SiO2 Spacer Layer Research Articles

Multiple high-Q resonances from mirror-coupled dielectric arrays

This paper reports how high-Q resonances can be created at multiple designated wavelengths in a low-index-contrast dielectric nanoparticle array coupled with a metal mirror. A rectangular array of TiO2 nanoparticles over a silver film (separated by a SiO2 spacer layer) can support collective resonances of magnetic and electric dipoles with their wavelengths determined mainly by the lattice spacings of the rectangular lattice. These resonances can be modulated by the spacer layer thickness to form accidental bound states in the continuum. Furthermore, resonances related to the periodicity along the diagonal of the rectangular unit cell can be produced by perturbing the lattice through modifying the dimensions of adjacent nanoparticles in the unit cell. Our result expands the potential of lattice resonances in low-index-contrast dielectric lattices, making them promising for applications in compact multi-wavelength and unidirectional light-emitting devices.

Enhancing red-light emission of CsPb(Br/I)3 quantum dots through noble metal nanostructure-mediated localized surface plasmon resonance

The luminescence properties of perovskite quantum dots are crucial for their applications in various optoelectronic devices. Enhancing the luminescence performance of CsPb(Br/I)3 quantum dots is a critical challenge for their application in optoelectronic devices. This paper explores the use of noble metal nanostructures to improve the luminescence intensity of these quantum dots through localized surface plasmon resonance (LSPR). We conducted electric field simulations to optimize the LSPR effect and prepared Au@Ag@SiO2 nanobipyramids with tailored LSPR peaks. The nanobipyramids were then connected to CsPbCl2Br, CsPbBr3, and CsPbBrI2 quantum dots using amine-functionalized SiO2 layers, forming a nanocomposite that showed a significant increase in luminescence. The study identifies the importance of the SiO2 spacer layer in preventing luminescence quenching and demonstrates that the LSPR effect can be harnessed to achieve up to a 15-fold enhancement in luminescence intensity. This research provides a promising avenue for advancing the efficiency of solar energy harvesting devices, light-emitting devices, and optical detectors.

An ultra-broadband high-performance solar energy perfect absorber from deep ultraviolet to mid-infrared

Metamaterial absorbers possess potential applications in a large number of fields, including solar energy harvesting, photovoltaics, stealth technology and sensors, owing to their capability to achieve perfect absorption of electromagnetic waves. In this study, a solar metamaterial absorber comprising a TiN substrate, a SiO2 spacer layer and an array of patterned-layer TiN-SiO2 double circular employing the finite-difference time-domain (FDTD) method is proposed to achieve perfect absorption in ultra-broadband. Optimization results indicate that the broadband absorption spectrum of the absorber extends from deep ultraviolet to mid-infrared wavelengths with an average absorption of 98.1%. The distribution of electric and magnetic fields suggests that the absorber achieves high absorption due to the interaction between surface plasmon resonance (SPR), cavity resonance (CR) and magnetic resonance (MR). Furthermore, the proposed absorber is insensitive to large-angle incidence and polarization-independent. Moreover, the absorber can be produced at a low cost due to its tolerance of the geometrical errors. The newly proposed metamaterial absorber is expected to be applied in solar energy related fields.

Narrow-band and highly absorbing fano resonance in a cavity-coupled dielectric metasurface

Metamaterial resonance offers a flexibility in engineering the frequency and bandwidth of light absorption for a variety of optoelectronic applications such as wavelength-selective photodetection, optical sensing and infrared camouflaging etc. In this paper, we demonstrate a class of metal-dielectric thin-film cavity-coupled dielectric metasurfaces, which feature Fano resonances with both narrow bandwidth and strong light absorption. Our fabricated metasurface consists of a Si cuboid array on top of a SiO2 film backed with a metallic Cu layer. The weak coupling between electric mie mode in Si cuboid and Fabry–Perot mode within the SiO2 spacer layer yields a Fano resonance at 4.19 μm wavelength, which exhibits a strong light absorption of 65.8% and a quality (Q) factor of 112. The strongly absorbing Fano resonance is tunable within the 3–5 μm band by varying geometric parameters of the metasurface. To reveal potential application of the metasurface, the Fano resonance is applied in refractive index sensing and exhibits a sensitivity of 518.75 nm RIU−1 and a figure-of-merit (FoM) of 14.82 RIU−1. These results suggest that cavity-coupling presents an effective way in reducing the resonance bandwidth and enhancing light absorption in dielectric metamaterials, which holds promise for expanding the properties and device functionalities of metamaterials.

