Subwavelength Grating Research Articles

Design of Polarization Beam Splitter Based on Al0.9Ga0.1As/Al2O3 Subwavelength Grating

Design of Polarization Beam Splitter Based on Al0.9Ga0.1As/Al2O3 Subwavelength Grating

Quasi-resonant cavity enhanced photodetector with a subwavelength grating

Quasi-resonant cavity enhanced photodetector with a subwavelength grating

A Silicon Tm Mode Multiplexer Based on Three Waveguide Directional Coupler with Subwavelength Grating

A Silicon Tm Mode Multiplexer Based on Three Waveguide Directional Coupler with Subwavelength Grating

Polarization Engineered Second Harmonic Generation Enhancement from Amorphous Silicon - Gallium Nitride Heterogeneous Resonant Metasurface

We experimentally demonstrate second harmonic generation (SHG) enhancement from a heterogeneous resonant metasurface consisting of amorphous silicon sub-wavelength gratings supporting resonances within an underlying gallium nitride (GaN) layer. Electromagnetic design studies used to choose the sub-wavelength grating dimensions, corresponding spectral resonances, its angular sensitivity, electric field and non-linear polarization profiles are discussed. The efficient excitation of second-order nonlinearities within the GaN layer requiring longitudinal fundamental field component and efficient extraction of on-axis SHG using in-plane nonlinear polarization components are simultaneously satisfied for fundamental excitation polarized at 45^o, resulting in excitation of TE and TM-like resonances. Experimental studies on the fabricated structures characterizing the linear and nonlinear SHG properties including power, polarization, and spectral dependence are also discussed. The SHG spectral measurements show 1300-times enhancement for 1460 nm fundamental excitation. The SHG enhancement observed for the higher order resonance at 1460 nm is attributed to the trade-off between the increase in the in-plane to out-of-plane nonlinear polarization ratio, resulting in relative increase in on-axis SHG radiation and the decrease in quality factor of the resonances, which lowers the resonance field strength with increasing guided-mode order.

Enhancement of photoluminescence of monolayer transition metal dichalcogenide by subwavelength TiO<sub>2</sub> grating

Transition metal dichalcogenide (TMDC) monolayers have direct band gaps and can produce strong photoluminescence(PL), thereby possessing a wide application prospect in photoelectric devices and photoelectric detection fields. However, their PL efficiency needs further improving because they are of atomic thickness only, besides, they have non-radiative recombination of excitons. In this study, a combination structure of a gold film, titanium dioxide subwavelength gratings and TMDC monolayers is designed, which can greatly improve PL efficiency of the TMDC monolayers. The spontaneous emission rate can be controlled by the Purcell effect, and the maximum enhancement of photoluminescence is as high as 3.4 times. In this paper, the PL signals of monolayer WS<sub>2</sub> and monolayer WSe<sub>2</sub> on the designed structure are studied. The feasibility of the enhancement of PL of the TMDC monolayers on the subwavelength grating structure is verified experimentally, which provides a new idea for the application of two-dimensional materials to optoelectronic devices.

Polarization independent 2×2 multimode interference coupler with bricked subwavelength metamaterial

The silicon-on-insulator (SOI) platform enables high integration density in photonic integrated circuits while maintaining compatibility with CMOS fabrication processes. Nevertheless, its inherently high modal birefringence hinders the development of polarization-insensitive devices. The dispersion and anisotropy engineering leveraging subwavelength grating (SWG) metamaterials makes possible the development of polarization agnostic waveguide components. In this work we build upon the bricked SWG metamaterial nanostructures to design a polarization independent 2×2 multimode interference (MMI) coupler for the 220 nm SOI platform, operating in the telecom O-band. The designed device exhibits a 160 nm bandwidth with excess loss, polarization dependent loss and imbalance below 1 dB and phase error lower than 5°.

