Μm Telecommunication Wavelength Research Articles

Optically enhanced single- and multi-stacked 1.55 μm InAs/InAlGaAs/InP quantum dots for laser applications

For the development of InAs/InP quantum dot (QD) lasers for 1.55 μm telecom wavelength, there are two main challenges: (1) morphological preference for quantum dashes over QDs, and (2) generally poor size uniformity of QDs (dashes). This study addresses the issues, in synchronous, by demonstrating the improved optical properties of 1.55 μm InAs/InP QDs at room temperature with excellent reproducibility. A high-density (∼4 × 1010 cm−2) dot-like morphology was initially attained via adjusting the growth parameters, albeit with a large full-width at half-maximum (FWHM) of ∼80 meV and a peak position of a wavelength longer than 1.55 μm. For improvement, the indium-flush technique was employed, which enhanced the uniformity of InAs QDs and substantially lowered the FWHM of five (single) stacked QDs to 50.9 meV (47.9 meV). This technique also blue-shifted the emission peak to 1530.2 nm (1522 nm). The InAs/InP QDs presented are appropriate for the fabrication of high-performance 1.55 μm lasers on InP (001) and, potentially, emerging light sources on the important Si (001).

(Invited) Coupled Quantum Defects of DNA Functionalized Carbon Nanotubes

Single wall carbon nanotubes (SWCNTs) with sp3 defects created via covalent bonding of organic functional groups are rapidly emerging as a new material system for chemistry, physics, material science, engineering and quantum technologies. They are often referred to as “organic color centers” as they can mimic quantum mechanical properties of solid-state color centers.1 Their unique properties include highly enhanced molecularly wavelength-tunable fluorescence and room temperature single photon generation with 99% single photon purity at 1.55 µm telecommunication wavelength.2 To date, diazonium chemistry has been the chemists’ tool of the choice for creating sp3 defects sparsely distributed at random positions along the sidewall of SWCNTs. While such distribution is ideally suited for single photon generation, an ability to incorporate multiple coupled quantum defects in a well-defined spatial pattern is highly desirable for more sophisticated quantum information science applications involving entanglement. Recent discovery of covalently functionalizing SWCNTs with single-stranded DNA (ssDNA)3 open a possible path toward achieving this goal. In this process, SWCNTs form covalent bonds to the guanine nucleotides when aqueous suspensions of SWCNTs coated with ssDNA are exposed to singlet oxygen. As a result, creating a regular array of covalent defect with spacing defined by the ssDNA sequence become a possibility. To further explore this exciting possibility, we investigated intrinsic PL emission properties of ssDNA functionalized SWCNTs through low temperature single tube PL spectroscopy. In contrast to individual aryl functionalized SWCNTs that usually emit a single sharp isolated PL peaks, multiple PL peaks with their emission wavelengths varying from 1050 to 1300 nm were observed from individual ssDNA functinalized SWCNTs. We further conducted 2nd order photon correlation (g(2)) and cross-correlation experiments to investigate their quantum light emission properties. We observed photon anti-bunching behavior when an individual PL emission peak is spectrally isolated indicating that ssDNA functionalization creates a series of quantum defects along the length of SWCNTs. Interestingly, when the g(2) experiment is performed on combined emission of multiple (2—4) PL peaks, an incomplete antibunching with g(2)(0) values in the range of 0.2-0.4 is observed. Because these values are significantly smaller than g(2)(0) values estimated for 2-4 independent quantum emitters (0.5-0.75), we interpret that the series of defects created via ssDNA functionalization are coupled. Our photon cross-correlation experiment performed between two emission peaks of the group also reveal evidence of coupling. 1 Srinivasan, K. & Zheng, M. Nat. Photon. 11, 535 (2017). 2 Brozena, A.H., Kim, M., Powell, L.R. & Wang, Y. Nat. Rev. Chem. 3, 375-392 (2019). 3 Zheng, Y., Bachilo, S. M., Weisman, R. B. ACS Nano, 13, 8222 (2019)

