Space-qualifying silicon photonic modulators and circuits.

Reducing the form factor while retaining the radiation hardness and performance matrix is the goal of avionics. While a compromise between a transistor's size and its radiation hardness has reached consensus in microelectronics, the size-performance balance for their optical counterparts has not been quested but eventually will limit the spaceborne photonic instruments' capacity to weight ratio. Here, we performed space experiments of photonic integrated circuits (PICs), revealing the critical roles of energetic charged particles. The year-long cosmic radiation exposure does not change carrier mobility but reduces free carrier lifetime, resulting in unchanged electro-optic modulation efficiency and well-expanded optoelectronic bandwidth. The diversity and statistics of the tested PIC modulator indicate the minimal requirement of shielding for PIC transmitters with small footprint modulators and complexed routing waveguides toward lightweight space terminals for terabits communications and intersatellite ranging.

Micro-dispenser-based optical packaging scheme for grating couplers.

Due to their sub-millimeter spatial resolution, ink-based additive manufacturing tools are typically considered less attractive than nanophotonics. Among these tools, precision micro-dispensers with sub-nanoliter volumetric control offer the finest spatial resolution: down to 50 µm. Within a sub-second, a flawless, surface-tension-driven spherical shape of the dielectric dot is formed as a self-assembled µlens. When combined with dispersive nanophotonic structures defined on a silicon-on-insulator substrate, we show that the dispensed dielectric µlenses [numerical aperture (NA) = 0.36] engineer the angular field distribution of vertically coupled nanostructures. The µlenses improve the angular tolerance for the input and reduces the angular spread of the output beam in the far field. The micro-dispenser is fast, scalable, and back-end-of-line compatible, allowing geometric-offset-caused efficiency reductions and center wavelength drift to be easily fixed. The design concept is experimentally verified by comparing several exemplary grating couplers with and without a µlens on top. A difference of less than 1 dB between incident angles of 7° and 14° is observed in the index-matched µlens, while the reference grating coupler shows around 5 dB contrast.

Cryptococcal Hsf3 controls intramitochondrial ROS homeostasis by regulating the respiratory process.

Mitochondrial quality control prevents accumulation of intramitochondrial-derived reactive oxygen species (mtROS), thereby protecting cells against DNA damage, genome instability, and programmed cell death. However, underlying mechanisms are incompletely understood, particularly in fungal species. Here, we show that Cryptococcus neoformans heat shock factor 3 (CnHsf3) exhibits an atypical function in regulating mtROS independent of the unfolded protein response. CnHsf3 acts in nuclei and mitochondria, and nuclear- and mitochondrial-targeting signals are required for its organelle-specific functions. It represses the expression of genes involved in the tricarboxylic acid cycle while promoting expression of genes involved in electron transfer chain. In addition, CnHsf3 responds to multiple intramitochondrial stresses; this response is mediated by oxidation of the cysteine residue on its DNA binding domain, which enhances DNA binding. Our results reveal a function of HSF proteins in regulating mtROS homeostasis that is independent of the unfolded protein response.

Integrated photonic metasystem for image classifications at telecommunication wavelength.

Miniaturized image classifiers are potential for revolutionizing their applications in optical communication, autonomous vehicles, and healthcare. With subwavelength structure enabled directional diffraction and dispersion engineering, the light propagation through multi-layer metasurfaces achieves wavelength-selective image recognitions on a silicon photonic platform at telecommunication wavelength. The metasystems implement high-throughput vector-by-matrix multiplications, enabled by near 103 nanoscale phase shifters as weight elements within 0.135 mm2 footprints. The diffraction manifested computing capability incorporates the fabrication and measurement related phase fluctuations, and thus the pre-trained metasystem can handle uncertainties in inputs without post-tuning. Here we demonstrate three functional metasystems: a 15-pixel spatial pattern classifier that reaches near 90% accuracy with femtosecond inputs, a multi-channel wavelength demultiplexer, and a hyperspectral image classifier. The diffractive metasystem provides an alternative machine learning architecture for photonic integrated circuits, with densely integrated phase shifters, spatially multiplexed throughput, and data processing capabilities.

Structural Phase Transitions between Layered Indium Selenide for Integrated Photonic Memory.

