This guest editorial introduces the special section on Nanostructured Thin Films IX: Design, Fabrication, Characterization, and Modeling, which appears in the October-December 2017 issue of the Journal of Nanophotonics.

Chiral sculptured thin films (STFs) were grown using the asymmetric serial-bideposition (ASBD) technique, whereby (i) two subdeposits of unequal heights are separated by a substrate rotation of 180 deg about the central normal axis, and (ii) consecutive subdeposit pairs are separated by a small substrate rotation δ on the order of a few degrees. Eight samples were prepared with subdeposit heights in ratios of 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 5:1, 7:1, and 9:1. A finely ambichiral STF was also grown. All nine samples were grown with the same vapor flux direction and to have 10 periods with the same thickness. The spectrums of all eight circular remittances of every sample were measured over a wide range of incidence angle θinc. Redshifting and narrowing of the circular Bragg regime was observed with increasing subdeposit-height ratio for all values of θinc, arriving at a limit with the 9:1 sample. The finely ambichiral sample has a circular Bragg regime similar to that of the 9:1 sample, but the latter exhibits much better discrimination between incident left circularly polarized light and right circularly polarized light than the former for θinc larger than about 20 deg.

Morpho butterflies living in Central and South America are well-known for their structural-colored blue wings. The blue coloring originates from the interaction of light with nano-sized dielectric structures that are equipped on the external surface of scales covering over their wings. The high-accuracy nonstandard finite-difference time domain (NS-FDTD) method is used to investigate the reflection characterization from the nanostructures. In the NS-FDTD calculation, a computational model is built to mimic the actual tree-like multilayered structures wherever possible using the hyperbolic tangent functions. It is generally known that both multilayer interference and diffraction grating phenomena can occur when light enters the nano-sized multilayered structure. To answer the question that which phenomenon is mainly responsible for the blue coloring, the NS-FDTD calculation is performed under various incidence angles at wavelengths from 360 to 500 nm. The calculated results at one incident wavelength under different incidence angles are visualized in a two-dimensional mapping image, where horizontal and vertical axes are incidence and reflection angles, respectively. The images demonstrate a remarkable transition from a ring-like pattern at shorter wavelengths to a retro-reflection pattern at longer wavelengths. To clarify the origin of the pattern transition, the model is separated into several simpler parts and compared their mapping images with the theoretical diffraction calculations. It can be concluded that the blue coloring at longer wavelengths is mainly caused by the cooperation of multilayer interference and retro-reflection while the effect of diffraction grating is predominant at shorter wavelengths.

The precise control of light–matter interaction has a wide range of applications and is currently driven by the use of nanoparticles (NPs) by the recent advances in nanotechnology. Taking advantage of the material, size, shape, and surrounding media dependence of the optical properties of plasmonic NPs, thin film layers with tunable optical properties are achieved. The NPs are synthesized by wet chemistry and embedded in a polyvinylpyrrolidone (PVP) polymer thin film layer. Spectrophotometer and spectroscopic ellipsometry measurements are coupled to finite-difference time domain numerical modeling to optically characterize the heterogeneous thin film layers. Silver nanoprisms of 10 to 50 nm edge size exhibit high absorption through the visible wavelength range. A simple optical model composed of a Cauchy law and a Lorentz law, accounting for the optical properties of the nonabsorbing polymer and the absorbing property of the nanoprisms, fits the spectroscopic ellipsometry measurements. Knowing the complex optical indices of heterogeneous thin film layers let us design layers of any optical properties.

The absorption characteristics of complex medium structures having metasurfaces comprised of columnar nanorods of gold were investigated. In this stream, a periodically arranged assembly of vertical gold nanorods of circular and elliptical cross sections, backed by chromium nanorods of the same cross-sectional size and shape, was considered to be the metasurface, and the comparative features of the absorption characteristics were emphasized. The results exhibit very high absorption corresponding to certain wavelengths in the visible span, and the absorber having elliptical gold nanorods yields a better performance than the one with circular nanorods in terms of the magnitude/smoothness of the absorption peaks.

