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We report high-gain, low-noise-figure, and high linearity analog photonic links at 1064 nm using InP/InGaAs modified uni-traveling-carrier (MUTC) photodiodes. Balanced photodiodes reach output powers of 17.7 dBm, 19.8 dBm, 20.7 dBm, and 22 dBm at 38 GHz, 29 GHz, 27 GHz, and 24.5 GHz, respectively. High common-mode rejection ratio (CMRR) of 25 dB and good linearity with a third-order intercept point (OIP3) of up to 34 dBm were measured at 38 GHz for 10-μm-diameter balanced photodiodes. We demonstrate two analog photonic links with different noise reduction techniques without electronic amplification. In the first link, we use balanced detection with a quadrature-biased Mach–Zehnder modulator (MZM) for noise cancellation. Link gain of 6.8 dB, noise figure (NF) of 25.8 dB and spurious free dynamic range (SFDR3) of 114 dB·Hz2/3 at 38 GHz were achieved. The second link works at low bias modulation for improved signal-to-noise ratio with a single photodetector. Record-high gain (19.3 dB at 26 GHz and 17 dB at 38 GHz) and low noise figure (14.5 dB at 26 GHz and 17 dB at 38 GHz) were demonstrated.

Summary Objective Clinicians make numerous decisions when 3D printing models for fabrication of thermoformed appliances, including printing solid or hollow models. While hollow models can reduce resin use, models intended for thermoformed appliance fabrication must be printed with sufficient thickness to withstand thermoforming. The aim of the study was to determine for hollow 3D printed orthodontic models if there is an effect of shell thickness on the dimensional accuracy of retainers thermoformed upon them as compared with solid models and, if so, to identify the minimum shell thickness that ensures dimensional accuracy of the thermoformed retainer under the conditions investigated. Material and methods Thermoformed appliances were fabricated on 3D printed models of six shell thicknesses: 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, 3.0 mm, and solid (n = 10/group). The models were scanned before and after thermoforming. Thermoformed appliances were captured by two methods: scanning a polyvinylsiloxane casting of the appliance and scanning the appliance interior surface (intaglio surface). Each model-appliance pair was compared using superimposition software. A generalized linear model and post-hoc Tukey contrasts (α = 0.05) were applied to compare each thickness. Results Model thickness has a statistically significant effect on dimensional accuracy of thermoformed appliances. Appliances fabricated on 1.0 mm and 1.5 mm models displayed poor accuracy, with a statistically significantly lower percentage of data points within tolerance (±0.250 mm) than appliances fabricated on models printed at 2.0 mm thickness and greater. Conclusions 3D printed model thickness affects the dimensional accuracy of a thermoformed retainer. To ensure minimal deformation and promote clinical utility of the thermoformed appliance, models should be printed with a minimum shell thickness of 2.0 mm for the materials investigated.

Back-illuminated flip-chip-bonded InP/InGaAs modified uni-traveling-carrier photodiodes working at 1064 nm wavelength are demonstrated. The RF output powers of photodiodes with diameters of 10 μm and 20 μm are 15 dBm at 60 GHz and 21.7 dBm at 39 GHz, respectively. The measured third-order output intercept point (OIP3) shows a high linearity of 33 dBm at 40 GHz. A circuit analysis based on Z-parameter extraction indicates that voltage-dependent and photocurrent-dependent capacitance components are the primary nonlinear mechanisms.

In this work, we demonstrate a novel top-illuminated avalanche photodiode (APD) with high-speed and wide dynamic range performance for coherent lidar applications, which needs simultaneous processing of weak light reflections from the object and a strong optical local-oscillator (LO) signal at the receiving end. By taking advantage of a partially depleted p-type In0.53Ga0.47As absorbing layer and a thin In0.52Al0.48As multiplication layer (88 nm), the demonstrated APDs exhibit a wide optical-to-electrical bandwidth (16 GHz) and high responsivity (2.5 A/W at 0.9 Vbr) near the saturation output current which is as high as >8 mA. Furthermore, the measured low excess noise (k = 0.14) characteristics of this device ensure its wide dynamic range performance. In addition, it should be noted that the measured nonlinear behaviors of these APDs are quite different from those of typical fast p-i-n photodiodes (PDs), which always show serious degradation in their O-E bandwidths at their saturation output. On the other hand, the measured bias dependent 3-dB O-E bandwidth of the demonstrated APDs can be pinned at 16 GHz without any degradation at around the saturation current output. Such distinct nonlinear APD behaviors can be attributed to the space-charge-screening effect induced gain reduction under high-power operation.

