The lotus leaf shows superhydrophobicity and extreme nonsticking properties due to the two-level roughness features of its surface in addition to the low surface energy. In the thin film of vertically aligned treelike carbon nanostructures, each nanostructure is a multiwalled carbon nanotube aligned vertically to the plane of the substrate with carbon films attached to the nanotubelike branches, giving the overall treelike appearance. These nanostructures also have two-degree roughness due to their unique geometry, which makes the wettability of the material an interesting property to study. The change in hydrophobicity of the material as a function of the deposition time is studied to understand the mutual dependence of the geometry of the nanostructures and the hydrophobicity of the film. The material exhibits superhydrophobicity, showing a static water contact angle as high as ${165}^{\circ }$ for certain deposition conditions with extreme nonsticking properties.

Geometric phase, which is acquired after a system undergoes cyclic evolution in Hilbert space, is believed to be noise resilient because it depends only on the global properties of the evolution path. We report geometric control of energy transfer between two parametrically coupled mechanical modes in an optomechanical system. The parametric pump is controlled along a closed loop to implement geometric Stückelberg interferometry of mechanical motion, in which the dynamical phase is eliminated using the Hahn-echo technique. We demonstrate that the interference based solely on the geometric phase is robust against certain noises. More remarkably, we show that the all-geometric approach can achieve a high-energy-transfer rate that is comparable to those using conventional dynamical protocols.

Eliminating the grating lobes is significant in ultrasonics. For example, highly efficient ultrasound focus and flexible focus steering without off-target heating for medical applications require the total rejection of grating lobes and side lobes. We investigate analytically, numerically, and experimentally the physical causes of grating lobes in ultrasound beam engineering. We find that the generation of grating lobes is attributed to two main factors, namely, the diffraction from the finite-sized source array and the different phase delays of the phased array. Based on these findings, we design and implement an active acoustic metasurface to completely eliminate the grating lobes. The lab-made metasurface comprises 16 × 16 square-lattice elements of subwavelength thickness, where each element is a supercell with 4 × 4 piezoelectric sheets in a matrix of epoxy resin. By tailoring the ratio of the supercell width over the lattice constant, the grating lobes due to the structure diffraction can be completely suppressed. With a programmable multichannel drive, we further impose Gaussian-shaped amplitude modulation on the metasurface to wipe out the grating lobes induced by inhomogeneous phase delays. Our experimental results convincingly demonstrate that the active metasurface with programmable phase and amplitude modulations shows a high performance in grating-lobe-proof and steerable ultrasound focusing.

Two centuries ago, Fresnel, Poisson, and Arago showed how the wave nature of light induces a bright spot behind an opaque disk. We develop an analytical model based on the Fresnel diffraction theory to show how small perturbations such as digs or scratches can yield intense light enhancements on the downstream axis. The impact of defects with complex morphology on light diffraction is shown to be accurately modeled by the Fresnel theory applied to a set of opaque disks characterized by phase shift and transmittance. This model can be used either to define the geometry of the defects that optimizes the light enhancement or to improve the defect specification on the surface of the optical components. We explain why partial information of the defect morphology can suffice to specify a safety distance beyond which light intensifications are not dangerous. The validity of this analytical approach is studied by measuring the intensifications created by three different microdefects.

We generate correlated photon pairs in an all-pass silicon microring resonator using continuous-wave pumped spontaneous four-wave mixing and we characterize in detail the optical bistability in the nonclassical single-photon regime. Special attention is given to the resonance-locked method by balancing the pump-induced heating and the active off-chip cooling, leading to stable operation of the photon-pair source. The maximal coincidence rate of $20.53\pm 0.34\mathrm{Hz}$, maximal coincidence-to-accidental ratio of $654\pm 125$, and minimal zero-delayed heralded second-order correlation of $0.14\pm 0.09$ are achieved. The wavelengths of photon pairs remain unchanged at different pump power, indicating that our source is more compatible with the common communication systems than those using conventional on-resonance strategies, that tunes the pump wavelength. This work not only experimentally validates the nonclassical bistability behaviors, but also puts forward a simple method of pump-resonance matching for high-quality performance.

