Strong Nonlinear Optical Activity Induced by Lattice Surface Modes on Plasmonic Metasurface.

Optical activity (polarization rotation of light) is one of the most desired features of chiral media, as it is important for many polarization‐related applications. However, in the THz region, chiral media with strong optical activity are not available in nature. Here, a chiral metamaterial (CMM) structure composed of pairs of vertical U‐shape resonators of “twisted” arms is studied, and it is revealed that it demonstrates large pure optical activity (i.e., optical activity associated with negligible transmitted wave ellipticity) in the low THz regime. The experimental data show polarization rotation up to 25° for an unmatched bandwidth of 1 THz (relative bandwidth 80%), from a 130 µm‐thickness structure, while theoretical optimizations show that the rotation can reach 45°. The enhanced chiral response of the structure is analyzed through an equivalent RLC circuit model, which also provides simple optimization rules for the enhancement of its chiral response. The proposed chiral design allows for easy fabrication via direct laser writing (DLW) and electroless metal plating, making the associated structures suitable candidates for polarization control applications.

We demonstrate the generation of solitons in the photorefractive materials with strong optical activity and absorption. The general wave equations are processed by rotation and affine transformations in order to decouple the effects of photorefractivity and optical activity. Analytical solutions of these equations are found in the case of slowly varying envelope and low-intensity approximations. The solutions show the occurrence of breathing solitons in sillenite crystals with strong optical activity and absorption in a particular orientation with respect to the external electric field. The absorption is decreasing the soliton intensity with the propagation distance (which set a limit in the soliton channel length) and is increasing the breathing period.

The Au102(p-MBA)44 cluster (p-MBA: para-mercaptobenzoic acid) is observed as a chiral compound comprised of achiral components in its single-crystal structure. So far the enantiomers observed in the crystal structure are not isolated, nor is the circular dichroism spectrum known. A chiral phase transfer method is presented which allows partial resolution of the enantiomers by the use of a chiral ammonium bromide, (-)-1R,2S-N-dodecyl-N-methylephedrinium bromide ((-)-DMEBr). At sufficiently low concentration of (-)-DMEBr, the phase transfer from water to chloroform is incomplete. Both the aqueous and organic phases show optical activity of near mirror image relationship. Differences in the spectra are ascribed to the formation of diastereomeric salts. At high concentrations of (-)-DMEBr, full phase transfer is observed. The organic phase, however, still displays optical activity. We assume that one of the diastereomers has very strong optical activity, which overrules the cancelation of the spectra with opposite sign. Comparison with computations further corroborates the experimental data and allows a provisional assignment of handedness of each fraction.

Wide bandgap semiconductors such as gallium nitride (GaN) are essential constituents of future optical circuits, as their optical response can accommodate a broad wavelength range while suppressing two-photon absorption and free-carrier absorption effects that are encountered with silicon (Si) structures. Their direct wide bandgap is also favourable for the incorporation of active elements. Furthermore, nonlinear optical processes can be harnessed by exploiting the higher-order susceptibility tensors of the crystal structure to achieve advanced light control modalities, enabling all-optical processing and the generation of entangled photon states. We will report on nonlinear frequency conversion from the telecom range via second harmonic generation (SHG) and third harmonic generation (THG) in suspended gallium nitride slab photonic crystal (PhC) cavities on silicon, under continuous-wave resonant excitation. Genetic optimization is applied to sweep parameter space for the highest cavity quality factors, and simultaneously accounting for power in-coupling. While there is a clear trade-off theoretically between coupling efficiency and Q-factor for a given cavity design, the upper limit on the Q-factor that is imposed by loss channels, given the disorder figure of current fabrication technology, makes room for introducing improved far-field coupling to enhance nonlinear processes without sacrificing the experimentally achievable light confinement. Far-field coupling is addressed through various PhC cavity designs, which enable the excitation of the fundamental mode with a Gaussian beam. Optimized two-dimensional PhC cavities with increased far-field coupling have been characterized with quality factors as high as 44′000, approaching the computed theoretical values. The strong enhancement in light confinement has enabled second harmonic generation (SHG) under continuous-wave excitation, with up-conversion from both 1300 nm and 1550 nm wavelength bands, confirmed by spectral and power dependence measurements. At 1550 nm, normalized SHG conversion efficiency as large as 2.4×10−3 W−1 are measured as well as simultaneous THG. SHG emission power of up to 0.74 nW has been detected without saturation

Recently, the terahertz (THz) chiral field control opens a new window to THz devices and their applications. In this Letter, the active manipulation for THz chiral states based on the cholesteric liquid crystal (CLC) has been demonstrated by THz time domain cross-polarization spectroscopy. The results show that the CLC has strong THz optical activity and circular dichroism (CD) effect, and the strongest THz CD of 22 dB and a polarization rotation angle of 88.4° occur around the phase transition temperature TS-N=250K. Rising to a room temperature of 300 K, the CLC turns from a chiral state to an isotropic state for THz waves with the phase transition processes of CLC molecules. Therefore, this CLC device can be performed as a thermally active THz circular polarizer, which brings potential applications in THz polarization imaging, broadband communication, and spectroscopy.

