Subwavelength Grating Research Articles

Infrared nearly complete absorption in hexagonal boron nitride crystals based on guided mode resonance

The near-complete absorption effect in hexagonal boron nitride (hBN) crystals for mid-infrared spectra with guided mode resonance (GMR) is studied. It is achieved by placing an hBN film on a substrate, which is separated from a subwavelength grating by a dielectric film. Under normal incidence, the absorber demonstrates approximately 100% absorption in the reststrahlen upper-frequency band of light. The underlying physical mechanism responsible for this near-complete absorption effect is examined through visualization of electric field distributions at the resonant frequency. Additionally, to provide practical guidance for fabrication, the influence of structural dimensions on the absorption spectra is also explored. These findings reveal that guided mode resonance can be utilized to achieve perfect absorption in hBN films, thereby enhancing their potential applications in the mid-infrared range.

Intertwined Fano resonances in sub-wavelength metallic gratings: omnidirectional and wideband optical transmission.

Metallic gratings can be used as infrared filters, but their performance is often limited by bandwidth restrictions due to metallic losses. In this work, we propose a metallic groove-slit-groove (GSG) structure that overcomes these limitations by exhibiting a large bandwidth, angularly independent, extraordinary optical transmission. Our design achieves high transmission efficiency in the longwave infrared range, driven by Fano-type resonances created through the interaction between the grooves and the central slit. This mechanism results in a tunable 2 µm transmission window with high rejection rate. We extend the concept to a two-dimensional GSG array, exhibiting a polarization-insensitive 80% transmission window for incident angles up to 50°, offering significant potential for infrared filtering applications.

Subwavelength-modulated silicon photonics for low-energy free-electron-photon interactions.

We investigate silicon waveguides with subwavelength-scale modulation for applications in free-electron-photon interactions. The modulation enables velocity matching and efficient interactions between low-energy electrons and co-propagating photons. Specifically, we design a subwavelength-grating (SWG) waveguide for interactions between 23-keV free electrons and ≈1500-nm photons. The SWG waveguide and electron system exhibit a coupling coefficient of |gQu| = 0.23, and as we corroborate with time-domain, particle-in-cell simulations, the system operates as a backward-wave oscillator. Overall, our results show that modulated waveguides could open the door to strong, extended interactions between photons and low-energy (10-keV-scale) electrons, like those typically present in scanning electron microscopes. Additionally, our SWG waveguide design suggests that periodic waveguides could offer intriguing dispersion engineering opportunities for tailoring these interactions.

Ultrahigh-reflectivity chiral mirrors based on the metasurface of the quarter-waveplate

A study is conducted on a GaAs-based high-contrast subwavelength chiral metasurface (HCCM) designed for 1064 nm. The metasurface integrates a high-contrast subwavelength grating (HCG) for TM mode modulation, a SiO2 support layer, and a compact quarter-waveplate (QWP) to convert linearly polarized light to circularly polarized light. The HCG achieves ultra-high reflectivity at 1064 nm, attributed to the large refractive index contrast between the Si grating and SiO2 layer. The designed HCCM has a reflectivity of more than 99 % for the TM mode and less than 80 % for the TE waves in the wavelength range of 1050 nm–1085nm, and it can also excite the circularly polarized light with an absolute error of less than 0.03 at the transmitting end in the case of the 1064 nm TM wavelength. These results suggest that circular dichroic metasurfaces could offer a promising approach for photonic manipulation in circular polarization VCSELs, potentially reducing the manufacturing challenges and costs associated with traditional methods.