Analysis of formation and evolution of double Fano resonances in sub-wavelength dielectric grating/MDM waveguide/periodic photonic crystal

Based on the diffraction principle and the mode coupling theory, a composite micro-nano structure of sub-wavelength dielectric grating/metal-dielectric-metal (MDM) waveguide/periodic photonic crystal is proposed. Combined with the angle spectrum of reflection, the transmission characteristics of the surface plasmon polaritons and the generation mechanism of double Fano resonances at different incident angles and fixed wavelength are analyzed. The studies show that the physical mechanism of double Fano resonances is that the surface plasmon resonance generated at the interface of sub-wavelength dielectric grating and upper metal Ag film, and the waveguide mode resonance occurring in the MDM waveguide, provide the independently tunable double discrete states, under the condition of satisfying wave vector matching, which can be respectively coupled in the near field with the continuous state formed by the photonic band gap effect in the photonic crystal, thereby achieving the double Fano resonances. Then the influence of the structural parameters on the double Fano characteristics is analyzed quantitatively, and the evolution law of the double Fano resonances is explored by the change of the reflection spectra of resonance curves. The results show that the tuning between double Fano resonance curves and the resonance angles can be realized by changing the structural parameters. And under optimal conditions, the figure of merit (FOM) values of FR a and FR b in resonance A region can be as high as 460.0 and <inline-formula><tex-math id="M3">\begin{document}$ 4.00 \times {10^4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211491_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211491_M3.png"/></alternatives></inline-formula>, and the FOM values of FR a and FR b in resonance B region can be as high as 269.2 and <inline-formula><tex-math id="M4">\begin{document}$ 2.22 \times {10^4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211491_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211491_M4.png"/></alternatives></inline-formula>. The structure can provide an effective theoretical reference for designing the refractive index sensors based on Fano resonances.

Enhancement of Diffraction Efficiency Based on the Addition Principle of Coded Digital Gratings

For the traditional subwavelength gratings, we can code different units to construct the coded metagratings. In this paper, we propose an innovative principle of adding coded gratings, which can be used to add or subtract two or more traditional coded metagratings sequences and obtain a new coded metagratings sequence. Based on the proposed addition principle, the coded metagratings can adjust the scattering angle flexibly. This method overcomes the limitation that traditional grating can only obtain discrete diffraction angle. Importantly, compared with traditional gratings, the coded metagrating sequences with addition operation have enhanced diffraction efficiency at the same diffraction angle. This phenomenon may be attributed to the enhanced interaction between the coding particles of the new disordered coding sequence metagratings constructed. Our proposed coded grating addition principle provides a new degree of freedom for the flexible modulation of the terahertz wavefront. At the same time, this method can be extended to optical band or microwave band, which opens a new path for electromagnetic wave modulation and beam scanning.

Dynamically Switchable Triple-Band Absorption Enhancement of Graphene by a Subwavelength Grating Coupled Hybrid Structure

Dynamically Switchable Triple-Band Absorption Enhancement of Graphene by a Subwavelength Grating Coupled Hybrid Structure

Synthetic Aperture Imaging Analysis of Sub-Wavelength Grating Based on Rigorous Coupled-Wave Analysis Method

Synthetic Aperture Imaging Analysis of Sub-Wavelength Grating Based on Rigorous Coupled-Wave Analysis Method

Ultra-Broadband Tunable Filter Based on Cascaded Chirped Subwavelength Grating Assisted Contra-Directional Coupler

Ultra-Broadband Tunable Filter Based on Cascaded Chirped Subwavelength Grating Assisted Contra-Directional Coupler

Diffraction Anomalies and Structural Design of One-dimensional TiO2 Subwavelength Gratings

Diffraction Anomalies and Structural Design of One-dimensional TiO2 Subwavelength Gratings

Numerical Optimization of Refractive Index Sensors Based on Diffraction Gratings with High Aspect Ratio in Terahertz Range.

Terahertz surface plasmon resonance (SPR) sensors have been regarded as a promising technology in biomedicine due to their real-time, label-free, and ultrasensitive monitoring features. Different authors have suggested a lot of SPR sensors, including those based on 2D and 3D metamaterials, subwavelength gratings, graphene, and graphene nanotube, as well as others. However, one of the traditional approaches to realize high sensitivity SPR sensors based on metal diffraction gratings has been studied poorly in the terahertz frequency range. In this article, a linear metal rectangular diffraction grating with high aspect ratio is studied. The influence of the grating structure parameters on the sensor sensitivity is simulated. Effects arising from different ratios of depth and width were discovered and explained. The results show that the sensitivity can be increased to 2.26 THz/RIU when the refractive index range of the gas to measure is between 1 and 1.002 with the resolution .

