Second harmonic generation (SHG) and sum frequency spectroscopy (SFS) have provided unique opportunities to probe surfaces and interfaces. They have found broad applications in many disciplines of ...science and technology. In recent years, there has been significant progress in the development of SHG/SFS technology that has significantly broadened the applications of SHG and SFS. In this article, we review the recent progress of the field with emphasis on SFS.
Optical materials with centrosymmetry, such as silicon and germanium, are unfortunately absent of second‐order nonlinear optical responses, hindering their developments in efficient nonlinear optical ...devices. Here, a design with an array of slotted nanocubes is proposed to realize remarkable second harmonic generation (SHG) from the centrosymmetric silicon, which takes advantage of enlarged surface second‐order nonlinearity, strengthened electric field over the surface of the air‐slot, as well as the resonance enhancement by the bound states in the continuum. Compared with that from the array of silicon nanocubes without air‐slots, SHG from the slotted nanocube array is improved by more than two orders of magnitude. The experimentally measured SHG efficiency of the silicon slotted nanocube array is high as 1.8 × 10−4 W−1, which is expected to be further engineered by modifying the air‐slot geometries. The result can provide a new strategy to expand nonlinear optical effects and devices of centrosymmetric materials.
An array of slotted nanocubes is proposed to achieve strong second harmonic generation (SHG) effects in Si. Bound states in the continuum‐induced electric field enhancement and slot‐induced surface effect play a crucial role in the SHG. The high conversion efficiency of 1.8 × 10−4 W−1 can be experimentally obtained. This strategy can boost efficient SHG even in centrosymmetric materials.
Access to the elusive deep‐ultraviolet by direct second harmonic generation (SHG) enabled by a new beryllium‐free zincoborate‐phosphate crystal is reported. Ba3(ZnB5O10)PO4, exhibits large SHG ...responses at 1064 and 532 nm and a short 180 nm absorption edge. Centimeter‐size crystals are grown, and quantum mechanical calculations demonstrate the key role played by ZnO4 tetrahedra in the enhanced optical responses.
Nonlinear optical microscopy has become a powerful tool in bioimaging research due to its unique capabilities of deep optical sectioning, high‐spatial‐resolution imaging, and 3D reconstruction of ...biological specimens. Developing organic fluorescent probes with strong nonlinear optical effects, in particular third‐harmonic generation (THG), is promising for exploiting nonlinear microscopic imaging for biomedical applications. Herein, a simple method for preparing organic nanocrystals based on an aggregation‐induced emission (AIE) luminogen (DCCN) with bright near‐infrared emission is successfully demonstrated. Aggregation‐induced nonlinear optical effects, including two‐photon fluorescence (2PF), three‐photon fluorescence (3PF), and THG, of DCCN are observed in nanoparticles, especially for crystalline nanoparticles. The nanocrystals of DCCN are successfully applied for 2PF microscopy at 1040 nm NIR‐II excitation and THG microscopy at 1560 nm NIR‐II excitation, respectively, to reconstruct the 3D vasculature of the mouse cerebral vasculature. Impressively, the THG microscopy provides much higher spatial resolution and brightness than the 2PF microscopy and can visualize small vessels with diameters of ≈2.7 µm at the deepest depth of 800 µm in a mouse brain. Thus, this is expected to inspire new insights into the development of advanced AIE materials with multiple nonlinearity, in particular THG, for multimodal nonlinear optical microscopy.
Aggregation‐induced nonlinear optical effects, in particular third‐harmonic generation, of an aggregation‐induced emission luminogen (AIEgen) are observed in nanoparticles, especially for crystalline nanoparticles. AIEgen nanocrystals enable ultradeep intravital third‐harmonic generation imaging (up to 800 µm) of mouse cerebral vasculature with high spatial resolution (≈2.7 µm) at the deepest depth of 800 µm, much superior to two‐ and three‐photon fluorescence imaging.