Design of high efficiency and wideband dual-functional metasurface for polarization conversion and asymmetric transmission

A wideband and high-efficiency bi-functional metasurface for polarization conversion and asymmetric transmission (AT) based on the gratings/ polarization converter/gratings structure is proposed. The proposed metasurface structure consists of a two double-split ring structure array sandwiched by two orthogonal metallic sub-wavelength gratings. Using two orthogonal metallicsub-wavelength gratings, the proposed metasurface reveals excellent AT effect and nearly perfect polarization conversion performance for linearly polarized waves. The proposed metasurface achieves a polarization conversion ratio (PCR) of above 0.99 from 3.7 GHz to 20.2 GHz with a relative bandwidth (RBW) of 138% and an AT parameter of over 0.9 from 5.8 GHz to 17 GHz withan RBW of 99.5%. Moreover, the proposed metamaterial maintains high PCR and AT performance for a wide range of incident angles. Due to its excellent features, such as the wideband and high efficiency of both PCR and AT performances, the proposed bi-functional metasurface can be useful for promising applications such as polarization imaging. radar, radiometer, and transmission.array antennas.

Double-slot micro-ring resonators with trapezoidal subwavelength grating as ultra-sensitive biochemical sensors

Silicon-based refractive index sensors are of significance in the detection of gases, biological substances and chemical compounds. Among these, optical microcavities can confine the optical field to the micrometre-scale region, and possess the advantages of high Q factor, small size and easy integration. In this paper, a trapezoidal subwavelength grating (SWG) is introduced into a slot micro-ring resonator, and the mode splitting is employed to enrich the supported standing wave modes and optimize the spatial profiles of the resonant modes. The modes’ Q factor is improved and the high sensitivity and low detection limit is achieved. The optimal trapezoidal subwavelength grating double slot micro-ring resonator (T-SWGDSMRR) structure is obtained by designing the structural parameters and analyzing their effects on the sensing performance parameters and spectral characteristics. The T-SWGDSMRR, designed for detecting the glucose solution, demonstrated a low detection limit of 3.3×10−5 RIU and an ultra-high Q factor of up to 100825, accompanied by a refractive index sensitivity of 424 nm/RIU. Finally, a cascaded double micro-ring sensor is proposed using the vernier effect, through cascading the T-SWGDSMRR with a referential ring, the sensitivity is enhanced to 12828 nm/RIU, and the limit of detection is 3.12×10−6 RIU.

Design and Analysis of Ridge and Sub-Wavelength Grating Waveguide Based Micro Ring-Resonator Sensor

The subwavelength grating microring resonator based filter shows great promise in optical sensing and communication. However, it spectrum nature makes it unsuitable for tracking or extracting a single wavelength from a broadband light source. In this paper, we used side-mode suppression, based on an angular grating- subwavelength microring resonator which offer high flexibility in wavelength design within the silicon-on-insulator (SOI) waveguide. By periodically arranging silicon waveguide pieces in three different widths, we can set the effective index along the inner sidewall of subwavelength microring resonator. thereby can achieve the precise wavelength selection. We examine the effects of various key parameters on the center wavelength, side-mode suppressed ratio, and quality factor of this filter.The proposed structure's isalso analysed for coupling strength between the bus and subwavelength grating ring resonator. The wavelength selection, compact size, compatibility with other similar nature waveguide-based devices and design flexibility highlight its significant potential in compact optical sensing and optical communication

Transmission enhancement mechanism of PT symmetric photonic crystal coupled with grooved sub-wavelength grating

In order to enhance the sensing characteristics of the sensor, based on the periodic sub-wavelength grating(SWG), a groove structure is introduced to make the Fano resonance waveform sharper. At the same time, based on the Parity-Time(PT) symmetry principle, by making the periodic photonic crystal meet the PT symmetry condition, the structure can absorb the energy of the emitted light on the original basis, produce the transmission enhancement effect, and make the transmission peak of the structure exceed 1. The sensing is realized by using the property that the different refractive indices of the detected medium corresponds to different transmission. When the incident light is 1550 nm, the sensitivity and FOM (Figure of Merit) value of the proposed structure reach 5.702 × 105RIU−1and 1.9 × 107 respectively, which greatly improves the performance of the sensor.

Zero-crosstalk silicon photonic refractive indexsensor with subwavelength gratings.