Ultrabright narrow-band telecom two-photon source for long-distance quantum communication

We demonstrate an ultrabright narrow-band two-photon source at the 1.5 µm telecom wavelength for long-distance quantum communication. By utilizing a bow-tie cavity, we obtain a cavity enhancement factor of 4.06 × 104. Our measurement of the second-order correlation function G(2)(τ) reveals that the linewidth of 2.4 MHz has been hitherto unachieved in the 1.5 µm telecom band. This two-photon source is useful for obtaining a high absorption probability close to unity by quantum memories set inside quantum repeater nodes. Furthermore, to the best of our knowledge, the observed spectral brightness of 3.94 × 105 pairs/(s·MHz·mW) is also the highest reported over all wavelengths.

Parametric study of high-performance 1.55 μm InAs quantum dot microdisk lasers on Si.

In this paper, we present a parametric study of high performance microdisk lasers at 1.55 μm telecom wavelength, monolithically grown on on-axis (001) Si substrates incorporating quantum dots (QDs) as gain elements. In the optimized structure, seven layers of QDs were adopted to provide a high gain as well as a suppressed inhomogeneous broadening. The same laser structure employing quantum wells (QWs) on Si was concurrently evaluated, showing a higher threshold and more dispersive quantum efficiency than the QDs. Finally, a statistical comparison of these Si-based QD microdisk lasers with those grown on InP native substrates was conducted, revealing somewhat higher thresholds but of the same order. The monolithically grown QD microlasers on Si also demonstrated excellent temperature stability, with a record high characteristic temperature of 277 K. This work thus offers helpful insight towards the optimization of reliable Si-based QD lasers at 1550 nm.

Passively mode-locked InAs quantum dot lasers on a silicon substrate by Pd-GaAs wafer bonding

We demonstrate an electrically pumped InAs quantum dot (QD) two-section passively mode-locked laser (MLL) on a silicon substrate by low temperature (250 °C) Pd-GaAs wafer bonding technology. The saturable absorber of the QD-MLL is electrically isolated by a 15-μm wide dry-etching gap which resulted in ∼30 kΩ resistance from the gain regions of the MLL. At room temperature, the laser operates in the O-band (1.3 μm) telecommunication wavelength regime with a threshold current of 94 mA and laser bar cavity and absorber lengths of 6 mm and 300 μm, respectively. The optimum mode-locked conditions are observed under injection current and reverse bias voltage of 124 mA and −7 V, which generates pulses at a repetition rate of 7.3 GHz, an optical bandwidth of 0.97 nm, and a nearly transform limited pulse width of 2 ps (sech2 pulse profile). These results enable QD-MLLs to be integrated with silicon photonic integrated circuits, such as optical time division multiplexing and optical clocks.

Ultralow Surface Recombination Velocity in Passivated InGaAs/InP Nanopillars.

The III–V semiconductor InGaAs is a key material for photonics because it provides optical emission and absorption in the 1.55 μm telecommunication wavelength window. However, InGaAs suffers from pronounced nonradiative effects associated with its surface states, which affect the performance of nanophotonic devices for optical interconnects, namely nanolasers and nanodetectors. This work reports the strong suppression of surface recombination of undoped InGaAs/InP nanostructured semiconductor pillars using a combination of ammonium sulfide, (NH4)2S, chemical treatment and silicon oxide, SiOx, coating. An 80-fold enhancement in the photoluminescence (PL) intensity of submicrometer pillars at a wavelength of 1550 nm is observed as compared with the unpassivated nanopillars. The PL decay time of ∼0.3 μm wide square nanopillars is dramatically increased from ∼100 ps to ∼25 ns after sulfur treatment and SiOx coating. The extremely long lifetimes reported here, to our knowledge the highest reported to date for undoped InGaAs nanostructures, are associated with a record-low surface recombination velocity of ∼260 cm/s. We also conclusively show that the SiOx capping layer plays an active role in the passivation. These results are crucial for the future development of high-performance nanoscale optoelectronic devices for applications in energy-efficient data optical links, single-photon sensing, and photovoltaics.

