Telecommunication Wavelengths Research Articles

Low-voltage-tunable electromechanical photonic directional coupler in silicon nitride for telecom wavelengths.

Silicon nitride is a low-loss photonic integrated circuit (PIC) platform. However, silicon nitride also shows small nonlinear optical properties and is dielectric, which makes the implementation of programmability challenging. Typically, the thermo-optic effect is used for this, but modulators based on this effect are often slow and cross talk-limited. Here, we present a different approach to programmability in silicon nitride photonics. Micro-electromechanical elements are added to a photonic directional coupler, forming two H-shaped structures. The coupling can be changed by applying a voltage to electrodes placed onto the H-structure, which are then attracted by an electrostatic force. These suspended directional couplers show an insertion loss of 0.67 dB and demonstrate switching with 1.1±0.1 µs rise times, representing a valuable addition to the thermal photonic modulators in silicon nitride technology that offer higher modulation speeds while keeping a comparable insertion loss.

Polarization purity and dispersion characteristics of nested antiresonant nodeless hollow-core optical fiber at near- and short-wave-IR wavelengths for quantum communications

Advancements in quantum communication and sensing require improved optical transmission that ensures excellent state purity and reduced losses. While free-space optical communication is often preferred, its use becomes challenging over long distances due to beam divergence, atmospheric absorption, scattering, and turbulence, among other factors. In the case of polarization encoding, traditional silica-core optical fibers, though commonly used, struggle with maintaining state purity due to stress-induced birefringence. Hollow core fibers, and in particular nested antiresonant nodeless fibers (NANF), have recently been shown to possess unparalleled polarization purity with minimal birefringence in the telecom wavelength range using continuous-wave (CW) laser light. Here, we investigate a 1-km NANF designed for wavelengths up to the 2-μm waveband. Our results show a polarization extinction ratio between ~−30 dB and ~−70 dB across the 1520 to 1620 nm range in CW operation, peaking at ~−60 dB at the 2-μm design wavelength. Our study also includes the pulsed regime, providing insights beyond previous CW studies, e.g., on the propagation of broadband quantum states of light in NANF at 2 μm, and corresponding extinction-ratio-limited quantum bit error rates (QBER) for prepare-measure and entanglement-based quantum key distribution (QKD) protocols. Our findings highlight the potential of these fibers in emerging applications such as QKD, pointing towards a new standard in optical quantum technologies.

Highly coherent mid-infrared wideband supercontinuum generation by a silica cladded silicon nitride core buried waveguide

Abstract Optical waveguides with semiconductor cores are drawing considerable research interest in the domain of supercontinuum (SC) generation in recent times. In this work, we design a square-core silicon nitride buried waveguide with a silica-clad, aiming for a wideband spectrum generation in the mid-IR region when operated at the standard telecommunication wavelength of 1550 nm. Among different such silicon nitride square-core buried waveguides, we propose a typical design with dimensions of 400 nm × 400 nm along its height and width, capable of producing a highly coherent broadband intensity spectrum ranging from 810 nm to 5441 nm after propagating through just a few millimeters of the waveguide. The group velocity dispersion maintains minimal value over a broad wavelength range in the mid-IR region, while the nonlinear coefficient is estimated to be sufficiently high. The nonlinear pulse propagation through such a waveguide leads to achieving an SC spanning over 2.76 octaves, sufficiently broader than previously reported silicon nitride-based waveguides. Furthermore, our calculations confirm the highly coherent nature of the generated SC. To the best of our knowledge, this is the first report of SC generation maintaining a high degree of coherence over such a wide wavelength range in the mid-IR zone using a square-core silicon nitride buried waveguide.

Electrically tunable metasurface in TN LC configuration with enhanced quality factor and metaatom-induced alignment.

Dynamic metasurface with subwavelength dimensions has emerged as a key optical technology in recent years. Although various active tuning mechanisms are being proposed, liquid crystal-based dynamic metasurface remains attractive due to its large index variation, electrical tunability, reliability, and mass fabricability. In this work, we report a dynamic metasurface for amplitude modulation in reflection, with twisted nematic liquid crystal configuration to reduce broadband reflection in the off state. The metasurface consists of coupled subwavelength grating fingers, which provide alignment for the liquid crystal without the need for an additional alignment material or process. The alignment of liquid crystal materials was examined between crossed polarizers, and the twist of nematic liquid crystal was confirmed. The coupled grating fingers exhibit a resonance quality factor of 27 at telecommunication wavelength and an amplitude modulation depth of 8 times of the minimum at 1630 nm. This work highlights the potential of liquid crystal-based tunable metasurface, combining polarization control via liquid crystal and spectrum control via metasurface. Furthermore, it also shows a way in which the interaction between liquid crystal and metasurface is used for an alignment layer-free cell assembly process.