Compact and broadband silicon-based polarization beam splitter using asymmetric directional couplers embedded with subwavelength gratings and slots.

A compact and broadband silicon-based polarization beam splitter (PBS) is proposed and investigated in detail, where the two arms of the directional coupler (DC) are, respectively, embedded with subwavelength gratings (SWGs) and vertical slots so that field distributions for the TE mode are significantly changed, effectively weakening coupling strength, whereas those for the TM mode are almost unaffected, nearly analogous to the DC with strip waveguides. By carefully optimizing structural parameters, efficient coupling will emerge between the two waveguides for the TM mode, while TE mode will be confined in the SWG-assisted strip waveguide. Consequently, the two modes can be effectively separated, and thus the realization of a PBS is accomplished. Results show that a compact PBS with a coupling length of 6.45 µm is achieved, together with the extinction ratio (ER) of 27.54/31.88 dB, the insertion loss of 0.12/0.14 dB, and the reflection loss of -43.67/-30.50dB, respectively, for TE/TM mode at the wavelength of 1.55 µm. The bandwidth, for both modes, is up to 230/100 nm when ER is larger than 15/20 dB. In addition, fabrication tolerances to the critical structural parameters and field evolution through the proposed device are analyzed.

Ultracompact silicon-based polarization splitter and rotator based on asymmetric directional couplers with subwavelength gratings

An ultrasmall polarization splitter and rotator (PSR) on a silicon-on-insulator (SOI) platform is proposed based on an asymmetric directional coupler (ADC) consisting of a bridged subwavelength grating (BSWG) and a strip silicon waveguide with another SWG embedded in the middle slot. The inputting TE mode is tightly confined in BSWG and directly passes through a bar port with little coupling to the adjacent waveguide because of the phase mismatch, while the phase-matching condition allows the injected TM mode to be evanescently decoupled from BSWG and simultaneously converted into the TE mode supported by a SWG slot waveguide before propagating out through the cross port. Both polarization-dependent behaviors within the proposed PSR can be readily realized and efficiently manipulated by independently controlling the attributes of the ADC waveguides, which are distinctly engineered by different SWG segments. We have verified what we believe, to the best of our knowledge, is the first combination of BSWG and a SWG slot waveguide that provides an additional degree of freedom in the design and a stronger modal hybridization due to the doubled effects from jointly used SWGs. Further, the increasing number of inserted SWGs in the present device drastically reduces the device footprint, leading to a coupling length of only 5.5 µm. From the results, the conversion loss of − 0.28 d B (polarization conversion ratio of 93.7%) has been obtained for a TM-to-TE conversion at 1.55 µm. The TM mode of the resulting PSR shows its cross talk is less than − 14.3 d B and the conversion loss is better than − 0.63 d B in a 95 nm bandwidth that has covered the entire C and L bands. The cross talk and insertion loss of the TE mode are lower than − 29 d B and − 0.42 d B , respectively, as the operating wavelength varies.

Design of an ultrasharp silicon multimode waveguide bend assisted by multiregional subwavelength grating structures

Sharp multimode waveguide bends are beneficial to improve the integration density of the on-chip mode-division multiplexing (MDM) systems, which can be used to increase the optical information transmission capacity. A method of designing a multimode bending structure based on subwavelength gratings is proposed in this paper, through the mode conversion and matching between the straight and bending waveguide. The proposed bending structure is composed of a 90-deg arc-bend with a radius of 5 μm combined with the shallow-etched grating structures on the top, and two-mode converters on the input and output straight waveguide realized also with the help of nonuniform grating-like structures, achieving a total effective bending radius of 8.25 μm and high transmission efficiencies of the first three TE modes (TE0 − TE2) in a wide wavelength range of 1500 to 1600 nm. At the center wavelength of 1550 nm, the calculated excess losses of the three modes TE0, TE1, and TE2 are 0.21, 0.19, and 0.59 dB, respectively, and the crosstalks for all modes are less than −26.58 dB. More importantly, we add a non-uniform grating converter in the straight waveguide region which pre-deviates the mode fields inside the straight waveguide to help further reduce the size of the bending part to only 5 μm, which will be helpful for large scale integration when a great number of bends are cascaded.

