High-speed carbon nanotube photodetector based on a planarized silicon waveguide.

The integration of silicon waveguides with low-dimensional materials with excellent optoelectronic properties can enable compact and highly integrated optical devices with multiple advantages for multiple fields. A carbon nanotube (CNT) photodetector integrated on the silicon waveguide has the potential to meet on-chip high-speed optical interconnection systems, based on the outstanding properties of CNTs such as picosecond-level intrinsic photoresponse time, high charge carrier mobility, broad spectral response, high absorption coefficient, and so on. However, the thermal stability of the device may be compromised due to the local suspension in the channel for the height difference between the WG and the substrate. Here, we report a low-cost and low-optical-loss method to achieve the planarized silicon waveguide. After that, the CNT photodetectors integrated on the original and planarized waveguide with asymmetric palladium (Pd)-hafnium (Hf) metal contacts are fabricated. The influence of this planarization method on the performance of devices is analyzed via comparing the dark leakage current, the leakage current rectification ratio (CRR), the series resistances (R S), and the photoelectric response. Finally, a CNT photodetector based on the planarized waveguide with a photocurrent (I p h ) ∼510.84n A, a photoresponsivity (R I) ∼51.04m A/W, the dark current ∼0.389µA, as well as a 3 dB bandwidth ∼34G H z at the large reverse voltage -3V is shown.

16QAM optical signal generation scheme based on weighted optimal Euclidean distance.

A two-dimensional signal constellation scheme for binary uniform memoryless source transmission in optical fiber channels is studied in this paper. In geometric shaping (GS), optimization algorithms are usually used to change the overall position of constellation points while maintaining the probability of constellation points unchanged. Different optimization functions are used to allocate the position of constellation symbols, thereby improving constellation performance. A 16 quadrature amplitude modulation (QAM) optical signal generation scheme based on weighted optimal Euclidean distance is proposed in this paper. In order to obtain the best constellation diagram and increase the shaping gain, the weighted optimal Euclidean distance that can minimize the bit error rate (BER) over multiple iterative optimizations is used as the objective function. On the one hand, the proposed 16QAM optical signal generation scheme based on weighted optimal Euclidean distance always outperforms the uniform square 16QAM and the uniform circle 16QAM schemes in the back to back (BTB) transmission. On the other hand, after analyzing the simulation demonstration in a 50GBaud coherent optical communication system over 3000 km, results demonstrate that the optical signal to noise ratio (OSNR) performance of this system is better than that of the uniform square 16QAM and the uniform circle 16QAM, which is improved by 0.52 dB and 0.85 dB, respectively. In addition, the proposed 16QAM system increases the transmission distance by 989 km and 741 km, respectively, compared to the other two systems. The performance confirms that the proposed novel 16QAM scheme, to the best of our knowledge, can effectively improve the reliability and transmission distance. Therefore, the proposed scheme has a certain development prospect in the future long-distance transmission of high-speed optical fiber communication.

Data rate measurement method with selectable range in linear optical sampling.

Linear optical sampling (LOS) is one of the most powerful techniques for high-speed signal monitoring. To measure the data-rate of signal under test (SUT) in optical sampling, multi-frequency sampling (MFS) was proposed. However, the measurable data-rate range of the existing method based on MFS is limited, which makes it very difficult to measure the data-rate of high-speed signals. To solve the above problem, a range selectable data-rate measurement method based on MFS in LOS is proposed in this paper. Through this method, the measurable data-rate range can be selected to match the data-rate range of SUT and the data-rate of SUT can be measured precisely, independently of the modulation format. What's more, the sampling order can be judged using the discriminant in the proposed method, which is key for plotting eye diagrams with correct time information. We experimentally measure the baud-rates of PDM-QPSK signal from 800 MBaud to 40.8 GBaud in different ranges and judge the sampling orders. The relative error of measured baud-rate is less than 0.17% while the error vector magnitude (EVM) is less than 0.38. Compared with the existing method, under the same sampling cost, our proposed method realizes the selectivity of the measurable data-rate range and the judgment of sampling order, greatly extends the measurable data-rate range of SUT. Hence, the data-rate measurement method with selectable range has great potential for high-speed signal data-rate monitoring.

