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Next-generation energy-efficient photonic integrated systems, such as neuromorphic computational chips require efficient heterogeneous integration of ultracompact light sources and photodetectors through highly dense waveguide circuits. However, interconnecting these devices in emitter-receiver communication circuits remains a challenge and an obstacle towards upscaling heterogeneous photonic chips. Here we report on versatile air-cladded free-standing 3D polymer waveguides (OrmoCore, n≈1.5) spanning up to 900 µm in length without intermediate mechanical support structures, microprinted via two-photon polymerization. The presented waveguides are suitable for on-chip out-of-plane light coupling as well as non-connected 3D crossings, needed for high density optical circuits. The waveguides show optical transmission losses of 1.93 dB mm−1 at λ = 635 nm, and of 3.71 dB mm−1 at λ = 830 nm in the wavelength range of GaAs-based microLEDs spectral emission. On-chip imaging for high-precision alignment and TPP microfabrication are performed seamlessly by utilizing the same laser source for both steps, allowing accurate 3D printing on microstructured substrates. As proof of concept, we interconnect two GaAs-based microLEDs via an on-chip microprinted 3D waveguide. Such combined systems can serve as building blocks of future complex integrated heterogeneous photonic networks. [ABSTRACT FROM AUTHOR]

Silicon-based optical switches are integral to on-chip optical interconnects, and mode-division multiplexing (MDM) technology has enabled modes to function as carriers in routing, further boosting optical switches' link capacity. However, traditional multimode optical switches, which typically use Mach–Zehnder interferometer (MZI) structures and mode (de)multiplexers, are complex and occupy significant physical space. In this paper, we propose and experimentally demonstrate a novel demultiplexing-free dual-mode 3 × 3 thermal-optical switch based on micro-rings (MRs) and mode exchangers (MEs). All MRs are designed to handle TE1 mode, while the ME converts TE0 mode to TE1 mode, enabling separate routing of both modes. Bezier curves are employed to optimize not only the ME, but also the dual-mode 45° and 90° waveguide bends, which facilitate the flexible and compact layout design. Moreover, our structure can support multiple wavelength channels and spacings by adding pairs of MRs, exhibiting strong WDM compatibility. The switch has an ultra-compact footprint of 0.87 × 0.52 mm2. Under both "all-bar" and "all-cross" configurations, its insertion losses (ILs) remain below 8.7 dB at 1,551 nm, with optical signal-to-noise ratios (OSNRs) exceeding 13.0 dB. Also, 32 Gbps data transmission experiments validate the switch's high-speed transmission capability. [ABSTRACT FROM AUTHOR]

Modulating characteristics of transmissive and reflective Fabry–Perot type multi-nanolayer conductor–dielectric electro-optical modulators (EOMs) for chip-to-chip free space optical interconnection are analyzed via wavelength-scale electromagnetic modelling. For electromagnetic analysis the frequency-domain method of single expression is used. The considered EOM structures consist of an electro-optical spacer of LiNbO3 covered by two thin ITO conducting nano-layers, surrounded by Si/SiO2 distributed Bragg reflectors (DBRs). In the case of transmissive EOM the DBRs are symmetric regarding the spacer, while in the case of reflective EOM they are asymmetric to provide high reflectance for the structure. From four possible types of DBRs the suitable structure has been chosen with layers of higher permittivity adjoining to the ITO nano-layers. ITO nano-layers serving as electric contacts for supplying modulating electrical signal to the electro-optical spacer are parts of the multi-nanolayer structures and are included in the electromagnetic models. An incident radiation from an external laser diode at 1.55 μm wavelength is considered. The optimal configurations of the EOM structures providing a high peak in the transmittance for the transmissive EOM and a narrow dip in the reflectance for the reflective EOM are proposed. Efficiency of optical wave intensity modulation is analyzed by means of influence of electro-optical spacer's permittivity change on the transmittance and the reflectance of the EOM structures. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated a tunable and high transmission efficiency plasmonic power splitter and also a wavelength demultiplexer based on metal–insulator-metal surface. By using Finite Difference Time Domain method, the power splitter exhibits high transmission peaks of about 45% in each output port. Additionally, the simulation results demonstrate that the equal transmitted wavelengths of the two output ports can be easily tuned by changing the geometric parameters of the waveguide. Subsequently, the structure was amended to become a demultiplexer with selecting wavelengths of 1272 and 912 nm, and transmission efficiencies of 76.44 and 88.03%, respectively. Following that, the dependence of demultiplexing wavelengths on geometrical parameters of the structure is investigated. Besides being highly compact and efficient, having narrow-band spectra and low reflectance coefficient are the other main advantages of these devices. The compact power splitter presented in this study is one of the critical components of an integrated processor and hence can be of great interest in a wide range of applications, from highly integrated photonic circuits to on-chip optoelectronic modules and other optical devices such as erbium-doped fiber amplifiers. [ABSTRACT FROM AUTHOR]

This two-part work develops a comprehensive toolbox for the statistical characterization of nonlinear distortions of DAC-generated signal constellations to be transmitted over communication links, be they electronic (wireline, wireless) or photonic, Mach–Zehnder modulator (MZM)-based optical interconnects in particular. The all-analytic toolbox developed here delivers closed-form expressions for the second-order statistics (means, variances) of all relevant constellation metrics of the DACs' building blocks and of DAC-driven MZM-based optical transmitters, all the way to the slicer in the optical receivers over a linear channel with coherent detection. The key impairment targeted by the model is the random current mismatch of the ASIC devices implementing the DAC drivers. In particular the (skew-)centrosymmetry of the DAC metrics is formally derived and explored. A key applicative insight is that the conventional INL/DNL (Integral NonLinearity/Differential NonLinearity) constellation metrics, widely adopted in the electronic devices and circuits community, are not quite useful in the context of communication systems, since these metrics are ill-suited to predict communication link statistical performance. To rectify this deficiency of existing electronic DAC metrics, we introduce modified variants of the INL|DNL, namely the integral error vector (IEV) and the differential error vector (DEV) constellation metrics. The new IEV|DEV represent straightforward predictors of relevant communication-minded metrics: error vector magnitude (EVM) treated here in Part I, and Symbol/Bit Error-Rates (SER, BER) treated in the upcoming Part II of this paper. [ABSTRACT FROM AUTHOR]

