Measuring the electron-transport properties of substrate-supported nanomaterials with the traditional two-point comparison method is difficult at electron energies below 50 eV, where core-level signals are too feeble to be detected against the strong secondary-electron background. Herein, a data-driven spectral analysis technique is used to study the low-energy electron-transport properties of substrate-supported target nanomaterials, while eliminating the influence of the substrate signal. Applying this technique, the electron-transport properties of the effective attenuation length and the inelastic mean free path (IMFP) can be determined with extremely high efficiency over the entire measured energy range of 6–600 eV. Further, these results show excellent agreement with other experimental and theoretical results. Significant differences are observed between monolayer graphene and the bulk graphite IMFP, which illustrates the importance of the nanometer effect in the electron-transport properties of the material. Furthermore, this technique is readily applicable to any ultrathin material that can be transferred onto a polycrystalline gold substrate.

Irradiation-induced lattice defects in silicon carbide ($\mathrm{Si}\mathrm{C}$) have already exceeded their previous reputation as purely performance inhibiting. With their remarkable quantum properties, such as long room-temperature spin coherence and the possibility of downscaling to single-photon-source level, they have proven to be promising candidates for a multitude of quantum-information applications. One of the most crucial parameters of any quantum system is how long its quantum coherence can be preserved. By using the pulsed optically detected magnetic resonance (ODMR) technique, we investigate the spin-lattice relaxation time ($T_1$) and spin-coherence time ($T_2$) of silicon vacancies in $4H$-$\mathrm{Si}\mathrm{C}$ created by neutron, electron, and proton irradiation in a broad range of fluences. We also examine the effect of irradiation energy and sample annealing. We establish a robustness of the $T_1$ time against all types of irradiation and reveal a universal scaling of the $T_2$ time with the emitter density. Our results can be used to optimize the coherence properties of silicon-vacancy qubits in $\mathrm{Si}\mathrm{C}$ for specific tasks.

Subwavelength nanoparticles can support electromagnetic resonances with distinct features depending on their size, shape, and nature. For example, electric and magnetic Mie resonances occur in dielectric particles, while plasmonic resonances appear in metals. Here, we experimentally demonstrate that the multipolar resonances hosted by ${\mathrm{VO}}_2$ nanocrystals can be dynamically tuned and switched thanks to the insulator-to-metal transition of ${\mathrm{VO}}_2$. Using both Mie theory and Maxwell-Garnett effective-medium theory, we retrieve the complex refractive index of the effective medium composed of a slab of ${\mathrm{VO}}_2$ nanospheres embedded in ${\mathrm{Si}\mathrm{O}}_2$ and show that such a resulting metamaterial presents distinct optical tunability compared to unpatterned ${\mathrm{VO}}_2$. We further show that this approach provides a new degree of freedom to design low-loss phase-change metamaterials with record large figure of merit ($\mathrm{\Delta }n/\mathrm{\Delta }k$) and designer optical tunability.

A large number of optimization problems of extremely complex systems in the real world remain highly challenging for conventional digital computers. Some such problems can be mapped into the Ising model, and then efficiently solved by searching for the global minimum of the Ising Hamiltonian. Finding an appropriate physical system for efficient simulation of the Ising model is a promising way of addressing such optimization problems that has recently emerged. Here we report on the realization of analog-spin chains of exciton-polariton condensates that mimic a chain of classical spins. This is done with a room-temperature system based on a one-dimensional polariton lattice that is induced by exciting a $\mathrm{Zn}\mathrm{O}$ microrod with a controllable periodic laser pattern. Depending on the lattice constant chosen, the spontaneously phase-locked condensates show either an antiphase ($\pi$) or an in-phase (zero) ordering in the steady state, which mimics the antiferromagnetic or ferromagnetic state in the one-dimensional classical Ising model. In addition, when the external excitation power is increased, a chain of coupled condensate pairs, characterized by a small phase shift between the neighboring condensates that is induced by the tunneling effect, arises at a lower energy. These observations pave the way to the realization at room temperature of analog-spin simulators based on periodic condensates of exciton polaritons.