We investigate the room-temperature bandwidth performance of AlInAsSb avalanche photodiodes under 2-μm illumination. Parameter characterization denotes RC-limited performance. While measurements indicate a maximum gain-bandwidth product of 44 GHz for a 60-μm-diameter device, we scale this performance to smaller device sizes based on the RC response. For a 15-μm-diameter device, we predict a maximum gain-bandwidth product of approximately 144 GHz based on the reported measurements.

Transitioning to electrified transport requires improvements in sustainability, energy density, power density, lifetime, and approved the cost of lithium-ion batteries, with significant opportunities remaining in the development of next-generation cathodes. This presents a highly complex, multiparameter optimization challenge, where developments in cathode chemical design and discovery, theoretical and experimental understanding, structural and morphological control, synthetic approaches, and cost reduction strategies can deliver performance enhancements required in the near- and longer-term. This multifaceted challenge requires an interdisciplinary approach to solve, which has seen the establishment of numerous academic and industrial consortia around the world to focus on cathode development. One such example is the Next Generation Lithium-ion Cathode Materials project, FutureCat, established by the UK’s Faraday Institution for electrochemical energy storage research in 2019, aimed at developing our understanding of existing and newly discovered cathode chemistries. Here, we present our perspective on persistent fundamental challenges, including protective coatings and additives to extend lifetime and improve interfacial ion transport, the design of existing and the discovery of new cathode materials where cation and cation-plus-anion redox-activity can be exploited to increase energy density, the application of earth-abundant elements that could ultimately reduce costs, and the delivery of new electrode topologies resistant to fracture which can extend battery lifetime.

We demonstrate low-dark-current, high-speed and high-power back-illuminated flip-chip-bonded modified uni-traveling carrier photodiodes operating at 1064 nm. The photodiodes with $15~\mu \text{m}$ and $18~\mu \text{m}$ diameters achieve 18.6 dBm RF output power at 55 GHz and 19.4 dBm RF output power at 41 GHz, respectively. The bandwidth is analyzed with parameters obtained from S-parameter fitting. There is good agreement between measured and calculated bandwidths.

Heat-induced antigen retrieval (HIAR) is routinely employed on aldehyde-fixed tissue sections to enhance the reactivity of antibodies that exhibit weak or no specific interactions with tissue antigens when applied in conventional immunohistochemical protocols. A major drawback of HIAR protocols is, however, the heat-induced detachment of sections from the microscope slide with resultant impaired tissue morphology or loss of the section. We developed a method in which tissue sections mounted on glass slides are temporally coverslipped, and a clamp is used to compress the sections on the microscope slide during HIAR treatment. This “pressurized coverslipping” during HIAR was tested on various formalin-fixed tissues (murine kidneys and temporal bones, human tonsils and temporal bones) that were embedded in paraffin or celloidin. The method reliably kept the sections adherent to the slide, preserved the tissue morphology, and effectively retrieved tissue antigens for improved results in immunohistochemical labeling, even for exceptionally delicate, large, and poorly adhering sections, that is, decalcified human temporal bone sections. In summary, we present a simple method for improved slide adherence and morphological preservation of tissue sections during HIAR treatment that can be combined with all HIAR protocols and that requires only basic lab equipment.

The amplitude-to-phase (AM-to-PM) conversion in photodetectors is one the primary noise sources in the generation of ultralow noise microwave signals with stabilized optical frequency combs. We have designed charge-compensated modified unitraveling carrier (CC-MUTC) photodiodes to reduce the AM-to-PM conversion. The photocurrent-induced phase change at 15 GHz is reduced by >3 times and the AM-to-PM is greatly suppressed, especially at large photocurrent. For 10-μm devices, the 3-dB bandwidth at −6 V is 35 GHz and the maximum RF output power is 13.7 dBm at 15 GHz. Compared with the original CC-MUTC structure, the maximum output power is increased by 2.7 dB.

