We present a quantum-optics approach for describing stimulated parametric down-conversion in the two-type-I crystal “sandwich” configuration, which allows for parametric interaction of vector vortex beams. We analyze the conditions for which phase conjugation of the seed vector beam occurs. We then use two strategies for defining generalized Stokes parameters to describe phase conjugation of vector vortex beams. These allow for geometrical representations, such as higher-order Poincaré spheres. Our results are useful for the description and design of stimulated and spontaneous parametric down-conversion experiments with vector vortex beams.

Optical and optoelectronic techniques for micro- and nano-object manipulation are becoming essential tools in nano- and biotechnology. The use of photovoltaic optoelectronic tweezers is a remarkable optoelectronic technique that has undergone rapid development in recent years, with excellent results and widespread potential applications. It is based on light-induced electric fields generated by the bulk photovoltaic effect in certain ferroelectrics, such as ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$. The technique is simple and versatile, enabling the successful manipulation of a large variety of micro- and nano-objects with only optical control, without the need for electrodes or power supplies. However, the handling of objects in aqueous solutions remains a challenge for this tool, due to the electric-screening effects of polar liquids. This has hindered their application in biotechnology and biomedicine, where most processes occur in aqueous solution. Here, an efficient route to overcome this problem is proposed and demonstrated. It uses photovoltaic optoelectronic tweezers to manipulate aqueous droplets, immersed in a nonpolar oil liquid, but hanging at the air-oil interface. In this singular configuration, the high electric fields generated by the bulk photovoltaic effect in the ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ substrate allow simple and flexible manipulation of aqueous droplets controlled by the light. Droplet guiding, trapping, merging, and splitting are achieved and efficient operation with water and a variety of biodroplets (DNA, sperm, and phosphate-buffered saline solutions) is demonstrated. A discussion on the physical mechanisms responsible for the manipulation processes is also provided. The reported results overcome the main limitation of these tweezers to handle biomaterials and promise high potential for biotechnological and biochemistry applications, including their implementation in optofluidic devices.

In this work we present a method to quantitatively measure the optical absorption of single nanowires that can be applied over a wide range of temperatures and with a high enough sensitivity to enable the measurement of below-band-gap absorption (as well as the absorption of single molecules). The method is based on accurately measuring the heat flow coming from a nanowire when it is illuminated by a laser beam. We experimentally verify this method by measuring the absorption of both a zincblende and a wurtzite GaAs, a wurtzite GaP, and a superlattice ${\mathrm{Zn}}_3{\mathrm{P}}_2$ nanowire. Furthermore, we find that the ${\mathrm{Zn}}_3{\mathrm{P}}_2$ nanowires have the largest absorption of all these materials. We analyze the advantages and disadvantages of the method and study its range of applicability.

The theory of quantum cryptography aims to guarantee unconditional information-theoretic security against an omnipotent eavesdropper. In many practical scenarios, however, the assumption of an all-powerful adversary is excessive and can be relaxed considerably. In this paper we study secret-key distillation across a lossy and noisy quantum wiretap channel between Alice and Bob, with a separately parameterized realistically lossy quantum channel to the eavesdropper Eve. We show that under such restricted eavesdropping, the key rates achievable can exceed the secret-key-distillation capacity against an unrestricted eavesdropper in the quantum wiretap channel. Furthermore, we show upper bounds on the key rates based on the relative entropy of entanglement. This simple restricted eavesdropping model is widely applicable, for example, to free-space quantum optical communication, where realistic collection of light by Eve is limited by the finite size of her optical aperture. Future work will include calculating bounds on the amount of light Eve can collect under various realistic scenarios.

Spin diodes are usually resonant in nature (GHz frequency) and tunable by magnetic field and bias current with performances, in terms of sensitivity and minimum detectable power, overcoming the semiconductor counterpart, i.e., Schottky diodes. Recently, spin diodes characterized by a low-frequency detection (MHz frequency) have been proposed. Here, we show a strategy to design low-frequency detectors based on magnetic tunnel junctions with the interfacial perpendicular anisotropy of the same order of the demagnetizing field out-of-plane component. Micromagnetic calculations show that, to reach this detection regime, a threshold input power has to be overcome and the phase shift between the oscillation magnetoresistive signal and the input radiofrequency current plays a key role in determining the value of the rectification voltage.

