Telecommunication Wavelengths Research Articles

Third-Order Optical Nonlinearities of 2D Materials at Telecommunications Wavelengths

All-optical signal processing based on nonlinear optical devices is promising for ultrafast information processing in optical communication systems. Recent advances in two-dimensional (2D) layered materials with unique structures and distinctive properties have opened up new avenues for nonlinear optics and the fabrication of related devices with high performance. This paper reviews the recent advances in research on third-order optical nonlinearities of 2D materials, focusing on all-optical processing applications in the optical telecommunications band near 1550 nm. First, we provide an overview of the material properties of different 2D materials. Next, we review different methods for characterizing the third-order optical nonlinearities of 2D materials, including the Z-scan technique, third-harmonic generation (THG) measurement, and hybrid device characterization, together with a summary of the measured n2 values in the telecommunications band. Finally, the current challenges and future perspectives are discussed.

Low Loss Vertical TiO2/Polymer Hybrid Nano-Waveguides.

This article proposes a novel demonstration of a low-loss polymer channel hybridized with a titania core leading to a nano-waveguide elongated in the normal direction to the substrate. It is aimed at using the quasi-transverse magnetic (TM) mode as the predominant mode in compact photonic circuitry. A detailed design analysis shows how a thin layer of a higher-refractive index material in a trench within the core of the waveguide can increase the confinement and reduce the propagation losses. This thin layer, produced by atomic layer deposition, covers the entire polymer structure in a conformal manner, ensuring both a reduction of the surface roughness and a stronger field confinement. The trench can be made at any place within the polymer channel and therefore its position can be tuned to obtain asymmetric modal distribution. The waveguide is demonstrated at telecom wavelengths, although the material's properties enable operation over a large part of the electromagnetic spectrum. We measured propagation losses as low as 1.75 ± 0.32 dB/cm in a 200 nm × 900 nm section of the waveguide core. All processes being mass-production compatible, this study opens a path towards easier integrated-component manufacture.

High quality factor resonant metasurface with etchless lithium niobate

Actively tunable metasurfaces have widespread applications in optics from light modulation, beam steering to LiDAR and many others. Recently, lithium niobate (LN) based metasurfaces have attracted significant attention for their great potentials in integrated electro-optically tunable metadevices. However, the hardness and chemical inactivity of LN make it difficult to process, which severely limits the quality (Q) factors and performance of LN based metasurfaces. In this paper, a type of high Q etchless LN resonant metasurface is experimentally demonstrated. The metasurface shows a measured Q factor of 563 around the telecom wavelength and even higher Q factors are expected to be realized using similar designs in the future. This work represents an important progress for LN resonant metasurfaces and paves the way for the development of high performance electro-optically tunable LN metadevices.

Pulsed Four-Wave Mixing at Telecom Wavelengths in Si3N4 Waveguides Locally Covered by Graphene.

Recently, the nonlinear optical response of graphene has been widely investigated, as has the integration of this 2D material onto dielectric waveguides so as to enhance the various nonlinear phenomena that underpin all-optical signal processing applications at telecom wavelengths. However, a great disparity continues to exist from these experimental reports, depending on the used conditions or the hybrid devices under test. Most importantly, hybrid graphene-based waveguides were tested under relatively low powers, and/or combined with waveguide materials that already exhibited a nonnegligible nonlinear contribution, thereby limiting the practical use of graphene for nonlinear applications. Here, we experimentally investigate the nonlinear response of Si3N4 waveguides that are locally covered by submillimeter-long graphene patches by means of pulsed degenerate four-wave mixing at telecom wavelength under 7 W peak powers. Our measurements and comparison with simulations allow us to estimate a local change of the nonlinearity sign as well as a moderate increase of the nonlinear waveguide parameter (γ∼-10 m-1W-1) provided by graphene. Our analysis also clarifies the tradeoff associated with the loss penalty and nonlinear benefit afforded by graphene patches integrated onto passive photonic circuits, thereby providing some guidelines for the design of hybrid integrated nonlinear devices, coated with graphene, or, more generally, any other 2D material.