The basis for superconducting electronics can broadly be divided between two technologies: the Josephson junction and the superconducting nanowire. While the Josephson junction (JJ) remains the dominant technology due to its high speed and low power dissipation, recently proposed nanowire devices offer improvements such as gain, high fanout, and compatibility with CMOS circuits. Despite these benefits, nanowire-based electronics have largely been limited to binary operations, with devices switching between the superconducting state and a high-impedance resistive state dominated by uncontrolled hotspot dynamics. Unlike the JJ, they cannot increment an output through successive switching and their operation speeds are limited by their slow thermal-reset times. Thus, there is a need for an intermediate device with the interfacing capabilities of a nanowire but a faster, moderated response allowing for modulation of the output. We present a nanowire device based on controlled fluxon transport. We show that the device is capable of responding proportionally to the strength of its input, unlike other nanowire technologies. The device can be operated to produce a multilevel output with distinguishable states, the number of which can be tuned by circuit parameters. Agreement between experimental results and electrothermal circuit simulations demonstrates that the device is classical and may be readily engineered for applications including use as a multilevel memory.

Understanding the relationship of photoexcited carrier lifetimes, mobilities, and recombination mechanisms to structural properties and processing of photovoltaic (PV) absorber materials is critical to the design of efficient solar cells. Carrier dynamics in PV absorbers have conventionally been characterized by time-resolved photoluminescence (TRPL), but TRPL may not be suitable or straightforward for all absorbers. Alternative noncontact methods can enable measurement of ultrafast carrier dynamics for a wider range of materials. Here, we demonstrate the complementary use of time-resolved terahertz spectroscopy (TRTS) and near-infrared transient reflectance (NIRTR) spectroscopy along with TRPL to elucidate photoexcited carrier dynamics in a high-quality copper-poor, zinc-rich kesterite ${\mathrm{Cu}}_2\mathrm{Zn}\mathrm{Sn}{\mathrm{Se}}_4$ (CZTSe) single crystal. The single-crystalline nature of the sample eliminates complications arising from grain boundaries, secondary phases, and interfaces associated with thin-film growth. A single-crystal-based PV device exhibited an efficiency of 5.7% and an open circuit voltage ($V_{\mathrm{oc}}$) of 400 mV, consistent with the quasi-Fermi-level splitting determined using absolute photoluminescence. NIRTR showed picosecond-scale cooling and relaxation of carriers into a distribution of band-tail states while TRTS revealed a characteristic time scale of 200–260 ps for recombination. Hall effect and TRTS measurements revealed electron and hole mobilities in the range of $50-100{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$. These dynamics result in a characteristic minority carrier diffusion length of less than 200 nm, leading to incomplete carrier collection, as confirmed by a strongly decreasing external quantum efficiency at long wavelengths. Our approach combining ultrafast spectroscopy and device measurements can lead to more detailed understanding of performance-limiting photophysical processes and can accelerate the development of more efficient PVs.

We present an experimental setup for monochromatic propagation-based x-ray phase-contrast imaging based on a conventional rotating-copper-anode source, capable of an integrated flux up to ${10}^8$ photons/s at 8 keV. In our study, the system is characterized in terms of spatial coherence, resolution, contrast sensitivity, and stability. Its quantitativeness is demonstrated by comparing theoretical predictions with experimental data on simple wire phantoms both in planar and computerized-tomography-scan geometries. Application to two biological samples of medical interest shows the potential for bioimaging on the millimeter scale with spatial resolution of the order of 10 $\mu \mathrm{m}$ and contrast resolution below 1%. All the scans are performed within laboratory-compatible exposure times, from 10 min to a few hours, and trade-offs between scan time and image quality are discussed.

A multiscale hydrodynamic-heat-transport model applicable to arbitrary geometries using finite-element methods is compared with the experimental effective thermal conductivity of silicon thin films and periodic holey membranes for different sizes and temperatures. The range of system length scales and temperatures in which the model predictions agree with experimental data is discussed and quantitatively determined. The model agrees with experimental results when the smallest system size is larger than twice the nonlocal length, an intrinsic property of the material that depends only on temperature. These results open the door to the use of the hydrodynamic equation instead of an effective Fourier model to interpret current heat-transport experimental data.

Nitrogen-vacancy (N-$V$) centers in nanodiamond (ND) are a promising single-photon-source candidate for quantum technology. However, the poor N-$V$ emission rate and low outcoupling of light significantly hinder their effective use in practical implementations. To overcome this limit, we place NDs hosting N-$V$ centers on silver columnar thin films (CTFs) and measure an increase in emission by an order of magnitude. The CTFs consist of silver nanocolumns the length of which is chosen to be half the wavelength of the emitted light. The silver nanocolumns act as efficient optical antennas that couple to the N-$V$ centers via the optical near field and outcouple the excitation energy of the N-$V$ centers effectively into the optical far field. A large distribution of radiated powers from different NDs is observed. Computer simulations show this distribution to arise from the different orientations of the emitting dipoles with respect to the columnar axis. We also report that further structuring of the silver CTF into gratings yields higher photon emission.