Layered oxide materials having alternating repeated layer thicknesses of 10 nm or less are difficult to make, especially with sharp interfaces. Nanostructured thin films having repeated layers of two different oxide materials were obtained by using pulsed laser deposition and two independent stationary targets consisting of Al2O3 and BaTiO3. Desired thicknesses were achieved by using a specific number of pulses from a 248-nm KrF excimer laser, at an energy of 450  mJ/pulse, a galvanometer mirror system, and a background pressure of oxygen. Trends in material properties were identified by systematically varying the number of pulses for multiple nanostructured thin films and comparing the resulting properties measured using in-situ spectroscopic ellipsometry and ex-situ capacitance measurements, including relative permittivity and loss. Four films were deposited with a goal of having 0.25-, 1-, 4-, and 10-nm thick layers, and each ∼220  nm thick. Ellipsometry data were modeled in situ to calculate thickness, n and k. A representative transmission electron microscopy measurement was also collected for the 10-nm sample with corresponding x-ray photoelectron spectroscopy and energy disperive x-ray spectroscopy. Ellipsometry and capacitance measurements were all performed on each of the samples, with one sample having calculated impedance greater than 30 GOhm at 0.001 Hz.

Surface-plasmon-polariton waves can be compounded when a sufficiently thin metal layer is sandwiched between two half spaces filled with dissimilar periodically nonhomogeneous dielectric materials. We solved the boundary-value problem for compound waves guided by a layer of a homogeneous and isotropic metal sandwiched between a structurally chiral material (SCM) and a periodically multilayered isotropic dielectric (PMLID) material. We found that the periodicities of the PMLID material and the SCM are crucial to excite a multiplicity of compound guided waves arising from strong coupling between the two interfaces.

A ZnO/Si heterojunction solar cell is studied with plasmonic nanoparticles embedded in the active layer. Two layers of silver nanoparticles are embedded in the ZnO layer. The effect of various parameters such as vertical-interparticle distance, horizontal-interparticle distance, relative dimensions of nanoparticles, and order of particle diameters are discussed in detail. Finite-difference time-domain studies suggest that particle dimensions of the top layer of silver nanoparticles should be less than the dimensions of the underneath layer of silver nanoparticles. The resulting structure acquires the benefits of each layer and improves the device performance over a broad spectrum. The dielectric separation of plasmonic layers is observed to be an important factor in favorable plasmonic response. Electric field diagrams are used to study the scattering of an incident field by proposed structure. Results are encouraging and suggest more concerted studies of multilayer plasmonic structures.

High-transmissivity all-dielectric metasurfaces have recently attracted attention toward the realization of ultracompact optical devices and systems. Silicon-based metasurfaces, in particular, are highly promising considering the possibility of monolithic integration with complementary metal–oxide–semiconductor very large scale integration circuits. Realization of silicon-based metasurfaces operational in the visible wavelengths, however, remains a challenge. A numerical study of bilayered truncated-cone shaped nanoantenna elements is presented. Metasurfaces based on the proposed stepped conical geometry can be designed for operation in the 700- to 800-nm wavelength window and can achieve full-cycle phase response (0 to 2π) with an improved transmittance in comparison with the previously reported cylindrical geometry. A systematic parameter study of the influence of various geometrical parameters on the achievable amplitude and phase coverage is reported.

A broadband band-stop filter based on a multiple side-coupled metal–dielectric–metal slots waveguide is analytically designed and numerically investigated. Beginning with a single-side-coupled slot, we use the dispersion equation and resonant condition to locate the stop band at the visible regime and achieve a narrow stop band filter. Then, we cascade several slots to enlarge the stop band to 2.5 times wider than the single-slot condition. Next, if the height of the slots is gradiently distributed, multiple stop bands will exist. Finally, as a refractive index sensor, the figure of merit of this structure as a sensor reaches 550  nm/RIU. This optimization design process significantly paves the way for high efficient filter or sensor production and fabrication in the visible range in practical scientific or engineering works.

