Continuous terahertz wave sources with narrow bandwidth and wide frequency tunability enable high-resolution terahertz spectroscopy and high-speed information communication. In this study, using the optical nonlinearity of excitons as the source of second-order nonlinear polarization, we realize a continuous terahertz electromagnetic wave demonstrating wide frequency tunability from 0.1 to 18 THz without a decrease in intensity due to phonon scattering. Because of excitation of two exciton states in a $\mathrm{Ga}\mathrm{As}$/$\mathrm{Al}\mathrm{As}$ multiple quantum well using two continuous-wave lasers, terahertz waves are emitted as a result of difference-frequency mixing, where the intensity shows a square dependence on the excitation intensity. Using the inhomogeneous width of exciton lines, we achieve wide frequency tunability without phonon effects.

We develop and characterize an atom-guiding technique that loads $3\times {10}^6$ cold rubidium atoms into a hollow-core optical fiber, an order-of-magnitude greater than previously reported results. This result is possible because it is guided by a physically realistic simulation that can provide the specifications for a loading efficiency of 3.0% and a peak optical depth of 600. The simulation further shows that the loading efficiency is limited solely by the geometric overlap of the atom cloud and the optical guide beam, and is thus open to further improvement with experimental modification. The experimental arrangement allows observation of the real-time effects of light-assisted cold-atom collisions and background-gas collisions by tracking the dynamics of the cold-atom cloud as it falls into the fiber. The combination of these observations, and physical understanding from the simulation, allows estimation of the limits to loading cold atoms into hollow-core fibers.

Phase-estimation protocols provide a fundamental benchmark for the field of quantum metrology. The latter represents one of the most relevant applications of quantum theory, potentially enabling the capability of measuring unknown physical parameters with improved precision over classical strategies. Within this context, most theoretical and experimental studies have focused on determining the fundamental bounds and how to achieve them in the asymptotic regime where a large number of resources are employed. However, in most applications, it is necessary to achieve optimal precision by performing only a limited number of measurements. To this end, machine-learning techniques can be applied as a powerful optimization tool. Here, we implement experimentally single-photon adaptive phase-estimation protocols enhanced by machine learning, showing the capability of reaching optimal precision after a small number of trials. In particular, we introduce an approach for Bayesian estimation that exhibits best performance for a very low number of photons $N$. Furthermore, we study the resilience to noise of the tested methods, showing that the optimized Bayesian approach is very robust in the presence of imperfections. Application of this methodology can be envisaged in the more general multiparameter case, which represents a paradigmatic scenario for several tasks, including imaging or Hamiltonian learning.

A Bloch surface wave (BSW) is a kind of resonant propagating wave sustained at the surface of a truncated dielectric multilayer. Spiral or concentric rings engraved on the top layer of the truncated multilayer, termed a “BSW lens,” will concentrate BSWs into the center. As a dielectric counterpart of a surface plasmon on a metal surface, a BSW propagates farther because of the absence of ohmic loss in a dielectric. Therefore, it is believed that a BSW lens can produce a much stronger focal spot if more rings are engraved. In this paper, we present a comprehensive study on the focusing properties of a BSW lens. We derive the dispersion relation of a BSW, analyze the focal pattern based on the interaction between the spiral-ring structure and light with specific spin and orbital angular momentum, and investigate transmittance and reflectance of the slot for the BSW. We find the reflectance of the slot introduces nonignorable loss for a BSW lens with multiple rings. In addition, we demonstrate that BSW waves can be efficiently concentrated into the focal spot by optimizing the trade-off between increasing number of rings and reflective loss of the ring slot, which is potentially helpful and valuable for integrated nanophotonic devices.

We report on luminescent centers contained in a few-layer-thick hexagonal boron nitride ($h$-BN) film grown on $\mathrm{Ni}$ by molecular beam epitaxy. After transfer to a ${\mathrm{Si}\mathrm{O}}_2$/$\mathrm{Si}$ substrate, sharp lines are observed in photo- and cathodoluminescence spectra in both the ultraviolet and the visible range. Spatially resolved measurements reveal that the luminescent centers responsible for these lines are localized within microscopic multilayer islands that form at the nucleation centers of the $h$-BN film. The comparison of their energy, polarization, and phonon-replica emission with previous theoretical predictions suggest that the ${\mathrm{N}}_{\mathrm{B}}{V}_{\mathrm{N}}$ antisite could be one of the light emitters present in our sample. Moreover, we also observe evidence of other kinds of centers that could be associated to defects containing carbon or oxygen. The characterized luminescent defects could have potential applications as quantum light sources.

