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<jats:title>Abstract</jats:title> <jats:p>Bright single-photon emitters (SPEs) are fundamental components in many quantum applications. However, it is difficult to simultaneously get large Purcell enhancements and quantum yields in metallic nanostructures because of the huge losses in the metallic nanostructures. Herein, we propose to combine an ultrathin metallic bowtie antenna with a silicon antenna above a metallic substrate to simultaneously get large Purcell enhancements, quantum yields, and collection efficiencies. As a result, the brightness of SPEs in the hybrid nanostructure is greatly increased. Due to the deep subwavelength field confinement (mode size &lt; 10 nm) of surface plasmons in the ultrathin metallic film (thickness &lt; 4 nm), the Purcell enhancement of the metallic bowtie antenna improves by more than 25 times when the metal thickness decreases from 20 nm to 2 nm. In the hybrid nanostructures by combining an ultrathin metallic bowtie antenna with a silicon antenna, the Purcell enhancement (Fp≈2.6 × 10<jats:sup>6</jats:sup>) in the hybrid nanostructures is 63 times greater than those (≤ 4.1 × 10<jats:sup>4</jats:sup>) in the previous metallic and hybrid nanostructures. Because of the reduced ratio of electromagnetic fields in the ultrathin metallic bowtie antenna when the high-index silicon antenna is under the quasi-BIC state, a high quantum yield (QY ≈ 0.70) is obtained. Moreover, the good radiation directivity of the quasi-BIC (bound state in the continuum) mode of the silicon antenna and the reflection of the metallic substrate result in a high collection efficiency (CE ≈ 0.71). Consequently, the overall enhancement factor of brightness of a SPE in the hybrid nanostructure is EF<jats:sup>*</jats:sup> ≈ Fp × QY × CE ≈ 1.3 × 10<jats:sup>6</jats:sup>, which is 5.6 × 10<jats:sup>2</jats:sup> times greater than those (EF<jats:sup>*</jats:sup> ≤ 2.2 × 10<jats:sup>3</jats:sup>) in the previous metallic and hybrid nanostructures.</jats:p>

<jats:p>The hidden <jats:inline-formula> <jats:tex-math> <?CDATA ${{\mathbb{Z}}}_{2}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="double-struck">Z</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_31_1_014210ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> symmetry of the asymmetric quantum Rabi model (AQRM) has recently been revealed via a systematic construction of the underlying symmetry operator. Based on the AQRM result, we propose an ansatz for the general form of the symmetry operators for AQRM-related models. Applying this ansatz we obtain the symmetry operator for three models: the anisotropic AQRM, the asymmetric Rabi–Stark model (ARSM), and the anisotropic ARSM.</jats:p>

<jats:p>The terahertz technology has attracted considerable attention because of its potential applications in various fields. However, the research of functional devices, including polarization converters, remains a major demand for practical applications. In this work, a reflective dual-functional terahertz metadevice is presented, which combines two different polarization conversions through using a switchable metasurface. Different functions can be achieved because of the insulator-to-metal transition of vanadium dioxide (VO<jats:sub>2</jats:sub>). At room temperature, the metadevice can be regarded as a linear-to-linear polarization convertor containing a gold circular split-ring resonator (CSRR), first polyimide (PI) spacer, continuous VO<jats:sub>2</jats:sub> film, second PI spacer, and gold substrate. The converter possesses a polarization conversion ratio higher than 0.9 and a bandwidth ratio of 81% in a range from 0.912 THz to 2.146 THz. When the temperature is above the insulator-to-metal transition temperature (approximately 68 °C) and VO<jats:sub>2</jats:sub> becomes a metal, the metasurface transforms into a wideband linear-to-circular polarization converter composed of the gold CSRR, first PI layer, and continuous VO<jats:sub>2</jats:sub> film. The ellipticity is close to −1, while the axis ratio is lower than 3 dB in a range of 1.07 THz–1.67 THz. The metadevice also achieves a large angle tolerance and large manufacturing tolerance.</jats:p>