We demonstrate the first experimental realization of the 8-srs network by direct laser writing in a dielectric medium. The optical properties of this photonic crystal are both numerically simulated and experimentally measured. The numerical simulations indicate a strong optical activity (OA) for regions of the spectrum close to the unit cell size (3 μm) and a high sensitivity to angular resolution. Experimentally, we observe 40° of OA for a region of the spectrum between 3 and 3.5 μm, confirming both simulation predictions.

We used the DNA origami method (Rothemund, 2006) for the fabrication of self-assembled nanoscopic materials (Seeman, 2010). In DNA origami, a virus-based 8 kilobase-long DNA single-strand is folded into shape with the help of ∼ 200 synthetic oligonucleotides. The resulting DNA nanostructures can be designed to adopt any three-dimensional shape and can be addressed through DNA hybridization or chemical modification with nanometer precision. We have realized that complex assemblies of nanoparticles, including magnetic, fluorescent, and plasmonic nanoparticles. Such nanoconstructs may exhibit striking optical properties such as strong optical activity in the visible range (Kuzyk et al., 2012). To this end, plasmonic particles were assembled in solution to form helices of controlled handedness. We achieved spatial control over particle placement better than 2 nm and attachment yields of 97% and above. As a collective optical response emerging from our dispersed nanostructures, we detected pronounced circular dichroism (CD) originating from the plasmon–plasmon interactions in the particle helices. In recent experiments, we were able to show that the optical response of chiral biomolecules can be transferred from the UV into the visible region in plasmonic hotspots. Thus, sensitive detection of chiral biomolecules may become feasible in the near future. We also found that the orientation of the helices in respect to the incoming light beam critically influences the resulting CD spectra. Our results can be explained with theoretical models based on plasmonic dipole interaction and demonstrate the potential of DNA origami for the assembly of metafluids with designed optical properties.

A complementary Y-shaped chiral metamaterial (CYCMM) is proposed for the realization of giant optical activity and circular polarization with strong circular dichroism (CD) simultaneously for terahertz (THz) waves. It is demonstrated that the proposed CYCMM can achieve 90° linear polarization rotation around 2.13 THz and a giant CD effect at 2.38 THz through full-wave simulations. The mechanism of the giant CD effect and optical activity is illustrated by simulated surface current distribution. Due to strong optical activity and the CD effect, the proposed CYCMM is useful for the development of integrated terahertz spectroscopic and imaging devices.

Multifunctional polarization controllers are highly desirable in numerous fields due to their potential applications. Although the emergence of metasurfaces provides a powerful platform for manipulating the polarization of electromagnetic waves, metasurfaces have not yet enabled multifunctional controllers. Here, this work demonstrates an effective strategy to design a multifunctional polarization controller through the integration of frequency and angular multiplexing. As an experimental proof, a non‐planar dual‐multiplexed metadevice composed of a chiral metagrating array and an anisotropic metasurface is fabricated by printed circuit board technology. The experiments show that at least five distinct functions, including ultra‐broadband linear‐to‐circular polarization conversion in 2.5–6.3 GHz, ultra‐broadband linear‐to‐linear polarization conversion in 6.6–11.0 GHz, strong optical activity between 11.4 and 12.2 GHz, giant linear and circular dichroism ≈5.96 and 12.97 GHz, respectively, can be accomplished by the proposed meta‐multiplexer. Hence, the proposed metadevice can act as high‐efficiency bifunctional waveplate, polarization rotator, linear polarizer, chiral metamirror, and so on, enabling more application flexibility. The concept of integrating multidimensional multiplexing proposed here will be of great benefit to the design of novel multifunctional polarization controllers.

