Nematic liquid crystals exhibit a fast optical response when the applied electric field modifies the degree of order but does not change the direction of molecular orientation. The effect requires a relatively high electric field, on the order of ${10}^8\mathrm{V}/\mathrm{m}$ for a field-induced birefringence change of 0.01. To address this detrimental issue, we explore electrically induced modification of the order parameter in a material with a giant dielectric anisotropy of +200. A relatively weak field of $3\times {10}^{7}V/m$ causes a significant change of birefringence of about 0.04. Both switching on and off times are approximately 10 µs. The effect is called a microsecond electrically modified order parameter effect and can be used in electro-optical devices, such as fast electro-optic shutters, phase modulators, and beamsteerers that require microsecond response times.

Recent progress has been made in creating terahertz magnons using ultrathin ferromagnetic layers. Due to their short lifetimes, these can be difficult to measure. Here we detail a calculation that shows that infrared magnons can be detected using standard reflection and attenuated total reflection measurements from a thin-film heterostructure made up of alternating ultrathin magnetic and nonmagnetic layers. We use an entire-cell effective-medium calculation to find the magnetic permeability of the heterostructure and then use electromagnetic boundary conditions to calculate reflectivity as a function of frequency. There are appreciable dips in the reflectivity at infrared magnon resonance frequencies, for realistic material parameters. Moreover, the strong coupling of magnon photons indicates the possible use of 50 GHz–1 THz magnons in integrated signal-processing devices.

We show that an intensity speckle can be directly interpreted as the properties of incident light – amplitude, phase, polarization, and coherency over spatial positions. Revisiting the speckle-correlation scattering matrix method [Lee and Park, Nat. Comm. 7, 13359 (2016)], we successfully extract the intact information of incident light from an intensity speckle snapshot as the form of a coherency matrix. The idea is experimentally verified by introducing the peculiar states of light that exhibit uneven amplitude, phase, polarization, and coherency features. We also find substantial practical advantage of the proposed method compared to the conventional coherency matrix measuring techniques such as Stokes polarimetry. We believe this physical interpretation of an intensity speckle could open a new avenue to study and utilize the speckle phenomenon in vast subfields of wave physics.

We propose an efficient optomechanical mass sensor operating at exceptional points (EPs), non-Hermitian degeneracies where eigenvalues of a system and their corresponding eigenvectors simultaneously coalesce. The benchmark system consists of two optomechanical cavities that are mechanically coupled, where we engineer mechanical gain (loss) by driving the cavity with a blue-detuned (red-detuned) laser. The system features an EP at the gain and loss balance, where any perturbation induces a frequency splitting that scales as the square root of the perturbation strength, resulting in a giant sensitivity-factor enhancement compared with conventional optomechanical sensors. For nonidentical mechanical resonators, quadratic optomechanical coupling is used to tune the frequency mismatch to get closer to the EP, extending the efficiency of our sensing scheme to mismatched resonators. This work paves the way toward new levels of sensitivity for optomechanical sensors, which could find applications in many other fields, including nanoparticle detection, precision measurement, and quantum metrology.

We propose and analyze an autonomous engine that combines ideas from electronic transport and self-oscillating heat engines. It is based on the electron-shuttling mechanism in conjunction with a rotational degree of freedom. We focus in particular on the isothermal regime, where chemical work is converted into mechanical work or vice versa, and we especially pay attention to using parameters estimated from experimental data of available single components. Our analysis shows that, for these parameters, the engine already works with remarkable stability, albeit with moderate efficiency. Moreover, it has the advantage that it can be upscaled to increase power and reduce fluctuations further.

We analyze pyroelectric energy conversion using a power-source-free characterization method, which can also be used to demonstrate electricity generation. In contrast to conventional pyroelectric-energy-conversion designs, our energy system is detached from any external power sources, operating only under periodically varying temperature. Such detachment unambiguously attributes the converted electricity to the heat that drives the change of polarization in the pyroelectric material, not to the electric field alternation caused by the external battery. We demonstrate electricity generation in consecutive temperature cycles by using three phase-transforming ferroelectric materials with different properties. In-depth thermodynamics analysis suggests that the work output is rate dependent: the work output per cycle is linearly dependent on the heating and/or cooling frequency below a predicted threshold. The linearity is confirmed by experiments, and the threshold frequency is derived by theory. Finally, we propose a figure of merit that separates the material’s intrinsic properties from the system design parameters. The figure of merit guides future material development and device improvement. Our work clarifies confusions and forms the foundation for the resurgence of pyroelectric material as a competitor for green electricity.

