Telecommunication Wavelength Range Research Articles

Rational design of efficient defect-based quantum emitters

Single-photon emitters are an essential component of quantum networks, and defects or impurities in semiconductors are a promising platform to realize such quantum emitters. Here, we present a model that encapsulates the essential physics of coupling to phonons, which governs the behavior of real single-photon emitters, and critically evaluate several approximations that are commonly utilized. Emission in the telecom wavelength range is highly desirable, but our model shows that nonradiative processes are greatly enhanced at these low photon energies, leading to a decrease in efficiency. Our results suggest that reducing the phonon frequency is a fruitful avenue to enhance the efficiency.

Vanadium in silicon carbide: telecom-ready spin centres with long relaxation lifetimes and hyperfine-resolved optical transitions

Vanadium in silicon carbide (SiC) is emerging as an important candidate system for quantum technology due to its optical transitions in the telecom wavelength range. However, several key characteristics of this defect family including their spin relaxation lifetime (T1), charge state dynamics, and level structure are not fully understood. In this work, we determine the T1 of an ensemble of vanadium defects, demonstrating that it can be greatly enhanced at low temperature. We observe a large spin contrast exceeding 90% and long spin-relaxation times of up to 25 s at 100 mK, and of order 1 s at 1.3 K. These measurements are complemented by a characterization of the ensemble charge state dynamics. The stable electron spin furthermore enables high-resolution characterization of the systems’ hyperfine level structure via two-photon magneto-spectroscopy. The acquired insights point towards high-performance spin-photon interfaces based on vanadium in SiC.

Silicon Nanoantennas for Ultra‐Compact, High‐Speed and Low‐Power Consumption Electro‐Optic Modulators

AbstractTo keep pace with the expanding data capacity in modern communications and increasing on‐chip integration density, it is highly desirable to have electro‐optic modulators featured with compact footprint, large modulation bandwidth, low energy consumption, and full complementary metal‐oxide‐semiconductor (CMOS) compatibility. Here, this work experimentally demonstrates a high‐speed electro‐optic modulator in the telecommunication wavelength range made of a 1D chain resonator of Mie‐resonant silicon nanoparticles. The modulator exploits the plasma dispersion effect and works in the carrier depletion configuration. The resulting large modulation bandwidth over 40 GHz and low energy consumption of 23 fJ bit−1 are attributed to its ultra‐compact footprint of 10 µm2. The open eye diagrams are measured up to 40 Gbits s−1. The modulators are fabricated at wafer level by a silicon photonic foundry using CMOS‐compatible process. This modulator shows promise for achieving higher integration density and larger modulation bandwidth. It can be added to the existing on‐chip device building blocks library and used for a wide range of applications, including data communication, quantum photonics, and non‐reciprocal optics.

Probing molecules in gas cells of subwavelength thickness with high frequency resolution

Miniaturizing and integrating atomic vapor cells is widely investigated for the purposes of fundamental measurements and technological applications such as quantum sensing. Extending such platforms to the realm of molecular physics is a fascinating prospect that paves the way for compact frequency metrology as well as for exploring light-matter interactions with complex quantum objects. Here, we perform molecular rovibrational spectroscopy in a thin-cell of micrometric thickness, comparable to excitation wavelengths. We operate the cell in two distinct regions of the electromagnetic spectrum, probing ν1 + ν3 resonances of acetylene at 1.530 µm, within the telecommunications wavelength range, as well as the ν3 and ν2 resonances of SF6 and NH3 respectively, in the mid-infrared fingerprint region around 10.55 µm. Thin-cell confinement allows linear sub-Doppler transmission spectroscopy due to the coherent Dicke narrowing effect, here demonstrated for molecular rovibrations. Our experiment can find applications extending to the fields of compact molecular frequency references, atmospheric physics or fundamental precision measurements.

Investigation on photonic crystal nanobeam cavity based on mixed diamond–circular holes

A photonic crystal nanobeam cavity (M-PCNC) with a structure incorporating a mixture of diamond-shaped and circular air holes is proposed. The performance of the cavity is simulated and studied theoretically. Using the finite-difference time-domain method, the parameters of the M-PCNC, including cavity thickness and width, lattice constant, and radii and numbers of holes, are optimized, with the quality factor Q and mode volume Vm as performance indicators. Mutual modulation of the lattice constant and hole radius enable the proposed M-PCNC to realize outstanding performance. The optimized cavity possesses a high quality factor Q = 1.45 × 105 and an ultra-small mode volume Vm = 0.01(λ/n) [Zeng et al., Opt Lett 2023:48;3981–3984] in the telecommunications wavelength range. Light can be progressively squeezed in both the propagation direction and the perpendicular in-plane direction by a series of interlocked anti-slots and slots in the diamond-shaped hole structure. Thereby, the energy can be confined within a small mode volume to achieve an ultra-high Q/Vm ratio.