The primary mechanism of optical memoristive devices relies on phase transitions between amorphous and crystalline states. The slow or energy-hungry amorphous-crystalline transitions in optical phase-change materials are detrimental to the scalability and performance of devices. Leveraging an integrated photonic platform, nonvolatile and reversible switching between two layered structures of indium selenide (In2 Se3 ) triggered by a single nanosecond pulse is demonstrated. The high-resolution pair distribution function reveals the detailed atomistic transition pathways between the layered structures. With interlayer "shear glide" and isosymmetric phase transition, switching between the α- and ß-structural states contains low re-configurational entropy, allowing reversible switching between layered structures. Broadband refractive index contrast, optical transparency, and volumetric effect in the crystalline-crystalline phase transition are experimentally characterized in molecular-beam-epitaxy-grown thin films and compared to ab initio calculations. The nonlinear resonator transmission spectra measure of incremental linear loss rate of 3.3 GHz, introduced by a 1.5 µm-long In2 Se3 -covered layer, resulted from the combinations of material absorption and scattering.

High-extinction-ratio low-voltage dual-racetrack modulator for low-power DAC-less PAM-4 modulation.

In a parallel-coupled dual-racetrack modulator, resonant light in two resonators can interfere with each other. In lieu of critical coupling, such interference is capable of producing high extinction ratios (ERs) for high-speed modulation. Experiments demonstrate ERs of over 9 dB at 50 Gb/s and 40-50% modulation depth enhancement compared with a single-resonator modulator at 50-56 Gb/s with a peak-to-peak driving voltage of 2.3 V. Furthermore, joint modulation of two racetracks offers the possibility to combine two separate bits of driving signals to generate four-level pulse-amplitude modulation (PAM-4) without an external digital-to-analog converter (DAC). To tackle the complex multi-variable transfer function of this modulator, a procedure for configuring PAM-4 states is theoretically developed. Finally, we demonstrate 100 Gb/s PAM-4 with an electro-optic modulation power consumption of < 40 fJ/bit for this device.

Focusing and defocusing switching of an indium selenide-silicon photonic metalens.

With a fixed geometric design, homogeneous change of Indium Selenide (In2Se3) switches the focusing length of a silicon photonic metalens between positive and negative values. This unique functionality of the hybrid metasurface is attributed to the fact that the silicon's refractive index is in the middle of the two convertible states in the optical phase change material. The infrared transparency of In2Se3 in both states enables near phase-only metasurface structures. The design is foundry compatible and feasible for implementing nonvolatile adaptive transformation optic systems on-chip.

Device architectures for low voltage and ultrafast graphene integrated phase modulators.

The atomic layer thin geometry and semi-metallic band diagram of graphene can be utilized for significantly improving the performance matrix of integrated photonic devices. Its semiconductor-like behavior of Fermi-level tunability allows graphene to serve as an active layer for electro-optic modulation. As a low loss metal layer, graphene can be placed much closer to active layer for low voltage operation. In this work, we investigate hybrid device architectures utilizing semiconductor and metallic properties of the graphene for ultrafast and energy efficient electro-optic phase modulators on semiconductor and dielectric platforms. (1) Directly contacted graphene-silicon heterojunctions. Without oxide layer, the carrier density of graphene can be modulated by the directly contact to silicon layer, while silicon intrinsic region stays mostly depleted. With doped silicon as electrodes, carrier can be quickly injected and depleted from the active region in graphene. The ultrafast carrier transit time and small RC constant promise ultrafast modulation speed (3dB bandwidth of 67 GHz) with an estimated Vπ·L of 1.19 V·mm. (2) Graphene integrated lithium niobite modulator. As a transparent electrode, graphene can be placed close to integrated lithium niobate waveguide for improving coupling coefficient between optical mode profile and electric field with minimal additional loss (4.6 dB/cm). Numerical simulation indicates 2.5× improvement of electro-optic field overlap coefficient, with estimated V π of 0.2 V.

Hydrogen Plasma Exposure of Monolayer MoS2 Field-Effect Transistors and Prevention of Desulfurization by Monolayer Graphene.

Atomic vacancies related to structural disorder and doping variation influence carrier transport in monolayer transition-metal dichalcogenide devices. Here, we investigate the effect of hydrogen plasma exposure (HPE) on monolayer MoS2 field-effect transistors (FETs). We observe that a 1% increase in sulfur vacancy after HPE results in incremental 0.06 eV of the Schottky barrier. Short-range scattering from the sulfur vacancies reduces the carrier mobility of monolayer MoS2 by 2 orders of magnitude. Despite the defects and grain boundaries formed during the chemical vapor deposition and transferring process, the surface desulfurization induced by the proton exposure and thermally accelerated oxidation can be blocked by monolayer graphene cladding with a van der Waals contact distance of 2.5 Å. The material-level study indicates a promising route for a low-cost and robust fabrication of smart sensor circuits on a monolithic MoS2 wafer, where the bare MoS2 FETs can serve as proton sensors, with their electronic readout processed by a logic circuit of graphene-protected pristine FETs with a high on/off ratio.