We implement a whispering-gallery-mode microbubble resonator (MBR) as a multifunctional component for all-optical photoacoustic inspection of materials in a platform combining microfluidic design and high sensitivity to spectral absorbance. In our setup, the MBR acts both as the acoustic transducer and as the vial for a sample causing the photoacoustic event, thus dictating its operational frequency and removing any need for impedance-matching media. We challenge our setup in the assessment of the photostability of gold nanorods to demonstrate its validity. We develop a numerical model to qualitatively interpret the experimental signal as a transient shift of the resonance condition of the MBR. Possible implementations of the MBR in photoacoustic sensors are discussed.

We demonstrate the structural imaging of magnetostatic spin-wave modes hosted in a millimeter-sized ferromagnetic sphere. Unlike for low-dimensional magnetic materials, there is no prior technique to image these modes in bulk magnetized solid of revolution. Based on resonant magnetic induction tomography in the microwave range, our approach ensures the robust identification of these nontrivial spin-wave modes by establishing their azimuthal and polar dependences, starting point of magnonic fundamental studies and hybrid systems with complex spin textures well beyond the uniform precession mode.

The frontiers of quantum electronics have been linked to the discovery of new refrigeration methods since the discovery of superconductivity at a temperature of around 4 K, enabled by the liquefaction of helium. Since then, advances in cryogenics have led to discoveries such as the quantum Hall effect and new technologies such as superconducting and semiconductor quantum bits. Presently, nanoelectronic devices typically reach electron temperatures of around 10–100 mK by use of commercially available dilution refrigerators. However, cooling electrons via the encompassing lattice vibrations, or phonons, becomes inefficient at low temperatures. Further progress toward lower temperatures requires new cooling methods for electrons on the nanoscale, such as direct cooling with nuclear spins, which themselves can be brought to microkelvin temperatures by adiabatic demagnetization. Here we introduce indium as a nuclear refrigerant for nanoelectronics and demonstrate that solely on-chip cooling of electrons is possible down to $3.2\pm 0.1\mathrm{mK}$, limited by the heat leak via the electrical connections of the device.

Efficient microresonators simultaneously require a large quality factor $Q$ and a small volume $V$. However, the former is ultimately limited by bending losses, the unavoidable radiation of energy of a wave upon changing direction of propagation. Such bending losses increase exponentially as $V$ decreases and eventually result in a drop of $Q$. Therefore, circular cavities are generally designed with radii that are much larger than the optical wavelength. The same leakage of energy by radiation limits the sharpness of bends in photonic integrated circuits. In this paper, we present a way to reduce bending losses in circular microresonators. The proposed scheme consists of one or more external dielectric rings that are concentric with the cavity. These rings alter the field outside the cavity where radial oscillations set in and thus control the far-field radiation. As a result, the $Q$ factor can be increased by several orders of magnitude while keeping a small cavity volume.

Practical implementation of quantum key distributions (QKD) requires efficient, real-time feedback control to maintain system stability when facing disturbance from either external environment or imperfect internal components. Usually, a “scanning-and-transmitting” program is adopted to compensate physical parameter variations of devices, which can provide accurate compensation but may cost plenty of time in stopping and calibrating processes, resulting in reduced efficiency in key transmissions. Here, we propose to employ a well-known machine learning model, i.e., the long short-term memory network (LSTM), to predict those physical parameter variations in advance and actively perform real-time control on corresponding QKD devices. Experimentally, we take the phase-coding scheme as an example and run the LSTM-model-based QKD system for more than 10 days. Experimental results show that we can keep the same level of quantum-bit error rate as the traditional scanning-and-transmitting program by employing our machine learning method, but dramatically reducing the scanning time and resulting in significantly enhanced key transmission efficiency. Furthermore, our current machine learning model should also be applicable to other QKD systems using any coding scheme or QKD protocols and thus seems a very promising candidate in large-scale application of quantum communication networks in the near future.

