Analysis of tissue condition based on interaction between inorganic and organic matter in cuttlefish bone

The absorption properties of an inner layer of cuttlefish bone were measured using a transmission terahertz time-domain spectrometer in a band from approximately 0.1 to 4 THz. For oriented samples, an absorption peak related to the behavior of calcium carbonate appeared at approximately 2 THz. The peak magnitude and frequency depended on the direction of the incident terahertz electric field, indicating that calcium carbonate crystals constituting the inner layer were oriented in a certain direction. The absorbance of a sample heated to 350 °C for 0 to 2 h to remove organic matter tended to decrease with heating time in the oriented direction, while the peak frequency shifted to higher frequencies. Furthermore, we showed that the peak frequency depended on the interaction area within the unheated sample and we thus obtained a two-dimensional image reflecting crystal regularity inside the cuttlefish bone from the spectral data at each position.     

Staked Graphene for Tunable Terahertz Absorber with Customized Bandwidth

Terahertz (THz) absorber with dynamically tunable bandwidth possesses huge application value in the fields of switches, sensors, and THz detection. However, the perfect absorbers based on photonic crystals and metamaterials are not intelligent enough to capture the electromagnetic wave in a tunable way. In this paper, we utilized only patterned graphene to tune the absorption positions and the bandwidth in the terahertz regime. More distinguished than some dynamic absorbers proposed before, the performances with peak frequency relative tuning range of 68 % and nearly unity absorbance are obtained by a single cross-shaped graphene layer. Additionally, the working bandwidth can be broadened with stacked structured graphene. The almost perfect absorption shifted from 2.36～3.2 to 3.26～3.99 THz continuously via changing the chemical potential of graphene.     

Polarization Management of Terahertz Extraordinary Optical Transmission through Ultracompact L-Shaped Subwavelength Patterns on Metal Films

Compact and efficient terahertz (THz) polarization conversion components are of importance for applications where the small dimension of the laser device/system is critical. Here, we propose an ultracompact L-shaped subwavelength patterns on metal films to realize the THz polarization management. By optimizing the geometric parameters of single-layered and double-layered patterns, the linear-polarized THz incidence can be converted to elliptical polarized output or rotated by 90° efficiently due to the THz extraordinary optical transmission phenomenon. The physical mechanism is explored by mode analysis using numerical and analytical modeling.     

A Three-Dimensional Plasmonic Nanostructure with Extraordinary Optical Transmission

We report a 3D plasmonic nanostructure having an extraordinary optical transmission due to localized surface plasmon (LSP) coupling between nanoholes and nanodisks. The nanostructure contains a free-standing gold nanohole array (NHA) film above a cavity and an array of nanodisks at the bottom of the cavity that is aligned with the NHA. For the device, the LSP-mediated resonance position was dependent on the hole and nanodisk diameter as well as the separation distance. Also, the effect of LSP coupling between each hole and corresponding nanodisk became negligible for cavities deeper than 200 nm as observed as a disappearance of the LSP resonance. The greatest LSP resonance transmission and the highest electric field intensity were observed for the structure with the shallowest cavity. In addition, the structure had high surface plasmon resonance sensitivity and may have potential for surface-enhanced Raman spectroscopy and optical trapping applications.     

Effects of Refractive Index Variations on the Optical Transmittance Spectral Properties of the Nano-Hole Arrays

The 3D finite difference time domain technique was carried out to study the optical transmission properties of nano-hole arrays in the gold thin film supported by materials with different index of refraction in the visible and infrared regions. A series of perforated nano-hole structures on the gold film at different hole radius, hole depth of 100 nm, and structural periodicity of 400 nm were studied. It was found that transmission properties (i.e., intensity, FWHM, and resonance position) were strongly affected by hole radius and surrounding medium index of refraction. The maximum optical transmittance was observed as 31.9 % in a nano-hole array of hole radius of 125 nm and refractive index of 1.3. The maximum sensitivity of 300 nm/RIU was obtained at index of refraction of 1.7, whereas the minimum one was calculated as 110 nm/RIU in a nano-hole array of hole radius of 50 nm. It was also found that on increasing the hole radius from 50 to 125 nm, the spectral sensitivity was decreased, whereas the index sensitivity was increased on increasing the refractive index.     

