Meta-Chirality: Fundamentals, Construction and Applications.

In this article, recent progress and development of terahertz chiral metamaterials including stereometamaterials are thoroughly reviewed. This review mainly focuses on the fundamental principles of design and arrangement of meta-atoms in metamaterials exhibiting chirality with various asymmetry and symmetry and 2D and 3D configuration. Related optical and propagation properties in chiral metamaterials, such as optical activity, circular dichroism, and negative refraction for each different chiral metamaterials, are compared and investigated. Finally, comparison between chiral metamaterials with stereometamaterials in terms of the polarization selective operation along with the similarity and the distinction is addressed as well.

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.

There has been a considerable effort recently in the development of planar chiral metamaterials. Owing to the lack of inversion symmetry, these materials have been shown to display interesting physical properties such as negative index of refraction and giant optical activity. However, the biosensing capabilities of these chiral metamaterials have not been fully explored. Ultrasensitive detection and structural characterization of proteins adsorbed on chiral plasmonic substrates was demonstrated recently using UV-visible circular dichroism (CD) spectroscopy. Second harmonic generation microscopy is an extremely sensitive nonlinear optical probe to investigate the chirality of biomaterials. In this study, we characterize the chiral response of chiral plasmonic metamaterials using second harmonic generation microscopy and CD spectroscopy. These planar chiral metamaterials, fabricated by electron-beam lithography, consist of right-handed and left-handed gold gammadions of length 400 nm and thickness 100nm, deposited on a glass substrate and arranged in a square lattice with a periodicity of 800nm.

Tunable optical activity in chiral metamaterials is demonstrated in simulation and shows actively tunable giant polarization rotation over a wide frequency band.

While single-mode waveguides are commonly used in integrated photonic circuits, emerging applications in nonlinear and quantum optics rely fundamentally on interactions between modes of different order. Here we propose several methods to evaluate the modal composition of both externally and device-internally excited guided waves and discuss a technique for efficient excitation of arbitrary modes. The applicability of these methods is verified in photonic circuits based on aluminum nitride. We control modal excitation through suitably engineered grating couplers and are able to perform a detailed study of waveguide-internal second harmonic generation. Efficient and broadband power conversion between orthogonal polarizations is realized within an asymmetric directional coupler to demonstrate selective excitation of arbitrary higher-order modes. Our approach holds promise for applications in nonlinear optics and frequency up/down-mixing in a chipscale framework.

Terahertz angle-insensitive 90° polarization rotator using chiral metamaterial

We propose a new uniaxial chiral metamaterial which is composed of four crescent shaped split-ring resonators mutually twisted by 90° and patterned on opposite sides of a dielectric substrate. It illustrates a strong optical activity and circular dichroism in the range from 5 GHz to 10 GHz. The presented design can be fabricated more easily due to its efficient metallic structures. Furthermore, chirality value of this proposed chiral metamaterial is large enough the optical applications. This study also shows that the proposed crescent shaped chiral metamaterials have good responses when compared with other studied samples in literature. All validations are performed by computer simulation program CST Microwave Studio in the 5–10 GHz frequency range.

Charge separated molecules with strong zwitterionic character make especially interesting species for quadratic Non Linear Optical (NLO) doped materials because of their large dipole moments and rather large quadratic hyperpolarizabilities. Furthermore, their sensitivity to the dielectric environment brands them as good candidates for checking the validity of formalisms relating geometrical structure and NLO properties. We have carried out an experimental and theoretical study on two phenoxide pyridinium derivatives with different inter ring twist angle, combining UV-visible, IR and NLO spectroscopies supplemented by semi-empirical and density functional calculations. Experimentally, changing the polarity of the solvent allows us to explore only a small range of twist angles. However, changing the pyridinium nitrogen atom from para to meta position gives essentially the same compound but with a larger twist angle. Changes in solvent polarity and position substitution give rise to a combined enhancement of the quadratic response by an order of magnitude, showing experimentally the possibility of tuning NLO properties of chromophores to be used as doping agents in NLO applications.

