Nondegenerate two-photon absorption (ND TPA) is studied in a nanophotonic silicon waveguide in a configuration such that the dispersion of the nonlinear absorption and refraction cannot be ignored. It is shown that a signal wave around 1330 nm can be strongly absorbed by ND TPA with a pulse of 6-W peak power at 1850 nm. Experiments are performed with pulses of about 200 fs in a dispersion engineered waveguide in which the pump and signal pulses propagate at a similar group velocity. The experimental results are very well reproduced by numerical simulations of two coupled generalized nonlinear Schrödinger equations (GNLSEs). We show that the nonlinear dynamics can be well described by a single GNLSE despite the wavelength separation between the pump and the signal waves. We also demonstrate that in silicon nanophotonic waveguides, the dispersion of the nonlinear absorption is much larger than the dispersion of the Kerr effect. This could have an impact in the design of all-optical functions based on ND TPA, as well as on the study of supercontinuum and frequency-comb generation in integrated semiconductor-on-insulator platforms.

Both experiments and simulations show that the polarization state and propagation path of the Bloch surface waves sustained on a dielectric multilayer can be manipulated with the grooves inscribed on this multilayer. These grooves can be easily producible, accessible, and controllable. Various nanodevices for the Bloch surface waves, such as the launcher, beam splitter, reflector, polarization rotator, and even the photonic single-pole double-throw switch, are all experimentally realized with the properly designed grooves, which are consistent with the numerical simulations. The proposed devices will be basic elements for the two-dimensional photonic system and will find numerous applications, including integrated photonics, molecular sensing, imaging, and micromanipulation.

Achieving exact balance between spatially separated gain and loss is generally regarded as a necessary condition for parity-time- (PT-)symmetric optical systems. We introduce generalized PT- (GPT-)symmetric optical structures, which have an asymmetric and unbalanced gain-loss profile, while exhibiting similar scattering properties and phase transitions as their traditional PT-symmetric counterparts. In particular, we show that the concept of GPT symmetry may help to reduce the threshold gain in achieving newly discovered PT-enabled applications, such as the coherent perfect absorber laser and exceptional points. The concept proposed herein will facilitate the practice of PT-symmetric optical and photonic devices by offering greater design flexibility to tailor gain-loss profiles and their thresholds.

We report a mode-locked laser design using $\mathrm{Yb}$-doped fiber lasers for low-noise frequency-comb generation. The frequency comb covers the spectral range from 700 to 1400 nm. Although this range is more practical for many measurements than that produced by the more commonly used Er-fiber lasers, it has been addressed in only a handful of reports, mainly due to the difficulty of generating a fully coherent supercontinuum at $1\mu \mathrm{m}$. We overcome this difficulty by a tailored design of the mode-locking dynamics that succeeds in generating energetic 33-fs-long pulses without even using higher-order-dispersion compensation, while ensuring that the laser operates with net zero cavity dispersion for low-noise supercontinuum generation. After locking to a $\mathrm{Cs}$ atomic clock, this frequency comb is used for absolute-frequency measurements of a $\mathrm{Nd}:\mathrm{YAG}\text{−}{\mathrm{I}}_2$ laser to verify its accuracy by comparison with results from the International Committee for Weights and Measures. After this verification, it is further used to measure the absolute frequency of a 543-nm two-mode stabilized He-Ne laser, which is routinely used for length measurements in our institute, thus verifying its practical utility in metrology applications. The entire setup is built with readily available components for easy duplication by other researchers.