Flexible plasmonic nanoparticle-on-a-mirror metasurface-enabled substrates for high-quality surface-enhanced Raman spectroscopy detection

Surface-enhanced Raman scattering (SERS) technology with the advantages of ultra-high sensitivity, non-destructive analysis, and quick measurement for molecular detection applications is receiving increasing attention. However, traditional rigid SERS substrates face challenges in in-suit conformal detection and weak structure coupling effect for real-life applications. Here we report a flexible polydimethylsiloxane (PDMS) substrate loaded with plasmonic nanoparticle-on-a-mirror (NPOM) metasurface for SERS detection that featured outstanding sensitivity, uniformity, repeatability, and excellent mechanical flexibility. The upper multilayered NPOM metasurface can be fabricated in a single-step process by ion beam sputtering of various targets. This NPOM configuration consists of dense silver nanoparticles over a silver mirror, separated by a SiO2 spacer layer, which can realize near-total power absorption and exhibits superior SERS ability. The bottom PDMS layer for support can provide excellent mechanical properties. In the test, the as-prepared NPOM/PDMS substrates show high SERS performance in detecting crystal violet and chlorpyrifos molecules. This flexible metasurface SERS substrate promises to provide an in-suit and efficient approach for trace substance detection.

Functionality-switchable terahertz metamaterial with perfect absorption and circular dichroism

Terahertz functionality-switchable devices have drawn more and more attention in the past few years. Here we report a graphene-VO2 based planar terahertz metamaterial model which can support switchable functions between perfect absorption and circular dichroism (CD) by changing the phase of VO2. In particular, when VO2 is in the metal phase or insulator phase, the device can respectively work as a broadband perfect absorber with maximum absorption larger than 99% or an absorber with a CD value as high as about 0.5 for the left-handed polarized and right-handed polarized beams. We employ the coherent light absorption theory and mode coupling model of the U-shaped resonators to interpret the underlying mechanism of the device. Meanwhile, the performance of the device can be modulated on purpose by changing the chemical potential of graphene or the thickness of the SiO2 spacer layer.

Augmentation of surface plasmon-enhanced second harmonic generation from Au nanoprisms on SiO2/Si: interference contribution.

We present an augmentation of Surface Plasmon (SP)-enhanced second harmonic generation (SHG) due to interference field enhancement in Au nanoprisms (AuNPs) on SiO2-coated Si substrates. The SiO2 spacer contributed for the optical interference and increased the coupling efficiency of the pump light with the SP polarization as well as a decoupling efficiency of the SHG waves from nonlinear polarization. The intensity of the SP-enhanced SHG signals increased 4.5-fold with respect to the AuNPs on the bare SiO2 substrate by setting the SiO2 spacer layer to the appropriate thickness. The numerical analysis revealed that the optimal SHG conversion was determined by the balance between the degree of the optical interference at the fundamental and SHG wavelengths.

Dual-function terahertz metasurface based on vanadium dioxide and graphene

A dual-function terahertz metasurface based on VO2 and graphene is proposed in this paper. It consists of a gold layer embedded with VO2 patches, a SiO2 spacer layer, a VO2 layer, graphene and a SiO2 spacer substrate. When the bottom VO2 layer is in the metallic state, the designed metasurface can achieve absorption. When the top VO2 patches are in the metallic state, the proposed metasurface can be used as a single-band absorber with terahertz absorptance of 99.7% at 0.736 THz. When the top VO2 patches are in the insulating state, the designed structure behaves as a dual-band absorber with an absorptance of 98.9% at 0.894 THz and 99.9% at 1.408 THz. In addition, the absorber is polarization insensitive and keeps good performance at large angles of incidence. When the bottom VO2 is in an insulating state, the metasurface shows electromagnetically induced transparency. The transparent window can be dynamically regulated by controlling the chemical potential of graphene. The proposed metasurface exhibits the advantages of terahertz absorption, electromagnetically induced transparency and dynamic control, which provides more options for the design of terahertz devices in the future.

