We report the design of a diamond-based honeycomblike phononic network in which a mechanical resonator couples to three distinct phononic crystal waveguides. This two-dimensional (2D) phononic network extends an earlier study on one-dimensional (1D) phononic networks with closed mechanical subsystems. With a special design for the phononic band structures of the waveguides, any two neighboring resonators in the 2D network and the waveguide between them can form a closed mechanical subsystem, which enables nearest neighbor coupling and at the same time circumvents the scaling problems inherent in typical large mechanical systems. The nearest neighbor mechanical coupling allows the theoretical implementation of Molmer-Sorensen quantum gates between spins in adjacent mechanical resonators through phonon-assisted spin transitions. Honeycomblike phononic networks of spin qubits with nearest neighbor coupling can serve as an experimental platform for quantum computing and especially topological quantum error corrections.

The availability of large amounts of data and the necessity of processing it efficiently have led to the rapid development of machine-learning techniques. To name a few examples, artificial-neural-network architectures are commonly used for financial forecasting, speech and image recognition, robotics, medicine, and even research. Direct hardware for neural networks is highly sought for overcoming the von Neumann bottleneck of software implementations. Reservoir computing (RC) is a recent and increasingly popular bioinspired computing scheme that holds promise for efficient temporal information processing. We demonstrate the applicability and performance of RC in a general complex Ginzburg-Landau lattice model, which adequately describes the dynamics of a wide class of systems, including coherent photonic devices. In particular, we propose that the concept can be readily applied in exciton-polariton lattices, which are characterized by unprecedented photonic nonlinearity, opening the way to signal processing at rates of the order of $1\mathrm{Tbit}{\mathrm{s}}^{-1}$.

We report on an experimental study of a laser regime in erbium-doped whispering-gallery-mode (WGM) microspheres under modal-coupling between the co- and counterpropagating modes. The evidence of modal coupling is observed in the relative intensity noise spectrum of several WGM lasers. Cross-correlation measurements are carried out in order to analyze precisely the emission regimes. It is shown that depending on the material constituting the WGM resonator, frequency-locked bidirectional emission or a self-modulated regime could be reached. The control of the laser emission regime of WGM microlasers is of great importance in the aim of applications in microwave optics or optical sensor miniaturization.

We study the structural, optical, and transport properties of sidewall quantum wires (QWRs) on $\mathrm{Ga}\mathrm{As}$(001) substrates. QWRs are grown by MBE on $\mathrm{Ga}\mathrm{As}$(001) substrates prepatterned with shallow ridges. They form as a consequence of material accumulation on the sidewalls of the ridges during the overgrowth of a quantum well (QW) on the patterned surface. The QWRs are approximately 200 nm wide and have emission energies red shifted by 27 meV with respect to the surrounding QW. Spatially resolved spectroscopic PL studies indicate that the QW thickness reduces around the QWRs, thus creating a 4-meV energy barrier for the transfer of carriers from the QW to the QWR. We show that the QWRs act as efficient channels for the transport of optically excited electrons and holes over tens of $\mu \mathrm{m}$ by a high-frequency surface acoustic wave (SAW). These results demonstrate the feasibility of efficient ambipolar transport in QWRs with submicrometer dimensions, photolithographically defined on $\mathrm{Ga}\mathrm{As}$ substrates.

Dielectric laser acceleration (DLA) represents a promising approach to building miniature particle accelerators on a chip. However, similar to conventional rf accelerators, an automatic and reconfigurable control mechanism is needed to scale DLA technology towards high-energy gains and practical applications. We propose a system providing control of the laser coupling to DLA using integrated optics and introduce a component for power distribution using a reconfigurable mesh of Mach-Zehnder interferometers. Using numerical simulations, we show how such a mesh may be sequentially and efficiently tuned to optimize power distribution in the circuit and find that this strategy has favorable scaling properties with respect to size of the mesh.

