Zero-contrast Gratings Research Articles

Transition from Quasi‐Unidirectional to Unidirectional Guided Resonances in Leaky‐Mode Photonic Lattices

AbstractUnidirectional light emission from planar photonic structures is highly advantageous for a wide range of optoelectronic applications. Recently, it has been demonstrated that unidirectional guided resonances (UGRs) can be realized by utilizing topological polarization singularities in momentum space. However, the practical application of these topological unidirectional emitters has been limited due to their intricate geometric configurations, requiring special efforts with high‐cost fabrication processes. In this study, it is demonstrated that unidirectional light emission can be achieved in conventional 1D zero‐contrast gratings (ZCGs), which can be easily fabricated using current nanofabrication technologies. In ZCGs, the interband coupling between even‐like and odd‐like waveguide modes leads to the formation of quasi‐UGRs, characterized by significantly higher decay rates in either the upward or downward direction compared to the opposite direction. It is demonstrated that these quasi‐UGRs evolve into genuine UGRs with an gradual increase in grating thickness. Moreover, the emission direction of UGRs can be selectively steered either upward or downward by adjusting the lattice parameters. In addition to quasi‐UGRs and UGRs, the study also reveals additional topological phenomena in ZCGs, including exceptional points and quasi‐BICs.

Nonreciprocal Thermal Radiation With Zero-Contrast Gratings for Extreme Small Incident Angle

Nonreciprocal Thermal Radiation With Zero-Contrast Gratings for Extreme Small Incident Angle

Large nonreciprocity of thermal radiation for transverse electric wave with extremely small incident angle

Violating of traditional Kirchhoff’s law has been a topic of active research because of the possibilities for engineering thermal radiation. An alternative paradigm consisting of zero-contrast gratings (ZCGs) and Weyl semimetal (WSM) interlayer sitting on a silver (Ag) substrate is proposed. Strong nonreciprocal thermal radiation (NTR) has been observed for transverse electric (TE) incidence wave. This structure is highly favorable for standard mask-based nano-fabrication processes. Moreover, the nonreciprocity larger than 0.8 is realized with an extremely small angle of 1°. The mechanism of this proposal is owing to the guided-mode resonance (GMR) supported by the ZCGs and nonreciprocal radiation of WSM, which is confirmed by investigating the amplitude distribution of electric field. The high performance of the NTR can be maintained within wide ranges of structural parameters, which is highly favorable for practical fabrication. The nonreciprocal emitter described herein allows simultaneous control of spectral distribution and polarization of radiation, which will facilitate the active design and application of mid-infrared (MIR) thermal emitters.

Polarization-independent long-wave infrared transmission bandpass filters based on guided mode resonance.

Subwavelength dielectric gratings based on guided mode resonances (GMRs) provide a compact, low-cost, and readily fabricable platform for narrowband spectral filters applied in numerous fields, such as sensors, spectrometers, and hyperspectral imaging. We numerically designed two long-wave infrared transmission bandpass filters based on Ge-ZnS-Ge thin film stacks and 2D zero-contrast gratings. The first filter is attributed to double GMR mode coupling, exhibiting a broadband high reflectance between 8 and 12 µm and a narrow transmission peak with a spectral linewidth of 53 nm and transmittance efficiency of 98% at the resonance. The other filter adopts a low-refractive-index material BaF2 as the substrate, which displays a lower-transmittance sideband from 8.6 to 9.4 µm, and a higher transmission efficiency up to 100% at the resonance. The physical mechanisms of two spectral responses are all relevant to guided mode coupling and Fabry-Perot resonance.

Two-dimensional guided-mode resonance gratings with an etch-stop layer and high tolerance to fabrication errors.

Guided-mode resonance (GMR) bandpass filters have many important applications. The tolerance of fabrication errors that easily cause the transmission wavelength to shift has been well studied for one-dimensional (1D) anisotropic GMR gratings. However, the tolerance of two-dimensional (2D) GMR gratings, especially for different design architectures, has rarely been explored, which prevents the achievement of a high-tolerance unpolarized design. Here, GMR filters with common 2D zero-contrast gratings (ZCGs) were first investigated to reveal their differences from 1D gratings in fabrication tolerance. We demonstrated that 2D ZCGs are highly sensitive to errors in the grating linewidth against the case of 1D gratings, and the linewidth orthogonal to a certain polarization direction has much more influence than that parallel to the polarization. By analyzing the electromagnetic fields, we found that there was an obvious field enhancement inside the gratings, which could have a strong effect on the modes in the waveguide layer through the field overlap. Therefore, we proposed the introduction of an etch-stop (ES) layer between the gratings and the waveguide-layer, which can effectively suppress the interaction between the gratings and modal evanescent fields, resulting in 4-fold increased tolerance to the errors in the grating linewidth. Finally, the proposed etch-stop ZCGs (ES-ZCGs) GMR filters were experimentally fabricated to verify the error robustness.