Lead-free halide double perovskites with the formula of quaternary $A_2^{+}{B}^{+}{B}^{{\mathrm{\prime }}^{3+}}{X}_6^{-}$ have recently attracted intense interest as alternatives to lead-halide-perovskite-based optoelectronic materials for their nontoxicity and enhanced chemical and thermodynamic stability. However, the understanding of intrinsic defect properties and their effects on carrier transport and Fermi level tuning is still limited. In this paper, we show that, by exploring the phase diagram of a halide double perovskite, one can control the effects of intrinsic defects on carrier trapping and Fermi-level pinning. We reveal the ideal growth conditions to grow p type ${\mathrm{Cs}}_2\mathrm{Ag}\mathrm{In}{\mathrm{Cl}}_6$ and ${\mathrm{Cs}}_2\mathrm{Ag}\mathrm{Bi}{\mathrm{Cl}}_6$ as well as semi-insulating ${\mathrm{Cs}}_2\mathrm{Ag}\mathrm{Bi}{\mathrm{Br}}_6$ with low trap density for targeted photovoltaic or visible light/radiation detection application.

Increasing coherence times of quantum bits is a fundamental challenge in the field of quantum computing. With long-lived qubits it is, however, inefficient to wait until the qubits have relaxed to their ground state after completion of an experiment. Moreover, for error-correction schemes it is important to rapidly reinitialize syndrome qubits. We present a simple pulsed qubit reset protocol based on a two-pulse sequence. A first pulse transfers the excited-state population to a higher excited qubit state and a second pulse transfer it into a lossy environment provided by a low-$Q$ transmission-line resonator, which is also used for qubit read-out. We show that the remaining excited-state population can be suppressed to $(1.7\pm 0.1)\mathrm{\%}$ and that this figure may be reduced by further improving the pulse calibration.

The field of low-loss dielectric metasurfaces has created a new paradigm of miniaturization for free-space optical elements. We study the effects of uniformly changing the refractive index of high-index metalenses. We experimentally demonstrate a modal refractive-index change $(\mathrm{\Delta }{n}_{\mathrm{MD}}=0.15)$ in the fundamental Mie resonances in an $\mathrm{In}\mathrm{Sb}$ resonator based on traditional thermo-optic effects. We develop a high aspect ratio metasurface design with simulated 75–90% transmission efficiency and 2π phase shift as a function of cylinder radius. A metalens is shown to have high (>60%) focusing efficiencies for large (up to 0.8) numerical aperture designs. The uniform thermal tuning of the metalens system is studied based on the variations in the spatial phase profile of the individual resonators. We demonstrate that both the operating wavelength $(\mathrm{\Delta }{\lambda }_f=500\mathrm{nm})$ and the focal length (Δf = 45 µm) can be dynamically modified based on the chromatic dispersion of the engineered metalens. The results show that static metasurfaces made of high-index semiconductors can be thermally actuated to tune the operating focal wavelength and focal length.

One-way edge states at the surface of photonic topological insulators are of significant interest for communications and nonlinear and quantum optics. Moreover, when reciprocity is broken in a photonic topological insulator, these states provide protection against disorder, which is of particular importance for slow-light applications. Achieving such a one-way edge state, however, requires the construction of a two-dimensional structure. Here, we show how unidiriectional Floquet bands can arise in purely one-dimensional, adiabatically modulated dynamic systems, in contrast with the higher dimensionality needed in topological insulators. We also show that, using realistic experimental parameters, the concept can be implemented using both a coupled-resonator optical waveguide and a photonic-crystal waveguide. Furthermore, we illustrate the associated protection against disorder and find it to be of a different nature when compared to Floquet topological insulators.

Experimental verifications of cloaking, which can make objects invisible, have long been limited to single-frequency operation for relatively small objects. In this paper, we demonstrate a metamaterial cloak enabling multifrequency operation for an electrically large object, which can be robustly controlled based on an intrinsic harmonics selection condition. By designing an inhomogeneous isotropic refractive index that collimates the scattered wave path into forward direction rather than bending the propagating wave around the object exactly, the harmonics selection condition for multifrequency cloaking is naturally derived by matching the phase delay induced from the cloak medium to $2\pi$ harmonics. Then, the cloaking harmonics are controlled robustly by adjusting the size of the hidden object, which determines the phase delay inside the cloak without any extrinsic schemes such as actively controllable metamaterials. From the merit of the isotropic nature, the proposed cloak is fabricated in the three-dimensional space and verified experimentally from the reduced electromagnetic scatterings of a copper double cone. The proposed cloak employs a distinct manner, which may open a new paradigm to design multiband apparatuses such as stealth or camouflage.