Effects of p-type doping and electric field on electronic band structure and optical properties of GaNAsBi/GaAs quantum well detectors operating at 1.55 μm

The optical properties of p-type doped lattice matched GaNAsBi/GaAs single quantum well (SQW) were theoretically studied in order to reach the maximum of absorption at 1.55μm telecommunication wavelength. The calculation was performed using a numerical method based on the mixture of a self-consistent procedure and the band anticrossing model. We investigated the effect of p doping density in the well GaNAsBi on the subband energies, Fermi level and the confining hole density distribution for the specific couple (well width, Bi composition), with respect to confinement conditions. The increase of doping density led to a shift of the fundamental transition to blue. The effect of the applied electric field on the physical parameters was also examined. It was found that unlike the n-type doping, p-type doping of the studied SQW obeys the restrictive conditions determined in this work.

Resonant cavities for efficient LT-GaAs photoconductors operating at λ = 1550 nm

We show that photoconductors based on low-temperature-grown GaAs (LT-GaAs) can be efficiently operated by 1.55 μm telecom wavelength by using metallic mirror based optical cavities. Two different semi-transparent front mirrors are compared: the first one is a thin gold layer, whereas the second one consists of a gold grating. Light absorption in grating mirror based optical cavities is numerically, analytically, and experimentally investigated allowing for an appropriate optical design. We show a 3 times improvement of the LT-GaAs photoconductor photoresponse by using, as front mirror, the gold grating once compared with the thin gold layer. It reaches around 0.5 mA/W under continuous wave, whereas a transient photoresistivity (Ron) as low as 5 Ω is deduced from dc photocurrents measured under femtosecond pulsed laser excitation. This work paves the way to efficient and reliable optoelectronics systems for GHz or THz waves sampling driven by 1.55 μm pulsed lasers widely available.

Black phosphorus polycarbonate polymer composite for pulsed fibre lasers

Recently, the re-discovered black phosphorus (BP) has been extensively investigated for both electronic and photonic applications. However, the intrinsic instability of BP caused by moisture or oxygen reaction in ambient atmosphere has overshadowed its practical applications. Here, we present a liquid phase exfoliation-based approach for the production of few-layer BP (FL-BP) and the subsequent mixing with polycarbonate (PC) for the fabrication of a composite, which significantly reduces BP degradation by PC passivation. Experimentally, the functionalized PC few-layer BP (PC/FL-BP) composite shows environmental stability if compared with mechanically exfoliated BP flakes. We then use the PC/FL-BP composite as saturable absorber to study the nonlinear absorption property in a fibre laser at the 1.55μm telecommunication wavelength. A Q-switched laser with pulse energy up to 25.2nJ and pulse duration down to 1.65μs is obtained at a low pump power of 71.7mW. Our results can boost further research and scalable photonic applications, where environmentally stable few-layer BP based devices are needed.

Thirty Gigahertz Optoelectronic Mixing in Chemical Vapor Deposited Graphene.

The remarkable properties of graphene, such as broadband optical absorption, high carrier mobility, and short photogenerated carrier lifetime, are particularly attractive for high-frequency optoelectronic devices operating at 1.55 μm telecom wavelength. Moreover, the possibility to transfer graphene on a silicon substrate using a complementary metal-oxide-semiconductor-compatible process opens the ability to integrate electronics and optics on a single cost-effective chip. Here, we report an optoelectronic mixer based on chemical vapor-deposited graphene transferred on an oxidized silicon substrate. Our device consists in a coplanar waveguide that integrates a graphene channel, passivated with an atomic layer-deposited Al2O3 film. With this new structure, 30 GHz optoelectronic mixing in commercially available graphene is demonstrated for the first time. In particular, using a 30 GHz intensity-modulated optical signal and a 29.9 GHz electrical signal, we show frequency downconversion to 100 MHz. These results open promising perspectives in the domain of optoelectronics for radar and radio-communication systems.