We exploit the nonlinearity arising from the spin-photon interaction in an $\mathrm{In}\mathrm{As}$ quantum dot to demonstrate phase shifts of scattered light pulses at the single-photon level. Photon phase shifts of close to ${90}^{\circ }$ are achieved using a charged quantum dot in a micropillar cavity. We also demonstrate a photon phase switch by using a spin-pumping mechanism through Raman transitions in an in-plane magnetic field. The experimental findings are supported by a theoretical model that explores the dynamics of the system. Our results demonstrate the potential of quantum-dot-induced nonlinearities for quantum information processing.

A wave plate consisting of dielectric subwavelength gratings (SWGs) with a certain thickness is typically used for polarization control. Since the phase retardation strongly depends on the wavelength, the wave plates made from dielectric SWGs usually operate in narrow or limited bandwidths, which severely hinders their device applications. In this paper, we demonstrate theoretically and experimentally that broadband quarter- and half-wave plates may be achieved in a single layer of high-index-contrast SWG by tailoring the dispersion relations of two orthogonal waveguide-array modes supported by the structure. The broadband SWG-based wave plates demonstrated in this work could be an important step forward in the development of functional polarization conversion devices for practical applications.

This paper pertains to a long-standing debate about the quantification criteria and mechanisms of optical superresolution in microsphere-based nanoscopy. Its super resolution capability has been closely linked in the literature to the width of photonic nanojets. A comprehensive and systematic comparison of the resolution obtained from the point-spread function (PSF) and the nanojet width is provided here. In both cases, the Maxwell equations are rigorously solved in a two-dimensional geometry using similar microsphere parameters. By carrying out the analysis in a rather broad range of parameters (microsphere size, object-to-microsphere separation), the assumption that the standard resolution defined by the PSF can be deduced from the nanojet focusing properties is disproved. The resolution values that result from the PSF and magnification are studied in two regimes: with and without the excitation of whispering-gallery-mode resonances in the microparticle. Unlike previous studies of nanojet focusing, the presented results summarize the true resolution capability of microspheres stemming from the classical point-emission geometry. It is also argued that a higher resolution capability can result from the coherence of illumination or the resonantly increased role of near fields in nanoplasmonic arrays.

A theoretical model for describing the emission spectra of microsphere cavities is presented and its predictions of detailed line shapes of emission spectra associated with whispering gallery modes (WGMs) of various orders in $\mathrm{Zn}\mathrm{O}$ microspheres (MSs) are verified experimentally by photoluminescence (PL) spectroscopy. The interplay of the Purcell effect, quality factor, and leaky modes in spontaneous and stimulated emission spectra related to WGMs of all orders is revealed. The key success of the theory is based on the expansion of the full Green’s function of the MS in terms of all possible resonance modes in complex frequency space, which allows incorporation of contributions from leaky modes, stimulated emission processes, and the Purcell effect. We show that the spontaneous emission spectrum calculated according to Mie theory (without the Purcell effect) is dominated by the contribution of leaky modes, while the spontaneous and stimulated emission enhanced by the Purcell effect is responsible for the main WGM resonance peaks observed experimentally. It is found that the stimulated emission peaks are doubly enhanced by their respective mode quality factor $Q$: one factor from the Purcell effect and the other factor from the photon number derived from the rate equation. After combining all these effects, the theory can provide a quantitative description of fine features of both TE and TM modes (including higher-order modes) observed in the PL spectra of $\mathrm{Zn}\mathrm{O}$ MSs. Surprisingly, it is found that for $\mathrm{Zn}\mathrm{O}$ MS with a diameter larger than 5 $\mu \mathrm{m}$, the PL emission spectrum is dominated by higher-order modes. The quantitative understanding of the interplay of these emission mechanisms should prove useful for optimizing the performance of light-emitting devices based on microresonators.

