Lasers with well-controlled relative frequencies are indispensable for many applications in science and technology. We present a frequency-offset locking method for lasers based on beat-frequency discrimination utilizing hybrid electronic $LC$ filters. The method is specifically designed for decoupling the tightness of the lock from the broadness of its capture range. The presented demonstration locks two free-running diode lasers at 780 nm with a 5.5-GHz offset. It displays an offset frequency instability below 55 Hz for time scales in excess of 1000 s and a minimum of 12 Hz at 10-s averaging. The performance is complemented with a 190-MHz lock-capture range, a tuning range of up to 1 GHz, and a frequency ramp agility of $200\mathrm{kHz}/\mu \mathrm{s}$.

The resolution of far-field imaging systems suffers from the diffraction limit. In this work, a blind structured illumination microscopy (blind-SIM) system that achieves subwavelength resolution in acoustic far-field imaging is developed. In blind-SIM, precise knowledge of the structured illumination (SI) patterns is not required. Instead, the acoustic SIs are generated by a randomly located acoustic diffraction grating with a subwavelength grating period. The spatial frequency mixing between the object and the SIs converts evanescent waves to propagating waves that can reach sensors in the far field. The image of the object is then reconstructed from multiple far-field measurements utilizing a joint-sparsity compressive sensing algorithm. Numerical simulations and experiments in kilohertz airborne sound are performed to validate the proposed system. A subwavelength imaging resolution of around 1/3 wavelength is achieved in experiments, and around 1/5 wavelength in simulations. The blind-SIM system largely reduces the imaging hardware complexity compared with existing acoustic subwavelength technologies that mostly rely on metamaterial lenses, time-reversal mirrors, or the precise localization of time-varying contrast agents. The combination of blind-SIM and the joint-sparsity compressive sensing algorithm for image reconstruction reduces the requirements for an object’s sparsity level and enhances the system’s robustness against noise. Our work has potential in improving the practicality of acoustic far-field subwavelength imaging in medical diagnoses, nondestructive testing, and underwater acoustic imaging and communications.

Due to challenging efficiency limits facing conventional and unconventional electronic architectures, information processors based on photonics have attracted renewed interest. Research communities have yet to settle on definitive techniques to describe the performance of this class of information processors. Photonic systems are different from electronic ones, and the existing concepts of computer performance measurement cannot necessarily apply. In this paper, we quantify the power use of photonic neural networks with state-of-the-art and future hardware. We derive scaling laws, physical limits, and platform platform performance metrics. We find that overall performance takes on different dominant scaling laws depending on scale, bandwidth, and resolution, which means that energy efficiency characteristics of a photonic processor can be completely described by no less than seven performance metrics over the range of relevant operating domains. The introduction of these analytical strategies provides a much needed foundation and reference for quantitative roadmapping and commercial value assignment for silicon photonic neural networks.

Lasers, light-emitting diodes, and other optoelectronic devices employing $\mathrm{In}\mathrm{As}$/$\mathrm{In}\mathrm{P}$ quantum dots (QDs) instead of quantum wells (QWs) as their active parts benefit from the quasi-zero-dimensional (0D) density of states while maintaining the emission at the communication-relevant range of $1.55\mu \mathrm{m}$. However, for certain application purposes, the substitution of QWs with QDs is advantageous only if QDs can either be treated as isolated objects or exhibit quantum-mechanical coupling between deeply confined states. Here, we compare two material systems of $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$/$\mathrm{In}\mathrm{P}$ and $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$/$\mathrm{In}\mathrm{P}$ elongated QDs (quantum dashes, QDashes) of comparable surface densities and interdash distances. We investigate the presence and type of coupling between QDashes, focusing on the direct tunnel coupling for both types of carriers manifested already at low temperature. In the time-resolved photoluminescence (PL) experiment, we observe a significant dispersion of the PL decay time for $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ QDashes, resulting from the migration of carriers from high- to low-energy QDashes due to substantial tunnel coupling. In the case of $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$ QDashes, the dispersionless time dependence points toward the absence of such coupling. We confirm this interpretation with multiband $\mathit{k}\cdot \mathit{p}$ calculations. We check that the proposed coupling scenario is possible and show its much higher probability for $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ than for $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$ QDashes. Finally, in temperature-dependent PL, we observe the redistribution of holes among QDashes via thermal excitation to wetting-layer states for both systems, constituting an additional interdash coupling channel at elevated temperatures. Our results indicate that the $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$ QDash system is preferential for optoelectronic applications where QD isolation is highly sought after, whereas $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ QDashes exhibit quantum-mechanical coupling between deeply confined states.

