Strong Mode Confinement Research Articles

Helical hollow channel waveguide in YAG fabricated by femtosecond laser enhanced wet etching.

Three-dimensional optical waveguides with hollow channels have many advantages, such as strong mode confinement and excellent dispersion control ability. Femtosecond laser enhanced wet etching is widely used to fabricate hollow channel waveguides in transparent dielectric materials. We propose a method for fabricating hollow channel waveguides in YAG using femtosecond laser enhanced wet etching with a simpler fabrication process and shorter etching time compared with the previous work. After 90 h of etching, a series of helical hollow channel waveguides with a length of 5 mm and a radius of 32 µm were successfully fabricated. At a pitch of 3 µm, the waveguide exhibited a loss (including coupling loss and transmission loss) as low as 0.68 dB at 1030 nm. The helical hollow channel waveguide also exhibited exceptional isotropic light confinement capability and remarkable supercontinuum-generating properties. Moreover, helical hollow channel waveguides with a radius of 2 µm were successfully fabricated. According to simulations, waveguides of such size can effectively control dispersion. Our work presents, to our knowledge, a novel approach to fabricating hollow channel waveguides with arbitrary lengths using femtosecond laser-enhanced wet etching.

A low-threshold single-mode higher-order topological photonic crystal nanowire array laser

A higher-order topological photonic crystal GaAs/AlGaAs/GaAs nanowire array laser based on 0D corner modes is proposed and analyzed. The structure is composed of a nanowire trivial photonic crystal sandwiched by two topological photonic crystals. Compared with the conventional deformed honeycomb lattice photonic crystal structure, an additional mode selection mechanism is introduced by adjusting the coupling strength of topological cavities formed at two topological interfaces. By optimizing the structure parameters, a large side-mode suppression ratio of 24 dB and a high-quality factor of 7935 are obtained. Due to the strong mode confinement, lasing is achievable at an ultrasmall diameter of 86 nm, with a high confinement factor of 7.96 and low threshold gain of 230 cm−1. Moreover, the laser exhibits a small beam divergence of 6.6°. This work may pave a way for the development of low-threshold single-mode miniaturized lasers.

Dual-band complementary metamaterial perfect absorber for multispectral molecular sensing.

Metamaterial perfect absorbers (MPAs) show great potential in achieving exceptional sensing performance, particularly in the realm of surface-enhanced infrared absorption (SEIRA) spectroscopy. To this aim, it is highly desirable for the localized hotspots to be readily exposed and accessible to analyte with strong mode confinement to enhance absorption. Here, we propose a quasi-three-dimensional MPA based on cross-shaped coupled complementary plasmonic arrays for highly sensitive refractive index sensing and molecular vibrational sensing. Dual-band perfect absorption can be approached with the two plasmonic resonances corresponding to the electric dipole-like mode of cross antenna array and the magnetic dipole-like mode of cross hole array, respectively. Large portions of the electric field of the hotspots are exposed and concentrated in the gap between the elevated cross antenna and its complementary structure on the substrate, leading to improved sensing sensitivities. An ultrathin polymethyl methacrylate (PMMA) film induces a significant redshift of the magnetic dipole-like mode with an 11.8 nm resonance shift per each nanometer polymer thickness. The value is comparable to the reported sensitivity of single molecule layer sensors. Additionally, the simultaneous detection of the C = O and C-H vibrations of PMMA molecules is enabled with the two plasmonic resonances adjusted by changing the lengths of the two cross branches. Remarkably, the observed mode splitting and anti-crossing behavior imply the strong interaction between plasmonic resonance and molecular vibration. Our dual-band MPA based on coupled complementary plasmonic arrays opens a new avenue for developing highly sensitive sensors for the detection of refractive index and multispectral molecular vibrations.

