High Index Silicon Research Articles

All-dielectric transmissive narrow-band filters adjustable within wide spectral range

The challenge of obtaining transmission spectra with both a wider tunable spectral range and high resolution persists. In this paper, a novel all-dielectric transmissive grating structure based on Mie resonance is proposed. The structure excites the Mie resonance by embedding a high refractive index contrast grating in a KF-Si film system band-stop filter. The dipole resonance energy is confined within the high refractive index silicon grating material, expanding the cutoff bandwidth of the KF-Si film system to over 400 nm and achieving single-peak transmission in the visible range. And the continuous transmission spectrum has an adjustable range of 300 nm with the highest spectral resolution (FWHM) (∼20 nm) can be achieved by adjusting the grating period. To suppress the coupling energy excitation between the higher-order magnetic dipole and the substrate, a silicon-rich nitride (SRN) matching layer is introduced between the KF-Si film system and the substrate. This layer enhances the intensity of the resonance peaks while simultaneously suppressing the short-wavelength sidebands, thereby improving the saturation and purity of the spectrum. In addition, we obtained a large angular tolerance of up to 15° by stacking the silicon film system. This work is of great significance for the advancement of hyperspectral imaging and display technology.

Dual-hollow-core anti-resonant fiber with silicon layers and nested resonant tubes for constructing a single-polarization 3 dB coupler.

A novel dual-hollow-core anti-resonant fiber (DHC-ARF) with silicon layers and nested resonant tubes is proposed for constructing a 3 dB single-polarization coupler. High-index silicon layers are deposited inside four cladding tubes along the x-direction and then nested resonant tubes are introduced into these cladding tubes, which is beneficial to promote the mode coupling between the x-polarized fundamental mode and the dielectric mode in the silica/silicon tubes so as to achieve single-polarization characteristics of the DHC-ARF. The properties of the DHC-ARF are optimized using finite element method combined with perfectly matched layer as a boundary condition. By using a 1.51 cm long DHC-ARF, a single-polarization 3 dB coupler is constructed with a relatively wide bandwidth of 29 nm covering the wavelength range from 1530 nm to 1559 nm, in which the polarization extinction ratio (PER) is lower than -20 dB and the coupling ratio can be guaranteed within 50 ± 2%. This bandwidth is more than 2.6 times higher than the bandwidth of currently reported single-polarization coupler. At the wavelength of 1550 nm, the lowest PER reaches -74.8 dB and the loss of the coupler is as low as 0.33 dB.

Bound states in the continuum in circular waveguides: toward the on-chip integration of nanofiber on silicon platform.

In previously reported researches on bound state in the continuum (BIC) waveguides, almost all of them are demonstrated with top-down fabrication procedures, leading to inconvenience for post-manipulation and size tuning. Nanofibers with circular cross sections are the fundamental components to transport energy due to their intrinsic advantages of high flexibility and adjustability, which is replaceable and can be readily manipulated over size and position on the substrate. In this work, we explore the possibility of achieving on-chip integration of silica nanofiber onto a silicon-on-insulator platform. By constructing additional leakage channels in coupled nanofiber waveguides, coherently destructive interferences are successfully achieved. The heavy leakage losses from the low-index nanofiber to a high-index silicon substrate are completely eliminated with BIC, and the propagation length of the nanofiber waveguide is significantly improved.

High-performance SiO2-SiNx distributed Bragg reflectors fabricated by ion-assisted reactive magnetron sputtering

With the rapid improvement of photonics, the demand for high-performance distributed Bragg reflectors (DBRs) is increasing. In this study, high-performance DBRs stacked by silica and high refractive index silicon nitride layers were designed and fabricated, which were deposited by ion-assisted reactive magnetron sputtering. TEM, EDX, and XPS were utilized to investigate the crystal properties, elemental composition, and bonding properties of the films. Additionally, AFM, SEM, and Raman spectra were used to analyze the morphology, roughness, and stress of DBRs. The result shows that the films prepared by ion-assisted reactive magnetron sputtering are mainly amorphous. DBRs have extremely smooth surfaces with a root mean square roughness (Rq) of approximately 0.5 nm. No obvious defects are found in their surface and cross-section. And the residual stresses of DBRs are relatively low. The reflectance results show that the maximum reflectance of 6-pair DBR on the silicon substrate is higher than 99.9%. The stopband bandwidth covers approximately 800–1150 nm. In this work, DBRs deposited by ion-assisted reactive magnetron sputtering feature high deposition quality, thinness, and broadband high reflection, which have great application potential in high-performance optical devices.