Spintronic emitters for super-resolution in THz-spectral imaging

We investigate local THz field generation using spintronic THz emitters to enhance the resolution for micrometer-sized imaging. Far-field imaging with wavelengths above 100 μm limits the resolution to this order of magnitude. By using optical laser pulses as a pump, THz field generation can be confined to the area of laser beam focusing. The divergence of the generated THz beam due to laser beam focusing requires the imaged object to be close to the generation spot at a distance below the THz field wavelength. We generate THz-radiation by fs-laser pulses in CoFeB/Pt heterostructures, based on spin currents, and detect them by commercial low-temperature grown-GaAs (LT-GaAs) Auston switches. The spatial resolution of THz radiation is determined by applying a 2D scanning technique with motorized stages allowing step sizes in the sub-micrometer range. Within the near-field limit, we achieve spatial resolution in the dimensions of the laser spot size on the micrometer scale. For this purpose, a gold test pattern is evaporated on the spintronic emitter separated by a 300 nm SiO2 spacer layer. Moving these structures with respect to the femtosecond laser spot, which generates THz radiation, allows for resolution determination. The knife-edge method yields a full-width half-maximum beam diameter of 4.9±0.4 μm at 1 THz. The possibility to deposit spintronic emitter heterostructures on simple glass substrates makes them attractive candidates for near-field imaging in many imaging applications.

New design strategy for broadband perfect absorber by coupling effects between metamaterial and epsilon-near-zero mode

We propose a novel and simple method to achieve perfect light absorption over a broadband by coupling a gap plasmon mode to an epsilon-near-zero (ENZ) mode, which is a new path for tailoring light-matter interactions. Near-perfect light absorption (over 99%) in the wavelength range of 1400 nm–1655 nm can be achieved utilizing a gold-disk periodically patterned metal-dielectric-metal (MDM) device by integrating an ultrathin ITO (indium tin oxide) film. The ENZ mode can be excited effectively by coupling the gap plasmon resonance to conducting ITO (indium tin oxide) nanolayer (ENZ wavelength of the ITO nanofilm close to 1403 nm) under normal incidence. Compared with the device without ITO nanolayer, the device integrating ITO ultrathin film shows wideband perfect light absorption. Besides, the thickness of SiO2 spacer layer is critical for the coupling strength of metamaterial resonator and ENZ mode. To achieve broadband absorption, the coupling strength of the ENZ mode and gap plasmon mode should be between the strong coupling and weak coupling regimes. The method is also effective for the design of multiband optoelectronic devices with simple structure and low cost.

Efficient monolithic dye sensitized solar cells with eco-friendly silica-titania spacer layers

Monolithic assembly of dye-sensitized solar cells (M−DSSCs) is technologically attractive due to its ease of fabrication, compactness and lower production cost. However, combining cost-effective materials with high power conversion efficiency in M−DSSCs is still a challenging task. New silica–based spacer layers for M−DSSCs, derived from entirely water-based paste formulation were developed. A commercial aqueous suspension of SiO2 nanoparticles (LUDOX-40), modified with hydroxypropyl cellulose as a binder, were used for formulating the silica paste. The reflectance of the spacer layers was tuned by adding anatase nanoparticles; when 30 wt% of titania was added to the SiO2-TiO2 composite, a sharp increase of the reflectivity of the spacer layers was observed rendering a device with the highest to date reported power conversion efficiency of 8.3% and full stability. The effect of titania concentration in the SiO2 spacer layer on the photovoltaic performance, the spectral response of the photocurrent and recombination in M−DSSCs is discussed.

AuAg Bimetallic Non-Alloyed Nanoparticles on SiO₂ Spacer Layer for Improved Light Absorption in Thin-Film c-Si Solar Cells.

We present a light trapping structure consisting of gold and silver (AuAg) bimetallic non-alloyed nanoparticles (BNNPs) on a silicon dioxide (SiO2) spacer layer over crystalline silicon (c-Si) film, designed to improve the absorption of thin-film c-Si solar cells. Prior to fabrication of the AuAg BNNPs on the SiO2 spacer layer, numerical investigations were carried out using electromagnetic field simulation following the finite-difference time-domain method. The hemispherical Au8Ag8 BNNPs were fabricated and deposited on a 15 nm-thick SiO2 spacer layer, which enhanced light trapping in the c-Si film over a broad wavelength range (450-1100 nm). Specifically, more than 85% of the incident light was absorbed in the c-Si film at 620 nm wavelengths due to the strong scattering of the Au8Ag8 BNNPs. To the best of our knowledge this is the first case presenting such a theoretical calculation and experimental study of the efficient light trapping by AuAg BNNPs on space layer for increasing the absorption in thin-film c-Si solar cells.