Superresolution Imaging Using a Tapered Bundle of High-Refractive-Index Optical Fibers

The spatial resolution of an optical fiber bundle is limited by the size of a mode propagating in an individual fiber, which can be as small as a single wavelength approximately $\ensuremath{\lambda}$ for conventional low-to-medium refractive-index optical fibers. High-refractive-index optical fiber bundles have a potential for the subwavelength-resolution imaging due to strong mode confinement in a fiber core, but they suffer from inconvenient image readout from the bundle output end, which is done by energy-inefficient near-field probes. To address this issue, this work introduces a tapered high-refractive-index terahertz (THz) optical fiber bundle. It comprises the subwavelength-diameter sapphire optical fibers, that are stacked tightly in the object plane to sample the near field with a subwavelength resolution. The fibers diverge from the object plane and, thus, stretch the captured near field for its further read out from the output bundle end using conventional diffraction-limited lens. Such a tapered fiber bundle was investigated numerically and then implemented experimentally at $0.33$ THz. The subwavelength resolution of this bundle was confirmed, and varies over its aperture and can be as high as $0.35\ensuremath{\lambda}$. The developed imaging principle allows the $\ensuremath{\simeq}0.5\ensuremath{\lambda}$ Abbe resolution limit of a free-space focusing to be overcome and almost any common diffraction-limited optics for the near-field applications to be adapted.

Hybrid silicon photonics DBR laser based on flip-chip integration of GaSb amplifiers and µm-scale SOI waveguides.

The development of integrated photonics experiences an unprecedented growth dynamic, owing to accelerated penetration to new applications. This leads to new requirements in terms of functionality, with the most obvious feature being the increased need for wavelength versatility. To this end, we demonstrate for the first time the flip-chip integration of a GaSb semiconductor optical amplifier with a silicon photonic circuit, addressing the transition of photonic integration technology towards mid-IR wavelengths. In particular, an on-chip hybrid DBR laser emitting in the 2 µm region with an output power of 6 mW at room temperature is demonstrated. Wavelength locking was achieved employing a grating realized using 3 µm thick silicon-on-insulator (SOI) technology. The SOI waveguides exhibit strong mode confinement and low losses, as well as excellent mode matching with GaSb optoelectronic chips ensuring low loss coupling. These narrow line-width laser diodes with an on-chip extended cavity can generate a continuous-wave output power of more than 1 mW even when operated at an elevated temperature of 45°C. The demonstration opens an attractive perspective for the on-chip silicon photonics integration of GaSb gain chips, enabling the development of PICs in a broad spectral range extending from 1.8 µm to beyond 3 µm.

Spin‐Lasing in Bimodal Quantum Dot Micropillar Cavities

AbstractSpin‐controlled lasers are highly interesting photonic devices and have been shown to provide ultrafast polarization dynamics in excess of 200 GHz. In contrast to conventional semiconductor lasers their temporal properties are not limited by the intensity dynamics, but are governed primarily by the interaction of the spin dynamics with the birefringent mode splitting that determines the polarization oscillation frequency. Another class of modern semiconductor lasers are high‐β emitters, which benefit from enhanced light–matter interaction due to strong mode confinement in low‐mode‐volume microcavities. In such structures, the emission properties can be tailored by the resonator geometry to realize for instance bimodal emission behavior in slightly elliptical micropillar cavities. This attractive feature is utilized to demonstrate and explore spin‐lasing effects in bimodal high‐β quantum dot micropillar lasers. The studied microlasers with a β‐factor of 4% show spin‐laser effects with experimental polarization oscillation frequencies up to 15 GHz and predicted frequencies up to about 100 GHz, which are controlled by the ellipticity of the resonator. These results reveal appealing prospects for very compact, ultrafast, and energy‐efficient spin‐lasers and can pave the way for future purely electrically injected spin‐lasers enabled by short injection path lengths.

Nonlinear nanophotonics based on surface plasmon polaritons

Surface plasmon polaritons (SPPs), elementary excitation of the hybrid states between collective motion of electrons and photons, are associated with strong local field enhancement and deep subwavelength mode confinement. The use of SPPs in nonlinear optics can amplify intrinsically weak nonlinear processes and shrink down the size of nonlinear optic devices to a nanometer scale. In this Perspective, we review the nonlinear optic processes using SPPs in the plasmonic waveguides and foresee their potential in developing compact nonlinear integrated circuits. We discuss the key factors to enhance the conversion efficiency from the plasmonic waveguide, including the spatial overlap between the interacting modes and the nonlinear materials, and the momentum conservation that allows the coherent constructive superposition. Strategies toward this goal include shrinking the effective mode area through adjusting the geometry of the plasmonic waveguide, proper incorporation of the nonlinear susceptibilities to the plasmonic near field, and the use of counter-propagating configurations or phase compensation techniques. We also forecast the future developments of nonlinear plasmonics based on propagating SPPs in active nonlinear devices.