Fano Resonances in Metal Gratings with Sub-Wavelength Slits on High Refractive Index Silicon.

The enhancement of optical waves through perforated plates has received particular attention over the past two decades. This phenomenon can occur due to two distinct and independent mechanisms, namely, nanoscale enhanced optical transmission and micron-scale Fabry-Perot resonance. The aim of the present paper is to shed light on the coupling potential between two neighboring slots filled with two different materials with contrasting physical properties (air and silicon, for example). Using theoretical predictions and numerical simulations, we highlight the role of each constituent material; the low-index material (air) acts as a continuum, while the higher-index material (silicon) exhibits discrete states. This combination gives rise to the so-called Fano resonance, well known since the early 1960s. In particular, it has been demonstrated that optimized geometrical parameters can create sustainable and robust band gaps at will, which provides the scientific community with a further genuine alternative to control optical waves.

High-Quality Amorphous Silicon Carbide for Hybrid Photonic Integration Deposited at a Low Temperature.

Integrated photonic platforms have proliferated in recent years, each demonstrating its unique strengths and shortcomings. Given the processing incompatibilities of different platforms, a formidable challenge in the field of integrated photonics still remains for combining the strengths of different optical materials in one hybrid integrated platform. Silicon carbide is a material of great interest because of its high refractive index, strong second- and third-order nonlinearities, and broad transparency window in the visible and near-infrared range. However, integrating silicon carbide (SiC) has been difficult, and current approaches rely on transfer bonding techniques that are time-consuming, expensive, and lacking precision in layer thickness. Here, we demonstrate high-index amorphous silicon carbide (a-SiC) films deposited at 150 °C and verify the high performance of the platform by fabricating standard photonic waveguides and ring resonators. The intrinsic quality factors of single-mode ring resonators were in the range of Qint = (4.7-5.7) × 105 corresponding to optical losses between 0.78 and 1.06 dB/cm. We then demonstrate the potential of this platform for future heterogeneous integration with ultralow-loss thin SiN and LiNbO3 platforms.

Inverse design of a Si-based high-performance vertical-emitting meta-grating coupler on a 220 nm silicon-on-insulator platform

Efficient extraction of light from a high refractive index silicon waveguide out of a chip is difficult to achieve. An inverse design approach was employed using the particle swarm optimization method to attain a vertical emitting meta-grating coupler with high coupling efficiency in a 220-nm-thick silicon-on-insulator platform. By carefully selecting the figure of merit and appropriately defining parameter space, unique L-shape and U-shape grating elements that boosted the out-of-plane radiation of light were obtained. In addition, a 65.7% (−1.82 dB) outcoupling efficiency and a 60.2% (−2.2 dB) fiber-to-chip vertical coupling efficiency with an 88 nm 3 dB bandwidth were demonstrated by numerical simulation. Considering fabrication constraints, the optimized complex meta-grating coupler was modified to correspond to two etching steps and was then fabricated with a complementary metal-oxide-semiconductor-compatible process. The modified meta-grating coupler exhibited a simulated coupling efficiency of 57.5% (−2.4 dB) with a 74 nm 3-dB bandwidth in the C-band and an experimentally measured coupling efficiency of 38% (−4.2 dB).