Traumatic brain injury (TBI) is a major public health concern, affecting as many as 3 million people each year in the U.S. Despite substantial research efforts in recent years, our physical understanding of the cause of injury remains rather limited. In this paper, we characterize the nonlinear dynamical behavior of the brain-skull system through modal analysis and advanced finite-element (FE) simulations. We observe nonlinear behavior in the deep-white-matter (WM) structures near the dural folds, with an energy redistribution of around 30% between the dominant modes. We find evidence of shear-wave redirection near the falx and the tentorium (approximately ${15}^{\circ }$ in the axial and $8^{\circ }$ in the coronal plane) as a result of geometric nonlinearities. The shift in the falx mode shape, which is perpendicular to the deformation of the brain, causes geometrical nonlinear effects at the falx-brain tissue boundary. This is accompanied by a lateral sliding of the tentorium below the brain tissue, which induces higher local strains at its interface with deep regions of the brain. We observe that deep regions of the brain with high principal strains coincide with the identified nonlinear regions. The onset of nonlinear behavior in brain tissue is closely associated with the previously reported concussion thresholds, suggesting a possible link between the damage mechanism and the underlying nonlinear brain biomechanics.

Graphene thermionic electron emission across a high interface barrier involves energetic electrons residing far away from the Dirac point, where the Dirac cone approximation of the band structure breaks down. Here, we construct a full-band model beyond the simple Dirac cone approximation for the thermionic injection of high-energy electrons in graphene. We show that the thermionic emission model based on the Dirac cone approximation is valid only in the graphene-semiconductor Schottky interface operating near room temperature, but breaks down in the cases involving high-energy electrons, such as the graphene-vacuum interface or heterojunction in the presence of photon absorption, where the full-band model is required to account for the band structure nonlinearity at high electron energy. We identify a critical barrier height, ${\mathrm{\Phi }}_B^{(\mathrm{c})}\approx 3.5$ eV, beyond which the Dirac cone approximation crosses over from underestimation to overestimation. In the high-temperature thermionic emission regime at the graphene-vacuum interface, the Dirac cone approximation severely overestimates the electrical and heat current densities by more than 50% compared to the more accurate full-band model. The large discrepancies between the two models are demonstrated using a graphene-based thermionic cooler. These findings reveal the fallacy of Dirac cone approximation in the thermionic injection of high-energy electrons in graphene. The full-band model developed here can be readily generalized to other 2D materials and provides an improved theoretical avenue for the accurate analysis, modeling, and design of graphene-based thermionic energy devices.

Pure topological insulating materials preserve a unique electronic state comprised of a bulk insulating gap and conducting surface states. We use bulk ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ single crystals possessing $\mathrm{Se}$ vacancy defects as a prototype topological insulator (TI) material for exploring the effect of nonmagnetic disorder on the conducting properties of TIs. We employ a sensitive, noncontact mutual-inductance-based technique for measuring the surface and bulk contributions to electrical conductivity in the TI. We discern the bulk and surface contributions by observing that the predominant surface electrical conduction shows a linear frequency dependence of the pickup signal while bulk conductivity gives rise to a quadratic frequency dependence. We also observe an algebraic temperature-dependent surface conductivity and an activated form of bulk electrical conductivity. Using the above, we uncover an interplay between surface and bulk contributions to electrical conductivity in the TI as a function of temperature. In the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystals, the transformation from surface to bulk-dominated electrical transport is found to occur close to a temperature of 70 K. This temperature matches well with our results from activated bulk electrical transport results, which show an activation energy scale Δ, which is in the meV range. The gap Δ is much less than the bulk band gap in ${\mathrm{Bi}}_2{\mathrm{Se}}_3,$ which we argue is associated with defect states in the TI material. To understand our results, we propose a model of TI comprised of an inhomogeneous low electrically conducting medium (bulk), which is sandwiched between two thin high electrically conducting sheets (surface). We argue that the inhomogeneous TI state is generated by selenium vacancy defects in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, which are responsible for producing an interplay between bulk and surface conductivities.

Carrier localization due to statistical fluctuations in indium gallium nitride alloys has been recognized to play an important role for the performance of light-emitting diodes, both experimentally and through theoretical studies. While a random-alloy assumption is usually made, in this work we take into account the presence of spatial nonuniformities in the indium content on the nanometer scale and we theoretically analyze their impact on the electronic and optical properties of the alloy and the device. We show that indium clustering induces tail states in both the conduction and valence bands. This causes a reduction of the band gap and a broadening of the optical absorption edge. Furthermore, compositional fluctuations in the active region of the device determine a substantial broadening of the optical emission spectrum and a decrease of the peak emission energy, in agreement with experimental results. Moreover, the radiative recombination coefficient increases for an increasing degree of clustering, suggesting a transition to a quantum-dot-like structure. Finally, the temperature dependence of the radiative coefficient derived for the nonuniform structures is in good agreement with the experimental results, which show a temperature behavior opposite to the trend expected from standard theoretical considerations.