Spin-orbit torque provides a powerful means of manipulating domain walls along magnetic wires. However, the current density required for domain-wall motion is still too high to realize low power devices. We experimentally demonstrate helicity-dependent domain-wall motion by combining synchronized femtosecond laser pulses and short current pulses in $\mathrm{Co}/\mathrm{Ni}/\mathrm{Co}$ ultrathin-film wires with perpendicular magnetization. The domain wall can remain pinned under one laser circular helicity while being depinned by the opposite circular helicity. Thanks to the all-optical helicity-dependent effect, the threshold current density due to spin-orbit torque can be reduced by more than 50%. Based on this joint effect combining spin-orbit torque and helicity-dependent laser pulses, an optoelectronic logic-in-memory device is experimentally demonstrated. This work enables an alternative class of low-power spintronic-photonic devices beyond the conventional approach of all-optical switching or all-current switching for data storage.

Carrier-lifetime measurements reveal that, contrary to common expectations, the high-current nonradiative recombination (droop) in III-nitride light emitters is comprised of two contributions that scale with the cube of the carrier density: an intrinsic recombination—most likely standard Auger scattering—and an extrinsic recombination that is proportional to the density of point defects. This second droop mechanism, which has not previously been observed, may be caused by an impurity-assisted Auger process. Further, it is shown that longer-wavelength emitters suffer from higher point-defect recombinations, in turn causing an increase in the extrinsic droop process. It is proposed that this effect leads to the green gap and that point-defect reduction is a strategy to both vanquish the green gap and more generally improve quantum efficiency at high current.

Entanglement between optical and microwave cavity modes is a critical issue in illumination systems. Optomechanical systems are utilized to introduce coupling between the optical and microwave cavity modes. However, due to some restrictions of the optomechanical system, especially sensitivity to the thermal photon noise at room temperature, an alternative optoelectronic system is designed to address the problem. We study a method by which it may be possible to remove the mechanical part of the previous systems to minimize the thermally generated photons. Unlike optomechanical systems, in our system, the optical mode is directly coupled to the microwave cavity mode through the optoelectronic elements without employing any mechanical parts. The utilized approach leads to generating the entangled modes at room temperature. For this purpose, the dynamics of the motion of the optoelectronic system is theoretically derived using the Heisenberg-Langevin equations from which one can calculate the coupling between optical and microwave cavity modes. The direct coupling between the optical and microwave cavity modes is the most important feature and is achieved through the combination of the photodetector and a Varactor diode. Hence, by controlling the photodetector current, that is, the photocurrent, depending on the optical cavity incident wave and the Varactor diode-biased voltage, the coupling between the optical and microwave cavity modes is established. The voltage across the Varactor diode also depends on the generated photocurrent. Consequently, our results show that the coupled modes are entangled at room temperature without the requirement for any mechanical parts.

We report that metamaterial-inspired one-dimensional gratings (or metagratings) can be used to control nonpropagating diffraction orders as well as propagating ones. By accurate engineering of the near field, it becomes possible to satisfy power-conservation conditions and achieve perfect control over all propagating diffraction orders with passive and lossless metagratings. We show that each propagating diffraction order requires 2 degrees of freedom represented by passive and lossless loaded thin “wires.” This provides a solution to the old problem of power management between diffraction orders created by a grating. The theory developed is verified by both three-dimensional full-wave numerical simulations and experimental measurements, and can be readily applied to the design of wavefront-manipulation devices over the entire electromagnetic spectrum as well as in different fields of physics.

We report on the isolation of single negatively-charged-silicon-vacancy ($\mathrm{Si}$-$V^{-}$) centers in nanodiamonds. We observe the fine structure of single $\mathrm{Si}$-$V^{-}$ centers with reduced inhomogeneous ensemble linewidth below the excited-state splitting, stable optical transitions, good polarization contrast, and excellent spectral stability under resonant excitation. On the basis of our experimental results, we develop an analytical strain model where we extract the ratio between strain coefficients of excited and ground states as well the intrinsic zero-strain spin-orbit splittings. The observed strain values are as low as the best values in low-strain bulk diamond. We achieve our results by means of $\mathrm{H}$-plasma treatment of the diamond surface and in combination with resonant and off-resonant excitation. Our work paves the way for indistinguishable, single-photon emission. Furthermore, we demonstrate controlled nanomanipulation by an atomic-force-microscope cantilever of one- and two-dimensional alignments with an accuracy of about 10 nm, as well as new tools including dipole rotation and cluster decomposition. Combined, our results show the potential to utilize $\mathrm{Si}$-$V^{-}$ centers in nanodiamonds for controlled interfacing via optical coupling of individually-well-isolated atoms for bottom-up assemblies of complex quantum systems.