Localized Strain Measurement in Molecular Beam Epitaxially Grown Chalcogenide Thin Films by Micro-Raman Spectroscopy.

We developed an experimental metrology for measuring local strain in molecular beam epitaxially (MBE) grown crystalline chalcogenide thin films through micro-Raman spectroscopy. For In2Se3 and Bi2Se3 on c-plane sapphire substrates, the transverse-optical vibrational mode (A1 phonon) is most sensitive to strain. We first calibrated the phonon frequency-strain relationship in each material by introducing strain in flexible substrates. The Raman shift-strain coefficient is -1.97 cm-1/% for the In2Se3 A1(LO + TO) mode and -1.68 cm-1/% for the Bi2Se3 A1g 2 mode. In2Se3 and Bi2Se3 samples exhibit compressive strain and tensile strain, respectively. The observations are compliant with predictions from the opposite relative thermal expansion coefficient between the sample and the substrate. We also map strain cartography near the edge of as-grown MBE samples. In In2Se3, the strain accumulates with increasing film thickness, while a low strain is observed in thicker Bi2Se3 films.

On-chip wavefront shaping with dielectric metasurface.

Metasurfaces can be programmed for a spatial transformation of the wavefront, thus allowing parallel optical signal processing on-chip within an ultracompact dimension. On-chip metasurfaces have been implemented with two-dimensional periodic structures, however, their inherent scattering loss limits their large-scale implementation. The scattering can be minimized in single layer high-contrast transmitarray (HCTA) metasurface. Here we demonstrate a one-dimensional HCTA based lens defined on a standard silicon-on-insulator substrate, with its high transmission (<1 dB loss) maintained over a 200 nm bandwidth. Three layers of the HCTAs are cascaded for demonstrating meta-system functionalities of Fourier transformation and differentiation. The meta-system design holds potential for realizing on-chip transformation optics, mathematical operations and spectrometers, with applications in areas of imaging, sensing and quantum information processing.

Correlation between in Situ Diagnostics of the Hydrogen Plasma and the Interface Passivation Quality of Hydrogen Plasma Post-Treated a-Si:H in Silicon Heterojunction Solar Cells.

Passivation of the interface defect states is crucial to mitigate the recombination losses in silicon solar cells. In this work, we have investigated the role of hydrogen plasma treatment (HPT) to passivate the interfacial defects between crystalline (c-Si) and hydrogenated amorphous silicon (a-Si:H) in silicon heterojunction (SHJ) solar cells. For the first time, we have found a correlation between the dynamic properties of hydrogen plasma and passivation quality of the films by using in situ optical emission spectroscopy and quasi-steady state photoconductance measurement. The optimum condition for saturation of the dangling bonds by HPT has been studied in detail by tuning the excited hydrogen (H) species and ion bombardment energies by controlling physical parameters like plasma current and chamber pressure. We have investigated the role of annealing after HPT to redistribute the H in the post-treated a-Si:H film and have obtained an i Voc of 755 mV, minority carrier lifetime (τeff) of 4.6 ms, and SRV of 1.5 cm/s on test structures having only an 10 nm intrinsic a-Si:H layer on textured silicon wafers. The H bond configuration at the interface of a-Si:H and c-Si has been investigated by Fourier transform infrared spectroscopy, which demonstrates improved monohydride bonding in the films after HPT derived from the analysis of microstructure parameter and H concentration values. Raman spectroscopy shows the absence of the nanocrystalline fraction after HPT and verifies reduced coordination defects due to annealing after HPT. The proof of concept has been validated by fabricated SHJ solar cells having a Voc of 729 mV and efficiency of 18.7% after HPT, with the best cell efficiency reaching 20.2% after doped layer optimization. The decrease in reverse saturation current and ideality factor after HPT verifies that the improvement in performance is from reduced recombination losses at the interface due to passivation of defects in midgap states.

Photoluminescence of Functionalized Germanium Nanocrystals Embedded in Arsenic Sulfide Glass.

Embedding metallic and semiconductor nanoparticles in a chalcogenide glass matrix effectively modifies the photonic properties. Such nanostructured materials could play an important role in optoelectronic devices, catalysis, and imaging applications. In this work, we fabricate and characterize germanium nanocrystals (Ge NCs) embedded in arsenic sulfide thin films by pulsed laser ablation in aliphatic amine solutions. Unstable surface termination of aliphatic groups and stable termination by amine on Ge NCs are indicated by Raman and Fourier-transform infrared spectroscopy measurements. A broad-band photoluminescence in the visible range is observed for the amine functionalized Ge NCs. A noticeable enhancement of fluorescence is observed for Ge NCs in arsenic sulfide, after annealing to remove the residual solvent of the glass matrix.

