Spectral Operation Research Articles

High-performance Teraherz photodetection in 2D materials and topological materials

Abstract Photodetectors leveraging two-dimensional (2D) materials and topological materials have garnered substantial interest due to their exceptional electronic and optoelectronic characteristics. These materials, including 2D semimetals like graphene, semiconducting transition metal dichalcogenides, and topological insulators such as bismuth selenide, exhibit a broad array of bandgap values and unique photon interaction properties. To date, numerous high-performance photodetectors using these materials have been documented, showing significant potential in terahertz (THz) frequency applications. This review presents a comprehensive examination of photodetectors based on 2D and topological materials, focusing on the THz frequency. Initially, an insight into the photocurrent generation mechanisms within these materials is provided, alongside a discussion of the figure-of-merits, such as responsivity and detectivity, which are crucial for evaluating photodetector performance. The recent advancements in THz photodetection are then highlighted, noting exceptional attributes such as high sensitivity, ultrafast response, broad spectral operation, and anisotropic detection capabilities, based on cutting-edge devices. Early-stage applications and the integration potential of these photodetectors in various technologies are also explored. Concluding, the manuscript offers a forward-looking perspective, outlining ongoing challenges, future research directions, and practical advice for developing next-generation THz photodetectors, aiming to inspire continued innovation in this rapidly evolving field.

Extending the spectral operation of multimode and polarization-independent power splitters through subwavelength nanotechnology

Power splitters play a crucial role in virtually all photonic circuits, enabling precise control of on-chip signal distribution. However, state-of-the-art solutions typically present trade-offs in terms of loss, bandwidth, and fabrication robustness, especially when targeting multimode operation. Here, we present a novel multimode 3-dB power splitter based on a symmetric Y-junction assisted by two regions of subwavelength grating metamaterials with different geometries. The proposed device demonstrates high-performance for multimode and dual-polarization operation with relaxed fabrication tolerances by leveraging the additional degrees of freedom offered by two distinct geometries of subwavelength metamaterials to control mode evolution. Our design achieves, to the best of our knowledge, the widest operational bandwidth reported to date for a nanophotonic multimode silicon power splitter. Simulations for a standard 220-nm-thick silicon-on-insulator platform predict minimal excess loss (< 0.2 dB) for the fundamental and the first-order transverse-electric modes over an ultra-broad 700 nm bandwidth (1300 – 2000 nm). For the fundamental transverse–magnetic mode, losses are less than 0.3 dB in the 1300 – 1800 nm range. Experimental measurements validate these predictions in the 1430 – 1630 nm wavelength range, demonstrating losses < 0.4 dB for all three modes, even in the presence of fabrication deviations of up to ± 10 nm. We believe that this device is suitable for the implementation of advanced photonic applications requiring high–-performance distribution of optical signals, such as programmable photonics, multi-target spectroscopy and quantum key distribution.

A Modified EMD Technique for Broken Rotor Bar Fault Detection in Induction Machines.

Induction machines (IMs) are commonly used in various industrial sectors. It is essential to recognize IM defects at their earliest stage so as to prevent machine performance degradation and improve production quality and safety. This work will focus on IM broken rotor bar (BRB) fault detection, as BRB fault could generate extra heating, vibration, acoustic noise, or even sparks in IMs. In this paper, a modified empirical mode decomposition (EMD) technique, or MEMD, is proposed for BRB fault detection using motor current signature analysis. A smart sensor-based data acquisition (DAQ) system is developed by our research team and is used to collect current signals wirelessly. The MEMD takes several processing steps. Firstly, correlation-based EMD analysis is undertaken to select the most representative intrinsic mode function (IMF). Secondly, an adaptive window function is suggested for spectral operation and analysis to detect the BRB fault. Thirdly, a new reference function is proposed to generate the fault index for fault severity diagnosis analytically. The effectiveness of the proposed MEMD technique is verified experimentally.

Optical color routing enabled by deep learning.