Controlling the spontaneous emission of the telecom O-band centers in Silicon-on-Insulator with coherent dipole-quadrupole interactions on a silicon pillar lattice

The G-center in silicon-on-insulator (SOI) has emerged as a relevant single photon source due to the zero-phonon line at 1278 nm, matching the O-band of optical telecommunication wavelengths. Due to the weak emission of the G-center from planar SOI, it is necessary to enhance its emission, in terms of collection efficiency and fluorescence enhancement, to further study its optical properties for application in quantum technologies. Here we design a fabrication-ready SOI Mie-resonator made of a lattice of silicon nanopillars on silica to achieve the spontaneous emission enhancement of single G-centers. Using Si nanopillars lattice on silica with period and dimensions optimized, owing to the coherent superposition of the electric dipolar and magnetic quadrupolar electromagnetic Mie-scattering moments of the individual pillars to the periodic lattice, we show the G-center emission/decay rate enhancement averaged over it's possible dipole orientations is about 13 times compared to bulk. This corresponds to a fluorescence enhancement of about 125 times compared to bulk and about 5-6 times compared to an optimized single pillar with photon collection with a confocal objective. These results provide a fabrication pathway for out-of-plane single photon collection from the SOI G-center or other telecom O-band single-photon sources for their potential deployment in CMOS-compatible quantum optical technologies.

Hole Spin Coherence in InAs/InAlGaAs Self‐Assembled Quantum Dots Emitting at Telecom Wavelengths

Measurements of the longitudinal and transverse spin relaxation times of holes in an ensemble of vertically tunnel‐coupled self‐assembled InAs/InAlGaAs quantum dots (QDs), emitting in the telecom spectral range, are reported. The spin coherence is determined using the spin mode‐locking method in the inhomogeneous ensemble of QDs. Modeling the signal allows us to extract the hole spin coherence time to be in the range of μs. The longitudinal spin relaxation time μs is measured using the spin inertia method. The parameters investigated allow us to suggest that the main relaxation mechanism at low magnetic fields is related to the electron–hole spin exchange.

Single photon detection up to 2 µm in pair of parallel microstrips based on NbRe ultrathin films

Superconducting microstrip single photon detectors (SMSPDs) are increasingly attracting the interest of the scientific community as a new platform for large area detectors with unprecedented advantaged in terms of fabrication. However, while their operativity at the telecommunication wavelength was achieved, working beyond 1.55 µm is challenging. Here, we experimentally demonstrate single-photon operation of NbRe microstrips at wavelengths of 1.55 and 2 µm. The devices are structured as pairs of parallel microstrips with widths ranging from 1.4 to 2.2 μm and lengths from 5 to 10 μm. This innovative design may assure large sensitive areas, without affecting the kinetic inductance, namely the time performance of the detectors. The results are discussed in the framework of the hot-spot two-temperature model.

Engineering quasi-bound states in the continuum in asymmetric waveguide gratings

Abstract The occurrence of quasi-bound states in the continuum (qBIC) in all-dielectric asymmetric grating waveguide couplers with different degrees of asymmetry under normal light incidence is analysed from the viewpoint of identifying the most promising configuration for realizing the highest quality (Q) factor under the condition of utmost efficiency (i.e. total extinction). Considering asymmetric gratings produced by altering every Nth ridge of a conventional (symmetric) grating coupler, we analyse different regimes corresponding to the interplay between diffractive coupling to waveguide modes and band gap effects caused by the Bragg reflection of waveguide modes. The symmetric and double- and triple-period asymmetric grating couplers are considered in detail for the same unperturbed two-mode waveguide and the grating coupler parameters that ensure the occurrence of total transmission extinction at the same wavelengths. It is found that the highest Q is expected for the double-period asymmetric grating, a feature that we explain by the circumstance that the first-order distributed Bragg resonator (DBR) is realized for this configuration while, for other configurations, the second-order DBR comes into play. Experiments conducted at telecom wavelengths for all three cases using thin-film Al2O3-on-MgF2 waveguides and Ge diffraction gratings exhibit the transmission spectra in qualitative agreement with numerical simulations. Since the occurrence of considered qBIC can be analytically predicted, the results obtained may serve as reliable guidelines for intelligent engineering of asymmetric grating waveguide couplers enabling highly resonant, linear and nonlinear, electromagnetic interactions.