$\mathrm{Ce}$-activated $A^{2+}{B}^{4+}{\mathrm{O}}_3$ perovskites represent a class of compounds currently under active exploration for their potential as scintillators. Depending on the chemistry and synthesis conditions, perovskites can crystallize in multiple crystal structures, and a $\mathrm{Ce}$ substitutional dopant in an $AB{\mathrm{O}}_3$ perovskite can adopt different charge states (i.e., ${\mathrm{Ce}}^{3+}$ or ${\mathrm{Ce}}^{4+}$) as well as different substitutional sites (namely, the 12-fold-coordinated $A$ site or the octahedrally coordinated $B$ site). Here, we use first-principles density-functional-theory- and hybrid-functional-based computations to study relative trends in the structure, energetics, and electronic structure of bulk $AB{\mathrm{O}}_3$ perovskites, where $A=\mathrm{Ca},\mathrm{Sr}$, or $\mathrm{Ba}$ and $B=\mathrm{Hf}$ or $\mathrm{Zr}$. Subsequently, we consider the relative energetics of preferential solution sites for $\mathrm{Ce}$ as a function of charge states, chemical potential, and defect configurations. Our results reveal that while ${\mathrm{Ce}}^{3+}$ or ${\mathrm{Ce}}^{4+}$ defects can be thermodynamically stable, depending on the choice of the substitutional site and synthesis conditions (i.e., prevailing chemical potential), only ${\mathrm{Ce}}^{3+}$ dopant at the $A$ site leads to an electronic structure that can exhibit scintillation. Our comparative analysis shows that while the positions of the $5d^1$ and $4f$ levels of ${\mathrm{Ce}}^{3+}$ as a dopant at the $A$ site are favorably placed in the band structure, these levels are consistently higher for the ${\mathrm{Ce}}^{4+}$ charge state and are unlikely to manifest any luminescence. The findings of this study are also discussed in relation to previously reported results and display excellent agreement with past experimental observations. In general, it is demonstrated that control of the $\mathrm{Ce}$ charge state and local chemical environment can be used—in addition to band-gap and band-edge engineering—to manipulate the relative position of scintillating states with respect to the valence-band maximum and conduction-band minimum. While this study specifically focuses on perovskites, the results (in particular, the relative alignment of the positions of the $5d^1$ and $4f$ levels of $\mathrm{Ce}$ dopant as a function of the activator’s charge state) are expected to be general and thus transferable to other chemistries.

Driven in part by the extreme requirements of gravitational-wave detection, recent research about interferometric phase estimation has focused on the use of nonclassical states of light in order to beat the shot-noise limit, allowing further improvements in sensitivity once the detected optical power has been maximized. Still, for a large class of interferometric applications, the efficient use of these states remains prohibitively complicated. Seeking simple improvements to instruments illuminated by classical laser light, we revisit the passive or unmodulated Mach-Zehnder interferometer by structuring the estimation problem in the form of a linear model for which an unbiased estimator attaining the Cramér-Rao bound is easily computed. This paper compares the performance of such linear and efficient estimators for several detection schemes used with two-output and four-output (in-phase and quadrature) interferometers. We find that the independent monitoring of all available output ports leads to an overall sensitivity that is, in most cases, better than that of balanced detection and single-output detection. In addition, it allows the cancellation of the technical amplitude noise contribution at all operating phases without resorting to modulation techniques. This seldom used but simple detection scheme should therefore be considered when designing instruments for operation close to and perhaps below the shot-noise limit.

We present how to physically realize the recently introduced active plasmon-injection loss compensation scheme. Particularly, we show that the characteristics of the auxiliary source described in the active plasmon-injection scheme, including tunable narrow band and selective amplification via convolution, can be realized by using a hyperbolic metamaterial functioning as a near-field spatial filter. Besides loss compensation, the proposed approach with the near-field spatial filter can be useful for enhanced near-field superlensing and real-time high-resolution edge detection.

We propose a single-photon detector based on a superconducting quantum interference device (SQUID) with superconductor–normal-metal–superconductor Josephson weak links. One of the two Josephson junctions is connected to an antenna, and is heated when a photon is absorbed. The increase of the weak-link temperature exponentially suppresses the Josephson critical current, thereby inducing an asymmetry in the SQUID. This generates a voltage pulse across the SQUID that can be measured with a threshold detector. Realized with realistic parameters, the device can be used as a single-photon detector—and as a calorimeter, since it is able to discriminate photon frequencies above $5\mathrm{THz}$ with a signal-to-noise ratio larger than $20$. The performance of the detector is robust with respect to working temperatures between $0.1$ and $0.5\mathrm{K}$, and with respect to thermal noise perturbation.