Time-of-Flight Model for the Extraordinary Transmission Through Periodic Arrays of Subwavelength Apertures at THz Frequencies

A qualitative model explaining the extraordinary optical transmission of terahertz (THz) radiation through two-dimensional periodic arrays of subwavelength apertures is presented. Systematic terahertz time-domain spectroscopy studies have been undertaken to investigate the combined effects of the lattice arrangement, aperture shape, area and aspect ratio on the transmission properties of electroformed copper arrays. The extensive results presented provide a unified example of how aperture geometry dictates SPP activity. The novel fabrication method creates exemplary peak resonances, allowing the onset of surface plasmon polariton (SPP) decoupling to be distinguished from direct transmission. Furthermore, we provide the first evidence as to how the temporal properties of SPPs are governed by the single-cycle THz pulse. The time-of-flight model presented can not only be used to explain the results observed in both the presented and previously published experiments but serves as a method to engineer specific resonances for sensor applications.     

Extraordinary Transmission of Three-Dimensional Crescent-like Holes Arrays

We developed a method to fabricate a periodic array of three-dimensional crescent-like holes (3DCLH) via an inverted hemispherical colloidal lithography. It is found that there exists an extraordinary optical transmission in this non-planar perforated periodic array of 3DCLH when the electric field of the incident light is perpendicular to the cross-line of the crescent-like hole. This extraordinary optical peak is insensitive with the incident angles and sensitive with the angle between the electric field of the incident light to the cross-line of the 3DCLH. Numerical simulation based on finite-difference time-domain method reveals that this peak is caused by an asymmetric localized surface plasmon resonance. This structure might be useful for the optical sensing and optical-integrated circuits.     

Enhanced and Selective Photodetection Using Graphene-Stabilized Hybrid Plasmonic Silver Nanoparticles

We report the fabrication and characteristics of a novel graphene-Ag⁰ hybrid plasmonic nanostructure-based photodetector exhibiting moderately high responsivity (～28 mA/W) and spectral selectivity (～510 nm) in the visible wavelength. The formation of highly stable Ag⁰ nanoparticles with an average size of 40 nm is observed within the graphene layers, resulting in n-type doping of hybrid material. The absorption peak of graphene-Ag⁰ hybrid is redshifted to the visible wavelength (～510 nm) from the plasmonic Ag peak (～380 nm) in agreement with the optical simulation results for embedded metal nanoparticles. The study demonstrates the synergistic effect of the graphene-metal nanocomposite, which appears attractive for applications in graphene-based photonic devices.     

Magneto-optical Studies of Noble Metal-Magnetic Dielectric Systems

The extraordinary optical transmission and Faraday effects of the bilayer heterostructure consisting of a metallic film perforated with subwavelength hole arrays and a uniform dielectric film magnetized perpendicular to its plane were systematically studied by three-dimensional finite-difference time-domain method. Results of the calculation found that for the magneto-plasmonic crystals under polarized incident light with transverse magnetic mode, the resonant transmittance reached 36.9%, the Faraday rotation acquired 1.216°, and the ellipticity got a positive value of 0.840. The value of Faraday rotation and ellipticity is respectively 15.2 and 93.3 times enhancement of the 0.08° and ?0.009 of the bare BIG film at the wavelength. In the transverse electric mode, the Faraday effects of the systems also had a large enhancement in contrast to the bare magnetic film. The magneto-optical effects of the systems could be manipulated by polarization mode of incident light, geometry of perforated subwavelength hole arrays, and thickness of metallic and magnetic films. Evolution of the magneto-optical properties on the structural parameters was also analyzed. Possible mechanisms underlying the extraordinary phenomena were profoundly discussed. All these results indicated that the systems could find potential applications in magneto-optical devices such as data storages, sensors, and telecommunications.     