Intensity-modulated direct-detection (IM/DD) optical systems are most widely employed in short-reach optical interconnects due to their simple structure and cost-effectiveness. However, IM/DD systems face mixed linear and nonlinear channel impairments, mainly induced by the combination of square-law detection and chromatic dispersion, as well as the utilization of low-cost non-ideal transceivers. To solve this issue, recent years have witnessed a growing trend of introducing machine learning technologies such as neural networks (NNs) into IM/DD systems for channel equalization. NNs usually present better system performance than traditional approaches, and various types of NNs have been investigated. Despite the excellent system performance, the associated high computational complexity is a major drawback that hinders the practical application of NN-based equalizers. This paper focuses on the performance and complexity trade-off of NNs employed in IM/DD systems, presenting a systematic review of the current status of NN-based equalizers as well as a number of effective complexity reduction approaches. The future trends of leveraging advanced NN in IM/DD links are also discussed. [ABSTRACT FROM AUTHOR]

With the advantages of high speed, small size, and easy integration, the silicon photonic resonant ring modulator has gradually become a critical device for emerging integrated optical platforms. Ring modulators are primarily used in optical communications, optical computing, artificial intelligence, and other fields. In this work, the proposed ring modulator can operate in both the O- and C-bands. The 3 dB electro-optical (EO) bandwidth of the ring modulator is 39 GHz and 34 GHz at −4 V in the O-band and C-band, respectively. The modulation efficiency of the device is 0.92 V·cm and 0.95 V·cm in the O-band and C-band, respectively. The eye diagram of an optical output signal from the device is tested using a 100 Gbit/s non-return-to-zero (NRZ) input signal with a 2.5 Vpp in both the O-band and C-band. The modulation speed can reach 140 Gb/s and 120 Gb/s in the O-band and C-band with four-level pulse amplitude modulation (PAM-4) formats at a voltage swing of 2.5 Vpp, respectively. [ABSTRACT FROM AUTHOR]

The hardware‐level construction of artificial synapses, mimicking the functionality of biological synapses, is widely acknowledged as a pivotal stride in artificial intelligence systems. Particularly, establishing all‐optical artificial synapses for neuromorphic computing, leveraging on‐chip optical interconnects instead of conventional wired connections, has garnered widespread attention. Here, high‐quality CsPbBr3 single crystals are synthesized, and intriguing relaxation processes associated with reversible photo‐induced phase transition (PIPT) are observed, leading to a corresponding change in the birefringence effect. The birefringence change is systematically analyzed by an optical system. The relaxation time of reversible PIPT is similar to the behavior patterns of biological synapses, enabling the device to mimic synaptic features. The device successfully mimics excitatory/inhibitory synaptic behaviors such as EPSP/IPSP, PPF, SDDP, SFDP, and SIDP by using 1310 nm signal. Furthermore, the device continues to exhibit excitatory/inhibitory synaptic functionality when 940 and 1550 nm signals are applied, achieving multi‐wavelength synaptic behaviors. The all‐optical devices have the advantages of low crosstalk, fast signaling, and wide optical bandwidth, which lays the foundation of all‐optical hardware for neuromorphic computing systems. [ABSTRACT FROM AUTHOR]

A novel bidirectional passive optical interconnection is proposed for data centers (DCs). The interconnection is based on a coupler of arrayed waveguide grating. The nodes distribute on both sides and simultaneously, bidirectional transmission is allowed. Compared with the traditional passive optical interconnected DCs, the scale is doubled, and the throughput is increased. The crosstalk in simultaneously bidirectional transmission is tested and the results show it is very little. The transmission between the nodes on the same side is relayed by one node on the other side. Some relay methods are proposed, and the relay efficiency is evaluated. [ABSTRACT FROM AUTHOR]

A Transimpedance amplifier (TIA)-based optical receiver in active voltage-current feedback topology is designed for optical interconnects at 0.18 μ m CMOS technology. The proposed inductorless TIA incorporates a gain boosting amplifier in its path to achieve high gain and bandwidth. This gain boosting amplifier is based on a regulated gain cascode (RGC) amplifier. The RGC stage raises the g m of the circuit, reducing the input resistance of the input stage; as a result, the bandwidth extends. To further amplify the gain, a common source (CS) stage is used at the output without much effect on the system's overall bandwidth. An analytical model of the proposed TIA is developed and later simulated at room temperature in the Cadence Virtuoso analog design environment. The pre-layout and post-layout simulation results of the proposed TIA have been shown to validate its performance. It achieves a transimpedance gain of 62 d B Ω while consuming power of 14.5 mW from a 1.8 V supply voltage. The bandwidth is extended to 8.2 GHz from 1.83 GHz with an input photodiode capacitance of 300 fF. The current spectral density of input-referred noise is 31 pA/ H z. [ABSTRACT FROM AUTHOR]

On-chip ultra-wideband optical waveguide amplifiers are crucial for short-distance optical interconnections that utilize wavelength division multiplexing (WDM) technology. Silicon optical amplifiers still face significant challenges due to the indirect-band-gap nature and low rare earth solubility of silicon. This paper prepared a series of Er3+-Tm3+ co-doped La 2 O 3 –Al 2 O 3 glasses that are expected to be used in on-chip ultra-broadband optical waveguide amplifiers by aerodynamic levitation (ADL) technique, and further studied their luminescence characteristics and energy transfer process. Research results indicate that the glass composition 5Er 2 O 3 -0.6Tm 2 O 3 -43.4La 2 O 3 –51Al 2 O 3 is an optimal optical waveguide amplifier host material due to its excellent luminescence properties and ultra-broadband flat photoluminescence spectrum extending up to 440 nm. Moreover, positive gain can be obtained with a population ratio greater than 0.2, which further illustrates its feasibility as a matrix material for ultra-broadband optical waveguide amplifiers. The advancement of this material is expected to significantly aid in the fabrication and progress of next-generation ultra-wideband optical waveguide amplifiers. [ABSTRACT FROM AUTHOR]

Recently, lumped Mach-Zehnder Modulators (MZMs) have received renewed attention due to their potential for low power consumption and compact size. However, the practicality of lumped MZMs with conventional lumped electrodes (C−LEs) is limited by their lower electro−optical (EO) bandwidth. The reduction in EO bandwidth results from the inherent trade−off between EO bandwidth and half−wave voltage length product ( V π L ) within the C−LE architecture. This paper proposes a thin−film lithium niobate (TFLN)−based lumped MZM with capacitively−loaded lumped electrodes (CL−LEs). The purely linear EO effect of the LN eliminates the parasitic capacitance in the doped PN junction and enhances the EO bandwidth. Furthermore, the CL−LE structure can break the limitation between EO bandwidth and V π L inherent in the C−LE design. Simulations show the proposed device achieves a high EO bandwidth of 32.4 GHz and a low V π L of 1.15 V·cm. Due to the reduced capacitance and lower V π L , the power consumption of the device is as low as 0.1 pJ/bit. Simulation results indicate that the open−eye diagrams are achieved at 64 Gb/s for 1.5 mm TFLN lumped MZM, with an ER of 2.97 dB. Consequently, the proposed device architecture substantially enhances the performance of lumped MZMs, showing promise for application in short−reach optical interconnects within data centers. [ABSTRACT FROM AUTHOR]

