Tatsunosuke Matsui

Transmission resonances through aperiodic arrays of subwavelength apertures

Resonantly enhanced light transmission through periodic subwavelength aperture arrays perforated in metallic films has generated significant interest because of potential applications in near-field microscopy, photolithography, displays, and thermal emission. The enhanced transmission was originally explained by a mechanism where surface plasmon polaritons (collective electronic excitations in the metal surface) mediate light transmission through the grating. In this picture, structural periodicity is perceived to be crucial in forming the transmission resonances. Here we demonstrate experimentally that, in contrast to the conventional view, sharp transmission resonances can be obtained from aperiodic aperture arrays. Terahertz transmission resonances are observed from several arrays in metallic films that exhibit unusual local n-fold rotational symmetries, where n  = 10, 12, 18, 40 and 120. This is accomplished by using quasicrystals with long-range order, as well as a new type of ‘quasicrystal approximates’ in which the long-range order is somewhat relaxed. We find that strong transmission resonances also form in these aperiodic structures, at frequencies that closely match the discrete Fourier transform vectors in the aperture array structure factor. The shape of these resonances arises from Fano interference of the discrete resonances and the non-resonant transmission band continuum related to the individual holes. Our approach expands potential design parameters for aperture arrays that are aperiodic but contain discrete Fourier transform vectors, and opens new avenues for optoelectronic devices.

Mechanism of optical terahertz-transmission modulation in an organic/inorganic semiconductor interface and its application to active metamaterials

Terahertz (THz) transmission modulation through copper phthalocyanine (CuPc)-coated Si under various laser light irradiation conditions was investigated using THz time-domain spectroscopy. The charge carrier transfer from Si to CuPc is crucial for photo-induced metallization, and the thickness of the CuPc layer is a critical parameter for achieving high charge carrier density for metallization. Transmission through a split-ring resonator array metamaterial, fabricated on CuPc-coated Si, can be efficiently modulated by laser light irradiation. Our findings may open the way for various types of metamaterials using organic conjugated materials that are suitable for easy device fabrication using printing technologies.

Resonantly-enhanced transmission through a periodic array of subwavelength apertures in heavily-doped conducting polymer films

We observed resonantly-enhanced 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 ‘anomalous 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, showing that the surface plasmon polaritons on the PPy(PF6) film surfaces have higher attenuation.

Terahertz transmission properties of quasiperiodic and aperiodic aperture arrays

Recent demonstrations have shown that the transmission through a subwavelength aperture in metal film placed in a periodic lattice or an aperiodic structure is significantly increased relative to a bare aperture. Using terahertz time-domain spectroscopy, we analyze the enhanced transmission properties of aperiodic and corresponding random 2D aperture arrays perforated in metallic films, which include quasicrystals and quasicrystal approximates. We demonstrate that the transmission enhancement phenomenon occurs for aperture arrays having discrete Fourier components in the 2D geometrical structure factor. We further show that the phenomenon is valid for a larger class of 2D aperture array designs that can be tailored to exhibit desired resonances and hence is more general. The inherent relationship between various features observed in the measured time-domain electric field, calculated transmission spectra, and the real and reciprocal space representation of the aperture array is discussed in detail. The results are interpreted in terms of Fano-type interference mechanism. The importance of antiresonance features observed in the transmission spectra is also discussed.

Finite-Difference Time-Domain Analysis of Laser Action in Cholesteric Photonic Liquid Crystal

We have numerically investigated lasing dynamics in cholesteric liquid crystal (CLC) with gain by an auxiliary differential equation finite-difference time-domain (ADE-FDTD) method in which the FDTD method is coupled with a rate equation in a four-level energy structure. Circularly polarized lasing was achieved at the photonic band edge above threshold pumping. Our model opens a way for a computational design of the CLC laser on the basis of numerical simulation to realize a more efficient device architecture for a greatly reduced lasing threshold, which is still under extensive study.

Electron-beam induced terahertz radiation from graded metallic grating.

