The capability to tailor the dispersion and the cut-off frequency of waveguides is of importance, as these essential parameters govern the operating frequency range and the waveguide dimension. Here, we propose the concept of substrate-integrated impedance surface (SIIS) that enables arbitrary control of propagation characteristics of closed-shape waveguides. Specifically, we develop a theoretical framework for the simplest form of SIIS constituted by a one-dimensional array of blind vias, which is equivalent to a homogenized surface capacitance embedded in the waveguide. We theoretically and experimentally demonstrate that loading a substrate-integrated waveguide (SIW) with a capacitive SIIS can effectively reduce its cut-off frequency, regardless of the transverse dimension of the SIW. In addition, a SIIS-loaded SIW exhibits several intriguing phenomena, such as the slow-wave guiding properties and the local field concentration. This SIIS-loading technique may open up new possibilities for miniaturization of various waveguide-based components and for enhancement of their uses in microwave sensing and nonlinear functions.

In conventional all-dielectric one-dimensional photonic crystals (1DPCs), photonic gaps will shift toward short wavelengths (blueshift) as the incident angle increases for both transverse magnetic (TM) and transverse electric (TE) polarizations. We theoretically and experimentally achieve redshift gaps in 1DPCs composed of alternative hyperbolic metamaterials (HMMs) and dielectrics for TM polarization based on the anomalous wavevector dispersion of HMMs. The HMM is mimicked by layered titanium dioxide and silver with a subwavelength unit cell. However, the gaps remain to be blue-shifted for TE polarization. Therefore, the edges of such gaps can be utilized for high-efficiency wide-angle polarization selection. We achieve efficient polarization selection for incident angles ranging from 50° to 80° at the wavelength of 365 nm.

We investigate refractive-index changes caused by femtosecond photoionization in a gas-filled hollow-core photonic-crystal fiber. Using spatially-resolved interferometric side-probing, we find that these changes live for tens of microseconds after the photoionization event — eight orders of magnitude longer than the pulse duration. Oscillations in the megahertz frequency range are simultaneously observed, caused by mechanical vibrations of the thin-walled capillaries surrounding the hollow core. These two nonlocal effects can affect the propagation of a second pulse that arrives within their lifetime, which works out to repetition rates of tens of kilohertz. Filling the fiber with an atomically lighter gas significantly reduces ionization, lessening the strength of the refractive-index changes. The results will be important for understanding the dynamics of gas-based fiber systems operating at high intensities and high repetition rates, when temporally nonlocal interactions between successive laser pulses become relevant.

We present analytical formulae for light propagation through a multilayer of dielectric media as a function of the “total charge” and the “accumulated charge” that we propose by the transfer-matrix method. We redefine the concept of the total charge introduced by Liu et al. [J. Phys. Condens. Matter. 29, 455303 (2017)] and newly define the accumulated charge to determine the magnitude of electric field inside the multilayers analytically. In addition, we derive an analytical expression for the group velocity of light in each layer. The purpose of this work is to predict which kind of sequence would provide the strongest amplification of the electric field inside the multilayer as well as to find the sequence that gives the lowest group velocity. We find that the symmetric alternating sequence gives the best amplification of the electric field and the lowest mean group velocity compared with the other sequences.