We report the noise characteristics of an AlInAsSb avalanche photodiode (APD) on an InP substrate. We observe low excess noise corresponding to an impact ionization coefficient ratio (k) of 0.012, and a dark current density of 55 μA/cm2 at a gain of 10 at room temperature. The performance of commercial APDs on InP substrates is limited by the excess noise and the performance of state of the art (SOA) APDs on InP substrates is limited by the dark current. The combination of low excess noise and low dark current of AlInAsSb leads to a significant performance improvement compared to commercial APDs and provides a potential candidate for low noise, SOA, commercial APDs for near-infrared applications. When combined in a separate absorber, charge and multiplication layer (SACM) architecture with an InGaAs absorption layer, the low noise characteristics of AlInAsSb point towards a superior InP substrate-based APD targeting 1.55 μm for applications such as optical communications and light detection and ranging (LiDAR).

We present gain, dark current and excess noise characteristics of PIN Al0.85Ga0.15As0.56Sb0.44 (hereafter AlGaAsSb) avalanche photodiodes (APDs) on InP substrates with 1000 nm thick multiplier layers. The AlGaAsSb APDs were grown by molecular beam epitaxy using a digital alloy technique (DA) to avoid phase separation. Current-voltage measurements give a peak gain of ~ 42, a breakdown voltage of – 54.3 V, and a dark current density at a gain of 10 of ~ 145 μA/cm2. Excess noise measurements of multiple AlGaAsSb APDs show that k (the ratio of electron and hole impact ionization coefficients) is ~ 0.01. This k-value is comparable to Si, which is widely used for visible and near-infrared APDs. The low dark current density and low excess noise suggest that such thick AlGaAsSb layers are promising multipliers in separate absorption, charge and multiplication (SACM) structures for short-wavelength infrared applications such as optical communication and LIDAR, particularly on a commercial InP platform.

Avalanche photodiodes (APDs) can provide high receiver sensitivity owing to their internal gain, however, the impact ionization gain mechanism is also a source of noise. Recently, AlxIn1-xAsySb1-y grown as a digital alloy has been used to fabricate APDs with excess noise as low as Si (k ~ 0.01). Operation of these APDs at telecommunication wavelengths (1300 nm to 1550 nm) or 2 µm has been achieved. This paper also discusses staircase APDs, which utilize compositional grading to mimic the dynodes of a photomultiplier tube, with a gain of 2x arising at each step providing extremely low noise avalanche gain.

III-V material based digital alloy Avalanche Photodiodes (APDs) have recently been found to exhibit low noise similar to Silicon APDs. The III-V materials can be chosen to operate at any wavelength in the infrared spectrum. In this work, we present a physics-based SPICE compatible compact model for APDs built from parameters extracted from an Environment-Dependent Tight Binding (EDTB) model calibrated to ab-initio Density Functional Theory (DFT) and Monte Carlo (MC) methods. Using this approach, we can accurately capture the physical characteristics of these APDs in integrated photonics circuit simulations., 8 pages, 11 figures

A detailed theoretical investigation of the underlying physics of low noise III-V digital alloy APDs is presented here. Based on our investigations, the criteria for developing low noise digital alloys are proposed. © 2021 The Author(s)

The measured capacitance, modulus and strength of carbon nanotube-polyaniline (CNT-PANI) composite electrodes render them promising candidates for structural energy storage devices. Here, CNT-PANI composite electrodes are manufactured with electrodeposition of PANI onto the bundle network of CNT mats produced via a floating catalyst chemical vapour deposition process. PANI comprises 0% to 30% by volume of the electrode. The composition, modulus, strength and capacitance of the electrodes is measured in the initial state, after the first charge, and after 1000 charge/discharge cycles. Electrode modulus and strength increase with increasing CNT volume fraction; in contrast, the capacitance increases with increasing PANI mass. Charging or cycling reduce the electrode modulus and strength due to a decrease in CNT bundle volume fraction caused by swelling; the electrode capacitance also decreases due to a reduction in PANI mass. A micromechanical model is able to predict the stress-strain response of pre-charged and cycled electrodes, based upon their measured composition after pre-charging and cycling. The electrodes possess up to 63% of their theoretical capacitance, and their tensile strengths are comparable to those of engineering alloys. Their capacitance and strength decrease by less than 15% after the application of 1000 charge/discharge cycles. These properties illustrate their potential as structural energy storage devices.