The dynamic process of a meta-atom (the classic double split-ring resonator) immersed in a low-loss liquid medium and illuminated by electromagnetic waves with different intensities is investigated, with detailed theoretical analysis and experimental verification. The intrinsic energy-coupling mechanism and state-changing conditions are provided to reveal the interesting and also important dynamic phenomena of the meta-atom in a liquid. The work presented here opens a new research direction for metamaterials in physics and engineering areas.

Directional sound beam forming is an area of interest in practical acoustic applications where leaky-wave principles and techniques may provide efficient solutions. Here, we present a type of high-gain acoustic leaky antenna used for acoustic beamforming using an admittance-modulation metasurface which consists of periodic subwavelength grooves with sinusoidal depth profiles. The acoustic metasurface converts the surface waves into far-field radiating waves forming a high-gain surface wave antenna. We numerically and experimentally demonstrate far-field propagation angles of −30°, 0°, and 30° at 19 300, 22 000, and 23 900 Hz, respectively, and show that the radiation angle and beamwidth can be independently controlled by modulating the sinusoidal profile. Also, we use a Laser Doppler Vibrometer to directly measure the surface wave damping along a circular sinusoidally modulated admittance surface (SMAS).

We perform a systematic theoretical analysis of the nature and importance of alloy disorder effects on the electronic and optical properties of $\mathrm{Ga}{\mathrm{N}}_y{\mathrm{As}}_{1-x-y}{\mathrm{Bi}}_x$ alloys and quantum wells (QWs), using large-scale atomistic supercell electronic structure calculations based on the tight-binding method. Using ordered alloy supercell calculations, we also derive and parametrize an extended-basis 14-band k$\cdot$p Hamiltonian for $\mathrm{Ga}{\mathrm{N}}_y{\mathrm{As}}_{1-x-y}{\mathrm{Bi}}_x$. Comparison of the results of these models highlights the role played by short-range alloy disorder—associated with substitutional nitrogen ($\mathrm{N}$) and bismuth ($\mathrm{Bi}$) incorporation—in determining the details of the electronic and optical properties. Systematic analysis of large alloy supercells reveals that the respective impacts of $\mathrm{N}$ and $\mathrm{Bi}$ on the band structure remain largely independent, a robust conclusion that we find to be valid even in the presence of significant alloy disorder where $\mathrm{N}$ and $\mathrm{Bi}$ atoms share common $\mathrm{Ga}$ nearest neighbors. Our calculations reveal that $\mathrm{N}$- ($\mathrm{Bi}$-)related alloy disorder strongly influences the conduction- (valence-)band edge states, leading in QWs to strong carrier localization, as well as inhomogeneous broadening and modification of the conventional selection rules for optical transitions. Our analysis provides detailed insight into key properties and trends in this unusual material system, and enables quantitative evaluation of the potential of $\mathrm{Ga}{\mathrm{N}}_y{\mathrm{As}}_{1-x-y}{\mathrm{Bi}}_x$ alloys for applications in photonic and photovoltaic devices.

A method for extracting the density and energetic distribution of the trap states in the semiconductor of a field-effect transistor from its measured transfer characteristics is investigated. The method is based on an established extraction scheme [M. Grünewald et al., Phys. Stat. Sol. B 100, K139 (1980)] and extends it to low-voltage thin-film transistors (TFTs). In order to demonstrate the significance of this extension, two types of TFTs are fabricated and analyzed: one with a thick gate dielectric and high operating voltage and one with a thin gate dielectric and low operating voltage. From the measured transfer characteristics of both TFTs, the density of states (DOS) is calculated using both the original and the extended Grünewald method. The results not only confirm the validity of the original Grünewald method for high-voltage transistors, but also indicate the need for the extended Grünewald method for the reliable extraction of the trap DOS in transistors with a thin gate dielectric and low operating voltage.