Line shape engineering of sharp Fano resonance in Al-based metal-dielectric multilayer structure

A systematic experimental study was performed on the Fano line shape exhibited by multilayer structures consisting of an Al layer, a SiO2 spacer layer, and an Al2O3 waveguide layer. In the structures studied, a sharp Fano resonance appears on the background of broad asymmetric resonance attributed to the excitation of a surface plasmon polariton at the Al/SiO2 interface. It is shown that the background asymmetric surface plasmon resonance can be well fitted to a single Fano function, and the sharp Fano line shape can be well fitted to a double Fano function expressed as a product of two single Fano functions. The results of measurements performed by varying the spacer layer thickness indicate that the width (Q factor) of the sharp Fano resonance decreases (increases) monotonously as the thickness increases. The Q factor achieved in the present study is as high as ∼1500. A comparison with the results of electromagnetic calculations suggests that not only the spacer layer thickness but also the imaginary part of the dielectric constant of the waveguide layer plays an important role in the Fano line shape engineering.

Tailoring the nickel nanoparticles anchored on the surface of Fe3O4@SiO2 spheres for nanocatalysis

Herein, we report an efficient and universal strategy for synthesizing a unique triple-shell structured Fe3O4@SiO2@C-Ni hybrid composite. Firstly, the Fe3O4 cores were synthesized by hydrothermal reaction, and sequentially coated with SiO2 and a thin layer of nickel-ion-doped resin-formaldehyde (RF-Ni2+) using an extended Stöber method. This was followed by carbonization to produce the Fe3O4@SiO2@C-Ni nanocomposites with metallic nickel nanoparticles embedded in an RF-derived thin graphic carbon layer. Interestingly, the thin SiO2 spacer layer between RF-Ni2+ and Fe3O4 plays a critical role on adjusting the size and density of the nickel nanoparticles on the surface of Fe3O4@SiO2 nanospheres. The detailed tailoring mechanism is explicitly discussed, and it is shown that the iron oxide core can react with the nickel nanoparticles without the SiO2 spacer layer, and the size and density of the nickel nanoparticles can be effectively controlled when the SiO2 layer exits. The multifunctional composites exhibit a significantly enhanced catalytic performance in the reduction of 4-nitrophenol (4-NP).

Visible Wavelength Color Filters Using Dielectric Subwavelength Gratings for Backside-Illuminated CMOS Image Sensor Technologies.

We report transmissive color filters based on subwavelength dielectric gratings that can replace conventional dye-based color filters used in backside-illuminated CMOS image sensor (BSI CIS) technologies. The filters are patterned in an 80 nm-thick poly silicon film on a 115 nm-thick SiO2 spacer layer. They are optimized for operating at the primary RGB colors, exhibit peak transmittance of 60-80%, and have an almost insensitive response over a ± 20° angular range. This technology enables shrinking of the pixel sizes down to near a micrometer.

Influence of stress on the properties of Ge nanocrystals in an SiO2 matrix

In this work, self-assembled Ge quantum dot (QD) formation in a dielectric matrix is explored. Of particular interest were their structural and optical properties, in order to understand the stress build-up in such a process and its impact on the material properties during processing. To this end, thin films consisting of (Ge + SiO2)/SiO2 multilayers grown by RF magnetron sputtering were deposited at room temperature. Annealing of such films at 873 K in inert N2 atmosphere produced, at the position of the Ge-rich SiO2 layers, a high lateral density (about 1012 cm−2) of Ge QDs with a good crystallinity. SiO2 spacer layers separated the adjacent Ge-rich layers, where the Ge QDs were formed with a diameter of about the size of the (Ge + SiO2) as-deposited layer thickness, and created a good vertical repeatability, confirmed by the appearance of a Bragg sheet in two-dimensional small-angle X-ray scattering patterns. The structural analysis, by wide-angle X-ray diffraction, grazing-incidence small-angle X-ray scattering and transmission electron microscopy, has shown that the described processing of the films induced large compressive stress on the formed QDs. Optical analysis by time-resolved photoluminescence (PL) revealed that the high density of crystalline Ge QDs embedded in the amorphous SiO2 matrix produced a strong luminescence in the visible part of the spectrum at 2–2.5 eV photon energy. It is shown that the decay dynamics in this energy range are very fast, and therefore the transitions that create such PL are attributed to matrix defects present in the shell surrounding the Ge QD surface (interface region with the matrix). The measured PL peak, though wide at its half-width, when analysed in consecutive short spectral segments showed the same decay dynamics, suggesting the same mechanism of relaxation.