Nanostructured hybrid plasmonic waveguide in a slot structure for high-performance light transmission.

Squeezing light to nanoscale is the most vital capacity of nanophotonic circuits processing on-chip optical signals that allows to significantly enhance light-matter interaction by stimulating various nonlinear optical effects. It is well known that plasmon can offer an unrivaled concentration of optical energy beyond the optical diffraction limit. However, the progress of plasmonic technology is mainly hindered by its ohmic losses, thus leading to the difficulty in building large-area photonic integrated circuits. To significantly increase the propagation distance of light, we develop a new waveguide structure operating at the telecommunication wavelength of 1,550 nm. It consists of a nanostructured hybrid plasmonic waveguide embedded in a high-index-contrast slot waveguide. We capitalize on the strong mode confinement of the slot waveguide and reduce mode areas with the nanostructured hybrid plasmonic configuration while maintaining extremely low ohmic losses using a nanoscale metal strip. The proposed design achieves a record propagation distance of 1,115 µm while comparing with that of other designs at a mode area of the order of 10-5A0 (A0 is the diffraction-limited area). The mode characterization considering fabrication imperfections and spectral responses show the robustness and broadband operation range of the proposed waveguide. Moreover, we also investigated the crosstalk to assess the density of integration. The proposed design paves the way for building nanophotonic circuits and optoelectronic devices that require strong light-matter interaction.

Ultrashort inverted tapered silicon ridge-to-slot waveguide coupler at 1.55 µm and 3.392 µm wavelength.

Herein, a compact and efficient inverted tapered ridge-to-slot waveguide coupler design based on the silicon-on-insulator platform is presented. The proposed device consists of three segments such as ridge waveguide, inverted taper segment, and slot waveguide. The coupling segment resembles a V shape, which provides good mode-matching between the ridge and slot waveguide. Two significant aspects of the proposed coupler design are discussed. In the first part of the paper, the coupler design optimized at 1.55 µm is suggested for optical interconnect. The propagation loss and coupling efficiency of 1.69 dB/µm and 91% are obtained for the 100 nm long tapered segment introduced between the ridge waveguide and slot waveguide, respectively. This propagation loss of the device includes the loss suffered by the ridge waveguide, tapered segment, and slot waveguide. Our proposed device design can be used in integrated optical platforms, where the efficient coupling of light to slot waveguides is required. Whereas, in the second part, the coupler design is optimized at the mid-infrared of 3.392 µm for an evanescent field absorption methane gas sensor. Slot waveguide offers excessive light-matter interaction due to its strong mode confinement in the low index material. The evanescent field ratio of ∼0.73 is obtained for the optimized waveguide geometry. As a result, 3 dB decay in the transmitted power can be obtained at 60% of gas concentration present in the ambient medium.

Numerical simulation of dispersion and nonlinear characteristics of microstructured silica fibres with a thin suspended core in a wide range of their parameters

Dispersion and nonlinear characteristics of microstructured silica fibres with a thin suspended core surrounded by three, four or six air holes have been studied theoretically in the wavelength range 1 – 2 μm. It has been shown that, owing to strong fundamental mode confinement near the core, the Kerr nonlinearity coefficient can exceed the nonlinearity coefficient of standard telecom fibre SMF28e by two orders of magnitude. The large waveguide contribution allows for effective group velocity dispersion management. Estimates are presented that demonstrate the feasibility of using suspended core fibre exhibiting Kerr nonlinearity for generating non-classical light: a state with squeezed quantum fluctuations in one of the quadrature components of a cw laser signal at a wavelength near 1.55 μm.

Mode analysis of a confocal elliptical dielectric nanowire coated with double-layer graphene

Mode properties of a confocal elliptical dielectric nanowire coated with double-layer graphene are analyzed by using the separation of variables method (SVM). The graphene surface plasmon modes could be tuned by changing the working wavelength, the distance between semi-major axes, the Fermi energy level of graphene and the geometry parameters. The results show that the semi-analytical results of SVM are completely consistent with the numerical simulation results of finite element method (FEM). Compared with the elliptical dielectric nanowire coated with single-layer graphene, the novel waveguide structure has stronger mode confinement, lower propagation loss, larger propagation length, wider adjustable working wavelength and wider adjustable Fermi energy level of graphene. The waveguide may be exploited for many applications such as plasmon resonances, detectors and microstrip antennas.