Tunable Optical Transparency in Self-Assembled Three-Dimensional Polyhedral Graphene Oxide.

The origami-like self-folding process is an intellectually stimulating technique for realizing three-dimensional (3D) polyhedral free-standing graphene oxide (GO) structures. This technique allows for easy control of size, shape, and thickness of GO membranes, which in turn permits fabrication of free-standing 3D microscale polyhedral GO structures. Unlike 2D GO sheets, the 3D polyhedral free-standing GO shows a distinct optical switching behavior, resulting from a combination of the geometrical effect of the 3D hollow structure and the water-permeable multilayered GO membrane that affects the optical paths.

An integrated low phase noise radiation-pressure-driven optomechanical oscillator chipset.

High-quality frequency references are the cornerstones in position, navigation and timing applications of both scientific and commercial domains. Optomechanical oscillators, with direct coupling to continuous-wave light and non-material-limited f × Q product, are long regarded as a potential platform for frequency reference in radio-frequency-photonic architectures. However, one major challenge is the compatibility with standard CMOS fabrication processes while maintaining optomechanical high quality performance. Here we demonstrate the monolithic integration of photonic crystal optomechanical oscillators and on-chip high speed Ge detectors based on the silicon CMOS platform. With the generation of both high harmonics (up to 59 th order) and subharmonics (down to 1/4), our chipset provides multiple frequency tones for applications in both frequency multipliers and dividers. The phase noise is measured down to -125 dBc/Hz at 10 kHz offset at ~400 µW dropped-in powers, one of the lowest noise optomechanical oscillators to date and in room-temperature and atmospheric non-vacuum operating conditions. These characteristics enable optomechanical oscillators as a frequency reference platform for radio-frequency-photonic information processing.

Graphene coated ZnO nanowire optical waveguides.

We report the fabrication and characterization of freestanding graphene coated ZnO nanowires (GZNs) for optical waveguiding. The GZNs are fabricated using a tape-assist transfer under micromanipulation. Owing to the deep-subwavelength diameter and high index contrast of the ZnO nanowire waveguide, light-graphene interaction is significantly enhanced by the strong surface optical fields, resulting in a linear absorption as high as 0.11 dB/µm in a 606-nm-diameter GZN at 1550-nm wavelength. Launched by 1550-nm-wavelength femto-second pulses, a 475-nm-diameter GZN with a graphene coating length of merely 24 µm exhibits evident nonlinear saturable absorption with a peak power threshold down to 1.3 W. In addition, we also demonstrate a transmission modulation for 1550-nm-wavelength signal with a 590-nm-diameter GZN, showing the possibility of using GZN waveguides as nanoscale bulding blocks for nanophotonic devices.

Cascaded uncoupled dual-ring modulator.

We demonstrate, by coherent driving two uncoupled rings in same direction, that the effective photon circulating time in the dual-ring modulator is reduced, with increased modulation quality. The inter-ring detuning-dependent photon dynamics, Q factor, extinction ratio, and optical modulation amplitude of two cascaded silicon ring resonators are studied and compared with that of a single-ring modulator. Experimentally measured eye diagrams, together with coupled mode theory simulations, demonstrate the enhancement of the dual-ring configuration at 20 Gbps with a Q∼20,000.

Molecular-absorption-induced thermal bistability in PECVD silicon nitride microring resonators.

The wavelength selective linear absorption in communication C-band is investigated in CMOS-processed PECVD silicon nitride rings. In the overcoupled region, the linear absorption loss lowers the on-resonance transmission of a ring resonator and increases its overall quality factor. Both the linear absorption and ring quality factor are maximized near 1520 nm. The direct heating by phonon absorption leads to thermal optical bistable switching in PECVD silicon nitride based microring resonators. We calibrate the linear absorption rate in the microring resonator by measuring its transmission lineshape at different laser power levels, consistent with coupled mode theory calculations.

Selective tuning of high-Q silicon photonic crystal nanocavities via laser-assisted local oxidation.

We examine the cavity resonance tuning of high-Q silicon photonic crystal heterostructures by localized laser-assisted thermal oxidation using a 532 nm continuous wave laser focused to a 2.5 µm radius spot-size. The total shift is consistent with the parabolic rate law. A tuning range of up to 8.7 nm is achieved with ∼ 30 mW laser powers. Over this tuning range, the cavity Qs decreases from 3.2×10(5) to 1.2×10(5). Numerical simulations model the temperature distributions in the silicon photonic crystal membrane and the cavity resonance shift from oxidation.