Silicon photonic index sensors have received significant attention for label-free bio and gas-sensing applications, offering cost-effective and scalable solutions. Here, we introduce an ultra-compact silicon photonic refractive index sensor that leverages zero-crosstalk singularity responses enabled by subwavelength gratings. The subwavelength gratings are precisely engineered to achieve an anisotropic perturbation-led zero-crosstalk, resulting in a single transmission dip singularity in the spectrum that is independent of device length. The sensor is optimized for the transverse magnetic mode operation, where the subwavelength gratings are arranged perpendicular to the propagation direction to support a leaky-like mode and maximize the evanescent field interaction with the analyte space. Experimental results demonstrate a high wavelength sensitivity of - 410nm/RIU and an intensity sensitivity of 395dB/RIU, with a compact device footprint of approximately 82.8μm2. Distinct from other resonant and interferometric sensors, our approach provides an FSR-free single-dip spectral response on a small device footprint, overcoming common challenges faced by traditional sensors, such as signal/phase ambiguity, sensitivity fading, limited detection range, and the necessity for large device footprints. This makes our sensor ideal for simplified intensity interrogation. The proposed sensor holds promise for a range of on-chip refractive index sensing applications, from gas to biochemical detection, representing a significant step towards efficient and miniaturized photonic sensing solutions.

Enhanced four-band absorption in a tunable graphene subwavelength grating structure for photodetection

This paper investigates a sub-wavelength multilayer dielectric grating structure based on a simple metasurface, achieving dynamically switchable four-band absorption enhancement. This enhancement spans the visible to near-infrared (NIR) range, with absorption exceeding 90%, representing nearly a 40-fold increase over the intrinsic absorption of a single graphene layer. The contributions of guided-mode resonance (GMR), optical Tamm state (OTS), and photonic crystal defect cavity resonance to the absorption spectra are analyzed separately, offering a method for independent multi-channel signal extraction. Additionally, we explore the impact of structural parameters on the absorption response and examine the broad-spectrum multiband detection mechanism. This mechanism, explained through the integration of resonant cavity perturbation theory and the dynamic tuning of absorption efficiency via graphene's Fermi energy levels, results in an independently tunable optical system for four key operating bands. These properties make the structure suitable for applications such as multiband biomedical photodetectors or components in optoelectronic devices with multiple operating bands.

Compact and fabrication tolerant polarization insensitive mode-order converter for MDM systems

With the increasing capacity demands of data communications, mode division multiplexing in on-chip optical interconnect systems is becoming an attractive solution. Compact, broadband, and fabrication-tolerant mode-order converters unaffected by polarizations are considered crucial for the progress of on-chip systems utilizing mode division multiplexing. A proposal for a design scheme for an on-chip mode-order converter that is insensitive to polarization is presented, utilizing a combination of 3D finite-difference time-domain (FDTD) and particle swarm optimization (PSO). The conversion of TE0/TM0-to-TE1/TM1 mode is achieved through phase matching between a subwavelength grating and an input–output tapered waveguide. Theoretical studies indicate that the TE0-to-TE1 and TM0-to-TM1 insertion losses are below 0.46 dB and 0.78 dB at a wavelength of 1550 nm. The TE0-to-TE1 and TM0-to-TM1 insertion loss is under 1.0 dB, with crosstalk below −15 dB within the 190 nm (1432∼1622 nm) and 81 nm (1515∼1596 nm) operating ranges. Experimental data indicates that the device measuring 7 × 1.58 μm2 can successfully convert mode orders regardless of polarization within an 81 nm bandwidth (1515∼1596 nm), effectively doubling the data transmission capacity of the MDM system. In addition, the devices fabricated by a standard CMOS process demonstrate the potential for mass production.

Resonantly Enhanced Infrared Up‐Conversion in Double‐Step Asymmetric Subwavelength Grating Structure

Resonantly Enhanced Infrared Up‐Conversion in Double‐Step Asymmetric Subwavelength Grating Structure

Highly reflective and high-Q thin resonant subwavelength gratings

Abstract We theoretically investigate the design of thin subwavelength gratings possessing high-reflectivity and high-Q resonances when illuminated at normal incidence by a Gaussian beam. We compare the performances of single-period and dual-period rectangular gratings using finite element method-based optimization and predict a close to two orders of magnitude improvement (×90) in their transmission loss-linewidth product, which is the relevant figure of merit for e.g. resonant mirror-based microcavity applications.

