Subwavelength plasmonic nanostructures have demonstrated many interesting phenomena and applications in recent decades. However, plasmonic resonance and associated properties, such as strong field enhancement, are limited to the visible regime due to the quasistatic nature of subwavelength plasmonic nanoparticles. Here, we numerically demonstrate spectral resonance tuning of subwavelength nanoparticles with a high-index core and a plasmonic shell while maintaining a sharp spectral peak. These core-shell-based nanoparticles have predominantly nonradiative loss and strong near-field enhancement due to their subwavelength feature size. This scheme can be further extended for magnetic and chiral meta-atoms. We suggest that these features of subwavelength core-shell-based nanoparticles are advantageous for many interesting applications including photothermal devices, enhanced light-matter interaction, and photonic circuitry.

Multiuse or reconfigurable rf front ends require rf filters capable of dynamically broadening and/or shifting their operating frequency. Utilizing frequency-agile filters, the burden of signal processing is shifted from application specific hardware (e.g., application specific integrated circuit) to software-based solutions such as software defined radio (SDR). Current filter tuning approaches electrically modify surface or bulk acoustic wave signal absorption, but typically require prohibitively large electric fields to achieve large tuning. Alternatively, multiferroic based approaches overcome this limitation by leveraging magnetoelasticity to tune the ferromagnetic resonance (FMR) of nanostructures. Four configurations of ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ nanoellipses are simulated to theoretically demonstrate how voltage-induced strain and dipole-coupling affect their FMR spectra. For a single ellipse, two FMR peaks are observed, signifying primary (bulk) and secondary (edge) modes that were shifted up to approximately 2 GHz by a voltage-induced strain. For dipole-coupled ellipses, a third resonance peak develops in addition to the bulk and edge modes and similar frequency shifting is observed with equal strain applied to both nanostructures. When strain is applied to only one ellipse, fourth and fifth resonance peaks develop. The results demonstrate the high tunability of magnetoelastic based designs for future strain-based filtering, antennas, or computing applications.

We propose the concept of selective excitation of a terahertz operating wave in a high-power electron maser with an oversized microwave system fed by a high-current relativistic electron beam. This concept is based on excitation of a supermode formed by a fixed set of transverse modes, rather than a fixed transverse mode of the operating cavity. A possible approach to form a high-Q supermode inside a simple cavity is the use of the Talbot effect, namely, periodic reproduction of the transverse structure of a multimode wave field in an oversized waveguide.

A time scale is a procedure for accurately and continuously marking the passage of time. It is exemplified by Coordinated Universal Time (UTC) and provides the backbone for critical navigation tools such as the Global Positioning System. Present time scales employ microwave atomic clocks, whose attributes can be combined and averaged in a manner such that the composite is more stable, accurate, and reliable than the output of any individual clock. Over the past decade, clocks operating at optical frequencies have been introduced that are orders of magnitude more stable than any microwave clock. However, in spite of their great potential, these optical clocks cannot be operated continuously, which makes their use in a time scale problematic. We report the development of a hybrid microwave-optical time scale, which only requires the optical clock to run intermittently while relying upon the ensemble of microwave clocks to serve as the flywheel oscillator. The benefit of using a clock ensemble as the flywheel oscillator instead of a single clock can be understood by the Dick-effect limit. This time scale demonstrates for the first time subnanosecond accuracy over a few months, attaining a fractional frequency stability of 1.45 × ${10}^{-16}$ at 30 days and reaching the ${10}^{-17}$ decade at 50 days, with respect to UTC. This time scale significantly improves the accuracy in timekeeping and could change the existing time-scale architectures.