Ultraviolet luminescence holds potential for diverse applications. Further development of ultraviolet-luminescence technology, however, is hindered by the common but very inconvenient photoluminescence form. Here we create a form of ultraviolet emission―persistent luminescence in the narrowband ultraviolet-B (NB UVB) region (309–313 nm) upon illumination by a blue light-emitting diode. On this basis, we study the dynamic competition between an up-conversion charging excitation and a photostimulated detrapping upon illumination by the light-emitting diode. Such a competition mechanism, as well as the associated manipulation technique, appears to be generally applicable for many existing phosphors. As a proof of concept, we present an imaging demonstration, showing the potential of the NB-UVB persistent luminescence in the optical tagging field. This work can potentially revolutionize the ways for utilizing persistent luminescence, leading NB-UVB applications to another level.

Magnetic tunnel junctions are nanoscale spintronic devices with microwave-generation and -detection capabilities. Here, we use the rectification effect called a “spin diode” in a magnetic tunnel junction to wirelessly detect the microwave emission of another junction in the auto-oscillatory regime. We show that the rectified spin-diode voltage measured at the receiving junction end can be reconstructed from the independently measured auto-oscillation and spin-diode spectra in each junction. Finally, we adapt the auto-oscillator model to the case of the spin-torque oscillator and the spin diode and we show that it accurately reproduces the experimentally observed features. These results will be useful for the design of circuits and chips based on spintronic nanodevices communicating through microwaves.

High-precision magnetic field measurement is a ubiquitous issue in physics and a critical task in metrology. Generally, a magnetic field has dc and ac components, and it is hard to extract both of these components simultaneously. Conventional Ramsey interferometry can easily measure dc magnetic fields, but it becomes invalid for ac magnetic fields, since the accumulated phase may average to zero. Here, we propose a scheme for simultaneous measurement of dc and ac magnetic fields by combining Ramsey interferometry and rapid periodic pulses. In our scheme, the interrogation stage is divided into two signal-accumulation processes linked by a unitary operation. In the first process, only the dc component contributes to the accumulated phase. In the second process, when multiple rapid periodic $\pi$ pulses are applied, only the ac component gives rise to an accumulated phase. By selecting suitable input states and the unitary operations used in the interrogation and readout stages, the dc and ac components can be extracted by population measurements. In particular, if the input state is a Greenberger-Horne-Zeilinger state and two interaction-based operations are applied during interferometry, the measurement precisions of both the dc and the ac component can simultaneously approach the Heisenberg limit. Our scheme provides a feasible way to achieve Heisenberg-limited simultaneous measurement of dc and ac fields.

All-optical thermometry presents a promising avenue of investigation for sensitive biological and electronic systems, with methods free of radio-frequency interference of particular importance. In this work, an all-optical scanning fiber thermometer is tested and shown to possess $5\mu \mathrm{m}$ spatial and $20\mathrm{mK}/\sqrt{\mathrm{Hz}}$ temperature resolution. This probe is then used to generate noncontact scanning thermal images of active circuit elements with microscale thermal features.

By identifying the similarities between the coupled-wave equations and the parametrically driven nonlinear Schrödinger equation, we show the existence conditions of quadratic solitons in continuous-wave pumped singly resonant degenerate optical parametric oscillators (SRDOPOs). Compared to the previously explored doubly resonant DOPOs, quadratic solitons in SRDOPOs are advantageous in their robustness against perturbations induced by dispersion of the effective third-order nonlinearity and temporal walk-off between the signal and the pump. Terahertz comb bandwidth and femtosecond pulse duration are attainable in an example periodically poled Lithium niobate waveguide resonator in the short-wave infrared. The working principle can be extended to other material platforms, making it a competitive ultrashort pulse and broadband comb source architecture at the mid-infrared spectral range.

