Image encoding and decoding are significant for information security, authenticity verification, and anticounterfeiting. We propose and demonstrate a polarization multiplexed metasurface for multi-image hiding and seeking in the terahertz band. The designed metasurface is composed of rod slot antennas and C-shaped slot antennas operating for both linear and circular polarizations. Different hidden images are revealed by controlling the polarization states of the incident and detection light. Three channels for object image encoding are demonstrated, indicating that this approach can increase the degree of freedom for optical image hiding and verification. This work may open an alternative avenue for polarization-controlled metasurface design, product identification, optical image encoding, and optical information security.

Trapping and manipulation of small particles underlies many scientific and technological applications. Recently, the precise manipulation of multiple small particles has been demonstrated using a Stokes trap that relies only on fluid flow, without the need for optical or electric fields. Active flow control generates complex flow topologies around suspended particles during the trapping process, yet the relationship between the control algorithm and the flow structure is not well understood. In this work, we characterize the flow topology during active control of particle trajectories using a Stokes trap. Our results show that optimal control of two particles unexpectedly relies on flow patterns with zero or one stagnation points, as opposed to positioning two particles using two distinct stagnation points. We characterize the sensitivity of the system with respect to the parameters in the control objective function, thereby providing a systematic understanding of the trapping process. Overall, these results will be useful in guiding applications involving the controlled manipulation of multiple colloidal particles and the precise deformation of soft particles in defined flow fields.

We report the behavior of a dense suspension (blood sample, $\phi >0.1$) coflowing with an aqueous buffer in a microchannel exposed to an acoustic standing wave studied by experiments and a theoretical model. The coupled effect of acoustophoretic particle migration and bulk relocation due to an inhomogeneous-fluid configuration leads to interesting phenomena. Our study reveals that depending on the acoustic impedance of the suspension ($Z_s$) and buffer ($Z_b$) streams, different flow regimes (complete relocation, acoustic migration, nonrelocation and nonmigration, and partial relocation) can be observed. The occurrence of different regimes is explained by the impedance contrast (i.e., $\mathrm{\Delta }{Z}_{sb}$ and $\mathrm{\Delta }{Z}_{pb}$) and is characterized in terms of the particle residence (${\tau }_{\mathrm{res}}$), acoustic relocation (${\tau }_{\mathrm{rel}}$), and acoustic migration (${\tau }_{\mathrm{mig}}$) timescales. We discover the partial-relocation regime, wherein acoustic migration of particles results in a band of a highly concentrated suspension at the suspension-buffer interface that alone relocates to the pressure node. The partial-relocation regime thus offers a technique for acoustic-driven splitting of a suspension stream into denser and diluted suspension streams.

A phased-array antenna is a device that generates radiation patterns whose shape and direction can be electronically controlled by tailoring the amplitude and phase of the signals that feed each element of the array. These devices provide identical responses in transmission and reception due to the constraints imposed by time-reversal symmetry. We introduce the concept of nonreciprocal phased-array antennas and demonstrate that they can exhibit drastically different radiation patterns when operated in transmission or in reception. The building block of the array consists of a time-modulated resonant antenna element that provides very efficient frequency conversion between only two frequencies: one associated with waves propagating in free space and the other related to guided signals. Controlling the tunable nonreciprocal phase response of these elements with the phase of low-frequency modulation signals permits to independently tailor the transmission and reception radiation patterns of the entire array. Measured results at microwaves confirm isolation levels over 40 dB at desired directions in space with an overall loss below 4 dB. We believe that this concept can be extended across the electromagnetic spectrum provided adequate tuning elements are available, with important implications in communication, sensing, and radar systems, as well as in thermal management and energy harvesting.

Intersubband transitions in $\mathrm{Zn}\mathrm{O}$ material systems are predicted to be promising candidates for infrared and terahertz (THz) optoelectronic devices due to their unusual material properties. In particular, the temperature performance of THz quantum cascade lasers is postulated to be significantly enhanced using $\mathrm{Zn}\mathrm{O}$ material systems due to their large optical phonon energy. Taking a step forward toward that goal, intersubband transitions in $\mathrm{Zn}\mathrm{O}/{\mathrm{Mg}}_{x}Z{\mathrm{n}}_{1-x}\mathrm{O}$ asymmetric coupled quantum wells are observed on a nonpolar m plane $\mathrm{Zn}\mathrm{O}$ substrate. Two absorption peaks are observed in the energy range from approximately 250 meV to approximately 410 meV at room temperature, unambiguously demonstrating the interwell coupling in the asymmetric coupled quantum wells. A theoretical model taking into account the interaction between intersubband transitions shows reasonable overall agreement with the experimental results, thus proving the strong coupling nature of the investigated system. As the building block of complex quantum structures based on intersubband transitions, the results presented show great potential applications of $\mathrm{Zn}\mathrm{O}/{\mathrm{Mg}}_{x}Z{\mathrm{n}}_{1-x}\mathrm{O}$ material systems in infrared and THz optoelectronics and physics.

