Large second harmonic generation in alloyed TMDs and boron nitride nanostructures.
ABSTRACT: First principles methods are used to explicitly calculate the nonlinear susceptibility (?(2)(2?, ?, ?)) representing the second harmonic generation (SHG) of two dimensional semiconducting materials, namely transition metal dichalcogenides (TMDs) and Boron Nitride (BN). It is found that alloying TMDs improves their second harmonic response, with MoTeS alloys exhibiting the highest of all hexagonal alloys at low photon energies. Moreover, careful examination of the relationship between the concentration of Se in MoxSeySz alloys shows that the SHG intensity can be tuned by modifying the stoichiometry. In addition, materials with curvature can have large second harmonic susceptibility. Of all the calculated monolayer structures, the hypothetical TMD Haeckelites NbSSe and Nb0.5Ta0.5S2 exhibit the highest ?(2), while one of the porous 3D structures constructed from 2D hBN exhibits a larger ?(2) than known large band gap 3-D materials.
Project description:Mid-far infrared (IR) non-linear optical (NLO) materials are of great importance in military and civil fields. However, commercial IR-NLO crystals (such as AgGaS<sub>2</sub>, AgGaSe<sub>2</sub> and ZnGeP<sub>2</sub>) do not currently satisfy the requirements of large second-harmonic generation (SHG) and high laser induced damage thresholds (LIDTs), which seriously limits their practical applications. Herein, we have developed a new series of salt-inclusion chalcogenides, [A<sub>3</sub>X][Ga<sub>3</sub>PS<sub>8</sub>] (A = K, Rb; X = Cl, Br), which are constructed from alternate stacking of adamantane-like [Ga<sub>3</sub>PS<sub>10</sub>]<sup>6-</sup> cluster layers and cationic [A<sub>3</sub>X]<sup>2+</sup> salt layers. Importantly, they display both large SHG responses of several-fold and high LIDTs for dozens of times that of commercial AgGaS<sub>2</sub>, which exhibit the highest LIDTs among the reported IR-NLO materials with a larger SHG conversion efficiency than that of AgGaS<sub>2</sub>. These properties together with wide transparent region, type I phase-matching features and congruent-melting behaviors indicate they are promising IR-NLO materials.
Project description:Nonlinear optical (NLO) switchable materials are important for photonic and optoelectronic technologies. One important issue for NLO photoswitching, the most studied physical switching approach, is how to improve the switching contrast of second harmonic generation (SHG) in crystals, because the known values are generally below 3 times. Thermoswitching, as another approach, has shown impressive high SHG-switching contrasts (4-? times), but the fast decay of thermally induced states demands constant heat sources to maintain specific SHG intensities. We have synthesized a photochromic and thermochromic bistable acentric compound, ?-[(MQ)ZnCl<sub>3</sub>] (MQ<sup>+</sup> = <i>N</i>-methyl-4,4'-bipyridinium), which represents the first crystalline compound with both photo- and heat-induced SHG-switching behavior and the first example of a thermoswitchable NLO crystal that can maintain its expected second-order NLO intensity without any heat source. The SHG-switching contrast can reach about 8 times after laser irradiation or 2 times after thermal annealing. The former value is the highest recorded for photoswitchable NLO crystals. This work also indicates that higher SHG-switching contrasts may be obtained through increasing electron-transfer efficiency, variation of permanent dipole moment, and self-absorption.
Project description:Band nesting occurs when conduction and valence bands are approximately equispaced over regions in the Brillouin zone. In two-dimensional materials, band nesting results in singularities of the joint density of states and thus in a strongly enhanced optical response at resonant frequencies. We exploit the high sensitivity of such resonances to small changes in the band structure to sensitively probe strain in semiconducting transition metal dichalcogenides (TMDs). We measure and calculate the polarization-resolved optical second harmonic generation (SHG) at the band nesting energies and present the first measurements of the energy-dependent nonlinear photoelastic effect in atomically thin TMDs (MoS2, MoSe2, WS2, and WSe2) combined with a theoretical analysis of the underlying processes. Experiment and theory are found to be in good qualitative agreement displaying a strong energy dependence of the SHG, which can be exploited to achieve exceptionally strong modulation of the SHG under strain. We attribute this sensitivity to a redistribution of the joint density of states for the optical response in the band nesting region. We predict that this exceptional strain sensitivity is a general property of all 2D materials with band nesting.
