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Fundamental effects in nanophotonic resonance systems focused on singular states and their properties are presented. Strongly related to lattice geometry and material composition, there appear resonant bright channels and non-resonant dark channels in the spectra. The bright state corresponds to high reflectivity guided-mode resonance (GMR) whereas the dark channel represents a bound state in the continuum (BIC). Even in simple systems, singular states with tunable bandwidth appear as isolated spectral lines that are widely separated from other resonance features. Under moderate lattice modulation, there ensues leaky-band metamorphosis, merging modal bands and resulting in offset dark states and reflective BICs along with transmissive BICs within a high-reflectance wideband. Rytov-type effective medium theory (EMT) is shown to be a powerful means to describe, formulate, and understand the collective GMR/BIC fundamentals in resonant photonic systems. Particularly, the discarded Rytov analytical solution for asymmetric fields is shown here to predict the dark BIC states essentially exactly for considerable modulation levels. The propagation constant of an equivalent EMT homogeneous film provides a quantitative evaluation of the eminent, oft-cited embedded BIC eigenvalue. The work concludes with experimental verification of key effects.

Metamaterials are important, as they possess properties not found in simple materials. Photonic device technology applying metamaterials supports many new and useful applications. Here, we address the fundamental physics of wideband metamaterial reflectors. We show that these devices operate because of resonant leaky Bloch modes propagating in the periodic lattice. Moreover, in contrast to published literature, we demonstrate that Mie scattering in individual array particles is not a causal effect. In particular, by connecting the constituent particles by a matched sublayer and thereby destroying the Mie cavity, we find that the resonance bandwidth actually expands even though localized Mie resonances have been extinguished. There is no abrupt change in the reflection characteristics on addition of a sublayer to any metamaterial array consisting of discrete particles. Thus, the physics of the discrete and connected arrays is the same. The resonant Bloch mode picture is supported by numerous additional examples and analyses presented herein.

Guided-mode resonant (GMR) thin films integrated on fiber tips are known to realize compact filters and sensors. However, limited progress in experimental realization has been reported to date. Here we provide a considerable advance in this technology, as we experimentally demonstrate efficient fiber-facet mounted device prototypes. To retain a large aperture for convenient coupling, we design and fabricate silicon nitride-based resonators on the tip of a multimode fiber. We account for light propagation along the multimode fiber with exact numerical methods. This establishes the correct amplitude and phase distribution of the beam incident on the tip-mounted GMR element, thus enabling us to properly predict the resonance response. To fabricate the integrated GMR structures on the tips of fibers, we employ standard microfabrication processes, including holographic interference lithography and reactive-ion etching. The experimental results agree with simulation with an example device achieving high efficiency of ∼77% in transmission. To investigate fiber sensor operation, an etched silicon nitride fiber tip filter is surrounded with solutions of various refractive indices, yielding an approximate sensitivity of 200 nm/RIU.

Resonant nanogratings and periodic metasurfaces express diverse spectral and polarization properties on broadside illumination by incident light. Cooperative resonance interactions may yield shaped spectra for particular applications, in contrast to a multilayer dielectric mirror. Here, we provide guided-mode resonance filters with flat-top spectra suitable for wavelength division multiplexing systems. Applying a single one-dimensional grating layer sandwiched by two waveguides, we theoretically achieve high-efficiency flat-top spectra in the near-infrared region. This result is obtained by inducing simultaneous nearly degenerate resonant modes. The resonance separation under this condition controls the width of the flat-top spectrum. This means we can implement spectral widths ranging from a sub-nanometer to several nanometers applying fundamentally the same device architecture.

We report the creation of hybrid energy cells based on hierarchical nano/micro-architectured polydimethylsiloxane (HNMA-PDMS) films with multifunctionality to simultaneously harvest mechanical, solar, and wind energies. These films consist of nano/micro dual-scale architectures (i.e., nanonipples on inverted micropyramidal arrays) on the PDMS surface. The HNMA-PDMS is replicable by facile and cost-effective soft imprint lithography using a nanoporous anodic alumina oxide film formed on the micropyramidal-structured silicon substrate. The HNMA-PDMS film plays multifunctional roles as a triboelectric layer in nanogenerators and an antireflection layer for dye-sensitized solar cells (DSSCs), as well as a self-cleaning surface. This film is employed in triboelectric nanogenerator (TENG) devices, fabricated by laminating it on indium–tin oxide-coated polyethylene terephthalate (ITO/PET) as a bottom electrode. The large effective contact area that emerged from the densely packed hierarchical nano/micro-architectures of the PDMS film leads to the enhancement of TENG device performance. Moreover, the HNMA-PDMS/ITO/PET, with a high transmittance of >90%, also results in highly transparent TENG devices. By placing the HNMA-PDMS/ITO/PET, where the ITO/PET is coated with zinc oxide nanowires, as the top glass substrate of DSSCs, the device is able to add the functionality of TENG devices, thus creating a hybrid energy cell. The hybrid energy cell can successfully convert mechanical, solar, and wind energies into electricity, simultaneously or independently. To specify the device performance, the effects of external pushing frequency and load resistance on the output of TENG devices are also analyzed, including the photovoltaic performance of the hybrid energy cells.