Generation of multi-photon Fock states at telecommunication wavelength using picosecond pulsed light

Multi-photon Fock states have diverse applications such as optical quantum information processing. For the implementation of quantum information processing, Fock states should be generated within the telecommunication wavelength band, particularly in the C-band (1530-1565 nm). This is because mature optical communication technologies can be leveraged for transmission, manipulation, and detection. Additionally, to achieve high-speed quantum information processing, Fock states should be generated in optical pulses with as short a duration as possible, as this allows embedding lots of information in the time domain. In this paper, we successfully generate picosecond pulsed multi-photon Fock states (single-photon and two-photon states) in the C-band with Wigner negativities for the first time, which are verified by pulsed homodyne tomography. In our experimental setup, we utilize a single-pixel superconducting nanostrip photon-number-resolving detector (SNSPD), which is expected to facilitate the high-rate generation of various quantum states. This capability stems from the high temporal resolution of SNSPDs (several tens of picoseconds in our case and also in general) allowing us to increase the repetition frequency of pulsed light from the conventional MHz range to the GHz range, although in this experiment the repetition frequency is limited to 10 MHz due to the bandwidth of the homodyne detector. Consequently, our experimental setup is anticipated to serve as a prototype of a high-speed optical quantum state generator for ultrafast quantum information processing at telecommunication wavelength.

Rare Earth‐Diamond Hybrid Structures for Optical Quantum Technologies

AbstractDiamond containing nitrogen‐vacancy centers (NV−) is one of the most investigated materials for quantum technologies, because of this system's exceptional spin properties. Although the NV− optical transition is very bright, it suffers from spectral diffusion and weak zero‐phonon line, and is in the visible range. This limits integration into quantum photonic structures and interfacing with optical fiber networks. In contrast, rare earth (RE) ions exhibit extremely narrow and stable optical transitions, including erbium's 1.5 µm telecom wavelength. Combining RE with NV− properties through short‐range interactions is however challenging as RE do not readily enter the diamond lattice. In this work, a thin‐film‐based architecture in which RE and NV− centers can interact while preserving their unique properties is introduced. Thin films of Er3+:Y2O3 are grown by chemical vapor deposition on diamond substrates with well‐crystallized and highly textured structures. An extensive spectroscopic study of Er3+ transitions at room and low temperatures further reveals that photoluminescence spectra and decays are close to bulk materials. It is also shown that Y2O3 thin film deposition has no detrimental effects on NV− optical and spin properties. RE thin films deposited on diamond can thus be suitable for building hybrid materials for new functionalities in quantum sensing, communication, and processing.

Germanium metasurface near-infrared high-q absorber with symmetry-protected bound states in the continuum

The all-dielectric germanium nanohole (GNH) metasurface with a sub-wavelength thickness supports simultaneous excitation of quasi bound state in the continuum (BIC) and super radiant mode. By selecting the different hole depths in a germanium slab, we present a trade-off metasurface between high Q-factor and high absorption in the photonic system. The presented device demonstrated absorption of super-radiant mode ∼98.5% and quasi-BIC ∼93% without back-metal reflector at the telecommunication wavelength. The numerical results, obtained by the finite difference time domain (FDTD) method are explained in the framework of temporal coupled mode theory (TCMT).

WITHDRAWN: Perfect asymmetric transmission for linear polarization in background reflection-free bilayer dielectric metasurfaces

In spite of the importance of asymmetric transmission (AT) with near-unity value and significantly high contrast of transmission, such a perfect AT has not yet been realized in reciprocal systems. For single resonance, it is already known that the upper limit of AT is identical to the background amplitude reflection coefficient. Nevertheless, if multiple resonances simultaneously contribute to AT, their coherent coupling enables us to obtain perfect AT, even when background reflection completely vanishes. Taking bilayer dielectric metasurfaces composed of orthogonal silicon nanorods embedded in silica glass as examples, here we show that perfect AT can be realized for linear polarization with characteristics comparable with bulky optical isolators based on Faraday effect: AT of 0.9994 with dramatically high contrast 35.1dB at telecom wavelength, the spectral position of which is tunable by properly changing the structural parameters. The analytical model based on the quasinormal-mode expansion well agrees with numerical results, revealing the concurrent contributions of multiple resonances. The presented perfect AT has the advantages of excellent performances of AT and compact structures without bulky devices for field bias or temporal modulations, enabling practical applications for optical computing, high-contrast polarization multiplexing and conversion.