High-power and broadband dual-comb sources in the terahertz frequency regime are much in demand for fast and high-resolution spectroscopies. The terahertz quantum cascade laser (QCL) is a powerful radiation source in the frequency range between 1 and 5 THz, which is an ideal candidate for frequency-comb and dual-comb generations in this frequency regime. In this work, we report on broadband on-chip dual-comb sources based on terahertz QCLs emitting at approximately 4.2 THz with a single plasmon waveguide configuration. Due to the fast intersubband transitions in QCLs, the dual-comb lines can be detected by employing the laser self-detection scheme. A finite-element analysis shows that a weak optical coupling between the two laser combs is sufficient for generating dual-comb lines via the nonlinear multiheterodyne frequency beatings. The noise-equivalent power of the QCL detector is also evaluated for different optical powers; furthermore, a microwave double injection is employed to stabilize the comb frequency and broaden the optical comb bandwidth. We find that even with weak radio-frequency powers, the optical comb bandwidth can be significantly broadened from 86 to 166 GHz. The double injection also allows us to investigate the carrier offset noise of QCL combs.

Heralding techniques are useful in quantum communication to circumvent losses without resorting to error-correction schemes or quantum repeaters. Such techniques are realized, for example, by monitoring for photon loss at the receiving end of the quantum channel while not disturbing the transmitted quantum state. We describe and experimentally benchmark a scheme that incorporates error detection in a direct quantum channel connecting two transmon qubits using traveling microwave photons. This is achieved by encoding the quantum information as a time-bin superposition of a single photon, which simultaneously realizes high communication rates and high fidelities. The presented scheme is straightforward to implement in circuit quantum electrodynamics and is fully microwave controlled, making it an interesting candidate for future modular quantum-computing architectures.

We report on microwave optomechanics measurements performed on a nuclear adiabatic demagnetization cryostat, whose temperature is determined by accurate thermometry from below 500 $\mu \mathrm{K}$ to about 1 K. We describe a method for accessing the on-chip temperature, building on the blue-detuned parametric instability and a standard microwave setup. The capabilities and sensitivity of both the experimental arrangement and the developed technique are demonstrated with a very weakly coupled silicon-nitride doubly clamped beam mode of about 4 MHz and a niobium on-chip cavity resonating around 6 GHz. We report on an unstable intrinsic driving force in the coupled microwave-mechanical system acting on the mechanics that appears below typically 100 mK. The origin of this phenomenon remains unknown, and deserves theoretical input. It prevents us from performing reliable experiments below typically 10–30 mK; however no evidence of thermal decoupling is observed, and we propose that the same features should be present in all devices sharing the microwave technology, at different levels of strengths. We further demonstrate empirically how most of the unstable features can be annihilated, and speculate how the mechanism could be linked to atomic-scale two-level systems. The described microwave and microkelvin facility is part of the European Microkelvin Platform, and shall be used for further experiments within and below the millikelvin range.

We propose a theoretical scheme to achieve strong photon blockade via a single atom in a cavity. By utilizing the optical Stark shift, the dressed-state splitting between the higher and lower branches is enhanced, which results in a significant increase in the lower (higher) branch and a decrease in the higher (lower) branch for a negative (positive) Stark shift, and dominates the time evolution of the photon-number oscillations as well. Furthermore, two-photon excitation is suppressed via quantum interference for the optimal phase and Rabi frequency of an external microwave field. It is shown that the interplay between quantum interference and the enhanced vacuum Rabi splitting gives rise to a strong photon blockade that can be used to realize a high-quality single-photon source beyond the regime of strong atom-cavity coupling. In particular, by tuning the optical Stark shift, the second-order correlation function in our scheme can be made three orders of magnitude smaller than in the Jaynes-Cummings model, and, correspondingly, a large cavity photon number appears as well. Our proposal may imply exciting opportunities for potential applications in quantum networks and quantum information processing.