Scalable Implementation of Temporal and Phase Encoding QKD with Phase‐Randomized States

AbstractQuantum key distribution (QKD), that is, exchanging cryptographic keys encoded in quantum particles exploiting the laws of quantum physics, is already a reality in our society. Current implementations are based on attenuated laser technique, a practical replacement of single photons which requires a random phase for each quantum state in order to achieve the highest level of security. In particular, the time‐bin and phase encoding techniques are mainly exploiting laser in gain‐switching modes combined with asymmetric interferometers or multiple laser sources in a master–slave configuration, which present limitations in terms of stability and scalability. In this work, a novel scheme for implementing a reconfigurable and scalable QKD transmitter based on the time‐bin encoding protocol with a decoy‐state method employing phase‐randomized weak coherent states is proposed and demonstrated. The scheme is tested and validated up to 26 dB‐attenuation channel using standard single‐photon detectors working in the telecom wavelength range.

Efficient Near‐Infrared Light‐Emitting Diodes Based on CdHgSe Nanoplatelets

AbstractCadmium mercury selenide (CdHgSe) nanocrystals exhibit a unique combination of low‐energy optical absorption and emission, which can be tuned from the visible to the infrared range through both quantum confinement and adjustment of their composition. Owing to this advantage, such nanocrystals have been studied as a promising narrow‐band infrared light emitter. However, the electroluminescence of CdHgSe‐based nanocrystals has remained largely unexplored, despite their potential for emitting light in the telecom wavelength range. Further benefits to their optical properties are expected from their shape control, in particular the formation of 2D nanocrystals, as well as from a proper design of their heterostructures. In this work, a colloidal synthesis of CdHgSe/ZnCdS core/shell nanoplatelets (NPLs) starting from CdSe template NPLs employing a cation exchange strategy is developed. The heterostructures synthesized exhibit photoluminescence that can be tuned from ≈1300 to 1500 nm. These near‐infrared‐active NPLs are employed in light‐emitting diodes, demonstrating low turn‐on voltage and high external quantum efficiency.

Strain‐Free GaSb Quantum Dots as Single‐Photon Sources in the Telecom S‐Band

AbstractGenerating single photons in the telecommunication wavelength range from semiconductor quantum dots (QDs) and interfacing them with spins of electrons or holes is of high interest in recent years, with research mainly focusing on indium‐based QDs. However, there is not much data on the optical and spin properties of gallium antimonide (GaSb) QDs, despite it being a physically rich system with an indirect to direct bandgap crossover in the telecom wavelength range. This work investigates the (quantum‐) optical properties of GaSb QDs, which are fabricated by filling droplet‐etched nanoholes in an aluminum gallium antimonide (AlGaSb) matrix. Photoluminescence (PL) features from isolated and highly symmetric QDs are observed that exhibit narrow linewidth in the telecom S‐band and show an excitonic fine structure splitting of up to µeV. Moreover, time‐resolved measurements of the decay characteristics of an exciton are performed and the second‐order photon autocorrelation function of the charge complex is measured to , revealing clear antibunching and thus proving the capability of this material platform to generate non‐classical light.

Tailoring the optical properties of polyvinyl alcohol-polyvinyl pyrrolidone based polymers

In this work, new polymeric based films are fabricated and optically characterized. The new films include poly(vinyl alcohol), plain and blended with poly(vinyl pyrrolidone), comprising glycerin, and acetic acid,as additives. XRD and FTIR analysis were conducted to give insight into the structure of the films. The reflectance and transmittance for these films are measured in the UV–vis–NIR regions. Using these measurements, the refractive index, the permittivity and many other optical constants are retrieved. The results reveal that the fabricated films possess an extinction coefficient that is almost 3 times less, and a refractive index that is 0.1 higher than plain PVA in the telecommunications wavelength range. Besides, a noticeable increase in the refractive index of the films is achieved, up to 0.35 higher compared with silicon dioxide at the near infrared spectral region. Moreover, when compared to silicon and Germanium, in the ultraviolet and visible spectral region, a substantial reduction of 3 to 4 orders of magnitudes in extinction coefficient is achieved, in the favor of the investigated films. Interestingly, the films also show small dispersion over a wide range of wavelength. Therefore, the newly proposed films can be candidates for optoelectronics, solar cells, and integrated optics applications where high refractive index and low loss are desired for high density of fabrication.

