One-dimensional Photonic Crystal Slab Research Articles

Optical moiré bound states in the continuum

Trapping electromagnetic waves within the radiation continuum holds significant implications in the field of optical science and technology. Photonic bound states in the continuum (BICs) present a distinctive approach for achieving this functionality, offering potential applications in laser systems, sensing technologies, and other domains. However, the simultaneous achievement of high Q-factors, flat-band dispersions, and wide-angle responses in photonic BICs has not yet been reported, thereby impeding their practical performance due to laser direction deviation or sample disorder. Here, we theoretically demonstrate the construction of moiré BICs in one-dimensional photonic crystal (PhC) slabs, where high-Q resonances in the entire moiré flat band are achieved. Specifically, we numerically validate that the radiation loss of moiré BICs can be eliminated by aligning multiple topological polarization charges with all diffraction channels, enabling the strong suppression of far-field radiation from the entire moiré band. This leads to a slow decay of Q-factors away from moiré BICs in the momentum space. Moreover, it is found that Q-factors of the moiré flat band can still maintain at a high level with structural disorder. In experiments, we fabricate the designed 1D moiré PhC slab and observe both high-Q resonances and a slow decrease of Q-factors for moiré flat-band Bloch modes. Our findings hold promising implications for designing highly efficient optical devices with wide-angle responses and introduce a novel avenue for exploring BICs in moiré superlattices.

Reflectionless graphene perfect absorber based on parity symmetric unidirectional guided resonance.

We propose a type of reflectionless graphene perfect absorber (GPA) in which the reflection channel is forbidden, while the transmission channel is open. Peak absorption of 99.97% in the near-infrared is numerically demonstrated for monolayer graphene loaded on a one-dimensional silicon photonic crystal slab with rhomboid cross sections that supports parity symmetric unidirectional guided resonances (UGRs). Based on the proposed GPA, a transmissive optical modulator with a modulation depth of about 28 dB and an insertion loss of 0.31 dB by varying the Fermi energy level graphene from 0.3 eV to 0.7 eV is numerically presented. Remarkably, the design strategy can be straightforwardly applied to other two-dimensional (2D) materials. Our study may find promising applications in 2D material-based optical modulators and filters.

General Bound States in the Continuum in Momentum Space.

Polarization singularities including bound states in the continuum (BICs) and circularly polarized states have provided promising opportunities in the manipulation of light waves. Previous studies show that BICs in photonic crystal slabs are protected by C_{2}T symmetry and hence normally exist on the high-symmetry lines of momentum space. Here, we propose an approach based on graph theory to study these polarization singularities in momentum space, especially in the region off the high-symmetry lines. With a polarization graph, it is demonstrated for the first time that BICs can stably exist off the high-symmetry lines of momentum space for both one-dimensional and two-dimensional photonic crystal slabs. Furthermore, two kinds of interesting processes, including the merging involved with this newly found BICs both on and off the high-symmetry lines, are observed by changing the geometrical parameters of photonic crystal slabs while keeping their symmetry. Our findings provide a new perspective to explore polarization singularities in momentum space and render their further applications in light-matter interaction and light manipulation.

Observation of terahertz transition from Fano resonances to bound states in the continuum.

Bound states in the continuum (BIC) in metamaterials have recently attracted attention for their promising applications in photonics. Here, we investigate the transition from Fano resonances to BIC, at terahertz (THz) frequencies, of a one-dimensional photonic crystal slab made of rectangular dielectric rods. Simulations performed by an analytical exact solution of the Maxwell equations showed that symmetry-protected, high-quality factor (Q), BIC emerge at normal incidence. For non-normal incidence, BIC couple with the freely propagating waves and appear in the scattering field as a Fano resonance. Simulations were verified by realizing the photonic crystal slab by 3D-printing technique. THz time-domain spectroscopy measurements as a function of the incidence angle matched the simulation to good accuracy and confirmed the evolution of Fano resonances to high-Q resonances typical of BIC. These results point out the design of highly sensitive and low-cost THz devices for sensing for a wide range of applications.

