Strong chiroptical effects recently reported result from the interaction of light with chiral plasmonic nanostructures. Such nanostructures can be used to enhance the chiroptical response of chiral ...molecules and could also significantly increase the enantiomeric excess of direct asymmetric synthesis and catalysis. Moreover, in optical metamaterials, chirality leads to negative refractive index and all the promising applications thereof. In this Progress Report, we highlight four different strategies which have been used to achieve giant chiroptical effects in chiral nanostructures. These strategies consecutively highlight the importance of chirality in the nanostructures (for linear and nonlinear chiroptical effects), in the experimental setup and in the light itself. Because, in the future, manipulating chirality will play an important role, we present two examples of chiral switches. Whereas in the first one, switching the chirality of incoming light causes a reversal of the handedness in the nanostructures, in the second one, switching the handedness of the nanostructures causes a reversal in the chirality of outgoing light.
Due to the recent development of bottom‐up and top‐down approaches for material design and fabrication at the nanoscale, giant chiroptical effects have been reported from plasmonic nanostructures. These effects are exhibited both in the linear and in the nonlinear optical regimes and are sensitive to the chirality of nanostructures, the chirality of the experiments and the chirality of light itself.
Nonlinear optical (NLO) materials are of intense academic and technological interest attributable to their ability to generate coherent radiation over a range of different wavelengths. The ...requirements for a viable NLO material are rather strict, and their discovery has mainly been serendipitous. This study reports synthesis, characterization, and, most importantly, growth of large single crystals of a technologically viable NLO material—Rb3Ba3Li2Al4B6O20F. Through the judicious selection of cations, Rb3Ba3Li2Al4B6O20F exhibits a 3D structure that facilitates the growth of large single crystals along the optical axis direction. Measurements on these crystals indicate that Rb3Ba3Li2Al4B6O20F exhibits a moderate birefringence of 0.057 at 1064 nm enabling Type I phase‐matching down to 243 nm. Theoretical calculations indicate the symmetry adapted mode displacement (SAMD) parameter scales with the second‐harmonic generation intensity.
A next‐generation technologically viable nonlinear optical crystal, Rb3Ba3Li2Al4B6O20F, is successfully designed, synthesized, characterized, and grown. This crystal not only solves the layering issue observed in KBe2BO3F and Sr2Be2B2O7, which has hindered their crystal growth, but also does not require the use of toxic BeO in the synthesis. More importantly, it exhibits a moderate birefringence and is nonhygroscopic.
We introduce a plasmonic-semiconductor hybrid nanosystem, consisting of a ZnO nanowire coupled to a gold pentamer oligomer by crossing the hot-spot. It is demonstrated that the hybrid system exhibits ...a second harmonic (SH) conversion efficiency of ∼3 × 10–5%, which is among the highest values for a nanoscale object at optical frequencies reported so far. The SH intensity was found to be ∼1700 times larger than that from the same nanowire excited outside the hot-spot. Placing high nonlinear susceptibility materials precisely in plasmonic confined-field regions to enhance SH generation opens new perspectives for highly efficient light frequency up-conversion on the nanoscale.
We present a thermal model to describe the interaction between the visible modes in micro-ring resonators from the third harmonic generation (THG) nonlinear process. We demonstrate that the model can ...predict the thermal characteristics of the orthogonal THG modes from the interaction and of the resonance coupling between them in micro-ring resonators having a strong nonlinear thermo-optic coefficient. Furthermore, the model can be used to extract the THG efficiency, the linear and nonlinear thermo-optic coefficients and the polarization dependency of the THG modes from the measurement data.
Ge‐based hybrid perovskite materials have demonstrated great potential for second harmonic generation (SHG) due to the geometry and lone‐pair induced non‐centrosymmetric structures. Here, we report a ...new family of hybrid 3D Ge‐based bromide perovskites AGeBr3, A=CH3NH3 (MA), CH(NH2)2 (FA), Cs and FAGe0.5Sn0.5Br3, crystallizing in polar space groups. These compounds exhibit tunable SHG responses, where MAGeBr3 shows the strongest SHG intensity (5×potassium dihydrogen phosphate, KDP). Structural and theoretical analysis indicate the high SHG efficiency is attributed to the displacement of Ge2+ along 111 direction and the relatively strong interactions between lone pair electrons of Ge2+ and polar MA cations along the c‐axis. This work provides new structural insights for designing and fine‐tuning the SHG properties in hybrid metal halide materials.
A new family of non‐centrosymmetric hybrid Ge‐based bromide perovskites are reported, showing second harmonic generation (SHG) responses. The largest SHG signal from CH3NH3GeBr3 is likely due to the perferable alignment of the polar cation with the lone pair electrons of Ge2+ along the 111 direction.