We report on engineering impact ionization characteristics of In0.53Ga0.47As/Al0.48In0.52As superlattice avalanche photodiodes (InGaAs/AlInAs SL APDs) on InP substrate to design and demonstrate an APD with low k-value. We design InGaAs/AlInAs SL APDs with three different SL periods (4 ML, 6 ML, and 8 ML) to achieve the same composition as Al0.4Ga0.07In0.53As quaternary random alloy (RA). The simulated results of an RA and the three SLs predict that the SLs have lower k-values than the RA because the electrons can readily reach their threshold energy for impact ionization while the holes experience the multiple valence minibands scattering. The shorter period of SL shows the lower k-value. To support the theoretical prediction, the designed 6 ML and 8 ML SLs are experimentally demonstrated. The 8 ML SL shows k-value of 0.22, which is lower than the k-value of the RA. The 6 ML SL exhibits even lower k-value than the 8 ML SL, indicating that the shorter period of the SL, the lower k-value as predicted. This work is a theoretical modeling and experimental demonstration of engineering avalanche characteristics in InGaAs/AlInAs SLs and would assist one to design the SLs with improved performance for various SWIR APD application.

Recently, advances in imaging and LIDAR applications have stimulated the development of high-sensitivity receivers that operate at wavelengths of ≥ 2 µm, which has driven research on avalanche photodiodes (APDs) that operate in that spectral region. High quantum efficiency is a key performance parameter for these photodetectors. Increasing the thickness of the absorption region is a straightforward approach to increase the quantum efficiency. However, the primary source of dark current is the narrow-bandgap material used for 2-µm detection. Increasing its thickness results in higher noise. In this paper, we describe two approaches to enhance the quantum efficiency, both of which are superior to a conventional anti-reflection (AR) coating. For normal incidence at 2 µm, finite-difference time-domain (FDTD) simulations show the absorption can be enhanced by more than 100% with a triangular-lattice photonic crystal, and nearly 400% by applying a metal grating. This is achieved by coupling normal incidence light into the laterally propagating modes in the device. Moreover, the significantly higher absorption of the metal grating compared to the photonic crystal is due to the high coupling efficiency provided by the metal grating. This work provides promising methods and physical understanding for enhancing the quantum efficiency for 2-µm detection without increasing absorber thickness, which also enables low dark current and high bandwidth.

Homojunction device architectures are explored through numerical simulations to extend the response of 4H-SiC APDs broadly throughout the UV spectrum. We report on device designs and experimental validation of approaches to improve both surface and bulk carrier collection, compatible with high gain Geiger mode operation.

Pulsed microwave signals with record high peak power of 44.2 dBm (26.3 W), 41.6 dBm (14.5 W) and 40.6 dBm (11.5 W) at 10 GHz, 17 GHz and 22 GHz, respectively, were generated using modified uni-travelling-carrier (MUTC) photodiodes.

The 2-μm optical window has recently become an area of great interest for imaging and LIDAR applications due to improved ranging capability and eye safety compared to common telecommunications wavelengths. Avalanche photodiodes (APDs) operating in this spectrum are highly desirable, as their intrinsic gain offers increased sensitivity over traditional photodiodes, further improving receiver sensitivity. HgCdTe, InAs, and InSb, as well as various superlattice materials have been used for this purpose, however, the combination of high electric field and narrow-bandgap absorber yields high dark current. As a result, these APDs are operated at cryogenic temperatures to suppress recombination mechanisms. At the high electric fields required for impact ionization, narrow bandgap materials are also susceptible to band-to-band tunneling, which cannot be suppressed by cooling. The separate absorption, charge, and multiplication (SACM) APD was designed to address this challenge by reducing the electric field in the absorber while maintaining sufficiently high enough field in the multiplication region for impact ionization [1] . This design spatially separates the absorption and multiplication layers, controlling the electric field in the absorber and multiplication layers through an intermediate charge layer. SACM APDs have been widely deployed in the InGaAs/InP and InGaAs/InAlAs materials systems for use in near-infrared telecommunications applications.