Exfoliated Flexible Photonic Crystal Slabs for Refractive Index and Biomolecular Sensing

Using the optical transduction properties of one-dimensional photonic crystal slabs is a promising approach for label-free biosensing in the field of flexible and wearable biosensing. In this work we demonstrate the fabrication of flexible photonic crystal slabs (f-PCS) via an exfoliation method from rigid photonic crystal slabs. We investigate the processing effect on the optical properties and show that the optical resonances are maintained, but affected. The quality factor changes from 50 to 31 for the TE mode and from 187 to 136 for the TM mode after the process. We demonstrate use of f-PCS for refractive index sensing and achieve a limit of detection (LOD) of 3.6 E-4 refractive index units (RIU). Furthermore, we introduce vapor-phase functionalization of the f-PCS and show first results of biosensing of antibodies from diluted feline serum.

Unidirectional asymmetry transmission based on quasi-accidental bound states in the continuum.

The unidirectional asymmetry transmission is demonstrated based on quasi-accidental bound states in the continuum by a one-dimensional chiral photonic crystal slab (CPhCs) composed of tilted silicon nano-pillars. The chirality breaks symmetries on the far field polarization and radiation Q-factor between the upward and downward radiation channels. Accordingly, the CPhCs only supports the unidirectional maximal asymmetry transmission at fixed incident and conical angles. The numerical simulation indicates that the CPhCs obtains a circular dichroism of 0.99 and Q-factor of 753.7 at λ = 1.565 μm. In addition, the handedness of polarization is also effectively converted between the incidence and transmission, and the handedness depends on the incident direction and conical angle. Our scheme provides a feasible route for applications in manipulating polarization and chiral sensing.

The generating and modulating characteristics of bound states in the continuum for both TE and TM polarizations by one-dimensional photonic crystal slabs

Optical bound states in the continuum (BICs) are capable of manipulating the polarization, confinement, and coupling of light with infinitely high-quality factor. The emerging design based on photonic crystal (PhC) slabs has led to the realization of novel platforms with strong light–matter interaction, but these structures usually are sensitive to the polarization of light. Here, we report theoretically that both transverse-electric (TE) and transverse-magnetic (TM) polarized BICs at telecommunication wavelengths can be simultaneously constructed based on one-dimensional dielectric PhC slabs. By means of the mode expansion method, the essential characteristics of these BICs are systematically investigated in this structure, including the formation and modulation of the symmetry-protected and non-symmetry-protected BICs for different polarizations. It is derived that these BICs can be achieved independent of the incident polarization and for a broad range of propagating constants. These results can provide potential applications for efficient biosensing and perfectly filtering as well as to study various nonlinear phenomena.

Enhanced directional emission of monolayer tungsten disulfide (WS2) with robust linear polarization via one-dimensional photonic crystal (PhC) slab

Abstract Objectives Monolayer transition metal dichalcogenides (TMDCs) have been regarded as promising candidates for the future light-emitting devices. To date, though the modulation of emission intensity and directionality in monolayer TMDCs has received considerable scholarly attention, there has been no systematic investigation on the underlying critical polarization. The intensity, directionality and robust polarization are highly favorable and pivotal for the future on-chip optoelectronic emission devices based on TMDCs. Methods We explore the emission features of the monolayer TMDCs in the photonic crystal (PhC) platform at room temperature. A monolayer tungsten disulfide (WS2) is specifically integrated with a tailored PhC structure. Angle-resolved photoluminescence (PL), time-resolved PL and polarized PL measurements are carried out to study the enhanced emission and polarization properties. Results The photoluminescence (PL) of WS2 is greatly enhanced by over 300-fold, resulting from a ∼fivefold enhancement (from 1.5 to 7.2%) of the PL efficiency with accelerated spontaneous emission rates. Additionally, the overall polarized emission is obtained with the degree of linear polarization (DLP) up to 60%, which is independent of the excitation polarization. Moreover, two branched directional emissions with horizontal polarization are also achieved at a divergency angle of only 3.5°, accompanied by a surprising near-100% DLP at ±8° directions. Conclusions This comprehensive study sets out to assess the feasibility of the high-performance light emission device based on the monolayer TMDCs and PhC structures.