As the semiconductor industry turns to alternative conductors to replace $\mathrm{Cu}$ for future interconnect nodes, much attention has been focused on evaluating the electrical performance of $\mathrm{Ru}$. The typical hcp phase has been extensively studied, but relatively little attention has been paid to the fcc phase, which has been shown to nucleate in confined structures and may be present in tight-pitch interconnects. Using ab initio techniques, we benchmark the performance of fcc $\mathrm{Ru}$. We find that the phonon-limited bulk resistivity of the fcc $\mathrm{Ru}$ is less than half of that of hcp $\mathrm{Ru}$, a feature we trace back to the stronger electron-phonon coupling elements in hcp $\mathrm{Ru}$ that are geometrically inherited from the modified Fermi-surface shape of the fcc crystal. Despite this benefit of the fcc phase, high grain-boundary scattering results in increased resistivity compared to $\mathrm{Cu}$-based interconnects with similar average grain size. We find, however, that the line resistance of fcc $\mathrm{Ru}$ is lower than that of $\mathrm{Cu}$ below 21-nm linewidth due to the conductor volume lost to adhesion and wetting liners. In addition to studying bulk transport properties, we evaluate the performance of adhesion liners for fcc $\mathrm{Ru}$. We find that it is energetically more favorable for fcc $\mathrm{Ru}$ to bind directly to silicon dioxide than through conventional adhesion liners such as $\mathrm{Ta}\mathrm{N}$ and $\mathrm{Ti}\mathrm{N}$. In the case that a thin liner is necessary for the $\mathrm{Ru}$ deposition technique, we find that the vertical resistance penalty of a liner for fcc $\mathrm{Ru}$ can be up to eight times lower than that calculated for conventional liners used for $\mathrm{Cu}$ interconnects. Our calculations, therefore, suggest that the formation of the fcc phase of $\mathrm{Ru}$ may be beneficial for advanced, low-resistance interconnects.

We use scanning tunneling microscopy (STM) and spectroscopy (STS), and x-ray photoelectron spectroscopy (XPS) to investigate the effect of nitrogen doping on the surface electronic and chemical structures of cutouts from superconducting $\mathrm{Nb}$ radio-frequency cavities. The goal of this work is to get insights into the fundamental physics and materials mechanisms behind the striking decrease of the surface resistance with the radio-frequency magnetic field, which has been observed on $N$-doped $\mathrm{Nb}$ cavities. Our XPS measurements reveal significantly more oxidized $\mathrm{Nb}$ $3d$ states and a thinner metallic suboxide layer on the $N$-doped $\mathrm{Nb}$ surfaces, which is also confirmed by tunneling spectroscopy measurements. In turn, tunneling measurements performed on native surfaces as well as on $\mathrm{Ar}$-ion sputtered surfaces allow us to separate the effect of $N$ doping on the surface-oxide layer from that on the density of states in the bulk. Analysis of our tunneling spectra in the framework of a model of a proximity-coupled normal layer at the surface [A. Gurevich and T. Kubo, Phys. Rev. B 96, 184515 (2017)] is consistent with the hypothesis that $N$-doping ameliorates lateral inhomogeneities of superconducting properties on the surface and shrinks the metallic suboxide layer. For the $\mathrm{Ar}$ sputtered surfaces, we also find evidence that $N$ doping changes the contact resistance between the metallic suboxide and the bulk niobium toward an optimum value corresponding to a minimum surface resistance. The totality of our experimental data suggests that the $N$ doping provides an effective tuning of the density of states in such a way that it can result in a decrease of the surface resistance with the radio-frequency field, as predicted by calculations of the nonlinear low-frequency electromagnetic response of dirty superconductors. Furthermore, STM imaging of vortex cores shows a slightly reduced average superconducting gap and a shorter coherence length in the $N$-doped $\mathrm{Nb}$ samples as compared to typically prepared $\mathrm{Nb}$ samples, indicating a stronger impurity scattering caused by nitrogen doping in a moderately disordered material.

We demonstrate the use of individual magnetic nanowires (NWs), grown by focused-electron-beam-induced deposition (FEBID), as scanning magnetic force sensors. Measurements of their mechanical susceptibility, thermal motion, and magnetic response show that these NWs possess high-quality flexural mechanical modes and a strong remanent magnetization pointing along their long axis. Together, these properties make the NWs excellent sensors of weak magnetic field patterns, as confirmed by calibration measurements on a micron-sized current-carrying wire and magnetic scanning-probe images of a permalloy disk. The flexibility of FEBID in terms of the composition, geometry, and growth location of the resulting NWs, makes it ideal for fabricating scanning probes specifically designed for imaging subtle patterns of magnetization or current density.

