Slab Waveguide Research Articles

Bragg grating based entangled biphoton dispersion management

In the realm of quantum information processing and precision measurements, a range of non-classical effects based on entangled photon pairs are employed. Non-local dispersion cancellation is one of these properties, in which the second photon (idler) can be manipulated to cancel the effects of dispersion on the first photon (signal). In the present study, a thorough investigation is conducted into non-local dispersion cancellation by using excitation modes within a Bragg grating slab waveguide. We present a complete theoretical analysis of non-local dispersion cancellation for frequency-limited entangled photon pairs. For the analysis of the effect of dispersion cancellation, we use a spontaneous parametric down-conversion (SPDC) photon pair source with frequency correlation and anti-correlation as the entangled source. To evaluate the dispersion cancellation effect for information processing applications, the second correlation function is used and calculated as a practical and tangible metric. Considering different cases for Bragg Grating waveguide the results on both photons are extracted and discussed. Moreover, we will show that the physical parameters of the waveguide that the second photon is propagated inside will be effective in minimizing the width of the temporal correlation function. Hence, this study underscores the feasibility of achieving non-local dispersion cancellation through strategic optimization of optical and geometric parameters in the Bragg Grating waveguide, with potential implications for quantum information processing and high-resolution sensing endeavors.

Evanescent Wave Sensitivity of Silica-Titania Rib Waveguides in the Single-Mode Propagation Regime

The analysis reported in this paper shows that the homogeneous sensitivity of both fundamental rib waveguide modes, HE00 and EH00, can slightly exceed the sensitivity of the optimized parent slab waveguide. The most crucial difference in the behavior of these two polarizations is that the sensitivity of the HE00 mode is the maximum for strip waveguides. In contrast, the sensitivity of the EH00 mode can either decrease monotonically or not-monotonically with increasing rib height or behave like a homogeneous sensitivity characteristic of the slab waveguide’s EH0 mode. The second important conclusion comes from comparing the sensitivity characteristics with the distributions of the fundamental mode’s optical power. Namely, the homogeneous sensitivity of the rib waveguide is at the maximum if, due to a slight variance in the cover refractive index, a variation in the weighted optical power carried by the mode is the maximum.

Nonlinear self-action of ultrashort guided exciton–polariton pulses in dielectric slab coupled to 2D semiconductor

Recently reported large values of exciton–polariton nonlinearity of transition metal dichalcogenide (TMD) monolayers coupled to optically resonant structures approach the values characteristic for GaAs-based systems in the regime of strong light-matter coupling. Contrary to the latter, TMD-based polaritonic devices remain operational at ambient conditions and therefore have greater potential for practical nanophotonic applications. Here, we present the study of the nonlinear properties of Ta2O5 slab waveguide coupled to a WSe2 monolayer. We confirm that the hybridization between the waveguide mode and the exciton resonance in WSe2 gives rise to the formation of guided exciton–polaritons with Rabi splitting of 36 meV. By measuring transmission of ultrashort optical pulses through this TMD-based polaritonic waveguide, we demonstrate the strong nonlinear dependence of the output spectrum on the input pulse energy. We develop a theoretical model that shows agreement with the experimental results and gives insights into the dominating microscopic processes which determine the nonlinear pulse self-action: Coulomb exciton–exciton interaction and scattering to an incoherent excitonic reservoir. Based on the numerical simulation of nonlinear phenomena in our polariton system, we conclude that it may support a quasi-stationary solitonic regime of pulse propagation at intermediate pump energies. Our results provide an important step for the development of nonlinear on-chip polaritonic devices based on 2D semiconductors.

Mechanically tuned cholesteric elastomer waveguide slab

In this research, a planar waveguide whose core is a cholesteric liquid crystalline elastomer whose helical axis is perpendicular to its planar boundaries, is studied. The electrodynamics equations are established, together with the constitutive equations. The mechanical stress is tuned through elastic deformations in the parallel direction along the helix axis, in order to control the propagation parameters. The resulting equations system is solved numerically by assuming the guide to be surrounded either by air or vacuum. The propagation parameter and the ratio between the magnetic and electrical modes are obtained as a function of frequency, which provides the band structure of the aforementioned modes. The amplitudes of the profiles of the electromagnetic fields and the Poynting vector distributions are also sought as a function of position. Finally, the mixing of transverse electric and magnetic modes to generate new eigenmodes and their conditions for propagation within the waveguide, are also studied.