Near-infrared emitting In-rich InGaN layers grown directly on Si: Towards the whole composition range

The authors report compact and chemically homogeneous In-rich InGaN layers directly grown on Si (111) by plasma-assisted molecular beam epitaxy. High structural and optical quality is evidenced by transmission electron microscopy, near-field scanning optical microscopy, and X-ray diffraction. Photoluminescence emission in the near-infrared is observed up to room temperature covering the important 1.3 and 1.55 μm telecom wavelength bands. The n-InGaN/p-Si interface is ohmic due to the absence of any insulating buffer layers. This qualitatively extends the application fields of III-nitrides and allows their integration with established Si technology.

Design of efficient surface plasmon polariton modulators using graphene

We design surface plasmon polariton (SPP) absorption modulators using monolayer graphene as the active layer in the 1.5 µm telecommunication wavelength window. Modulation is achieved by electrically modulating the Fermi energy level in the graphene layer. We investigated a silver slab SPP waveguide on graphene and found that there are limitations on the modulation performance as the guided mode only weakly interacts with the graphene layer. To overcome these limitations, we propose a silver–air–silver SPP waveguide on graphene configuration. We show that the latter design can achieve a two times improvement in the figure of merit, defined by the ratio of the extinction ratio to the voltage-off propagation loss, compared to previous indium–tin oxide based SPP absorption modulators.

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.

Site-controlled InAs/GaAs quantum dots emitting at telecommunication wavelength

We demonstrate site-controlled InAs/GaAs quantum dot (QD) emission at 1.3 µm telecommunication wavelength. The samples were fabricated by molecular beam epitaxy on patterned substrates, which have been prepared by electron beam lithography and wet chemical etching. By embedding a single layer of positioned QDs in a strain reducing InGaAs quantum well layer, we successfully shifted the emission band beyond the important telecommunication wavelength of 1.3 µm. Furthermore, the resulting deep carrier confinement allowed us to preserve strong QD luminescence up to room temperature.

Low-loss ultra-subwavelength hybrid plasmonic waveguide based on metallic bump structures

Sharp metal edges (like wedge or triangular geometries) have been widely studied as low-loss surface plasmon polariton (SPP) waveguiding structures. However, fabrication difficulties lead to round-shaped tips which have larger mode size and shorter propagation length. As a solution, here we present an easy to realize structure for turning an edge-less bump metallic geometry (whose fabrication is less troublesome) into a hybrid one. Through investigating the geometrical dispersion, propagation lengths of up to half a millimetre and effective mode areas as small as λ2/330 are obtained for the 1.55 µm telecommunication wavelength. Gain-assisted SPP propagation is also considered by introducing gain in the InGaAsP material, for which critical gain as low as 11 cm−1 is calculated. Finally, obtained results suggest that the designed waveguide can find various applications in nanoscale waveguiding.

Pulsed laser deposited amorphous chalcogenide and alumino-silicate thin films and their multilayered structures for photonic applications