We have numerically analyzed, based on a simplified particle-in-cell finite-difference time-domain (PIC-FDTD) method, an electron-beam (e-beam) induced terahertz (THz) radiation from metallic grating structures with graded depths (graded grating). Upon exciting with e-beam, directional THz radiations with wide-band spectrum containing several sharp peaks are obtained only from the one of the edge of the grating, which cannot be expected from the conventional theory of Smith-Purcell radiation. It was clarified that each modes originate from different locations on the graded grating reflecting different dispersion characteristics of spoof surface plasmon polariton (spoof SPP) at each locations, and they can propagate toward only the shallower groove as a surface wave due to the cutoff at each locations, and all of these modes eventually emitted from the one of the edge of the graded grating. These directional radiations can be directed toward either backward or forward by making the groove depth deeper or shallower. The lowest and the highest frequency of the radiation can be chosen by appropriately designing the deepest and the shallowest groove depths, respectively. These unique radiations cannot be obtained from the uniformly grooved grating. Our findings may open the way for a development of novel THz radiation source based on the spoof SPP on the wide variety of metallic grating structures or metasurfaces.

Extraordinary optical transmission through metallic films perforated with aperture arrays having short-range order

Using terahertz time-domain spectroscopy, we have measured the optical properties of metallic films perforated with arrays of subwavelength apertures that possess short-range order (SRO), but lack long-range orientational order (LRO). We demonstrate that extraordinary transmission enhancement still occurs through the SRO aperture structures, despite the absence of LRO. The dielectric response of these arrays is characterized by a superposition of a broad principal resonance that is due to the random rotations of the building block (BB) units, and discrete resonances arising from well-defined reciprocal vectors in the structure factor that result from a virtual lattice associated with the BB units.

Finite-difference time-domain analysis of photonic nanojets from liquid-crystal-containing microcylinder

The photonic nanojet (PNJ) from a microcylinder with liquid crystals (LCs) showing tangential molecular alignment inside the microcylinder has been numerically analyzed on the basis of the finite-difference time-domain method. By introducing a small degree of birefringence, the characteristics of the PNJ, such as propagation length and polarization state, can be drastically changed. The azimuth angle and the ellipticity of the elliptically polarized PNJ obtained from the LC microcylinder changes within the propagation lengths in the micrometer range even in the isotropic matrix, which might be attributed to the jet like spatial profile of the PNJ. By using LC microcylinders or microspheres, we may obtain a rich variety of PNJs with unique polarization characteristics, which might open a new avenue for the development of novel optical devices with electrical tunability.

Efficient optical terahertz-transmission modulation in solution-processable organic semiconductor thin films on silicon substrate

Terahertz (THz)-transmission modulation through the Si substrate coated with four π-conjugated materials under various laser light irradiation conditions was investigated using THz time-domain spectroscopy. Two of the π-conjugated low molecules, the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), and two of the π-conjugated polymer materials, poly[5-(2-ethylhexyloxy)-2-methoxycyanoterephthalyliden] (MEH-CN-PPV) and poly(benzimidazobenzophenanthroline) (BBL), were investigated. Among these materials, PCBM and TIPS-pentacene showed higher modulation efficiencies and it was also shown that thermal annealing is quite effective in obtaining THz modulation efficiencies higher than those previously reported for copper phthalocyanine. Utilizing these solution-processable π-conjugated materials, various types of THz materials and devices could be fabricated by printing technologies. Our findings may open the way to fabricating various types of THz active devices.

Numerical Simulation of Lasing Dynamics in Cholesteric Liquid Crystal

We have numerically simulated lasing dynamics in cholesteric liquid crystal (CLC) on the basis of an auxiliary differential equation finite-difference time-domain (ADE-FDTD) method. In our ADE-FDTD procedure, the FDTD method is coupled with a rate equation in a four-level energy structure and equation of motion of polarization. Circularly polarized lasing at the edge of the stop band was successfully reproduced. Time-dependent field distributions were also investigated. Our model can be a powerful tool to study the underlying physics of CLC laser and also can be utilized to pursue the more efficient laser device architecture with reduced lasing threshold.