<jats:p>Obtaining the vertical distribution profile of trace gas is of great significance for studying the diffusion procedure of air pollution. In this article, a look-up table method based on multi-axis differential optical absorption spectroscopy (MAXDOAS) technology is established for retrieving the tropospheric NO<jats:sub>2</jats:sub> vertical distribution profiles. This method retrieves the aerosol extinction profiles with minimum cost function. Then, the aerosol extinction profiles and the atmospheric radiation transfer model (RTM) are employed to establish the look-up table for retrieving the NO<jats:sub>2</jats:sub> vertical column densities (VCDs) and profiles. The measured NO<jats:sub>2</jats:sub> differential slant column densities (DSCDs) are compared with the NO<jats:sub>2</jats:sub> DSCDs simulated by the atmospheric RTM, and the NO<jats:sub>2</jats:sub> VCDs, the weight factor of NO<jats:sub>2</jats:sub> in the boundary layer, and the boundary layer height are obtained by the minimization process. The look-up table is established to retrieve NO<jats:sub>2</jats:sub> VCDs based on MAX-DOAS measurements in Huaibei area, and the results are compared with the data from Copernicus Atmospheric Monitoring Service(CAMS) model. It isfound that there are nearly consistent and the correlation coefficient R<jats:sup>2</jats:sup> is morethan 0.86. The results show that this technology provides a more convenient and accurate retrieval method for the stereoscopic monitoring of atmospheric environment.</jats:p>

<jats:p>We report on an idler-resonant femtosecond optical parametrical oscillator (OPO) based on BiB<jats:sub>3</jats:sub>O<jats:sub>6</jats:sub> (BiBO) crystal, synchronously pumped by a frequency-doubled, mode-locked Yb:KGW laser at 515 nm. The idler wavelengths of OPO can be tuned from 1100 nm to 1540 nm. At a repetition rate of 75.5 MHz, the OPO generates as much as 400 mW of idler power with 3.1 W of pump power, the corresponding pulse duration is 80 fs, which is 1.04 times of Fourier transform-limited (FTL) pulse duration at 1305 nm. In addition, the OPO exhibits excellent beam quality with <jats:italic>M</jats:italic> <jats:sup>2</jats:sup> &lt; 1.8 at 1150 nm. To the best of our knowledge, this is the first idler-resonant femtosecond OPO pumped by 515 nm.</jats:p>

<jats:p>The optical response of metal nanoparticles can be modified through near-field or far-field interaction, yet the lattice plasmon modes (LPMs) considered can only be excited from the latter. Here instead, we present a theoretical evaluation for LPM excitation via the near-field coupling process. The sample is an arrayed structure with specific units composed of upper metal disks, a lower metal hole and a sandwiched dielectric post. The excitation process and underlying mechanism of the LPM and the influence of the structure parameters on the optical properties have been investigated in detail by using a finite-difference time-domain (FDTD) numerical method. Our investigation presented here should advance the understanding of near-field interaction of plasmon modes for LPM excitation, and LPMs could find some potential applications, such as in near-field optical microscopes, biosensors, optical filters and plasmonic lasers.</jats:p>