Two-dimensional materials-based van der Waals heterojunctions (vdWHs) are promising candidates for tunnel field-effect transistors (TFET) because of their atomically clean and electronically sharp junction interfaces and lattice mismatch-free characteristics. The tunneling behaviors of the TFETs greatly depend on the energy band alignments between different layers. By first-principles analyses, the band offsets in vdWHs are found to be very sensitive to the vdW interaction and temperature, especially for the type-III heterojunctions, which can be switched to the type-II configuration. This band-offset degradation is attributed to the interlayer charge transfer, generating built-in electric fields and potential differences, consistent with the plate condenser model. The charges are mainly transferred by two mechanisms: spontaneous and thermal excitation-assisted electron tunneling. The first one is caused by the electron wave function overlaps and energy difference of the valence band maximum and conduction band minimum, which belong to different materials, whereas the latter one is strongly correlated to the thermal excitations. The band-offset degradation is a general phenomenon in vdWHs, especially for those with a type-III band alignment. Therefore, attention should be paid in TFET investigations, since they are switched on by changing the band alignments to the type-III configuration.

Circular photon drag effect (CPDE) is important for helicity-dependent optoelectronic emitters and detectors yet is less studied in graphene due to the relatively weak light-matter interaction and is submerged by other nonlinear optical effects. We give experimental evidence of CPDE in vertically grown graphene (VGG) by terahertz (THz) emission spectroscopy. The emitted THz polarization states can be tuned to linear, left-handed, and right-handed elliptical polarizations by changing the helicity of the pump laser. Polarity reversal of the THz time-domain signal occurs with the opposite helicity of pump laser excitation due to the CPDE. Theory analysis suggests that both the linear photon drag effect and CPDE-induced transient photocurrents contribute to the THz emission from which the contribution weight of CPDE can be tuned by different elliptical states of the excitation light. The photon-helicity-dependent THz emission from VGG based on CPDE offers an alternative thought of graphene-based polarization sensitive THz sources for chiral analysis in the THz field.

Scanning nanoscale superconducting quantum interference devices (SQUIDs) are gaining interest as highly sensitive microscopic magnetic and thermal characterization tools of quantum and topological states of matter and devices. We introduce a technique of collimated differential-pressure magnetron sputtering for versatile self-aligned fabrication of SQUID-on-tip (SOT) nanodevices, which cannot be produced by conventional sputtering methods due to their diffusive, rather than the required directional point source, deposition. The technique provides access to a broad range of superconducting materials and alloys beyond the elemental superconductors employed in the existing thermal deposition methods, opening the route to greatly enhanced SOT characteristics and functionalities. Utilizing this method, we have developed molybdenum-rhenium (${\mathrm{Mo}}_{66}{\mathrm{Re}}_{34}$) SOT devices with sub-50-nm diameter, magnetic flux sensitivity of 1.2 $\mu {\mathrm{\Phi }}_0/\mathrm{H}{\mathrm{z}}^{1/2}$ up to 3 T at 4.2 K, and thermal sensitivity better than 4 $\mu \mathrm{K}/\mathrm{H}{\mathrm{z}}^{1/2}$ up to 5 T—about five times higher than any previous report—paving the way to nanoscale imaging of magnetic and spintronic phenomena and of dissipation mechanisms in previously inaccessible quantum states of matter.

We analyze the effects of time-dependent magnetic and rf fields on the spin dynamics of the Carr-Purcell-Meiboom-Gill sequence. The analysis is based on the decomposition of the magnetization into the eigenmodes of the propagator of a single refocusing cycle. For sufficiently slow changes in the external fields, the magnetization follows the changing eigenmodes adiabatically. This results in echo amplitudes that show regular modulations with time. Faster field changes can induce transitions between the eigenmodes. Such nonadiabatic behavior occurs preferentially at particular offsets of the Larmor frequency from the rf frequency where the eigenmodes become nearly degenerate. We introduce the instantaneous adiabaticity parameter $\mathcal{A}(t)$ that accurately predicts the crossover from the adiabatic to the nonadiabatic regime and allows the classification of field fluctuations. $\mathcal{A}(t)$ is determined solely by the properties of a single refocusing cycle under static conditions and the instantaneous value of the field offset and its temporal derivative. The analytical results are compared with numerical simulations.