Luminescence (nano)thermometry is an important technique for remote temperature sensing. The recent development of lanthanide-doped nanoparticles with temperature-dependent emission has expanded the field of applications, especially for ratiometric methods relying on the temperature variation of relative emission intensities from thermally coupled energy levels. Analysis and calibration of the temperature dependence is based on a Boltzmann equilibrium for the coupled levels. To investigate the validity of this assumption, we analyze and model thermal equilibration for ${\mathrm{Eu}}^{3+}$ ${}^5{D}_1$ and ${}^5{D}_0$ emission in $\mathrm{Na}{\mathrm{YF}}_4$. The results show that for low ${\mathrm{Eu}}^{3+}$ concentrations, temperature-dependent multiphonon relaxation can accurately explain both the intensity ratio and emission decay dynamics. The analysis also reveals that a Boltzmann equilibrium is not realized in the temperature regime investigated (300–900 K). By increasing the ${\mathrm{Eu}}^{3+}$ concentration, cross relaxation between neighboring ${\mathrm{Eu}}^{3+}$ ions enhances ${}^5{D}_1-{}^5{D}_0$ relaxation rates and extends the temperature range in which emission intensity ratios can be used for temperature sensing (500–900+ K). The results obtained are important for recognizing, understanding, and controlling deviations from Boltzmann behavior in luminescence (nano)thermometry. By varying the dopant concentration, the range for accurate temperature sensing can be adjusted. These insights are crucial in the development and understanding of reliable temperature sensors.

The enhancement of stimulated Raman scattering (SRS) with a $\mathrm{Ga}\mathrm{As}$/$\mathrm{Al}\mathrm{As}$ intermixed superlattice that works as a ${\chi }^{(3)}$-quasi-phase-matched structure is studied, where such Kerr-induced effects as four-wave mixing (FWM), self-phase-modulation (SPM), cross-phase-modulation (XPM), and two-photon absorption (TPA) are included. In particular, the efficiency of anti-Stokes generation is enhanced here; anti-Stokes generation inherently has an extremely small efficiency due to a phase mismatch in the interaction of the pump, Stokes, and anti-Stokes waves (while the efficiency of Stokes generation is sufficiently large because of no such phase mismatch). The superlattice enhances the anti-Stokes efficiency up to the order of ${10}^3$ when compared with that without the superlattice, particularly at a small pump intensity. In this enhancement, it is seen that there is an efficiency boost via simultaneous FWM. In this situation, it is shown how much SPM and XPM degrade the efficiency enhancement. Furthermore, an optimal superlattice length is identified that provides the highest efficiency. The degradation of the efficiency at the optimized length due to TPA is also analyzed. Finally, to gain more anti-Stokes efficiency (or control the sizes of the Stokes and anti-Stokes efficiencies), a photonic-band-gap cavity structure is proposed.

Phase-change materials are attracting much attention in the scientific and engineering communities owing to their applications and underlying basic phenomena. ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ is reversible-phase-change material (amorphous to crystalline and vice versa) that is used for optical data storage and phase-change random-access memory and has recently been explored for use as a reversible near-infrared (NIR) window [Singh et al., Appl. Phys. Lett. 111, 261102 (2017)]. For a reversible NIR window, large transmission contrast between two phases and low phase-transition temperature are required to reduce the power consumption. In the present work, phase transition in thermally deposited $({\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5{)}_{100-x}{\mathrm{Ag}}_x$ ($x=0$, 1, 3, 5, and 10) thin films is achieved by vacuum thermal annealing. Transmission sharply decreases with phase transition in the NIR region. ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ shows large transmission contrast (more than $50$%) in the wavelength range from 1600 to 3200 nm with phase transition from an amorphous to a hexagonal-close-packed structure at $260{}^{\circ }\mathrm{C}$. In $({\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5{)}_{90}{\mathrm{Ag}}_{10}$ thin films, a similar transmission contrast is achieved at a comparatively lower temperature ($160{}^{\circ }\mathrm{C}$) due to reduction of the rocksalt phase. Distortion of the host lattice with addition of 10% $\mathrm{Ag}$ is confirmed from the drastic change in the density of states in the valence band and the shift in core-level (3d) spectra of $\mathrm{Ag}$, $\mathrm{Sb}$, and $\mathrm{Te}$. This distortion enables a hexagonal-close-packed phase in $({\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5{)}_{90}{\mathrm{Ag}}_{10}$ thin films to be obtained at $160{}^{\circ }\mathrm{C}$. $({\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5{)}_{90}{\mathrm{Ag}}_{10}$ could be a potential candidate for a reversible NIR window as it requires less power to achieve phase transition and high transmission contrast.