Avalanche photodiodes (APDs) are used in short- and mid-wave infrared applications such as optical communication, LIDAR and 3D imaging [1] due to their internal gain, which improves the signal to noise ratio (SNR). However, the multiplication gain ( M ) gives rise to excess noise, caused by the stochastic nature of impact ionization, which can significantly degrade the SNR of APDs. The excess noise is quantitatively measured by excess noise factor, F(M) that is expressed by McIntyre’s local field theory [1] , F(M) = kM + (1-k)[2-(1/M)] where k is the ratio of the impact ionization coefficients for electrons and holes. According to the equation above, the low excess noise factor in APDs can be attained by a low k value.

High-speed high-power photodiodes are key components in microwave photonic applications including antenna remoting, generation of microwave signals, and analog optical links. With the rapid emergence of Si photonics, it is becoming increasing important to develop approaches to incorporate these photodiodes into a Si platform. For integration of III-V photodiodes on silicon, various integration approaches have been reported, including adhesive wafer bonding, direct molecular wafer bonding, and direct III-V material growth on Si. This paper will report the integration of InGaAs/InGaAsP/InP modified uni-traveling carrier photodiodes using each of these techniques. In addition, high-power Ge on Si photodiodes will be discussed.

Optical atomic clocks are poised to redefine the SI second, thanks to stability and accuracy more than one hundred times better than the current microwave atomic clock standard. However, the best optical clocks have not seen their performance transferred to the electronic domain, where radar, navigation, communications, and fundamental research rely on less stable microwave sources. By comparing two independent optical-to-electronic signal generators, we demonstrate a 10 GHz microwave signal with phase that exactly tracks that of the optical clock phase from which it is derived, yielding an absolute fractional frequency instability of 1*10-18 in the electronic domain. Such faithful reproduction of the optical clock phase expands the opportunities for optical clocks both technologically and scientifically for time-dissemination, navigation, and long-baseline interferometric imaging., 19 page, 10 figures (including Supplementary Text)

We demonstrate the modified uni-travelling-carrier (MUTC) photodiodes achieving RF power levels of 21.7 dBm to 15 dBm in the frequency range of 39 GHz to 60 GHz, respectively, at 1064 nm wavelength. The photodiodes show good linearity with a third-order intercept point (OIP3) up to 33 dBm at 40 GHz.

We examine the optical pulse width and illumination spot size dependence on the flicker phase noise of a photodiode, resulting in a 10 dB noise reduction to -143/f dBc/Hz on a 1 GHz carrier.

We investigate phase-modulated analog photonic link performance using interferometric detection with a Mach–Zehnder interferometer (MZI) under different bias conditions and a high-power high-linearity modified unitraveling carrier (MUTC) photodiode (PD). Expressions for radio frequency (RF) small signal gain and the optimal MZI bias point for RF gain are derived, and noise figure (NF) and third-order spurious free dynamic range (SFDR3 ) are analyzed. Using a 28- μ m-diameter MUTC single PD with 23 dBm RF output power under 155-mA saturation current at 10 GHz, we measured a record-high link gain of 25 dB, 18 dB NF, and ${\text{114}}\;{\text{dB}} \cdot {\rm{H}}{{\rm{z}}^{{\text{2/3}}}}$ SFDR3 at 130 mA photocurrent with an optimally biased MZI. By using a 24- μ m-diameter balanced MUTC PD pair with 21 dBm RF output power at 108 mA at 10 GHz, we obtained a 16 dB link gain, 16 dB NF, and ${\text{118}}\;{\text{dB}} \cdot {\rm{H}}{{\rm{z}}^{{\text{2/3}}}}$ DR3 at 100 mA total dc photocurrent when the MZI was biased at quadrature. The RF gain at both MZI bias points is in good agreement with our calculations.

We report avalanche photodiodes (APDs) fabricated from the digital alloy Al0.8In0.2As0.23Sb0.77 (lattice-matched to GaSb). The APDs exhibit high avalanche multiplication and low excess noise.