Radical-Mediated Creation of Fluorescent Defects in Carbon Nanotubes

The fluorescent defects in carbon nanotubes have been demonstrated to give exceeding optical properties such as room temperature single photon emissions at telecom wavelengths. A handful of methods for creating these defects have been invented in the past 13 years and provides practical guidelines for obtaining desired functional groups. However, most reactions are performed in the aqueous solution, and the transfer of the single-wall carbon nanotubes (SWCNTs) from aqueous to organic solvents are usually challenging. This gives barriers for applications that require SWCNTs to be dissolved in an organic solvent. Here, we introduce controlled creations of three different fluorescent defects in carbon nanotube hosts in organic solvents using a single radical initiator. We achieved selective creations of benzyl, phenyl, and benzoyloxy defects by controlling the reaction conditions such as dissolved oxygen concentration, reaction temperature, solvents, and addition of radical scavengers. It is worth noting that the both radical initiator and solvent can provide functional groups at the defect sites through radical reaction. We believe that this method will serve as a practical platform for designing new fluorescent defects in the organic phase.

Scanning Near-Field Optical Microscopy: Recent Advances in Disordered and Correlated Disordered Photonics

Disordered and correlated disordered photonic materials have emerged in the past few decades and have been rapidly proposed as a complementary alternative to ordered photonics. These materials have thrived in the field of photonics, revealing the considerable impact of disorder with and without structural correlations on the scattering, transport, and localization of light in matter. Scanning near-field optical microscopy (SNOM) has proven to be a fundamental tool for the study of the interaction between light and matter at the nanoscale in such systems, allowing for the investigation of optical properties and local electromagnetic fields with extremely high spatial resolution, surpassing the diffraction limit of conventional optical microscopy. In this review, the most important and recent advances obtained for disordered and correlated disordered luminescent structures by means of the aperture SNOM technique are addressed, showing how it allows the tailoring of local density of states (LDOS), as well as providing access to statistical analysis for multi-resonance disordered and hyperuniform disordered structures at telecom wavelengths.

Integrated photon pairs source in silicon carbide based on micro-ring resonators for quantum storage at telecom wavelengths

We present the design of an on-chip integrated photon pair source based on Spontaneous Four Wave Mixing (SFWM), implemented on a ring resonator in the 4H Silicon Carbide On Insulator (4H-SiCOI) platform, compatible with a solid state quantum memory in the telecommunications band. Through careful engineering of the waveguide dispersion and micro-ring resonator dimensions, we found solutions where the signal photons are emitted at 1536.48 nm with a bandwidth of ∼150\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 150$$\\end{document} MHz, enabling the interaction with the hyperfine structure of Er3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} ions. Simultaneously, the idler photons are generated at 1563.86 nm, matching the central wavelength of a specific channel in a commercial dense wavelength division multiplexing system. The proposed device fulfill all the spectral requirements in a simple ring-bus coupled waveguide configuration with design parameters within the range of reported values for similar resonators, making feasible its manufacturing with current fabrication capabilities.

Fast topological pumps via quantum metric engineering on photonic chips.

Topological pumps have garnered substantial attention in physics. However, the requirement for slow evolution speed to satisfy adiabaticity greatly restricts their application in on-chip devices. Here, we discover a direct link between adiabaticity and quantum metric, the real part of quantum geometry that has been relatively less explored compared to its imaginary counterpart, the Berry curvature. We demonstrate that the evolution speed of topological pumps between nontrivial edge states can be increased by reducing the quantum metric via introduction of long-range coupling to the celebrated Rice-Mele model. This fast topological pump can occur without affecting the bulk state evolution, which challenges the common understanding. We experimentally confirm our findings by using a platform consisting of bilayer integrated silicon waveguides operating at telecommunication wavelengths. Our work provides possibilities for lifting topological pumps from the constraints of slow evolution and paves the way toward compact photonic integration.