Narrow bandpass filters are applied in laser systems, imaging, telecommunications, and astronomy. Traditionally implemented with thin-film stacks, there is recent interest in alternative means incorporating photonic resonance effects. Here, we demonstrate a new approach to bandpass filters that engages the guided-mode resonance effect working with a cavity-based Fabry–Perot resonance to flatten and steepen the pass band. Both of these resonance mechanisms are native to simple resonant bandpass filters placed in a cascade. Numerical examples provide quantitative spectral properties including pass-band shape and sideband levels. Thus, we compare the spectra of single-layer 1D and 2D resonant gratings with the dual-cascade design incorporating identical gratings. Two- and three-cavity designs are measured against a classic multi-cavity thin-film filter with 151 layers. Whereas these initial results show comparable and improved results achieved with sparse structures, the challenge remains of developing a suitable fabrication technology to capitalize on this promise.

Bandpass filters based on subwavelength dielectric gratings are grounded in physical principles that are totally distinct from their thin-film counterparts. Ease in fabrication, design scalability, material sparsity, and on-chip integration compatibility makes them a promising alternative especially for long-wavelength applications. Here we demonstrate the interesting attribute of resonant bandpass filters of high angular stability for fully conical light incidence. Fashioning an experimental bandpass filter with a subwavelength silicon grating on a quartz substrate, we show that fully conical incidence provides an angular full width at half-maximum linewidth of ∼9.5° compared to a linewidth of ∼0.1° for classical incidence. Slow angular variation of the central wavelength with full conical incidence arises via a corresponding slow angular variation of the resonant second diffraction orders driving the pertinent leaky modes. Moreover, full conical incidence maintains a profile with a single passband as opposed to the formation of two passbands characteristic of resonant subwavelength gratings under classical incidence. Our experimental results demonstrate excellent stability in angle, spectral profile, linewidth, and efficiency.

Highly transparent and flexible triboelectric nanogenerators (TENGs) were fabricated using the subwavelength-architectured (SWA) polydimethylsiloxane (PDMS) with a nanoporous anodic aluminum oxide (AAO) template as a replica mold. The SWA PDMS could be utilized as a multifunctional film for a triboelectric layer, an antireflection coating, and a self-cleaning surface. The nanopore arrays of AAO were formed by a simple, fast, and cost-effective electrochemical oxidation process of aluminum, which is relatively impressive for fabrication of the TENG device. For electrical contacts, the SWA PDMS was laminated on the indium tin oxide (ITO)-coated polyethylene terephthalate (PET) as a bottom electrode, and the bare ITO-coated PET (i.e., ITO/PET) was used for the top electrode. Compared to the ITO/PET, the SWA PDMS on the ITO/PET improved the transmittance from 80.5 to 83% in the visible wavelength region and also had high transmittances of >85% at wavelengths of 430–455 nm. The SWA PDMS also exhibited the hydrophobic surface with a water contact angle (θCA) of ∼115°, which can be useful for self-cleaning applications. The average transmittance (Tavg) of the entire TENG device was observed to be ∼70% over a broad wavelength range. At an external pushing frequency of 0.5 Hz, for the TENG device with the ITO top electrode, open-circuit voltage (VOC) and short-circuit current (ISC) values of ∼3.8 V and ∼0.8 μA were obtained instantaneously, respectively, which were higher than those (i.e., VOC≈ 2.2 V, and ISC≈ 0.4 μA) of the TENG device with a gold top electrode. The effect of external pushing force and frequency on the output device performance of the TENGs was investigated, including the device robustness. A theoretical optical analysis of SWA PDMS was also performed.