A major challenge in operating multiqubit quantum processors is to mitigate multiqubit coherent errors. For superconducting circuits, besides crosstalk originating from imperfect isolation of control lines, dispersive coupling between qubits is a major source of multiqubit coherent errors. We benchmark phase errors in a controlled-phase gate due to dispersive coupling of either of the qubits involved in the gate to one or more spectator qubits. We measure the associated gate infidelity using quantum-process tomography. We point out that, due to coupling of the gate qubits to a noncomputational state during the gate, two-qubit conditional-phase errors are enhanced. Our work is important for understanding limits to the fidelity of two-qubit gates with finite on-off ratio in multiqubit settings.

It is well known that dielectric breakdown in a liquid generates cavitation bubbles and shock waves. Here we demonstrate that when the liquid is bounded by two solid glass boundaries (10–20-$\mu \mathrm{m}$ separation), rings of microscopic bubbles can be nucleated around the laser-induced cavitation bubble. While generally acoustic nucleation is achieved with longitudinal waves of sufficient tension, this work demonstrates that acoustic cavitation can also be generated from transverse waves. Our experiments identify three waves originating at the boundaries: the fastest is the bulk wave in the solid, followed by a leaky Rayleigh wave at the liquid-solid contact, which is trailed by a Lamb-type wave. For the latter, the two solid boundaries act as a wave guide and generate intense and short-lived cavitation activity within the gap. Streak photography and high-speed photography reveal the microsecond-duration cavitation-bubble dynamics, and subpicosecond strobe photography visualizes the mechanism of bubble nucleation from the accelerated surface. Simulations coupling the solid mechanics with the acoustics support the experimentally observed mechanisms of transverse-wave-induced cavitation inception.

Single-mode operation in a rectangular waveguide is very important, since the existence of multimodes may arouse distortions of carried signals due to the modal dispersion. High-order mode means multiple periods of field in the transverse cross section, with distinct electromagnetic properties from the dominant mode. Although numerous methods are proposed to achieve mode selection within the waveguide, they only permit specific high-order modes to survive in a frequency range. However, it still remains a great challenge to control the bandwidth freely for single-mode operation. Here, we propose an approach for high-order mode generation, in which the desired mode operates in the band gap of other modes, and the working bandwidth can be engineered independently by reducing the symmetry of the loaded periodic metallic ridges in the waveguide. Two examples are demonstrated, where only ${\mathrm{TE}}_{20}$ and ${\mathrm{TE}}_{30}$ modes are supported. The proposed method can also be applied to generate other high-order TE modes in the future.

We demonstrate that threading dislocations in $(\mathrm{Al},\mathrm{Ga})\mathrm{N}/\mathrm{Ga}\mathrm{N}$ heterostructures can be spontaneously decorated by acceptors during the epitaxial process. First-principles calculations show that the threading dislocation introduces detrimental deep electronic states both above the valence-band maximum (VBM) and below the conduction-band minimum (CBM) of $\mathrm{Ga}\mathrm{N}$. Because of the electron transfer between the occupied level above the VBM of the threading dislocation and the defect states of acceptors, acceptors will decorate the threading dislocation, which leads to the shift of the dislocation states. For the occupied deep states above the VBM, the acceptor decoration shifts the deep states toward the VBM, which may constructively contribute to the dislocation tolerance of $(\mathrm{Al},\mathrm{Ga})\mathrm{N}/\mathrm{Ga}\mathrm{N}$ heterostructures. For the unoccupied states below the CBM, the acceptor-decorated dislocation provides an additional electron-transfer channel besides that through the pure threading dislocations. These two distinct electron-transfer channels are observed in reverse-biased $(\mathrm{Al},\mathrm{Ga})\mathrm{N}/\mathrm{Ga}\mathrm{N}$ Schottky diodes, which is characterized by two distinct Frenkel-Poole-emission states.

Tunable optical phased arrays and metasurfaces play an important role in a diverse range of applications from imaging and remote sensing to communications and displays. However, as the number of tunable elements grows, the required control architecture becomes insurmountably complex. Here, we discuss the concept of perimeter-controlled tuning to shape far-zone radiation. We discuss applications of our approach to beam forming, holography, and image projection. We show that, with a proper design, the complexity of a control architecture may be dramatically simplified. We further discuss the use of our method to time-sharing image projection and holography. Our concept is applicable to a variety of systems, including phased array optical antennas and metasurfaces.