<jats:p>Non-Hermitian systems can exhibit unconventional spectral singularities called exceptional points (EPs). Various EP sensors have been fabricated in recent years, showing strong spectral responses to external signals. Here we propose how to achieve a nonlinear anti-parity-time (<jats:inline-formula> <jats:tex-math><?CDATA ${\mathscr{A}}{\mathscr{P}}{\mathscr{T}}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="script">A</mml:mi> <mml:mi mathvariant="script">P</mml:mi> <mml:mi mathvariant="script">T</mml:mi> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_31_1_014215_ieqn1-3.gif" xlink:type="simple" /> </jats:inline-formula>) gyroscope by spinning an optical resonator. We show that, in the absence of any nonlinearity, the sensitivity or optical mode splitting of the linear device can be magnified up to 3 orders compared to that of the conventional device without EPs. Remarkably, the <jats:inline-formula> <jats:tex-math><?CDATA ${\mathscr{A}}{\mathscr{P}}{\mathscr{T}}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="script">A</mml:mi> <mml:mi mathvariant="script">P</mml:mi> <mml:mi mathvariant="script">T</mml:mi> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_31_1_014215_ieqn1-4.gif" xlink:type="simple" /> </jats:inline-formula> symmetry can be broken when including the Kerr nonlinearity of the materials and, as a result, the detection threshold can be significantly lowered, i.e., much weaker rotations which are well beyond the ability of a linear gyroscope can now be detected with the nonlinear device. Our work shows the powerful ability of <jats:inline-formula> <jats:tex-math><?CDATA ${\mathscr{A}}{\mathscr{P}}{\mathscr{T}}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="script">A</mml:mi> <mml:mi mathvariant="script">P</mml:mi> <mml:mi mathvariant="script">T</mml:mi> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_31_1_014215_ieqn1-5.gif" xlink:type="simple" /> </jats:inline-formula> gyroscopes in practice to achieve ultrasensitive rotation measurement.</jats:p>

<jats:p>Resonance cavity is a basic element in optics, which has wide applications in optical devices. Coupled cavities (CCs) designed in metal-insulator-metal (MIM) bus waveguide are investigated through the finite difference time domain method and coupled-mode theory. In the CCs, the resonant modes of the surface plasmon polaritons (SPPs) split with the thickness decreasing of the middle baffle. Through the coupled-mode theory analysis, it is found that the phase differences introduced in opposite and positive couplings between two cavities lead to mode splitting. The resonant wavelength of positive coupling mode can be tuned in a large range (about 644 nm) through adjusting the coupling strength, which is quite different from the classical adjustment of the optical path in a single cavity. Based on the resonances of the CCs in the MIM waveguide, more compact devices can be designed to manipulate SPPs propagation. A device is designed to realize flexible multiple-wavelength SPPs routing. The coupling in CC structures can be applied to the design of easy-integrated laser cavities, filters, multiple-wavelength management devices in SPPs circuits, nanosensors, etc.</jats:p>

<jats:p>We report that using asymmetric lattice periods can enhance the quality factor of plasmonic surface lattice resonances (SLRs) in two-dimensional array of metal-insulator-metalnanopillars in asymmetric dielectric environment. Simulation results show that by adopting appropriate asymmetric lattice periods, the SLR quality factor can be enhanced by 24\% compared with the scenario of symmetric periods. We find that the SLR quality factor is optimized when the resonance wavelength is closest to the Rayleigh cutoff wavelength. We also find that the SLRs effect is polarization sensitive in the proposed structure. We expect this work will advance the engineering of SLRs especially in asymmetric dielectric environments, and will promote their applications in sensing.</jats:p>

<jats:p>A novel Lamb wave frequency-mixing technique is proposed for locating microcracks in a thin plate, which does not require the resonance condition of Lamb wave mixing and can accurately locate the microcracks through only one-time sensing. Based on the bilinear stress-strain constitutive model, a two-dimensional finite element (FE) model is built to investigate the frequency-mixing response induced by the interaction between two primary Lamb waves and a microcrack. When twoprimary Lamb waves of A0 and S0 modes with different frequencies excited on the same side of the plate simultaneously impinge on the examined microcrack, under the modulation of the contact acoustic nonlinearity, the microcrack itself can be deemed as the secondary sound source and it will radiate the Lamb waves of new combined frequencies. Based on the timeof flight of the generated A0 mode at difference frequency, an indicator named normalized amplitude index (NAI) is defined to directly locate the multi-microcracks in the given plate. It is found that the number and location of the microcracks can be intuitively visualized by using the NAI based frequency-mixing technique. It is also demonstrated that the proposed frequency mixing technique is a promising approach for the microcrack localization.</jats:p>