One electron and exciton states in toroidal quantum dot (QD) have been considered. The convenient coordinate system has been defined and the Schrodinger equation has been solved in these coordinates. The electron energy spectrum and wave function dependence on the geometrical parameters of toroidal QD have been obtained. The comparison with the results obtained by numerical methods has been done. Optical absorption has been considered in both cases of “small” toroid when the interaction of an electron and a hole is neglected and cases of relatively “large” toroid when the correlation between the particles has been taken into account. Interband optical transitions have been considered in the ensemble of toroidal QDs. The selection rules for quantum transitions and absorption edge dependence on the geometrical parameters have been obtained.

The coupling between thick-shell CdSe/CdS colloidal nanocrystals with the hot spots of a semicontinuous gold film is characterized by measuring simultaneously the photoluminescence decay rate and the linear polarization ratio. The absence of correlations between the two quantities is demonstrated. In contrast with the results obtained with continuous gold films, polarization ratios higher than 80% are achieved for the smallest nanocrystals. This ratio decreases quickly when the nanocrystals size is increased.

Surface plasmons are excited at a metal/dielectric interface, through the coupling between conduction electrons and incident photons. The surface plasmon generation is therefore strongly determined by the accessibility of the surface to the incoming electromagnetic field. We demonstrate the role of this surface for plasmonic nanoantennas with identical volumes and resonant lengths. An antenna is stratified parallel to the plane of its main dipolar resonance axis and the influence of the number of layers and the spacing between them on the optical properties of the antenna are investigated experimentally. We show that increasing the number of layers and, hence, increasing the total accessible surface of the antenna, results in an enhanced scattering cross section and a redshift which indicates that lower energy photons are required to couple to the metal electrons. In particular, the far-field enhancement observed for double-layer nanostructures suggests that standard single-layer metal deposition can be easily and advantageously substituted with metal/dielectric/metal deposition to boost light scattered by a plasmonic antenna.

Nonlinear emission properties of aggregated 50-nm gold nanoparticles (GNPs) excited by a femtosecond laser at 1560 nm are characterized. Aggregate forms are correlated to emission by scanning electron microscope imaging and pattern matching. Broad spectra in the visible region are obtained from aggregated GNPs, and their emission power exhibits a quadratic power dependence and an exponential decay in time due to morphology change. Polarization analysis reveals that longitudinal plasmonic modes play important roles for nonlinear emission. Relationships between brightness and morphology show that a large number of aggregates produce luminescence enhancement but with associated photo damage. It is proposed that characteristics of nonlinear emission from GNPs are explained by plasmon enhanced polarized hot electrons.

A finite-difference time-domain (FDTD) discretization of the dispersive conductivity of monolayer graphene has been done at the wavelength of 1.55  μm, the third telecommunication window. The FDTD discretization is used to simulate a monolayer graphene plasmonic waveguide and a graphene surface plasmon polariton add–drop filter. We utilize the auxiliary equation method to solve Maxwell’s equations in time-domain and simulate monolayer graphene-based devices without using the subcell method. The results show that the implementation of this method is surprisingly consistent with the results of the previous works and theoretical method. Our proposed method does not suffer from the deficiencies of the subcell and surface boundary condition methods. Both the interband and the intraband terms of the graphene conductivity are used to implement the graphene simulation. Our proposed method is useful for the analysis of monolayer graphene devices at near-infrared wavelengths with less computational complexity.