Easy-to-fabricate, large-area, and inexpensive microstructures that realize control of the polarization of terahertz (THz) radiation are of fundamental importance to the development of the field of THz wave photonics. However, due to the lack of natural materials that can facilitate strong THz radiation-matter interactions, THz polarization components remain an undeveloped technology. Strong resonance-based responses offered by THz metamaterials have led to the recent development of THz metadevices, whereas, for polarization control devices, micrometer-scale fabrication techniques including aligned photolithography are generally required to create multilayer microstructures. In this work, leveraging a two-step textile manufacturing approach, a chiral metamaterial capable of exhibiting strong chiroptical responses at THz frequencies is demonstrated. Chiral-selective transmission and pronounced optical activity are experimentally observed. In sharp contrast to smart-clothing-related devices (e.g., textile antennas), the investigated chiral metamaterials gain their THz properties directly from the yarn-twisting enabled microhelical strings. It is envisioned that the interplay between meta-atom designs and textile manufacturing technology will lead to a new family of metadevices for complete control over the phase, amplitude, and polarization of THz radiation.

The growth and overlap of research in the fields of photonic crystals (PhCs) and nonlinear optics should lead to new exciting, active functions and high-efficiency nonlinear optical applications because nonlinear optical effects in PhCs may allow for significant advances in various optical processes by using resonantly stored light and anomalous band dispersions, in combination with nonlinear optical host materials. To show the experimental and theoretical evidences of great potentials of the nonlinear PhCs for the realization of the high-efficient and very compact nonlinear optical devices and applications, we have examined the typical second- and third-order nonlinear optical responses and the photonic band structure of the nonlinear two-dimensional (2D) PhC waveguides. We have demonstrated for the first time the second harmonic generation (SHG), the sum-frequency mixing (SFM) in the ultraviolet (UV) region, and the strong enhancement of the SHG intensity originating from the photonic band resonance in this waveguide. Moreover, by probing the nonlinear optical changes of the band resonances in angle-resolved reflectivity,we have shown direct evidence that the nonlinear optical changes arising from modifications of the photonic bands by purely optical means are dominated by the dispersion nature and the group velocity of the photonic bands, which are essential for the realization of desirable nonlinear applications, such as practical all-optical switching devices with very low operational power and ultra-small dimensions. These results agree well with the bhavior predicted from band structures, indicating that the design of nonlinear optical properties of material systems is realistically possible by band dispersion and group velocity engineering.

Integrated photonics has been pursued on a large variety of materials and platforms over more than half a century. In recent decades, silicon has emerged as the preferred optical and substrate material. However, since many optical functionalities cannot be, inherently or conveniently, implemented on silicon-based devices, heterogeneous integration of other materials has been an inevitable constituent of silicon photonics from its inception. This paper reviews recent progress in the heterogeneous integration of dielectrics and compound semiconductors as the core optical material for ultracompact nonlinear integrated photonics, preferably on silicon substrates. Occasionally, important devices on native substrates of the core nonlinear material are also reviewed. The focus is on ultracompact photonic devices and circuits formed on submicron films of lithium niobate (LiNbO3), and the aluminium-gallium-arsenide (AlGaAs) and aluminium-gallium-nitride (AlGaN) families for second-order nonlinear processes, such as second-harmonic generation, and techniques to address phase-matching in them. Integration of second- and third-order nonlinear devices on the same chip appears to be a future trend, thus the related preliminary results are reviewed. A broader introduction to heterogeneously integrated photonics, shortcomings of conventional silicon photonics for application in nonlinear optics, as well as an overview of modeling second-order nonlinear integrated-waveguide devices are also provided for completeness.