This work investigates carrier transport in tunnel junctions for vertical-cavity surface-emitting lasers (VCSELs). The study is performed with a quantum-corrected semiclassical approach, where tunneling is described rigorously with a nonequilibrium Green’s function formalism based on a multiband description of the electronic structure. Validated with experimental results, the proposed approach provides a quantum-kinetic perspective of the tunneling process and paves the way toward a comprehensive theory of VCSELs, bridging the gap between semiclassical and quantum simulations.

Quantum key distribution (QKD) is one of the most important aspects of quantum cryptography. Using laws of quantum mechanics as the basis for security, the key-distribution process makes information theoretically secure in QKD. With the advancement and commercialization of QKD, an end-to-end QKD simulation software is required that can include experimental imperfections. Software of this kind will ensure that resources are invested only after prior performance analysis, and is faithful to experimental capacities and limitations. In this work, we introduce our QKD simulation toolkit qkdSim, which is ultimately aimed at being developed into such a software package that can precisely model and analyze any generic QKD protocol. We present the design, implementation, and testing of a prototype of qkdSim that can accurately simulate our own experimental demonstration of the B92 protocol. The simulation results match well with experiment; a representative key rate and the quantum bit error rate from experiment is $51\pm 0.5$ kbit/sec and $4.79\mathrm{\%}\pm 0.01\mathrm{\%}$ respectively, wherein the simulation yields $52.83\pm 0.36$ kbit/sec and $4.79\mathrm{\%}\pm 0.01\mathrm{\%}$, respectively.

Metasurfaces that enable wave-front manipulation within the subwavelength range exhibit fascinating capabilities and application potentials in ultrathin functional devices. Therein, various metasurfaces to realize delicate transverse and even three-dimensional structured light fields have been proposed for applications such as holographic displays, imaging, optical manipulation, etc. However, a metasurface with the capability of tailoring the axial structure of a light field has not been reported so far. Here, we propose and experimentally demonstrate a dielectric metalens to tailor the axial intensity distribution of a light field, based on the independent control of amplitude and phase. The metalens is designed according to an optimized Fourier spectrum encoding method, which allows construction of an ultrasmall nondiffractive light field with any arbitrary preestablished axial structure. This axial modulation scenario enriches the three-dimensional wave-front modulation functionality of the metasurface, and can be implemented for other waves beyond optics, from acoustic and elastic waves to matter waves.

The origin of the relationship between fill factor (FF) and light intensity (I) in organic disordered-semiconductor-based solar cells is studied. An analytical model describing the balance between transport and recombination of charge carriers, parameterized with a factor, ${\mathrm{\Gamma }}_m$, is introduced to understand the FF-I relation, where higher values of ${\mathrm{\Gamma }}_m$ correlate to larger FFs. Comparing the effects of direct and tail-state-mediated recombination on the FF-I plot, we find that, for low-mobility systems, direct recombination with constant transport mobility can deliver only a negative dependence of ${\mathrm{\Gamma }}_{m,\mathrm{dir}}$ on light intensity. By contrast, tail-state-mediated recombination with trapping and detrapping processes can produce a positive ${\mathrm{\Gamma }}_{m,t}$ versus sun dependency. The analytical model is validated by numerical drift-diffusion simulations. To further validate our model, two material systems that show opposite FF-I behavior are studied: poly{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-[4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6-diyl]} (PTB7-Th):[6,6]-phenyl-${\mathrm{C}}_{71}$-butyric acid methyl ester (${\mathrm{PC}}_{71}$BM) devices show a negative FF-I relation, while PTB7-Th:(5Z,5′Z)-5,5′-{[7,7′ -(4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl)]bis(methanylylidene)}bis(3-ethyl-2-thioxothiazolidin-4-one) (O-IDTBR) devices show a positive correlation. Optoelectronic measurements show that the O-IDTBR device presents a higher ideality factor, stronger trapping and detrapping behavior, and a higher density of trap states, relative to the ${\mathrm{PC}}_{71}$BM device, supporting the theoretical model. This work provides a comprehensive understanding of the correlation between FF and light intensity for disordered-semiconductor-based solar cells.

