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The past decade has witnessed rapid progress in deep learning for molecular design, owing to the availability of invertible and invariant representations for molecules such as simplified molecular-input line-entry system (SMILES), which has powered cheminformatics since the late 1980s. However, the design of elemental components and their structural arrangement in solid-state materials to achieve certain desired properties is still a long-standing challenge in physics, chemistry and biology. This is primarily due to, unlike molecular inverse design, the lack of an invertible crystal representation that satisfies translational, rotational, and permutational invariances. To address this issue, we have developed a simplified line-input crystal-encoding system (SLICES), which is a string-based crystal representation that satisfies both invertibility and invariances. The reconstruction routine of SLICES successfully reconstructed 94.95% of over 40,000 structurally and chemically diverse crystal structures, showcasing an unprecedented invertibility. Furthermore, by only encoding compositional and topological data, SLICES guarantees invariances. We demonstrate the application of SLICES in the inverse design of direct narrow-gap semiconductors for optoelectronic applications. As a string-based, invertible, and invariant crystal representation, SLICES shows promise as a useful tool for in silico materials discovery. The lack of invertible and invariant crystal representations hinders the inverse design of crystals. Here the authors develop SLICES, an invertible and invariant representation, empowering property-driven inverse design of crystals using generative AI. [ABSTRACT FROM AUTHOR]

As a quantum material, Weyl semimetal has a series of electronic-band-structure features, including Weyl points with left and right chirality and corresponding Berry curvature, which have been observed in experiments. These band-structure features also lead to some unique nonlinear properties, especially high-order harmonic generation (HHG) due to the dynamic process of electrons under strong laser excitation, which has remained unexplored previously. Herein, we obtain effective HHG in type-II Weyl semimetal β-WP2 crystals, where both odd and even orders are observed, with spectra extending into the vacuum ultraviolet region (190 nm, 10th order), even under fairly low femtosecond laser intensity. In-depth studies have interpreted that odd-order harmonics come from the Bloch electron oscillation, while even orders are attributed to Bloch oscillations under the "spike-like" Berry curvature at Weyl points. With crystallographic orientation-dependent HHG spectra, we further quantitatively retrieved the electronic band structure and Berry curvature of β-WP2. These findings may open the door for exploiting metallic/semimetallic states as solid platforms for deep ultraviolet radiation and offer an all-optical and pragmatic solution to characterize the complicated multiband electronic structure and Berry curvature of quantum topological materials. Weyl semimetals have interesting band-structure features that lead to unique properties. Here, the authors observe and study high-harmonic generation in type-II Weyl semimetal β-WP2 crystals. [ABSTRACT FROM AUTHOR]

A novel Tm3+: CaY 0.65 Gd 0.35 AlO 4 (Tm3+: CYGA) disordered crystal was successfully grown for the first time, to the best of our knowledge, using the Czochralski method. The optical properties and laser performance were investigated in details. Owing to the addition of Gd3+ ions to the Tm3+: CYA crystal, the Tm3+: CYGA crystal possesses a wider Tm3+ absorption and emission spectra compared with those of the Tm3+: CYA and Tm3+: CGA crystals. In particular, the full width at half-maximum (FWHM) of the emission spectra at approximately 2 µm for the Tm3+: CYGA crystal was 221.3 nm, which is larger than those of the Tm3+: CYA (200 nm) and Tm3+: CGA (151 nm) crystals. Furthermore, an efficient laser operation of the Tm3+: CYGA crystal at 1974 nm was achieved for the first time, with a slope efficiency of up to 31.5 %, corresponding to a maximum output power of 810 mW. The results show that Tm3+: CYGA disordered crystals have the potential to be applied in ∼ 2-μm broadband tunable lasers and ultra-short pulse lasers. [Display omitted] • A novel Tm3+: CaY 0.65 Gd 0.35 AlO 4 (Tm3+: CYGA) disordered crystal has been successfully grown for the first time. • The Tm3+ absorption and emission spectrum has been effectively broadened with the addition of Gd3+. • An efficient laser operation of Tm3+: CYGA crystal at 1974 nm was achieved for the first time. • Tm3+: CYGA crystal could be a promising disordered laser host for ∼ 2 µm broadband tunable laser and ultra-short pulse laser [ABSTRACT FROM AUTHOR]