Vertically-grown ZnO nanorod arrays (NRAs) on indium tin oxide (ITO)-coated polyethylene terephthalate (PET), as a top electrode of nanogenerators, were investigated for the antireflective property as well as an efficient contact surface in bare polydimethysiloxane (PDMS)-based triboelectric nanogenerators. Compared to conventional ITO-coated PET (i.e., ITO/PET), the ZnO NRAs considerably suppressed the reflectance from 20 to 9.7% at wavelengths of 300–1100 nm, creating a highly transparent top electrode, as demonstrated by theoretical analysis. Also, the interval time between the peaks of generated output voltage under external pushing forces was significantly decreased from 1.84 to 0.19 s because the reduced contact area of the PDMS by discrete surfaces of the ZnO NRAs on ITO/PET causes a rapid sequence for triboelectric charge generation process including rubbing and separating. Therefore, the use of this top electrode enabled to operate the transparent PDMS-based triboelectric nanogenerator at high frequency of external pushing force. Under different external forces of 0.3–10 kgf, the output voltage and current were also characterized.

We reported the light-extraction properties of InGaN/GaN multiple quantum wells blue light-emitting diodes (LEDs) with ZnO nanorod arrays (NRAs) on Ni/Al-doped ZnO (AZO) films as a current spreading layer (CSL). The Ni/AZO bilayer exhibited a high optical transmittance of ${\sim}80$% at $\lambda \sim 460$ nm. The electrical property of AZO films was improved by inserting a thin Ni layer, which leads to the better current--voltage characteristics of LEDs. The ZnO nanorods can be easily grown on the AZO surface of Ni/AZO CBL as the same materials by a simple wet chemical growth. For $450 \times 450$ μm2LED with Ni/AZO CSL, the incorporation of ZnO NRAs into the AZO surface improved the light output power by ${\sim}14$% at 100 mA without causing any electrical degradation compared to the conventional LED without ZnO NRAs.

We report the diffuse light‐scattering properties of nanocracked and porous molybdenum trioxide (MoO3) films deposited on indium tin oxide (ITO) coated glass as a transparent conductive oxide (TCO), caused by the enhanced light diffraction due to high surface roughness as well as narrow slit‐like structures. By utilizing the electrodeposition and thermal annealing processes, the hydrous and amorphous molybdenum oxide films were changed into crystallized MoO3films with self‐formed nanocracks and rough surface. From the theoretical analysis of the light behavior passing through the morphology of MoO3films, a considerable diffuse light scattering was predicted. In order to optimize the light‐scattering property, the structural and optical characteristics of MoO3films on ITO/glass at different applied voltages were investigated. For a proper applied voltage of 2 V, widely distributed nanocracks and rough surface were observed, leading to the enhanced diffuse transmittance of 54% at λ∼ 410 nm. Additionally, the fabricated MoO3films on ITO/glass exhibited a more hydrophilic surface.

The silver (Ag) nanoparticle (NP) decorated zinc oxide (ZnO) nanorod arrays (NRAs) on aluminum‐doped ZnO (AZO) coated glass substrates were fabricated by the wet chemical synthesis of ZnO and a subsequent thermal evaporation of Ag without thermal annealing. The optical properties of ZnO NRAs with different sized Ag NPs were investigated for light absorption enhancement, in comparison with the Ag on glass substrate. The nominal size and the amount of Ag NPs were controlled by changing the evaporation time. As the evaporation time was increased from 200 to 1000 s, the amount of Ag NPs of ≲ 50 nm attached to ZnO NRAs was increased, leading to a significant increase of light absorption due to the enhanced localized surface plasmon resonance (LSPR) compared to the Ag on glass substrate. However, at a long evaporation time of 2000 s, the evaporated Ag did not exhibit the peak of LSPR because the deposited Ag NPs become excessively agglomerated. At evaporation times of 200–1000 s, the resonance peaks of the ZnO NRAs with Ag NPs were observed at λ∼ 463–508 nm. After evaporating at 1000 s, the peak absorptance of 94% was achieved.