Ultra-low complexity random forest for optical fiber communications.

In this paper, we present an efficient equalizer based on random forest for channel equalization in optical fiber communication systems. The results are experimentally demonstrated in a 120 Gb/s, 375 km, dual-polarization 64-quadrature magnitude modulation (QAM) optical fiber communication platform. Based on the optimal parameters, we choose a series of deep learning algorithms for comparison. We find that random forest has the same level of equalization performance as deep neural networks as well as lower computational complexity. Moreover, we propose a two-step classification mechanism. We first divide the constellation points into two regions and then use different random forest equalizers to compensate the points in different regions. Based on this strategy, the system complexity and performance can be further reduced and improved. Furthermore, due to the plurality voting mechanism and two-stage classification strategy, the random forest-based equalizer can be applied to actual optical fiber communication systems.

Silicon Waveguide-Integrated Carbon Nanotube Photodetector with Low Dark Current and 48 GHz Bandwidth.

Low-dimensional materials with excellent optoelectronic properties and complementary metal-oxide-semiconductor (CMOS) process compatibility have the potential to construct high-performance photodetectors used in a cost-efficient monolithic or hybrid integrated optical communication system. Carbon nanotubes (CNTs) have attracted a lot of attention due to special geometric structure and broad band response, high optical absorption coefficient, ps-level intrinsic light response, high carrier mobility and wafer-scaled production process. Here, we demonstrated a high-performance waveguide-integrated CNT photodetector with asymmetric palladium (Pd) and hafnium (Hf) contact electrodes. The ideal photodetector structure was realized via comparing with simulation and experimental results, where the optimized device achieved a high 3 dB bandwidth ∼48 GHz at 0 V, as well as a responsivity ∼73.62 mA/W and dark current ∼0.157 µA at -2 V bias voltage. This waveguide-integrated CNT photodetector with low dark current and high bandwidth is helpful for next-generation optical communication and high-speed optical interconnects.

Secure key real-time update and dynamic DNA encryption for CO-OFDM-PON based on a hybrid 5-D chaos.

A double key (DK) real-time update and hybrid five-dimensional (5-D) hyperchaotic deoxyribonucleic acid (DNA) dynamic encryption scheme is proposed, which can ensure the security in the orthogonal frequency division multiplexing passive optical network (OFDM-PON). Chaotic sequences for DNA dynamic encryption are produced using a four-dimensional (4-D) hyperchaotic Lü system and a one-dimensional (1-D) logistic map. In this scheme, the DK consists of an external key set, which is stored locally, and an internal key, which is associated with the plaintext and external key. In addition, a pilot cluster is used as the carrier of key transmission and key embedding is achieved by converting key to phase information of the pilot. To verify the feasibility of the scheme, a simulation validation is performed on a 46.5Gb/s 16 quadrature amplitude modulation (QAM) coherent OFDM-PON system transmitted over an 80 km transmission distance. The results show that the proposed scheme can improve the security performance of OFDM-PON at a low OSNR cost of 0.3 dB and the key space is expanded to (8.514 × 10102)S. When the correlation redundancy (CR) G⩾7, the 0 bit error rate (BER) of key can be achieved and the key can be updated and distributed in real-time without occupying additional secure channels.

Jointly recognizing OAM mode and compensating wavefront distortion using one convolutional neural network.

In this work, a new recognition method of orbital angular momentum (OAM) is proposed. The method combines mode recognition and the wavefront sensor-less (WFS-less) adaptive optics (AO) by utilizing a jointly trained convolutional neural network (CNN) with the shared model backbone. The CNN-based AO method is implicitly applied in the system by providing additional mode information in the offline training process and accordingly the system structure is rather concise with no extra AO components needed. The numerical simulation result shows that the proposed method can improve the recognition accuracy significantly in different conditions of turbulence and can achieve similar performance compared with AO-combined methods.

Carrier phase recovery friendly probabilistic shaping scheme based on a quasi-Maxwell-Boltzmann distribution model.