Time-domain Brillouin scattering is a unique tool for determining depth-dependent material properties. Here, we show the influence of doping level in $\mathrm{Ga}\mathrm{As}$ on Brillouin oscillations. Measurements are performed on intrinsic, $n$-type, and $p$-type $\mathrm{Ga}\mathrm{As}$ samples. The results show high sensitivity of the amplitude of Brillouin oscillations to the doping concentration. Theoretical calculations are in good agreement with experimental data. This work provides an insight into specific dopant profiling as a function of depth.

Nonlinear imaging preserves more detailed information by exploiting the presence and interaction of many photons in nonlinear optics. Despite significant progress, nonlinear methods for white-light image recovery are still lacking. We report the white-light image reconstruction via seeded modulation instability, which is called dynamical stochastic resonance. The underlying signals are illuminated with white light emitted from a light-emitting diode and are reinforced by coupling with scattering noise in a noninstantaneous self-focusing medium. Our experimental results show that this nonlinear approach is effective in improving the contrast and visibility of completely blurred white-light images by guiding noise rather than by filtering noise. The signals of the entire temporal spectrum collectively seed a potential in the medium. In turn, the potential leads to the enhancement of signal modes by concentrating scattering noise. A theoretical growth rate is derived and the numerical simulations are presented to match the experimental observation. We also demonstrate that the underlying signals can be preferably reproduced by asynchronously probing the seeded potential with another white-light beam. Our work reveals an alternative scheme for white-light image detection.

Realization of magnetic-free nonreciprocal transmission for two or more wavelengths simultaneously is in great demand for multiwavelength optical communication and signal processing. Here, making use of optical pumping, we propose and experimentally demonstrate a magnetic-free multiwavelength isolator based on warm V-type atoms with the assistance of the Doppler effect. Applying ${}^{85}\mathrm{Rb}$ atoms, isolation ratios of 25 and 28 dB are obtained simultaneously for signal fields at 795 and 780 nm with transmissions higher than 0.85. This work provides an effective way to realize a multiwavelength optical isolator for optical information processing applications.

We show that the antireflection performance of nanoparticle arrays on top of solar-cell stacks is related to two conditions: a high enough degree of discrete rotational symmetry of the array and the ability of the system to suppress crosstalk between the two handednesses (helicities) of the electromagnetic field upon light-matter interaction. For particle-lattice systems with a high enough degree of discrete rotational symmetry $2\pi /n$ for $n\ge 3$, our numerical studies link the suppression of backscattering to the ability of the system to avoid the mixing between the two helicity components of the incident field. In an exemplary design, we optimize an array of ${\mathrm{Ti}\mathrm{O}}_2$ disks placed on top of a flat heterojunction solar-cell stack and obtain a threefold reduction of the current loss due to reflection with respect to an optimized flat reference. We numerically analyze the helicity-preservation properties of the system, and also show that a hexagonal array lattice, featuring a higher degree of discrete rotational symmetry, can improve over the antireflection performance of a square lattice. Importantly, the disks are introduced in an electrically decoupled manner such that the passivation and electric properties of the device are not disturbed.

An effective approach to isolation of submicrometer-sized particles is desired to separate cancer cells and healthy cells or in therapy for Parkinson’s disease and Alzheimer’s disease. However, since bioparticles span a large size range covering several orders of magnitude, the development of an adequate separation method is a challenging task. We consider a collection of noninteracting Brownian particles of various sizes moving in a symmetric periodic potential and subjected to external unbiased harmonic driving as well as a constant bias. We reveal a nonintuitive, yet efficient, separation mechanism based on a thermal-fluctuation-induced negative-mobility phenomenon in which particles of a given size move in a direction opposite to the applied bias. By varying solely the temperature of the system, one can separate particles of various strictly defined sizes. This approach may be an important step toward the development of point-of-care lab-on-a-chip devices.

We analyze the evolution of the normal and superconducting properties of epitaxial TiN films, characterized by high Ioffe-Regel parameter values, as a function of the film thickness. As the film thickness decreases, we observe an increase of the residual resistivity, that becomes dominated by diffusive surface scattering for $d\le 20\mathrm{nm}$. At the same time, a substantial thickness-dependent reduction of the superconducting critical temperature is observed compared to the bulk $\mathrm{Ti}\mathrm{N}$ value. In such high-quality material films, this effect can be explained by a weak magnetic disorder residing in the surface layer with a characteristic magnetic defect density of approximately ${10}^{12}{\mathrm{cm}}^{-2}$. Our results suggest that surface magnetic disorder is generally present in oxidized $\mathrm{Ti}\mathrm{N}$ films.

