resonators, and nano-antennas. [7][8][9][10][11][12][13] In addition to these structures, their organic equivalent can be a fascinating alternative. Additionally, observing this weak chiral event in the (usually feeble) NLO (nonlinear optical) signal is a challenging endeavor. In this context, we wondered about the possibility of using organic micro-optical waveguides [14][15][16][17] to produce and guide chirality-controlled NLO signals as these structures are known to confine photons in two dimensions and allow 1D propagation. Though a few NLO waveguides are reported, [18][19][20][21][22][23][24][25] to the best of our knowledge, we are not able to access any reports on organic waveguides exhibiting chirality effect in the NLO signal. To fabricate nano/microcrystalline [25,26] chiral waveguides, we envisioned organic molecules possessing chirality and electron donor-acceptor characters. [26,27] Generally, electron push-pull organic molecules due to their large dipole moment provide large Frenkel exciton binding energy (up to 1 eV), [1,27] high photoluminescence (PL) efficiency, [26,27] tunable optical band gap, [1,25,26] easy solution processability, [1] size and shape control of nanostructures, [14,29,30] and more importantly, easy access to generate chirality information encoded NLO signals [12] such as multiphoton PL [12,21,28] and second-harmonic generation (SHG). [13] The latter signal arises as a result of the nonzero second-order electric susceptibility (χ (2) ≠ 0) emerging from the non-centrosymmetric crystal packing provided by the molecular chirality.For our investigations, we chose enantiomeric R-and S-4,4′-(2,2′-diethoxy-1,1′-binaphthyl-6,6′-diyl)-dibenzaldehyde (1-R and 1-S) molecules [12] (Figure 1A). The molecular structures of 1-R and 1-S possess: i) chirality, which emerges from their axially chiral nature of the molecule provided by the chiral π-conjugated spacer, ii) abilities to crystallize in chiral non-centrosymmetric crystallographic space group, iii) multiphoton absorbance and SHG as a result of charge-transfer (CT) character provided by the electron donating ethoxy and electron accepting benzaldehyde functional groups attached to enantiomers, and iv) CD effect in the NLO signal due to chiral nature of the molecules.Here in this work, we report the first realization of multiphoton pumped organic chiral micro-rod waveguides self-assembled from 1-R and 1-S enantiomers displaying chiro-NLO effects. These enantiomeric micro-rods generate one-, two-, three-photon pumped PL including SHG and self-guide Production of chiral light and its manipulation down to nanolevel is a very challenging endeavor in the area of nanophotonics. In this work, the above demanding requirements are realized explicitly in two R-and S-type chiral organic optical waveguides which are self-assembled from charge-transfer type axially chiral enantiomeric molecules. These enantiomerically pure micro-optical waveguides generate one-, two-, three-photon pumped optical emissions. Remarkably, these waveguides also demonstrate...
Highly pure, organic, crystalline materials with nonlinear optical (NLO) properties are in great demand due to their potential to be utilized in miniaturized nanophotonic device applications. Perylene dye is one of the celebrated near‐direct bandgap NLO materials. It crystallizes in two distinctive polymorphic forms (square‐shaped, α, and rhombus‐shaped, β) emitting yellow and green fluorescence, respectively. However, selective access to any one of the polymorphic microcrystals possessing qualities such as smooth‐surface and mirror‐like light‐reflecting sharp edges is a challenging task. On the other hand, these qualities are indispensable for a microcrystal to operate as an optical cavity. Here, a cost‐effective and straightforward, yet promising sublimation technique to grow microscale perylene crystals with the above qualities in a polymorph‐selective manner at ambient pressure is presented. As a result, both polymorphic microcrystals act as whispering gallery mode (WGM) cavities in the linear and notably, NLO regime as well. In agreement with the experiments, finite difference time domain numerical calculations support the WGM‐cavity‐type and also reveal the intricate localization of electric‐field within these cavities. Further, the quadratic dependence of emission intensity as a function of laser power establishes the two‐photon absorption nature of the optical cavities pumped by infrared lasers.
Organic polar crystals are a promising material platform for achieving unique nonlinear nanophotonic properties. Mainly, tunable second‐harmonic generation (SHG) with continuous‐wave (CW) pump sources is considered as breakthrough technology for optical switching, actuation, sensing, chip‐integrated coherent light sources, and low‐threshold tunable microlasers. Here, the discovery of tunable CW SHG in a polar (non‐centrosymmetric) electro‐mechanical microcrystal cavity of 4‐(4‐(methylthio)phenyl)‐2,6‐di(1H‐pyrazol‐1‐yl)pyridine (UOH1) is demonstrated. As a result of total internal reflection in mirror‐like light‐reflecting facets and its high second‐order nonlinear susceptibility, the octahedron‐shaped cavities exhibit both bright femtosecond and CW‐pumped SHG with coherent whispering gallery mode (WGM) and bow‐tie‐like (BT) optical modes excited in its spectrum. Moreover, for an external dc electric field of about 23 kV cm−1 the microcavity displays 0.18 nm resonant wavelength peak shift in the second harmonic signal due to electrostriction (strain effect).
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