Nano-color routing has emerged as an immensely popular and widely discussed subject in the realms of light field manipulation, image sensing, and the integration of deep learning. The conventional dye filters employed in commercial applications have long been hampered by several limitations, including subpar signal-to-noise ratio, restricted upper bounds on optical efficiency, and challenges associated with miniaturization. Nonetheless, the advent of bandpass-free color routing has opened up unprecedented avenues for achieving remarkable optical spectral efficiency and operation at sub-wavelength scales within the area of image sensing applications. This has brought about a paradigm shift, fundamentally transforming the field by offering a promising solution to surmount the constraints encountered with traditional dye filters. This review presents a comprehensive exploration of representative deep learning-driven nano-color routing structure designs, encompassing forward simulation algorithms, photonic neural networks, and various global and local topology optimization methods. A thorough comparison is drawn between the exceptional light-splitting capabilities exhibited by these methods and those of traditional design approaches. Additionally, the existing research on color routing is summarized, highlighting a promising direction for forthcoming development, delivering valuable insights to advance the field of color routing and serving as a powerful reference for future endeavors.

State of health and remaining useful life prediction of lithium-ion batteries with conditional graph convolutional network

Graph convolutional networks (GCNs) have been increasingly used to predict the state of health (SOH) and remaining useful life (RUL) of batteries. However, conventional GCNs have limitations. Firstly, the correlation between features and the SOH or RUL is not considered. Secondly, temporal relationships among features are not considered when projecting aggregated temporal features into another dimensional space. To address these issues, two types of undirected graphs are introduced to simultaneously consider the correlation among features and the correlation between features and the SOH or RUL. A conditional GCN is built to analyze these graphs. A dual spectral graph convolutional operation is introduced to analyze the topological structures of these graphs. Additionally, a dilated convolutional operation is integrated with the conditional GCN to consider the temporal correlation among the aggregated features. Two battery datasets are used to evaluate the effectiveness of the proposed method. Experimental results show that the proposed method outperforms other machine learning methods reported in the literature.

“A Name Which to Them is Sacred”: Quakers and Religious Trademarks in American Enterprise

Abstract This article examines disputes surrounding the ownership and appropriation of the name “Quaker” in American enterprise in the late nineteenth and early twentieth century. I focus specifically on efforts by the Society of Friends – colloquially known as the Quakers – to prohibit the use of the name of any religious church, denomination, society, or association in commerce, thereby rendering religious trademarks unregistrable. By exploring disputes around the Quaker name, I argue that the Society of Friends strategically appropriated terminology provided by trademark law (product origin, goodwill, distinctiveness) to claim ownership in the Quaker name while simultaneously arguing that religious names were sacred and thus could not be owned as a form of intellectual property. This historical debate provides insights to crucial questions about the spectral operation of trademark law in the United States. For instance, how are the taxonomies and logics of trademark law ‘possessed’ or spectrally animated by other pre-existing cultural/theological dimensions of ownership? Conversely, how does trademark law then reconstitute pre-existing logics around naming within religious organizations, for instance by providing new frames (the name as defense for consumer confusion or as guarantor of divine source) for understanding American religious practice?

Dosimetry for radiobiological in vivo experiments at laser plasma-based proton accelerators

Objective. Laser plasma-based accelerators (LPAs) of protons can contribute to research of ultra-high dose rate radiobiology as they provide pulse dose rates unprecedented at medical proton sources. Yet, LPAs pose challenges regarding precise and accurate dosimetry due to the high pulse dose rates, but also due to the sources’ lower spectral stability and pulsed operation mode. For in vivo models, further challenges arise from the necessary small field dosimetry for volumetric dose distributions. For these novel source parameters and intended applications, a dosimetric standard needs to be established. Approach. In this work, we present a dosimetry and beam monitoring framework for in vivo irradiations of small target volumes with LPA protons, solving aforementioned challenges. The volumetric dose distribution in a sample (mean dose value and lateral/depth dose inhomogeneity) is provided by combining two independent dose measurements using radiochromic films (dose rate-independent) and ionization chambers (dose rate-dependent), respectively. The unique feature of the dosimetric setup is beam monitoring with a transmission time-of-flight spectrometer to quantify spectral fluctuations of the irradiating proton pulses. The resulting changes in the depth dose profile during irradiation of an in vivo sample are hence accessible and enable pulse-resolved depth dose correction for each dose measurement. Main results. A first successful small animal pilot study using an LPA proton source serves as a testcase for the presented dosimetry approach and proves its performance in a realistic setting. Significance. With several facilities worldwide either setting up or already using LPA infrastructure for radiobiological studies with protons, the importance of LPA-adapted dosimetric frameworks as presented in this work is clearly underlined.