We investigate a hybrid quantum system where an ensemble of nitrogen-vacancy (NV) centers in diamond is interfaced with a piezomagnetic superlattice that supports surface-phonon polaritons (SPHPs). We show that the strong magnetic coupling between the collective spin waves in the NV spin ensemble and the quantized SPHPs can be realized, due to the subwavelength nature of the SPHPs and relatively long spin-coherence times. The magnon-polariton coupling allows different modes of the SPHPs to be mapped and orthogonally stored in different spatial modes of excitation in the solid medium. Because of its easy implementation and high tunability, the proposed hybrid structure with NV spins and piezoactive superlattices could be used for quantum memory and quantum computation.

As an attractive degree of freedom in electromagnetic (EM) waves, the orbital angular momentum (OAM) enables infinite communication channels for both classical and quantum communications. The exploration of OAM generation inspires various designs involving spiral phase plates, antenna arrays, metasurfaces, and computer-generated holograms. In this work, we theoretically and experimentally demonstrate an approach to producing OAM carrying EM waves by a point defect in three-dimensional (3D) photonic crystals (PCs). Simultaneous excitation of two vibrational-defect states with an elaborately engineered phase retardation generates a rotational state carrying OAM. Through converting guided waves in a line defect to localized waves in a point defect and then to radiated vortex waves in free space, the lowest four OAM-mode emitters, i.e., OAM indices of $\pm 1$ and $\pm 2$, are successfully realized. This work offers a physical mechanism to generate OAM by PCs, especially when the OAM generation is to be integrated with other designs.

The influence of UV light-induced pitch contraction and dilation on the electroconvection patterns (ECPs) of a chiral nematic liquid crystal containing a photoresponsive chiral dopant is investigated in planar-aligned cells. It is observed that the helical twisting power of the dopant changes (even undergoes handedness inversion) under UV irradiation; consequently, the pitch and the direction of the convection rolls in ECPs (being either parallel with or perpendicular to the surface alignment) could be controlled by the UV intensity and the ac voltage. In contrast to traditional methods, where the pitch is modulated by electric or thermal fields, our method of applying a light field allows a remote and contactless manipulation of the pitch, which is easily detectable via the morphological changes of ECPs. The ability to control the orientation of ECPs by tuning the light intensity can conveniently be utilized as an optical grating, allowing switchable, dual-mode operation.

Electrically active defects have a significant impact on the performance of electronic devices based on wide-band-gap materials. This issue is ubiquitous in diamond science and technology, since the presence of charge traps in the active regions of different classes of diamond-based devices (detectors, power diodes, transistors) can significantly affect their performance, due to the formation of space charge, memory effects, and the degradation of the electronic response associated with radiation-induced damage. Among the most common defects in diamond, the nitrogen-vacancy (N-V) center possesses unique spin properties that enable high-sensitivity field sensing at the nanoscale. Here, we demonstrate that N-V ensembles can be successfully exploited to perform direct local mapping of the internal electric-field distribution of a graphite-diamond-graphite junction exhibiting electrical properties dominated by trap- and space-charge-related conduction mechanisms. By means of optically detected magnetic resonance measurements, we performed both point-by-point readout and spatial mapping of the electric field in the active region at different bias voltages. In this novel “self-diagnostic” approach, defect complexes represent not only the source of detrimental space-charge effects but also a unique tool for their direct investigation, by providing an insight on the conduction mechanisms that could not be inferred in previous studies on the basis of conventional electrical and optical characterization techniques.