Dual-band electro-optic modulator based on tunable broadband metamaterial absorber

Tunable metamaterials unfolded new scopes for active control of light absorption by engineering meta-atoms. In this study, for the first time, we propose a broadband electrically-tunable metamaterial absorber that can simultaneously modulate reflected light amplitude in both the key telecommunication wavelength bands, i.e., O-band and C-band. We leverage gap plasmon resonance (GPR) of a metal–insulator–metal (MIM) structure and epsilon-near-zero (ENZ) phenomenon of indium-tin-oxide (ITO) to enable real-time control of reflected light amplitude. The proposed metamaterial consists of an array of Au-nanograting on top of Al2O3–ITO–SiO2–Au stack. The coupling of GPR-mode with ENZ-mode is observed when an external bias-voltage is applied to accumulate free-carriers at Al2O3–ITO interface. This results in a broadening of resonance with more than 80 % absorption over a 370 nm spectral window ranging from 1.25 μm to 1.62 μm. Such dynamic control of absorption properties is then utilized for amplitude modulation of the reflected light. We achieved amplitude modulation with an extinction ratio as high as 20.5 dB and 16.7 dB at 1.55 μm and 1.31 μm of wavelength, respectively, while switching between 0 V and 5 V biasing conditions. Thus, the tunable metamaterial absorber could help realizing optical modulators operating at different wavelength bands for integrated nanophotonic systems.

Quasi-BIC Modes in All-Dielectric Slotted Nanoantennas for Enhanced Er3+ Emission.

In the quest for new and increasingly efficient photon sources, the engineering of the photonic environment at the subwavelength scale is fundamental for controlling the properties of quantum emitters. A high refractive index particle can be exploited to enhance the optical properties of nearby emitters without decreasing their quantum efficiency, but the relatively modest Q-factors (Q ∼ 5-10) limit the local density of optical states (LDOS) amplification achievable. On the other hand, ultrahigh Q-factors (up to Q ∼ 109) have been reported for quasi-BIC modes in all-dielectric nanostructures. In the present work, we demonstrate that the combination of quasi-BIC modes with high spectral confinement and nanogaps with spacial confinement in silicon slotted nanoantennas lead to a significant boosting of the electromagnetic LDOS in the optically active region of the nanoantenna array. We observe an enhancement of up to 3 orders of magnitude in the photoluminescence intensity and 2 orders of magnitude in the decay rate of the Er3+ emission at room temperature and telecom wavelengths. Moreover, the nanoantenna directivity is increased, proving that strong beaming effects can be obtained when the emitted radiation couples to the high Q-factor modes. Finally, via tuning the nanoanntenna aspect ratio, a selective control of the Er3+ electric and magnetic radiative transitions can be obtained, keeping the quantum efficiency almost unitary.

Ultrafast quantum key distribution using fully parallelized quantum channels.

The field of quantum information processing offers secure communication protected by the laws of quantum mechanics and is on the verge of finding wider application for the information transfer of sensitive data. To improve cost-efficiency, extensive research is being carried out on the various components required for high data throughput using quantum key distribution (QKD). Aiming for an application-oriented solution, we report the realization of a multichannel QKD system for plug-and-play high-bandwidth secure communication at telecom wavelengths. We designed a rack-sized multichannel superconducting nanowire single photon detector (SNSPD) system, as well as a highly parallelized time-correlated single photon counting (TCSPC) unit. Our system is linked to an FPGA-controlled QKD evaluation setup for continuous operation, allowing us to achieve high secret key rates using a coherent-one-way protocol.

Thermal tunable silicon valley photonic crystal ring resonators at the telecommunication wavelength.

Tunable ring resonators are essential devices in integrated circuits. Compared to conventional ring resonators, valley photonic crystal (VPC) ring resonators have a compact design and high quality factor (Q-factor), attracting broad attention. However, tunable VPC ring resonators haven't been demonstrated. Here we theoretically demonstrate the first tunable VPC ring resonator in the telecommunication wavelength region, the resonance peaks of which are tuned by controlling the temperature based on the thermal-optic effect of silicon. The design is ultracompact (12.05 µm by 10.44 µm), with a high Q-factor of 1281.00. By tuning the temperature from 100 K to 750 K, the phase modulation can reach 7.70 π, and the adjustment efficiency is 0.062 nm/K. Since thermal tuning has been broadly applied in silicon photonics, our design can be readily applied in integrated photonic circuits and will find broad applications. Furthermore, our work opens new possibilities and deepens the understanding of designing novel tunable VPC photonic devices.

Strong Er3+ radiative emission enhancement by quasi-BIC modes coupling in all-dielectric slot nanoantenna arrays

We have designed and realized all-dielectric lossless nanoantennas, in which a thin SiO2 layer doped with erbium ions is placed inside slotted silicon nanopillars arranged in a square array. The modal analysis has evi-denced that the slotted nanopillar array supports optical quasi-BIC resonances with ultra-high Q-factors (up to Q∼109), able to boost the electromagnetic local density of optical states in the optically active layer. Up to 3 orders of magnitude photoluminescence intensity increment and 2 orders of magnitude decay rate enhancement have been measured at room temperature when the Er3+ emission at about λ=1540 nm couples with the quasi-BIC resonances. Furthermore, by tailoring the nanopillar aspect ratio, the slot geometry has been exploited to obtain selective enhancements of the electric or magnetic dipole contribution to Er3+ radiative transitions in the NIR, keeping the emitter quantum efficiency almost unitary. Finally, by computing the angularly resolved elec-tromagnetic field enhancement inside the nanoslot, the nanoantenna directivity has been designed, proving that strong beaming effects can be obtained. Our findings have a direct impact on the development of bright and effi-cient photon sources operating at telecom wavelength that are of primary importance for quantum nanophotonic applications.