Processor fault diagnosis is intended to identify faulty processors of a multicomputer system, guaranteeing its high reliability and availability. The g -good-neighbor conditional diagnosability is a novel fault diagnosis method for various networks. Under the PMC model and comparison model, the g -good-neighbor conditional diagnosabilities of hypermesh optical interconnection networks are determined, respectively. Directly applying the results, the g -good-neighbor conditional diagnosabilities of hypercubes are derived under the PMC and comparison models. [ABSTRACT FROM AUTHOR]

Several types of silicon-nanowire-based optical demultiplexers (DeMUXs) for use in short-reach targeted datacenter applications were proposed and their spectral responses were experimentally verified. First, a novel 100-GHz-spaced 16λ polarization-diversified optical DeMUX consisting of 2λ delayed interferometer (DI) type interleaver and 8λ arrayed waveguide gratings will be discussed in the spectral regimes of C-band, together with experimental characterizations showing static and dynamic spectral properties. Second, a novel 800-GHz-spaced 8λ optical DeMUX was targeted for use in LR (long reach) 400 Gbps Ethernet applications. Based on multiple cascade-connected DIs, by integrating the extra band elimination cutting area, discontinuous filtering response was analytically identified with a flat-topped spectral window and a low spectral noise of <−20 dB within an entire LR-8 operating wavelength range. Finally, a 20-nm-spaced 4λ coarse wavelength division multiplexing (CWDM)-targeted optical DeMUX based on polarization diversity was experimentally verified. The measurement results showed a low excessive loss of 1.0 dB and a polarization-dependent loss of 1.0 dB, prominently reducing spectral noises from neighboring channels by less than −15 dB. Moreover, TM-mode elimination filters were theoretically analyzed and experimentally confirmed to minimize unwanted TM-mode-oriented polarization noises that were generated from the polarization-handling device. The TM-mode elimination filters functioned to reduce polarization noises to much lower than −20 dB across the entire CWDM operating window. [ABSTRACT FROM AUTHOR]

Efficient energy transfer is crucial in electromagnetic communication. Therefore, producing a waveguide coupler that achieves broadband, nonreflective transmission is a challenging task. With the advancement of silicon-based integrated photonic circuits, fiber-to-chip coupling has become increasingly important. Although various couplers have been developed for fiber-to-chip coupling, they often have limitations such as long coupling length, low coupling efficiency, and narrow bandwidth. This is due to the inability to eliminate reflections between the two waveguides. Here, we introduce a method using universal impedance matching theory and transformation optics to eliminate reflections between two waveguides. The coupler, called the universal impedance matching coupler, using this method has the shortest subwavelength coupling length, a 99.9 % coupling efficiency, and a broad bandwidth. [ABSTRACT FROM AUTHOR]

This paper demonstrates with FDTD simulations a silicon nitride polarization independent coarse wavelength division multiplexing (CWDM) receiver platform based on square waveguides for optical interconnects at 1 $\mu$m. Here, the channel spacing of the demultiplexers is much smaller than standard CWDM systems (25 nm) and is set to 8 nm. To avoid high waveguide propagation and radiation losses, several waveguide dimensions were considered. The 450 nm (width) × 450 nm (height) waveguide presented a good tradeoff between low loss and single-mode behavior and was therefore selected. The demultiplexer has a simulated polarization dependence on the waveguide’s width variations of 0.17 nm/nm. In addition, we present edge coupler designs with coupling loss within 2.5 dB for TE and TM polarization.

This article demonstrates with simulations two polarization independent wavelength division multiplexing receiver platforms based on thin silicon nitride waveguides for optical interconnects at 1 $\mu$m. The chosen waveguide base geometry (width = 900 nm × height = 160 nm) is a good tradeoff between mode confinement and propagation loss. We first propose a design using a polarization splitter with an 1×4 demultiplexer based on an arrayed waveguide grating (AWG). This receiver has a reduced size and requires only one etching step. We later propose another simplified receiver design using a polarization splitter-rotator with two identical 1×4 demultiplexers based on cascaded Mach-Zehnder interferometers. The rotator is based on a thicker waveguide (width = 500 nm × height = 400 nm) and is partially etched to rotate the electric field by 90$^{\circ }$. Thus, it requires the use of mode size converters at the in/output ports. To keep the fabrication complexity as low as possible for the second design, we limited ourselves to only two etching steps. Therefore, the thickness of the slab of the mode converters and of the rotator is the same as for the main 900 nm (wide) × 160 nm (thick) waveguide. The simulated extinction ratio of the polarization splitter at 1035 nm is 18 dB and the calculated TM-TE and TE-TM polarization conversion efficiency of the polarization rotator at 1035 nm is 99.9$\%$.

Grating couplers that interconnect photonic chips to off-chip components are crucial for various optoelectronics applications. Despite numerous efforts in past decades, the existing grating couplers are still far from optimal in energy efficiency and thus hinder photonic integration toward a larger scale. Here, we propose a strategy to achieve ultralow-loss grating couplers by using unidirectional guided resonances (UGRs), suppressing the useless downward radiation with no mirror on the bottom. By engineering the dispersion and apodizing the geometry of grating, we experimentally realize a grating coupler with a record-low loss of -0.34 dB and 1-dB bandwidth exceeding 30 nm at the telecom wavelength of 1550 nm and further demonstrate an optic via with a loss of only -0.94 dB. Given that UGRs ubiquitously exist in a variety of grating geometries, our work sheds light on a systematic method to achieve energy-efficient optical interconnect and paves the way to large-scale photonic integration. [ABSTRACT FROM AUTHOR]

