Surface-normal coupled four-wave mixing in a high contrast gratings resonator

Inverse design of sub-wavelength convergent grating based on the "stepped particle swarm algorithm"

In this Letter, a novel "stepped particle swarm optimization" (SPSO) based on field intensity adjustment is proposed. After that, we used this algorithm to design a sub-wavelength converging grating that could be integrated with the detector on the back. Firstly, the advantages of reverse design in the process of two-dimensional or multi-element grating pattern design were summarized by comparing the theory of forward and reverse design. Then, the common "particle swarm optimization" (PSO) and our proposal were compared in the reverse design process; we found that the field strength at the focal point obtained by the improved algorithm was approximately twice of the conventional PSO, and the SPSO had higher computational efficiency and better global convergence. The value of the SPSO had been steadily enlarged, which significantly improved the performance of the sub-wavelength convergent grating. Finally, the property of the sub-wavelength converging grating was simulated, and we found that the grating could achieve good convergence in the focal length range of 100-150 µm, and had a strong ability to compensate for the position deviation of the incident light. In actual optical communications, this sub-wavelength converging grating will play an important role in alleviating the bandwidth and quantum efficiency, improving the performance of the detectors.

Ultrathin multi-band coherent perfect absorber in graphene with high-contrast gratings

High-contrast gratings (HCGs) can be designed as a resonator with high-quality factor and surface-normal emission, which are excellent characters for designing optical devices. In this work, we combine HCGs with plasmonic graphene structure to achieve an ultrathin five-band coherent perfect absorber (CPA). The presented CPA can achieve multi- and narrow-band absorption with high intensity under a relatively large incident angle. The good agreement between theoretical analysis and numerical simulated results demonstrates that our proposed HCGs-based structure is feasible to realize CPA. Besides, by dynamically adjusting the Fermi energy of graphene, we realize the active tunability of resonance frequency and absorption intensity simultaneously. Benefitting from the combination of HCGs and the one-atom thickness of graphene, the proposed device possesses an extremely thin feature. Our work proposes a novel method to manipulate coherent perfect absorption and is helpful to design tunable multi-band and ultrathin absorbers.

Nonlinear Optics in Dielectric Guided-Mode Resonant Structures and Resonant Metasurfaces

Nonlinear optics is an important area of photonics research for realizing active optical functionalities such as light emission, frequency conversion, and ultrafast optical switching for applications in optical communication, material processing, precision measurements, spectroscopic sensing and label-free biological imaging. An emerging topic in nonlinear optics research is to realize high efficiency optical functionalities in ultra-small, sub-wavelength length scale structures by leveraging interesting optical resonances in surface relief metasurfaces. Such artificial surfaces can be engineered to support high quality factor resonances for enhanced nonlinear optical interaction by leveraging interesting physical mechanisms. The aim of this review article is to give an overview of the emerging field of nonlinear optics in dielectric based sub-wavelength periodic structures to realize efficient harmonic generators, wavelength mixers, optical switches etc. Dielectric metasurfaces support the realization of high quality-factor resonances with electric field concentrated either inside or in the vicinity of the dielectric media, while at the same time operate at high optical intensities without damage. The periodic dielectric structures considered here are broadly classified into guided-mode resonant structures and resonant metasurfaces. The basic physical mechanisms behind guided-mode resonances, electromagnetically-induced transparency like resonances and bound-states in continuum resonances in periodic photonic structures are discussed. Various nonlinear optical processes studied in such structures with example implementations are also reviewed. Finally, some future directions of interest in terms of realizing large-area metasurfaces, techniques for enhancing the efficiency of the nonlinear processes, heterogenous integration, and extension to non-conventional wavelength ranges in the ultra-violet and infrared region are discussed.