We report avalanche photodiodes (APDs) fabricated from high-aluminum-content Al x In 1− x As y Sb1− y lattice matched to GaSb that is grown within the miscibility gap using a digital alloy approach. The material was initially characterized through a series of Al x In1− x As y Sb1− y ( x = 0.3, 0.4, 0.5, 0.6, 0.7) p-i-n structures. In order to achieve operation at telecommunications wavelengths, an Al x In1− x As y Sb 1− y separate absorption, charge, and multiplication APD has been developed. Very low excess noise, as characterized by k ∼0.01−0.05, has been achieved.

With the increasing availability of treated-color diamonds on the market, their characterization is becoming more and more critical to the jewelry testers and customers. In this investigation, ten color diamonds treated by irradiation (4 pieces), HPHT (3 pieces), and multiprocess (3 pieces) were examined by spectroscopic methods. These diamonds are classified to be type Ia according to their FTIR characteristics. Using microscope and DiamondView, the internal features (such as distinctive color zoning and graphitized inclusions) and complex natural growth structures were observed, which show that the samples are more likely artificially colored natural diamonds. Through photoluminescence spectroscopy, a combination of optical centers was detected, including N-V0 at 575 nm, N-V− at 637 nm, H3 at 503 nm, H2 at 986 nm, and GR1 at 741 and 744 nm. Combining with the previous studies, treatment conditions for the studied diamonds were estimated depending on the presence and/or absence of the optical centers. In addition, the coloration mechanism of the samples (blue, green, and red) during the treatment process was also discussed. It is suggested that a number of techniques should be combined in order to make a reliable identification for such diamonds.

We demonstrate high-speed and high-power evanescently coupled waveguide integrated modified unitraveling carrier photodiodes. Over 105-GHz bandwidth along with 2 and 1.3 dBm RF output power at 100 and 105 GHz, respectively, has been achieved.

Background doping polarity is a key parameter in the design of numerous electrical and optoelectronic devices. It is especially critical for avalanche photodiodes (APDs). Recently, high-performance APDs have been demonstrated based on the AlInAsSb digital alloy materials system. A critical element of this work was the determination of the background doping polarity of the molecular beam epitaxial grown wafers. In this work, we determine the unintentional background doping polarity of Al0.7InAsSb using the double mesa capacitance-voltage technique. We fabricated two p-i-n Al0.7InAsSb structures: one with p-type background polarity and the other with n-type. The measurements indicate that devices with different background doping polarities show different capacitance relations to the mesa diameters; moreover, the relationship reverses at high voltage in a p-type background device. Subsequent simulations reveal that this reversal is caused by electrical field confinement after the depletion reaches the smaller top mesa. These findings are consistent with reports of reduced surface leakage current in double and triple mesa structures.

The performance of high-power photodiodes flip-chip bonded on polycrystalline aluminum nitride (AlN), single-crystal AlN, and diamond submounts are compared. The thermal boundary conductance (inverse of the thermal boundary resistance) between submount/titanium interfaces was measured and found to be the primary impedance to heat dissipation. Thermal profiles of the flip-chip-bonded devices were simulated to project the power density at failure, which is found to be inversely proportional to the diameter of the photodiodes.

The phase noise, i.e., amplitude modulation to phase modulation (AM-PM) in the photodetector. In general, AM-PM of charge-compensated modified uni-travelling carrier (CC-MUTC) photodiodes originates from the combined nonlinearities of the detector impedance and carrier transit times, which depend on the electric field distribution and the velocity-field characteristics of carrier transport. In this work, we report measurements of AM-PM in transit-time-limited CC-MUTC photodiodes. A Crosslight-based simulation of the photodetection process is used to quantitatively estimate the AM-PM of CC-MUTC photodiodes. Simulated AM-PM of a 15-GHz microwave signal shows good agreement with experimental results. The measured and simulated AM-PM versus photocurrent exhibit a null point that originates from the competition between hole deceleration due to the collapsed electric field in the depleted absorption region and electron acceleration in the self-induced electric field in the undepleted absorption region.