The electro‐optic (EO) modulator is a crucial device for achieving high‐speed optical communication and optical interconnection by loading the high‐frequency electrical signals onto the optical carrier. To date, modern communication technique demands for a low energy consumption for green data center establishment. In this work, a thin‐film BaTiO3 crystal waveguide Mach–Zehnder interferometer (MZI) type EO intensity modulator is experimentally implemented and its dynamic‐polarization interference process involving a phase‐polarization modulation (PPM) scheme other than conventional optical phase modulation (OPM) is carefully analyzed. Its findings show that the PPM scheme can efficiently reduce the half‐wave voltage (Vπ${V}_\pi$) and the equivalent half‐wave voltage (Veπ${V}_{e\pi }$) of the modulator. Then, a new theory of deriving the bit energy (BE) from Veπ${V}_{e\pi }$ is put forward, which has proven that the BE value under PPM is lower than the OPM counterpart. The EO modulator prototype featuring a length of 1.4 mm, Veπ${V}_{e\pi }$= 3.0 V and BE = 1.2 pJ/bit match well with the numerical calculations, thus validating the feasibility to achieve a BE<0.2 pJ/bit for an EO modulator through optimization of the electrode gap and state‐of‐the‐art fabrication techniques in the near future. [ABSTRACT FROM AUTHOR]

Optical interconnects are being discussed as a replacement for conventional electrical interconnects and are expected to be applied for future generations of high-performance supercomputers and data centers. Benes networks have attracted much attention because they require only 2 × 2 optical switches, which reduce the cost of rearrangeable nonblocking. However, optical power imbalances can significantly challenge receiver sensitivity. In this work, insertion loss (IL) fairness has been proposed and applied to the field of switches to achieve a relative balance of optical path data transmission in Benes networks. Fairness can be achieved when the port count is small (4 × 4) if the IL between ports is balanced. When the number of ports is moderate (8 × 8), we must use a suitable algorithm or determine the appropriate operating wavelength to minimize the power imbalance. An efficient two-step algorithm (ETS) has particular advantages in solving the path fairness problem and mitigating the power imbalance. As the number of ports increases, the switch states and topology jointly deteriorate the power imbalance. Finally, the ETS algorithm narrows the dynamic range requirement to 13.66 dB, with a 2 dB improvement. It achieves an extinction ratio of 24 dB and a bandwidth of 375 GHz, which outperforms the conventional 32 × 32 Benes network, respectively. [ABSTRACT FROM AUTHOR]

The exponential growth of global data traffic is driven by the unprecedented digitalization of the world. To meet the demands of next‐generation high‐capacity data communication systems, innovative solutions are required. Silicon photonics has gained significant attention due to its ability to integrate multiple core functions of optical interconnects into small‐scale chips, offering cost‐effective and scalable manufacturing capabilities. By leveraging silicon photonics, it becomes possible to address the challenge of scaling system complexity while reducing size, cost, and energy consumption. Despite its advantages, silicon has inherent limitations that restrict its application range, such as the weak plasma dispersion effect and relatively large bandgap. Recently, graphene has emerged as a promising solution for traditional silicon photonics because of its exceptional electrical and optical properties. For instance, graphene on a silicon chip enables nearly pure phase modulation and ultrafast photodetection beyond 500 GHz. Moreover, the demonstrated compatibility of graphene with wafer‐level integration paves the way for mass production of graphene‐based silicon photonic devices, offering improved yield, scalability, and cost‐effectiveness. In this work, a comprehensive review is provided, focusing on graphene‐based silicon photonic devices and their applications in optical interconnects, aiming to explore the potential of graphene in advancing silicon photonic technology. [ABSTRACT FROM AUTHOR]

We report a continuous-wave, O-band quantum-dot semiconductor comb laser for WDM optical interconnects exhibiting a 2.2 THz optical bandwidth with up to 89 comb wavelengths spaced at 25 GHz, over 30% peak ex-facet electrical-to-optical power conversion efficiency, up to 270 mW of usable laser power, relative intensity noise below − 135 dB/Hz per individual mode, individual laser mode linewidth of 140 kHz, mode beating linewidths of 50 kHz across all modes, and stable far-field output with 75% coupling efficiency to PM fiber in a butterfly package. [ABSTRACT FROM AUTHOR]

Intensity modulation/direct detection (IM/DD) optical interconnect systems are susceptible to waveform distortion caused by complex nonlinearities. To reduce the effect of waveform distortion on signal transmission accuracy, in this paper, a random forest (RF) equalization algorithm based on decision tree (DT) integration is proposed for cost-sensitive IM/DD systems, which has the characteristics of nonlinear classification and regression. The performance of the RF equalizer is verified by simulation experiments on a 120 Gbps 8-level pulse amplitude modulation (PAM8) transmission system using an electro-absorption-modulated laser (EML). Simulation studies show that whether it is a back-to-back (B2B) transmission system or a 10 km optical fiber transmission system, the RF equalization scheme can achieve much better bit error rate (BER) performance than the traditional equalization scheme. In addition, in both transmission cases, the RF equalization scheme enables efficient classification of signals and is much less simpler than the artificial neural network (ANN) equalization scheme. [ABSTRACT FROM AUTHOR]

Silicon microring modulator plays a critical role in energy-efficient optical interconnect and optical computing owing to its ultra-compact footprint and capability for on-chip wavelength-division multiplexing. However, existing silicon microring modulators usually require more than 2 V of driving voltage (Vpp), which is limited by both material properties and device structures. Here, we present a metal-oxide-semiconductor capacitor microring modulator through heterogeneous integration between silicon photonics and titanium-doped indium oxide, which is a high-mobility transparent conductive oxide (TCO) with a strong plasma dispersion effect. The device is co-fabricated by Intel's photonics fab and our in-house TCO patterning processes, which exhibits a high modulation efficiency of 117 pm/V and consequently can be driven by a very low Vpp of 0.8 V. At a 11 GHz modulation bandwidth where the modulator is limited by the RC bandwidth, we obtained 25 Gb/s clear eye diagrams with energy efficiency of 53 fJ/bit. Silicon microring resonator plays crucial role in optical computing owing to the compact footprint and energy-efficiency, yet existing modulators require >2 V to drive it. Here, the authors present a solution to this by using metal-oxide-semiconductor capacitor microring that brings down the driving voltage to 0.8 V. [ABSTRACT FROM AUTHOR]

Optical communication has become a research hotspot in modern non‐contact communication systems, and high‐performance photodetectors, as a key component, are crucial for achieving high‐fidelity signal transmission. Photoconductive devices (PCDs) are expected to find applications in the field of optical communication due to their advantages in high gain. However, it is difficult to achieve an effective balance between high responsiveness and fast response times in PCDs, leading to significant challenges in their commercial applications in the field of optical communication. In this paper, by employing both the mask method and channel width tuning method, a high‐performance PCD is developed based on the Ag‐MAPbI3‐Ag structure, demonstrating a high responsivity of 45.5 A W−1, a high detectivity of 7 × 1012 Jones, and a fast response time of 0.35 ms. Finally, this device is integrated into an optical communication system using embedded technology, effectively achieves byte transmission, and demonstrates the practical application of single‐crystal MAPbI3 PCD as a signal receiver in the system. This research provides a noteworthy approach to facilitating the seamless integration and commercialization of optical communication systems utilizing perovskite single‐crystal devices, while also paving the way for optical interconnects based on perovskite. [ABSTRACT FROM AUTHOR]