Amorphous chalcogenide and alumino-silicate thin films were fabricated by the pulsed laser deposition technique. Prepared films were characterized in terms of their morphology, chemical composition, and optical properties. Multilayered thin film stacks for reflectors and vertical microcavities were designed for telecommunication wavelength and the window of atmosphere transparency (band II) at 1.54μm and 4.65μm, respectively. Bearing in mind the benefit coming from the opportunity of an efficient wavelength tuning or, conversely, to stabilize the photoinduced effects in chalcogenide films as well as to improve their mechanical properties and/or their chemical durability, several pairs of materials from pure chalcogenide layers to chalcogenide/oxide layers were investigated. Different layer stacks were fabricated in order to check the compatibility between dissimilar materials which can have a strong influence on the interface roughness, adhesion, density, and homogeneity, for instance. Three different reflector designs were formulated and tested including all-chalcogenide layers (As40Se60/Ge25Sb5S70) and mixed chalcogenide-oxide layers (As40Se60/alumino-silicate and Ga10Ge15Te75/alumino-silicate). Prepared multilayers showed good compatibility between different material pairs deposited by laser ablation despite the diversity of chemical compositions. As40Se60/alumino-silicate reflector showed the best parameters; its stop band (R>97% at 8° off-normal incidence) has a bandwidth of ~100nm and it is centered at 1490nm. The quality of the different mirrors developed was good enough to try to obtain a microcavity structure for the 1.5μm telecommunication wavelength made of chalcogenide layers. The microcavity structure consists of Ga5Ge20Sb10S65 (doped with 5000ppm of Er3+) spacer surrounded by two 10-layer As40Se60/Ge25Sb5S70 reflectors. Scanning and transmission electron microscopies showed a good periodicity, great adherence and smooth interfaces between the alternating dielectric layers confirming a suitable compatibility between the different materials.

On-chip optical isolation via unidirectional Bloch oscillations in a waveguide array

We propose to use the unidirectionality of the optical Bloch oscillation phenomenon achievable in a magneto-optic asymmetric waveguide array to achieve optical isolation. At the 1.55 μm telecommunication wavelength, our isolator design exhibits an isolation ratio of 36 dB between forward- and backward-propagating waves. The proposed design consists of a waveguide array made in a silicon-on-insulator substrate with a magnetic garnet cover layer. A key role is played by the transverse-magnetic mode nonreciprocal phase shift effect.

Integrated fiber-coupled launcher for slow plasmon-polariton waves

We propose and numerically demonstrate an integrated fiber-coupled launcher for slow surface plasmon-polaritons. The device is based on a novel plasmonic mode-converter providing efficient power transfer from the fast to the slow modes of a metallic nanostripe. Total coupling efficiency with standard single-mode fiber approaching 30% (including ohmic losses) has been numerically predicted for a 25-µm long gold-based device operating at 1.55 µm telecom wavelength.

Brillouin Scattering in Polymer Optical Fibers: Fundamental Properties and Potential Use in Sensors

We review the fundamental properties of Brillouin scattering in a perfluorinated graded-index polymer optical fiber (PFGI-POF) with 120 μm core diameter. The experiments were performed at 1.55 μm telecommunication wavelength. The Brillouin frequency shift (BFS) and the Brillouin bandwidth were 2.83 GHz and 105 MHz, respectively. The Brillouin gain coefficient was calculated to be 3.09 × 10−11 m/W, which was comparable to that of fused silica fibers. The Brillouin threshold power of the 100 m POF was estimated to be as high as 24 W, which can be, for practical applications, reduced by using POFs with smaller cores. These properties were compared with those of silica-based graded-index multi-mode fibers. We also investigated the BFS dependences on strain and temperature. They showed negative dependences with coefficients of −121.8 MHz/% and −4.09 MHz/K, respectively, which are −0.2 and −3.5 times as large as those in silica fibers. These BFS dependences indicate that the Brillouin scattering in PFGI-POFs can be potentially applied to high-accuracy temperature sensing with reduced strain sensitivity.

Surface plasmon polariton absorption modulator

An electrically controlled ultra-compact surface plasmon polariton absorption modulator (SPPAM) is proposed. The device can be as small as a few micrometers depending on the required extinction ratio and the acceptable loss. The device allows for operation far beyond 100 Gbit/s, being only limited by RC time constants. The absorption modulator comprises a stack of metal/insulator/metal-oxide/metal layers, which support a strongly confined asymmetric surface plasmon polariton (SPP) in the 1.55 μm telecommunication wavelength window. Absorption modulation is achieved by electrically modulating the free carrier density in the intermediate metal-oxide layer. The concept is supported by proof-of-principle experiments.