Recently, there has been a rapid rise in experiential research investigating the potential of exploiting magnetic induction tomographic techniques to improve security, develop industry, and enhance medicine. This investigation explored the functionality of a fully automated magnetic imaging system able to capture 2-D images of featured metallic objects in unshielded and beneath conductively shielded configurations. Images were captured using a programmable x–y stage that moved objects relative to, and when magnetically coupled with, a stationary induction sensor coil; thus, position-resolved measurements of variations in sensor inductance (L) clearly defined the features of conductive objects and distinguished different material types. This investigation concentrated first on exploring the system’s ability to distinguish different metals from images that represented proportional conductivity maps, and second, on the effects that a range of shielding conductivity materials had on resultant image quality and resolution. This investigation has provided a greater understanding when assessing potential capability improvements that magnetic induction tomographic techniques could offer a security and/or industrial application.

We report uni-traveling-carrier photodiodes with a quantum efficiency of 98% ±0.8% at 1064 nm. For 50 μm devices, the measured 3-dB bandwidth is 2 GHz and the dark current is 10 nA at a bias voltage of -5 V. The dark current is dominated by generation-recombination at a bias voltage of -10 V and the activation energy is 0.23 eV. At higher bias voltage, tunneling becomes a significant component of the dark current.

We demonstrate back-illuminated flip-chip-bonded modified uni-traveling carrier photodiodes designed for operation at 1064 nm wavelength. The bandwidth is 40 GHz and the RF output power is 23.5 dBm at 25 GHz. The photodiode has responsivity of 0.46 A/W at 1064 nm and low dark current of 2 nA at −8 V.

Superior performance of digital alloy APDs is attributed to the formation of "minigaps" in the material band-structure. However, no improvement is observed in dilute nitride APDs in presence of minigaps. We propose criteria which can judge the effectiveness of these minigaps.

A high-sensitivity and a high-power photo-receiver is preferred in coherent lidar system. In this work, we demonstrate top-illuminated APDs, which can simultaneously exhibit low excess noise (k

Recently, using Al x In 1-y As y Sb 1-y grown as a digital alloy we have demonstrated APDs with noise as low as Si at telecommunication wavelengths (1300 nm to 1550 nm). In this paper, we a noise suppression mechanism in digital alloys. This is related to the fundamental issue of transport in ordered materials and provides the potential for "designer APDs" covering a broad range of spectral bands.

We report separate absorption, charge, and multiplication (SACM) avalanche photodiodes (APDs) in the Al x In 1-x As y Sb 1-y material system for 2-μm detection. Gain, dark current, and low excess noise are demonstrated.

We demonstrate a III-V avalanche photodiode (APD) grown by heteroepitaxy on silicon. This InGaAs/InAlAs APD exhibits low dark current, gain >20, external quantum efficiency >40%, and similar low excess noise, k∼0.2, as InAlAs APDs on InP.

We report the gain, noise, and dark current characteristics of random alloy Al0.79In0.21As0.74Sb0.26 (hereafter AlInAsSb)-based avalanche photodiodes (APDs) on InP substrates. We observe, at room temperature, a low excess noise corresponding to a k value (ratio of impact ionization coefficients) of 0.018 and a dark current density of 82 μA/cm2 with a gain of 15. These performance metrics represent an order of magnitude improvement of the k-value over commercially available APDs with InAlAs and InP multiplication layers grown on InP substrates. This material is also competitive with a recently reported low noise AlAsSb on InP [Yi et al., Nat. Photonics 13, 683 (2019)], with a comparable excess noise and a room temperature dark current density almost three orders of magnitude lower at the same gain. The low excess noise and dark current of AlInAsSb make it a candidate multiplication layer for integration into a separate absorption, charge, and multiplication layer avalanche photodiode for visible to short-wavelength infrared applications.

