To optimize the opto-electronic properties of compound semiconductors, a detailed understanding and control of compositional gradients forming during their synthesis is crucial. A common fabrication process for $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ (CIGS) thin-film solar cells uses annealing at high temperatures, which—contrary to what could be expected from simple Fickian diffusion—results in the formation of steep and stable $\mathrm{Ga}$ gradients, deviating from the optimal $\mathrm{Ga}$ profile for high-efficiency CIGS absorbers. Here, we show that the formation of elastic stresses inside the material during the interdiffusion can have a profound effect on the final $\mathrm{Ga}$ distribution, resulting in persistent $\mathrm{Ga}$ gradients inside CIGS absorber layers. A comparison of numerical diffusion and stress-formation calculations with real-time synchroton-based energy-dispersive x-ray diffraction data acquired in-situ during selenization of CIGS thin films demonstrates that the model can reproduce the stagnation of $\mathrm{In}$-$\mathrm{Ga}$ interdiffusion. We discuss that a detailed understanding of the interplay between stress and diffusion processes in thin films may open alternative fabrication strategies for generating desired stable compositional gradients to improve the opto-electronic properties of compound semiconductors, such as chalcopyrite, kesterite, and perovskite solar cell absorbers for solar cells.

Heterostructures composed of dissimilar perovskite oxides with different properties provide an opportunity to observe emergent phenomena, and have promising applications. A key feature of oxide heterostructures is interfacial electronic and orbital reconstruction. In this paper, we demonstrate a dramatic variation in magnetic anisotropy caused by electric tuning of the charge-transfer process in a ${\mathrm{La}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{Co}\mathrm{O}}_3/{\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}\mathrm{O}}_3$ bilayer structure. By repeatedly changing the valence state of $\mathrm{Co}$ ions in the ${\mathrm{La}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{Co}\mathrm{O}}_3$ top layer using ionic-liquid gating, reversible switching of the magnetic easy axis of the bottom ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}\mathrm{O}}_3$ layer between the out-of-plane and the in-plane direction is achieved, accompanying a modulation of the interfacial exchange coupling. $\mathrm{Mn}$-to-$\mathrm{Co}$ charge transfer and its effect on the interfacial orbital occupancy are further confirmed by x-ray absorption spectroscopy and x-ray-linear-dichroism analysis. The considerable interfacial charge transfer causes an overlap of the $\mathrm{Mn}$ and $\mathrm{Co}$ 3d orbitals, resulting in orbital reconstruction in the ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}\mathrm{O}}_3$ layer and thus magnetic anisotropy. This work demonstrates a promising method for tuning the orbital occupancy and related properties of perovskite heterostructures.

Superconducting quantum circuits are typically housed in conducting enclosures in order to control their electromagnetic environment. As devices grow in physical size, the electromagnetic modes of the enclosure come down in frequency and can introduce unwanted long-range cross-talk between distant elements of the enclosed circuit. Incorporating arrays of inductive shunts such as through-substrate vias or machined pillars can suppress these effects by raising these mode frequencies. Here, we derive simple, accurate models for the modes of enclosures that incorporate such inductive-shunt arrays. We use these models to predict that cavity-mediated interqubit couplings and drive-line cross-talk are exponentially suppressed with distance for arbitrarily large quantum circuits housed in such enclosures, indicating the promise of this approach for quantum computing. We find good agreement with a finite-element simulation of an example device containing more than 400 qubits.

Observations of ferroelectric domains and domain walls are essential for understanding ferroelectric switching characteristics. Here, we demonstrate that the use of optical birefringence, coupled with a first-order electro-optic effect (Pockels effect), allows the visualization of ferroelectric domains in transparent ferroelectric materials. We utilize a field-modulation imaging technique, with the use of a complementary metal-oxide-semiconductor (CMOS) area image sensor to detect slight differences in parallel- and crossed-Nicols polarized optical microscopy images between forward- and reverse-electric-field applications. The ferroelectric domains with antiparallel polarizations are clearly discriminated from each other by the sign of the ellipticity modulation of outgoing elliptically polarized light. The observed images are consistent with piezoresponse force microscopy images of transparent single-crystal films of 2-methylbenzimidazole (MBI). The modulation signal at both polarization configurations exhibit clear sign changes in the wavelength dispersion, owing to the anisotropic nature of the MBI films. The unique optical-probe nature also reveals the existence of two types of domain walls with different three-dimensional orientations.