Project description:The new non-centrosymmetric tin fluoride borate Sn<sub>3</sub> [B<sub>3</sub> O<sub>7</sub> ]F was synthesized hydrothermally, and was characterized by single-crystal and powder X-ray diffraction, vibrational spectroscopy, DFT calculations, second harmonic generation (SHG) measurements, thermogravimetry, and differential scanning calorimetry. Its SHG response is about 12 times that of quartz. The compound crystallizes in the non-centrosymmetric orthorhombic space group Pna2<sub>1</sub> with lattice parameters a=922.4(2), b=769.8(4), and c=1221.9(6)?pm (Z=4). Characteristic for the structure are isolated B<sub>3</sub> O<sub>7</sub> moieties, consisting of two corner-sharing BO<sub>3</sub> units and one BO<sub>4</sub> tetrahedron. These occupy half of the octahedral voids of a slightly distorted hexagonal closest packing of Sn<sup>2+</sup> atoms, with [SnF]<sup>+</sup> units in the other half of the octahedral voids. Sn<sub>3</sub> [B<sub>3</sub> O<sub>7</sub> ]F is transparent over a wide spectral range with a UV cut-off edge at about 263?nm.
Project description:Nonlinear optical processes, such as harmonic generation, are of great interest for various applications, e.g., microscopy, therapy, and frequency conversion. However, high-order harmonic conversion is typically much less efficient than low-order, due to the weak intrinsic response of the higher-order nonlinear processes. Here we report ultra-strong optical nonlinearities in monolayer MoS<sub>2</sub> (1L-MoS<sub>2</sub>): the third harmonic is 30 times stronger than the second, and the fourth is comparable to the second. The third harmonic generation efficiency for 1L-MoS<sub>2</sub> is approximately three times higher than that for graphene, which was reported to have a large ? <sup>(3)</sup>. We explain this by calculating the nonlinear response functions of 1L-MoS<sub>2</sub> with a continuum-model Hamiltonian and quantum mechanical diagrammatic perturbation theory, highlighting the role of trigonal warping. A similar effect is expected in all other transition-metal dichalcogenides. Our results pave the way for efficient harmonic generation based on layered materials for applications such as microscopy and imaging.Harmonic generation is a nonlinear optical process occurring in a variety of materials; the higher orders generation is generally less efficient than lower orders. Here, the authors report that the third-harmonic is thirty times stronger than the second-harmonic in monolayer MoS2.
Project description:Three-dimensional second-harmonic fields, sample orientation, and susceptibility ratios of biological samples are measured using polarization-resolved second-harmonic generation (SHG) microscopy. The three-dimensional (3D) polarization is gathered by measurement of a series of holograms for which excitation and analyzer polarizations are systematically varied, and the 3D SHG field is recovered through numerical back propagation. Harmonophore orientation is resolved in 3D from a sub-set of polarization-resolved SHG holograms. We further expand on previous approaches for the determination of susceptibility ratios, adding the calculation of multiple ratio values to allow intrinsic verification.
Project description:A new acentric metal borosilicate, namely Ba<sub>4</sub>Bi<sub>2</sub>(Si<sub>8-<i>x</i></sub> B<sub>4+<i>x</i></sub> O<sub>29</sub>) (<i>x</i> = 0.09), has been synthesized by a standard solid-state reaction. The title compound crystallizes in noncentrosymmetric (NCS) space group <i>I</i>4[combining macron]2<i>m</i> with lattice parameters <i>a</i> = 11.0254(4) Å and <i>c</i> = 10.3961(9) Å. Structure refinements indicate that mixing of B atoms and Si atoms exists for a few atomic sites. In the "ideal" Ba<sub>4</sub>Bi<sub>2</sub>(Si<sub>8</sub>B<sub>4</sub>O<sub>29</sub>), BO<sub>4</sub> or SiO<sub>4</sub> tetrahedra are inter-connected by corner-sharing to cyclic B<sub>4</sub>O<sub>12</sub> or Si<sub>4</sub>O<sub>12</sub> units. These B<sub>4</sub>O<sub>12</sub> and Si<sub>4</sub>O<sub>12</sub> units are further interconnected <i>via</i> corner-sharing to an "ideal" [Si<sub>8</sub>B<sub>4</sub>O<sub>29</sub>]<sup>14-</sup> 3D network. The Ba<sup>2+</sup> and Bi<sup>3+</sup> act as the counter cations and are located at the cavities of the structure. Ba<sub>4</sub>Bi<sub>2</sub>(Si<sub>8-<i>x</i></sub> B<sub>4+<i>x</i></sub> O<sub>29</sub>) (<i>x</i> = 0.09) melts incongruently at a high temperature of 929 °C. Powder second-harmonic generation (SHG) measurements reveal that Ba<sub>4</sub>Bi<sub>2</sub>(Si<sub>8-<i>x</i></sub> B<sub>4+<i>x</i></sub> O<sub>29</sub>) (<i>x</i> = 0.09) is a type I phase-matching compound with a good SHG response of about 5.1 times that of KDP (KH<sub>2</sub>PO<sub>4</sub>), which is the highest among the borosilicates reported so far. The SHG source has been studied by DFT theoretical calculations. Our preliminary results indicate that Ba<sub>4</sub>Bi<sub>2</sub>(Si<sub>8-<i>x</i></sub> B<sub>4+<i>x</i></sub> O<sub>29</sub>) (<i>x</i> = 0.09) is a new second-order nonlinear-optical crystalline material candidate.