In this paper, a dualband chiral metamaterial (CMM) based on cross-wire structure is proposed and studied numerically. It exhibits dual-band giant optical activity and negative refractive index in terahertz region. The surface current distributions are calculated to explain original physics. The further numerical results show that the efiective frequency bands of the CMMs can be independent adjusted easily by changing the structure geometrical parameter. The designed dual-band terahertz CMMs ofier ∞exibility in the investigation of novel terahertz device application.

In this paper, a multi-band chiral metamaterials (CMMs) based on complementary cross-wire structure is proposed and studied numerically. The finite element method (FEM) method was used to simulate the EM properties of the designed structure. Numerical results indicate that the giant optical activity and multiband negative refractive index can be realized in terahertz region. The proposed CMMs may have some potential applications in novel terahertz device.Particle size of bayerite (d50) was about 24 μm. Al2O3was obtained after calcination at 1050 °C for 1.5 h.

The three-dimensional, geometric handedness of chiral optical metamaterials allows for the rotation of linearly polarized light and creates a differential interaction with right and left circularly polarized light, known as circular dichroism. These three-dimensional metamaterials enable polarization control of optical and spin excitation and detection, and their stimuli-responsive, dynamic switching widens applications in chiral molecular sensing and imaging and spintronics; however, there are few reconfigurable solid-state implementations. Here, we report all-solid-state, thermally reconfigurable chiroptical metamaterials composed of arrays of three-dimensional nanoparticle/metal bilayer heterostructures fabricated from coassemblies of phase change VO2 and metallic Au colloidal nanoparticles and thin films of Ni. These metamaterials show dynamic switching in the mid-infrared as VO2 is thermally cycled through an insulator-metal phase transition. The spectral range of operation is tailored in breadth by controlling the periodicity of the arrays and thus the hybridization of optical modes and in position through the mixing of VO2 and Au nanoparticles.

We fabricated Au nanoparticles on the surface of TiO2 by photo-reduction method. The structural property of film is investigated using transmission electron microscopy technique. The surface plasmon resonance absorption peaks of TiO2 and Au-TiO2 were recorded using Shimadzu (UV-2450) UV-visible spectrophotometer. The work function of films was measured by scanning Kelvin probe microscopy (SKP5050) from KP technology, United Kingdom. The nonlinear optical refractive index and nonlinear optical absorption coefficient of Au-TiO2 nanocomposites were simultaneously measured using z-scan technique. The nonlinear optical response of Au-TiO2 nanocomposites is due to pure electronic transfer effects in Au nanoparticles. The observed nonlinearity is due to the dielectric constant of Au, which is due to the surface plasmon resonance and surface polarization between Au nanoparticles and TiO2. The dielectric constant of Au shows maximum value at 532 nm wavelength (2.33 eV). Surface plasmon resonance effect of Au nanoparticles is directly related to the metal dielectric constant, therefore it is increased. The increase in surface plasmon rsonance of Au-TiO2 nanocomposites can be observed from UV-visible absorption spectra. The optical nonlinearity depends strongly on the dielectric constant of Au. Therefore, the optical nonlinearity increases with the increase in surface plasmon resonance peak. The absorption peak of surface plasmon resonance at 544 nm is inversely proportional to the work function. Therefore, we can say that the Au dopant decreases the work function that effectively increases the surface plasmon resonance absorption peak at 544 nm, so that the nonlinear response is enhanced. The systematic change in the work function with Au concentration plays a major role in optical nonlinearity. The estimated optical nonlinearity was found to increase from 3.80×10-6 to 9.69×10-6 esu with increase in Au concentrations from 0 to 1.0x10-2 mole. This observed increment in nonlinearity is due to the enhancement of local electric field created by excitation of surface plasmon resonance that affects the work function. Therefore, the surface plasmon resonance and work function help in tuning the optical nonlinearity. The tunable nonlinear optical response of the Au-TiO2 nanocomposites may find applications in nonlinear optics at wavelength 532 nm and the development of materials for generating the higher order harmonics.