Leaky Bloch modal evolution of wideband reflectors with zero-contrast gratings from symmetric trapez to triangle ridge shapes

We treat wideband subwavelength guided-mode resonant gratings with grating-depth dependent duty cycles. By rigorous numerical computations, we visualize Bloch modal evolution while transforming the grating profile from trapezoid to triangle. Parametric optimization is achieved using the coordinate transformation method of Chandezon. With the increase of the profile base angle, the higher mixed resonant modes TM1 , 1 & 2 attract and combine and interact with the modes at both sides, forming wide reflection bands. This modal combination and interaction are the key determinant of broadband reflectivity spectral location and width. The optimized structure exhibits 99% reflectivity across a 613-nm spectral range, spanning a 1438- to 2051-nm wavelength range with a fractional bandwidth of ∼35 % . It is shown that gratings with trapezoidal profiles possess a good tolerance to groove depth variation, thus being easier to fabricate with a diamond-tip than triangular profile-based gratings.

Leaky Bloch modal evolution of wideband reflectors with zero-contrast gratings from symmetric trapez to triangle ridge shapes

Leaky Bloch modal evolution of wideband reflectors with zero-contrast gratings from symmetric trapez to triangle ridge shapes

Resonant four-wave mixing microscopy on silicon-on-insulator based zero-contrast gratings

In this paper, we report large area four-wave mixing microscopy studies on silicon-on-insulator based partially etched two-dimensional zero-contrast gratings. The zero-contrast gratings offer an additional degree of freedom for the design of spectral resonances by varying the etch depth of the grating structures. This is leveraged by designing signal resonance at 1580 nm, operating in the sub-wavelength, zeroth-order diffraction region and pump fixed at 1040 nm operating in the higher order diffraction region. The zero-contrast gratings are fabricated on standard 220 nm silicon-on-insulator substrates with etch depth chosen as 140 nm. The fabricated structures are characterized to measure the linear transmission and nonlinear four-wave mixing performance. Multi-spectral four-wave mixing images acquired across the grating structures for varying input signal wavelength show maximum enhancement of four-wave mixing signal at 1575 nm, with the on-grating four-wave mixing signal enhanced by ∼ 450 times when compared to the un-patterned film. Zero-contrast gratings present a promising platform for realizing sub-wavelength scale nanostructured surfaces for nonlinear wave-mixing applications using standard silicon-on-insulator substrates. Such structures can find potential applications in wavelength conversion across widely separated wavelength bands and as substrates for nonlinear frequency conversion.

Highly-resonant two-polarization transmission guided-mode resonance filter

We theoretically demonstrate a mid-infrared polarization-independent guided-mode-resonance transmission filter. We designed a structure based on a deeply-etched 2D grating above a thin slab of the same material respectively supporting transverse magnetic and transverse electric fundamental modes with identical effective index, which propagate along orthogonal directions. This device relates to multi-resonant guided-mode-resonance filters, and more particularly to the concept of zero-contrast gratings (ZCG), which can operate either as wideband reflectors [R. Magnusson, Optics Letters 39, 4337 (2014)] or bandpass filters [M. Niraula, J. W. Yoon, and R. Magnusson, Optics Letters 40, 5062 (2015)]. However, contrary to the latter, this new generation of filters is not bound by stringent material requirements inherent to conventional ZCGs. In particular, ZCGs are demonstrated with high to low refractive index ratio below 2, using germanium as high-index material over a low-index zinc sulfide substrate. These filters exhibit a transmission peak with a full-width at half-maximum of about 30 pm, and a maximum transmission close to 100 % lying in a 46-nm-wide rejection window.

Design of angularly tolerant zero-contrast grating filters for pixelated filtering in the mid-IR range.

Zero-contrast gratings (ZCG) can be used to implement narrow bandpass transmission filters. However, they suffer from poor angular tolerance, which hinders their use in pixelated applications. Combining ZCG with double-corrugation grating, we increase the resonance width and angular tolerance of the filter by more than 1 order of magnitude. Filters tunable around 4.6 μm with more than 90% transmission and compatibility with 140 μm pixel size are demonstrated.