Generating broadband cylindrical vector modes based on polarization-dependent acoustically induced fiber gratings using the dispersion turning point

Cylindrical vector beams (CVBs) with special polarization distribution have been extensively investigated due to the unique ways of interacting with matter. Although several configurations have been developed to generate CVBs, such as Q-plates and subwavelength gratings, the bandwidth of a single CVB is inherently narrow due to the phase geometry, which would limit its application for femtosecond lasers. Here, a broadband CVB mode converter based on an acoustically induced fiber grating (AIFG) and a tuning method of dispersion turning point (DTP) is demonstrated both theoretically and experimentally with the 3-dB bandwidth of 125 nm, which is more than 10 times that of conventional AIFGs. Not only can the DTP wavelength be tuned from the original 1500 nm to 1650 nm by thinning the fiber, but also the stable generation of a single broadband HE21odd/even mode can be controllably implemented by adjusting the polarization state of the incident light, owing to the larger beat length difference between HE21 and other CV modes. Additionally, the femtosecond CVBs and orbital angular momentum (OAM) modes are successfully generated and amplified by combining the broadband AIFG with a figure-9 mode-locked fiber laser. Meanwhile, it is verified by simulation that the choice of broadband CV mode and the tunability of DTP wavelength can be realized by designing ring-core fibers with different structures, which can furthermore improve the flexibility of generating high purity CVBs. This study provides a highly controllable technique for the generation of broadband CVBs and OAMs paving the way for high-capacity CVBs communication.

From high- to low-contrast: the role of asymmetries in dielectric gratings supporting bound states in the continuum.

One-dimensional sub-wavelength gratings are versatile photonic platforms supporting diverse resonances, including symmetry-protected bound states in the continuum. However, practical access to these bound modes relies on their quasi-bound form, which necessitates the introduction of perturbations in either geometry or material properties. Despite having a large, finite quality factor, quasi-bound modes retain their characteristically strong field confinement. Gaining control over field localization and leakage of quasi-bound modes requires an investigation not limited to studying the degree of asymmetry and the incoming polarization. Here, we demonstrate that by carefully combining specific types of asymmetries and refractive index contrast between the grating and its surrounding environment, one can tailor field localization and Q-factor almost at will. Our findings reveal a strategic roadmap for optimizing quasi-bound mode implementation, dramatically improving their use in applications such as optical communication, sensing, and nonlinear optical processes.

Superconducting nanowire single photon detector based on a subwavelength metallic-dielectric grating

Subwavelength gratings (SWGs) can separate transverse electric (TE) waves and transverse magnetic (TM) waves, offering a feasible approach for polarization detection in optical detectors. In this paper, a superconducting nanowire single photon detector (SNSPD) based on a subwavelength metallic-dielectric grating is designed. By embedding NbN nanowires into the grating structure with an Au reflector, the detector realizes both polarization detection and broadband high efficiency detection. The detector maintains commendable performance even at low filling factors (f), making it possible to improve the detection speed. Numerical simulation results show that the SNSPD with f = 50% achieves an absorption efficiency over 90% in the entire wavelength range from 1 to 3 μm, and the corresponding extinction ratio exceeds 2.3 × 103. The SNSPD maintains an absorption efficiency over 80% from 1000 to 1900 nm when f is reduced to 16.7%, and the corresponding extinction ratio exceeds 1.8 × 103. The SNSPD can still achieve an absorption efficiency of 75.6% at the wavelength of 1550 nm when f is reduced to only 10%, and the corresponding extinction ratio is 825. The results of this work may provide access for the design and fabrication of polarization-sensitive, broadband and high speed SNSPDs.