Broadband second harmonic generation by birefringent phase matching in an X-cut LiNbO3 membrane

We designed an X-cut lithium niobate (LiNbO3) membrane dedicated to type I second harmonic generation (SHG) at telecom wavelength. A competitive conversion efficiency compared to a quasi-phase-matched configuration with the advantage of a broadband response of 100nm is shown.

A waveguide-integrated self-powered van der Waals heterostructure photodetector with high performance at the telecom wavelength.

Two-dimensional (2D) materials are attractive candidates for high-performance photodetectors due to their wide operating wavelength and potential to integrate with silicon photonics. However, due to their limited atomic thickness and short carrier lifetime, they suffer from high driving source-drain voltages, weak light-matter interactions and low carrier collection efficiency. Here, we present a high-performance van der Waals (vdWs) heterostructure-based photodetector integrated on a silicon nitride photonic platform combining p-type black phosphorus (BP) and n-type molybdenum disulfide (MoS2). Owing to the efficient carrier separation process and dark current suppression at the junction interface of the vdWs heterostructure, high photodetectivity and a fast response speed can be achieved. A fast response time (∼2.08/3.54 μs), high responsivity (11.26 mA W-1), and a high light on/off ratio (104) operating in the near-infrared telecom band are obtained at zero bias. Our research highlights the great potential of the high-efficiency waveguide-integrated vdWs heterojunction photodetector for integrated optoelectronic systems, such as high-data-rate interconnects operated at standardized telecom wavelengths.

Spatially entangled states of light in nonlinear waveguide arrays - INVITED

We demonstrate a nonlinear AlGaAs photonic chip generating biphotons with nonclassical spatial correlations. Photon pairs are generated by parametric down conversion in a waveguide array and simultaneously spread through quantum walks along the various waveguides. This concept implements a compact and versatile source of spatially entangled states, operating at room temperature and telecom wavelength, that could serve as a workbench for simulating condensed matter problems on-chip.

Broadband Light Generation in Nonlinear Silicon Nitride Strip-Loaded Lithium Niobate Waveguides

This work demonstrates SiN strip-loaded lithium niobate waveguides with nonlinear optical properties, focusing on their performance when pumped at telecom wavelength. Experimental results show second and third harmonic generation in periodically poled SiN/LiNbO3. Furthermore, simulations reveal that these waveguides can be dispersion engineered to generate supercontinuum. The findings highlight the potential of SiN strip-loaded lithium niobate platform in sustaining broadband nonlinear light sources.

Third-order Optical Nonlinearities of 2D Materials at Telecommunications Wavelengths in the 1550nm Band

Third-order Optical Nonlinearities of 2D Materials at Telecommunications Wavelengths in the 1550nm Band

Спектроскопия и фотонное эхо на переходе Er-=SUP=-3+-=/SUP=- с малым неоднородным уширением и телекоммуникационной длиной волны в кристалле YPO-=SUB=-4-=/SUB=-

The results of investigations of Er3+ ions at an optical transition with a telecommunication wavelength (λ~1530 nm) in a YPO4 crystal by using photon echo and high-resolution laser spectroscopy in magnetic fields up to 4 T are presented. The maximum coherence time (T2) was 113 μs in a magnetic field of 4 T when it is oriented along the optical axis c of the crystal. The main sources of decoherence are discussed.

Systematic study of InP/InGaAsP heated plasma etching and roughness improvement for integrated optical devices

Indium Phosphide (InP) is one of the most widely commercialized III–V semiconductor materials for making efficient lasers operating in the O-band and C-band. It is also gaining significant attention as a material platform for passive integrated optical devices operating in the telecommunication wavelength range for optical communication networks and sensing. Fabrication of such devices requires a process of lithography for pattern writing followed by plasma etching for transferring the pattern into the semiconductor material. InP is one of the most difficult materials to etch due to the fact that the etching by-products (InClx) are not volatile at temperatures less than 150 °C. There have been some studies showing InP etching at lower temperatures and room temperatures. However, after carefully studying these processes using multiple plasma etching tools, we found that the claimed processes are not repeatable because of the low volatility of the by-products at room temperature. In this work, we demonstrate a systematic study of etching InP using methane-hydrogen-based chemistry at low temperatures (60 °C) and chlorine-based chemistry at high temperatures (190 °C).