A novel probabilistic shaping (PS) scheme based on the quasi-Maxwell-Boltzmann (quasi-MB) distribution model is proposed in order to solve the incompatibility between PS and carrier phase recovery (CPR) algorithms, such as blind phase search (BPS) and principal component-based phase estimation (PCPE). In the proposed quasi-MB model, the same occurrence probability is assigned to each constellation point on the same square-ring, rather than on the same circle. Signals obeying the quasi-MB model have superior CPR friendliness compared to traditional PS signals. For a PS 64 quadrature amplitude modulation system, the simulation results indicate up to 51% and 21% normalized generalized mutual information (NGMI) improvements for PCPE and BPS, respectively. Experimental verification of the proposed quasi-MB scheme was demonstrated in a 10 Gbaud coherent detection system. The results show that when the optical signal-to-noise ratio (OSNR) is low, the quasi-MB model can help the BPS algorithm to achieve better NGMI performance and, when the OSNR is high, the proposed model can also solve the incompatibility between the PCPE algorithm and PS.

Silicon-Waveguide-Integrated Carbon Nanotube Optoelectronic System on a Single Chip.

Monolithic optoelectronic integration based on a single material is a major pursuit in the fields of nanophotonics and nanoelectronics in order to meet the requirements of future fiber-optic telecommunication systems and on-chip optical interconnection systems. However, the incompatibility between silicon-based electronics and germanium or compound semiconductor-based photonics makes it very challenging to realize optoelectronic integration based on a single material. Here, the integration between silicon waveguides and a carbon nanotube (CNT) optoelectronic system is demonstrated. Waveguide-integrated photodetectors based on the CNT exhibit 12.5 mA/W photoresponsivity at 1530 nm, which presents an improvement of 97.6 times enhanced absorption efficiency compared to that without the waveguide. Multiplied output signals of cascading photodetectors are used to control the output of CNT-based logic gates, thereby demonstrating that the CNT-based optoelectronic integration system is compatible with silicon photonics. Our work indicates that carbon nanotubes have the potential for future integration between nanophotonics and nanoelectronics on a single chip.

Mixture-of-Gaussian clustering-based decision technique for a coherent optical communication system.

A decision technique using mixture-of-Gaussian (MoG) clustering algorithms is proposed in the context of a coherent optical communication system. For an 80-Gb/s single-carrier polarization-division multiplexed 16 quadrature amplitude modulation (QAM) transmission system at 975 km, an improvement in $Q$Q factors up to 0.41 dB is observed for the entire range of the considered optical signal-to-noise SNR. Compared with the traditional minimum Euclidean distance-based decision, the MoG clustering-based decision achieves a transmission distance increase of 175 km at a $Q$Q factor of 9.96 dB. Experiment results show that the proposed decision technique is insensitive to the system's nonlinear impairments and can effectively improve nonlinear tolerance of the system. We also propose a majorization method for decision-directed least mean square (LMS) using the MoG clustering-based decision algorithm, called MoG-LMS. The performance improvement of the MoG-LMS algorithm is verified by experiments.

Robust weighted K-means clustering algorithm for a probabilistic-shaped 64QAM coherent optical communication system.

A novel weighted K-means scheme for a probabilistic-shaped (PS) 64 quadrature amplitude modulation (QAM) signal is proposed in order to locate the decision points more accurately and enhance the robustness of clustering algorithm. By using a weighting factor following the reciprocal of Maxwell-Boltzmann distribution, the proposed algorithm can combine the advantages of PS and K-means robustly while reducing the overall computational complexity of the clustering process. Experimental verification of the proposed clustering technique was demonstrated in a 120-Gb/s probabilistic-shaped 64QAM coherent optical communication system. The results show that the proposed algorithm has outperformed K-means with respect to bit error rate (BER), clustering robustness and iteration times in both back-to-back and 375km transmission scenarios. For the back-to-back situation, the proposed algorithm is capable of achieving about 0.6dB and 1.8dB OSNR gain over K-means clustered signals and unclustered signals. For the case of transmission, the proposed clustering procedure can robustly locate the optimal decision points with launched signal power ranging from -5dBm to 5dBm, while the working range for K-means procedure is only -4dBm to 2dBm. In addition, the proposed weighted algorithm takes less iteration times than K-means to converge, especially when the signal impairments caused by fiber Kerr nonlinearity is severe.