Non-collinear generation of ultra-broadband parametric fluorescence photon pairs using chirped quasi-phase matching slab waveguides.

Many optical quantum applications rely on broadband frequency correlated photon pair sources. We previously reported a scheme for collinear emission of high-efficiency and ultra-broadband photon pairs using chirped quasi-phase matching (QPM) periodically poled stoichiometric lithium tantalate (PPSLT) ridge waveguides. However, collinearly emitted photon pairs cannot be directly adopted for applications that are based on two-photon interference, such as quantum optical coherence tomography (QOCT). In this work, we developed a chirped QPM device with a slab waveguide structure. This device was designed to produce spatially separable (photon pair non-collinear emission) parametric fluorescence photon pairs with an ultra-broadband bandwidth in an extremely efficient manner. Using a non-chirped QPM slab waveguide, we observed a photon pair spectrum with a full-width-at-half-maximum (FWHM) bandwidth of 26 nm. When using a 3% chirped QPM slab waveguide, the FWHM bandwidth of the spectrum increased to 190 nm, and the base-to-base width is 308 nm. We also confirmed a generation efficiency of 2.4×106 pairs/(μW·s) using the non-chirped device, and a efficiency of 8×105 pairs/(μW·s) using the 3% chirped device under non-collinear emission conditions after single-mode fiber coupling. This is, to the best of our knowledge, the first report of frequency correlated photon pairs generation using slab waveguide device as a source. In addition, using slab waveguides as photon pair sources, we performed two-photon interference experiments with the non-chirped device and obtained a Hong-Ou-Mandel (HOM) dip with a FWHM of 7.7 μm and visibility of 98%. When using the 3% chirped device as photon pair source, the HOM measurement gave a 2 μm FWHM dip and 74% visibility.

Photonic Dirac cone and topological transition in a moving dielectric slab

The moving media theory is applied to a photonic confined structure which is a continuous dielectric slab waveguide with the uniaxial anisotropy and without the discrete translational symmetry. The moving effect not only brings about non-reciprocity to the whole photonic band structure in the co-moving and counter-moving directions, but also leads to the topological transition of local degenerate points within the band diagram. We demonstrate through calculation that the type-II Dirac point can be turned into type-I Dirac point when the uniaxial slab is moving over certain speed. Our results provide a new approach to regulate the topology of degeneracy for two-dimensional photonic bands in the continuous translational symmetry condition.

Impact of film thickness in laser-induced periodic structures on amorphous Si films

We report self-organized periodic nanostructures on amorphous silicon thin films by femtosecond laser-induced oxidation. The dependence of structural periodicity on the thickness of silicon films and the substrate materials is investigated. The results reveal that when silicon film is 200 nm, the period of self-organized nanostructures is close to the laser wavelength and is insensitive to the substrates. In contrast, when the silicon film is 50 nm, the period of nanostructures is much shorter than the laser wavelength, and is dependent on the substrates. Furthermore, we demonstrate that, for the thick silicon films, quasi-cylindrical waves dominate the formation of periodic nanostructures, while for the thin silicon films, the formation originates from slab waveguide modes. Finite-difference time-domain method-based numerical simulations support the experimental discoveries.Graphical abstract

Planar Fourier optics for slab waveguides, surface plasmon polaritons, and 2D materials.

Recent experimental work has demonstrated the potential of combining the merits of diffractive and on-chip photonic information processing devices in a single chip by making use of planar (or slab) waveguides. Here, arguments are developed to show that diffraction formulas familiar from 3D Fourier optics can be adapted to 2D under certain mild conditions on the operating speeds of the devices in question. In addition to serving those working in on-chip photonics, this Letter provides analytical tools for the study of surface plasmon polaritons, surface waves, and the optical, acoustic, and crystallographic properties of 2D materials.