Supercontinuum generation in ICP-CVD silicon-rich silicon nitride waveguides

Inductively coupled plasma chemical vapor deposition was used to obtain thin films of siliconrich silicon nitride with a refractive index of 2.44 at optical telecommunications wavelength. The resulting layer was patterned into a 1.6 μm wide waveguide and tested for its nonlinear behavior using a 90-fs all-fiber laser centered at 1630 nm. A significant spectral broadening is demonstrated with a supercontinuum generation from 1300 nm to 1985 nm. Simulations are in fair agreement with the experiments, assuming a nonlinear index of 2 x 10-18 m2/W.

InP- and GaAs-Based Photonic Power Converters Under O-Band Laser Illumination: Performance Analysis and Comparison

Photonic power converters (PPCs), which convert narrow-band light to electricity, are essential components in power-by-light systems. When designed for telecommunications wavelengths such as the O-band, near 1310 nm, the devices are well-suited to power-over-fiber applications. Despite the potential for very high power conversion efficiencies ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf {&gt;50\%}$</tex-math></inline-formula> ), PPCs can be adversely affected by high-intensity nonuniform illumination conditions. In this work, we characterized two O-band PPC designs based on: high-quality InGaAsP absorber material lattice-matched to an InP substrate, and metamorphic InGaAs absorber material lattice-mismatched to a GaAs substrate, a more cost-effective and scalable alternative. We measured each device under O-band laser illumination with five beam profiles having peak-to-average ratios ranging from 2 to 11. Both devices were insensitive to the beam uniformity for input illumination with average irradiance below 2 W/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf {^{2}}$</tex-math></inline-formula> over their 5.4-mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf {^{2}}$</tex-math></inline-formula> active areas, but exhibited better open-circuit voltages under larger, more uniform illumination profiles at higher incident powers. Measured efficiencies reached 52.8% and 48.7% for the lattice-matched and mismatched devices, respectively. Distributed circuit modeling results suggested that both lateral conduction losses and localized heating effects were responsible for the measured dependence on beam-size. Our work demonstrates the potential for O-band PPCs, presenting two highly efficient designs suitable for powering devices requiring <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf {\lesssim 250}$</tex-math></inline-formula> mW, with an appropriate illumination profile.

Fabrication of an erbium–ytterbium-doped waveguide amplifier at communication wavelengths for integrated optics applications

Erbium–ytterbium-doped waveguide amplifiers provide a considerable gain at telecom wavelengths, low noise, nonlinearity, and compatibility with optical networks, making it an outstanding amplification module for telecommunication systems. This study reports on the fabrication of an optical waveguide amplifier for integrated optics. The signal can be amplified by using rare-earth dopings such as erbium (Er), which works at telecommunication wavelengths, i.e., 1.55 μm. Er-doped phosphate glass waveguides can be deposited using the sol–gel method, which is convenient for preparing active films on several substrates. The Er concentration was 1–2 × 1020/cm3. The confinement and the gain of the waveguide can be increased by reducing the width of the waveguide to 0.5 μm. In such a case, more than 1dB net gain can be achieved without additional pump power. The other material used as a dopant in optical amplifiers is ytterbium (Yb). For Er energy levels, a more significant pump intensity is necessary for inversion due to the limited absorption cross-section. This issue is solved by including a substance with a large absorption cross-section that transfers energy to Er. The Ag–Na ion exchange process is then used to fabricate the buried waveguide. In such a process, ions trade between the core material and the molten salt. Then, the waveguide is immersed in the molten salt. The fabricated waveguide has low loss, and a net gain of around 2 dB at a wavelength of approximately 1.55 μm in Er:Yb:Al: phospho silicate glass is achieved. The focus of the research is on the fabrication procedure (materials and methods) of the waveguide.

Heavily Doped Zinc Oxide with Plasma Frequencies in the Telecommunication Wavelength Range

Heavy and hyper doping of ZnO by a combination of gallium (Ga) ion implantation using a focused ion beam (FIB) system and post‐implantation laser annealing is demonstrated. Ion implantation allows for the incorporation of impurities with nearly arbitrary concentrations, and the laser‐annealing process enables dopant activation close to or beyond the solid‐solubility limit of Ga in ZnO. Heavily doped ZnO:Ga with free‐carrier concentrations of ≈1021 cm−3, resulting in a plasma wavelength of 1.02 μm, which is substantially shorter than the telecommunication wavelength of 1.55 μm is demonstrated. Thus, this approach enables the control of the plasma frequency of ZnO from the far infrared down to 1.02 μm, thus, providing a promising plasmonic material for applications in this regime.