Generation of Parabolic pulse by nonlinear pulse reshaping inside a Silicon on Insulator (SOI) Waveguide

In recent times, silicon photonics attracts a lot of attraction of researchers. It is the technology that converts many functions of the circuit on the thumb-size chip. Nowadays, various types of technology platforms are being used to design photonic integrated circuits using various materials such as high index glass, semiconductors, polymers, and silicon. In our work, we have used silicon to design rectangular silicon on Insulator (SOI) buried waveguide. This type of waveguide has shown great potential in the field of pulse reshaping. We have used the effective index method for the calculation of group velocity dispersion and nonlinearity. Though the two-photon absorption and free-carrier generation contribute significantly to loss parameters, the highly nonlinear buried waveguide is found to be capable of reshaping super-Gaussian pulse input into a parabolic shape. Moreover, the values of input chirp, pulse width, and peak power are further optimized for the generation of a high-quality parabolic pulse at shorter lengths. The length required for pulse reshaping is much less when compared to optical fibers. Thus, our design waveguide has potential in the domain of pulse generation, signal processing, and many more.

Anisotropic slot waveguides with bulk transition metal dichalcogenides for crosstalk reduction and high-efficiency mode conversion.

Conventional slot waveguides (CSWs) consisting of an isotropic low-index material sandwiched by two high-index silicon wires have been extensively used in functional photonic devices, including chemical sensing, optical modulating, and all-optical signal processing, due to its significantly enhanced electric field perpendicular to the interfaces in the slot layer. However, there are two drawbacks to be improved if the CSWs are used for signal transmission in photonic integrated circuits, including the crosstalk between waveguides and direct butting mode conversion efficiency (MCE) to a silicon (Si)-strip waveguide. In this study, we propose an anisotropic SW with bulk transition metal dichalcogenide (ASWTMD) to relieve the two shortcomings by replacing the isotropic low-index slot layer with a bulk molybdenum disulfide layer having a high refractive index and giant optical anisotropy. We demonstrated the crosstalk reduction (CR) of the proposed ASWTMD by analyzing the mode profile, power confinement, and coupling strength. We also investigated the MCE by examining the mode overlap ratio and power evolution. The proposed ASWTMD shows significant CR and superior MCE for the transverse electric and transverse magnetic modes compared to those of a CSW with a SiO2-slot layer. The present design paves the possible extensibility to other transition metal dichalcogenides (TMDs) for designing state-of-the-art TMD-based photonic devices exploiting their extraordinary optical properties.

Ultra-compact hybrid silicon:chalcogenide waveguide temperature sensor.

We demonstrate a real-time, reusable, and reversible integrated optical sensor for temperature monitoring within harsh environments. The sensor architecture combines the phase change property of chalcogenide glasses (ChG) with the high-density integration advantages of high index silicon waveguides. To demonstrate sensor feasibility, ChG composition Ge40S60, which is characterized by a sharp phase transition from amorphous to crystalline phase around 415 °C, is deposited over a 50 µm section of a single mode optical waveguide. The phase transition changes the behavior of Ge40S60 from a low loss to high loss material, thus significantly affecting the hybrid waveguide loss around the phase transition temperature. A transmission power drop of over 40dB in the crystalline phase compared to the amorphous phase is experimentally measured. Moreover, we recover the amorphous phase through the application of an electrical pulse, thus showing the reversible nature of our compact temperature sensor. Through integrating multiple compositions of ChG with well-defined phases transition temperatures over a silicon waveguide array, it is possible to determine, in real-time, the temperature evolution within a harsh environment, such as within a nuclear reactor cladding.

Brightening single-photon emitters by combining an ultrathin metallic antenna and a silicon quasi-BIC antenna