We suggest a single-photon thermal detector based on the abrupt jump of the critical current of a temperature-biased tunnel Josephson junction formed by different superconductors, working in the dissipationless regime. The electrode with the lower critical temperature is used as a radiation-sensing element, so it is thermally floating and is connected to an antenna or absorber. The warming up resulting from the absorption of a photon can induce a drastic measurable enhancement of the critical current of the junction. We propose a detection scheme based on a threshold mechanism for single- or multiphoton detection. This Josephson-threshold detector indeed has calorimetric capabilities, being able to determine the energy of the incident photon. So, for the realistic setup that we discuss, our detector can work efficiently as a calorimeter for photons from the midinfrared, through the optical, into the ultraviolet, specifically, for photons with frequencies in the range 30 THz to $9\times {10}^4\mathrm{THz}$. Over the whole range of detectable frequencies, we obtain a resolving power significantly larger than one. In order to reveal the signal, we suggest the fast measurement of the Josephson kinetic inductance. Indeed, the photon-induced change in the critical current affects the Josephson kinetic inductance of the junction, which can be read noninvasively through an $LC$ tank circuit that is inductively coupled to the junction. Finally, this readout scheme shows remarkable multiplexing capabilities.

All-dielectric bianisotropic metasurfaces with tailored polarization and spectral responses are studied, and a systematic approach to their design is developed. It is shown that a wide range of polarization transformations over broad bandwidths, as well as multiple bands, can be implemented by cascading subwavelength dielectric gratings. The subwavelength gratings, modeled as anisotropic layers, are cascaded to form metasurfaces that control the polarization of impinging waves. These metasurfaces are a low-loss alternative to their lossy, plasmonic counterparts at optical frequencies, and can replace conventional, bulky optical components. The multifunctional performance and compactness (wavelength-sized thickness) of the proposed devices will also find use at millimeter-wave frequencies. Example metasurfaces are presented and their performance is verified through full-wave simulation. This work demonstrates the flexibility with which cascaded subwavelength gratings can realize unprecedented polarization control with varied spectral responses.

Large-scale fiber-based quantum networks will likely employ telecommunication-wavelength photons of around 1550 nm wavelength to exchange quantum information between remote nodes, and quantum memories, ideally operating at the same wavelength, that allow the transmission distances to be increased, as key elements of a quantum repeater. However, the development of a suitable memory remains an ongoing challenge. Here, we demonstrate the storage and reemission of single heralded 1532-nm-wavelength photons using a crystal waveguide. The photons are emitted from a photon-pair source based on spontaneous parametric down-conversion and the memory is based on an atomic frequency comb of 6 GHz bandwidth, prepared through persistent spectral-hole burning of the inhomogeneously broadened absorption line of a cryogenically cooled erbium-doped lithium niobate waveguide. Despite currently limited storage time and efficiency, this demonstration represents an important step toward quantum networks that operate in the telecommunication band and the development of integrated (on-chip) quantum technology using industry-standard crystals.

Optical studies of single self-assembled semiconductor quantum dots (QDs) have been a topic of intensive investigation over the past two decades. Due to their solid-state nature, their electronic and optical emission properties are affected by the particular crystal structure as well as many-body-carrier interactions and dynamics. In this work, we use a master equation for microstates (MEM) model to study the carrier capture and escape from single QDs under optical nonresonant excitation and under the influence of a two-dimensional (2D) carrier reservoir (the wetting layer). This model reproduces carrier dynamics from power-dependent and time-resolved microphotoluminescence experiments. Due to the random nature of the carrier capture and escape processes, when a single QD is pumped with enough excitation power, the carrier redistribution across the available QD microstates produces an effective double-peaked excitonic decay. This double peak is characterized by a first ultrafast (subnanosecond) and a second conventional (approximately nanosecond) decay. The effective transient photoluminescence shape of the population dynamics is governed by the wetting-layer radiative decay and the exciton capture time.

We demonstrate that a photonic gauge potential, which arises from the phase degree of freedom in dynamic refractive-index modulation and hence is achormatic, can be used to achieve a broadband optical switch using the configuration of a photonic Aharonov-Bohm interferometer (ABI). The resulting ABI switch has a far larger bandwidth and lower cross talk than the conventional Mach-Zehnder interferometer. Using coupled-mode theory and full-wave numerical modeling, we compare the response of the two interferometers in the presence of nonidealities. Our results indicate the importance of the photonic gauge potential for broadband optical signal processing.

