Water-Splitting Photoelectrolysis Reaction Rate via Microscopic Imaging of Evolved Oxygen Bubbles

Bubble formation and growth on a water-splitting semiconductor photoelectrode under illumination with above-bandgap radiation provide a direct measurement of the gas-evolving reaction rate. Optical microscopy was used to record the bubble growth on single-crystal strontium titanate immersed in basic aqueous electrolyte and illuminated with UV light at 351/364 nm from a focused argon laser. By analyzing the bubble size as a function of time, the water-splitting reaction rate was determined for varying light intensities and was compared to photocurrent measurements. Bubble nucleation was explored on an illuminated flat surface, as well as the subsequent light scattering and electrode shielding due to the bubble. This technique allows a quantitative examination of the actual gas evolution rate during photoelectrochemical water splitting, independent of current measurements

Solar energy conversion via hot electron internal photoemission in metallic nanostructure: Efficiency estimates

Collection of hot electrons generated by the efficient absorption of light in metallic nanostructures, in contact with semiconductor substrates can provide a basis for the construction of solar energy-conversion devices. Herein, we evaluate theoretically the energy-conversion efficiency of systems that rely on internal photoemission processes at metal-semiconductor Schottky-barrier diodes. In this theory, the current-voltage characteristics are given by the internal photoemission yield as well as by the thermionic dark current over a varied-energy barrier height. The Fowler model, in all cases, predicts solar energy-conversion efficiencies of <1% for such systems. However, relaxation of the assumptions regarding constraints on the escape cone and momentum conservation at the interface yields solar energy-conversion efficiencies as high as 1%–10%, under some assumed (albeit optimistic) operating conditions. Under these conditions, the energy-conversion efficiency is mainly limited by the thermionic dark current, the distribution of hot electron energies, and hot-electron momentum considerations

Synchronous micromechanically resonant programmable photonic circuits

Programmable photonic integrated circuits (PICs) are emerging as powerful tools for the precise manipulation of light, with applications in quantum information processing, optical range finding, and artificial intelligence. The leading architecture for programmable PICs is the mesh of Mach-Zehnder interferometers (MZIs) embedded with reconfigurable optical phase shifters. Low-power implementations of these PICs involve micromechanical structures driven capacitively or piezoelectrically but are limited in modulation bandwidth by mechanical resonances and high operating voltages. However, circuits designed to operate exclusively at these mechanical resonances would reduce the necessary driving voltage from resonantly enhanced modulation as well as maintaining high actuation speeds. Here we introduce a synchronous, micromechanically resonant design architecture for programmable PICs, which exploits micromechanical eigenmodes for modulation enhancement. This approach combines high-frequency mechanical resonances and optically broadband phase shifters to increase the modulation response on the order of the mechanical quality factor $Q_m$, thereby reducing the PIC's power consumption, voltage-loss product, and footprint. The architecture is useful for broadly applicable circuits such as optical phased arrays, $1$ x $N$, and $N$ x $N$ photonic switches. We report a proof-of-principle programmable 1 x 8 switch with piezoelectric phase shifters at specifically targeted mechanical eigenfrequencies, showing a full switching cycle of all eight channels spaced by approximately 11 ns and operating at >3x average modulation enhancement across all on-chip modulators. By further leveraging micromechanical devices with high $Q_m$, which can exceed 1 million, our design architecture should enable a new class of low-voltage and high-speed programmable PICs.Comment: 18 pages, 5 figures, 5 supplementary figure

High-speed photonic crystal modulator with non-volatile memory via structurally-engineered strain concentration in a piezo-MEMS platform

Numerous applications in quantum and classical optics require scalable, high-speed modulators that cover visible-NIR wavelengths with low footprint, drive voltage (V) and power dissipation. A critical figure of merit for electro-optic (EO) modulators is the transmission change per voltage, dT/dV. Conventional approaches in wave-guided modulators seek to maximize dT/dV by the selection of a high EO coefficient or a longer light-material interaction, but are ultimately limited by nonlinear material properties and material losses, respectively. Optical and RF resonances can improve dT/dV, but introduce added challenges in terms of speed and spectral tuning, especially for high-Q photonic cavity resonances. Here, we introduce a cavity-based EO modulator to solve both trade-offs in a piezo-strained photonic crystal cavity. Our approach concentrates the displacement of a piezo-electric actuator of length L and a given piezoelectric coefficient into the PhCC, resulting in dT/dV proportional to L under fixed material loss. Secondly, we employ a material deformation that is programmable under a "read-write" protocol with a continuous, repeatable tuning range of 5 GHz and a maximum non-volatile excursion of 8 GHz. In telecom-band demonstrations, we measure a fundamental mode linewidth = 5.4 GHz, with voltage response 177 MHz/V corresponding to 40 GHz for voltage spanning -120 to 120 V, 3dB-modulation bandwidth of 3.2 MHz broadband DC-AC, and 142 MHz for resonant operation near 2.8 GHz operation, optical extinction down to min(log(T)) = -25 dB via Michelson-type interference, and an energy consumption down to 0.17 nW/GHz. The strain-enhancement methods presented here are applicable to study and control other strain-sensitive systems

Measurement of minority-carrier diffusion lengths using wedge-shaped semiconductor photoelectrodes