Although the cornea is the major refractive element of the eye, the mechanisms controlling corneal shape and hence visual acuity remain unknown. To begin to address this question we have used ...multiphoton, non-linear optical microscopy to image second harmonic generated signals (SHG) from collagen to characterize the evolutionary and structural changes that occur in the collagen architecture of the corneal stroma. Our studies show that there is a progression in complexity of the stromal collagen organization from lower (fish and amphibians) to higher (birds and mammals) vertebrates, leading to increasing tissue stiffness that may control shape. In boney and cartilaginous fish, the cornea is composed of orthogonally arranged, rotating collagen sheets that extend from limbus to limbus with little or no interaction between adjacent sheets, a structural paradigm analogous to 'plywood'. In amphibians and reptiles, these sheets are broken down into broader lamellae that begin to show branching and anastomosing with adjacent lamellae, albeit maintaining their orthogonal, rotational organization. This paradigm is most complex in birds, which show the highest degree of lamellar branching and anastomosing, forming a 'chicken wire' like pattern most prominent in the midstroma. Mammals, on the other hand, diverged from the orthogonal, rotational organization and developed a random lamellar pattern with branching and anastomosing appearing highest in the anterior stroma, associated with higher mechanical stiffness compared to the posterior stroma.
High-order harmonic generation in solid state materials is viewed as a means of studying strong field nonperturbative physics. Researching ellipticity dependence can help understand the dynamics of ...electron–hole pairs in the presence of strong laser field. In this work, ellipticity dependence of high harmonic generation in few layer MoS2 is experimentally investigated with a midinfrared laser centered at 3.9 um. We irradiate monolayer, bilayer and trilayer MoS2 with intense mid-infrared light to generate non-perturbative harmonics up to 11th order. We find that the normalized yield of each harmonic descends in a form of Gaussian-like profile as increasing the ellipticity of driving pulse. It is astonishing to see that ellipticity dependence of monolayer, bilayer and trilayer MoS2 follows the same rules. We ascribe this similarity to the same deviation distance of electron–hole pairs in three sets of samples. Meanwhile, the higher the harmonic order is, the more sensitive it is to ellipticity, which shows agreement with gas-phase harmonics. To explain the phenomena, we explore the role of intraband and interband dynamics with multiband semiconductor Bloch equations, which helps to delve deeper into ultrafast and strong-field features of MoS2.
•Experimental comparison of HHG ellipticity dependence in few-layer MoS2.•Simulations using multi-band model.•Comparison of above- and below-bandgap harmonics ellipticity dependence.
The new compounds Li2Mg2Si2S6 and Li2Mg2Ge2S6 have been prepared via traditional high‐temperature, solid‐state synthesis. The title compounds crystallize in the polar, noncentrosymmetric, trigonal ...space group P31m (No. 157) and adopt a new structure type featuring staggered, ethane‐like (T2S6)6− units, where T=Si or Ge. These (T2S6)6− units are nestled within the holes of magnesium‐sulfide “layers” that are created through the edge‐sharing of MgS6 octahedra. The holes found in the lithium‐sulfide “layers”, created by LiS6 edge‐sharing octahedra, remain vacant, containing no (T2S6)6− anionic group. Through the face sharing of the respective MgS6 and LiS6 octahedra, the magnesium‐sulfide and lithium‐sulfide “layers” are stitched together resulting in an overall three‐dimensional structure. The optical bandgaps of Li2Mg2Si2S6 and Li2Mg2Ge2S6 are 3.24 and 3.18 eV, respectively, as estimated from optical diffuse reflectance UV‐Vis‐NIR spectroscopy. The compounds exhibit second harmonic generation responses of ∼0.24×KDP and ∼2.92×α‐quartz for Li2Mg2Si2S6 and ∼0.17×KDP and ∼2.08×α‐quartz for Li2Mg2Ge2S6, using a Nd:YAG laser at 1.064 μm. Electronic structure calculations were performed using density functional theory and the linearized augmented plane‐wave approach within the WIEN2k software package. Examination of the electronic band structures shows that these compounds are indirect bandgap semiconductors.
Second Harmonic Generation
In article number 2100409, Lei Wang, Xiuquan Zhang, and Feng Chen demonstrate efficient second harmonic generation via modal phase matching in a dual‐layer thin‐film ...lithium niobate on insulator (LNOI) nanophotonic waveguide. The novel thin‐film lithium niobate consists of two layers of x‐cut LiNbO3, and their polarizations are antiparallel. Conversion efficiency up to 5540% W‐1cm‐2 has been achieved in waveguides under optical excitation at fundamental wavelength of ∼1.5 μm.
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