Silicon Nanodisk Arrays Research Articles

Soft and hard trimming of imprint resist masks to fabricate silicon nanodisk arrays with different edge roughness

Soft and hard trimming of imprint resist masks to fabricate silicon nanodisk arrays with different edge roughness

Ultra-narrow dual-band perfect absorber based on double-slotted silicon nanodisk arrays

Due to its unique advantage of optical properties, nanophotonic metamaterials have gained extensive applications in perfect absorbers. However, achieving both dual-band and ultra-narrow linewidth in absorbers simultaneously remains a challenge for typical metal-dielectric based metamaterials. In this work, a dual-band ultra-narrow perfect absorber consisting of a double slotted silicon nanodisk array that located on a silver film with a silica spacer layer is proposed theoretically. By combining the hybrid mode excited by the coupling of diffraction wave mode and magnetic dipole mode with the anapole–anapole interaction, two absorption peaks can be induced in the near-infrared regime, achieving nearly perfect absorbance of 99.31% and 99.61%, with ultra-narrow linewidths of 1.92 nm and 1.25 nm respectively. In addition, the dual-band absorption characteristics can be regulated by changing the structural parameters of the as-proposed metamaterials. The as-designed metamaterials can be employed as efficient two-channel refractive index sensors, with sensitivity and figure of merit (FOM) of 288 nm RIU−1 and 150 RIU−1 for the first band, and sensitivity and FOM of up to 204 nm RIU−1 and 163.2 RIU−1 for the second band. This work not only opens up a new design idea for the realization of dual-band perfect absorber synchronously with ultra-narrow linewidth, but also provides potential attractive candidates for developing dual-frequency channel sensors.

Mechanism of formation and evolution of bound states in the continuum of split-hole all-dielectric metasurface under asymmetric displacement perturbation

Bound states in the continuum (BICs) with ultra-high Q properties have attracted much attention for their perfect localization in the continuous spectral range coexisting with extended waves. In this study, breaking the traditional excitation form of structure breakage or excitation field asymmetry, a monolithic silicon nanodisk array with relative displacement generated by the complete splitting of square nanopores is proposed based on the unique electromagnetic properties of all-dielectric metamaterials. During the introduction of perturbations by asymmetric displacements of splitting holes, it is shown by numerical simulations that two BICs at different wavelengths can be realized. Combined with eigenmodes of group theory, the symmetric matching relationship between the symmetry-protected BICs and the free-space radiation during the evolution process is analytically demonstrated, and the formation mechanism and the evolution law of the BICs excited by this metasurface are deeply investigated. meanwhile, it also provides a theoretical basis for the polarization dependence of quasi-BICs excitation and the ultra-high Q factor expression of BICs. Furthermore, near-field distribution and multipole decomposition show that the field distribution and surface currents support the excitation of BIC-driven toroidal dipole and magnetic quadrupole dual modes. This study not only provides an effective reference for the stability of high-Q resonance wavelengths, but also solves the problem of the lack of universality in analyzing the resonance mechanism based on resonance phenomena, and provides solid theoretical support for the study of displacement-mediated BICs resonance excitation and evolution.

Observation of tunable accidental bound state in the continuum in silicon nanodisk array

Abstract We experimentally demonstrate the tuning of accidental bound states in the continuum (A-BICs) in silicon nanodisk arrays. The A-BIC emerges of the destructive interference of multipoles, which are the dominating out-of-plane electric dipole and in-plane magnetic dipole, and weak electric quadrupole and magnetic quadrupole. We further show that the spectral and angular position of the A-BIC can be conveniently tuned by varying the nanodisk size or the lattice period. Remarkably, the angular position can be tuned even to 0°, suggesting an interesting transition of the A-BIC from an off-Γ-BIC to an at-Γ-BIC. Our work provides a new strategy for light trapping with high quality factors, and the obtained tunable A-BICs can find potential applications in low-threshold lasing, enhanced nonlinear optics, and optical sensing.