Design of a Narrow Band Filter Based on a Photonic Crystal Cavity for CO2 Sensing Application

This paper investigates the use of a miniaturized filter based on a triangular lattice of holes in a photonic crystal (PhC) slab. The plane wave expansion method (PWE) and finite-difference time-domain (FDTD) techniques were utilized to analyze the dispersion and transmission spectrum, as well as the quality factor and free spectral range (FSR) of the filter. A 3D simulation has demonstrated that for the designed filter, an FSR of more than 550 nm and a quality factor of 873 can be attained by adiabatically coupling light from a slab waveguide into a PhC waveguide. This work designs a filter structure that is implemented into the waveguide and is suitable for a fully integrated sensor. The small size of the device provides a strong potential for the realization of large arrays of independent filters on a single chip. The fully integrated character of this filter has further advantages such as reducing power loss in coupling light from sources to filters and also from filters to waveguides. The ease of fabrication is another benefit of completely integrating the filter.

How to suppress radiative losses in high-contrast integrated Bragg gratings

High-contrast slab waveguide Bragg gratings with 1D periodicity are investigated. For specific oblique excitation by semi-guided waves at sufficiently high angles of incidence, the idealized structures do not exhibit any radiative losses, such that reflectance and transmittance for the single port mode add strictly up to one. We consider a series of symmetric, fully and partly etched finite gratings, for parameters found in integrated silicon photonics. These can act as spectral filters with a reasonably flattop response. Apodization can lead to more box shaped reflectance and transmittance spectra. Together with a narrowband Fabry–Perot filter, these configurations are characterized by reflection bands, or transmittance peaks, with widths that span three orders of magnitude.

Simultaneous Dual‐Color Amplified Spontaneous Emission and Lasing from Colloidal Quantum Well Gain Media in their Own Layered Waveguide and Cavity

AbstractMicro/nanoscale semiconductor multicolor lasers offer great potential for enhanced‐performance photonic circuits. Colloidal quantum wells (CQWs) are excellent candidates as active materials for these platforms owing to their superior properties including suppressed Auger recombination and large absorption cross‐section. In this work, multicolor optical gain and lasing from the heterostructures of CQWs as the gain media in their own all‐solution processed optical cavity are proposed and demonstrated for the first time. Here, using a simple waveguide slab consisting of the thin films of green‐emitting CdSeS/Cd0.1Zn0.9S core/hot‐injection‐shell grown CQWs and red‐emitting CdSe/CdS@CdZnS core/crown@shell CQWs, a transparent low refractive index colloidal spacing layer of silica nanoparticles (NPs) is devised that critically suppresses otherwise detrimental nonradiative energy transfer between the green and red‐emitting CQWs. This multilayer configuration is key to enabling simultaneous amplified spontaneous emission behavior in two colors with low threshold levels. This layered architecture is further adapted to a whispering‐gallery‐mode cavity by fabricating a microdisk pattern directly out of these CQWs‐NPs‐CQWs colloids. The resulting device exhibits dual‐color multimode lasing both at 569 and 648 nm at the same time. This unique multicolor lasing layered architecture holds great promise for on‐chip photonic applications such as dual‐color biological imaging.

Exotic light transition between superluminal and subluminal group velocity in a [formula omitted] coupled slab waveguide

We propose a rigorous full-wave modal analysis based on the eigenmodes approach (“supermodes”), that allows the calculation of the transverse electric (TE) and transverse magnetic (TM) stationary propagation eigenmodes in a coupled PT-symmetric slab waveguide. Our findings show that the group velocity, for the fundamental TE0 supermode, passes from superluminal to subluminal speed over both sides of a critical point (CP), located near to the exceptional point (EP). We also demonstrate that the first higher-order supermode, TE1, exhibits a slow light phenomenon around the EP.

Highly sensitive terahertz fingerprint sensing based on the quasi-guided modes in a distorted photonic lattice.