Extraordinary Optical Transmission Performances of Nanosandwiched Grating for Wideband Multi-Function Integration

In this paper, we present a peculiar metal-dielectric-metal (MDM) nanosandwich grating structure that can achieve extraordinary optical transmission performances at normal incidence in the ultraviolet-visible-near infrared (UV-VIS-NIR) regions. The proposed structure shows three obvious spectrum characteristics: it can obtain high transmittance up to 80 % in NUV region and efficiently blocking visible wavelengths for transverse-magnetic (TM) polarized incidence; a broadband NIR polarizer can be inspired in the wavelength range from 950 to 1400 nm; more surprisingly, these performances do not deteriorated until 30° tilting angle. Compared to other grating structures with single metal overlayer, it shows wider band-stop characteristics and higher broadband transmission transmittance and extinction ratio (ER) in the investigated wavebands. We analyze the underlying physical mechanism by using numerical simulation, which is primarily attributed to metal ultraviolet transparency, surface plasmon polariton (SPP) at metal/dielectric interface, Fabry–Perot (FP)-like cavity mode within this dielectric grating, and optical magnetic resonance especially in the dielectric interlayer of the MDM sandwiched structure. This structure is very important for developing high-performance subwavelength multifunctional integrated optical devices.     

Tunable Plasmon–Induced Transparency Based on Dirac Semimetals

Numerical and theoretical studies were conducted on the plasmon induced transparency (PIT) of the symmetrical structure of Dirac semi-metal films (DSFS). The films have a parallel strip and split resonant ring structure. After analysing the surface current intensity and distribution, it was found that the electromagnetically induced transparency is as a result of destructive interference between these two structures, with the amplitude modulation depth of the frequency of the transmission window reaching as high as 99.09%. Moreover, by adjusting the Fermi level of the DSFS, the Fermi level changed from 50 to 90 meV, and the transmission window blue-shifted from 0.529 to 0.799 THz. The transmission peak frequency was found to have a linear relationship with the Fermi level. Similarly, the transmission phase and group delay under different Fermi levels was investigated. The positive group delay of the film reaches 7.026 ps, which provides a direction for new applications of terahertz, such as optical storage and slow optical devices.

     

Optical Properties of Noncontinuous Gold Shell Engineered on Silica Mesosphere

The optical properties of individual noncontinuous shells with different gold coverage are investigated by the single-particle dark field scattering measurements and single-particle surface-enhanced Raman scattering (SERS) measurements at different excitation wavelengths. By controlling the growth of gold seeds, multi-metallic nanogaps/crevices with different optical responses are assembled on silica mesospheres forming noncontinuous shells that can be confirmed through the transmission electron microscope images. We find the surface plasmon resonance of single shell red-shifts from 510 to 680 nm with the increase of gold coverage. At the excitation of 532 and 785 nm, the best enhancements about 2.0?×?10⁵ and 1.1?×?10⁷ are obtained on spheres with ～60 and 83 % gold coverage, respectively. The weak polarization-dependent SERS indicates that the enhancement is from the multi-gaps on single noncontinuous shell. This optical tunable and SERS active noncontinuous gold shell can be applied in biosensing, ultra trace detection, and molecule analysis needing multi-wavelengths excitation.     

Extraordinary denaturant tolerance of keratinolytic protease complex assemblies produced by Meiothermus ruber H328