We investigate the local structures of a $\mathrm{Ga}\mathrm{N}$ substrate through an energy-resolved white x-ray diffraction topography method. A section topography geometry is implemented at various sample positions for the purpose of wafer mapping. The obtained images at each position are piled up for the three-dimensional matrix and then sliced at a same energy position. The sliced images of the lower and higher diffraction energy show periodic bottomless and topless features originated from the local lattice tilting, respectively. It is revealed that the local lattice planes are tilted by $\pm {0.03}^{\circ }$ around points in the periodic structures. This tilting angle is consistent with results obtained from monochromatic x-ray diffraction topography measurements. X-ray rocking curves obtained by a single-crystal x-ray diffraction shows the tilting of local structure that is also consistent with the energy-resolved x-ray diffraction topography results.

A quantum photonic integrated circuit (QPIC) is a promising tool for constructing integrated devices for quantum-technology applications. In the optical regime, silicon photonics empowered by complementary-metal-oxide-semiconductor (CMOS) technology provides optical components useful for realizing large-scale QPICs. Optical nonlinearity at the single-photon level is required for QPICs to facilitate photon-photon interaction. However, to date, realization of optical elements with deterministic (i.e., not probabilistic) single-photon nonlinearity by use of silicon-based components is challenging despite the enhancement of the functionality of QPICs based on silicon photonics. In this study, we realize a strongly coupled $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{As}$ quantum-dot–cavity-quantum-electrodynamics system on a CMOS-processed silicon photonic chip. The heterogeneous integration of the $\mathrm{Ga}\mathrm{As}$ cavity on the silicon chip is performed by transfer printing. The cavity-quantum-electrodynamics system on the CMOS photonic chip realized in this work is a promising candidate for an on-chip single-photon nonlinear element, which constitutes the fundamental component for future applications based on QPICs, such as coherent manipulation and nondestructive measurement of qubit states via a cavity, and an efficient single-photon filter and router.

Quantum key distribution (QKD) guarantees the secure communication between legitimate parties with quantum mechanics. High-dimensional QKD (HDQKD) not only increases the secret key rate but also tolerates higher quantum bit error rate (QBER). Many HDQKD experiments have been realized by utilizing orbital-angular-momentum (OAM) photons as the degree of freedom (DOF) of OAM of the photon is a prospective resource for HD quantum information. In this work, we propose that a high-quality HDQKD based on polarization-OAM hybrid states can be realized by utilizing state mapping between different DOFs and we characterize this HDQKD. Both the preparation and measurement procedures of the proof-of-principle verification experiment are simple and stable. Our experiment verifies that a $0.60\mathrm{\%}\pm 0.06\mathrm{\%}$ QBER and a $1.849\pm 0.008$-bit secret key rate per sifted signal can be achieved for a four-dimensional QKD with the weak coherent light source and decoy state method.

Fullerenes and their derivatives are well-known electron-transporting materials used in organic solar cells and transistors. However, the extent to which fullerenes are able to transport holes is heavily disputed. Using selective Ohmic contacts, we study the bulk hole and electron transport in a bisadduct fullerene derivative. Trap-free space-charge-limited hole and electron currents are measured, with a hole mobility equivalent to the electron mobility. Our results identify the bisadduct fullerene as an organic semiconductor with balanced bipolar bulk transport with excellent electron and hole mobilities.

With recent advances in dynamic scanning probe microscopy techniques, it is now routine to image the submolecular structure of molecules with atomically engineered tips, which are prepared via controlled modification of the tip termination and are chemically well defined. The enhanced spatial resolution is possible as atomically engineered tips can preserve their integrity in the repulsive interaction regime. Although the mechanism of improved spatial resolution has been investigated both experimentally and theoretically, the ultimate temporal resolution while preserving picometer-scale spatial resolution still remains an open question. We computationally analyze the temporal resolution of atomic force microscopy imaging with atomically engineered tips. Our computational results reveal that nonmetal terminated tips, for example, oxygen-terminated copper, are well suited for enhanced temporal resolution up to video-rate imaging velocities while preserving picometer-range spatial resolution. On the other hand, the highest attainable spatial resolution of atomically engineered tips with low stiffness, for example, CO terminated, deteriorate with increasing imaging velocity. Our results reveal that when atomically engineered tips terminated with molecules are in use, imaging velocities of the order of nanometers per second at most are inevitable even for atomically flat surfaces to retain atomic resolution and avoid slip-stick motion. In addition to shedding light on the temporal resolution of atomic force microscopy imaging with atomically engineered tips, our numerical results provide an outlook to the scalability of atom-by-atom fabrication using scanning probe microscopy techniques.