Routing electromagnetic energy at a scale smaller than the wavelength is a highly sought functionality in a variety of applications, including compact lightweight satellite communications, slow-waves sensors, all-optical information processing, and energy harvesting. Unfortunately, strong field confinement at this scale requires the use of coupled subwavelength resonators, implying a large sensitivity to geometrical imperfections and disorder-induced backscattering. We propose a very unconventional solution to this problem by exploiting the interface modes occurring at the boundary between two chiral metamaterials composed of resonant metamolecules with opposite chirality. Our numerical and experimental results demonstrate the inherent robustness of these interface states to disorder in both the position and resonance frequency of the metamaterial’s meta-atoms. By computing transmission averages over many realizations of disorder, we quantitatively demonstrate the superiority of this form of subwavelength routing over previously proposed designs, including frequency-defect lines, symmetry-based topological edge modes, and Valley-Hall interface states.

As the hyperentanglement of photon systems presents lots of unique opportunities in high-capacity quantum networking, the hyperentanglement purification protocol (hyper-EPP) becomes a vital project work and the quality of its accomplishment attracts considerable attention recently. Here we present the first theoretical scheme of faithful hyper-EPP for nonlocal two-photon systems in two degrees of freedom (DOFs) by constructing several fidelity-robust quantum circuits for hyper-encoded photons. With this faithful hyper-EPP, the bit-flip errors in both the polarization and spatial-mode DOFs can be efficiently corrected and the maximal hyperentanglement in two DOFs could in principle be achieved by performing the hyper-EPP multiple rounds. Moreover, the fidelity-robust quantum circuits, parity-check quantum nondemolition detectors, and swap gates make this hyper-EPP works faithfully as the errors coming from practical scattering, in these quantum circuits, are converted into a detectable failure rather than infidelity. Furthermore, this hyper-EPP can be directly extended to purify photon systems entangled in single polarization or spatial-mode DOF and that hyperentangled in polarization and multiple-spatial-mode DOFs.

The realization of high-$\beta$ lasers is one of the prime applications of cavity QED promising ultra-low thresholds, integrability, and reduced power consumption in the field of green photonics. In such nanolasers, spontaneous emission can play a central role even above the threshold. By going beyond rate-equation approaches, we revisit the definition of a laser threshold in terms of the input-output characteristics and the degree of coherence of the emission. We demonstrate that there are new regimes of cavity-QED lasing, realized, e.g., in high-$Q$ nanolasers with extended gain material, for which the two can differ significantly such that coherence is reached at much higher pump powers than required to observe the thresholdlike intensity jump. Against the common perception, such devices do not benefit from high-$\beta$ factors in terms of power reduction, as a significant amount of stimulated emission is required to quieten the spontaneous emission noise.

We theoretically demonstrate the nontrivial transmission properties of a graphene-insulator-metal waveguide segment of deeply subwavelength scale. We show that, at midinfrared frequencies, the graphene-covered segment allows for the resonant transmission through the graphene-plasmon modes as well as the nonresonant transmission through background modes, and that these two pathways can lead to a strong Fano interference effect. The Fano interference enables a strong modulation of the overall optical transmission with a very small change in graphene Fermi level. By engineering the waveguide junction, it is possible that the two transmission pathways perfectly cancel each other out, resulting in a zero transmittance. We theoretically demonstrate the transmission modulation from 0% to 25% at 7.5-µm wavelength by shifting the Fermi level of graphene by a mere 15 meV. In addition, the active region of the device is more than 50 times shorter than the free-space wavelength. Thus, the reported phenomenon is of great advantage to the development of on-chip plasmonic devices.