We optimized and fabricated an ultra-bend-resistant 4-core simplex cable (SXC) employing 4-core multicore fiber (MCF) suitable for short-reach dense spatial division multiplexing (DSDM) optical transmission in the O-band. The characteristics of transmission loss, macro-bending and cross-talk (XT) between adjacent cores after cabling were firstly clarified. By introducing the trapezoid index and optimizing the cabling process, the maximum values of added XT of 1.17 dB/km due to 10 loops with a bending radius of 6 mm imposed over the 4-core SXC and a macro-bending loss of 0.37 dB/10 turns were, respectively, achieved.P Then, the optical transmission with low bit error rate (BER) was presented using a 100GBASE-LR4 transceiver over the 1.2 km long 4-core SXC. The excellent bending resistance of the 4-core SXC may pave the way for a reduction in space pressure and increase in access density on short-reach optical interconnect (OI) based on DSDM. [ABSTRACT FROM AUTHOR]

We present a high-efficiency silicon grating coupler design based on a left–right mirror-symmetric grating and a metal mirror. The coupler achieves nearly perfect 90-degree vertical coupling. When two SOI chips are placed face to face with a vertical working distance of 50 μm, the chip-to-chip interlayer coupling efficiency reaches as high as 96%. When the vertical working distance ranges from 45 μm to 55 μm, the coupling loss remains below 1 dB. We also verified the effectiveness of our designed vertical coupler through 3D FDTD full-model simulation. The results demonstrate that our proposed vertical coupling structure represents a high-efficiency solution for 3D optical interconnects. The integration of multiple photonic chips in a 3D package with vertical optical and electrical interconnects is also feasible in the foreseeable future. [ABSTRACT FROM AUTHOR]

Photonic integrated circuits (PICs) are coupled with the laser to create active devices for optical communication, optical interconnection, and optical computing. Consequently, the light source remains a focal point of extensive research and development. Currently, the utilization of carbon quantum dots as a gain medium in photonic cavity lasers is accomplished and suggested as a viable approach for achieving active PICs. Nevertheless, challenges persist in achieving narrowband lasing beam output in one direction that is real‐time tunable while minimizing energy losses. These issues represent an active area of ongoing research. This article presents a self‐assembled microstructure and proposes a method for obtaining tunable, narrowband, unidirectional lasing beams. The objective is to stimulate further development in integration of PICs with lasers, extending beyond these applications, to realize substantial performance gains, eco‐friendly solutions, and efficient mass production. [ABSTRACT FROM AUTHOR]

In recent datacenters, considerable data communication has been performed between servers and storages; therefore, an optical interconnection with a wider band is required. Silicon photonics has attracted attention as a technology that places optical engines closer to switch application‐specific integrated circuits and realizes broadband optical interconnection. To utilize silicon photonics, it is necessary to overcome high‐precision alignment between silicon waveguides and single‐mode fibers (SMFs) having vastly different mode field diameters. Optical self‐coupling using light‐induced self‐written (LISW) optical waveguide technology is a potential solution. Previously, near‐infrared (NIR) LISW optical waveguides are fabricated using an SMF with an approximate 10 μm core through irradiation with 10 μW NIR light (1310 and 1550 nm). However, the cladding is liquid, which motivates to fabricate all‐solid LISW optical waveguides. In this study, all‐solid NIR LISW optical waveguides are produced using a core‐selective polymerization method, and single‐mode LISW optical self‐coupling between two SMFs is realized. To enhance the performance of the waveguides, additional resins are incorporated into the high‐refractive‐index base monomer during the cladding formation process. Through the experimentation, the potential for achieving even greater loss reduction by optimizing the hybridization between the base monomer and the supplementary resins is demonstrated. [ABSTRACT FROM AUTHOR]

Incoherent addition of three monochromatic laser sources is used to create four distinctive power levels to match the profile of standard quaternary pulse amplitude modulation (PAM4). The proposed PAM4 signal generation results in a lower symbol error rate as compared to standard PAM4 systems. This design improves the signal-to-noise ratio as compared to existing PAM4 schemes and can reduce the cost and power consumption of existing 200 G or higher optical transceivers. [ABSTRACT FROM AUTHOR]

Efficient mode conversion is crucial for hybrid photonic systems. We present efficient light transition from a standard single-mode fiber (SMF) to a subwavelength-diameter microfiber via a relatively short tapered fiber. Numerical simulations were performed to design the tapered morphology with high transmittance (approximately 86%) for the fundamental modes. The designed tapered fiber was successfully fabricated on the top of a cleaved SMF tip by the direct laser writing (DLW) method. For the 1550 nm wavelength, the transmittance from the standard SMF to the subwavelength-diameter microfiber was determined to be 77%, accompanied by a change in the effective mode area from 38 μm2 to 0.47 μm2 within a very short length of 150 μm. Our result demonstrated the versatility of the DLW technique for boosting the mode conversion efficiency of fiber-to-chip devices, enabling various applications in the future. [ABSTRACT FROM AUTHOR]

The growth of artificial intelligence applications demands ever larger and more complex deep learning models, dominating today's—and tomorrow's—data center and high-performance computing systems. While traditional electronics are failing to keep pace with application demands, silicon photonic (SiPh) interconnects have emerged as a necessary technology to support these systems. SiPh-driven wavelength-division multiplexing (WDM) offers a particularly promising path toward supporting incredibly high-aggregate link bandwidth in a compact and efficient form factor. One of the basic building blocks of these integrated WDM interconnects is the SiPh resonator. Their inherent wavelength selectivity and compact footprint allow for efficient data transmission multiplexed across dozens of carrier wavelengths. Used as add-drop (AD) filters, SiPh resonators are critical to constructing integrated tunable wavelength-selective optical circuit switches as well as for demultiplexing the different carrier wavelengths toward independent wavelength-insensitive photodiodes in a dense wavelength-division multiplexing receiver. Resonators in the all-pass (AP) configuration are widespread as well, allowing for wavelength-selective modulation to drive aggregate link bandwidths far beyond the individual channel data rate. Unlike SiPh Mach–Zehnder modulators (MZM), resonant modulators can be driven using low, complementary metal-oxide-semiconductor drive voltages, allowing for tight co-integration between photonic integrated circuits, fabricated with larger process node technologies, and electronic integrated circuits, designed to exploit the advantages of the latest node. To push toward practical peta-scale interconnects, a comprehensive review of SiPh resonators is required, addressing bottlenecks and design constraints at both the architecture and device levels. We first describe the predominant integrated link architectures and identify their limits. We then discuss the device-level design considerations that can be made for both AD and AP configuration resonators to overcome the system level limits with novel resonator device designs. Analytical models and numerical simulation of resonators are validated by experimental measurement of devices fabricated in a commercial 300-mm foundry, showing a clear path toward volume manufacturing. The demonstrated resonant modulators and filters support the feasibility of increasing the aggregate bandwidth of resonator-driven SiPh interconnects into the peta-scale regime. [ABSTRACT FROM AUTHOR]