The anapole state excited by an oblique incidence

Anapole states supported by high-refractive-index dielectric nanoparticles have mostly been studied under normal incidence, but this work explores the oblique incidence excitation. For a single silicon nanodisk, as the incident angle (θ) increases, the anapole wavelength undergoes a gradual blueshift, while the wavelength of maximum near-field enhancement remains almost unchanged with increasing E-field enhancement factor (|E/E 0 |) due to phase retardation effect caused by oblique incidence, and some unique features in field distributions differed from normal excitation are exhibited. In the case of a silicon nanodisk array, the anapole state and near-field enhancement are affected by near-field coupling and the phase retardation effect is weakened. With increasing θ, the coupling between the units is enhanced, and the anapole wavelength and maximum field enhancement wavelength both blue shift. The field distributions in anapole wavelength and maximum enhancement wavelength have obvious near-field coupling characteristics. Oblique incident excitation gives us a deeper understanding of anapole state and may have potential applications in nanophotonics.

Anapole-assisted ultra-narrow-band lattice resonance in slotted silicon nanodisk arrays

Anapole modes supported by well-designed dielectric nanostructures have attracted extensive attention in the field of nanophotonic applications owing to their unique strong near-field enhancement and non-radiative far-field scattering characteristics, yet it is still difficult to achieve high Q-factor resonance features with a narrow linewidth. In this work, a periodic slotted silicon nanodisk array is theoretically proposed to realize narrow linewidth and high Q-factor resonance in the near-infrared wavelength range. Through introducing the coupling between the anapole modes in the single dielectric nanostructure and the diffractive wave mode arising from the periodic array, the as-designed dielectric nanostructure synchronously manifests excellent spectral features with a bandwidth as narrow as about 2.0 nm, a large Q-factor of 599, an almost-perfect transmission amplitude of 96% and a relatively high electric field intensity (>2809 times) in the middle of the slotted silicon nanodisk. The as-designed nanostructure possessing these outstanding optical features can work as a high-efficiency refractive index sensor, whose sensitivity can reach 161.5 nm RIU−1 with its figure of merit attaining 80.8 RIU−1, efficiently distinguishing an index change of less than 0.01. The proposed slotted silicon nanodisk array exhibits tremendous potential for expanding applications such as label-free biochemical sensing, plasmonic refractive index sensing and surface enhancement spectroscopy.

Bound states in the continuum (BIC) in silicon nanodisk array on mirror structure: Perfect absorption associated with quasi-BIC below the bandgap

A structure composed of a hexagonal array of Si nanodisks having toroidal dipole resonances and a reflecting mirror separated by a SiO2 spacer is proposed as a platform that exhibits narrow-band perfect absorption in the Si sub-bandgap wavelength range for a CMOS compatible Si based photodetector operating below the bandgap range. The numerical simulation reveals that the structure possesses Fabry–Pérot bound states in the continuum at proper spacer thicknesses due to the interference between the toroidal dipole and its image dipole. By slightly detuning the spacer thickness to meet the critical coupling condition, narrow-band perfect absorption appears despite assumption of a very small extinction coefficient (5 × 10−4). The wavelength of the perfect absorption is controlled in a wide range by the structural parameters of a Si nanodisk hexagonal array and is insensitive to the fluctuation of the extinction coefficient and the choice of a metallic mirror. In the structure, over 90% of incident power can be absorbed in the Si region. This suggests that the structure can be used as a narrow-band photodetector operating in the Si sub-bandgap wavelength range. We also evaluate the sensing performance of the proposed structure as an intensity based refractive index sensor operating in the near-infrared range.

All-dielectric chiral-field-enhanced Raman optical activity

Raman optical activity (ROA) is effective for studying the conformational structure and behavior of chiral molecules in aqueous solutions and is advantageous over X-ray crystallography and nuclear magnetic resonance spectroscopy in sample preparation and cost performance. However, ROA signals are inherently minuscule; 3–5 orders of magnitude weaker than spontaneous Raman scattering due to the weak chiral light–matter interaction. Localized surface plasmon resonance on metallic nanoparticles has been employed to enhance ROA signals, but suffers from detrimental spectral artifacts due to its photothermal heat generation and inability to efficiently transfer and enhance optical chirality from the far field to the near field. Here we demonstrate all-dielectric chiral-field-enhanced ROA by devising a silicon nanodisk array and exploiting its dark mode to overcome these limitations. Specifically, we use it with pairs of chemical and biological enantiomers to show >100x enhanced chiral light–molecule interaction with negligible artifacts for ROA measurements.