Conventional Marchenko redatuming can retrieve the Green’s function at a virtual receiver inside a heterogeneous medium from single-sided reflection data. It requires minimal a priori knowledge of medium properties but assumes the medium to be lossless. The dissipative Marchenko scheme, on the other hand, can retrieve the Green’s function inside a lossy medium but requires double-sided access to the lossy medium in order to obtain the reflection response of its corresponding effectual medium with negative dissipation. Here, we implement the dissipative Marchenko scheme both numerically and experimentally, and successfully redatum full sound wavefields in a one-dimensional (1D) waveguide. To achieve this, we first quantify the medium attenuation using the nearly constant $Q$ model to facilitate numerical modeling. Then, we numerically retrieve two sets of focusing functions that are required by the scheme, by iteratively reemitting the time-windowed and time-reversed signal from a single side of the dissipative and effectual media. Without any knowledge of the medium properties, we experimentally retrieve the desired focusing and Green’s functions inside the physical wave tube from double-sided scattering data measurements. We show that both offline processing and real-time focusing can achieve similar accuracy. True-amplitude Green’s functions can also be obtained but require a scaling correction. The reconciliation of numerical modeling and physical experiments is an important step in establishing the applicability of the Marchenko methods for attenuative media.

We show that flexural waves can be focused in a beam to an extent that induces precisely controlled dynamic fracture of the beam. Flexural waves are excited at one end of a finite beam and focus at another location along the beam to form a shorter but much larger bending-moment pulse. The strong focusing is achieved by use of the frequency dependence of the phase and group velocity of flexural waves, as well as the superposition of multiple reflections. Amplification of the actuator input by a factor of 20 is achieved solely by wave focusing, proving the potential of such a technique.

Graphene, the two-dimensional monolayer of carbon, is an essential building block for advanced electronic applications. Graphene is often integrated with bulk conductors, metals, and semiconductors for optoelectronic applications. The semimetal bismuth, that shares many electronic properties with these other conductors, is interesting because like graphene, it is a gapless conductor of high electronic mobility with linear dispersion relations and low electronic density. We study the doping of graphene by $\mathrm{Bi}$ and the interfacial electric dipole. Graphene Raman spectroscopy results show that there is a very large charge transfer between graphene and bismuth. This doping is larger than in the interfaces of graphene with metals such as $\mathrm{Cu}$ and semiconductors such as $\mathrm{Si}$. Our findings are in good agreement with recent theoretical results for graphene bismuth interfaces. We also present a demonstration of zero-bias photocurrent generation that is enabled by the electric dipole at the graphene-bismuth interface.

We demonstrate the generation of multimode entangled states of propagating microwaves. The entangled states are generated by our parametrically pumping a multimode superconducting cavity. By combining different pump frequencies, applied simultaneously to the device, we can produce different entanglement structures in a programable fashion. The Gaussian output states are fully characterized by our measuring the full covariance matrices of the modes. The covariance matrices are absolutely calibrated by our using an in situ microwave calibration source, a shot-noise tunnel junction. Applying a variety of entanglement measures, we demonstrate both full inseparability and genuine tripartite entanglement of the states. Our method is easily extensible to more modes.

Formation of nanoporous structures is an effective approach to manipulate heat conduction and has been experimentally demonstrated to greatly reduce thermal conductivity. Thermal conductivity reduction of nanoporous materials depends on structural parameters such as size, shape, and position of the pores and their distributions, which are hard to explore in experiments. In this work, by systematically performing ray-tracing Monte Carlo simulations of nanoporous silicon crystal, we evaluate the impacts of the structural parameters on the thermal conductivity reduction. As a result, we find that the thermal conductivity reduction with spherical pores is insensitive to the spatial configuration and size distribution of the pores, even in the regime of quasi-ballistic phonon transport. Although the sensitivity does increase as phonon scattering by the pores becomes directional (as for rectangular pores) and porosity increases, the overall results deliver aspects that are useful in practice: the thermal conductivity of nanoporous structures can be well described by a phonon scattering model with a single length scale.

Preserving coherence long enough to perform meaningful calculations is one of the major challenges on the pathway to large-scale quantum-computer implementations. Noise coupled in from the environment is the main factor contributing to decoherence but can be mitigated via engineering design and control solutions. However, this is possible only after acquisition of a thorough understanding of the dominant noise sources and their spectrum. In the work reported here, we use a silicon quantum dot spin qubit as a metrological device to study the noise environment experienced by the qubit. We compare the sensitivity of this qubit to electrical noise with that of an implanted silicon-donor qubit in the same environment and measurement setup. Our results show that, as expected, a quantum dot spin qubit is more sensitive to electrical noise than a donor spin qubit due to the larger Stark shift, and the noise-spectroscopy data show pronounced charge-noise contributions at intermediate frequencies (2–20 kHz).