We demonstrate a programmable quantum Hall circuit that implements an iterative voltage bisection scheme and allows any binary fraction $(k/2^n)$ of the fundamental resistance quantum $R_K/2=h/2e^2$ to be obtained. The circuit requires a number $n$ of bisection stages that only scales logarithmically with the resolution of the set of possible output fractions. The value of $k$ can be set to any integer between 1 and $2^n$ by proper and easily predictable gate configuration. The architecture exploits gate-controlled routing, mixing, and equilibration of edge modes of robust quantum Hall states. The device does not contain internal Ohmic contacts and is thus naturally robust towards stray-resistance effects. Our scheme offers an alternative way to obtain custom quantum Hall resistance standards, and its potential advantages are discussed. The basic viability of the approach is demonstrated in a proof-of-principle two-stage bisection circuit built on a high-mobility $\mathrm{Ga}\mathrm{As}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ heterostructure operating at a temperature of $260\mathrm{mK}$ and a magnetic field of $4.1\mathrm{T}$. Our prototype achieves a relative quantization precision of the order of ${10}^{-4}$, which is limited by the experimental setup rather than by the circuit itself.

The spectral theory of thermal fields subject to arbitrary boundary surface flux conditions developed in this work establishes a universal approach to non-steady-state (photo)thermal responses of solids under transient or modulated thermal excitation through a combined Fourier-Laplace formalism. The self-consistent evolution from the nonsteady state to the steady state under single frequency or arbitrary pulsed or multifrequency thermal-wave excitation waveforms allows for defining experimental criteria in terms of excitation waveform repetition periods to attain steady state in frequency-modulated photothermal (and other general experimental) systems. This approach is crucial for ensuring accurate material property measurements using lock-in or correlation and spectral analysis demodulation of thermal waves, especially under linear frequency modulation (LFM) when the modulated steady state is time dependent. The theory is validated using experimental photothermal radiometry LFM responses from black anodized aluminum. We also define the associated thermal diffusion length in terms of linear superpositions of partial thermal waves (wavelets), which are shown to result in improved depth, lateral, and axial resolution in thermal-wave radar imaging applications compared with sequential single-modulation frequency detection.

Materials with properties that can be described by a near-zero index of refraction have received significant attention in the fields of electromagnetics, optics, and acoustics, due to their extraordinary capabilities in wave manipulation. It was recently demonstrated theoretically and experimentally that acoustic waves could manifest near-zero-index propagation based on the effective compressibility of a waveguide channel approaching zero. In turn, this allows tunneling of acoustic waves with nearly infinite wavelength (or equivalently, uniform phase) when the channel cross section is much larger than the feeding lines. Here, we show that these concepts can be leveraged to realize an acoustic power divider and multiplexer offering tunneling of sound to an arbitrary number of output ports, with phase shifts equal to 0° or 180° and robust response to variations in the port position. We present analytical and numerical models describing the properties of this device and study limitations and trade-offs that occur in the presence of losses as the device size is scaled.

On nanomaterials characterization, the evolution from the broadly utilized micro-Raman spectroscopy to a nanoscale regime is taking place with the improvements in tip-enhanced Raman spectroscopy (TERS). It is not clear, however, to which extent protocols developed within the diffraction-limited far-field configuration apply to the near-field regime. Defects in graphene nanoflakes are investigated in this work in both micro and nano regimes, showing that the Raman characteristics utilized to quantify defects in the far-field regime actually change in the presence of the TERS tip, generating incompatible results. The micro-to-nano spectral mismatch can be modeled using a theory of spatial coherence in near-field Raman scattering, from which a parameterization procedure can be derived in order to obtain consistent results. The incompatibilities observed here for graphene should also happen in other structures relevant to nanoscience and nanotechnology when explored in the nano-Raman regime, and they can be resolved similarly.

Periodic photonic structures have attracted much interest due to their versatility for controlling light propagation. The dispersion of photon modes in photonic lattices exhibits an energy band structure analogous to the electronic one in crystals. An important consequence of the photonic band structure is the localization of light in a band gap, which can be engineered by breaking the translational symmetry introducing a defect in the lattice. Here, we demonstrate experimentally the localization of light in a two-dimensional periodic system of polaritons confined in a patterned microcavity. We generate a self-trapping of light by optically inducing a local breaking of the strong-coupling regime of excitons to photons. In the photon lasing regime we show the existence of confined modes that can be controlled by the shape of the generated defect. We demonstrate single on-site localization with lasing mode at the edge of the Brillouin zone similar to a gap soliton. Our results pave the way for useful tools to optically control localization and propagation of light towards the generation of microlasers.