Quantum bits or qubits naturally decohere by becoming entangled with uncontrollable environments. Dynamical decoupling is, therefore, required to disentangle qubits from an environment by periodically reversing the qubit bases, but this causes rotation error to accumulate. Whereas a conventional qubit is rotated within the SU(2) two-level system, a geometric qubit defined in the degenerate subspace of a V-shaped SU(3) three-level system is geometrically rotated via the third ancillary level to acquire a geometric phase. We demonstrate that simply by introducing detuning, the dynamical decoupling of the geometric qubit on a spin triplet electron in a nitrogen-vacancy center in diamond can be made to spontaneously suppress error accumulation. The geometric dynamical decoupling extends the coherence time of the geometric qubit up to 1.9 ms, limited by the relaxation time, with 128 decoupling gates at room temperature. Our technique opens a route to holonomic quantum memory for use in various quantum applications requiring sequential operations.

Theoretical models allow design of acoustic traps to manipulate objects with radiation force. A model of the acoustic radiation force by an arbitrary beam on a solid object is validated against measurement. The lateral force in water of different acoustic beams is measured and calculated for spheres of different diameters (2–6 wavelengths λ in water) and compositions. This is the first effort to validate a general model, to quantify the lateral force on a range of objects, and to electronically steer large or dense objects with a single-sided transducer. Vortex beams and two other beam shapes having a ring-shaped pressure field in the focal plane are synthesized in water by a 1.5-MHz, 256-element focused array. Spherical targets (glass, brass, ceramic, 2–6 mm dia.) are placed on an acoustically transparent plastic plate that is normal to the acoustic beam axis and rigidly attached to the array. Each sphere is trapped in the beam as the array with the attached plate is rotated until the sphere falls from the acoustic trap because of gravity. Calculated and measured maximum obtained angles agree on average to within 22%. The maximum lateral force occurs when the target diameter equals the beam width; however, objects up to 40% larger than the beam width are trapped. The lateral force is comparable to the gravitation force on spheres up to 90 mg (0.0009 N) at beam powers on the order of 10 W. As a step toward manipulating objects, the beams are used to trap and electronically steer the spheres along a two-dimensional path.

The demand for higher-density, higher-speed, and more energy-efficient magnetoelectric RAM (MeRAM) requires the search of promising materials and magnetic-tunnel-junction stacks with voltage-controlled magnetic anisotropy (VCMA) efficiency greater than the 1000 fJ/(Vm). Using first-principles electronic structure calculations, we propose a double-barrier ferromagnetic heterostructure with an atomically thin late transition metal $X$ ($\mathrm{Rh}$, $\mathrm{Ir}$, $\mathrm{Pt}$), which exhibits both giant perpendicular magnetic anisotropy (PMA) and VCMA efficiency, where the former (latter) is 1 (1 to 2) order of magnitude higher than the values reported to date. We demonstrate that the dominant contribution to both the PMA and VCMA arises from the late heavy metal $X$ due to the large biaxial tensile strain-induced magnetism in $X$. Furthermore, we predict a sign reversal of the VCMA efficiency from the $\mathrm{Ir}$- to the $\mathrm{Pt}$-monolayer cap. We elucidate that the underlying mechanism is the electric-field-induced energy shift of the spin-polarized $d_{z^2}$-derived projected states on the $X$ layer. These findings provide useful guiding rules in exploiting the large spin-orbit coupling and biaxial tensile strain-induced magnetism in the late $5d$-transition metals for the design of the next generation of ultra-low energy MeRAM devices.

Recent demonstration of magnetization manipulation has been focused on the utilization of pure spin current converted by charge current in nonmagnetic materials with strong spin-orbit coupling (SOC), which stimulates intensive studies on the exploration of materials with larger SOC, such as heavy metals and topological insulators. We suggest an alternative approach for enhancing the effective charge-to-spin conversion efficiency by applying strain in $\mathrm{Ta}/\mathrm{Fe}/\mathrm{Pt}$ films grown on flexible mica substrates. We experimentally demonstrate a large and tunable charge-to-spin conversion efficiency in $\mathrm{Ta}/\mathrm{Fe}/\mathrm{Pt}$ films by applying compressive strain within a flexible substrate, and over 50% enhancement of effective spin Hall angle ${\mathrm{\Theta }}_{\mathrm{SHE}}^{\mathrm{eff}}$ of up to approximately 0.2 is achieved in a 6.26‰ strained film. Our findings may spur further work on the integration of flexible electronics and SOC and may potentially lead to the innovation of alternative flexible spintronics devices.

Reducing noise to the shot-noise limit is a challenge for laser development for ultrasensitive applications in precision sensing and fundamental science. We use hollow-core fiber in a collinear-balanced-detection scheme to suppress 2.6 dB of amplitude noise to within 0.01 dB of the shot-noise limit, while simultaneously preserving the spectrum and temporal profile of picosecond laser pulses. We also provide an enhanced version of the scheme that concatenates multiple circuits for suppression over many frequencies and broad frequency ranges. We perform a first demonstration of this method and reduce total excess amplitude noise, between 2 and 6 MHz, by 85%. These demonstrations enable passive, broadband, all-guided fiber-laser technology operating at the shot-noise limit.

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.