First-order perturbation theory for changes in the surrounding of open optical resonators.

The single-mode approximation of resonant state expansion has proven to give accurate first-order approximations of resonance shifts and linewidth changes when modifying the material properties inside open optical resonators. Here, we extend this first-order perturbation theory to modifications of the material properties in the surrounding medium. As a side product of our derivations, we retrieve the already known analytical normalization condition for resonant states. We apply our theory to two example systems: a metallic nanosphere and a periodic array of metallic nanoslits.

Disk-based one-dimensional photonic crystal slabs for label-free immunosensing

One-dimensional photonic crystal slabs are periodic optical nanostructures that produce guided-mode resonance. They couple part of the incident light into the waveguide generating bandgaps in the transmittance spectrum, whose position is sensitive to refractive index variations on their surface. In this study, we present one-dimensional photonic crystal slab biosensors based on the internal nanogrooved structure of Blu-ray disks for label-free immunosensing. We demonstrated that this polycarbonate structure coated with a critical thickness of TiO2 generates guided-mode resonance. Its optical behavior was established comparing it with other compact disk structures. The results were theoretically calculated and experimentally demonstrated, all them being in agreement. The bioanalytical performance of these photonic crystals was experimentally demonstrated in a model assay to quantify IgGs as well as in two immunoassays to determine the biomarkers C-reactive protein and lactate dehydrogenase (detection limits of 0.1, 87, and 13 nM, respectively). The results are promising towards the development of new low-cost, portable, and label-free optical biosensors that join these photonic crystals with dedicated bioanalytical scanners based on compact disk drives.

Nonlinear Bound States in the Continuum in One-Dimensional Photonic Crystal Slab

We explore the nonlinear optical response of the silicon one-dimensional photonic crystal slab supporting bound states in the continuum (BIC). We show the emergence of such nolinear effects as multistable behaviour, self-tuning of BIC and breaking of symmetry protected BIC. We define a class of the nonlinear solutions generated by the linear BIC state and analyze the modulation instability of the obtained solutions and the effect of the finite system size on the stability.

Nonlinear bound states in the continuum of a one-dimensional photonic crystal slab

Optical bound state in the continuum (BIC) is characterized by infinitely high quality factor resulting in drastic enhancement of light-matter interaction phenomena. We study the optical response of a one-dimensional photonic crystal slab with Kerr focusing nonlinearity in the vicinity of BIC analytically and numerically. We predict a strong nonlinear response including multistable behaviour, self-tuning of BIC to the frequency of incident wave, and breaking of symmetry protected BIC. We show that all of these phenomena can be observed in silicon photonic structure at the pump power of several $\mu$W/cm$^2$. We also analyze the modulation instability of the obtained solutions and the effect of the finite size of the structure on the stability. Our findings have strong implications for nonlinear photonics and integrated optical circuits.

Design of guided Bloch surface wave resonance bio-sensors with high sensitivity

The sensing performance of bio-sensors based on guided Bloch surface wave (BSW) resonance (GBR) is studied. GBR is realized by coupling the propagating electromagnetic wave with BSW on one side of a one-dimensional photonic crystal slab via the grating on the other side. The sensitivity of the designed bio-sensors is proportional to the grating constant when the wavelength spectrum is analyzed, and inversely proportional to the normal wave vector of the incident electromagnetic wave when the angular spectrum is resolved. For a GBR bio-sensor designed to operate near 70° angle of incidence from air, the angular sensitivity is very high, reaching 128degRIU−1. The sensitivity can be substantially increased by designing bio-sensors for operating at larger angles of incidence.

Bound state in the continuum in the one-dimensional photonic crystal slab

In this work we developed a design of one-dimensional Si/SiO2 photonic crystal slab supporting so called optical bound states in the continuum - infinitely high-Q optical states with energies lying above the light line of the surrounding space. Such high-Q states are very perspective for many potential applications ranging from on-chip photonics and optical communications to biological sensing and photovoltaics.