Spatial differentiation is a fundamental mathematical operation used in many fields of science or engineering. Recently, optical analog computing of spatial differentiation, which can overcome the speed and energy limitations of digital differentiation techniques, has been realized in some nano systems such as the excitation of surface plasmon polaritons (SPPs). However, the inevitable backscattering of SPP propagation around defects or discontinuities may bring some undesired noises to the output signal. In this work, we firstly design a backscattering-immune spatial differentiator with the utilization of the nonreciprocal plasmonic platform, where the topologically protected one-way SPP leaky mode exists, in the terahertz (THz) region. Guided by the nonreciprocal spatial coupled-mode theory, we show that the ideal transfer function of the first-order spatial differentiation is able to be realized by subtly tuning the balance between the asymmetric leaky rate stemming from the nonreciprocal nature and intrinsic absorption rate of the system. Full wave simulations show that the first-order differentiation without backscattering around defects can be precisely implemented by using the designed nonreciprocal differentiator. Our proposed backscattering-immune spatial differentiator can find widespread applications for robust edge detection and image processing in the THz frequency range.

We demonstrate an absolute frequency measurement using the ultrabroadband comb generated in a periodically poled lithium niobate waveguide (PPLN WG). Based on the measured frequency information, we determine that the spectral broadening in the PPLN WG is due to quadratic nonlinearity. We also show that the self-referenced carrier-envelope offset beat signal observed in the ultrabroadband comb is generated based on the 2f–3f interference of different comb series. This study is not only useful for the practical application of frequency measurement but is also suitable for basic research of nonlinear optics.

We present a powerful method to implement a metasurface with the ability to convert a wave with linear polarization into multiple beams with orbital angular momentum (OAM) such that each beam possesses a flexible topological charge, independent direction, and polarization state. As opposed to conventional geometric-phase-based metasurfaces, the proposed method provides highly dense scalar and full tensor impedance textures governed by holography theory, thereby continuously controlling the main beam directions and broadening the operational bandwidth. In order to demonstrate the validity of this approach, we design two metasurfaces converting a linearly polarized beam emitted from an ordinary horn into OAM-carrying waves that have linear and circular polarizations with diverse topological charges. The theoretical and numerical results show that the two metasurfaces can generate OAM beams from 14 to 16 GHz with stable phase and intensity profiles of the near-field wave front over the entire band.

We theoretically and experimentally examine the spectroscopy of warm atoms traversing the evanescent field of a guided mode in silica glass. The subwavelength scale of the evanescent field leads to a large spectral broadening dominated by transit-time dephasing, creating a lineshape quite different from that of atoms in free space, and also distinct from that arising from transit-time-limited interactions in a field with a Gaussian intensity profile. We show reasonable agreement between our modeling of this scenario and experimental $D_2$-line spectra obtained from an exposed-core fiber immersed in rubidium vapor.

Laguerre-Gaussian (LG) beams and their orbital angular momentum (OAM) have found a plethora of applications—from quantum information to gravitational-wave detection. In this work, we probe the OAM of LG beams by using a digital micromirror device programmed as a single slit. We observe the diffraction patterns of the beams through the slit and we show that we can determine the OAM of the incoming beams from the changes in the diffraction pattern as the slit is moved transversely. Moreover, we demonstrate that we can access higher OAM modes compared to previous works involving slit diffraction. Our results are valuable in the study of OAM especially for beams where the current technology for detection is not yet mature, for example, beams whose wavelengths are in the terahertz regime or beams of higher energy such as, electron vortex beams.

A resistive coupling circuit is used to model the recently discovered dissipative coupling in a hybridized cavity photon-magnon system. With this model as a basis we design a planar cavity in which a controllable transition between level attraction and level repulsion can be achieved. This behavior can be quantitatively understood using an LCR-circuit model with a complex coupling strength. Our work therefore develops and verifies a circuit method to model level repulsion and level attraction and confirms the universality of dissipative coupling in the cavity photon-magnon system. The realization of both coherent coupling and dissipative coupling in a planar cavity may provide new avenues for the design and adaptation of dissipatively coupled systems for practical applications in information processing.