In this study, we elaborate on the recent concept of metagratings proposed in Ra’di et al. [Phys. Rev. Lett. 119, 067404 (2017)] for efficient manipulation of reflected waves. Essentially, a metagrating is a set of 1D arrays of polarization line currents which are engineered to cancel scattering in undesirable diffraction orders. We consider a general case of metagratings composed of $N$ polarization electric line currents per supercell. This generalization is a necessary step to totally control diffraction patterns. We show that a metagrating having $N$ equal to the number of plane waves scattered in the far field can be used to control the diffraction pattern. To validate the developed theoretical approach, anomalous and multichannel reflections are demonstrated with 3D full-wave simulations in the microwave regime at $10\mathrm{GHz}$. The results are interesting for the metamaterials community as they allow one to significantly decrease the number of used elements and simplify the design of wavefront manipulation devices, which is very convenient for optical and infrared frequency ranges. Our findings may also provide a way to develop efficient tunable antennas in the microwave domain.

The Pockels effect is theoretically forbidden in centrosymmetric media. However, centrosymmetry is broken at the interface and second-order nonlinear effects such as the Pockels effect can be detected. In this work, we report the experimental observation of the Pockels effect at the interface of two isotropic solids, amorphous sol-gel spin-coated $\mathrm{Ti}{\mathrm{O}}_2$ and polycrystalline magnetron-sputtered metal. A micron-thick voltage-driven Fabry-Perot resonator is designed using these materials to experimentally detect the optical response of their interface to an applied electric field. Using the experimental data, nonlinear two-dimensional susceptibility ${\chi }_{2\mathrm{D}}^{(2)}(\omega ;\omega ,0)$ for the interface is obtained with two different theoretical approaches and found to be $3.5\pm 1.9\times {10}^7{\mathrm{pm}}^2/\mathrm{V}$. The order of magnitude is similar to a recent report of ${\chi }_{\mathrm{surface}}^{(2)}(2\omega ;\omega ,\omega )\sim {10}^6,{\mathrm{pm}}^2/\mathrm{V}$ for the silicon-air interface obtained from surface second-harmonic generation measurements. Isotropy usually prohibits the design of thin-film active photonic components, such as electro-optic modulators, using amorphous materials which can be deposited by simple sol-gel spin coating or sputtering. Our demonstration of the Pockels effect at the $\mathrm{Ti}{\mathrm{O}}_2$ and metal interface presents it as a new electro-optic material with potential applications for integrated active plasmonic devices.

Light manipulation through spin-orbit coupling opens new perspectives in photonics and particularly in integrated optics. The reverse spin Hall effect, where the guided wave direction affects the polarization properties of the light scattered by a nanostructure, is a key effect for the development of new functionalities at the connection between integrated structures and free space, and could find application in chiral quantum optics, modulation, and multiplexing. We show that metasurfaces represent a promising platform for the reverse spin Hall effect. Using a periodic array of Λ-shaped metallic nanoantenna, we control the polarization of the extracted light with the guided wave-propagation direction, but also the number of output directions and the polarization of each one. These results and the versatility of metasurfaces for the spin Hall effect could be extended to various frequencies and materials, such as silicon photonics at telecom wavelength.

We study theoretically the dynamics of a driven-dissipative qubit-resonator system. Specifically, a transmon qubit is coupled to a transmission-line resonator; this system is considered to be probed via a resonator, by means of either continuous or pulsed measurements. Analytic results obtained in the semiclassical approximation are compared with calculations in the semiquantum theory as well as with the previous experiments. We demonstrate that the temperature dependence of the resonator frequency shift can be used for the system thermometry and that the dynamics, which display a pinched-hysteretic curve, can be useful for realization of the memory devices termed quantum memcapacitors.

Optical frequency combs enable state-of-the-art applications including frequency metrology, optical clocks, astronomical measurements, and sensing. Recent demonstrations of microresonator-based Kerr frequency combs or microcombs pave the way to scalable and stable comb sources on a photonic chip. Generating microcombs in the short-wavelength range, however, has been limited by large material dispersion and optical loss. Here we demonstrate a scheme for efficiently generating microcombs near the edge of the visible spectrum in a high-$Q$ aluminum nitride microring resonator. The enhanced Pockels effect strongly couples infrared and near-visible modes into hybrid mode pairs, which participate in the Kerr microcomb generation process and lead to strong Cherenkov-like radiation in the near-visible band an octave apart. A surprisingly high on-chip conversion efficiency of 22% is achieved from a pulsed pump laser to the near-visible comb. As a result of pulse pumping, the generated microcombs are in the chaotic state. We further demonstrate a robust frequency tuning of the near-visible comb by more than one free spectral range and apply it to the absorption spectroscopy of a water-based dye molecule solution. Our work is a step towards high-efficiency visible microcomb generation and its utilization, and it also provides insights into the significance of the Pockels effect and its strong coupling with Kerr nonlinearity in a single microcavity device.