Artificial intelligence is currently attracting unprecedented attention for its ability to tackle hard problems with huge datasets, which have been rendered tractable by the giant computational power and amount of training data available today. Photonic inverse design, in which one seeks to find objects of desired electromagnetic response, belongs to this class of complex problems that can greatly benefit from these ideas. In this work, artificial intelligence concepts are applied to advance the quest for invisible particles that do not perturb the background field; in particular, a fully connected neural network is proposed to address such a problem by learning the dynamics of visible-light interaction with low-scattering multilayered nanospheres. By swapping the roles between inputs and outputs, the same network can then be used for the inverse design of invisible nanoparticles, obtaining superior performance with respect to the best elements of the training set. The proposed approach can be generalized to approximate Maxwell interactions by simulating the electromagnetic response of more complicated optical configurations, and accomplish their inverse design directly, without successive iterations.

We report on the production of cold, state-selected ${\mathrm{H}}_2^{+}$ molecular ions in a linear rf trap. The ions are produced by ($3+1$) resonance-enhanced multiphoton ionization (REMPI) of ${\mathrm{H}}_2$, and sympathetically cooled by laser-cooled ${\mathrm{Be}}^{+}$ ions. After demonstrating and characterizing the REMPI process, we use photodissociation by a deep UV laser at 213 nm to verify the high vibrational purity of the produced ${\mathrm{H}}_2^{+}$ ion samples. Moreover, the large difference between the photodissociation efficiencies of ions created in the $v=0$ and $v=1$ levels provides a way to detect a $v=0\to 1$ transition. These results pave the way towards high-resolution vibrational spectroscopy of ${\mathrm{H}}_2^{+}$ for fundamental metrology applications.

The acoustic radiation force acting on a spherical particle placed close to the interface of two infinite half-spaces that is excited by a normally incident traveling wave is investigated using the finite element method (FEM). The medium surrounding the particle is water, whereas different material models are used for the particle itself and the second half-space. Recently, acoustic force spectroscopy has been developed, a technology that uses acoustic forces to measure the mechanical properties of small filaments. If acoustics are used to manipulate particles that are placed very close to an interface then the acoustic interactions between the particle and interface lead to an acoustic interaction force. This interaction force was neglected in previous studies and is included in the total acoustic radiation force obtained by our FEM model. Comparing our results to the primary radiation force that can be obtained using the Gor’kov potential, we found that, for cells (red blood cells or fat cells) immersed in water and placed close to a PMMA, glass, or silicon domain, the magnitude of the acoustic interaction force can exceed 10% of the magnitude of the primary radiation force. In other cases, the acoustic interaction force can be even larger in amplitude and of opposite sign than the primary force.

Phase-locking of oscillators leads to super-radiant amplification of the emission power. This is particularly important for development of terahertz sources, which suffer from low emission efficiency. In this work we study large Josephson junction arrays containing several thousand $\mathrm{Nb}$-based junctions. Using low-temperature scanning laser microscopy, we observe that at certain bias conditions two-dimensional standing-wave patterns are formed, manifesting themselves as global synchronization of the arrays. Analysis of standing waves indicates that they are formed by surface plasmon–type electromagnetic waves propagating at the electrode-substrate interface. Thus, we demonstrate that surface waves provide an effective mechanism for long-range coupling and phase-locking of large junction arrays.

The present-day nanodevice dimensions continuously shrink, with the aim to prolong Moore’s law. As downsizing meticulously persists, undesirable dynamic defects, which cause low-frequency noise and structural instability, play detrimental roles on limiting the ultimate performance and reliability of miniaturized devices. A good understanding and a meaningful control of the defect kinetics then become fundamental and urgent issues. Here we report observations of thermally activated atomic defect motion as well as nanocrystalline defect motion through electrical noise processes in metallic ${\mathrm{Ru}\mathrm{O}}_2$ rutile nanowires around room temperature. First, we extract the energy-distribution function and the number density of mobile atomic defects (oxygen vacancies). Second, we obtain the geometrical size, grain-boundary bonding strength, and relaxation times of dynamic nanocrystallites. Our results show clearly a powerful probe for effective and noninvasive characterizations of nanostructures and nanomaterials for which quantitative information about mechanical hardness, breakdown current density, and/or resistance noise is essential.

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.