DNN-based aberration correction in a wavefront sensorless adaptive optics system.

Existing wavefront sensorless (WFS-less) adaptive optics (AO) generally require a search algorithm that takes lots of iterations and measurements to get optimal results. So the latency is a serious problem in the current WFS-less AO system, especially in applications to free-space optics communication. To solve this issue, we propose a deep neural network (DNN)-based aberration correction method. The DNN model can detect the wavefront distortion directly from the intensity images, thereby avoiding time-consuming iterative processes. Since the tip-and-tilt mode of Zernike coefficients are considered, the tip-tilt correction system is not necessarily required in the proposed method. From our simulation results, the proposed method can effectively reduce the computation time and has an impressive improvement of root mean square (RMS) in different turbulence conditions.

Piezotronic Effect on Rashba Spin-Orbit Coupling in a ZnO/P3HT Nanowire Array Structure.

A key concept in the emerging field of spintronics is the voltage-gate control of spin precession via the effective magnetic field generated by the Rashba spin-orbit coupling (SOC). Traditional external gate voltage usually needs a power supply, which can easily bring about background noise or lead to a short circuit in measurement, especially for nanoscale spintronic devices. Here, we present a study on the circular photogalvanic effect (CPGE) in a ZnO/P3HT nanowire array structure with the device excited under oblique incidence. We demonstrate that a strong Rashba SOC is induced by the structure inversion asymmetry of the ZnO/P3HT heterointerface. We show that the Rashba SOC can be effectively tuned by inner-crystal piezo-potential created inside the ZnO nanowires instead of an externally applied voltage. The piezo-potential can not only ensure the stability of future spin-devices under a static pressure or strain but also work without the need of extra energy; hence this room-temperature generation and piezotronic effect control of spin photocurrent demonstrate a potential application in large-scale flexible spintronics in piezoelectric nanowire systems.

Enhancing the Efficiency of Silicon-Based Solar Cells by the Piezo-Phototronic Effect.

Although there are numerous approaches for fabricating solar cells, the silicon-based photovoltaics are still the most widely used in industry and around the world. A small increase in the efficiency of silicon-based solar cells has a huge economic impact and practical importance. We fabricate a silicon-based nanoheterostructure (p+-Si/p-Si/n+-Si (and n-Si)/n-ZnO nanowire (NW) array) photovoltaic device and demonstrate the enhanced device performance through significantly enhanced light absorption by NW array and effective charge carrier separation by the piezo-phototronic effect. The strain-induced piezoelectric polarization charges created at n-doped Si-ZnO interfaces can effectively modulate the corresponding band structure and electron gas trapped in the n+-Si/n-ZnO NW nanoheterostructure and thus enhance the transport process of local charge carriers. The efficiency of the solar cell was improved from 8.97% to 9.51% by simply applying a static compress strain. This study indicates that the piezo-phototronic effect can enhance the performance of a large-scale silicon-based solar cell, with great potential for industrial applications.

Surface modification effect on photoluminescence of individual ZnO nanorods with different diameters.

Optical properties of Al2O3 coated individual ZnO nanorods (NRs) with different diameters were studied by confocal micro-photoluminescence spectroscopy. The one-dimensional ZnO/Al2O3 core-shell NRs showed enhanced near band-edge emission compared with the same ZnO NRs before Al2O3 coating at room temperature. Besides, the relative intensity of the deep-level emission with respect to the near band-edge emission was reduced. A model was proposed to explain these spectral changes. For ZnO NRs with diameters above 360 nm, a multi-mode behavior resulting from whispering gallery resonance was observed. In addition, selective enhancement or quenching of different whispering gallery modes in ultraviolet (UV) emission was observed after Al2O3 coating at room temperature, which is due to the larger refractive index of Al2O3 compared with air. We proposed a model to explain these spectral changes as well. By comparing the optical properties before and after surface coating, our results suggest that surface coating of an Al2O3 layer is an effective way to tailor the optical properties of ZnO NRs.