Using photonic structures resonating at the characteristic absorption frequency of the target molecules is a widely-adopted approach to enhance the absorption and improve the sensitivity in many spectral regions. Unfortunately, the requirement of accurate spectral matching poses a big challenge for the structure fabrication, while active tuning of the resonance for a given structure using external means like the electric gating significantly complicates the system. In this work, we propose to circumvent the problem by making use of quasi-guided modes which feature both ultra-high Q factors and wavevector-dependent resonances over a large operating bandwidth. These modes are supported in a distorted photonic lattice, whose band structure is formed above the light line due to the band-folding effect. The advantage and flexibility of this scheme in terahertz sensing are elucidated and exemplified by using a compound grating structure on a silicon slab waveguide to achieve the detection of a nanometer scale α-lactose film. The spectral matching between the leaky resonance and the α-lactose absorption frequency at 529.2 GHz by changing the incident angle is demonstrated using a flawed structure which exhibits a detuned resonance at normal incidence. Based on the high dependence of the transmittance at the resonance on the thickness of α-lactose, our results show it is possible to achieve an exclusive detection of α-lactose with the effective sensing of thickness as small as 0.5 nm.

All-Silicon Photoelectric Biosensor on Chip Based on Silicon Nitride Waveguide with Low Loss.

Compared to the widely used compound semiconductor photoelectric sensors, all-silicon photoelectric sensors have the advantage of easy mass production because they are compatible with the complementary metal-oxide-semiconductor (CMOS) fabrication technique. In this paper, we propose an all-silicon photoelectric biosensor with a simple process and that is integrated, miniature, and with low loss. This biosensor is based on monolithic integration technology, and its light source is a PN junction cascaded polysilicon nanostructure. The detection device utilizes a simple refractive index sensing method. According to our simulation, when the refractive index of the detected material is more than 1.52, evanescent wave intensity decreases with the growth of the refractive index. Thus, refractive index sensing can be achieved. Moreover, it was also shown that, compared to a slab waveguide, the embedded waveguide designed in this paper has a lower loss. With these features, our all-silicon photoelectric biosensor (ASPB) demonstrates its potential in the application of handheld biosensors.

Chiral hybrid waveguide-plasmon resonances.

We investigate the chiroptical responses of the hybrid systems consisting of metal-insulator-metal (MIM) gammadion arrays on top of a dielectric slab waveguide. We demonstrate that both the transverse magnetic (TM) and transverse electric (TE) waveguide modes could be coupled to the antisymmetric localized surface plasmon resonances (LSPRs) of the individual MIM-gammadions, leading to the formation of narrow hybrid waveguide-plasmon resonances (WPRs), of which the TM-WPR is less dependent while the TE-WPR is highly dependent on the handedness of the incident light. Associated with the excitation of the TE-WPRs, strong negative and positive circular dichroism (CD) peaks with high quality factors could be obtained on the short-wavelength and long-wavelength side of the LSPRs of the MIM-gammadion, respectively. Moreover, we show that the variation on either the lattice period or slab waveguide thickness allows for easily tuning the TE-WPRs based CD peaks over a relative wide spectral range. Our proposed hybrid system provides tunable and strong CD responses with narrow linewidth, which may have applications in chiral selective imaging, chiral plasmonic bio-sensing and spectroscopy.

Polariton lasing in AlGaN microring with GaN/AlGaN quantum wells

Microcavity polaritons are strongly interacting hybrid light–matter quasiparticles, which are promising for the development of novel light sources and active photonic devices. Here, we report polariton lasing in the UV spectral range in microring resonators based on GaN/AlGaN slab waveguides, with experiments carried out from 4 K up to room temperature. Stimulated polariton relaxation into multiple ring resonator modes is observed, which exhibit threshold-like dependence of the emission intensity with pulse energy. The strong exciton-photon coupling regime is confirmed by the significant reduction of the free spectral range with energy and the blueshift of the exciton–like modes with increasing pulse energy. Importantly, the exciton emission shows no broadening with power, further confirming that lasing is observed at electron–hole densities well below the Mott transition. Overall, our work paves the way toward the development of novel UV devices based on the high-speed slab waveguide polariton geometry operating up to room temperature with the potential to be integrated into complex photonic circuits.