Low Contrast Sub-wavelengths Grating Lenses

ABSTRACTIn this work, we have demonstrated the light concentration from zero-contrast gratings (ZCG) subwavelength structures and compared the light concentration properties of previously proposed high-contrast gratings (HCG) to the ZCG micro concentrating lenses. To address the challenges of potential HCG fabrication, the difference between ZCG and HCG micro lenses is investigated numerically and found both of these subwavelength grating structures have similar light concentration characteristics. To gain deeper understanding of this phenomenon, we have explored the light concentration formation process and discussed the concentration mechanism in detail. Our work will be promising to provide a new ZCG micro lens potentially with easier and more controllable fabrication and could be utilized for various integrated nanophotonics applications, from optical cavities, read/write heads and concentrating photovoltaics.

Comparison between high- and zero-contrast gratings as VCSEL mirrors

This study presents a comparison between high-contrast gratings (HCGs) and zero-contrast gratings (ZCGs) for high-speed vertical-cavity surface-emitting lasers (VCSELs). Both types of gratings exhibit high reflectivities beyond 99.5% due to the destructive interference at the output plane, but the HCG has a broader high reflectivity band. The HCG has a lower reflection delay time and smaller energy penetration length than the ZCG. The HCG has poorer mode selectivity for the VCSEL than the ZCG. The fabrication of the ZCG is less complex but with tight fabrication tolerances.

Subwavelength grating wideband reflectors with tapered sidewall profile

AbstractOne main difference between practical device and ideal design for subwavelength grating structure is the tapered sidewall profile of grating, which is normally obtained by the practical CMOS-compatible fabrication and etching process. Our work has investigated the impacts of tapered sidewall profile on the subwavelength grating wideband reflector characteristics. Both zero-contrast gratings (ZCG) and high- contrast gratings (HCG) are numerically investigated in detail and the results show a distinct differences of the impacts of tapered sidewall profile of grating. The simulation results reveal that this factor play a critical role in determining the reflection bandwidth, average reflectance, and the band edge. Our study has potential in guiding the utilization of subwavelength grating wideband reflector on application of a variety of nanophotonic devices and their integration, as well as to facilitate the design of the fabrication process on the control of tapered sidewall profile.

Impacts of tapered sidewall profile on subwavelength grating wideband reflectors

We have demonstrated the significant impacts of grating tapered sidewall profile on the subwavelength grating wideband reflector characteristics when taking into account the practical fabrication process. Two different classes of wideband reflectors, referred to as zero-contrast gratings and high-contrast gratings, are numerically investigated in detail and the distinct differences of the impacts due to a grating tapered sidewall are observed. Our works reveal that this tapered sidewall profile plays a critical role in determining the reflection bandwidth, average reflectance, and the band edge. The results could be widely utilized in applications of a variety of nanophotonic devices and their integration, as well as facilitate the design of the fabrication process on how to control the degree of tapered sidewall profile for the integrated subwavelength grating nanophotonic devices.

Properties of two-dimensional resonant reflectors with zero-contrast gratings.

The spectral properties of high-reflection mirrors imbued with two-dimensional (2D) subwavelength periodicity are investigated. The reflectors are designed in a silicon-on-glass film that is partially etched to implement a zero-contrast interface between the grating pillars and the sublayer, thereby annulling the local reflections and phase changes associated with hard interfaces. This approach has shown impressive results for 1D polarized reflectors; here we strive to discover analogous wideband unpolarized reflectors. Using particle swarm optimization, we report wideband unpolarized reflectors in the 1.4-2.0μm wavelength band. A 2D reflector with square pillars exhibits 99% reflectance across a bandwidth exceeding 350nm and possesses tolerance against angular deviations. The complementary structure with rectangular periodic voids achieves a bandwidth of 370nm. A comparable, optimized 2D high-contrast grating reflector with grating pillars residing directly on the substrate yields a 99% bandwidth of 240nm.

Wideband reflectors with zero-contrast gratings.

We present wideband resonant reflectors designed with gratings in which the grating ridges are matched to an identical material, thereby eliminating local reflections and phase changes. This critical interface thus possesses zero refractive-index contrast; hence "zero-contrast gratings." We design reflectors with zero-contrast gratings and high-contrast gratings and compare the results. For simple gratings with two-part periods, we show that zero-contrast grating reflectors outperform comparable high-contrast grating reflectors. An example silicon-on-glass reflector exhibits a 99% reflectance bandwidth of ∼700 nm for zero refractive-index contrast Δn=0, whereas a high-contrast device with Δn=2 yields a bandwidth of ∼600 nm. It follows that local Fabry-Perot modes residing in the grating ridges and reflecting off a high-contrast interface are not the root cause of wideband reflection.