Broadband and scalable silicon-based mode multiplexer using subwavelength-grating-based adiabatic coupler

A broadband and scalable mode multiplexer (MUX) using an adiabatic coupler (AC) assisted with subwavelength grating (SWG) is proposed and analyzed in detail. The adiabatic coupler consists of a strip bus waveguide and a tapered waveguide whose width decreases along the propagation direction so that the fundamental TE mode cannot be supported well and will be coupled into the high-order mode in the bus waveguide via mode evolution due to the phase matching. On the contrary, the fundamental TE mode injected into the bus waveguide will keep propagating in the same waveguide with negligible coupling. The SWG is incorporated into the gap between the bus and tapered waveguides for effectively enhancing the mode coupling, leading to a reduction in length of the device. By carefully choosing the structural parameters, the proposed TE0 / TE1 mode MUX shows a low insertion loss (IL) of 0.07 dB (0.01 dB) and crosstalk (CT) of −24.01 dB (−21.25 dB) for the coupled TE1 mode (the through TE0 mode) at wavelength 1550 nm while the coupling length is compact as ∼8.8 μm. The device achieves a broad bandwidth of 165 nm for CT < − 19 dB and IL < 0.11 dB over a wavelength range of 1460 to 1625 nm, covering the whole S, C, and L bands. Moreover, the device also shows a good fabrication tolerance to the deviations of the waveguide width, the total height of the device and the grating duty cycle. In addition, the TE2 and TE3 multiplexing are also presented, showing the scalability of the proposed scheme.

Plasmon-induced transparency sensor for detection of minuscule refractive index changes in ultra-low index materials

Detection of low-index materials such as aerogels and also detection of refractive index variations in these materials is still a challenging task. Here, a high figure of merit (FOM) sensor based on plasmon-induced transparency (PIT) is proposed for the detection of aerogel refractive index changes. In the proposed PIT sensor, the transparency window in an opaque region arises from the coupling between surface plasmon polariton (SPP) mode and planar waveguide mode. By comprising sub-wavelength grating (SWG) in the planar waveguide region, the maximum of the electric field of waveguide occurs in a low index media. This facilitates detection of the aerogels when they are used as the low index material (sensing material). Application of the subwavelength grating waveguide also improves the sensitivity of the sensor by a factor of six compared to a conventional structure with a homogenous waveguide. The proposed structure has a quality factor of Q ≥ 1800, and a reflection of 86%, and can detect the refractive index changes as low as Δn = 0.002 (around n = 1.0). The lineshape, Q-factor, and resonant wavelength of the transparency spectrum can be controlled by tailoring the structural parameters. Our work also has potential application in switching, filtering, and spectral shaping.

Metamaterial-Engineered Silicon Beam Splitter Fabricated with Deep UV Immersion Lithography.

Subwavelength grating (SWG) metamaterials have garnered a great interest for their singular capability to shape the material properties and the propagation of light, allowing the realization of devices with unprecedented performance. However, practical SWG implementations are limited by fabrication constraints, such as minimum feature size, that restrict the available design space or compromise compatibility with high-volume fabrication technologies. Indeed, most successful SWG realizations so far relied on electron-beam lithographic techniques, compromising the scalability of the approach. Here, we report the experimental demonstration of an SWG metamaterial engineered beam splitter fabricated with deep-ultraviolet immersion lithography in a 300-mm silicon-on-insulator technology. The metamaterial beam splitter exhibits high performance over a measured bandwidth exceeding 186 nm centered at 1550 nm. These results open a new route for the development of scalable silicon photonic circuits exploiting flexible metamaterial engineering.

Angular Goos-Hanchen shift in subwavelength gratings enhanced by surface plasmon resonance

The angular Goos-Hanchen shift dependence on the subwavelength grating parameters and the incident Gaussian beam width is investigated theoretically. High sensitivity of the angular Goos-Hanchen shift to the incident angle of a light beam near the surface plasmon resonance is demonstrated. Splitting of the reflected beam into two angularly separated beams is shown for strongly focused beam incident at the surface plasmon resonance angle.