We advance a protocol to measure the complex spatial degree of coherence of a partially coherent light field obeying Gaussian statistics through a generalized Hanbury Brown–Twiss (HBT) experiment. The proposed generalized HBT experiment amounts to combining a partially coherent field with a pair of coherent reference fields and measuring the intensity-intensity cross-correlation of the cumulative field. The real or imaginary part of the complex spatial degree of coherence can be extracted directly from the intensity-intensity cross-correlation by adjusting the phase delay between the two reference fields to be $0$ or $\pi /2$. We test our method by carrying out a proof-of-principle experiment to measure the complex spatial degree of coherence of inhomogeneous light fields obeying Gaussian statistics. We find excellent agreement between the experimental results and our general theory. We show that our approach is extremely robust against the environmental fluctuations (e.g., the atmospheric turbulence) during the measurement. We demonstrate experimentally that a moving target hiding behind a ground-glass disk can be tracked with the aid of our complex-spatial-coherence measurement protocol.

From fundamental discovery to practical application, advances in the optical and quantum sciences rely upon precise control of light-matter interactions. Systems of coupled optical cavities are ubiquitous in these efforts, yet the design and active modification of the hybridized mode properties remains challenging. Here, we demonstrate the design, fabrication, and analysis of a tunable heterogeneous photonic molecule consisting of a ring resonator strongly coupled to a nanobeam photonic crystal cavity. Leveraging the disparity in mode volume between these two strongly coupled cavities, we combine theory and experiment to establish the ability to actively tune the mode volume of the resulting supermodes over a full order of magnitude. As the mode volume determines the strength of light-matter interactions, this work illustrates the potential for strongly coupled cavities with dissimilar mode volumes in applications requiring designer photonic properties and tunable light-matter coupling, such as photonics-based quantum simulation.

Integrating multiple functionalities into a single metasurface is becoming of great interest for future intelligent communication systems. While such devices have been extensively explored for reciprocal functionalities, in this work, we integrate a wide variety of nonreciprocal applications into a single platform. The proposed structure is based on spatiotemporally modulated impedance sheets supported by a grounded dielectric substrate. We show that, by engineering the excitation of evanescent modes, nonreciprocal interactions with impinging waves can be configured at will. We demonstrate a plethora of nonreciprocal components, such as wave isolators, phase shifters, and circulators, on the same metasurface. This platform allows switching between different functionalities by modifying only the pumping signals (harmonic or nonharmonic), without changing the main body of the metasurface structure. This solution opens the door for future real-time reconfigurable and environment-adaptive nonreciprocal wave controllers.

We investigate a solid-state reversible alkali-ion battery (AIB) capable of regulating the density of alkali atoms in a vacuum system used for the production of laser-cooled atoms. The cold-atom sample can be used with in-vacuum chronoamperometry as a diagnostic for the voltage-controlled electrochemical reaction that sources or sinks alkali atoms into the vapor. In a combined reaction-diffusion-limited regime, we show that the number of laser-cooled atoms in a magneto-optical trap can be increased both by initially loading the AIB from the vapor for longer and by using higher voltages across the AIB when atoms are subsequently sourced back into the vapor. The time constants associated with the change in atom number in response to a change in AIB voltage are in the range of 0.5–40 s. The AIB alkali reservoir is demonstrated to survive oxidization during atmospheric exposure, simplifying reservoir loading prior to vacuum implementation as a replacement for traditional resistively heated dispensers. The AIB capabilities may provide an improved atom-number stability in next-generation atomic clocks and sensors, while also facilitating fast loading and increased interrogation times.

The principle of reciprocity underpins one of the fundamental effects in optics that signifies symmetric transmission with respect to the interchange of the source and observation points. Breaking reciprocity, however, enables additional functionalities and greatly enriches the applications of photonics in terms of nonreciprocal devices. Here, a realistic nanoscale cascaded-cavity system that envisages nonreciprocal optical isolation and efficient wavelength conversion based on spatiotemporally modulated index of refraction are proposed and numerically demonstrated. The on-chip, linear, magnetic-free nonreciprocal isolation and dynamically controllable wavelength conversion can be a promising candidate for silicon nanophotonic and optoelectronic devices.