Subwavelength Grating Cascaded Microring Resonator Biochemical Sensors with Record-High Sensitivity.

Photonic integrated circuit biochemical and biomedical sensors show promising applications in medical diagnosis, food security, healthcare, and environmental monitoring. Silicon-on-insulator subwavelength grating waveguides and cascaded microring resonator structures enhance photon-analyte interaction, offering superior sensing performance (higher sensitivity with lower limit of detection and larger free spectral range) compared to traditional strip and slot waveguide microring resonator structures. In this study, we design, simulate, and experimentally demonstrate a novel and compact biochemical sensor integrating subwavelength grating cascaded microring resonators and multibox subwavelength grating straight waveguides on a silicon-on-insulator platform. We achieve a record-high refractive index sensitivity of 810 nm/RIU with a limit of detection value of 2.04 × 10-5 RIU. The measured concentration sensitivity for sodium chloride solutions is 1430 pm/% with a limit of detection of 0.04%. The free spectral range is 35.8 nm, and the measured Q factor is 1.9 × 103. By combining the advantages of cascaded microring resonators with those subwavelength gratings, our sensor offers unprecedented sensitivity for biochemical sensing applications, promising significant enhancements in healthcare diagnostics and environmental monitoring systems.

Broadband and low-loss power splitter with polarization manipulation using subwavelength gratings

A broadband and low-loss power splitter with polarization manipulation using subwavelength gratings (SWGs) is proposed and analyzed, where SWGs are embedded in input taper/output inverse tapered waveguides in the bottom layer to form an SWG-based structure, and a silicon nitride layer is placed above the bottom layer to form a low-index guiding structure. In the bottom layer, the TE mode is cut off as the input strip waveguide is tapered down. In this way, the injected TE mode is evenly coupled to two adjacent output ports with high efficiency by the assistance of SWGs. By contrast, the input TM mode is well supported by the above silicon nitride layer and directly transmitted to the Through port, which is almost not influenced by the bottom structures. Consequently, both power splitting and polarization handling are achieved in the designed device simultaneously. Simulation results show that an extinction ratio (ER) of 32.63 dB (18.56 dB) and insertion loss (IL) of 0.16 dB (0.16 dB) for the TE (TM) mode are obtained at 1.55 µm. The bandwidth is up to 191 nm (1450–1641 nm) for the TE mode and 200 nm (1450–1650 nm) for the TM mode with an ER exceeding 15 dB and IL below 0.3 dB. Furthermore, fabrication tolerances and power distribution with polarization manipulation through the device are also presented.

Ultranarrow and multiband perfect absorbers based on dual quasi-bound states in the continuum in near infrared

High quality factor (Q-factor) and multiband perfect absorbers are highly desired in light-harvesting efficiency related optical devices, yet they are always limited by conventional mechanisms like electromagnetically induced transparency, Tamm or plasmonic resonance. In this paper, through constructing the strong coupling between two guided-mode resonances in subwavelength gratings, we propose an ultranarrow and multiband perfect absorber based the dual-band quasi-bound states in the continuum (quasi-BIC) design in near infrared. Benefiting from its angle-dependent characteristic, the device can be straightforwardly utilized for light-switched lamps and optical sensors. Q-factor 8337 and the modulation depth 98.1% are realized while the insertion loss is only 0.07 dB. Compared with its conventional counterparts, the quasi-BIC based absorber enables an excellent hexa-frequency and ultranarrow asynchronous switch function. The quatary-band perfect absorption below 1 nm bandwidth can be applied to the optical sensor to detect four light signals with weak signal strength. This work provides a theoretical basis for multifunctional absorbers with high Q-factor, high modulation depth, and low insertion loss.

Envelope Spectrum of the Subwavelength Grating Slot Waveguide Microring Resonator for High-Sensitivity and Wide-Range Refractive Index Sensing Applications

Envelope Spectrum of the Subwavelength Grating Slot Waveguide Microring Resonator for High-Sensitivity and Wide-Range Refractive Index Sensing Applications