Active research on mesoscopic quantum systems has increased our understanding of and ability to control quantum objects, allowing the construction of a universal form for quantum networks that consist of more than one physical system. This kind of quantum network is anticipated to enable the building of quantum infrastructure, such as long-distance quantum communication and distributed quantum computers, and motivates the establishment of photonic quantum interfaces that are compatible with physical systems. Here, a universal photonic quantum interface is experimentally developed with the benefit of a unique, specially designed entangled photon source. The detailed experimental results show that this configuration can satisfy all the urgent demands for a photonic quantum interface, including the accurate matching of the working wavelength and bandwidth and specifically, the entanglement ability ($F=89.6\mathrm{\%}$, $S=2.36\pm 0.03$). The realization of this universal photonic quantum interface is expected to expedite the construction of much more complex quantum networks and to be a major step in the area of optical engineering and control.

Based on standard silicon-on-insulator platforms, we propose an interferometer structure operating at telecom wavelengths that is driven by pump-induced nonlinear Kerr-effect. Special attention is given to the waveguide design in achieving the smallest full-vectorial effective mode area corresponding to the largest nonlinear phase shift. We present that this work is so far, in theory, the most promising approach for high-speed optical-driven quantum interference, whose realizability is reflected in two advantages: (1) the proposed strategy mitigates the pump-induced noise from the sideband, spontaneous four-wave mixing, and spontaneous Raman scattering and (2) the pulsed pump at $2.2\mu \mathrm{m}$ facilitates a relatively higher nonlinear response and negligible free-carrier absorption.

In this work the polarizability of a subwavelength core-shell sphere is considered, where the shell exhibits a radially inhomogeneous permittivity profile. The mathematical treatment of the electrostatic polarizability is formulated in terms of the scattering potentials and the corresponding scattering amplitudes. As a result, a generalized expression of the polarizability is presented to be dependent of the radial inhomogeneity function. The extracted general model is applied for two particular cases, i.e., a power-law profile and a new class of permittivity profiles that exhibit exponential radial dependence. The proposed analysis quantifies in a simple manner the inhomogeneity effects, allowing the direct implementation of naturally or artificially occurring permittivity inhomogeneities for a wide range of applications within and beyond the metamaterial paradigm. Specifically, a special case of symmetric-antisymmetric resonant plasmonic degeneracy is identified and shown for the case of a core-shell sphere with a power-law permittivity profile. This degeneracy could be used for the experimental identification of inhomogeneity-induced effects or for applications where a strong coupling resonant regime is required. Furthermore, the described analysis opens avenues towards the phenomenological and first-principles modeling of the electrodynamic scattering effects for graded-index plasmonic particles at the nanoscale. Finally, such a description can be readily used either for the benchmarking of novel computational methods incorporating inhomogeneous materials or for inverse scattering purposes.

Subnatural-linewidth single photons are generated from atomic ensembles and temporally reshaped as desired so that their temporal modes are eligible for constructing qubits. However, the photon-counting measurement scheme most commonly used in the time-bin basis does not directly give the complex coefficients between time-bins. On the other hand, a mode-sensitive measurement technique, such as homodyne tomography, is hard to implement since a mode-restricting cavity is not allowed in this reshaped photon source. Here, we utilize a cavity-free homodyne detection scheme for reconstructing the temporal-density matrix for the subnatural-linewidth single photons generated from a cold atomic cloud. The characterization of the pure temporal-spectral state of subnatural-linewidth single photons paves the way for exploiting the temporal-spectral degree of freedom to develop photonic-quantum information processing.

We report a one-dimensional planar optical waveguide with a transverse distribution of the inhomogeneous loss profile, which exhibits an exceptional point (EP). The waveguide hosts two leaky resonant modes, where the interaction between them in the vicinity of the EP is controlled by proper adjustment of the inhomogeneity in the attenuation profile only. We study the adiabatic dynamics of propagation constants of the coupled modes by quasistatic encirclement of control parameters around the EP. Realizing such an encirclement with the inhomogeneous loss distribution along the direction of light propagation, we report the breakdown of adiabatic evolution of two coupled modes through the waveguide in the presence of an EP. During conversion the output mode is irrespective of the choice of the input excited mode but depends on the direction of light transportation. This topologically controlled, robust scheme of asymmetric mode conversion in the platform of the proposed all-lossy waveguide structure may provide a means for implementation of state-transfer applications in chirality-driven waveguide-based devices.