We fabricated a variety of periodic and random III-V, IV, and II-IV semiconductor nanostructures based on various materials, such as Si, GaAs, ZnO, sapphire, etc., by top-down and bottom-up processes for optoelectronic device applications. The periodic arrays of nanostructures were fabricated by the dry etching after forming nanopatterns using the laser interference lithography or monolayer assembly of silica nanospheres. The random nanostructures were formed by a thermal dewetting process of continuous metal thin films and a subsequent dry etching process. Also, the porous films with nanocolumns were formed by a glancing angle deposition. The ZnO nanostructures were fabricated by the hydrothermal growth and electrochemical deposition. The optical properties of the fabricated nanostructures were measured, together with theoretical analyses using the rigorous coupled-wave analysis method. To improve the device performance, these semiconductor nanostructures were applied to the devices such as light emitting diodes, solar cells, and sensors.

Optical waveguide structures of InP‐based quantum cascade lasers (QCLs) are theoretically analyzed and designed using the finite element method. The optical confinement factor (Γ) and absorption coefficient (α) of the waveguide structures for λ∼ 4.6, 6, 8.8, and 9.5 µm QCLs with different numbers of active/injector stages (Ns) are calculated. As Nsis decreased at longer wavelength, Γis gradually decreased and αis increased. For double‐channel (DC) ridge waveguide structures of QCLs operating at λ∼ 9.5 µm, a relatively high optical loss is caused by the lossy insulation layer. To improve the performance, the buried heterostructure (BH) by InP regrowth is used for waveguide design in terms of Nsand waveguide width. For a narrow BH width of ∼10 µm, a low αvalue of 3.92 cm−1is obtained compared to 12.32 cm−1in the DC ridge waveguide structure. With the calculated optical properties of waveguide structures, the device characteristics of QCLs operating at λ∼ 4.6 and 9.5 µm are theoretically investigated in comparison with the experimental results.

We report the enhancement in optical absorption of embedded silver (Ag) nanoparticles (NPs), which are self‐assembled from Ag films with different thicknesses by a rapid thermal annealing treatment, with zinc oxide (ZnO) nanorod arrays (NRAs) on fluorine‐doped SnO2/glass by a simple wet‐chemical synthesis using a thin sputtered ZnO seed layer. The optical absorption and photoluminescence (PL) emission are enhanced by the resonant coupling between the localized surface plasmons of Ag NPs. For optimized ZnO NRAs with embedded Ag NPs, the PL peak intensity is increased up to 4.1 times and the optical absorption of Ag NPs is maximized with dipole and quadrupole resonance peaks.

By a simulation optimization technique, we design In0.53Ga0.47AsIn0.52Al0.48As quantum cascade laser QCL structures, based on a fourquantum well QW active region. The design of QCL structures is performed by maximizing an objective function, i.e., $z_{{\rm UL}}^2 (1 - \tau _{\rm L} /\tau _{{\rm UL}} )\tau _{\rm U}$, related to the optical gain, including dipole matrix element $z_{{\rm UL}}$ and population inversion between subbands. For a specific lasing wavelength, each barrierwell layer in the active region can be optimized to maximize the objective function, which results from the increase in the dipole matrix element and the reduction in the lifetime ratio of $\tau _{\rm L} /\tau _{{\rm UL}}$, by an iterative procedure. The optimized QCL structures are obtained for the target wavelengths from 11 to 12 µm by a step of 0.5 µm. For a QCL structure operating at λ  11.98, it exhibits the maximized objection function of 6.30 psnm2with a large dipole matrix element under an electric field of 45 kVcm. The influence of the percent variation of thickness in In0.53Ga0.47As and In0.52Al0.48As layers by the change in growth rate on the lasing wavelength and objective function is also investigated.

A guided-mode resonance mirror (GMRM) consisting of a subwavelength grating integrated in an optical waveguide on a highly reflective substrate was predicted to give interesting characteristics of high reflectance with a steep reflection-phase spectrum. This time, the characteristics were experimentally demonstrated for the first time. A GMRM of 1535 nm resonance wavelength was designed and fabricated for a vertically injected wave from the air with TE polarization. The reflectance was measured to be higher than ?1 dB over the wavelength from 1520 to 1560 nm. The reflection phase varied by ? for a wavelength change of 10 nm.

We fabricate various ZnO (zinc oxide) nanostructures, such as nanorods, nanotips and nanoflowers, as well as ZnO subwavelength grating structures for applications in optoelectronic devices such as solar cells, light emitting diodes, and biosensors. The optical properties are theoretically analyzed using the rigorous coupled-wave analysis method. The fabricated ZnO nanostructures are of wurzite crystal structure. The reflection and absorption characteristics depend strongly on the shape and geometry of Zn nanostructures. The ZnO nanostructures with Au (or Ag) particles, based on surface plasmons, are also investigated.