Telecommunication Wavelength Region Research Articles

Photon Phase Delay Sensing with Sub-Attosecond Uncertainty.

The application of statistical estimation theory to Hong-Ou-Mandel interferometry led to enticing results in terms of the detection limit for photon reciprocal delay and polarisation measurement. In the following paper, a fully fibre-coupled setup operating in the telecom wavelength region proves to achieve, for the first time, in common-path Hong-Ou-Mandel-based interferometry, a detection limit for photon phase delay at the zeptosecond scale. The experimental results are then framed in a theoretical model by calculating the Cramer-Rao bound (CRB) and, after comparison with the obtained experimental results, it is shown that our setup attains the optimal measurement, nearly saturating CRB.

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.

Saturable absorption performance of large area monolayer MoS2 coated planarized optical waveguide

The development of compact waveguide-based pulsed lasers has been of great interest in the past two decades. This is supported by the rapid development of two-dimensional (2D) materials saturable absorbers (SAs). In this paper, we integrate a large area monolayer molybdenum disulphide (MoS2) on a planarized silica optical waveguide and studied its performance as an SA. A single layer MoS2 film on polydimethylsiloxane substrate is mechanically transferred to the planarized optical waveguide. The single layer MoS2 can interact with the evanescent field of the waveguide core mode, thus achieving SA. The MoS2-coated waveguide is then integrated into an erbium-doped fibre laser cavity that operates in the telecommunication wavelength region. Q-switching is achieved with a repetition rate, pulse duration and maximum pulse energy of 22.5 kHz, 5.24 µs and 4.1 nJ, respectively at a pump power of 122.8 mW. The results show that 2D material thin films can be integrated onto a planarized optical waveguide to act as SAs. These findings show the potential of using 2D materials in developing compact, integrated waveguide pulsed laser sources.

Carbon defect qubit in two-dimensional WS2

Identifying and fabricating defect qubits in two-dimensional semiconductors are of great interest in exploring candidates for quantum information and sensing applications. A milestone has been recently achieved by demonstrating that single defect, a carbon atom substituting sulphur atom in single layer tungsten disulphide, can be engineered on demand at atomic size level precision, which holds a promise for a scalable and addressable unit. It is an immediate quest to reveal its potential as a qubit. To this end, we determine its electronic structure and optical properties from first principles. We identify the fingerprint of the neutral charge state of the defect in the scanning tunnelling spectrum. In the neutral defect, the giant spin-orbit coupling mixes the singlet and triplet excited states with resulting in phosphorescence at the telecom band that can be used to read out the spin state, and coherent driving with microwave excitation is also viable. Our results establish a scalable qubit in a two-dimensional material with spin-photon interface at the telecom wavelength region.

Femto-Joule All-Optical Switching Using Epsilon-Near-Zero High-Mobility Conductive Oxide

Recently discovered extraordinary optical nonlinearities in epsilon-near-zero (ENZ) materials open a new realm of ultra-energy-efficient, all-optical switching devices for future optical communication and computation. In this paper, we propose a sub-micron, sub-pico-second, femto-joule level all-optical switch (AOS) using hybrid plasmonic-silicon waveguides driven by high mobility transparent conductive oxides (HMTCOs) such as cadmium oxide. Our analysis indicates that the ENZ-induced nonlinear optical effect is greatly enhanced by the high free carrier mobility in the telecom wavelength region. We propose an electrically tunable AOS device which is precisely biased at the ENZ condition, or the high-loss “OFF” state. Then the AOS device can be switched to the “ON” state with an unprecedented modulation strength of 15.9 dB/μm and a rapid switching time of 230 fs at the cost of an ultralow switching energy of 13.5 fJ. By defining a comprehensive metric using the product of device size, switching energy and switching time, the proposed ENZ-enabled device shows superior performance than any existing on-chip AOS device. We envision that such HMTCO-driven AOS device can be integrated with silicon photonic integrated circuits to realize on-chip optical signal processing and computation with ultra-low energy and ultra-high bandwidth.

68 MHz Fundamental Repetition Rates for Mode-Locked Erbium Doped Fiber Laser based Carbon Nanotube Saturable Absorber

A carbon nanotube thin film as saturable absorber in a cavity design of mode-locked fiber laser is reported in this work. Measured by power dependent absorption setup, 22% modulation depth of the saturable absorber is estimated which is ideal for stabilizing the mode-locked operation. Based on the scanning image from atomic force microscope (AFM), Single wall carbon nanotube components are identified existed in the thin film. The laser cavity employed a ~40 cm long erbium doped fiber as a gain medium which corresponds to the laser wavelength of 1550 nm (telecommunication wavelength region). Total length cavity of the fiber laser is about 3.1 m where shorter cavity design is required for higher repetition rate. This proposed system result in a firm soliton spectrum at centre wavelength of ~1561 nm with ~0.7 mW average output power and pulse duration of 410 fs. The pulses are generated at fundamental repetition rate of 68 MHz without any distortion and excellent spectral quality.