Sculptured porous Bragg microcavities (BMs) formed by the successive stacking of columnar SiO2 and TiO2 thin films with a zig-zag columnar microstructure are prepared by glancing angle deposition. These BMs act as wavelength-dependent optical retarders. This optical behavior is attributed to a self-structuration of the stacked layers involving the lateral association of nanocolumns in the direction perpendicular to the main flux of particles during the multilayer film growth, as observed by focused ion beam scanning electron microscopy. The retardance of these optically active BMs can be modulated by dynamic infiltration of their open porosity with vapors, liquids, or solutions with different refractive indices. The tunable birefringence of these nanostructured photonic systems has been successfully simulated with a simple model that assumes that each layer within the BMs stack has uniaxial birefringence. The sculptured BMs have been incorporated as microfluidic chips for optical transduction for label-free vapor and liquid sensing. Several examples of the detection performance of these chips, working either in reflection or transmission configuration, for the optical monitoring of vapor and liquids of different refractive indices and aqueous solutions of glucose flowing through the microfluidic chips are described.

The nonlinear TE surface waves guided by an interface of a semi-infinite Kerr-type dielectric medium and a one-dimensional graphene-based photonic crystal have been investigated using the transfer-matrix method. It is shown that increasing the intensity of the incident beam drastically changes dispersion behavior inside the graphene-induced photonic band gap as a result of nonlinearity. Also regarding the simulation results, it reveals that the peculiar dispersion that is governed by a nonlinear structure appeared to be tuned by graphene conductivity via an external electric field. Applying such an electric field makes it possible to easily tune the nonlinear surface waves at the desired frequency only by adjusting the chemical potential of the graphene. Also, it is shown that for application purposes, the results of studies can be applied to the case in which graphene nanolayers are embedded by identical dielectric constant, i.e., zero permittivity contrast of dielectric.

Various metamaterial absorber designs operating in the microwave, infrared, visible, and ultraviolet frequency regions have been proposed in the literature. However, only a few studies have been done on the metamaterials that absorb in both visible and ultraviolet solar spectra. A triple-band polarization-insensitive metamaterial absorber structure with semiconducting single-walled carbon nanotube as the dielectric layer is proposed to efficiently absorb the incident electromagnetic radiations in visible and ultraviolet frequency regions. A unit cell of this design comprises three basic components in the form of metal–semiconductor–metal layers. The metallic part of the structure is aluminum, and the (5,4) single-walled carbon nanotube is used as the semiconducting material. The electromagnetic response of the proposed design is numerically simulated in the visible and ultraviolet regions with the maximum absorption rates of 99.75% at 479.4 THz, 99.94% at 766.9 THz, and 97.33% at 938.8 THz with corresponding skin depths of 13.0, 12.8, and 12.9 nm, respectively. Thus, solar cells based on this metamaterial absorber can offer nearly perfect absorption in the suggested frequency regions. The simple configuration of the design provides flexibility to control geometric parameters to be used in the solar cell and possesses the capability to be rescaled for other solar spectrum.

An implementation of plasmon-induced transparency (PIT) composed of a bus waveguide coupled to a rectangular nanocavity and an H-shape resonator is studied. It is shown that the wavelength, transmission, and width of the PIT peak can be tuned by adjusting the geometrical parameters of the H-resonator, such as the gap between the resonators, width, and position of the middle slit among the branches. For avoiding oxidization of the structure, the effect of coating a thin graphene layer on the inner walls of the H-resonator is investigated. It is shown that the proposed structure can be used as a nanosensor with a refractive index sensitivity of 1300 nm. RIU−1 and figure of merit equal to 59.1  RIU−1 and also as a compact nanoswitch with transmission 76% and a modulation depth equal to 19.77 dB.

Fivefold photonic crystal (PC) structure is originally presented with theoretical investigation about its photonic band gap through transmission spectrum calculated by finite-difference time-domain methods. The existence of PC band gap makes it possible for defect structures with high-quality factor. Two types of fivefold PC microcavity are proposed of which the quality factor, normalized frequency, and electromagnetic field profile have been simulated. With some preliminary structural parameters optimization, monopole mode in one-defect fivefold PC microcavity with a high- Q factor ( ∼2212) and whispering gallery mode in six-defect fivefold PC microcavity with a high- Q factor ( ∼13,300) are obtained.