A moderately thermophilic bacterial strain, Meiothermus ruber H328, can efficiently solubilize intact chicken feathers by aerobic cultivation at 55 °C for 6 days. The keratinolytic proteases extracellularly secreted by the strain were partially purified by an ultrafiltration system and a size-exclusion column chromatography, and thus were found to be two different sizes of macromolecules with an extremely high molecular mass like the sizes of virus and DNA (peak 1 fraction) and with a molecular mass of larger than 500 kDa (peak 2 fraction). They formed protein complex assemblies that were composed of multiple but different proteins. The peak 1 fraction showed more thermophilic characteristics than did the peak 2 fraction in temperature dependence and thermal stability. By contrast, they comparably showed extraordinary resistance to powerful denaturants, SDS at 30 % (w/v) and organic solvents (methanol, ethanol, acetonitrile, acetone, and chloroform) at 40 % (v/v) at 60 °C for 30 min. The extraordinary denaturant tolerance and the large molecular size of the keratinolytic protease complex assemblies suggest the possibility that those may be lipophilic and have the structure of partial membrane fractions, or membrane vesicles, which are exfoliated from the outer membrane of the cells.     

A Broadband Nanosensor based on Multi-Interference of Surface Plasmon Polaritons

A novel broadband refractive index nanosensor based on multi-interference of surface plasmon polaritons is reported. It is composed of a metallic nanoslit flanked by periodical grooves on its two sides. Extraordinary high-throughput, high-resolution, and high-sensitivity detections can be realized by observing the shift of the resonant wavelength. The sensor covers a large range of the refractive index change due to both the narrow linewidth of the single resonant peak in the broadband spectrum and the sensitive shift of the peak position withthe refractive index change. A theoretical model is developed to well predict the optical response of the sensor. An excellent linearity between the resonant wavelength and the refractive index can be achieved. The sensitivity, which is 620 nm/refractive index unit, can be further increased by tuning the period of the grooves and the high throughput; high resolution can be simultaneously achieved by adding the number of grooves.     

Dispersionless and Giant Optical Activity in Terahertz Chiral Metamaterials

In this paper, we propose a new structure which is achieved via the combination of twist conjugated gammadion and four-L resonators pairs. The proposed chiral metamaterial can achieve dispersionless and giant optical activity simultaneously. The polarization ellipticity is lower than 0.46° through all function bands, and the polarization azimuth rotation angle is larger than 90.3° from 2.37 to 2.69 THz. Specifically, the structure can achieve 90° dispersionless polarization rotation at f?=?2.57 THz. The optical activity is optimized through changing the parameters of the chiral structure and the physical mechanism is also analyzed based on surface current distribution.     

Tunable Salisbury Screen Absorber Using Square Lattice of Plasmonic Nanodisk

We propose a novel polarization independent Salisbury screen absorber to provide tunable resonant absorption at terahertz (THz) frequencies. The Salisbury screen absorber is designed by using a planar array of thin gold nanodisks arranged in a square lattice. Certain configurations of Salisbury screen have multiple distinctive absorption bands that support near-unity/FWHM absorption bandwidth reaching 36 THz/169 THz, respectively. Moreover, the absorption bandwidth depends upon the optical thickness of the dielectric spacer between the metasurface and the metallic ground plane. The proposed tunable Salisbury screen absorber can find practical applications in photonic detection, imaging, sensing, and solar cells at optical frequencies.     

Solvent-Induced Morphological Changes of Polyhedral Silver Nanoparticles in Epoxy Resin

The use of just one material as reducing and capping agent during the synthesis of noble metal nanoparticles is of great interest for potential applications. This paper reports a simple method to prepare polyhedral silver nanoparticles at 80 °C using an epoxy resin (Araldite 506) as both reducing and capping agent. The formation of metal nanoparticles was investigated by Fourier-transform infrared spectroscopy, atomic force microscopy, transmission electronic microscopy, high-resolution transmission electronic microscopy and UV-vis spectroscopy. The proposed mechanism for the reduction of silver ions involves radicals as precursors of ketones and other products originated by thermo-oxidation of the resin. The nucleation of small spherical and polyhedral seeds (～7 nm) of polycrystalline silver and twin planes lead to big polyhedral silver nanoparticles of average size, 68 nm. On the other hand, the polyhedral silver nanoparticles dispersed in toluene changed to prolate-like particles, and their dispersion in dimethyl-sulphoxide and formamide originated elongated polyhedrons and concave nanostructures, respectively. These structural changes lead to unusual solvent-induced optical properties. For instance, the polyhedral nanoparticles dispersed in toluene red-shifted their surface plasmon resonance from 425 to 540 nm, in dimethyl-sulphoxide the spectrum exhibited a peak at 418 nm and a shoulder at 520 nm, and for the silver nanoparticles in formamide a broad band with maximum peak at 420 nm was observed. It is showed that the solvent/resin system works itself as structure-directing agent of silver nanoparticles. These results open the doors to achieve silver nanostructures highly sensitive to the dielectric environment, an ideal condition for applications in colorimetric sensors of molecules of biological or chemical interest.     

