymerized cholesteric liquid crystal (PCLC), which originates from band edge effect of the one-dimensional photonic band gap. On the other hand, defect mode laser action has also been experimentally demonstrated at the defect mode wavelength within the band gap. This laser action is based on the photon localization at the twist defect of the composite film consisting of two PCLC layers. Twist defect mode (TDM) is induced by the introduction of twist defect which is a discontinuity of the director rotation around the helix axis at an interface of two PCLC layers. We also propose a new type of tunable defect mode based on the chiral defect in which the partial modulation of the helix pitch is introduced.
Electrically tunable laser action has been demonstrated in a dye-doped nematic liquid crystal (NLC) waveguide by holographic excitation. The optical feedback were provided by a transient grating induced by two-beam interference using Lloyd mirror configuration, and the distributed feedback (DFB) laser action was observed. Electrical tuning of lasing wavelength was realized due to the change of the effective refractive index of the NLC core layer caused by the reorientation of NLC molecules. The total shift of lasing wavelength was about 30 nm, which could be realized with less than about 1.4 V of applied voltage. Based on a waveguiding mode theory, numerical analysis of TM-guided mode in the presence of applied electric field was performed, and field-induced tuning of the lasing wavelength was investigated in detail. Prospects for the realization of a single-mode operation and tuning of the lasing wavelength was also shown. Based on the numerical results, single-mode operation of lasing was experimentally realized utilizing NLC with low refractive indices.
Various types of tunable lasing in dye-doped liquid crystals with one dimensional periodic structure have been demonstrated. An electrical tuning of lasing wavelength has been demonstrated in a dye-doped chiral smectic liquid crystal mixture with a short pitch helical structure which is so-called ferroelectric liquid crystal (FLC). Waveguide configuration of FLC laser has also been proposed, also in which the lasing wavelength widely can be tuned upon the electric field. The electrically tunable lasing has been observed also in a focal conic structure of dye-doped cholesteric liquid crystal. This laser action is based on a helix micro-cavity in focal conic domains. Optically pumped distributed feedback lasing has been proposed in a dye-doped nematic liquid crystal (NLC) waveguide by holographic excitation, in which continuous tuning of the lasing wavelength is performed upon applying electric field. Electrical tuning of the wavelength of the defect mode lasing in a one-dimensional periodic structure has been demonstrated using a dye-doped NLC as a defect layer in the periodic structure. Lasing wavelength is widely tuned upon applying the electric field, which is due to the refractive index change in the defect layer caused by the field-induced realignment of the NLC molecules.