Machine learning using the ANN approach is presented in this paper to design 101 channels of optical DEMUX in the range of 1300–1400 nm. This work is used to envisage the optical parameters of silicon grating structure such as the thickness of SiO and Si which are the decisive factors to allow the desired signal through the structure to realize optical DEMUX. This is accomplished by training an ANN model with a few pre-defined optical DEMUX parameters which are obtained beforehand by the plane wave expansion method. Here, the ANN model is implemented in the Tensor Flow framework of Google Colaboratory Network (Colab) which consists of five hidden layers of ReLU activation besides the input and output linear activation nodes. Besides this, the optical parameters collected from the ANN model are used to simulate through the PWE method for validation by observing the reflectance characteristics curve and found that most of the values are satisfied to realize the optical VLSI DEMUX. The outcomes of the present work could be beneficial in the field of optical VLSI systems pertaining to the optical interconnect. [ABSTRACT FROM AUTHOR]

Mode-division multiplexing (MDM) is a promising multiplexing technique to further improve the transmission capacity of optical communication and on-chip optical interconnection systems. Furthermore, the multimode optical switch is of great importance in the MDM system, since it makes the MDM system more flexible by directly switching multiple spatial signals simultaneously. In this paper, we proposed a mode-independent optical switch based on the graphene–polymer hybrid waveguide platform that could process the TE11, TE12, TE21 and TE22 modes in a few-mode waveguide. The presented switch is independent of the four guided modes, optimizing the buried position of graphene capacitors in the polymer waveguide to regulate the coplanar interaction between the graphene capacitors and spatial modes. The TE11, TE12, TE21 and TE22 modes can be regulated simultaneously by changing the chemical potential of graphene capacitors in a straight waveguide. Our presented switch can enable the independent management of the spatial modes to be more flexible and efficient and has wide application in the MDM transmission systems. [ABSTRACT FROM AUTHOR]

Mode division multiplexing (MDM) technology is an effective solution for high-capacity optical interconnection, and multimode power splitters, as essential components in MDM systems, have attracted widespread attention. However, supporting a wide range of modes and arbitrary power splitting ratios with large bandwidth in power splitters remains a significant challenge. In this paper, we designed a power splitter based on a subwavelength grating (SWG) structure with tilted placement on a silicon-on-insulator (SOI) substrate. We achieve arbitrary TE0–TE9 mode-insensitive power distribution by altering the filling coefficient of the SWG. Thanks to our specific selection of cladding materials and compensatory design for the optical wave transmission and reflection shifts induced by SWG, our device demonstrates low additional loss (EL < 1.1 dB) and inter-mode crosstalk (−18.8 < CT < −60 dB) for optical modes ranging from TE0 to TE9, covering a wavelength range from 1200 nm to 1700 nm. Furthermore, our proposed device can be easily extended to higher-order modes with little loss of device performance, offering significant potential in MDM platforms. [ABSTRACT FROM AUTHOR]

Optical interconnects have emerged as promising solutions for assisting electrical interconnects in short‐reach scenarios, where high bandwidth density and energy efficiency are of particular importance. Mode‐division multiplexing (MDM), capable of enhancing the bandwidth density and energy efficiency of optical systems, has drawn tremendous interest. However, the chip‐to‐chip MDM optical interconnects are impeded by the multimode chip‐fiber interfaces, where the transverse modes on the MDM chip mismatch with the linear polarization modes in the circular core few‐mode fiber (CCF). Moreover, the high differential group delays (DGDs) of a conventional CCF make the digital signal processing (DSP) computation complex thus consuming high power. To overcome these bottlenecks, a new multimode coupling solution based on a rectangular core few‐mode fiber with ultra‐low DGDs and an integrated multimode coupler is proposed. Based on this coupling scheme, a chip‐to‐chip MDM transmission experiment is performed on the TE00, TM00, TE10, and TM10 modes, and the highest bandwidth density of 19.4 Tb s mm −1 is realized at the chip facet. The DSP power consumption is ≈6% of a conventional CCF‐based system. It is believed that this work will pave the way for ultra‐compact, high density, low cost, and low computation‐complexity optical interconnects, such as data centers, cloud computing, and telecommunications. [ABSTRACT FROM AUTHOR]

On‐chip integration of two‐dimensional (2D) materials holds immense potential for novel optoelectronic devices across diverse photonic platforms. In particular, indium selenide (InSe) is a very promising 2D material due to its ultra‐high carrier mobility and outstanding photoresponsivity. Herein, a high‐speed photodetector based on a multilayer 90 nm thick InSe integrated on a silicon nitride (SiN) waveguide is reported. The device exhibits a remarkable low dark current density of ≈40 nA μm−2, a photoresponsivity of 0.38 A W−1, and an external quantum efficiency of ≈48.4%. Tested under ambient conditions at near‐infrared 976 nm wavelength, it exhibits an absorption coefficient of 0.11 dB μm−1. Additionally, the photodetector demonstrates a 3‐dB radiofrequency bandwidth of 85 MHz and an open‐eye diagram at data transmission of 1 Gbit s−1. Based on these exceptional optoelectronic advantages, integrated multilayer InSe enables the realization of active photonic devices for a range of applications, such as short‐reach optical interconnects, LiDAR imaging, and biosensing. [ABSTRACT FROM AUTHOR]