Bright single-photon emitters (SPEs) are fundamental components in many quantum applications. However, it is difficult to simultaneously get large Purcell enhancements and quantum yields in metallic nanostructures because of the huge losses in the metallic nanostructures. Herein, we propose to combine an ultrathin metallic bowtie antenna with a silicon antenna above a metallic substrate to simultaneously get large Purcell enhancements, quantum yields, and collection efficiencies. As a result, the brightness of SPEs in the hybrid nanostructure is greatly increased. Due to the deep subwavelength field confinement (mode size < 10 nm) of surface plasmons in the ultrathin metallic film (thickness < 4 nm), the Purcell enhancement of the metallic bowtie antenna improves by more than 25 times when the metal thickness decreases from 20 nm to 2 nm. In the hybrid nanostructures by combining an ultrathin metallic bowtie antenna with a silicon antenna, the Purcell enhancement (Fp≈2.6 × 106) in the hybrid nanostructures is 63 times greater than those (≤ 4.1 × 104) in the previous metallic and hybrid nanostructures. Because of the reduced ratio of electromagnetic fields in the ultrathin metallic bowtie antenna when the high-index silicon antenna is under the quasi-BIC state, a high quantum yield (QY ≈ 0.70) is obtained. Moreover, the good radiation directivity of the quasi-BIC (bound state in the continuum) mode of the silicon antenna and the reflection of the metallic substrate result in a high collection efficiency (CE ≈ 0.71). Consequently, the overall enhancement factor of brightness of a SPE in the hybrid nanostructure is EF* ≈ Fp × QY × CE ≈ 1.3 × 106, which is 5.6 × 102 times greater than those (EF* ≤ 2.2 × 103) in the previous metallic and hybrid nanostructures.

High-efficiency all-silicon metasurfaces with 2π phase control based on multiple resonators

Flat all-silicon metasurfaces have significant potential as ultrathin optical devices and systems that offer the advantages of mature fabrication technology and complementary metal–oxidesemiconductor compatibility. However, previously reported all-silicon metasurfaces suffered from the drawback of low transmission owing to the large intrinsic reflection loss induced by high-index silicon substrates. To achieve high transmission, silicon-based metasurfaces are usually composed of low-index dielectric substrates and silicon pillars. It is still a substantial challenge for an all-silicon metasurface to realize high transmission. In this paper, we present a design to overcome this drawback by integrating two-dimensional periodic arrays of subwavelength silicon cylinders as perfect antireflection resonators on the bottom and top surfaces of the silicon substrate. Using this design, a uniform array of all-silicon unit cells comprising a metasurface can exhibit high transmission, up to 99.5%, at the target wavelength of 10.6 µm, which indicates a significant improvement over the design without the antireflection scheme. Furthermore, as an example of a metasurface application, an all-silicon polarization-insensitive metalens is built based on a series of all-silicon unit cells with high transmission and 2π phase control, which indicates subwavelength-focusing capability. The transmission of the entire metalens reached 81.9%, and the focusing efficiency was 71%. This study not only recommends a means to develop ultrathin all-silicon optical devices with high efficiency but also proposes a general approach to achieve high-quality metasurfaces based on a single high-index dielectric.

Tailoring Third Harmonic Generation From Anapole Mode in a Metal-Dielectric Hybrid Nanoantenna

An all-dielectric nanoantenna is expected to greatly boost the nonlinear performance owing to its large mode volume and low material loss. However, it still suffers from the relatively weak near-field enhancement and the inferior far-field directivity. In this work, by successively introducing an upper metal ring and a bottom metal film, the plasmonic resonance and the perfect electric conductor (PEC) mirror effect are employed to significantly enhance and tailor the third harmonic generation (THG) from the high-index silicon nanoantenna. Numerical results show that benefiting from both of these two metal components, not only the THG efficiency is increased by more than 1 and 3.5 orders of magnitude compared with those with only single metal component and without any metal component, respectively, but also an improved radiation directivity can be obtained. The dependences of geometric parameters are discussed as well. This work suggests a route to enhance and manipulate the nonlinear radiation by a hybrid nanoantenna and facilitates its practical applications such as biosensing and nonlinear light sources.

Millimeter-long metamaterial surface-emitting antenna in the silicon photonics platform.