Colloidal crystals exhibit structural color without any color pigment due to the crystals’ periodic nanostructure, which can interfere with visible light. This crystal structure is iridescent as the resulting color changes with the viewing or illumination angle, which limits its use for printing or displays. To eliminate the iridescent property, it is important to make the packing of the colloidal nanoparticles disordered. Here, we introduce a drop-casting method where a droplet of a water-ethanol mixture containing monodisperse polymer-coated silica nanoparticles creates a relatively uniform and noniridescent deposit after the droplet evaporates completely on a heated substrate. The uniformity is caused by a thermal Marangoni flow and fast evaporation effects due to the heated substrate, whereas noniridescence is the outcome of short-range-ordered packing of nanoparticles by depletion attraction and friction effects produced by polymer brushes. We show that the colors of the final deposits from individual droplets remain unchanged while the viewing angle is varied under ambient light. We expect the coating method to be compatible with ink-jet printing and the uniformly coated self-assembled noniridescent nanostructures have potential for color displays using reflection mode and other optical devices.

The reconfigurability of integrated photonic chips plays a key role in current experiments in the area of linear-optical quantum computing. We demonstrate a reconfigurable multiport interferometer implemented as a femtosecond-laser-written integrated photonic device. The device includes a femtosecond-laser-written $4\times 4$ multiport interferometer equipped with 12 thermo-optical phase shifters, making it a universal programmable linear-optical circuit. We achieve a fast switching time for a single nested Mach-Zender interferometer of $\sim 10$ ms and quantitatively analyse the reconfigurability of the optical circuit. We believe that our results will improve the current state of quantum-optical experiments utilizing femtosecond-laser-written photonic circuits.

A Bloch surface wave (BSW) is a kind of resonant propagating wave sustained at the surface of a truncated dielectric multilayer. Spiral or concentric rings engraved on the top layer of the truncated multilayer, termed a “BSW lens,” will concentrate BSWs into the center. As a dielectric counterpart of a surface plasmon on a metal surface, a BSW propagates farther because of the absence of ohmic loss in a dielectric. Therefore, it is believed that a BSW lens can produce a much stronger focal spot if more rings are engraved. In this paper, we present a comprehensive study on the focusing properties of a BSW lens. We derive the dispersion relation of a BSW, analyze the focal pattern based on the interaction between the spiral-ring structure and light with specific spin and orbital angular momentum, and investigate transmittance and reflectance of the slot for the BSW. We find the reflectance of the slot introduces nonignorable loss for a BSW lens with multiple rings. In addition, we demonstrate that BSW waves can be efficiently concentrated into the focal spot by optimizing the trade-off between increasing number of rings and reflective loss of the ring slot, which is potentially helpful and valuable for integrated nanophotonic devices.

We report on luminescent centers contained in a few-layer-thick hexagonal boron nitride ($h$-BN) film grown on $\mathrm{Ni}$ by molecular beam epitaxy. After transfer to a ${\mathrm{Si}\mathrm{O}}_2$/$\mathrm{Si}$ substrate, sharp lines are observed in photo- and cathodoluminescence spectra in both the ultraviolet and the visible range. Spatially resolved measurements reveal that the luminescent centers responsible for these lines are localized within microscopic multilayer islands that form at the nucleation centers of the $h$-BN film. The comparison of their energy, polarization, and phonon-replica emission with previous theoretical predictions suggest that the ${\mathrm{N}}_{\mathrm{B}}{V}_{\mathrm{N}}$ antisite could be one of the light emitters present in our sample. Moreover, we also observe evidence of other kinds of centers that could be associated to defects containing carbon or oxygen. The characterized luminescent defects could have potential applications as quantum light sources.