Remaining useful life prediction of bearings with attention-awared graph convolutional network

Graph Convolutional Networks (GCNs) have recently been used to predict the remaining useful life (RUL) of bearings due to its effectiveness in revealing correlations in condition monitoring data. However, traditional GCNs use a single graph only, either a temporal-correlated graph or a feature-correlated graph without considering both temporal and feature correlations of condition monitoring data. Additionally, traditional GCNs rely heavily on pre-defined graphs to aggregate correlated features. However, the topology of these pre-defined graphs may vary depending on a pre-defined threshold for cosine similarity or covariance which might affect prediction accuracy and robustness. To address these issues, we introduce a spectral graph convolutional operation that can handle both temporal-correlated and feature-correlated graphs, which allows one to consider both the temporal and feature correlations simultaneously. Moreover, we introduce a self-attention mechanism to construct the temporal-correlated and feature-correlated graphs automatically without defining a threshold. Such a mechanism allows the predictive model to learn graphs automatically during training so that the prediction accuracy and robustness can be significantly improved. The proposed method is demonstrated on two bearing datasets, and the experimental results have shown that it outperforms both traditional GCNs and other deep-learning methods in predicting RUL of bearings.

Trajectory monitoring method based on optical sensing spectral energy and adaptive threshold

Distributed optical fiber sensing technology is widely used in structural health monitoring, security systems and other fields. The traditional monitoring method of electrical measurement can ensure monitoring accuracy of train trajectory, while this method is easily affected by thunderstorm weather. In order to simultaneously improve the anti-interference ability and monitoring accuracy, this paper proposes a trajectory monitoring method based on optical sensing spectral energy and adaptive threshold. This method only needs to monitor the backward Rayleigh scattering optical signal of a redundant communication fiber, and perform Fourier transform and spectral energy operation to analyze vibration signals of the entire line. An adaptive threshold algorithm is utilized to autonomously locate fiber position of the train and form a spatial–temporal trajectory map. The method proposed in this paper is a passive monitoring method using optical sensing, which greatly improves system’s ability to resist electromagnetic interference, reduces the monitoring cost of the entire system, and effectively ensures the positioning accuracy. Experimental results demonstrate the effectiveness, safety, and accuracy of the train trajectory monitoring method based on optical sensing spectral energy and adaptive threshold.

Ultrafast Carbon Nanotube Photodetectors with Bandwidth over 60 GHz

The future interconnect links in intra- and inter-chip require the photodetector with high bandwidth, ultra-wide waveband, compact footprint, low-cost, and compatible integration process with silicon complementary metal-oxide-semiconductor (CMOS) technology. Here, we demonstrate a CMOS-compatible carbon nanotube (CNT) photodetector that exhibits high responsivity, high bandwidth and broad spectral operation over all optical telecommunication band based on high-purity CNT arrays. The ultrafast CNT photodetector demonstrates the 100 Gbit/s Nyquist-shaped on-off-keying (OOK) signal transmission, which can address the demand for high-speed optical interconnects in and between data centers. Furthermore, the photodetector exhibits a bandwidth over 60 GHz by scaling down the active area to 20 {\mu}m2. As the CNT photodetectors are fabricated by doping-free process, it also provides a cost-effective solution to integrate CNT photonic devices with CNT-based CMOS integrated circuits. Our work paves a way for future CNT-based high-speed optical interconnects and optoelectronic integrated circuits (OEICs).

Spectral characterization and laser operation of Ho:SrF2 single-crystal fiber

An efficient Ho:SrF2 single-crystal fiber (SCF), lightly Ho-doped to a concentration of 0.5 at%, was successfully grown using the temperature gradient technique, and its laser characteristics were studied. Several optical parameters of the Ho:SrF2 SCF were determined using the Judd-Ofelt (J-O) theory, including the J-O intensity parameters, radiative lifetimes, and fluorescence branching ratios. In addition, the continuous-wave (CW), tunable, and passive Q-switched (PQS) laser operations of Ho:SrF2 SCF pumped by a thulium-doped fiber laser were demonstrated in this study. While the CW laser was operating, average output power greater than 1 W and a continuous tuning range of 62 nm were obtained. For the PQS operation, a minimum pulse duration of 779 ns with a repetition rate of 36.27 kHz was achieved using silver nanorods as saturable absorbers. These results indicate that the Ho:SrF2 SCF has potential application in tunable or ultrafast laser at 2.1 µm.