Integrated optical antennas are key components for on-chip light detection and ranging technology (LIDAR). In order to achieve a highly collimated far field with reduced beam divergence, antenna lengths on the order of several millimeters are required. In the high-index contrast silicon photonics platform, achieving such long antennas typically demands weakly modulated gratings with lithographic minimum feature sizes below 10 nm. Here, we experimentally demonstrate a new, to the best of our knowledge, strategy to make long antennas in silicon waveguides using a metamaterial subwavelength grating (SWG) waveguide core loaded with a lateral periodic array of radiative elements. The mode field confinement is controlled by the SWG duty cycle, and the delocalized propagating mode overlaps with the periodic perturbations. With this arrangement, weak antenna radiation strength can be achieved while maintaining a minimum feature size as large as 80 nm. Using this strategy, we experimentally demonstrate a 2-millimeter-long, single-etched subwavelength-engineered optical antenna on a conventional 220 nm SOI platform, presenting a measured far-field beam divergence of 0.1° and a wavelength scanning sensitivity of 0.13°/nm.

A High-Sensitivity SPR Sensor with Bimetal/Silicon/Two-Dimensional Material Structure: A Theoretical Analysis

In this paper, we reported a theoretical study of a novel Surface plasmon resonance (SPR) biosensor composed of BK7 prism, gold (Au)/silver (Ag) bimetallic layer, silicon and two-dimensional (2D) materials. The bimetallic layer combines the advantages of Au and Ag and the high refractive index silicon layer enhances the electric field on the surface of the sensor, so that the sensor has a better overall performance in terms of sensitivity and figure of merit (FOM). Compared with ordinary dielectrics, 2D materials have excellent photoelectric properties, such as larger specific surface area, higher carrier density and stronger adsorption capacity, which improve the detection ability of the sensor. The sensitivity of the optimized sensor achieves 297.2°/RIU, 274°/RIU and 246°/RIU when the silicon layer is covered with graphene, MXene (Ti3T2Cx) and MoS2, respectively. Compared with the traditional SPR sensor, the sensitivity of the structure has been significantly improved, and its excellent performance has broad application prospects in biosensing and other fields.

On the Roughness Analysis of High-Index Silicon Wafers Using KOH-IPA Solutions

Bulk Micromachining on some stable high-index silicon substrates are ever more interesting because they are opening new possibilities for the development of novel 3D microstructures and surface nanostructures useful for diverse quantum applications. Considering crystallography-oriented etching, several mechanisms are poorly understood and hence, some experimental work leading to 3D device fabrication are being developed and reported without analyzing the morphology evolution. For bulk micromachining, when 3D etching is developed using some anisotropic solution, the structure evolution takes place under a competition of fast-etching planes and unstable facets against slow-etching planes. According to the etchant composition, the resulting surface roughness can be considered as one of the main issues to be addressed. Wet chemical etching on (1 1 4), (1 1 3), and (5 5 12) silicon substrates at 60 °C, using aqueous KOH and KOH+IPA solutions, is carried on this work. The absolute etching rate trends and surface roughness are compared to reported data obtained from (0 0 1) silicon substrates. The overall etching mechanism is analyzed following some crystallographic rules and compared with high-index surfaces obtained from (0 0 1) silicon substrates. According to the best etching conditions achieved on this study, some novel 3D microstructures are presented.

Genetically optimized dual-wavelength all-dielectric metasurface based on double-layer epsilon-near-zero indium-tin-oxide films

Following the pioneering works on electrically tunable conducting oxide-based reflectarray metasurfaces, it has been shown that maximum phase modulation can be realized at a wavelength, where the transition from over- to under-coupling regimes coincides with the epsilon-near-zero (ENZ) phenomenon inside the indium-tin-oxide (ITO) active layer. However, the ENZ transition is restricted to a narrow bandwidth in the near-infrared regime, which limits the maximal achievable phase span at the wavelengths exterior to this bandwidth. Here, we present the realization of a dual wavelength all-dielectric metasurface with a large wavelength-contrast ratio between the operating channels, which is integrated by double-layer ITO films. The doping densities inside the ITO films are judiciously controlled to facilitate the ENZ-crossing of the relative permittivities at the corresponding working wavelengths. The all-dielectric metasurface is comprised of the arrays of cross-shaped holes made inside a high-index silicon slab supporting two resonances that are 300 nm apart. Numerical analysis of the near-field resonant modes reveals the excitation of guided-mode and magnetic dipole resonances, which strongly overlap with ITO active layers. Leveraging from the double-ENZ effect, considerable phase modulations of almost 220° and 240° are attained with a single metasurface platform at the wavelengths of λ1=1200 nm and λ2=1500 nm under the bias voltage application. The design parameters including the geometrical sizes and plasma frequencies of the differently doped ITO layers are carefully optimized by multi-objective genetic algorithm. The proposed metasurface illustrates a great promise in tunable beam splitting of the reflected light and dynamic conversion of the polarization states.