Detection of damage in welded joints using high order feature guided ultrasonic waves

Abstract Topographical features such as welds, stiffeners, and bends in plate-like structures have been identified as special local waveguides, which are able to render ultrasonic wave energy preferentially guided along themselves. Such feature guided waves (FGW) at low frequencies have been reported in the literature as an effective screening tool for long-range features, which, however, can only detect relatively large defects. In this study, the existence of high order FGW modes, categorized into multiple wave families, are revealed via the modal analysis of an unbounded welded plate at high frequencies, by using the semi-analytical finite element (SAFE) approach. These FGWs exhibit strong mode confinement in specific regions of the weld, which can potentially offer enhanced interrogation of the localized area in long welds and provide improved detection sensitivity to small defects. A series of high order shear horizontal (SH) type weld-guided modes are investigated in both three-dimensional finite element (3D FE) simulations and experiments, for detecting axial and transverse cracks of sub-wavelength size in the weld bead. A new superposition technique is proposed to coherently enhance the amplitude of the selected mode in a multi-modal waveform, leading to an attractive single-mode inspection capability at high frequencies. Both small cracks are identified numerically and experimentally in the superposed reflection of the SH 1 -like weld-guided wave, and results are in close agreement. The feasibility of using high order weld-guided modes for inspecting small defects inside the weld has been demonstrated.

Nanoscale C‐Rich SixC1−x Bus/Ring Waveguide Based Cross‐Wavelength Data Converter

AbstractThe carbon‐rich silicon carbide (C‐rich SixC1−x) micro‐ring channel waveguide with asymmetric core aspect is demonstrated for all‐optical cross‐wavelength pulsed return‐to‐zero on‐off keying (PRZ‐OOK) data conversion. Enhanced nonlinear optical Kerr switching enables 12‐Gbit per second data processing with optimized modulation depth. The inverse tapered waveguide at end‐face further enlarges the edge‐coupling efficiency, and the asymmetric channel waveguide distinguishes the polarization modes. To prevent data shape distortion, the bus/ring gap spacing is adjusted to control the quality factor (Q‐factor) of the micro‐ring. Designing the waveguide cross section at 500 × 350 nm2 provides the C‐rich SixC1−x channel waveguide to induce strong transverse electric mode (TE‐mode) confinement with a large Kerr nonlinearity of 2.44 × 10−12 cm2 W−1. Owing to the trade‐off between the Q‐factor and the on/off extinction ratio, the optimized bus/ring gap spacing of 1400 nm is selected to provide a coupling ratio at 5–6% for compromising the modulation depth and the switching throughput. Such a C‐rich SixC1−x micro‐ring with asymmetric channel waveguide greatly enhances the cross‐wavelength data conversion efficiency to favor its on‐chip all‐optical data processing applications for future optoelectronic interconnect circuits.

Design of Room Temperature Electrically Pumped Visible Semiconductor Nanolasers

This paper presents a comprehensive theoretical study of the optical and thermal properties of an electrically pumped semiconductor nanolaser (SNL) having an GaN/(InGaN/GaN MQWs)/GaN core shell structure. Numerical results show that the lasing whispering-gallery mode has a threshold gain of 413 cm $^{-1}$ . Furthermore, it is shown that when it is operated well-above threshold, the device temperature increases by only 22 K above an ambient temperature of 300 K. These promising results are attributed to the strong mode confinement in the active region and the good thermal properties of the material system of the proposed structure. The results presented in this paper offer guidelines for fabrication of electrically pumped room temperature continuous wave SNL operating in the visible spectral region.

Aluminum plasmonic waveguides co-integrated with Si3N4 photonics using CMOS processes