Carbon nanotube photoelectronic and photovoltaic devices and their applications in infrared detection.

Semiconducting carbon nanotubes (CNTs) are direct bandgap materials with outstanding electronic and optoelectronic properties and have been investigated for various electronic and optoelectronic device applications, such as light-emitting diodes, photodetectors and photovoltaic cells. Here, a brief review of the various types of CNT diodes is presented, with a focus on one particular type of CNT diodes fabricated via a doping-free process. Their application for constructing high-performance optoelectronic and photovoltaic devices is also discussed, as well as the newly discovered photovoltage multiplication effect in CNTs and its application in improving the efficiency of CNT-based infrared detector.

Pointwise plucking of suspended carbon nanotubes.

Vibration of nanotubes/wires is significant for fundamental and applied researches. However, it remains challenging to control the vibration with point-level precision. Herein, individual suspended carbon nanotubes are plucked point by point to vibrate in scanning electron microscope with the electron beam as a nanoscale pointer. The vibration is directly imaged, and its images fit well with simulations from the plucking mechanism. This demonstrates a new way to manipulate the nanotube vibration with unprecedented precision.

Channel-length-dependent transport and photovoltaic characteristics of carbon-nanotube-based, barrier-free bipolar diode.

Carbon nanotube (CNT) diodes with different channel length between L = 0.6 µm to 3.5 µm are fabricated on the same tube, and the electric and photovoltaic characteristics are investigated. It is found that although the open voltage of the diode increases rapidly for channel length L less than 1.0 µm, it saturates for longer channel devices. On the other hand, the short circuit current of the diode exhibites a clear peak at intermediate channel length of about 1.5 µm, a large leakage current via tunneling for short channel device and significantly decreased current for long channel device due to the increased recombination and channel resistance. The optimal channel length for a CNT diode in photovoltaic application is thus determined to be about 1.5 µm.

High-performance carbon nanotube light-emitting diodes with asymmetric contacts.

Electroluminescence (EL) measurements are carried out on a two-terminal carbon nanotube (CNT) based light-emitting diode (LED). This two-terminal device is composed of an asymmetrically contacted semiconducting single-walled carbon nanotube (SWCNT). On the one end the SWCNT is contacted with Sc and on the other end with Pd. At large forward bias, with the Sc contact being grounded, electrons can be injected barrier-free into the conduction band of the SWCNT from the Sc contact and holes be injected into the valence band from the Pd electrode. The injected electrons and holes recombine radiatively in the SWCNT channel yielding a narrowly peaked emission peak with a full width at half-maximum of about 30 meV. Detailed EL spectroscopy measurements show that the emission is excitons dominated process, showing little overlap with that associated with the continuum states. The performance of the LED is compared with that based on a three-terminal field-effect transistor (FET) that is fabricated on the same SWCNT. The conversion efficiency of the two-terminal diode is shown to be more than three times higher than that of the FET based device, and the emission peak of the LED is much narrower and operation voltage is lower.

Growth and performance of yttrium oxide as an ideal high-kappa gate dielectric for carbon-based electronics.

High-quality yttrium oxide (Y(2)O(3)) is investigated as an ideal high-kappa gate dielectric for carbon-based electronics through a simple and cheap process. Utilizing the excellent wetting behavior of yttrium on sp(2) carbon framework, ultrathin (about few nm) and uniform Y(2)O(3) layers have been directly grown on the surfaces of carbon nanotube (CNT) and graphene without using noncovalent functionalization layers or introducing large structural distortion and damage. A top-gate CNT field-effect transistor (FET) adopting 5 nm Y(2)O(3) layer as its top-gate dielectric shows excellent device characteristics, including an ideal subthreshold swing of 60 mV/decade (up to the theoretical limit of an ideal FET at room temperature). The high electrical quality Y(2)O(3) dielectric layer has also been integrated into a graphene FET as its top-gate dielectric with a capacitance of up to 1200 nF/cm(2), showing an improvement on the gate efficiency and on state transconductance of over 100 times when compared with that of its back-gate counterpart.