The Cover Feature shows a small section of a layer in the structure of the new Li4Si(NCN)4. The solid‐state metathesis reaction (SSM) of mixtures of SiI4 and Li2(CN2) are controlled in situ by ...differential scanning calorimetry (DSC), revealing a polymeric reaction stage and the subsequent product formation under successive elimination of LiI. Tetracyanamidosilicates are closely related to orthosilicates and can exist with several cations of the PSE. As demonstrated in our previous research, metal tetracyanamidosilicates can act as robust host structures for luminescent materials to show second harmonic generation (SHG). More information can be found in the Research Article by H.‐J. Meyer and co‐workers.
The introduction of chirality into organic–inorganic hybrid perovskites (OIHPs) is expected to achieve excellent photoelectric and nonlinear materials related to circular dichroism. Owing to the ...existence of asymmetric center and intrinsic chirality in the chiral OIHPs, the different efficiencies of second harmonic generation (SHG) signal occurs when the circularly polarized light (CPL) with different phases passes through the chiral crystal, which is defined as second harmonic generation circular dichroism (SHG‐CD). Here, the SHG‐CD effect is developed in bulk single crystals of chiral one‐dimensional (1D) (R/S)‐3‐aminopiperidinePbI4. It is the first time that CPL is distinguished using chirality‐dependent SHG‐CD effect in OIHPs bulk single crystals. Such SHG‐CD technology extends the detection range to near infrared region (NIR). In this way, the anisotropy factor (gSHG‐CD) through SHG‐CD signal is as high as 0.21.
Bulk organic–inorganic hybrid perovskite single crystals are used to effectively distinguish the circularly polarized light in the near infrared region by chirality‐dependent second harmonic generation circular dichroism (SHG‐CD) effect with a high anisotropy factor (gSHG‐CD=0.21).
Exploring significant ultraviolet/deep‐ultraviolet nonlinear optical (NLO) materials is hindered by rigorous and contradictory requirements, especially, possessing a moderate optical birefringence to ...meet phase‐matching (PM). Except for suitable birefringence, small chromatic dispersion is also crucial to blue‐shift the PM wavelength. Here, the introduction of a fluorinated tetrahedral boron‐centred chromophore strategy was proposed to optimize the chromatic dispersion. Herein, the BF4− unit with a large HOMO–LUMO band gap was introduced to the Na−B−O−F system and Na4B8O9F10 was designed and synthesized successfully for the first time. Na4B8O9F10 with an optimized chromatic dispersion can achieve a short second harmonic generation PM wavelength of 240 nm with a relatively small birefringence (0.036@1064 nm). Notably, Na4B8O9F10 is the first acentric crystal with BF4− units among the reported metal–fluorooxoborate systems, involving isolated BF4− and novel B7O10F65− fundamental building blocks.
Na4B8O9F10, the first non‐centrosymmetric crystal with BF4− units among the reported metal‐fluorooxoborate systems, was designed and synthesized. It features isolated BF4− groups and novel B7O10F65− fundamental building blocks, and achieves an unexpected short λPM of 240 nm induced by optimized chromatic dispersion with such a small birefringence (0.036@1064 nm).
Electronic states and their dynamics are of critical importance for electronic and optoelectronic applications. Here, various relevant electronic states in monolayer MoS2, such as multiple excitonic ...Rydberg states and free‐particle energy bands are probed with a high relative contrast of up to ≥200 via broadband (from ≈1.79 to 3.10 eV) static third‐harmonic spectroscopy (THS), which is further supported by theoretical calculations. Moreover, transient THS is introduced to demonstrate that third‐harmonic generation can be all‐optically modulated with a modulation depth exceeding ≈94% at ≈2.18 eV, providing direct evidence of dominant carrier relaxation processes associated with carrier–exciton and carrier–phonon interactions. The results indicate that static and transient THS are not only promising techniques for the characterization of monolayer semiconductors and their heterostructures, but also a potential platform for disruptive photonic and optoelectronic applications, including all‐optical modulation and imaging.
An advanced optical technique is introduced: broadband static and transient third‐harmonic spectroscopies capture clear fingerprints of relevant electronic states in monolayer semiconductors with an unprecedentedly high contrast ratio up to 200 and further offer a dramatic advance in understanding of exciton dynamics with an ultrafast speed of ≈1 ps and high modulation depth up to 94%.
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