Double-layer metasurface for enhanced photon up-conversion

We present a double-layer dielectric metasurface obtained by stacking a silicon nanodisk array and a silicon photonic crystal slab with equal periodicity on top of each other. We focus on the investigation of electric near-field enhancement effects occurring at resonant excitation of the metasurface and study its optical properties numerically and experimentally. We find that the major difference in multi-layer metasurfaces when compared to conventional single-layer structures appears to be in Rayleigh–Wood anomalies: they are split into multiple different modes, which are themselves spectrally broadened. As a proof of concept, we cover a double-layer metasurface with a lanthanide-doped up-conversion particle layer and study its interaction with a 1550 nm photoexcitation. We observe a 2.7-fold enhanced up-conversion photoluminescence by using the stacked metasurface instead of a planar substrate, although only around 1% of the up-conversion material is exposed to enhanced near fields. Two mechanisms are identified explaining this behavior: First, enhanced near fields when exciting the metasurface resonantly, and second, light trapping by total internal reflection in the particle layer when the metasurface redirects light into high angle diffraction orders. These results pave the way for low-threshold and, in particular, broadband photon up-conversion in future solar energy and biosensing applications.

Fluorescence-Detected Circular Dichroism of a Chiral Molecular Monolayer with Dielectric Metasurfaces.

Strong enhancement of molecular circular dichroism (CD) has the potential to enable efficient asymmetric photolysis, a method of chiral separation that has conventionally been impeded by insufficient yield and low enantiomeric excess. Here, we study experimentally how predicted enhancements in optical chirality density near resonant silicon nanodisks boost CD. We use fluorescence-detected circular dichroism (FDCD) spectroscopy to measure indirectly the differential absorption of circularly polarized light by a monolayer of optically active molecules functionalized to silicon nanodisk arrays. Importantly, the molecules and nanodisk antennas have spectrally coincident resonances, and our fluorescence technique allows us to deconvolute absorption in the nanodisks from the molecules. We find that enhanced FDCD signals depend on nanophotonic resonances, in good agreement with simulated differential absorption and optical chirality density, while no signal is detected from molecules adsorbed on featureless silicon surfaces. These results verify the potential of nanophotonic platforms to be used for asymmetric photolysis with lower energy requirements.

Slow light using magnetic and electric Mie resonances.

The ability to slow down light leads to strong light-matter interaction, which is important for a number of optical applications such as sensing, nonlinear optics, and optical pulse manipulation. Here, we show that a dramatic reduction in the speed of light can be realized through the interference of electric and magnetic dipole resonances in Mie-type resonators made of a dielectric material with a high refractive index. We present a general theory that links the maximal speed reduction of light to resonator radiation losses and then consider a specific realization based on silicon nanodisk arrays.

Probing optical resonances of silicon nanostructures using tunable-excitation Raman spectroscopy.

Optical materials with a high refractive index enable effective manipulation of light at the nanoscale through strong light confinement. However, the optical near field, which is mainly confined inside such high-index nanostructures, is difficult to probe with existing measurement techniques. Here, we exploit the connection between Raman scattering and the stored electric energy to detect resonance-induced near-field enhancements in silicon nanostructures. We introduce a Raman setup with a wavelength-tunable laser, which allows us to tune the Raman excitation wavelength and thereby identify Fabry-Pérot and Mie type resonances in silicon thin films and nanodisk arrays, respectively. We measure the optical near-field enhancement by comparing the Raman response on and off resonance. Our results show that tunable-excitation Raman spectroscopy can be used as a complimentary far-field technique to reflection measurements for nanoscale characterization and quality control. As proof-of-principle for the latter, we demonstrate that Raman spectroscopy captures fabrication imperfections in the silicon nanodisk arrays, enabling an all-optical quality control of metasurfaces.

Optical properties and fabrication of dielectric metasurfaces based on amorphous silicon nanodisk arrays.

Dielectric metasurfaces based on amorphous silicon (a-Si) nanodisks are interesting for nanophotonic applications due to the high refractive index and mature/low temperature fabrication of a-Si. The investigated metasurfaces consist of a-Si nanodisk arrays embedded in a transparent film. The diameter-dependent optical properties of the nanodisk Mie resonators have been investigated by finite-difference time-domain (FDTD) simulations and spectrally-resolved reflectivity and transmission measurements. Well-ordered substrate-free a-Si nanodisk arrays were fabricated and characterized with regard to their broadband anti-reflection properties when placed on a crystalline silicon (c-Si) surface, and reflectivity/ transmission properties when embedded in a polydimethylsiloxane (PDMS) film. Our results confirm broadband anti-reflection when placed on silicon, while the optical characteristics of the nanodisks embedded in PDMS are shown to be potentially useful for color/NIR filter applications as well as for coloring on the micro/nanoscale.