Feasibility of Using High-Contrast Grating as a Point-of-Care Sensor for Therapeutic Drug Monitoring of Immunosuppressants

Point-of-care (POC) testing has demonstrated great transformative potential in personalized medicine. In particular, patients undergoing transplantation require POC testing to ensure appropriate serum immunosuppressant levels so as to maintain adequate graft function and survival. However, no suitable POC device for monitoring immunosuppressant levels is currently available. Exploiting the latest advances in metamaterials can lead to a breakthrough in POC testing. A high-contrast grating (HCG) biosensor is a low-cost, compact, simple-to-fabricate, and easy-to-operate structure. It is highly sensitive and robust in surface-based biomarker detection, which is favorable for the efficiency of a POC device. In this study, the feasibility of using an HCG as a POC sensor for therapeutic drug monitoring of immunosuppressants was evaluated. The detection efficiency of the most commonly prescribed immunosuppressive medication cyclosporine A by using this sensor was demonstrated to be comparable to those of conventional commercial kits, suggesting that the sensor has the potential to be used as a rapid detection and feedback platform for increasing drug compliance and improving new organ transplant survival.

Microscopic study of resonant third-harmonic generation from amorphous silicon nanodisk arrays

A detailed microscopic study of third-harmonic generation (THG) from two-dimensional arrays of sub-wavelength spaced amorphous silicon nanodisks is reported. The arrays are designed to support broadband, minimally angle-sensitive resonances for the fundamental excitation wavelength in the 1500 nm region. This results in resonantly enhanced visible THG in the green spectral range with ∼500-fold enhancement on-resonance, compared to the unpatterned a-Si thin-film. THG multispectral microscopic imaging reveals individual nanodisks with enhanced nonlinear signal on-resonance. For increasing pump intensities, spatially dependent saturation effects are observed for the first time, to the best of our knowledge, in such dielectric nanostructure arrays with THG images showing a reversal of contrast.

Demonstration of a thermo-optic phase shifter by utilizing high-Q resonance in high-index-contrast grating

A thermo-optic phase shifter is proposed and demonstrated by utilizing the high-Q resonance in high-index-contrast grating (HCG). The Q-factor up to ∼12000 is measured in a footprint of 110  μm×300  μm. By heating the HCG with paired metal strip micro-heaters, the optical resonance shifts, which induces phase modulation. A phase shift of ∼1.2π under heating power of ∼32  mW is directly observed and demodulated from the fringes shifting in a Michelson interferometer. The proposed configuration can also be extended to realize high-speed phase shift by adopting electro-optical modulation.

Surface-normal electro-optic spatial light modulator using graphene integrated on a high-contrast grating resonator

We demonstrate efficient optical modulation of surface-normal reflection in a novel device structure integrating graphene on a high contrast grating (HCG) resonator. As high as 11 dB extinction ratio is achieved by varying the voltage applied to a single atomic layer of graphene on a HCG resonator. The device topology facilitates easy fabrication of large 2D arrays, and free-space operation. We also demonstrate a graphene-oxide-graphene structure which can potentially operate at MHz operation speed. The devices are fully fabricated by standard CMOS compatible processes indicating that the integrated structure of graphene-on-HCG shows great promise for display, imaging and interconnects applications with low-cost and large scalability.

Mode splitting in high-index-contrast grating with mini-scale finite size

The mode-splitting phenomenon within finite-size, mini-scale high-index-contrast gratings (HCGs) has been investigated theoretically and experimentally. The high-Q resonance splits into a series of in-plane modes due to the confinement of boundaries but can still survive even on a mini-scale footprint. Q factors up to ∼3300 and ∼2200 have been observed for the HCGs with footprints that are only 55  μm×300  μm and 27.5  μm×300  μm, which would be promising for realizing optical communication and sensing applications with compact footprint.

High-contrast grating resonators for label-free detection of disease biomarkers

A label-free optical biosensor is described that employs a silicon-based high-contrast grating (HCG) resonator with a spectral linewidth of ~500 pm that is sensitive to ligand-induced changes in surface properties. The device is used to generate thermodynamic and kinetic data on surface-attached antibodies with their respective antigens. The device can detect serum cardiac troponin I, a biomarker of cardiac disease to 100 pg/ml within 4 mins, which is faster, and as sensitive as current enzyme-linked immuno-assays for cTnI.