Silicon nitride ($\mathrm{Si}\mathrm{N}$) is one of the emerging semiconductor materials that are used in both linear and nonlinear all-optical integrated devices. Its excellent dielectric properties, high material stability, and dispersion controllability are attractive to on-chip optical communications, optical signal processing, and even imaging devices. However, a large-aperture $\mathrm{Si}\mathrm{N}$ metalens with high numerical aperture (NA) is limited by the low refractive index and nanofabrication technologies, particular in the visible spectrum. Here, we experimentally realize the visible-spectrum $\mathrm{Si}\mathrm{N}$ divergent metalenses by fabricating the 695-nm-thick hexagonal arrays with a minimum space of 42 nm between adjacent nanopillars. A micro-size divergent metalens with $\mathrm{NA}\sim 0.98$ and subwavelength resolution enables objects to be shrunk as small as a single-mode fiber core. Another centimeter-size $\mathrm{Si}\mathrm{N}$ divergent metalens with over half a billion nanopillars, made by using the mature CMOS-compatible fabrication process, exhibits high-quality wide-angle imaging. Our findings may open a new door for the miniaturization of optical lenses in the fields of optical fibers, microendoscopes, and smart phones, as well as the applications in all-sky telescopes, large-angle beam shaping, and near-eye imaging.

We provide detailed insights into a link between structural and optical disorder of resonant optical structures, in particular, distributed Bragg reflectors (DBRs) and resonant microcavities ($\mu \mathrm{Cs}$). The standard (targeted) DBR structures have periodic square-wave–like refractive-index profiles, and their optical performance is determined by the refractive-index ratio of the two applied materials ($n_{12}=n_1/n_2$, $n_1>n_2$) and the number of DBR periods ($N$). It is well known that its structural disorder strongly affects its optical properties, but, despite that, this influence has not been quantitatively addressed in the literature. We propose a precise quantitative definition for a structural disorder of a single DBR unit cell (disorder factor $D_F$), completing the set of DBR fundamental parameters ($n_{12}$, $N$, $D_F$). Then we expose the basis for the effective refractive-index approximation (ERIA), showing that, as long as DBR optical properties are concerned, the influence of increasing structural disorder ($D_F\uparrow$) is virtually identical to the influence of decreasing refractive-index ratio ($n_{12}\downarrow$), with the latter influence being easily quantified. Making use of the ERIA method, simple analytical formulas, which enable rapid insights into the reflectivity and stop-band width of DBRs with different types of transient layers at the heterointerfaces, are derived and the results validated, via both transfer-matrix simulations and direct experimental measurements of imperfect DBRs. The insights of the ERIA method are then further applied on resonant $\mu \mathrm{Cs}$, providing a comprehensive link between their structural disorder and subsequent deterioration of their quality ($Q$) factor.

We experimentally demonstrate that a linear dipole is not restricted to emit linearly polarized light, provided that it is embedded in the appropriate nanophotonic environment. We observe emission of various elliptical polarizations by a linear dipole, including circularly polarized light, without the need for birefringent components. We further show that the emitted state of polarization can theoretically span the entire Poincaré sphere. The experimental demonstration is based on elongated gold nanoparticles (nanorods) deposited on an optical nanofiber and excited by a free-space laser beam. The light directly collected in the guided mode of the nanofiber is analyzed in regard to the azimuthal position and orientation of the nanorods, observed by means of scanning electron microscopy. We demonstrate a mapping between purely geometrical degrees of freedom of a light source and all polarization states that could open the way to alternative methods for polarization control of light sources at the nanoscale.