Properties on Bearing Fault Diagnosis of Water Injection Motor

Water injection motor in western Daqing Oilfield is the key equipment of water injection to the oil field. Due to the great pressure of water injection to the oil field, the water injection motor is often overloaded and runs at high speed, so that the rolling bearing of the motor is worn, deformed and other faults. In order to detect the small faults that are not easy to detect in the early stage of the motor bearing, this paper proposes a method combining variational mode decomposition (VMD) and fast spectral kurtosis. The optimal number of modes is determined by correlation and energy ratio. The fast spectral kurtosis operation is carried out on the modes with large kurtosis. The fault signal is enveloped and decomposed by the optimal filter parameters. The experimental results show that the algorithm can successfully identify the rolling bearing fault of water injection motor and determine the fault location.

Compact Remote Spectral Terahertz Imager

We report on development of an array of spectral sensitive detectors cooled down to 70 K by the compact cryocooler. The spectral sensitive operation of the detectors, explored between 0.16 THz and 0.22 THz, is due to the resonant excitation of the plasma waves in the two-dimensional electron gas of GaAs/AlGaAs heterostructure. A typical responsivity of the detectors is 0.01 A/W at 70 K while it increases by two orders of magnitude when the detector array is cooled down to 0.5 K. The photo-response has surprisingly a narrow peak of spectral sensitivity, sim 2–5%, which are highly likely due to the dimensional resonances of the plasma waves. As a demonstration of the spectral sensitive operation we detect a spectral feature of LiNbO_{3} crystal at 0.174 THz.

Spectral operation in locally convex algebras

We show that if A is a spectrally bounded algebra, then all functions operate spectrally on A if and only if SpAx is finite for every x ∈ A. We also prove that if A is a commutative Q-l.m.c.a, then all functions operate spectrally on A if and only if A/RadA is algebraic. Furthermore, if A is a semi-simple commutative Q-l.m.c.a. which is a Baire space, all functions operate spectrally on A if and only if it is isomorphic to Cn for some n ∈ N. A structure result concerning semi-simple commutative complete m-convex algebras of countable dimension is also given.

High-quality all-fluorescent white organic light-emitting diodes obtained by balancing carriers with hole limit layer

Broad-spectrum white organic light-emitting diodes (WOLEDs) based on all-fluorescent materials with excellent color stability were realized by precisely optimizing the doping concentrations of guests and the thickness of each functional layer. High-efficiency blue fluorescent emissive material 9,10-bis[4-(6-methylbenzothiazol-2-yl)phenyl]anthracene (DBzA) was selected as blue emitter and doped into first light emitting layer (EML), while red-emitting dopant 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidn-4-yl-vinyl)- 4H-pyran (DCJTB) was doped into green-emitting host material tris(8-hydroxyquinoline) aluminum (Alq3) as the second EML. Thin hole limit layer (HLL) was inserted to balance carriers' distribution within the two EMLs and to modulate the luminous intensity ratio of different emissions. Finally, the optimal WOLED exhibited the maximum current efficiency of 9.34 cd/A, power efficiency of 10.06 lm/W, brightness up to 29,364 cd/m2 and turn-on voltage of only 2.7 V. In addition, this device displayed stable Commission International de I'Eclairage coordinates from (0.339, 0.382) to (0.324, 0.354) with increasing current density. The highest color rendering index and corresponding correlated color temperature reach 85 and 5492 K, respectively, and the T50 lifetime reaches 7912 h. The achievement of these results fully exhibits the effectiveness of the HLL in improving spectral stability and operation lifetime.

DAC-Less PAM-4 Slow-Light Silicon Photonic Modulator Providing High Efficiency and Stability

We report a slow-light silicon modulator that enables high-speed PAM operation without using an electrical digital-to-analogue converter (DAC). Bragg grating resonators, integrated into each arm of a Mach-Zehnder modulator, enhance the phase modulation through the slow light effect. The optical 4-level PAM signal is generated by driving directly the segmented phase shifter design with two binary signals. This modulator presents an ultra-compact footprint (L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SL-MZM</sub> = 570 μm), low energy consumption (73 fJ/bit), large electro-optic bandwidth (>40 GHz). Up to 90 Gb/s is achieved over an nm-range spectral operation bandwidth (Δλ = 2 nm). Compared to other low-energy resonator-based modulators, such as micro-rings, this operating bandwidth confers higher stability with a potential operating temperature range of ΔT = 50 °C. We further examine the robustness of the proposed design to fabrication variations by measurements of spectral properties across the wafer. This modulator is of particular interest for applications, such as short-range data communications that require multiple compact and energy-efficient modulators on a single chip.