We report an artificial acoustic compressibility-near-zero medium made of a phononic crystal composed of epoxy blocks arranged in a square lattice. Its anisotropic effective density leads to a linear cross in its isofrequency contour in the vicinity of the Brillouin zone center, as its effective compressibility approaches zero. When a Gaussian beam is normally incident on the phononic crystal, a splitting effect is achieved at the frequency of the crossing point. Based on such a beam-splitting effect, an acoustic cloaking of an irregular-shaped object embedded in the phononic crystal is demonstrated both theoretically and experimentally. Such an anisotropic zero-index material offers a potential method to control acoustic waves.

Multiple reflections between transducer and imaged object can naturally occur in ultrasound imaging and other acoustic sensing applications such as sonar. The repeated interaction of the emitted wave front with the imaged object is traditionally regarded as an undesired reverberation artifact, often misinterpreted as fictitious acoustic boundaries. We introduce high-order reflection pulse-echo (HOPE) ultrasound, a method that leverages high-order reflections to improve on several aspects of conventional ultrasound imaging. HOPE is experimentally demonstrated to resolve submicrometer features by breaking through the sampling limit. The major contrast enhancement of the high reflection orders allowed defects within materials invisible to conventional scanning acoustic microscopy to be revealed. The technique is further shown to improve accuracy of frequency-dependent ultrasound attenuation measurements from biological tissues. HOPE ultrasound requires no additional hardware and is easy to implement, underscoring its potential to boost imaging performance in biomedical imaging, nondestructive testing, and other acoustic sensing applications.

Quantum networks with the ability to coherently transfer quantum states between different nodes open up possibilities for upscaling quantum technologies. In this work, we propose a theoretical protocol to implement the multinode state transfer and nonlocal state preparation in a one-dimensional network consisting of many quantum nodes coupled to a common unidirectional channel. By time-dependently modulating the coupling strength between the nodes and the channel, a quantum state stored in one node can be distributed to the other nodes through the channel, while a series of dark-state conditions ensure a desired final state. This protocol can also be used to prepare nonlocal entanglement states of these nodes, such as the W -type state. Importantly, it can be realized in various quantum platforms with currently available technologies.

The bull’s-eye structure in the terahertz (THz) frequency region has ample applications owing to its ability to focus free-propagating waves into subwavelength apertures, resulting in enhanced transmission, that is, extraordinary transmission. However, its coupling properties have been primarily discussed in terms of the normal plane-wave incidence to the structure. In this study, we investigate the multiple resonances in extraordinary transmission with normal and oblique incident waves. The experiment using a widely tunable and high-power THz wave source revealed two types of resonances. The main resonance split depends on the incident angle, and the other corresponds to the side lobe of the main resonances. The results are explained by a simple analytical model using a finite number of scattering media. The analysis is supported by the full-wave simulation using the finite-element method, which agrees with the experimental results. The coupling mechanisms will be applicable to design devices, such as THz biosensing devices or THz antennas for rapid communication systems.

Orbital-angular-momentum (OAM) complex-valued spectrum reconstruction routinely necessitates the measurements of a complete set of observables, which is rather time consuming and inefficient for high-dimensional OAM superposition state. Here we break this barrier to realize the full reconstruction of a high-dimensional OAM complex-valued spectrum via only a single-shot measurement, which benefits from the use of a residual convolutional neural network for learning the diffraction behaviors. In our experiment, the high-dimensional OAM states are diffracted by a simple aperture of pentagram that breaks the OAM conjugate symmetry. We achieve fidelity over 97.8% and 92.1% for high-dimensional pure and mixed OAM states (up to $d=15$), respectively. We also confirm the excellent robustness under strong noise environment. Our method will find potential applications in high-dimensional information encoding, such as large-alphabet communications, quantum imaging, and real-time quantum sensing.

Non-Markovian noise presents a particularly relevant challenge in understanding and combating decoherence in quantum computers, yet is challenging to capture in terms of simple models. Here we show that a simple phenomenological dynamical model known as the post-Markovian master equation (PMME) accurately captures and predicts non-Markovian noise in a superconducting qubit system. The PMME is constructed using experimentally measured state dynamics of an IBM Quantum Experience cloud-based quantum processor, and the model thus constructed successfully predicts the non-Markovian dynamics observed in later experiments. The model also allows the extraction of information about crosstalk and measures of non-Markovianity. We demonstrate definitively that the PMME model predicts subsequent dynamics of the processor better than the standard Markovian master equation.