Photonic Crystal Cavity with a Thin Low-Index Layer for Silicon-Compatible Nanolight Source

The development of an efficient silicon-based nanolight source is an important step for silicon-based photonic integrated circuits. We propose a high quality factor photonic crystal nanocavity consisting of silicon and silica, which can be used as a silicon-compatible nanolight source. We show that this cavity can effectively confine lights in a low-index silica layer with a high confinement factor of 0.25, in which rare-earth dopants can be embedded as gain materials. The cavity is optimized to have a high quality factor of 15,000 and a mode volume of 0.01 μm3, while the resonance has a wavelength of 1537 nm. We expect that the high confinement factor in the thin silica layer and the high quality factor of the proposed cavity enable the cavity to be a good candidate for silicon-compatible nanolight sources for use in nanolasers or light-emitting diodes in the telecommunication wavelength region.

Observation of Third-order Nonlinearities in Graphene Oxide Film at Telecommunication Wavelengths

All-optical switches have been considered as a promising solution to overcome the fundamental speed limit of the current electronic switches. However, the lack of a suitable third-order nonlinear material greatly hinders the development of this technology. Here we report the observation of ultrahigh third-order nonlinearity about 0.45 cm2/GW in graphene oxide thin films at the telecommunication wavelength region, which is four orders of magnitude higher than that of single crystalline silicon. Besides, graphene oxide is water soluble and thus easy to process due to the existence of oxygen containing groups. These unique properties can potentially significantly advance the performance of all-optical switches.

Si-rich Silicon Nitride for Nonlinear Signal Processing Applications

Nonlinear silicon photonic devices have attracted considerable attention thanks to their ability to show large third-order nonlinear effects at moderate power levels allowing for all-optical signal processing functionalities in miniaturized components. Although significant efforts have been made and many nonlinear optical functions have already been demonstrated in this platform, the performance of nonlinear silicon photonic devices remains fundamentally limited at the telecom wavelength region due to the two photon absorption (TPA) and related effects. In this work, we propose an alternative CMOS-compatible platform, based on silicon-rich silicon nitride that can overcome this limitation. By carefully selecting the material deposition parameters, we show that both of the device linear and nonlinear properties can be tuned in order to exhibit the desired behaviour at the selected wavelength region. A rigorous and systematic fabrication and characterization campaign of different material compositions is presented, enabling us to demonstrate TPA-free CMOS-compatible waveguides with low linear loss (~1.5 dB/cm) and enhanced Kerr nonlinear response (Re{γ} = 16 Wm−1). Thanks to these properties, our nonlinear waveguides are able to produce a π nonlinear phase shift, paving the way for the development of practical devices for future optical communication applications.

Effects of Annealing on GaAs/GaAsSbN/GaAs Core-Multi-shell Nanowires.

The effects of ex-situ annealing in a N2 ambient on the properties of GaAs/GaAsSbN/GaAs core-multi-shell nanowires on Si (111) substrate grown by self-catalyzed molecular beam epitaxy (MBE) are reported. As-grown nanowires exhibit band edge emission at ~0.99 eV with a shoulder peak at ~0.85 eV, identified to arise from band tail states. A large red shift of 7 cm−1 and broadened Raman spectra of as-grown nanowires compared to that of non-nitride nanowires confirmed phonon localization at N-induced localized defects. On annealing nanowires to 750 °C, there was no change in the planar defects in the nanowire with respect to the as-grown nanowire; however, vanishing of the photoluminescence (PL) peak corresponding to band tail states along with enhanced band edge PL intensity, recovery of the Raman shift and increase in the Schottky barrier height from 0.1 to 0.4 eV clearly point to the efficient annihilation of point defects in these GaAsSbN nanowires. A significant reduction in the temperature-induced energy shift in the annealed nanowires is attributed to annihilation of band tail states and weak temperature dependence of N-related localized states. The observation of room temperature PL signal in the 1.3 μm region shows that the strategy of adding small amounts of N to GaAsSb is a promising route to realization of efficient nanoscale light emitters with reduced temperature sensitivity in the telecommunication wavelength region.

