Atomic layer deposited second-order nonlinear optical metamaterial for back-end integration with CMOS-compatible nanophotonic circuitry

Chiral metal-organic frameworks for photonics

Recently, there has been significant interest in the use of chiral metal-organic frameworks (MOFs) and coordination polymers (CPs) for photonics applications. The promise of these materials lies in the ability to tune their properties through judicious selection of the metal and ligand components. Additionally, the interaction of guest species with the host framework can be exploited to realise new functionalities. In this review, we outline the methods for synthesising chiral MOFs and CPs, then analyse the recent innovations in their use for various optical and photonics applications. We focus on two emerging directions in the field of MOF chemistry - circularly polarised luminescence (CPL) and chiroptical switching - as well as the latest developments in the use of these materials for second-order nonlinear optics (NLO), particularly second-harmonic generation (SHG). The current challenges encountered so far, their possible solutions, and key directions for further research are also outlined. Overall, given the results demonstrated to date, chiral MOFs and CPs show great promise for use in future technologies such as optical communication and computing, optical displays, and all-optical devices.

Maximized Frequency Doubling through the Inverse Design of Nonlinear Metamaterials

The conventional process for developing an optimal design for nonlinear optical responses is based on a trial-and-error approach that is largely inefficient and does not necessarily lead to an ideal result. Deep learning can automate this process and widen the realm of nonlinear geometries and devices. This research illustrates a deep learning framework used to create an optimal plasmonic design for a nonlinear metamaterial. The algorithm produces a plasmonic pattern that can maximize the second-order nonlinear effect of a nonlinear metamaterial. A nanolaminate metamaterial is used as a nonlinear material, and plasmonic patterns are fabricated on the prepared nanolaminate to demonstrate the validity and efficacy of the deep learning algorithm. The optimal pattern produced yielded second-harmonic generation from the nanolaminate with normal incident fundamental light. The deep learning architecture applied in this research can be expanded to other optical responses and light-matter interaction processes.

Application of Nanooptics in Photographic Imagery and Medical Imaging

Background. At present, with the continuous development of nanotechnology, great changes have taken place in people’s lives in medical treatment, production, daily leisure, and so on. Nanooptical technology is entirely based on nanotechnology that laser and visible light are limited to submicron structures (nanopores, nanoslits, and nanoneedles). Due to the great development potential of nanooptical technology in nanoscale sensors, TOF camera applications, THz imaging technology, and other imaging equipment materials and applications, people have been interested in it, recently. Scope and Approach. In this review, the importance of good practices for nanooptical technology used in equipment as both nanometer scale sensors and optical auxiliary equipment is described. Based on recent reports, this work discussed the development of nanooptical technology in daily photography and medical imaging from both the positive and the negative sides and compared the engineering techniques. Key Findings and Conclusions. As a kind of new optical technology, nanooptical technology can produce the plasmonic effect under the intense collision of atoms and electrons in nanostructures. It has significant effects in superresolution nanolithography, high-density data storage, near-field optics, and other fields. Although the current nanooptic technology is not extremely mature, the results obtained from current works are pointing out that nanooptical technology is the future of daily imaging and medical imaging, and it also will play a positive role in the improvement of people’s health and ecological environment quality. As a trend, nanooptical technology is developing in the direction of energy-saving, portability, high efficiency, and low pollution, and in the upsurge of environmental protection in the world, nanooptical technology will surely achieve amazing development in the field of daily photography and medical imaging. Under the huge market demand and innovation power, nanophotonics technology will cover all emerging technologies that share the same research field with it and take advantage of each technology (terahertz, cell and molecular microscopy, and nanoscale probes) to develop an unprecedented new century in nanoscience. The future trends of research contain finding new imaging equipment with nanostructure, designing nanooptical products, and improving engineering techniques.

Strong second-harmonic response from semiconductor-dielectric interfaces

In this study, an analysis of the second-harmonic generation (SHG) response from surfaces containing dielectric-semiconductor interfaces with sub-wavelength features is presented. The investigated medium is a metamaterial where the SHG response is governed by the symmetry breaking between consecutive layers. The examined material is composed of a periodic structure based on 50 nm silicon nitride and 10 nm indium gallium zinc oxide (IGZO) fabricated on a quartz glass substrate. The elementary cell consists of a pair of materials in an exchangeable order. The preliminary results show a promising application of the amorphous IGZO as a nonlinear optical material, whose optical characteristics can be controlled by the fabrication process itself. Prepared structures give a remarkably high SHG response. For an effective thickness of the structure equal to 240 nm, a more than 250-fold increase in SHG compared to the reference substrate is observed.

Enhancement of bulk second-harmonic generation from silicon nitride films by material composition

We present a comprehensive tensorial characterization of second-harmonic generation from silicon nitride films with varying compositions. The samples were fabricated using plasma-enhanced chemical vapor deposition, and the material composition was varied by the reactive gas mixture in the process. We found a six-fold enhancement between the lowest and highest second-order susceptibility, with the highest value of approximately 5 pm/V from the most silicon-rich sample. Moreover, the optical losses were found to be sufficiently small (below 6 dB/cm) for applications. The tensorial results show that all samples retain in-plane isotropy independent of the silicon content, highlighting the controllability of the fabrication process.

