Highly efficient nonlinear optical organic crystals are very attractive for various photonic applications including terahertz (THz) wave generation. Up to now, only two classes of ionic crystals based on either pyridinium or quinolinium with extremely large macroscopic optical nonlinearity have been developed. This study reports on a new class of organic nonlinear optical crystals introducing electron-accepting benzothiazolium, which exhibit higher electron-withdrawing strength than pyridinium and quinolinium in benchmark crystals. The benzothiazolium crystals consisting of new acentric core HMB (2-(4-hydroxy-3-methoxystyryl)-3-methylbenzo[d]thiazol-3-ium) exhibit extremely large macroscopic optical nonlinearity with optimal molecular ordering for maximizing the diagonal second-order nonlinearity. HMB-based single crystals prepared by simple cleaving method satisfy all required crystal characteristics for intense THz wave generation such as large crystal size with parallel surfaces, moderate thickness and high optical quality with large optical transparency range (580-1620 nm). Optical rectification of 35 fs pulses at the technologically very important wavelength of 800 nm in 0.26 mm thick HMB crystal leads to one order of magnitude higher THz wave generation efficiency with remarkably broader bandwidth compared to standard inorganic 0.5 mm thick ZnTe crystal. Therefore, newly developed HMB crystals introducing benzothiazolium with extremely large macroscopic optical nonlinearity are very promising materials for intense broadband THz wave generation and other nonlinear optical applications.

Efficient broadband organic terahertz (THz) generators using X‐shaped alignment of the nonlinear optical chromophores, as an alternative to the parallel alignment of chromophores in benchmark organic crystals, are reported. All the developed six organic benzothiazolium crystals exhibit an isomorphic X‐shaped alignment of chromophores, resulting in an unprecedentedly large off‐diagonal optical nonlinearity (>100 × 10−30 esu), which presents one of the largest off‐diagonal optical nonlinearity of organic crystals. The benzothiazolium crystals exhibit efficient broadband THz wave generation employing the off‐diagonal optical nonlinearity, in contrast to the present state‐of‐the‐art organic THz generators that mostly utilize diagonal optical nonlinearity. For using off‐diagonal and diagonal optical nonlinearities, the polarization of the optical pump is perpendicular and parallel, respectively, to the polar axis of crystals. In addition to a large THz wave generation efficiency with one order of magnitude higher peak‐to‐peak THz electric field than that generated in a 1.0‐mm‐thick inorganic benchmark ZnTe crystal, the benzothiazolium crystals generate broadband THz spectra with an upper cut‐off frequency of near 8 THz, and the absence of strong absorption dimples in the range of 0.5−4 THz. Therefore, the X‐shaped alignment of chromophores presents an interesting potential alternative for efficient broadband organic THz generators.

Enhanced terahertz (THz) wave generation is demonstrated in nonlinear organic crystals through refractive index engineering, which improves phase matching characteristics substantially. Unlike conventional low‐bandgap nonlinear organic crystals, the newly designed benzimidazolium‐based HMI (2‐(4‐hydroxy‐3‐methoxystyryl)‐1,3‐dimethyl‐1H‐benzoimidazol‐3‐ium) chromophore possesses a relatively wide bandgap. This reduces the optical group index in the near‐infrared, allowing better phase matching with the generated THz waves, and leads to high optical‐to‐THz conversion. A unique feature of the HMI‐based crystals, compared to conventional wide‐bandgap aniline‐based crystals, is their remarkably larger macroscopic optical nonlinearity, a one order of magnitude higher diagonal component in macroscopic nonlinear susceptibility than NPP ((1‐(4‐nitrophenyl)pyrrolidin‐2‐yl)methanol) crystals. The HMI‐based crystals also exhibit much higher thermal stability, with a melting temperature Tm above 250 °C, versus aniline‐based crystals (116 °C for NPP). With pumping at the technologically important wavelength of 800 nm, the proposed HMI‐based crystals boost high optical‐to‐THz conversion efficiency, comparable to benchmark low‐bandgap quinolinium crystals with state‐of‐the‐art macroscopic nonlinearity. This performance is due to the excellent phase matching enabled by decreasing optical group indices in the near‐infrared through wide‐bandgap chromophores. The proposed wide‐bandgap design is a promising way to control the refractive index of various nonlinear organic materials for enhanced frequency conversion processes.