Manipulations of free‐space light are usually achieved through various metasurfaces and spatial light modulators. However, an external light source is required to excite these devices, making complete on‐chip integrations difficult. Integrated photonics enables the miniaturization and multi‐functionalization of optical systems by densely packing numerous optical components on a single chip. Particularly, plasmons have recently attracted extensive attention due to their unique abilities to enable the routing and manipulation of light at the nanoscale. Here optical wavefront shaping achieved by holographic metal‐insulator‐metal plasmonic gap waveguides is reported. An arbitrary free‐space wave's amplitude and phase information can be recorded in the waveguide by the amplitude variation of the guided wave. By elaborately designing the plasmonic gap waveguide, the guided waves can be molded into any desired free‐space light field, thus enabling complex free‐space functions including highly directional beams, dual beams with arbitrarily tailorable power ratio and radiation angles, focusing beams, and Airy beam generation. This study opens a new route for optical wavefront shaping via modulating guided waves. It paves the way for optical interconnects across multifunctional photonic integrated devices and free space, holding potential in optical communications, light detection and ranging, imaging, and displays. [ABSTRACT FROM AUTHOR]

The current approach to 2D optical phased array (OPA) encounters challenges, such as the requirement for a highly tunable laser that is incompatible with certain 2D beam-steering applications or significant power consumption, large antenna spacing and complex wiring resulting from independent control of array elements. To address these challenges, we propose an OPA architecture based on cascaded periodically poled LiNbO3 sequences, a multi-layered domains engineered structure within the LiNbO3 electro-optic crystal, only two control electronics to program the 2D beam-steering trajectory with a range of approximately θy = ±20° and θz = ±16° through simulations. This structure enables the uniform distribution of phase differences between adjacent array elements (adjacent domains) upon beam exit from the crystal, ensuring optimal performance. The aim of this study is to develop a methodology that employs domain engineering techniques for designing high-performance phase-controlled devices with customized functional units and sequences in electro-optical crystals. Our research has implications for emerging optoelectronic applications, such as customizable optical interconnects and integrated LiDAR systems. [ABSTRACT FROM AUTHOR]

In this regard, we directed a theoretical study with numerical simulations. This study allowed us to illustrate how a photonic crystal fiber (PCF) structure could divide an optical signal. One of the most fundamental components used to construct photonic integrated circuits (PIC) is the splitter, which is using light coupling between the cores as a control until the output ports by using pure silica to replace some air-hole zones along the PCF axis and split the single signal on two ports with almost equal intensity in each port. Optical interconnects are one of the most basic components of integrated optics, and splitters for photonic power are a key element of a connected family. With the least amount of loss, a competent photonic splitter can guide light input of a certain wavelength to several ports at various intensities. [ABSTRACT FROM AUTHOR]

The performance of free-space optical interconnects (FSOIs) system is significantly influenced by noise, similar to any wireless communication system. This noise has a notable impact on both the bandwidth density and data rate of FSOIs system. To address these challenges, this study proposes the utilization of vertical-cavity-surface-emitting laser (VCSEL) arrays on the transmitter side and photodetector arrays on the receiver side for FSOIs. The study investigates the bandwidth density of the system with and without coding while maintaining a specific bit error rate. An analysis is conducted in the presence of higher-order modes in the laser beams of the FSOIs system and a fundamental Gaussian operating mode. The presence of the higher-order modes leads to degradation in the performance of the FSOIs system in terms of bandwidth density. In addition, we examine the impact of the signal-to-noise ratio (SNR) on the system's bandwidth density for each considered operating mode. The provided simulation results clearly demonstrate that coding significantly enhances the bandwidth density of the systems, with the extent of improvement being closely tied to the employed code rate and codeword length. [ABSTRACT FROM AUTHOR]

Nanoscale spontaneous light sources are promising alternatives to lasers for high-speed optical communications and interconnections through energy-efficient integrated circuits. Yet, developing the spontaneous light sources faster than lasers is hampered by the detection means (e.g., time-resolved fluorescence spectroscopy). Here, by coupling monolayer WSe2 to individual plasmonic nanocavities, we achieved an efficient spontaneous light source with potential ultrafast modulation bandwidth and superior brightness. The ultrafast radiative decay rates can be determined and derived solely from the experimental parameters by combining the coupling strength and the photoluminescence enhancement in a single nanocavity-WSe2 hybrid. As a result, the hybrid light source has a radiative lifetime down to 350 fs, indicating a potential modulation bandwidth up to 440 GHz, which is 10 times of the traditional semiconductor lasers. Furthermore, the quantum yield is enhanced by a factor of over 300-folds up to 20.8% through making full use of the highly confined nanocavity mode. The nanocavity-WSe2 hybrid we built provides a promising approach for constructing high-speed light-emitting devices. [ABSTRACT FROM AUTHOR]

Definition: Integrated optics is a field of study and technology that focuses on the design, fabrication, and application of optical devices and systems using integrated circuit technology. It involves the integration of various optical components, such as waveguides, couplers, modulators, detectors, and lasers, into a single substrate. One of the key advantages of integrated optics is its compatibility with electronic integrated circuits. This compatibility enables seamless integration of optical and electronic functionalities onto the same chip, allowing efficient data transfer between optical and electronic domains. This synergy is crucial for applications such as optical interconnects in high-speed communication systems, optical sensing interfaces, and optoelectronic integrated circuits. This entry presents a brief study on some of the widely used and commercially available optical platforms and fabrication methods that can be used to create photonic integrated circuits. [ABSTRACT FROM AUTHOR]

Single‐frequency semiconductor lasers represent a critical role in optical communications, light detection and ranging systems, photonics integrated circuits, etc. Here, combining atom‐like quantum dot (QD) materials and advanced lateral gratings, a high‐power, ultra‐low‐noise 1.3 µm InAs/GaAs QD distributed feedback laser is demonstrated. Stable single‐longitudinal‐mode output power of 100 mW from 25 ℃ to 85 ℃ is achieved with a maximum side mode suppression ratio of 62.6 dB, and the variations of threshold current and slope efficiency over the temperature range are slight, indicting a high temperature stability. A record‐narrow intrinsic linewidth of 1.62 kHz is achieved at 55 ℃ with a white noise level of merely 515 Hz2 Hz–1, and a minimum averaged relative intensity noise of only –166 dB/Hz between 0.1 GHz and 20 GHz is obtained at 25 ℃. Furthermore, a strong tolerance to external optical feedback (〉 –14 dB) is demonstrated in the range from 25 ℃ to 85 ℃, with a maximum value of –8 dB at 85 ℃. This high‐quality single‐frequency laser fabricated with simplified processes and compact size paves the way for its future large‐scale applications such as high‐capacity optical communication, high‐precision optical detection, high‐speed optical interconnections, etc. [ABSTRACT FROM AUTHOR]