The intercalation behavior of the ${\mathrm{Al}\mathrm{Cl}}_4$ molecule in graphite and its diffusion behavior within the galleries formed between two adjacent graphene layers are studied using density-functional approximations and ab initio molecular-dynamics simulations. We confirm that the ${\mathrm{Al}\mathrm{Cl}}_4$ prefers to stay in its tetrahedral geometry even after intercalation into graphite. More importantly, our molecular-dynamics simulations reveal an intriguing dynamical behavior of the resulting graphite intercalation compound: (i) the intercalated ${\mathrm{Al}\mathrm{Cl}}_4$ molecule additionally rotates around an axis perpendicular to the graphene planes while migrating inside the gallery, and (ii) the graphene planes of the host graphite slide over each other during the diffusion of the molecule. We find that the synergetic motion of the graphene planes significantly enhances the mobility of the ${\mathrm{Al}\mathrm{Cl}}_4$ in the host graphite, and such a cooperative behavior is a key factor behind the ultrahigh rate capability of the recently reported aluminum-ion battery (AIB) [Lin et al., Nature 520, 325 (2015)]. Furthermore, as a result of investigating the intercalation and diffusion behavior of the ${\mathrm{Al}\mathrm{F}}_4$ molecule in graphite, we propose to use ${\mathrm{Al}\mathrm{F}}_4^{-}$ as the charge-carrying anion for an $\mathrm{Al}$-graphite battery cell. We predict that an ${\mathrm{Al}\mathrm{F}}_4$-based AIB cell, if experimentally realizable, will have a larger cathode specific capacity, higher voltage, and higher rate capability.

The joint impact of Anderson localization and many-body interaction is observed in the optical properties of strongly disordered III-nitride quantum wells, a system where the Coulomb interaction and the fluctuating potential are pronounced effects with similar magnitude. A numerical method is introduced to solve the six-dimensional coupled Schrodinger equation in the presence of disorder and Coulomb interaction, a challenging numerical task. It accurately reproduces the measured absorption and luminescence dynamics of (In,Ga)N quantum wells at room-temperature: absorption spectra reveal the existence of a broadened excitonic peak, and carrier lifetime measurements show that luminescence departs from a conventional bimolecular behavior. These results reveal that luminescence is governed by the interplay between localization and Coulomb interaction, and provide practical insight into the physics of modern light-emitting diodes.

Bendable signal transmission based on accelerating beams has become an increasingly interesting topic in recent years due to not only its fascinating propagation characteristics but also the potential to evade or resist obstructions within the transmission link. In this work, we propose a type of image-signal transmission based on nonconvex accelerating beams in contrast to convex accelerating beams such as Airy beams, which is expected to be more resilient to obstruction due to the special spectrum-position mapping characteristic. A constructed helical beam is used as the nonconvex accelerating beam for demonstration, and an image encoded in its angular spectrum is successfully retrieved after multiple self-bending without and with obstruction in the transmission link, which confirms the above idea and may open a new door for image-signal transmission in the fields of imaging, signal processing, and free-space communication.

The recently discovered phenomena of photomagnetic recording using femtosecond laser pulses in magnetic dielectrics opens up intriguing opportunities for ultrafast least-dissipative magnetic writing. Fundamental questions remain, however, about the processes, which limit the repetition rate of such writing. We report the magnetization dynamics and switching caused by a double-pulse excitation with various orientations of pump polarization. In particular, we study write and erase sequences, showing that mutual interference of the effects caused by two laser pulses can occur if the pulses are temporally separated by less than 60 ps. This imposes the fundamental limit for the repetition rate of writing (20 GHz), but this could be increased even further by strengthening the photoinduced magnetic anisotropy.