Symmetry-Aware Semantic Segmentation of a 3D Human Body Model

<p indent="0mm">An automatic segmentation method for a 3D human model is proposed for the symmetry-aware semantic segmentation. Firstly, a symmetry-aware kinematic skeleton is extracted from the input human body model via template skeleton embedding. Secondly, the number of the segments is determined by the kinematic skeleton; some key points corresponding to the segments are extracted meanwhile. Thirdly, while taking the key points as the boundary constraints, the harmonic fields are constructed to obtain a set of isolines as the potential cutting boundaries. Finally, the key points are taken as the clustering centers to conduct a spectral clustering operation, which guides the selections of the cutting boundaries. In this way, the semantic information in the kinematic skeleton can be directly transferred to the segments, and the number of segments can be controlled by the structure of the kinematic skeleton. Results on the SCAPE dataset, the MPI-FAUST dataset and the Princeton Segmentation Benchmark show that the proposed method can automatically achieve symmetry-aware semantic segmentation, and is robust to the shape and the pose of the human body model.

Spectral properties and laser operation of Nd3+-doped mixed sesquioxide LuYO3 ceramic

The absorption spectrum, fluorescence spectrum and fluorescence time of a mixed sesquioxide Nd:LuYO3 ceramic were measured at room temperature. Judd-Ofelt theory was applied to calculate the spectral parameters of Nd:LuYO3. The absorption cross section at 812 nm was calculated to be 0.99 × 10−20 cm2 with full width at half maximum (FWHM) of 47.69 nm. The emission cross-section at 1077 nm was calculated to be 5.24 × 10−20 cm2 with FWHM of 11.62 nm. The fluorescence lifetime and quantum efficiency of the 4F3/2 multiplet were determined to be 228.4 μs and 85.2%, respectively. Continuous-wave (CW) laser operation of Nd:LuYO3 ceramic was demonstrated. Simultaneous laser operation at 1075.24 and 1080.03 nm was achieved with maximum output power of 490 mW and slope efficiency of 6.36%.

QoT assessment of the optical spectrum as a service in disaggregated network scenarios

The potential to operate third-party terminals over multi-domain transparent optical networks attracts operators and customers to implement optical spectrum as a service (OSaaS). Because infrastructure information cannot always be shared with OSaaS end customers, alternatives to off-line quality of transmission (QoT) estimation tools are required to assess the performance of the spectrum slot to estimate achievable throughput. In this paper, commercially available sliceable coherent transceivers are used to assess the generalized signal-to-noise ratio (GSNR) based QoT of the OSaaS in a live production network for both narrowband and wideband OSaaS configurations. Extended channel probing based on symbol rate variability is combined with spectral sweeping and operation regime detection to characterize OSaaS implementations on 17 links with different underlying infrastructure configurations to maximize capacity and increase service margins in a low-margin operation regime. We achieve 0.05 dB estimation accuracy in the GSNR for wideband spectrum services and 0.32 dB accuracy for narrowband spectrum services. Based on the GSNR profile, spectral misalignment, spectral ripple, and operation regime are detected and service margin improvements are demonstrated. Finally, we discuss the network optimization perspective based on acquired data from channel probing and propose use cases for continuous channel probing in transparent optical networks.

Polarized spectral properties and laser operation of Nd:SrAl12O19 crystal

Nd:SrAl11O19 (Nd:SRA) crystal has been grown by the Czochralski method. Polarized absorption spectra, polarized fluorescence spectra and fluorescence lifetime were investigated. For σ-polarization, the peak absorption cross section at 798 nm is 2.08 × 10−20 cm2 with full width at half maximum (FWHM) of 10.2 nm and the peak emission cross section is 3.36 × 10−20 cm2 at 1048.5 nm with FWHM of 4.80 nm. The Judd-Ofelt parameters Ω2,4,6 were obtained to be 0.51 × 10−20 cm2, 2.08 × 10−20 cm2, and 2.12 × 10−20 cm2, respectively. Continuous-wave laser operation of the a-cut and c-cut Nd:SRA sample has been demonstrated. The maximum output power of 8.33 W was achieved from c-cut sample at an absorbed power of 27.73 W, corresponding to a slope efficiency of 31.3%.