Large-area, low-cost near-infrared meta-surface reflector based on a pixelated two-dimensional silicon disk array.

All-dielectric meta-surfaces composed of dielectric meta-atoms with electric and magnetic multipole resonances provide a low loss alternative to plasmonic meta-surfaces in some optical research fields such as meta-lens and meta-surface holography. We utilize the digital holography lithography technique to obtain the large area meta-surface perfect reflector made of high refractive index and low loss silicon discs arrays, with the capability to delicately control the optical response in the near infrared spectrum. Three types of meta-surface reflectors (discs, truncated cones and diamond-shaped discs) were fabricated, which correspondingly exhibited nearly 1 peak reflectance and greater than 97% average reflectance in their respective perfect reflectance spectral regions. Digital holography lithography only takes 4 min to fabricate millions of photoresist disks over an area of 100 mm2, which is high processing efficiency and low cost. The fabrication strategy opens a new avenue for the production of large-area meta-surfaces in the optical field, especially in the mass production of optical communication devices, semiconductor lasers, etc.

CPW-Fed Wide Band Micro-machined Fractal Antenna with Band-notched Function

In this paper, a straightforward yet effective design methodology to design wideband antenna with band notched characteristics has been proposed. Sierpinski carpet fractal geometry has been used to realize the antenna structure. Co-planar waveguide feed is used with a novel structure to achieve larger impedance bandwidth and band notching characteristics. Proposed antenna is designed using High Frequency Structure Simulator (HFSS) on a low cost FR4 substrate (ɛr=4.4) which resonates at three frequencies 1.51 GHz (1.19-2.06GHz), 6.53 GHz and 8.99 GHz (4.44-9.54 GHz) while a band is notched at 10.46 GHz (9.32-11.92 GHz). The proposed antenna has an electrical dimension of 0.36 λm× 0.24 λm, here λm is the wavelength with respect to lowest resonating frequency of the antenna. The resonating and radiation characteristics of the antenna are verified experimentally. Further, investigations are made to achieve easy integration of the antenna to the monolithic microwave integrated circuits. For that the antenna has been designed on micro-machined high index Silicon substrate which improve matching and gain of the antenna. The results of the micro-machined Sierpinski carpet fractal antenna are highly convincing over the conventional FR4 based antenna.

Performance analysis of slot waveguide using aluminum nitride in slot region

The major obstacle to accomplish an efficient photonic slot waveguide is its comparatively high propagation loss. In the current work, the silicon-on-insulator based slot waveguide with ultra-low propagation loss has been presented using aluminum nitride (AlN) in the slot region. The performance analysis of the slot waveguide has been done by varying the slot gaps from 20 to 240 nm and by varying the arm width from 160 to 340 nm. As compared with the recently reported works based on different slot waveguides, relatively smaller propagation loss of ~ 0.7 dB/cm has been predicted using the AlN based slot waveguide having a slot gap of 120 nm and arm width of 240 nm. Simulation results have demonstrated that the presence of AlN in the slot gap/low-index region, sandwiched between two high-index (silicon) regions, causes the discontinuity of the electric field of quasi-TE mode between the high-index regions. This results in high confinement of light with the smaller propagation loss in the slot region. Along with this, it has also been observed that the presented slot waveguide has the appreciably good propagation length, with low dispersion characteristics. Hence, this kind of analysis can be extended to realize the various optical devices/applications, such as for coupling, sensing, switching, etc.