Photoelectron emission from solids, as driven by high-intensity lasers, offers a platform for the coherent control of electron motion on ultrashort spatiotemporal scales. By solving the time-dependent Schrödinger equation, we present an exact analytical solution for the nonlinear ultrafast electron emission from a dc-biased metal surface illuminated by two-color laser fields. We systematically examine the combined effects of a dc electric field and two-color laser fields. In addition to the remarkable tunability of electron emission processes due to interference from two-color laser fields, we find that a strong dc electric field not only opens up more tunneling emission channels, but also introduces intense modulation to the emission current. We find surprising results that strong current modulation (with respect to the phase difference of the two-color lasers) persists (>70%), even with a large dc bias (i.e., ratio of the electric fields for dc : fundamental laser : second-harmonic laser $\cong 1:0.5:0.07$). In the meantime, the average emission current level increases by about three orders of magnitude relative to the case of zero dc bias. Application of our model to time-resolved photoelectron spectroscopy is exemplified and shows that the dynamics of the n-photon excited states depends strongly on the applied dc field. Our study suggests a practical way to maintain a strong modulation to high current photoemission, by the addition of a large dc bias for two-color laser-induced electron emission.

Many advanced solar cell concepts propose surpassing the Shockley-Queisser limit by introducing multiple quasi-Fermi-level separations that are arranged in series and/or in parallel. Exceeding the Shockley-Queisser limit with any parallel arrangement involves intermediate states that deliver additional charge carriers at, ideally, the same electrochemical potential as the other elements in the parallel network. This can be thought of as voltage matching individual parallel components, and in intermediate-band materials is intricately linked to solar concentration and étendue mismatch between absorption and emission. Generally, to achieve voltage matching under suboptimal conditions, an additional degree of freedom in the absorption thresholds of the material through a carrier relaxation or ratchet step is required. We explain why the ideal ratchet step decreases with solar concentration and how it depends on the radiative efficiency and emission étendue of the individual transitions and provide illustrative examples. The ideal ratchet step at 1 sun is largely given by the étendue mismatch between incoming sunlight and emitted light, which results in a step on the order of 270 meV that increases logarithmically with the ratio of luminescence-extraction efficiencies of the transitions. For solar cell concepts that use Auger-type carrier-carrier interactions or molecular triplet states for energetic up-conversion or down-conversion, the ideal band-gap combinations and achievable efficiencies also depend on interaction rates. We show that Auger-assisted solar cells suffer more strongly from finite interaction rates than carrier-multiplication devices.

Quantum processors rely on classical electronic controllers to manipulate and read out the state of quantum bits (qubits). As the performance of the quantum processor improves, nonidealities in the classical controller can become the performance bottleneck for the whole quantum computer. To prevent such limitation, this paper presents a systematic study of the impact of the classical electrical signals on the qubit fidelity. All operations, i.e., single-qubit rotations, two-qubit gates, and readout, are considered, in the presence of errors in the control electronics, such as static, dynamic, systematic, and random errors. Although the presented study could be extended to any qubit technology, it currently focuses on single-electron spin qubits, because of several advantages, such as purely electrical control and long coherence times, and for their potential for large-scale integration. As a result of this study, detailed electrical specifications for the classical control electronics for a given qubit fidelity can be derived. We also discuss how qubit fidelity is affected by the limited performance of the general-purpose room-temperature equipment typically employed to control the few qubits available today. Ultimately, we show that tailor-made electronic controllers can achieve significantly lower power, cost, and size, as required to support the scaling up of quantum computers.

The phase state of a parametric resonator provides important information about the nonlinear damping and the ability to employ feedback to achieve self-sustained oscillations. We derive the minimum nonlinear damping value required to enable phase control of Duffing parametric resonators and confirm this using tunable nonlinear micromechanical devices with nonlinear damping values below and above the critical value. For softening resonators with nonlinear damping values beyond the critical value, we demonstrate parametric phase control to experimentally operate on the unstable branches of the parametric response. Parametric phase locking is an alternate method for forming a timing reference from a micromechanical resonator, provides a sensitive technique for measuring the nonlinear parameters, and enables future closed-loop measurements of the dynamics of driven single and coupled parametric resonators.