We theoretically investigated detection accuracy (DA) of a surface plasmon resonance (SPR) sensor with a top dielectric layer. The effects of the variation of thickness and the material of the dielectric layer on the DA of the SPR sensor were investigated. The result indicated that adding a top dielectric layer with a small refractive index (less than the refractive index of the analyte) can improve the DA of a traditional SPR sensor. In addition, by comparing the variation law of the DA and the electric field intensity at the analyte–dielectric interface of the SPR sensor, we studied the physical mechanism of DA variation. This study can be helpful for high-performance SPR sensor development.

We proposed and theoretically investigated a hybrid plasmonics waveguide consisting of a tiptilted quadrate nanowire, which was embedded in a low-permittivity dielectric and placed on a metal substrate with a small gap distance. Due to the corner effect, the hybrid mode with extremely local field enhancement has the long propagation length and strong coupling strength between the dielectric nanowire and metal. By employing the simulations with different geometric parameters, the proposed waveguide can obtain better performances than the previous hybrid plasmonics waveguide, particularly in the subwavelength confinement (as small as λ2/1600), long-range propagation (millimeter range), and optical trapping forces ( 2.12  pN/W). The use of a naturally dielectric wedge tip of quadrate nanowire that can be chemically synthesized provides an efficient approach to circumvent the fabrication difficulty of shape wedge tips. The present structure provides an excellent platform for nanophotonic waveguides, nanolasers, nanoscale optical tweezers, and biosensing.

Accurate determination of a material’s optical band gap lies in the precise measurement of its absorption coefficients, either from its absorbance via the Beer–Lambert law or diffuse reflectance spectrum via the Kubelka–Munk function. Absorption coefficients of powder-form nanomaterials calculated from absorbance spectrum do not match those calculated from diffuse reflectance spectrum, implying the inaccuracy of the traditional optical band gap measurement method for such samples. We have modified the Beer–Lambert law and the Kubelka–Munk function with proper approximations for powder-form nanomaterials. Applying the modified method for powder-form nanomaterial samples, both absorbance and diffuse reflectance spectra yield exactly the same absorption coefficients and therefore accurately determine the optical band gap.

Light trapping in thin-film solar cells is important for improving efficiency and reducing cost. We propose a hybrid nanostructure based on the anodic aluminum oxide grating and Si3N4 double-layer antireflection coatings combined with Ag nanoparticles to achieve advanced light trapping property in gallium arsenide (GaAs) solar cells with 500-nm thickness. The finite-element method is used to study the relationship between geometrical parameters of hybrid nanostructure and optical characteristics of thin-film GaAs solar cells. The light trapping ability is systematically studied by COMSOL multiphysics. The simulation results show that the hybrid nanostructure can highly increase the light absorption in the wavelengths from 300 to 860 nm. The average absorption in 500-nm-thick GaAs layer is 96.7%. The short circuit current density in 500-nm-thick GaAs layer is 30.2  mA/cm2, representing a 58.9% enhancement compared with that ( 19.0  mA/cm2) of the reference cell. Research paves the way for designing highly efficient light trapping structures in thin-film GaAs solar cells.

The resolution limit, i.e., the lower limit to the size of a luminous object that can be determined accurately by ordinary optical microscopy, was determined over a century ago on the basis of the imprecision that is imparted to the details of an image by diffraction. Such criteria assume one is making observations at distances greater than the wavelength of the light emitted or scattered by the luminous object, i.e., in the so-called far field. Although near-field microscopy is an established imaging technique, the analogous resolution criterion pertinent to the near field, in which the distance between the observer or the observing instrument and the light emitter is smaller than the wavelength, has not yet been quantitatively enunciated. Here I propose two ansätze pertinent to optical resolution in the near field. The first, based on classical physics, concludes that the minimal lateral near-field resolution limit is equal to the (perpendicular) distance between the near-field probe and the object. The second is a quantum limitation based on the uncertainty principle, which shows that the momentum uncertainty arising from atomic vibrations results in a minimal resolvable size, which in most instances is of the order of 10 pm.