Unfolding the Hong–Ou–Mandel interference between heralded photons from narrowband twin beams

The Hong–Ou–Mandel (HOM) interference is one of the most intriguing quantum optical phenomena and crucial in performing quantum optical communication and computation tasks. Lately, twin beam emitters such as those relying on the process of parametric down-conversion (PDC) have become confident sources of heralded single photons. However, if the pump power is high enough, the pairs produced via PDC—often called signal and idler—incorporate multiphoton contributions that usually distort the investigated quantum features. Here, we derive the temporal characteristics of the HOM interference between heralded states from two independent narrowband PDC sources. Apart from the PDC multiphoton content, our treatment also takes into account effects arriving from an unbalanced beam splitter ratio and optical losses. We perform a simulation in the telecommunication wavelength range and provide a useful tool for finding the optimal choice for PDC process parameters. Our results offer insight in the properties of narrowband PDC sources and turn useful when driving quantum optical applications with them.

Linear and nonlinear characterization of a broadband integrated Si-rich silicon nitride racetrack ring resonator for on-chip applications.

We demonstrate the linear and nonlinear characterization of a plasma-enhanced chemical vapor deposited silicon-rich silicon nitride (SRSN) racetrack ring resonator for on-chip applications within the telecommunication wavelength range. The SRSN waveguide parameters are optimized by employing the refractive index profile measured by ellipsometry to achieve flat dispersion in the telecom band. Furthermore, we measure the thermo-optic coefficient of the micro-resonator by analyzing the temperature-dependent transmission spectra and assess it to be 3.2825×10-5 ∘ C -1. Additionally, we study power-dependent transmission spectra to investigate the effect of local heating and nonlinear absorption. The power-dependent transmission spectra exhibit a blueshifting of the resonance peak in the visible and near-IR regions, which indicates the presence of nonlinear losses in that range. The power-dependent transmission spectra almost remain unchanged in the telecom band, revealing the absence of nonlinear losses and excellent thermal stability in that wavelength range. Our experimental results reveal that the SRSN-based structure can be employed potentially to realize linear and nonlinear applications in the telecom band.

Fabrication of low-loss III-V Bragg-reflection waveguides for parametric down-conversion

Entangled photon pairs are an important resource for many types of quantum protocols. Semiconductor Bragg-reflection waveguides are a promising photon-pair source due to mature fabrication, integrability, large transparency window in the telecom wavelength range, integration capabilities for electro-optical devices as well as a high second-order nonlinear coefficient. To increase performance, we improved the fabrication of Bragg-reflection waveguides by employing fixed-beam-moving-stage optical lithography, low-pressure, and low chlorine concentration etching, and resist reflow. The reduction in sidewall roughness yields a low optical loss coefficient for telecom wavelength light of αreflow = 0.08 (6) mm−1. Owing to the decreased losses, we achieved a photon-pair production rate of 8800 (300) (mW · s · mm)−1, which is 15-fold higher than in previous samples.

High-efficiency and easy-to-fabricate integrated plasmonic grating couplers for the telecommunication wavelength range

The paper describes results of the numerical simulation and experimental characterization of plasmonic gratings and waveguides in the optical range of 1520 – 1630 nm. Gold structures were coated with a 630-nm thick tetraethylorthosilicate layer to achieve higher coupling efficiency for plasmonic gratings using lensed optical probes. We experimentally demonstrated gold input/output plasmonic gratings with an efficiency of about – 3.8 ± 0.2 dB per grating. The measured loss of the plasmonic mode propagating over the surface of the gold waveguide was 0.14 ± 0.02 dB / μm. The work was carried out with the aim of studying integrated plasmonic structures for the development of a hybrid photonic-plasmonic integrated technology.

Temperature Tunable 4D Polymeric Photonic Crystals

AbstractPhotonic crystals owe their multitude of optical properties to the way their structure creates interference effects. It is therefore possible to influence the photonic response by acting on their physical structure. In this article, tunable photonic crystals made by elastic polymers that respond to their environment are explored, in particular with a physical deformation under temperature variation. This creates a feedback process in which the photonic response depends on its physical structure, which itself is influenced by the environmental changes. By using a multi‐photon polymerization process specifically optimized for soft responsive polymers such as Liquid Crystalline Networks, highly resolved, reproducible, and mechanically self‐standing photonic crystals are fabricated. The physical structure of the 3D woodpile can be tuned by an external temperature variation creating a reversible spectral tuning of 50 nm in the telecom wavelength range. By comparing these results with finite element calculations and temperature induced shape‐change, it is confirmed that the observed tuning is due to an elastic deformation of the structure. The achieved nanometric patterning of tunable anisotropic photonic materials will further foster the development of reconfigurable photonic crystals with point defects acting as tunable resonant cavities and, more in general, of 4D nanostructures.