Nitrogen-vacancy (N-V) center defects in diamond enable local, optical detection of magnons, owing to their sensitivity to gigahertz frequency magnetic field noise. Applied to ferromagnetic dynamics sensing, N-Vs can be used to sense an increase in the population of N-V-resonant magnons in a nearby ferromagnetic film, which arise from microwave drive of the uniform-mode ferromagnetic resonance and the subsequent scattering of uniform-mode magnon population throughout the magnon states. In this work we utilize N-V sensing to detect ferromagnetic resonance in films of the low magnetization, ferrimagnetic, coordination compound vanadium tetracyanoethylene (V$[\mathrm{TCNE}{]}_x$). From an applications standpoint V$[\mathrm{TCNE}{]}_x$ has excellent magnetic resonance properties rivalling the best magnetic oxides and it can be deposited on a wide range of substrates, unlike most oxides. The remarkably low magnetization of V$[\mathrm{TCNE}{]}_x$ offers the chance to probe N-V-ferromagnet coupling in a yet-unexplored regime. We simultaneously detect broadband FMR spectroscopy of V$[\mathrm{TCNE}{]}_x$ films by both conventional microwave absorption and N-V techniques, finding few-gauss magnetic resonance linewidth and a magnetization of 49 G by both methods. Using spin-wave theory, we calculate the N-V-resonant spin-wave wavelengths to be on the order of tens of nanometers, with wavevectors on the order of ${10}^8{\mathrm{m}}^{-1}$. Our measurements highlight that the low magnetization of V$[\mathrm{TNCE}{]}_x$ results in very short wavelength spin waves at few-GHz frequencies, which can have significant advantage for future nanoscale magnonic applications.

Nonreciprocal optical devices, such as isolators and circulators, provide highly desirable functionalities in routing signals. This motivates the active research on high-efficiency nonreciprocal devices, especially for miniaturized and integrable magnetic-free schemes. All the present experimental demonstrations of circulators in optical domains are based on the high-$Q$ factor cavity. In this work, we demonstrate a cavity-free optical circulator with low insertion loss by making use of a far-detuned nonlinear Raman process with the assistance of the atomic Doppler effect. Due to the large detuning, the excited state can be adiabatically eliminated, resulting in nearly vanishing absorption. An optical circulator with average transmission of more than $85\mathrm{\%}$ and fidelity above $0.98$ is well achieved in an experiment with hot atoms. Our scheme shows the possibility of cavity-free nonreciprocal optical devices with low insertion loss.

Ideality factors are used to identify the dominant form of recombination in many types of solar cells and guide future development. Unusual noninteger and voltage-dependent ideality factors, which are difficult to explain using the classical diode theory, have been reported for perovskite solar cells and remain unexplained. Experimental measurements and theoretical simulations of the electric potential profile across a planar perovskite solar cell show that significant potential drops occur across each of the perovskite- and transport-layer interfaces. Such potential profiles are fundamentally distinct from the single potential drop that characterizes a $p$-$n$ or a $p$-$i$-$n$ junction. We propose an analytical model, developed specifically for perovskite devices, in which the ideality factor is replaced by a systematically derived analog, which we term the ectypal factor. In common with the classical theory, the ectypal diode equation is derived as an approximation to a drift-diffusion model for the motion of charges across a solar cell, however, crucially, it incorporates the effects of ion migration within the perovskite absorber layer. The theory provides a framework for analyzing the steady-state performance of a perovskite solar cell (PSC) according to the value of the ectypal factor. Predictions are verified against numerical simulations of a full set of drift-diffusion equations. An important conclusion is that our ability to evaluate PSC performance, using standard techniques such as the analysis of dark $J$-$V$ or Suns-$V_{\mathrm{OC}}$ measurements, relies on understanding how the potential distribution varies with applied voltage. Implications of this work on the interpretation of data from the literature are discussed.

We develop a form of nondestructive optical thermometry to deterministically measure the temperature of the selected segment of a cold-atom ensemble. The essence is to monitor the thermal expansion of the targeted cold atoms after their labeling through manipulation of the internal states, and the nondestructive property relies on nearly lossless detection via the driving of a cycling transition. The temperature information is extracted from the change of densities via a bucket detector without the necessity of a complicated imaging setup. We also focus on the application of this method to isotropic laser cooling, and it has the capability of addressing only the atoms on the optical detection axis within the enclosure, which is readily compatible with typical configurations of atomic clocks or quantum sensing. Our results confirm the sub-Doppler-cooling features of isotropic laser cooling, and we further investigate the relevant cooling properties. Furthermore, we demonstrate the recently developed optical configuration with injection of cooling-laser light in the form of hollow beams, which helps to enhance the cooling performance and accumulate more cold atoms in the central regions.