Gain measurements on VCSEL material using segmented contact technique

We report direct measurements of the optical gain on vertical-cavity surface-emitting laser (VCSEL) material using a stripe-length method featuring segmented contacts. We utilise the similarity of the in-plane transverse electric (TE) polarised matrix element and that of the VCSEL lasing mode and a simple method to reduce round trip effects. The confinement factor is determined from cold-cavity simulations of the in-plane TE polarised slab waveguide mode and used to convert the measured in-plane modal gain into the vertical-cavity modal gain, as required for the VCSEL structure. This gives a threshold material gain of 1440 ± 140 cm−1 at 30 °C for this structure. A comparison with the threshold material gain values determined from the lasing condition, where internal optical losses due to doping induced absorption is included using parameters taken from the literature, indicates the presence of an additional source of optical loss in the experiment which increases the threshold material gain by ∼450 cm−1. A best fit is obtained by increasing the optical loss in the n-DBR (distributed Bragg reflectors) layers to 40 cm−1, which is consistent with previous work on additional scattering losses due to interface roughening in the n-DBR layers. To further demonstrate the utility of this method for rapid optimisation, the gain-peak wavelength is measured directly, and its temperature dependence is compared to the lasing wavelength.

Predicting Fundamental Transverse Electric Mode of Slab Waveguide Based on Physics-Informed Neural Networks

Over the past few years, deep learning has proven to be an effective and fast tool in many areas, especially in the field of photonics. The design of integrated optical devices relies on optical waveguides which requires a reliable and fast methods for determining the waveguide’s characteristics before the fabrication process. In this work, the newly emerging paradigm of physics-informed neural networks (PINNs) is employed for analyzing and predicting the fundamental transverse electric (TE) mode and effective refractive index ( ) of a slab waveguide. PINNs is particularly useful as it is a data and mesh-free method, which solve the most critical problems of computational modeling which are the speed and computational hardware cost. The suggested model has a prediction accuracy of up to 99% and effective refractive index relative error ranges between 10-5 and 10-6. Model results are validated against finite difference time domain method using Lumerical software and variational method.

Parametric optical rectification due to the near-field interaction between nanosized metallic domains.

In this paper, we study parametric optical rectification that is not due to material properties but emerges from the electrostatic near-field interaction between nanosized metallic domains. The ability to demonstrate this effect comes from samples based on a unique slab waveguide with deeply buried nanometer-thin metallic layers. These samples intensify the presumed rectification mechanism while suppressing competing effects. We describe three experiments that, combined, indicate a non-material-based nonlinear mechanism in our samples. The origin of the nonlinear mechanism responsible for rectification is elucidated by invoking a toy model whose sole nonlinearity comes from the interaction between strictly linear oscillators.

Quasi-Guided Modes Supported by a Composite Grating Structure with Alternating Element Widths

The efficiency of many optical processes is significantly dependent on the magnitude of the electric field. In this context, many artificially made resonating structures have been investigated to enhance light–matter interactions and facilitate the creation of practical applications. While metal–based terahertz metamaterials have been extensively investigated for this purpose, their performances are mainly limited by the poor confinement of terahertz waves on metal surfaces, exhibiting low resonance quality factors. In this work, we propose and investigate a simple yet novel scheme of enhancing wave–matter interactions in the terahertz region by exploiting the phenomenon of quasi–guided modes. The quasi-guided modes with ultra–high quality factors and huge local field enhancement can be achieved by manipulating the guided modes supported by a slab waveguide. The guided modes with the dispersion lines below the light line have infinite Q factors and can not be accessed from external space. By using a new type of composite grating composed of two ridge grating arrays with alternating ridge widths, the grating period is doubled, leading to a folding of the first Brillouin Zone and the flipping of the dispersion lines to be above the light line. Then, the guided modes will be transitioned into new quasi–guided modes with the possibility of free–space excitation while the Q factors are determined by the level of period–doubling perturbation. The presented results of realizing quasi–guided modes can be extended to other structures, providing a novel means of manipulating light–matter interactions.