Active Erbium‐Doped Silicon Nanoantenna

AbstractThe development of silicon‐based radiative nanostructures is an important step toward silicon‐based optoelectronic systems. Here an approach for the fabrication of photoluminescent Er‐doped silicon nanoantennas is demonstrated. The combination of electron beam lithography and laser annealing of a thin Er film deposited after fabrication creates an active dielectric nanoantenna operating in the standard telecommunication wavelength range (C‐band). Using the multipole decomposition method the geometrical parameters of the silicon nanoparticle when the first‐order Mie‐resonances are tuned to the erbium emission in C‐band range ( transition) is calculated. It is observed experimentally that the presence of the Mie‐resonances provides enhancement of the photoluminescence intensity up to 40% compared to the non‐resonant case. The proposed approach allows for the creation of computational optical chips compatible with existing nanofabrication technologies.

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).

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.

Mid-infrared timing jitter of superconducting nanowire single-photon detectors

Detector timing jitter is a key parameter in advanced photon counting applications. Superconducting nanowire single-photon detectors offer the fastest timing jitter in the visible to telecom wavelength range and have demonstrated single-photon sensitivity in the mid-infrared spectral region. Here, we report on timing jitter in a NbTiN nanowire device from 1.56 to 3.5 μm wavelength, achieving a FWHM jitter from 13.2 to 30.3 ps. This study has implications for emerging time-correlated single-photon counting applications in the mid-infrared spectral region.

Resonant channeling of light near metal surface by passive and active silicon nanoparticles

We investigate the optical response of passive (homogeneous) and active (doped by a magnetic-dipole source) silicon nanospheres placed above the gold substrate supporting excitation and propagation of surface plasmon polaritons (SPPs). In the case of the passive system, the influence of the particle electric and magnetic dipole resonances on the channeling of scattered light into the two channels: light-into-light and light-into-SPPs, is studied around the telecommunication wavelength range. Scattering efficiencies of both channels are analytically and numerically calculated and compared for different gaps between the nanosphere and gold substrate. Conditions, under which the resonant distribution of energy between SPPs and light channels can be approximately equal to each other, are revealed. In the case of an active system, associated with a magnetic dipole source embedded in a silicon resonator, two separate emission channels into light and into SPPs are considered as well. The radiation efficiency in each channel as a function of the gap is examined and compared. In particular, it is revealed the conditions when the efficiency of resonant light energy propagation along the two channels is practically equal to each other, and the Purcell factor achieves the value almost two times higher than for the case without the substrate. The obtained results elucidate the features introduced by the Mie dipole resonances of silicon nanospheres to control the light scattering (radiation) and the SPP excitation in dielectric/metal interface systems. The results obtained are perspectives for the development of new compact photonic and plasmonic devices operating at standard telecommunication wavelength ranges.

Tuning and switching effects of quasi-BIC states combining phase change materials with all-dielectric metasurfaces

Emission enhancement of quantum emitters is particularly relevant in the development of single-photon sources, which are key elements in quantum information and quantum communications. All-dielectric metasurfaces offer a route towards strong enhancement of local density of optical states via engineering of high quality factor optical modes. In particular, the recently proposed concept of quasi-bound states in the continuum (quasi-BIC) allows for precise control of such resonances in lattices with an asymmetric unit cell. Still, the spectral band of emission enhancement is usually fixed by the geometric parameters of the metasurface. Here, we propose to utilize phase change materials to tune the properties of light-emitting metasurfaces designed to support quasi-BIC states in the telecom wavelength range. In our design, a thin layer of a phase change material, which provides strong contrast of refractive index when switched from the amorphous to the crystalline state, is located on top of the resonators made of amorphous silicon (a-Si). Depending on the selected phase change material, we numerically demonstrate different functionalities of the metasurface, In particular, for low-loss Sb2Se3 we evidence spectral tuning effects, whereas for Ge2Sb2Te5, we report an “on”/“off” switching effect of the quasi-BIC resonance. Furthermore, we investigate the influence of the crystallization fraction and the asymmetry parameter of the metasurface on the results. This work provides concrete design blueprints for switchable metasurfaces, offering new opportunities for nanophotonics devices or integrated photonic circuits.