A design strategy is proposed for electron‐transporting materials (ETMs) with homochiral asymmetric‐shaped groups for highly efficient non‐fullerene perovskite solar cells (PSCs). The electron transporting N,N′‐bis[(R)‐1‐phenylethyl]naphthalene‐1,4,5,8‐tetracarboxylic diimide (NDI‐PhE) consists of two asymmetric‐shaped chiral (R)‐1‐phenylethyl (PhE) groups that act as solubilizing groups by reducing molecular symmetry and increasing the free volume. NDI‐PhE exhibits excellent film‐forming ability with high solubility in various organic solvents [about two times higher solubility than the widely used fullerene‐based phenyl‐C61‐butyric acid methyl ester (PCBM) in o‐dichlorobenzene]. NDI‐PhE ETM‐based inverted PSCs exhibit very high power conversion efficiencies (PCE) of up to 20.5 % with an average PCE of 18.74±0.95 %, which are higher than those of PCBM ETM‐based PSCs. The high PCE of NDI‐PhE ETM‐based PSCs may be attributed to good film‐forming abilities and to three‐dimensional isotropic electron transporting capabilities. Therefore, introducing homochiral asymmetric‐shaped groups onto charge‐transporting materials is a good strategy for achieving high device performance.

A new organic three-component single crystals are developed using the so-called "pseudo-isomorphic cocrystallization" for nonlinear optical and terahertz (THz) photonic applications. The pseudo-isomorphic cocrystallization is based on two homocrystals exhibiting similar molecular ordering feature in the crystalline state, but different crystallographic space groups. Such new organic cocrystals consist of three components, highly nonlinear optical 2-(4-hydroxystyryl)-1-methylquinolinium (OHQ) cation, and two different counter anions. Compared to homocrystals having two components (OHQ cation and a single anion type), OHQbased cocrystals by isomorphic cocrystallization from isomorphic homocrystals exhibit an isomorphic crystal structure with very similar physical properties. In contrast, OHQ-based cocrystals by pseudoisomorphic cocrystallization provide a different molecular ordering with a different crystallographic space group, resulting in remarkably distinguishable crystal characteristics and physical properties, while maintaining large macroscopic optical nonlinearity with excellent optical quality and morphology suitable for diverse optical experiments. To show a potential for nonlinear optical applications, THz wave generation is demonstrated by optical rectification pumped at fundamental wavelength of 1300 nm. A 0.92 mm thick OHQ-based cocrystal by pseudo-isomorphic cocrystallization delivers efficient optical-to-THz conversion with one order of magnitude higher peak-to-peak THz electric field than the 1.0 mm thick inorganic standard ZnTe crystal and presents a broad spectral bandwidth of up to 8 THz.

of the existing organic and inorganic THz wave generators and detectors are hindered by fundamental limitations in spectral generation and detection as well as by bandwidth limits due to intrinsic phonon and molecular vibrational modes. [2][3][4][5] In nonlinear optical frequency conversion processes, such as optical rectification and difference frequency generation, organic electro-optic crystals are more beneficial for broadband THz wave generation and detection as compared to their inorganic counterparts. [6][7][8][9] This is because organic electro-optic crystals exhibit large macroscopic optical nonlinearity with good phase matching ability between optical and THz frequencies and a relatively low absorption coefficient in high-frequency THz spectral regions.When considering THz wave generation and detection, current state-of-the-art organic electro-optic pyridinium-, quinolinium-, and benzothiazolium-based ionic crystals and nonionic phenolic polyene-based crystals exhibit a macroscopic nonlinear optical coefficient one order of magnitude larger than that of inorganic crystals (e.g., ZnTe and GaP) with an effective hyperpolarizability tensor β iii eff > 100 × 10 −30 esu and/or an electro-optic coefficient >50 pm V −1 . [6,[10][11][12][13][14] In addition, benchmark organic electro-optic crystals exhibit much lower absorption coefficients in the high THz frequency region when compared to inorganic crystals. While the upper New organic electro-optic crystals containing orthogonally oriented electronwithdrawing groups are developed for efficient and gap-free THz wave generation with a very flat broadband spectral shape. These crystals consist of 2-(4-hydroxystyryl)-1-methylquinolinium (OHQ) cationic chromophores and nonplanar 4-(trifluoromethoxy)benzenesulfonate (TFO) anions with orthogonally oriented highly electronegative trifluoromethoxy groups capable of strong hydrogen bonds. OHQ-TFO crystals exhibit enhanced macroscopic optical nonlinearity with a second harmonic generation efficiency 2.3 times greater than that of benchmark OHQ-based crystals with conventional planar anions. This enhancement is attributed to reduced edge-to-face π-π interactions between cations and anions due to the orthogonal orientation and electron-withdrawing characteristics of trifluoromethoxy groups. Moreover, OHQ-TFO crystals exhibit excellent THz wave characteristics generated by optical rectification; 0.52 mm thick OHQ-TFO crystal generate a peak-to-peak THz electric field 15 times higher than that of inorganic standard 1.0 mm thick ZnTe crystal and a broader spectrum with an upper cut-off frequency near 8 THz at pump wavelengths of 1140-1500 nm. Unlike previously reported state-of-the-art organic electro-optic salt crystals with strong phonon absorption in the frequency range of 0.8-3 THz, OHQ-TFO crystals facilitate gap-free broadband THz wave generation without strong absorption modulations due to the strong hydrogen bond ability of trifluoromethoxy groups.