Theory of pulsed four-wave mixing in one-dimensional silicon photonic crystal slab waveguides

We present a comprehensive theoretical analysis and computational study of four-wave mixing (FWM) of optical pulses co-propagating in one-dimensional silicon photonic crystal waveguides (Si-PhCWGs). Our theoretical analysis describes a very general set-up of the interacting optical pulses, namely we consider nondegenerate FWM in a configuration in which at each frequency there exists a superposition of guiding modes. We incorporate in our theoretical model all relevant linear optical effects, including waveguide loss, free-carrier (FC) dispersion and FC absorption, nonlinear optical effects such as self- and cross-phase modulation (SPM, XPM), two-photon absorption (TPA), and cross-absorption modulation (XAM), as well as the coupled dynamics of FCs and optical field. In particular, our theoretical analysis based on the coupled-mode theory provides rigorously derived formulae for linear dispersion coefficients of the guiding modes, linear coupling coefficients between these modes, as well as the nonlinear waveguide coefficients describing SPM, XPM, TPA, XAM, and FWM. In addition, our theoretical analysis and numerical simulations reveal key differences between the characteristics of FWM in the slow- and fast-light regimes, which could potentially have important implications to the design of ultra-compact active photonic devices.

Analytic approach to the directed diffraction in a one-dimensional photonic crystal slab

Analytic approach to the directed diffraction in a one-dimensional photonic crystal slab

Analysis of line shapes and strong coupling with intersubband transitions in one-dimensional metallodielectric photonic crystal slabs

This work reports a detailed study of quasi-guided mode resonances in metallodielectric photonic crystal slabs, focusing on the interaction with the intersubband transition supported by a multiple quantum well embedded in the photonic crystal itself. The strong light-matter coupling regime is obtained at room temperature in the mid-infrared spectral range, and its spectral fingerprints are analyzed in the framework of a Fano line-shape model. We propose an estimate of the light-matter coupling coefficient for arbitrarily shaped resonant cavities embedding multiple quantum wells supporting intersubband transitions, obtaining good agreement between theoretical calculations and experimental results.

High-Q microcavities in low-index one-dimensional photonic crystal slabs based on modal gap confinement

Recently, various high quality factor photonic crystal microcavities have been demonstrated theoretically and experimentally with only one-dimensional periodicity. However, in most cases high-index materials such as silicon were chosen for easily achievable large photonic bandgap and elaborate refractive index modulation or taper structure is required for reducing radiation loss. Here, we present a design of high-Q microcavities in one-dimensional multilayer polystyrene photonic crystal slab structures with a low-index contrast of 1.59:1. Microcavities are introduced by simply decreasing the thickness of layers at the center region to form a double-heterostructure. A resonant mode with a quality factor up to 20 000 is obtained and found to originate from the modal gap confinement by comparing with a Fabry–Perot cavity. The dependence of the maximal quality factor on the cavity length further reveals that the small group velocity of light within the heterostructure cavity contributes significantly to the high-Q. In terms of the high quality factor, ease of fabrication, and large Kerr nonlinearity of polystyrene, our double-heterostructure microcavities will find potential application in realizing all-optical modulation devices.

Air waveguide in a hybrid 1D and 2D photonic crystal hetero-structure

An air waveguide in hybrid one-dimensional photonic crystal and two-dimensional photonic crystal slab hetero-structure is designed. Light propagating in air waveguide can be confined by two-dimensional photonic crystal slab in x– y plane and one-dimensional photonic crystal films in z direction. Theoretical calculations show that air waveguide in the hetero-structure can achieve some functions as 3D PhCs but could be made more easily than 3D PhCs.

Resonant diffraction of symmetric and antisymmetric Bloch surface waves on a corrugated periodic multilayer slab

A mechanism for resonantly diffracting surface waves is presented. Symmetric and antisymmetric Bloch surface modes, prism coupled via evanescent coupling on a one-dimensional photonic crystal slab, are backdiffracted by means of a very shallow sinusoidal grating located on the top of the slab. At a given wavelength, resonant diffraction is produced when the grating vector equals the spatial beat frequency between the two modes. The presented mechanism is particularly attractive in optical sensing schemes in which both a high surface sensitivity and high diffraction efficiencies are required.