Co-integrating CMOS plasmonics and photonics became the “sweet spot” to hit in order to combine their benefits and allow for volume manufacturing of plasmo-photonic integrated circuits. Plasmonics can naturally interface photonics with electronics while offering strong mode confinement, enabling in this way on-chip data interconnects when tailored to single-mode waveguides, as well as high-sensitivity biosensors when exposing Surface-Plasmon-Polariton (SPP) modes in aqueous environment. Their synergy with low-loss photonics can tolerate the high plasmonic propagation losses in interconnect applications, offering at the same time a powerful portfolio of passive photonic functions towards avoiding the use of bulk optics for SPP excitation and facilitating compact biosensor setups. The co-integration roadmap has to proceed, however, over the utilization of fully CMOS compatible material platforms and manufacturing processes in order to allow for a practical deployment route. Herein, we demonstrate for the first time Aluminum plasmonic waveguides co-integrated with Si3N4 photonics using CMOS manufacturing processes. We validate the data carrying credentials of CMOS plasmonics with 25 Gb/s data traffic and we confirm successful plasmonic propagation in both air and water-cladded waveguide configurations. This platform can potentially fuel the deployment of co-integrated plasmonic and photonic structures using CMOS processes for biosensing and on-chip interconnect applications.

Scattering-loss reduction of ridge waveguides by sidewall polishing.

Ridge waveguides provide a large refractive index contrast and thus strong mode confinement, making them highly attractive for building compact photonic integrated circuits. However, ridge waveguides suffer from scattering losses. We demonstrate scattering-loss reduction of ridge waveguides made of lithium-niobate-on-insulator (LNOI) substrates by more than one order of magnitude. This is achieved by gently polishing of the ridge's sidewalls and simultaneous protection of the top surfaces by a metal layer. Whispering-gallery-resonator loss measurements reveal ultra-low losses down to 0.04 dB/cm of the processed waveguides. Our approach pushes ridge waveguides further towards their fundamental absorption-loss limit, enabling highly efficient integrated devices.

Graphene-coated nanowires with drop-shaped cross section for the low loss propagation of THz waves with sub-micron mode widths

Both the strong mode confinement and the low propagation loss are very important for the design of highly integrated THz devices, but they are often contradictory to each other and difficult to be achieved at the same time. In this paper, we study the inside-dominant graphene surface plasmon polaritons (GSPPs) propagating on the graphene-coated nanowires with drop-shaped cross section using COMSOL multiphysics. We find that the THz GSPPs achieve not only the sub-micron mode width but also the long propagation length. Furthermore, the inside-dominant GSPPs have very high energy utilization efficiency, because almost all the mode power is inside the nanowire. These excellent characteristics make the mode very useful in the nanophotonics, bio-photonics and highly integrated THz circuits.

Nonlinear frequency conversion in one dimensional lithium niobate photonic crystal nanocavities

We demonstrate flexible nonlinear frequency up-conversion in high-Q lithium niobate photonic crystal nanobeam resonators. The high optical Q together with strong optical mode confinement allows us to observe clear second harmonic generation and sum frequency generation with an optical power around only tens of microWatts. These demonstrations show that high-Q lithium niobate photonic crystal nanoresonators are of great promise for nonlinear photonic applications.

Storage and Retrieval of Surface Polaritons

We investigate the memory of surface polariton (SP) via the electromagnetically induced transparency (EIT) of quantum emitters doped at the interface between a dielectric and a metamaterial. We show that, due to the strong mode confinement provided by the interface, the EIT effect can be largely enhanced; furthermore, the storage and retrieval of the SP can be realized by switching off and on of a control laser field; additionally, the efficiency and fidelity of the SP memory can be improved much by using a weak microwave field. The results reported here are helpful not only for enhancing the understanding of SP property but also for promising applications in quantum information processing and transmission.

Near-field spectroscopy and tuning of sub-surface modes in plasmonic terahertz resonators

Highly confined modes in THz plasmonic resonators comprising two metallic elements can enhance light-matter interaction for efficient THz optoelectronic devices. We demonstrate that sub-surface modes in such double-metal resonators can be revealed with an aperture-type near-field probe and THz time-domain spectroscopy despite strong mode confinement in the dielectric spacer. The sub-surface modes couple a fraction of their energy to the resonator surface via surface waves, which we detected with the near-field probe. We investigated two resonator geometries: a λ/2 double-metal patch antenna with a 2 μm thick dielectric spacer, and a three-dimensional meta-atom resonator. THz time-domain spectroscopy analysis of the fields at the resonator surface displays spectral signatures of sub-surface modes. Investigations of strong light-matter coupling in resonators with sub-surface modes therefore can be assisted by the aperture-type THz near-field probes. Furthermore, near-field interaction of the probe with the resonator enables tuning of the resonance frequency for the spacer mode in the antenna geometry from 1.6 to 1.9 THz (~15%).