Research on artificial neural network computing based on conventional integrated circuit chips has made significant progress, but it faces technical bottlenecks such as reduced energy consumption, computing speed, and efficiency. So, it is seeking integrated chips based on optical interconnections to solve the current dilemma. Fortunately, the small size and very localized graphene surface plasmon waves offer the possibility of optical integrated chips. In this paper, we propose a graphene-based one-bit optical numerical comparator. The comparator is located on the top of a 0.4-μm2 rectangular dielectric layer, mainly consisting of Y-shaped graphene nanoribbons. The on/off effect of the graphene nanoribbons is achieved by applying an external voltage to change the chemical potential energy of the graphene switching bands. The proposed optical numerical comparator with 9.55-μm TM mode light achieves a minimum extinction ratio of 31.12 dB and amplitude modulation of 0.77 dB, as shown by the finite-difference time-domain (FDTD) method. Compared with the current optical numerical comparators, it has the advantages of a high extinction ratio, small size, low loss, and high stability. In addition, the effect of process deviation of graphene nanoribbons on the reliability of the designed optical numerical comparator is analyzed by simulation. It is beneficial to developing integrated photonic devices and has some significance for developing ultra-high-frequency and integrating artificial neural network computing. [ABSTRACT FROM AUTHOR]

Lithium niobate Mach–Zehnder modulators (MZMs) with compact footprint and fast electro-optics (EO) responses are highly demanded for the next-generation optical interconnect systems. Here, we demonstrate slow-light (SL) effect using a coupled Bragg resonator structure on the thin-film lithium niobate (TFLN) platform, and an ultra-compact SL-MZM with length L of ∼370 μm is also constructed. The fabricated SL waveguides show a large optical passband width of ∼8 nm, an insertion loss of 2.9 dB, and a maximal optical group index of 7.50, corresponding to 3.4 times as large as that of regular TFLN rib waveguide. The fabricated SL-MZM exhibits a large EO bandwidth of >50 GHz in an operating wavelength band of ∼8 nm as well. High-speed OOK transmissions at data rates of 64 Gbit/s and 80 Gbit/s are successfully achieved. To our best knowledge, it is first time to build SL waveguides and compact SL-MZMs with large EO bandwidths of >50 GHz on the monolithic TFLN platform. [ABSTRACT FROM AUTHOR]

We put forward and demonstrate a silicon photonics (SiPh)-based mode division multiplexed (MDM) optical power splitter that supports transverse-electric (TE) single-mode, dual-mode, and triple-mode (i.e., TE0, TE1, and TE2). An optical power splitter is needed for optical signal distribution and routing in optical interconnects. However, a traditional optical splitter only divides the power of the input optical signal. This means the same data information is received at all the output ports of the optical splitter. The powers at different output ports may change depending on the splitting ratio of the optical splitter. The main contributions of our proposed optical splitter are: (i) Different data information is received at different output ports of the optical splitter via the utilization of NOMA. By adjusting the power ratios of different channels in the digital domain (i.e., via software control) at the Tx, different channel data information can be received at different output ports of the splitter. It can increase the flexibility of optical signal distribution and routing. (ii) Besides, the proposed optical splitter can support the fundamental TE0 mode and the higher modes TE1, TE2, etc. Supporting mode-division multiplexing and multi-mode operation are important for future optical interconnects since the number of port counts is limited by the chip size. This can significantly increase the capacity besides wavelength division multiplexing (WDM) and spatial division multiplexing (SDM). The integrated SiPh MDM optical power splitter consists of a mode up-conversion section implemented by asymmetric directional couplers (ADCs) and a Y-branch structure for MDM power distribution. Here, we also propose and discuss the use of the Genetic algorithm (GA) for the MDM optical power splitter parameter optimization. Finally, to provide adjustable data rates at different output ports after the MDM optical power splitter, non-orthogonal multiple access—orthogonal frequency division multiplexing (NOMA-OFDM) is also employed. Experimental results validate that, in three modes (TE0, TE1, and TE2), user-1 and user-2 achieve data rates of (user-1: greater than 22 Gbit/s; user-2: greater than 12 Gbit/s) and (user-1: greater than 12 Gbit/s; user-2: 24 Gbit/s), respectively, at power-ratio (PR) = 2.0 or 3.0. Each channel meets the hard-decision forward-error-correction (HD-FEC, i.e., BER = 3.8 × 10−3) threshold. The proposed method allows flexible data rate allocation for multiple users for optical interconnects and system-on-chip networks. [ABSTRACT FROM AUTHOR]

Plasmonic nanolasers as a new class of coherent laser beyond the diffraction limit have attracted a lot of attention. However, the ultrahigh optical confinement caused by the plasmon effect is inevitably accompanied by metal absorption loss, thus increasing the pump threshold of the plasmonic nanolaser. In the past decade, many good results about low threshold plasmonic nanolasers have been realized and successfully extended to various applications. Here, this advance is discussed and some opinions are offered. First, based on the theoretical model and key parameters of the plasmonic nanolaser, the factors affecting the threshold are analyzed. Subsequently, the experimental efforts on the realization of low threshold plasmonic nanolasers from optical pumping to electrical pumping are reviewed. Their applications in on‐chip optical interconnects, biochemical analysis, and far‐field structure are then considered. Finally, feasible approaches to threshold reduction are discussed, as well as more possible applications in the future. [ABSTRACT FROM AUTHOR]

Reduced thermal budget is required for back-end-of-line (BEOL) integration of application specific functionality into the multilevel metal stack of a processor "substrate." We report 400 °C BEOL-compatible Ge-on-Si growth (LT Ge) that is epitaxial and single crystalline with a defect density similar to high temperature growth and a small 0.05% tensile strain. Room temperature methanol–iodine passivation is employed pre-growth in lieu of the typical 800 °C oxide removal step. Undoped LT Ge exhibits p-type conductivity initially and n-type conductivity conversion upon annealing. Hall effect measurements following post growth heat treatment between 400 and 600 °C reveal an acceptor removal reaction that follows first-order kinetics with an activation energy of 1.7 ± 0.5 eV and a pre-exponential factor of 2.3 × 10 7 s−1 consistent with a point defect, diffusion limited process. We also observe that 90° sessile dislocations identified via transmission electron microscopy are annihilated in the same temperature regime, which is evidence for point defect-mediated climb. Ensuring high-quality epitaxy by characterizing defect reactions in a BEOL-compatible Ge-on-Si process flow is key to enabling vertical integration of optical interconnects. [ABSTRACT FROM AUTHOR]