Topological Laser in Anomalous Quadrupole Topological Phases

AbstractTopological photonics shows considerable promise in revolutionizing photonic devices through the use of topological phases, leading to innovations like topological lasers that enhance light control. One of the recent breakthroughs is reducing the size of these systems by utilizing lower‐dimensional boundary states, notably via higher‐order topological phases. This paper presents the experimental demonstration of a topological laser in an anomalous quadrupole topological phase, an instance of higher‐order phases. To facilitate this, a topological nanocavity with a quality factor near 6000 is engineered through a twisting operation. The topological nature of this system is validated by the calculation of nested Wannier centers and the emergency condition of corner states. The experimental observations reveal the manifestation of corner states and the achievement of single‐mode pulsed laser, driven by an optical gain from multiple quantum wells at telecommunication wavelengths and a temperature of 4 K. A lasing threshold of 23 µW and a cold quality factor of 1500 are deduced through rate equation. This work gives a new potential in the application of topological principles to advance nanophotonic technologies.

Avoiding lateral mode leakage in thin film lithium niobate waveguides for the generation of spectrally pure photons at telecom wavelengths

Photonic integrated optical components, notably straight waveguides, serve as pivotal elements for on-chip generation and manipulation of quantum states of light. In this work, we focus on optimizing waveguides based on lithium niobate on insulator (LNOI) to generate photon pairs at telecom wavelengths using spontaneous parametric down-conversion (SPDC). Specifically, we investigate lateral leakage for all possible SPDC processes involving type 0, type I, and type II phase matching conditions in an X-cut lithium niobate waveguide and provide a recipe to avoid leakage loss for the interacting photons. Furthermore, focusing on type II phase matching, we engineer the waveguide in the single-mode regime such that it also satisfies group index matching for generating spectrally pure single photons with high purity (99.33%). We also address fabrication imperfections of the optimized design and find that the spectral purity of the generated photons is robust to fabrication errors. This work offers guidance for the suitable selection of morphological parameters to obtain lossless, single-mode LNOI waveguides for building linear optical circuits and photon pair generation at telecom wavelengths using desired phase-matching conditions.

Multiphoton-initiated laser writing of semiconductors using nanosecond mid-infrared pulses

The advent of more powerful mid-infrared nanosecond laser sources allows using them as attractive tools in material laser processing. For semiconductor bulk processing, laser intensities must remain modest to avoid detrimental nonlinear propagation and pre-focal plasma screening effects. Accordingly, nanosecond pulses are usually appropriate to locally initiate energy deposition by multiphoton absorption and subsequent modification, hardly achievable with ultrashort pulses. Employing a 2.8-μm 2.5-ns laser source, we explore the possibility to modify silicon and other semiconductors. With interactions initiated by three-photon absorption, we modify and determine the surface and bulk modification thresholds of Si, GaAs, and InP. For Si, compared to previous studies at telecom wavelengths, we report on a reduction of the energy threshold for volume modification with processing depth, and repeatable rear-surface modifications. Compared to Si, we find for GaAs and InP important fluctuations in the modification responses, which we attribute to an absorption mechanism initially triggered by a greater presence of defects. We also study germanium, as this narrower bandgap semiconductor very interesting for mid-infrared electro-optical applications is transparent at the newly introduced wavelength. Despite the ability to modify the surface, the bulk of Ge remains inaccessible in this two-photon regime, mainly attributed to beam aberrations and nonlinearities. Nonetheless, we demonstrate the ability to process Si chips integrating Ge layers. All these results showcase the potential of mid-infrared wavelengths to offer new degrees of flexibility for 3D laser processing technologies turned to silicon photonics and microelectronics packaging.

Efficient Poling‐Free Wavelength Conversion in Thin Film Lithium Niobate Harnessing Bound States in the Continuum

AbstractOver the past two decades, there has been a surge of interest in photonic integrated circuits (PICs) to enable low‐cost industrial mass manufacture of miniaturized optical systems while simultaneously improving performance and energy efficiency following the trajectory of electronics and large‐scale integration. An essential characteristic of any PIC lies in the ability of waveguides to efficiently confine light. The phenomenon of bound states in the continuum (BIC) shows promise in achieving confinement of light for modes that otherwise would suffer from high leakage losses, which can occur in many PIC platforms. In this contribution, the usefulness of operating near a BIC to achieve efficient second harmonic generation of ≈300% W−1 cm−2 in Silicon Nitride (SiN) loaded lithium niobate on insulator (LNOI) waveguides without needing to either pole or etch the lithium niobate is experimentally demonstrated. The guided fundamental mode is phase matched at the telecom wavelength to a higher order mode at the second harmonic wavelength that is engineered as a bound state in the continuum to mitigate leakage losses. This demonstrates the promise of using BIC in weakly confining waveguides for nonlinear optical applications all while avoiding the poling process by engineering waveguide geometry.