We report on the demonstration of Al0.85Ga0.15As0.56Sb0.44 (hereafter, AlGaAsSb) avalanche photodiodes (APDs) with a 1000 nm-thick multiplication layer. Such a thick AlGaAsSb device was grown by a digital alloy technique to avoid phase separation. The current-voltage measurements under dark and illumination conditions were performed to determine gain for the AlGaAsSb APDs. The highest gain was ∼ 42, and the avalanche initiation occurred at 21.6 V. The breakdown voltage was found to be around −53 V. The measured dark current densities of bulk and surface components were 6.0 μA/cm2 and 0.23 μA/cm, respectively. These values are about two orders of magnitude lower than those for previously reported 1550 nm-thick AlAs0.56Sb0.44 APDs [Yi et al., Nat. Photonics 13, 683 (2019)]. Excess noise measurements showed that the AlGaAsSb APD has a low k of 0.01 (the ratio of electron and hole impact ionization coefficients) compared to Si APDs. The k of the 1000-nm AlGaAsSb APD is similar to that of the thick AlAsSb APDs (k ∼ 0.005) and 5–8 times lower than that of 170 nm-thick AlGaAsSb APDs (k ∼ 0.5–0.8). Increasing the thickness of the multiplication layer over 1000 nm can also reduce k further since the difference between electron and hole impact ionization coefficients becomes significant in this material system as the thickness of the multiplication layer increases. Therefore, this thick AlGaAsSb-based APD on an InP substrate shows the potential to be a high-performance multiplier that can be used with available short-wavelength infrared (SWIR) absorption layers for a SWIR APD.

We report on improvement in the deep ultraviolet (DUV) photoresponse of SiC based detectors through the development of n- AlxGa1-xN / i-p SiC heterojunction photodiodes. Fabricated photodiodes have high external quantum efficiency (EQE), greater than 60%, over a wide spectral range from 215-255 nm that is ~10x enhancement in performance over comparable homogenous SiC photodiodes at the shortest wavelength. This is attributed to photogeneration of carriers within the SiC depletion region by DUV illumination of the diode through the n- AlxGa1-xN “window”, as compared to a typical homogenous SiC n-i-p structure where the carriers are photogenerated in the n-type neutral region, resulting in more efficient collection of holes through drift

Back-illuminated flip-chip-bonded charge-compensated modified uni-traveling-carrier photodiodes (PDs) with bandwidths in excess of 110 GHz are demonstrated. PDs with 10- and 6-μm-diameters deliver RF output power levels as high as 9.6 dBm at 100 GHz and 7.8 dBm at 110 GHz, respectively. An analytical model based on parameter extraction from S-parameter fitting was used to assess the bandwidth limiting factors.

Until the early 2000's, the avalanche photodiode (APD) was widely deployed in high-performance optical receivers that operated up to 10 Gb/s. In subsequent years, the use of APDs for high-capacity systems declined as a result of their limited gain bandwidth, the transition to coherent detection, and the development of high-efficiency modulation techniques. Recently, the rapid growth of optical-fiber communications systems that utilize baud rates up to 25 Gb/s represented by a 100-Gb/s Ethernet has led to a resurgence of research on APDs and the development of low-noise APDs with enhanced gain bandwidth. This paper presents a brief review of APD fundamentals and describes some of the recent advances.

InP-based high-power and high-speed modified unitraveling carrier photodiodes heterogeneously integrated on silicon-on-insulator waveguides are demonstrated. Internal responsivity up to 0.95A/W and bandwidth up to 48 GHz have been achieved. The maximum RF output power of a $20 \times 35$ μm2 photodiode was 16.6, 15.8, and 13.5 dBm at 10, 20, and 30 GHz, respectively. The maximum output RF power of a $10 \times 35$ μm2 photodiode was 12 dBm at 40 GHz. Using the same integration technology, we show that balanced waveguide photodiodes reach 0.78-A/W internal responsivity, 14-GHz bandwidth, and >20-dB common-mode rejection ratio. In the differential mode, the unsaturated RF output power was 17.2 dBm at 10 GHz and 15.2 dBm at 20 GHz.

This paper reviews progress in the field of heterogeneously integrated photodiodes with a focus on InP-based high-speed high-power modified uni-traveling carrier photodiodes on silicon-on-insulator waveguides. Discrete, balanced, and photodiode arrays are described.

Photonic microwave generation of high-power pulsed signals in the X-, Ku- and K-band using charge-compensated MUTC photodiodes is demonstrated. The impulse photoresponse without modulation showed a maximum peak voltage of 38.3 V and full-width at half-maximum of 30 ps. High power pulsed microwave signals at 10, 17 and 22 GHz with peak power up to 44.2 dBm (26.3 W), 41.6 dBm (14.5 W) and 40.6 dBm (11.5 W) were achieved, respectively.