Influence of SiO2 Spacer Layer Thickness on Performance of Plasmonic Textured Silicon Solar Cell

We investigated the effect of SiO2 spacer layer thickness between the textured silicon surface and silver nanoparticles (Ag NPs) on solar cell performance using quantum efficiency analysis. Separation of Ag NPs from high index silicon with SiO2 layer led to modified absorption and scattering cross-sections due to graded refractive index medium. The forward scattering from Ag NPs is very sensitive to SiO2 layer thickness in plasmonic silicon cell performance due to the evanescent character of generated near-fields around the NPs. With the optimized ∼30–40 nm SiO2 spacer layer, we observed an enhancement of solar cell efficiency from ∼8.7 to ∼10 %, which is due to the photocurrent enhancement in the off-resonance surface plasmon region. We also estimated minority carrier diffusion lengths (Leff) from internal quantum efficiency data, which are also sensitive to SiO2 spacer layer thickness. We observed that the Leff values are enhanced from ∼356 to ∼420 μm after placing Ag NPs on ∼40 nm spacer layer due to improved forward (angular) scattering of light from the Ag NPs into silicon.

Strong Coupling of Plasmon and Nanocavity Modes for Dual-Band, Near-Perfect Absorbers and Ultrathin Photovoltaics

When optical resonances interact strongly, hybridized modes are formed with mixed properties inherited from the basic modes. Strong coupling therefore tends to equalize properties such as damping and oscillator strength of the spectrally separate resonance modes. This effect is here shown to be very useful for the realization of near-perfect dual-band absorption with ultrathin (∼10 nm) layers in a simple geometry. Absorber layers are constructed by atomic layer deposition of the heavy-damping semiconductor tin monosulfide (SnS) onto a two-dimensional gold nanodot array. In combination with a thin (55 nm) SiO2 spacer layer and a highly reflective Al film on the back, a semiopen nanocavity is formed. The SnS-coated array supports a localized surface plasmon resonance in the vicinity of the lowest order antisymmetric Fabry–Perot resonance of the nanocavity. Very strong coupling of the two resonances is evident through anticrossing behavior with a minimum peak splitting of 400 meV, amounting to 24% of the pla...

Formation of Fe3O4@SiO2@C/Ni hybrids with enhanced catalytic activity and histidine-rich protein separation.

In this paper, we have developed an extended Stöber method to construct a Ni(2+)-polydopamine (PDA) complex thin coating on Fe3O4@SiO2 spheres, which can be carbonized to produce hybrid composites with metallic nickel nanoparticles embedded in a PDA-derived thin graphitic carbon layer (named Fe3O4@SiO2@C/Ni). Interestingly, by introducing a thin SiO2 spacer layer between PDA-Ni(2+) and Fe3O4, the reverse electron transfer from PDA to Fe3O4 is probably able to be suppressed in the calcination process, which leads to the in situ reduction of only Ni(2+) by PDA instead of Fe3O4 and Ni(2+). Consequently, the size and density of nickel nanoparticles on the surface of SiO2@Fe3O4 can be finely adjusted. Moreover, it is found that the ability of tuning nickel nanoparticles is mainly dependent on the thickness of the spacer layer. When the thickness of the SiO2 spacer is beyond the electron penetration depth, the size and density of nickel nanoparticles can be exactly tuned. The as-prepared Fe3O4@SiO2@C/Ni was employed as the catalyst to investigate the catalytic performance in the reduction of 4-nitrophenol (4-NP); furthermore, nickel nanoparticles decorated on Fe3O4@SiO2@C spheres display a strong affinity to His-tagged proteins (BHb and BSA) via a specific metal affinity force between polyhistidine groups and nickel nanoparticles.

Size confinement of Si nanocrystals in multinanolayer structures.

Si nanocrystals (NCs) are often prepared by thermal annealing of multiple stacks of alternating sub-stoichiometric SiOx and SiO2 nanolayers. It is frequently claimed that in these structures, the NC diameter can be predefined by the thickness of the SiOx layer, while the NC concentration is independently controlled by the stoichiometry parameter x. However, several detailed structural investigations report that the NC size confinement to within the thickness of the SiOx layer is not strictly obeyed. In this study we address these contradicting findings: based on cross-correlation between structural and optical characterization of NCs grown in a series of purposefully prepared samples of different stoichiometry and layer thickness, we develop a comprehensive understanding of NC formation by Si precipitation in multinanolayer structures. We argue that the narrow NC size distribution generally observed in these materials appears due to reduction of the Si diffusion range, imposed by the SiO2 spacer layer. Therefore, both the SiOx layer thickness and composition as well as the actual thickness of the SiO2 spacer play an essential role in the NC formation.