Dielectric Dispersion in Ga<Subscript>2</Subscript>S<Subscript>3</Subscript> Thin Films

In this work, the structural, compositional, optical, and dielectric properties of Ga₂S₃ thin films are investigated by means of X-ray diffraction, scanning electron microscopy, energy dispersion X-ray analysis, and ultraviolet—visible light spectrophotometry. The Ga₂S₃ thin films which exhibited amorphous nature in its as grown form are observed to be generally composed of 40.7 % Ga and 59.3 % S atomic content. The direct allowed transitions optical energy bandgap is found to be 2.96 eV. On the other hand, the modeling of the dielectric spectra in the frequency range of 270–1,000 THz, using the modified Drude-Lorentz model for electron-plasmon interactions revealed the electrons scattering time as 1.8 (fs), the electron bounded plasma frequency as ~0.76–0.94 (GHz) and the reduced resonant frequency as 2.20–4.60 ×10¹⁵ (Hz) in the range of 270–753 THz. The corresponding drift mobility of electrons to the terahertz oscillating incident electric field is found to be 7.91 (cm ²/Vs). The values are promising as they nominate the Ga₂S₃ thin films as effective candidates in thin-film transistor and gas sensing technologies.     

Multiband Metamaterial Absorber Design Based on Plasmonic Resonances for Solar Energy Harvesting

A new metamaterial absorber is designed and characterized numerically for the harvesting of solar energy. The design is composed of three layers in which the interaction among them gives rise to the plasmonic resonances. The main operation frequency range of the proposed structure is chosen to be the visible regime. However, the design is also analyzed for the infrared and ultraviolet regimes. In order to characterize the absorber, some parametric studies with respect to the dimensions of the structure are carried out. According to the results, it is found that the proposed metamaterial absorber has 98.2 % absorption capability at 445.85 THz and 99.4 % absorption capability between 624 and 658.3 THz. Moreover, the polarization dependency of the structure is examined and it is found that the design operates well as a perfect absorber with polarization independency for the studied frequency range. As a result, the proposed metamaterial absorber can be used for solar energy harvesting as it provides multiple perfect absorption bands in the visible regime.     

Plasmonically Induced Absorption and Transparency Based on Stub Waveguide with Nanodisk and Fabry-Perot Resonator

A novel metal-insulator-metal (MIM) plasmonic waveguides structure, which is composed by stub waveguide with nanodisk and Fabry-Perot (F-P) resonator, has been proposed and numerically simulated with the finite-difference time-domain (FDTD). Based on the three-level system, the extreme destructive interference between bright and dark resonators gives rise to the distinct plasmonically induced absorption (PIA) response with the abnormal dispersion and novel fast-light feature. Simultaneously, the dramatic double plasmonically induced transparency (PIT) effect with slow-light characteristic can also be achieved in the system. The relationship between the transmission characteristics and the geometric parameters is studied in detail. By optimum design, the modulation depth of the PIA transmission spectrum of 90 % with 0.145 and 0.14 ps fast-light effect can be gained simultaneously, and the peak transmissivity of the double PIT system of 75.2 and 72.8 % with ?0.38 ps slow light-effect can be achieved. The simulated transmission features are in agreement with the temporal-coupled mode theory (CMT). The characteristics of the system indicate an important potential application in integrated optical circuits such as slow-light and fast-light devices, high-performance filter, and optical storage.