Sandwich structures, constructed from a planar defect of rhodamine-B (RhB)-doped titania (TiO2) and two photonic crystals, were synthesized via the self-assembly method combined with spin-coating. The modification of the spontaneous emission of RhB molecules in such structures was investigated experimentally. The spontaneous emission of RhB-doped TiO2 film with photonic crystals was reduced by a factor of 5.5 over a large bandwidth of 13% of the first-order Bragg diffraction frequency when compared with that of RhB-doped TiO2 film without photonic crystals. The angular dependence of the modification and the photoluminescence lifetime of RhB molecules demonstrate that the strong and wide suppression of the spontaneous emission of the RhB molecules is due to the presence of the photonic band gap. [ABSTRACT FROM AUTHOR]

Wave trapping and manipulation are at the heart of modern integrated photonics and acoustics. Grand challenges emerge on increasing the integration density and reducing the wave leakage/noises due to fabrication imperfections, especially for waveguides and cavities at subwavelength scales. The rising of robust wave dynamics based on topological mechanisms offers possible solutions. Ideally, in a three-dimensional (3D) topological integrated chip, there are coexisting robust two-dimensional (2D) interfaces, one-dimensional (1D) waveguides and zero-dimensional (0D) cavities. Here, we report the experimental discovery of such a dimensional hierarchy of the topologically-protected 2D surface states, 1D hinge states and 0D corner states in a single 3D system. Such an unprecedented phenomenon is triggered by the higher-order topology in simple-cubic sonic crystals and protected by the space group P m 3 ¯ m . Our study opens up a new regime for multidimensional wave trapping and manipulation at subwavelength scales, which may inspire future technology for integrated acoustics and photonics. Here, the authors report the experimental discovery of such a dimensional hierarchy of the topologically-protected 2D surface states, 1D hinge states and 0D corner states in a single 3D acoustic system by using higher-order topological sonic crystals. [ABSTRACT FROM AUTHOR]

An exact formula for the equivalent circuit of a crystal resonator in series with a capacitor is derived. Network analysis is used to obtain an exact formula for the equivalent circuit parameters without applying any approximations. The result is an expansion of that obtained by those who used assumptions regarding high frequency, high quality factor, high capacitance ratio, and so on. Hence, this formula can be used for instances of low quality factor or low capacitance ratio, and even for the actual inductor-capacitor network as long as these devices have the same equivalent circuits. The enhanced accuracy of the new formula extends itself to oscillator frequency calibration, temperature compensation, and electronic frequency control. [ABSTRACT FROM PUBLISHER]

In this paper, 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) single crystal/epoxy 1/3 composite was used as the active material of the endoscopic ultrasonic radial array transducer, because this composite exhibited ultrahigh electromechanical coupling coefficient (kt = 0.81%), very low mechanical quality factor (Qm = 11) and relatively low acoustic impedance (Zt = 12 MRayls). A 6.91 MHz PMN-PT/epoxy 1/3 composite radial array transducer with 64 elements was tested in a pulseecho response measurement. The -6-dB bandwidth of the composite array transducer was 102%, which was ~30% larger than that of traditional lead zirconate titanate array transducer. The two-way insertion loss was found to be -32.3 dB. The obtained results show that this broadband array transducer is promising for acquiring high-resolution endoscopic ultrasonic images in many clinical applications. [ABSTRACT FROM PUBLISHER]

High-frequency (25 MHz) ultrasonic transducers with Na0.5Bi0.5TiO3-BaTiO3 (NBT-BT) lead-free piezoelectric single crystal as the active elements are fabricated and characterized. The impedance measurement reveals that the poled [001]-oriented NBT-BT single crystal exhibits a high thickness electromechanical coefficient kt of 0.52 and a low clamped dielectric constant of 80. The -6-dB bandwidth of the transducer is 46.16% and the insertion loss at the center frequency is -31.89 dB. The good performance of the transducer indicates that the NBT-BT single crystal would be a promising lead-free material for ultrasonic transducer applications. [ABSTRACT FROM PUBLISHER]

Phononic crystals have been proposed about two decades ago and some important characteristics such as acoustic band structure and negative refraction have stimulated fundamental and practical studies in acoustic materials and devices since then. To carefully engineer a phononic crystal in an acoustic “atom” scale, acoustic metamaterials with their inherent deep subwavelength nature have triggered more exciting investigations on negative bulk modulus and/or negative mass density. Acoustic surface evanescent waves have also been recognized to play key roles to reach acoustic subwavelength imaging and enhanced transmission. [Copyright &y& Elsevier]