Electronic Metamaterials with Tunable Second-order Optical Nonlinearities

The ability to engineer metamaterials with tunable nonlinear optical properties is crucial for nonlinear optics. Traditionally, metals have been employed to enhance nonlinear optical interactions through field localization. Here, inspired by the electronic properties of materials, we introduce and demonstrate experimentally an asymmetric metal-semiconductor-metal (MSM) metamaterial that exhibits a large and electronically tunable effective second-order optical susceptibility (χ⁽²⁾). The induced χ⁽²⁾ originates from the interaction between the third-order optical susceptibility of the semiconductor (χ⁽³⁾) with the engineered internal electric field resulting from the two metals possessing dissimilar work function at its interfaces. We demonstrate a five times larger second-harmonic intensity from the MSM metamaterial, compared to contributions from its constituents with electrically tunable nonlinear coefficient ranging from 2.8 to 15.6 pm/V. Spatial patterning of one of the metals on the semiconductor demonstrates tunable nonlinear diffraction, paving the way for all-optical spatial signal processing with space-invariant and -variant nonlinear impulse response.

On the determination of χ(2) in thin films: a comparison of one-beam second-harmonic generation measurement methodologies

The determination of the second-order susceptibility (χ⁽²⁾) of thin film samples can be a delicate matter since well-established χ⁽²⁾ measurement methodologies such as the Maker fringe technique are best suited for nonlinear materials with large thicknesses typically ranging from tens of microns to several millimeters. Here we compare two different second-harmonic generation setups and the corresponding measurement methodologies that are especially advantageous for thin film χ⁽²⁾ characterization. This exercise allows for cross-checking the χ⁽²⁾ obtained for identical samples and identifying the main sources of error for the respective techniques. The development of photonic integrated circuits makes nonlinear thin films of particular interest, since they can be processed into long waveguides to create efficient nonlinear devices. The investigated samples are ABC-type nanolaminates, which were reported recently by two different research groups. However, the subsequent analysis can be useful for all researchers active in the field of thin film χ⁽²⁾ characterization.

Bulk second-harmonic generation from thermally evaporated indium selenide thin films

We investigate bulk second-order nonlinear optical properties of amorphous indium selenide thin films fabricated by thermal evaporation. Such films are shown to exhibit strong and photostable second-harmonic generation (SHG). We report strong thickness dependence of the second-harmonic signals as characterized by the Maker-fringe method. The absolute value of the nonlinear susceptibility tensor of the film is addressed by analyzing the interference of SHG signals from the film and the glass substrate. The value of the joint non-diagonal component of the susceptibility is found to be 4 pm/V, which is comparable to that of widely used second-order nonlinear materials.

Sub-wavelength modulation of χ(2) optical nonlinearity in organic thin films

Modulating the second-order nonlinear optical susceptibility (χ⁽²⁾) of materials at the nanoscale represents an ongoing technological challenge for a variety of integrated frequency conversion and nonlinear nanophotonic applications. Here we exploit the large hyperpolarizability of intermolecular charge transfer states, naturally aligned at an organic semiconductor donor–acceptor (DA) interface, as a means to control the magnitude and sign of χ⁽²⁾ at the nanoscale. Focusing initially on a single pentacene-C₆₀ DA interface, we confirm that the charge transfer transition is strongly aligned orthogonal to the heterojunction and find that it is responsible for a large interfacial nonlinearity probed via second harmonic generation that is sufficient to achieve d₃₃>10 pm V⁻¹, when incorporated in a non-centrosymmetric DA multilayer stack. Using grating-shadowed oblique-angle deposition to laterally structure the DA interface distribution in such multilayers subsequently enables the demonstration of a χ⁽²⁾ grating with 280 nm periodicity, which is the shortest reported to date.

Materials with spatially modulated nonlinear optical properties are used for quasi-phase matching. Here, Yan et al. exploit the nonlinearity of intermolecular charge transfer states together with oblique-angle deposition to achieve nanoscale modulation of the second-order susceptibility.

Enhancement of second-harmonic generation in nonlinear nanolaminate metamaterials by nanophotonic resonances

Nanolaminate metamaterials recently attracted a lot of attention as a novel second-order nonlinear material that can be used in integrated photonic circuits. Here, we explore theoretically and numerically the opportunity to enhance the nonlinear response from such nanolaminates by exploiting Fano resonances supported in grating-coupled waveguides. The enhancement factor of the radiated second harmonic signal compared to a flat nanolaminate can reach values as large as 35 for gold gratings and even 7000 for MgF₂ gratings. For the MgF₂ grating, extremely high-Q Fano resonances are excited in such all-dielectric system that result in strong local fields in the nonlinear waveguide layer to boost the nonlinear conversion. A significant portion of the nonlinear signal is also strongly coupled to a dark waveguide mode, which remains guided in the nanolaminate. The strong excitation of a dark mode at the second harmonic frequency provides a viable method for utilizing second-order nonlinearities for light generation and manipulation in integrated photonic circuits.

Recognition of multipolar second-order nonlinearities in thin-film samples

We use two-beam second-harmonic generation to address thin films of silicon nitride (SiN). This technique is able to distinguish between the dipolar and higher-multipolar (magnetic and quadrupolar) contributions to the nonlinearity, as earlier shown for bulk samples. Our results for the SiN films exhibit strong multipolar signatures. Nevertheless, the results can be fully explained by the strong dipolar response of SiN once multiple reflections of the fundamental and second-harmonic fields within the film are properly taken into account. The results show that the recognition of multipolar nonlinearities requires extreme care for samples typically used for the characterization of new materials.