Nonlinear infrared plasmonic waveguide arrays

The large negative permittivity of noble metals in the infrared region prevents the possibility of highly confined plasmons in simple waveguide structures such as thin films or rods. This is a critical obstacle to applications of nonlinear plasmonics in the telecommunication wavelength region. We theoretically propose and numerically demonstrate that such limitation can be overcome by exploiting inter-element coupling effects in a plasmonic waveguide array. The supermodes of a plasmonic array span a large range of effective indices, making these structures ideal for broadband mode-multiplexed interconnects for integrated photonic devices. We show such plasmonic waveguide arrays can significantly enhance nonlinear optical interactions when operating in a high-index, tightly bound supermode. For example, a third-order nonlinear coefficient in such a waveguide can be more than three orders of magnitude larger compared to silicon waveguides of similar dimensions. These findings open new design possibilities towards the application of plasmonics in integrated optical devices in the telecommunications spectral region.

High-Quality, Ultraconformal Aluminum-Doped Zinc Oxide Nanoplasmonic and Hyperbolic Metamaterials.

Aluminum-doped zinc oxide (AZO) is a tunable low-loss plasmonic material capable of supporting dopant concentrations high enough to operate at telecommunication wavelengths. Due to its ultrahigh conformality and compatibility with semiconductor processing, atomic layer deposition (ALD) is a powerful tool for many plasmonic applications. However, despite many attempts, high-quality AZO with a plasma frequency below 1550 nm has not yet been realized by ALD. Here a simple procedure is devised to tune the optical constants of AZO and enable plasmonic activity at 1550 nm with low loss. The highly conformal nature of ALD is also exploited to coat silicon nanopillars to create localized surface plasmon resonances that are tunable by adjusting the aluminum concentration, thermal conditions, and the use of a ZnO buffer layer. The high-quality AZO is then used to make a layered AZO/ZnO structure that displays negative refraction in the telecommunication wavelength region due to hyperbolic dispersion. Finally, a novel synthetic scheme is demonstrated to create AZO embedded nanowires in ZnO, which also exhibits hyperbolic dispersion.

Self-catalyzed growth of dilute nitride GaAs/GaAsSbN/GaAs core-shell nanowires by molecular beam epitaxy

Bandgap tuning up to 1.3 μm in GaAsSb based nanowires by incorporation of dilute amount of N is reported. Highly vertical GaAs/GaAsSbN/GaAs core-shell configured nanowires were grown for different N contents on Si (111) substrates using plasma assisted molecular beam epitaxy. X-ray diffraction analysis revealed close lattice matching of GaAsSbN with GaAs. Micro-photoluminescence (μ-PL) revealed red shift as well as broadening of the spectra attesting to N incorporation in the nanowires. Replication of the 4K PL spectra for several different single nanowires compared to the corresponding nanowire array suggests good compositional homogeneity amongst the nanowires. A large red shift of the Raman spectrum and associated symmetric line shape in these nanowires have been attributed to phonon localization at point defects. Transmission electron microscopy reveals the dominance of stacking faults and twins in these nanowires. The lower strain present in these dilute nitride nanowires, as opposed to GaAsSb nanowires having the same PL emission wavelength, and the observation of room temperature PL demonstrate the advantage of the dilute nitride system offers in the nanowire configuration, providing a pathway for realizing nanoscale optoelectronic devices in the telecommunication wavelength region.

Photonic crystals composed of β-FeSi2 with amorphous Si cladding layers

Photonic crystals (PhCs) composed of β-FeSi2 with amorphous Si (a-Si) cladding layers are systematically studied to realize silicon photonics devices incorporating Si-based light-emitting layers. The bandgap characteristics of two types of triangular-lattice-type PhC in the telecommunication wavelength region of approximately 1.55 µm are calculated. They are composed of a-Si circular holes surrounded by β-FeSi2 (hole type) and β-FeSi2 circular columns surrounded by a-Si (column type). As a result, the bandgap for the hole-type PhC is obtained for TE polarization, while that for the column-type PhC is obtained for TM polarization. Furthermore, the PhC lattice constant range giving a bandgap for the column-type PhC is much wider than that for the hole-type PhC. The column-type PhC can be easily fabricated compared with the hole-type PhC. Thus, the column-type PhC is useful for actual applications from the viewpoint of fabrication and the bandgap characteristics themselves.

An all-optical switch/photodetection at 1550 nm, based on a micro-ring resonator array

In this paper, a photodetector based on InGaAs micro-ring resonator array with 5μm radius, in telecommunication wavelength region has been investigated. Then for the first time to our knowledge, with the change of refractive index due to optical nonlinear effects (such as: Kerr effect, two photon absorption, free carrier dispersion and free carrier absorption), an all-optical switch, based on pump-probe configuration using this photodetector have been proposed. So, the suggested device, will introduce a dual functionality that is important from view point of economically purposes and in term of integration. By numerically solving the light propagation equations, with FDTD and Crank–Nicolson methods, in addition to steady state and transient response, also we have found the frequency response, in both configurations. With increasing reverse bias voltage, and optimizing the free carrier transportation from absorption region, switching frequency can be increased up to 200GHz. Also by optimal choosing of parameters such as the number of rings and spaces between two adjacent rings and so on, the box-like spectral response of photodetector with high efficiency is achievable.