A scheme to produce mega-electron-volt- (MeV) level gamma rays with a high level of brilliance, a small divergence angle, and controllable angular momentum from Laguerre-Gaussian (LG) laser-pulse interactions with underdense plasma is proposed. Three-dimensional particle-in-cell simulations show that the gamma photon beam acquires angular momentum from the helically distributed relativistic electrons driven by the LG laser and the self-generated fields in the plasma bubble created by the laser. The divergence angle and the orbital angular momentum (OAM) of the gamma photons can be controlled by manipulating the laser parameters. It is found that a 1-MeV helical gamma-ray pulse with peak brightness $2.23\times {10}^{24}\mathrm{photons}/\mathrm{s}/{\mathrm{mm}}^2/{\mathrm{mrad}}^2/0.1\mathrm{\%}\mathrm{BW}$ and angular momentum $7.1\times {10}^{-15}\mathrm{kg}{\mathrm{m}}^2/\mathrm{s}$, as well as a $<{10}^{\circ }$ divergence angle, can be generated by a $2\times {10}^{22}\mathrm{W}/{\mathrm{cm}}^2$ right-hand circularly polarized LG laser pulse. Such bright gamma rays with OAM offer an additional degree of freedom, which is relevant for understanding quantum-electrodynamics phenomena involving angular momentum, astrophysical phenomena, time-resolved probing of the nucleus, the generation of a vortical positron beam, etc.

Circularly polarized light can be obtained by using either polarization conversion or structural chirality. Here we reveal a fundamentally unrelated mechanism of generating circularly polarized light using coupled nonequilibrium sources. We show that thermal emission from a compact dimer of subwavelength, anisotropic antennas can be highly circularly polarized when the antennas are at unequal temperatures. Furthermore, the handedness of emitted light is flipped upon interchanging the temperatures of the antennas, thereby enabling reconfigurability of the polarization state lacked by most circularly polarized light sources. We describe the fundamental origin of this mechanism using rigorous fluctuational electrodynamic analysis and further provide practical examples for its experimental implementation. Apart from the technology applications in reconfigurable devices, communication, and sensing, this work motivates new inquiries of angular-momentum-related thermal-radiation phenomena using thermal nonequilibrium, without applying magnetic field.

A striking characteristic of superconducting circuits is that their eigenspectra and intermode coupling strengths are well predicted by simple Hamiltonians representing combinations of quantum-circuit elements. Of particular interest is the Cooper-pair-box Hamiltonian used to describe the eigenspectra of transmon qubits, which can depend strongly on the offset-charge difference across the Josephson element. Notably, this offset-charge dependence can also be observed in the dispersive coupling between an ancillary readout mode and a transmon fabricated in the offset-charge-sensitive (OCS) regime. We utilize this effect to achieve direct high-fidelity dispersive readout of the joint plasmon and charge-parity state of an OCS transmon, which enables efficient detection of charge fluctuations and nonequilibrium-quasiparticle dynamics. Specifically, we show that additional high-frequency filtering can extend the charge-parity lifetime of our device by 2 orders of magnitude, resulting in a significantly improved energy relaxation time $T_1\sim 200\mu \mathrm{s}$.

An important step in diagnostics is the isolation of specific cells and microorganisms of interest from blood. Since such bioparticles are often present at very low concentrations, throughput needs to be as high as possible. In addition, to ensure simplicity, a minimum of sample preparation is important. Therefore, sorting schemes that function for whole blood are highly desirable. Deterministic lateral displacement (DLD) devices have proven to be very precise and versatile in terms of a wide range of sorting parameters. To better understand how DLD devices perform for blood as the hematocrit increases, we carry out measurements and simulations for spherical particles in the micrometer range which move through DLD arrays for different flow velocities and hematocrits ranging from pure buffer to concentrated erythrocyte suspensions mimicking whole blood. We find that the separation function of the DLD array is sustained even though the blood cells introduce a shift in the trajectories and a significant dispersion for particles whose diameters are close to the critical size in the device. Simulations qualitatively replicate our experimental observations and help us identify fundamental mechanisms for the effect of hematocrit on the performance of the DLD device.

We demonstrate, numerically, the generation of supercontinuum using a high-power ultrafast source at 1.06-$\mu \mathrm{m}$ wavelength and a nonlinear hollow-core fiber. We yield a system that is less sensitive to noise by pumping in the normal-dispersion regime, yet still achieves enhanced spectral broadening by exciting an optical soliton in the midinfrared region. The pump pulse goes through a series of nonlinear effects leading to a supercontinuum output that spans across the midinfrared, 0.5–4.2 $\mu \mathrm{m}$.