Optomechanical systems offer unique opportunities to explore macroscopic quantum state and related fundamental problems in quantum mechanics. Here, we propose a quantum optomechanical system involving exchange interaction between spin angular momentum of light and a torsional oscillator. We demonstrate that this system allows coherent control of the torsional quantum state of a torsional oscillator on the single-photon level, which facilitates efficient cooling and squeezing of the torsional oscillator. Furthermore, the torsional oscillator with a macroscopic length scale can be prepared in Schrödinger catlike state. Our work provides a platform to verify the validity of quantum mechanics in macroscopic systems on the micrometer and even centimeter scale.

Differential absorption spectroscopy techniques serve as powerful techniques to study the excited species in organic solar cells. However, it has always been challenging to employ these techniques for characterizing thick-junction organic solar cells, especially when a reflective top contact is involved. In this work, we present a detailed and systematic study on how a combination of the presence of the interference effect and a nonuniform charge-distribution profile, severely manipulates experimental spectra and the decay dynamics. Furthermore, we provide a practical methodology to correct these optical artifacts in differential absorption spectroscopies. The results and the proposed correction method generally apply to all kinds of differential absorption spectroscopy techniques and various thin-film systems, such as organics, perovskites, kesterites, and two-dimensional materials. Notably, it is found that the shape of differential absorption spectra can be strongly distorted, starting from 150-nm active-layer thickness; this matches the thickness range of thick-junction organic solar cells and most perovskite solar cells and needs to be carefully considered in experiments. In addition, the decay dynamics of differential absorption spectra is found to be disturbed by optical artifacts under certain conditions. With the help of the proposed correction formalism, differential spectra and the decay dynamics can be characterized on the full device of thin-film solar cells in transmission mode and yield accurate and reliable results to provide design rules for further progress.

The nonlinear dynamics of microelectromechanical resonators has recently garnered significant attention for their potential use in high-precision detection and information processing. In this paper, we report resonant excitation as a powerful tool for generating nonlinear mode coupling in a cantilever microresonator. It is discovered that resonant driving at its second flexural mode $f_2$ under moderate driving amplitude gives rise to blue and red sidebands due to mode coupling between the fundamental mode and the second flexural mode. Detuning the driving frequency enables a continuous shift in the frequencies of the blue and red sidebands with a frequency span of 220 Hz. Further increase in the drive amplitude at $f_2$ mode splitting, and broadband acoustic frequency comb with a frequency span exceeding 500 kHz. Our findings pave the way towards parametric control based on injection locking for manipulating multimode mechanical resonator systems.

Versatile twisted optical bottles (TOBs) generated from a series of abruptly autofocusing beams are derived numerically and experimentally. This kind of TOB beam utilizes dual-autofocus, optical-vortex, and astigmatic phase properties to structure one or more three-dimensional closed dark regions, the bottle space. Closed off-axis multiple optical bottles are generated and observed experimentally. The twist angles can individually affect the rotation of the TOB. The experimental results agree well with our numerical simulations. The relationships between the astigmatic phase, the length of the TOB, the topological order of the vortex, and the width of the TOB are determined. Additionally, stable mesocarbon particles trapped by TOB beams are observed in side view successfully. These TOBs offer more possibilities to trap, sort, and clean microparticles or nanoparticles and improve optical manipulation systems in the future.

The frequency band 1–15 GHz provides exciting prospects for resonant axion haloscopes as indicated by cosmological and astrophysical arguments. Among the challenges currently addressed to reach the required sensitivity, the development of high-quality factor cavities that tolerate multitesla fields plays a central role. We report a three-dimensional resonator based on a right-circular copper cavity with hollow cylinders that confine higher-order modes around the cylinder axis. Its effective volume at 10.3 GHz is $3.4\times {10}^{-2}$ l, and under an 8-T field we measure an internal quality factor of more than nine million. A fine-tuning system limited to approximately $1$-MHz range is tested, useful for understanding the conditions required to run a haloscope with a cavity exceeding the axion quality factor. The cavity parameters we report demonstrate the potential of this unique resonator to probe galactic dark-matter axion at scan rates exceeding 2 MHz/day when the cavity is readout by a quantum-limited receiver.