Measurement of the photocurrent as a function of the thickness of a light absorber has been shown herein both theoretically and experimentally to provide a method for determination of the minority-carrier diffusion length of a sample. To perform the measurement, an illuminated spot of photons with an energy well above the band gap of the material was scanned along the thickness gradient of a wedge-shaped, rear-illuminated semiconducting light absorber. Photogenerated majority carriers were collected through a back-side transparent ohmic contact, and a front-side liquid or Schottky junction collected the photogenerated minority carriers. Calculations showed that the diffusion length could be evaluated from the exponential variation in photocurrent as a function of the thickness of the sample. Good agreement was observed between experiment and theory for a solid-state silicon Schottky junction measured using this method. As an example for the application of the technique to semiconductor/liquid-junction photoelectrodes, the minority-carrier diffusion length was determined for graded thickness, sputtered tungsten trioxide and polished bismuth vanadate films under back-illumination in contact with an aqueous electrolyte. This wedge technique does not require knowledge of the spectral absorption coefficient, doping, or surface recombination velocity of the sample

Scalable photonic integrated circuits for programmable control of atomic systems

Advances in laser technology have driven discoveries in atomic, molecular, and optical (AMO) physics and emerging applications, from quantum computers with cold atoms or ions, to quantum networks with solid-state color centers. This progress is motivating the development of a new generation of "programmable optical control" systems, characterized by criteria (C1) visible (VIS) and near-infrared (IR) wavelength operation, (C2) large channel counts extensible beyond 1000s of individually addressable atoms, (C3) high intensity modulation extinction and (C4) repeatability compatible with low gate errors, and (C5) fast switching times. Here, we address these challenges by introducing an atom control architecture based on VIS-IR photonic integrated circuit (PIC) technology. Based on a complementary metal-oxide-semiconductor (CMOS) fabrication process, this Atom-control PIC (APIC) technology meets the system requirements (C1)-(C5). As a proof of concept, we demonstrate a 16-channel silicon nitride based APIC with (5.8$\pm$0.4) ns response times and -30 dB extinction ratio at a wavelength of 780 nm. This work demonstrates the suitability of PIC technology for quantum control, opening a path towards scalable quantum information processing based on optically-programmable atomic systems

Multiplexed control of spin quantum memories in a photonic circuit

A central goal in many quantum information processing applications is a network of quantum memories that can be entangled with each other while being individually controlled and measured with high fidelity. This goal has motivated the development of programmable photonic integrated circuits (PICs) with integrated spin quantum memories using diamond color center spin-photon interfaces. However, this approach introduces a challenge in the microwave control of individual spins within closely packed registers. Here, we present a quantum-memory-integrated photonics platform capable of (i) the integration of multiple diamond color center spins into a cryogenically compatible, high-speed programmable PIC platform; (ii) selective manipulation of individual spin qubits addressed via tunable magnetic field gradients; and (iii) simultaneous control of multiple qubits using numerically optimized microwave pulse shaping. The combination of localized optical control, enabled by the PIC platform, together with selective spin manipulation opens the path to scalable quantum networks on intra-chip and inter-chip platforms.Comment: 10 pages, 4 figure

Distortion of the major histocompatibility complex class I binding groove to accommodate an insulin-derived 10-Mer peptide

The non-obese diabetic mouse model of type 1 diabetes continues to be an important tool for delineating the role of T-cell-mediated destruction of pancreatic β-cells. However, little is known about the molecular mechanisms that enable this disease pathway. We show that insulin reactivity by a CD8+ T-cell clone, known to induce type 1 diabetes, is characterized by weak T-cell antigen receptor binding to a relatively unstable peptide-MHC. The structure of the native 9- and 10-mer insulin epitopes demonstrated that peptide residues 7 and 8 form a prominent solvent-exposed bulge that could potentially be the main focus of T-cell receptor binding. The C terminus of the peptide governed peptide-MHC stability. Unexpectedly, we further demonstrate a novel mode of flexible peptide presentation in which the MHC peptide-binding groove is able to “open the back door” to accommodate extra C-terminal peptide residues

A Novel Redox Method for Rapid Production of Functional Bi-Specific Antibodies For Use in Early Pilot Studies

We demonstrate here a rapid alternative method for the production of functional bi-specific antibodies using the mild reducing agent 2-mercaptoethanesulfonic acid sodium salt (MESNA). Following reduction of a mixture of two monoclonal antibodies with MESNA to break inter heavy chain bonds, this solution is dialysed under oxidising conditions and antibodies are allowed to reform. During this reaction a mixture of antibodies is formed, including parental antibodies and bi-specific antibody. Bi-specific antibodies are purified over two sequential affinity columns. Following purification, bi-specificity of antibodies is determined in enzyme-linked immunosorbent assays and by flow cytometry. Using this redox method we have been successful in producing hybrid and same-species bi-specific antibodies in a time frame of 6–10 working days, making this production method a time saving alternative to the time-consuming traditional heterohybridoma technology for the production of bi-specific antibodies for use in early pilot studies. The use of both rat and mouse IgG antibodies forming a rat/mouse bi-specific antibody as well as producing a pure mouse bi-specific antibody and a pure rat bi-specific antibody demonstrates the flexibility of this production method