Microscopic study of resonant third-harmonic generation from amorphous silicon nanodisk arrays.

A detailed microscopic study of third-harmonic generation (THG) from two-dimensional arrays of sub-wavelength spaced amorphous silicon nanodisks is reported. The arrays are designed to support broadband, minimally angle-sensitive resonances for the fundamental excitation wavelength in the 1500nm region. This results in resonantly enhanced visible THG in the green spectral range with ∼500-fold enhancement on-resonance, compared to the unpatterned a-Si thin-film. THG multispectral microscopic imaging reveals individual nanodisks with enhanced nonlinear signal on-resonance. For increasing pump intensities, spatially dependent saturation effects are observed for the first time, to the best of our knowledge, in such dielectric nanostructure arrays with THG images showing a reversal of contrast.

Design of highly transmissive all-dielectric metasurface based on silicon nanodisks

Using the developed discrete dipole approximation method, we investigate the behavior of silicon nanodisks acting as Huygens sources. First, we study the correlation between the radius of a silicon nanodisk and its electric resonance and also between the height of a nanodisk and its magnetic resonance and find a significant correlation between them. Then, we investigate the transmission of an array of silicon nanodisks. Using the overlap between the electric and magnetic resonances, we obtain a nearly perfect impedance match between the environment and silicon nanodisk array. We achieve a transmission above 99% over the entire phase range of 0–2π radians at the wavelength of 1190 nm. Our results can be extended to microwave, terahertz and optical frequencies and be used to design metasurfaces for different applications such as beam shaping lenses, beam deflectors and holograms.

Silicon nanodisk array with a fin field-effect transistor for time-domain weighted sum calculation toward massively parallel spiking neural networks

A nanodisk array connected with a fin field-effect transistor is fabricated and analyzed for spiking neural network applications. This nanodevice performs weighted sums in the time domain using rising slopes of responses triggered by input spike pulses. The nanodisk arrays, which act as a resistance of several giga-ohms, are fabricated using a self-assembly bio-nano-template technique. Weighted sums are achieved with an energy dissipation on the order of 1 fJ, where the number of inputs can be more than one hundred. This amount of energy is several orders of magnitude lower than that of conventional digital processors.

Fano resonance in all-dielectric binary nanodisk array realizing optical filter with efficient linewidth tuning.

In this research paper, we study the Fano resonance originating from the interaction of in-phased lattice collective resonance and anti-phased lattice collective resonance supported by a binary silicon nanodisk array. Experimental results agree well with the calculations using finite-difference-time-domain method and show a strong dependence of such Fano lineshapes on the radius difference of the particles in the array. Further calculations demonstrate that such binary silicon nanodisk array can be used as an optical filter and offers an efficient way to tune the linewidth simply by changing the radius of the particles, linewidth from 12 nm to 0.7 nm and corresponding Q factor from 72 to 1290 as the radius R(2) increasing from 60 nm to 115 nm. Such scheme possessing the merits of being easily fabricated, simulated, and tuned is very promising for practical applications.

Multicolor generation using silicon nanodisk absorber

A multicolored matrix that spans the visible range was demonstrated by using silicon nanodisk arrays. A nanostructured silicon substrate, which featured periodic silicon nanodisk arrays of various diameters, inter-nanodisk distances, and heights, was fabricated using electron-beam lithography and reactive ion etching. These silicon nanodisks were able to support HE1m leaky modes, which depended on the diameter of the nanodisks, resulting in wavelength-dependent reflection spectra. The resonant wavelength redshifted linearly with the increasing nanodisk diameter. The output color lay in the visible range and was observed to be tunable when varying the diameter, interdistance, and height. The results of finite-difference time-domain simulations exhibited close agreement with the observed optical properties of the periodic silicon nanodisk arrays.

Modeling miniband for realistic silicon nanocrystal array

Within the envelop-function framework, we developed the finite element method to calculate the minibands in the realist Silicon nanocrystal array. This method clearly reveals the miniband formation and accurately calculates the E–K dispersion relationship. In the simple 1D array, the deduced miniband structure matches well with the analytic Kronig–Penney method. More importantly, it can better simulate the 2D and 3D nanocrystal array, which avoids approximations of the quantum cubic box and the independent periodic potential of the multi-dimension Kronig–Penney method. Further, this model is utilized to calculate miniband structure of realistic 2D-array Silicon nanodisk array for guiding quantum dot solar cell design.