An approach for the automated whispering gallery mode (WGM) signal decomposition and its parameter estimation is discussed. The algorithm is based on the peak picking and can be applied for the preprocessing of the raw signal acquired from the multiplied WGM-based biosensing chips. Quantitative estimations representing physically meaningful parameters of the external disturbing factors on the WGM spectral shape are the output values. Derived parameters can be directly applied to the further deep qualitative and quantitative interpretations of the sensed disturbing factors. The algorithm is tested on both simulated and experimental data taken from the bovine serum albumin biosensing task. The proposed solution is expected to be a useful contribution to the preprocessing phase of the complete data analysis engine and is expected to push the WGM technology toward the real-live sensing nanobiophotonics.

Here, we have successfully designed a photonic crystal structure to improve the light absorption by increasing the optical path length of the incident light inside the absorbing material to enhance the efficiency of a thin film silicon solar cell. The current design is based on the high temperature superconducting-dielectric photonic crystals (HTcSD PCs). We have substituted the indium tin oxide as a front contact and antireflection coating in the conventional cell by HTcScD PCs. Also, we have substituted the back contact and the back reflector of the conventional cell construction by HTcScD PCs. The aim of these substitutions is to reduce the power dissipation in a thin film silicon solar cell. Numerical results of the proposed structure are obtained based on the transfer matrix method, the finite element method, and COMSOL Multiphysics software. The HTcScD PCs reduced the power dissipation in the thin film silicon solar cell due to the increase of the optical generation term of electron–hole pairs. Finally, using high temperature superconducting photonic crystals in photovoltaic applications is promising and may be of potential use in the future.

A type of silicon photodiode has been designed and simulated to probe the optical near field and detect evanescent waves. These waves convey subwavelength resolution. This photodiode consists of a truncated conical shaped, silicon Schottky diode having a subwavelength aperture of 150 nm. Electrical and electro-optical simulations have been conducted. These results are promising toward the fabrication of a new generation of photodetector devices.

We apply a hybrid nonequilibrium Green’s functions and relaxation rate approximation approach to investigate frequency multiplication in the gigahertz to terahertz range delivering results in good agreement with experimental data for different input frequencies. A discussion of the current limitation of housing designs for the superlattices is given, based on a simple model for the local electric field and in comparison with measured input powers delivered by a backward wave oscillator.

This paper theoretically studies how the optics of multiple grooves in a metal change as the number of grooves gradually increased from a single groove to infinitely many arranged in a periodic array. In the case of a single groove, the out-of-plane scattering (OUP) cross section at resonance can significantly exceed the groove width. On the other hand, a periodic array of identical grooves behaves radically different and is a near-perfect absorber at the same wavelength. When illuminating multiple grooves with a plane wave, the OUP cross section is found to scale roughly linearly with the number of grooves and is comparable with the physical array width even for widths of many wavelengths. The normalized OUP cross section per groove even exceeds that of a single groove, which is explained as a consequence of surface plasmon polaritons generated at one groove being scattered out of the plane by other grooves. In the case of illuminating instead with a Gaussian beam and observing the limit as the incident beam narrows and is confined within the multiple-groove array, it is found that the total reflectance becomes very low and that there is practically no OUP. The well-known result for periodic arrays is thus recovered. All calculations were carried out using Green’s function surface integral equation methods taking advantage of the periodic nature of the structures. Both rectangular and tapered grooves are considered.

We present an experimental technique based on modified multiple beam interference approach to generate various types of photonic crystal (PhC) cavities used in study of nanolasers and cavity quantum electro dynamics. Here, we propose an improvement to the existing method for better realization of PhC cavities and a method to control the hole radius to realize high Q-factors. We used phase-modified Bessel beam of zero and second order and superposed them to get a sidelobe-suppressed Bessel beam. This beam was further interfered with lattice wave field to get defined PhC cavity. We used spatial light modulator-assisted phase-engineered masks with 4F filtering setup to confirm the simulated patterns experimentally.