are not colorless because of the inherent absorption of visible light by melanin. Traditionally, inorganic materials, such as ZnO, or TiO 2 nanoparticles [3,6] have been widely used as UV blocking materials because of their strong light scattering capability or UV absorption owing to their wide band gaps. However, these particles also strongly reflect visible light via scattering, thus appearing white color rather than transparent. In addition, nanoparticles are not preferred especially for cosmetic applications because growing evidence suggests that the nanoparticles could be hazardous to human health. Furthermore, a multilayer film, Bragg reflector, can be fabricated by repeated vapor deposition of ZrO 2 and SiO 2 , which can also efficiently reflect UV, [4,7] but it has angle-dependency. By contrast, colloidal glasses or correlated amorphous colloidal structures [8][9][10][11][12][13][14][15][16][17][18][19] may show strong angle-independent UV reflection when "intraparticle" or "interparticle" backscattering resonance conditions [4,7] are matched for UV.For photonic glasses made of core-shell particles, the form and structure factors can be decoupled by adjusting the shell thickness in the index-matched matrix. [20,21] Amorphous structure of hollow silica nanospheres assembled in a polymer matrix with the same refractive index as silica, also produced angle-independent colors in the visible range. [17,20,21] In those photonic glasses made of hollow particles, the core diameter was sufficiently small enough to produce backscattering peak from "intraparticle resonance" or form factor in the UV region and the structure factor peak appeared in the visible region, which corresponded to the "interparticle" distance (=d ≈ λ/2).In this report, to obtain both peaks from form and structure factors in the UV region, the core and overall diameters of hollow silica nanospheres were reduced to less than 100 nm and 200 nm, respectively. For optical transparency, the refractive index of the hollow silica nanoparticles is adjusted in the polymer matrix. At the end, we assembled hollow nanospheres into the micron sized spherical aggregates or "supraballs" by emulsion encapsulation and shrinkage for practical applications, such as UV-protection ingredients in cosmetics or coating solutions for display and photovoltaic devices.For hollow silica particles, we have first core-shell polystyrene-silica particles by coating organosilica on polystyrene (PS) particles. [22,23] In order to avoid the formation For an optically transparent, UV-reflective film, hollow silica nanospheres smaller than the visible wavelength (<λ vis ) are prepared and assembled into colloidal glasses, of which interstices are then backfilled with a polymer. The polymer refractive index is matched with the silica shell to minimize backscattering in the visible range, and the average distance between the hollow silica particles is adjusted by tuning the shell thickness to satisfy the interference resonance condition for a UV selective reflection. The resulting ...

A new non‐fullerene electron transporting material based on naphthalene diimide with indane groups having simultaneous alicyclic and aromatic characteristics is developed by Sang Hyuk Im, Jong H. Kim, O‐Pil Kwon, and co‐workers in article number https://doi.org/10.1002/adfm.201800346. Low‐temperature solution‐processable perovskite solar cells using this material show up to 20.2% power conversion efficiency and excellent long‐term temporal stability.