Project description:Stacked atomically thin transition metal dichalcogenides (TMDs) exhibit fundamentally new physical properties compared to those of the individual layers. The twist angle between the layers plays a crucial role in tuning these properties. Having a tool that provides high-resolution, large area mapping of the twist angle, would be of great importance in the characterization of such 2D structures. Here we use polarization-resolved second harmonic generation (P-SHG) imaging microscopy to rapidly map the twist angle in large areas of overlapping WS2 stacked layers. The robustness of our methodology lies in the combination of both intensity and polarization measurements of SHG in the overlapping region. This allows the accurate measurement and consequent pixel-by-pixel mapping of the twist angle in this area. For the specific case of 30° twist angle, P-SHG enables imaging of individual layers.
Project description:The first bismuth borosulfate (H<sub>3</sub> O)Bi[B(SO<sub>4</sub> )<sub>2</sub> ]<sub>4</sub> is only the second featuring a three-dimensional anion, the first tectosilicate-analogous borosulfate synthesised solvothermally without a precursor (from Bi(NO<sub>3</sub> )<sub>3</sub> ?5?H<sub>2</sub> O and B(OH)<sub>3</sub> in oleum); moreover, it is the first comprising two differently charged cations and crystallises in a new structure type in space group I 4? (no.?82) (a=11.857(1), c=8.149(1)?Å, 1947?refl., 111?param., wR2=0.037), confirmed by a second harmonic generation (SHG) measurement. The B(SO<sub>4</sub> )<sub>4</sub> supertetrahedra are connected via all four sulfate tetrahedra resulting in a three-dimensional anion with both H<sub>3</sub> O<sup>+</sup> and Bi<sup>3+</sup> cations in channels. Additionally, the crystal structure of a further bismuth borosulfate, Bi<sub>2</sub> [B<sub>2</sub> (SO<sub>4</sub> )<sub>6</sub> ], is elucidated crystallising isotypically to the rare-earth borosulfates R<sub>2</sub> [B<sub>2</sub> (SO<sub>4</sub> )<sub>6</sub> ] in space group C2/c (No.?15) (a=13.568(2), b=11.490(2), c=11.106(2)?Å, 3127?refl., 155?param., wR2=0.035). Moreover, the optical and thermal properties of both compounds are discussed.
Project description:Second-order nonlinear optical materials are used to generate new frequencies by exploiting second-harmonic generation (SHG), a phenomenon where a nonlinear material generates light at double the optical frequency of the input beam. Maximum SHG is achieved when the pump and the generated waves are in phase, for example through birefringence in uniaxial crystals. However, applying these materials usually requires a complicated cutting procedure to yield a crystal with a particular orientation. Here we demonstrate the first example of phase matching under the normal incidence of SHG in a biaxial monoclinic single crystal of zinc tungstate. The crystal was grown by the micro-pulling-down method with the (102) plane perpendicular to the growth direction. Additionally, at the same time white light was generated as a result of stimulated Raman scattering and multiphoton luminescence induced by higher-order effects such as three-photon luminescence enhanced by cascaded third-harmonic generation. The annealed crystal offers SHG intensities approximately four times larger than the as grown one; optimized growth and annealing conditions may lead to much higher SHG intensities.