Refractive index and dispersion of highly fluorinated acrylic monomers in the 1.5 µm telecom wavelength region measured with a spectroscopic Abbe refractometer

Fluorinated polymers are very promising for integrated optics devices due to their high transparency in all three telecom wavelength regions around 0.85, 1.3 and 1.5 µm. Knowledge of the refractive index nλ and dispersion dnλ/dλ of these materials as a function of wavelength λ is necessary for fabricating polymer waveguides with a well-defined numerical aperture and group velocity dispersion in the above-mentioned spectral regions. We present a high resolution critical angle spectroscopic refractometer for measuring nλ and dnλ/dλ of liquid and solid samples in the 0.95–1.65 µm wavelength range. The device is based on extensions to a standard Abbe refractometer. The absolute accuracy of the spectroscopic refractometer in the near-IR equals ±2 × 10−4, the measurement range at 1.5 µm being nλ = 1.08–1.65. Finally, refractive index data in the 1.5 µm telecom wavelength region are given for a number of new highly fluorinated acrylic monomers, which have previously not been investigated to the best of our knowledge.

Design of a GaP/Si composite waveguide for CW terahertz wave generation via difference frequency mixing.

We design a GaP/Si composite waveguide to achieve efficient terahertz (THz) wave generation under collinear phase-matched difference frequency mixing (DFM) between near-infrared light sources. This waveguide structure provides a strong mode confinement of both near-infrared sources and THz wave, resulting in an efficient mode overlapping. The numerical results show that the waveguide can produce guided THz wave (5.93 THz) with a power conversion efficiency of 6.6×10(-4) W(-1). This value is larger than previously obtained with the bulk GaP crystal: 0.5×10(-9) W(-1) [J. Lightwave Technol.27, 3057 (2009)]. Our proposed composite waveguide can be achieved by bridging the telecom wavelength and THz frequency region.

Preparation, photoisomerization, and microfabrication with two‐photon polymerization of crosslinked azo‐polymers

ABSTRACTWe report the preparation, photoisomerization properties, and three‐dimensional (3D) microstructure fabrication with two‐photon polymerization of crosslinked azo‐polymers. A series of bi‐acrylate‐substituted azobenzene derivatives were designed and synthesized as the monomers and/or crosslinkers of the crosslinked azo‐polymers. The doping concentration of the derivatives in pre‐polymer resins was significantly increased due to the introduction of bulky tert‐butyl and flexible alkyl chains. The double‐exponential dynamics of trans‐to‐cis photoisomerization of the azo‐polymers indicated the coexistence of different processes for the azobenzene moieties in the polymeric crosslinked networks. The crosslinked azo‐polymers exhibited ideal “on–off” switching performance in the highly reversible trans–cis–trans isomerization cycles. Furthermore, we prepared a photoresist containing the azobenzene derivative for 3D microstructure fabrication based on two‐photon polymerization. A woodpile photonic crystal with a photonic bandgap at telecommunication wavelength region was successfully fabricated with the azobenzene‐containing photoresist, which would open the way for the design and manufacturing of miniature optical communication devices. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2947–2956, 2013

Four-photon quantum interferometry at a telecom wavelength

We report on an experimental demonstration of four-photon quantum interference using telecom-wavelength photons. The realization of multiphoton quantum interference is essential to linear optics quantum information processing and measurement-based quantum computing. We have developed a source that efficiently emits photon pairs in a pure spectrotemporal mode at a telecom-wavelength region, and have demonstrated quantum interference exhibiting reduced fringe intervals that correspond to a reduced de Broglie wavelength of up to the four-photon NOON state. Our result should open a path to practical quantum information processing using telecom-wavelength photons.

Chalcogenide optical parametric oscillator

We demonstrate the first optical parametric oscillator (OPO) based on chalcogenide glass. The parametric gain medium is an As(2)Se(3) chalcogenide microwire coated with a layer of polymer. The doubly-resonant OPO oscillates simultaneously at a Stokes and an anti Stokes wavelength shift of >50 nm from the pump wavelength that lies at λ(P) = 1,552 nm. The oscillator has a peak power threshold of 21.6 dBm and a conversion efficiency of >19%. This OPO experiment provides an additional application of the chalcogenide microwire technology; and considering the transparency of As(2)Se(3) glass extending far in the mid-infrared (mid-IR) wavelengths, the device holds promise for realizing mid-IR OPOs utilizing existing optical sources in the telecommunications wavelength region.