The preparation of the vast majority of nonlinear optical metal metasurfaces currently relies on complex top-down methods such as electron beam or ion beam etching, which are expensive and difficult to meet the requirement of large area preparation. In this paper, an easily prepared quasi-periodic silver dendritic metasurface model with high Q factor is established in the near-infrared band based on a simple and easy-to-operate electrochemical deposition method. The simulations prove that the silver dendritic metasurface has a high Q factor (exceeds 104) because of its strong electric field localization ability, which is analogous to the superposition of multiple split-ring resonators. It is demonstrated that the second harmonic generation (SHG) intensity of the dendritic metasurface at a large incident angle (such as 85°) is about 30 times that of the metasurface at a small incident angle when the x-polarized pump light is incident obliquely to break the centrosymmetry of the metasurface. The influences of the incident angle or dendritic structure’s dimensions on the Q factor and SHG efficiency have also been researched through a lot of simulation. This easily prepared quasi-periodic silver dendritic metasurface SHG device may provide a new avenue for the development and application of miniature, integratable nonlinear optical devices.

Applying the optical activity of cholesteric liquid crystal for measuring small wavelength differences

Terahertz angle-insensitive 90° polarization rotator using chiral metamaterial

The established approach to design a molecule with strong second-order nonlinear optical (NLO) activity is to connect an electron-donor to an electron-acceptor via a π-conjugated bridge, to generate push–pull system. Surprisingly, we have found that dyes with large first hyperpolarizabilities, and which exhibit strong second harmonic generation (SHG), can be created just by attaching an electron-donor to a porphyrin. The free-base porphyrin core is sufficiently electron-deficient that the hyperpolarizability does not increase on addition of a pyridinium electron-acceptor.

Abstract2D materials are excellent platforms for nonlinear optical (NLO) response, especially second harmonic generation (SHG), due to its large surface to volume ratio and sub‐nanometer thickness. The SHG susceptibility strongly relies on the symmetry of materials. Constructing Janus structures can break the out‐of‐plane mirror symmetry, bringing about asymmetric charge distribution and leading to a built‐in electric field. Consequently, SHG response along the out‐of‐plane direction can be improved. In the current work, combining first‐principles calculations with independent particle approximation, the SHG response of nine Janus group‐III chalcogenide monolayers (M2XX', MM'X2) are systematically evaluated. Both extraordinary in‐plane and out‐of‐plane SHG response are revealed in all the Janus structures. Besides, cation MM'X2 Janus structures exhibit systemically higher SHG response than that of anion M2XX’ due to stronger dipole. Among them, GaInTe2 possesses extremely high out‐of‐plane SHG response with d31 up to 10490.4 pm V−1 at photon energy (PE) of 4.7 eV, enabling promising applications in ultraviolet NLO devices. The SHG intensity polar plots from Janus structures display unusual rotational symmetry at different PEs allowed by the out‐of‐plane SHG components. This work provides theoretical guidelines for further experimental explorations in 2D group‐III monochalcogenide Janus structures and paves the way for their utilization in NLO devices.

Graphene magnetoplasmons, collective excitations of massless Dirac fermions, can be actively tuned by an external magnetic field in addition to electrostatic gating. Their magneto-optic properties are highly determined by the edges of the structure and their interaction. Here, we focus on the fundamental dipolar magnetoplasmon, which has strong optical activity, and report its extinction spectra in graphene circular rings, eccentric rings, and split rings. We find that, as compared to traditional graphene disks, circular rings still sustain symmetrical mode splitting (${\ensuremath{\omega}}_{+}$ and ${\ensuremath{\omega}}_{\ensuremath{-}}$) but with less energy difference $\mathrm{\ensuremath{\Delta}}\ensuremath{\omega}={\ensuremath{\omega}}_{+}\ensuremath{-}{\ensuremath{\omega}}_{\ensuremath{-}}$. Increasing the ratio of inner radius to outer radius, $\mathrm{\ensuremath{\Delta}}\ensuremath{\omega}$ will decrease further and eventually vanish at a critical ratio, which can be well described as a function of the only product of cyclotron frequency and relaxation time. In eccentric rings, a small translation of the inner hole will not change the mode splitting and thus the value of $\mathrm{\ensuremath{\Delta}}\ensuremath{\omega}$, but for a larger translation, $\mathrm{\ensuremath{\Delta}}\ensuremath{\omega}$ increases dramatically, with one magnetoplasmonic mode (polarization dependent) getting dark gradually. In split rings, due to symmetry breaking abruptly, even a very tiny split will cause the disappearance of mode splitting. Our findings contribute to a basic understanding of graphene magnetoplasmons which, in turn, can boost the application of graphene magneto-optic devices at terahertz and infrared frequencies.