In the recent decade, the research field using arrays of high-index-contrast near-wavelength dieletric structures on flat surfaces, known as high-contrast metastructures (HCMs) or metasurfaces, has emerged and expanded rapidly. Although the HCMs and metasurfaces share great similarities in physical structures with photonic crystals (PhCs), i.e. periodic nanostructures, many differences exist in their design, analysis, operation conditions, and applications. In this paper, we provide a generalized theoretical understanding of the two subjects and show their intrinsic connections. We further discuss the simulation and design approaches, categorized by their functionalities and applications. The similarity and differences between HCMs, metasurfaces and PhCs are also discussed. New findings are presented regarding the physical connection between the PhC band structures and the 1D and 2D HCM scattering spectra under transverse and longitudinal tilt incidence. Novel designs using HCMs as holograms, spatial light modulators, and surface plasmonic couplers are discussed. Recent advances on HCMs, metasurfaces and PhCs are reviewed and compared for applications such as broadband mirrors, waveguides, couplers, resonators, and reconfigurable optics.
The electronic band structures and optical properties of type-II superlattice (T2SL) photodetectors in the mid-infrared (IR) range are investigated. We formulate a rigorous band structure model using the 8-band k · p method to include the conduction and valence band mixing. After solving the 8 × 8 Hamiltonian and deriving explicitly the new momentum matrix elements in terms of envelope functions, optical transition rates are obtained through the Fermi's golden rule under various doping and injection conditions. Optical measurements on T2SL photodetectors are compared with our model and show good agreement. Our modeling results of quantum structures connect directly to the device-level design and simulation. The predicted doping effect is readily applicable to the optimization of photodetectors. We further include interfacial (IF) layers to study the significance of their effect. Optical properties of T2SLs are expected to have a large tunable range by controlling the thickness and material composition of the IF layers. Our model provides an efficient tool for the designs of novel photodetectors.
We report an electrically pumped hybrid cavity AlGaInAs-silicon long-wavelength VCSEL using a high contrast grating (HCG) reflector on a silicon-on-insulator (SOI) substrate. The VCSEL operates at silicon transparent wavelengths ~1.57 μm with >1 mW CW power outcoupled from the semiconductor DBR, and single-mode operation up to 65 °C. The thermal resistance of our device is measured to be 1.46 K/mW. We demonstrate >2.5 GHz 3-dB direct modulation bandwidth, and show error-free transmission over 2.5 km single mode fiber under 5 Gb/s direct modulation. We show a theoretical design of SOI-HCG serving both as a VCSEL reflector as well as waveguide coupler for an in-plane SOI waveguide, facilitating integration of VCSEL with in-plane silicon photonic circuits. The novel HCG-VCSEL design, which employs scalable flip-chip eutectic bonding, may enable low cost light sources for integrated optical links.
We present improved performance in strain-balanced InAs/GaSb type-II superlattice photodetectors grown using InSb interfacial layers, measured using a cross-sectional electron beam induced current (EBIC) technique to obtain minority carrier diffusion characteristics. We detail a modified EBIC model that accounts for the long absorber regions in photodetectors and fit the experimental data. We find a significant increase in the minority hole lifetime (up to 157 ns) and increased minority electron lifetime due to the interfacial layers. Additionally, electrical characterization of the device temperature-dependent resistance-area product reveals that the interfacial treatment improves the device dark current at lower temperatures.
On the demand of single‐photon entangled light sources and high‐sensitivity probes in the fields of quantum information processing, weak magnetic field detection and biosensing, the nitrogen vacancy (NV) color center is very attractive and has been deeply and intensively studied, due to its convenience of spin initialization, operation, and optical readout combined with long coherence time in the ambient environment. Although the application prospect is promising, there are still some problems to be solved before fully exerting its characteristic performance, including enhancement of emission of NV centers in certain charge state (NV− or NV0), obtaining indistinguishable photons, and improving of collecting efficiency for the photons. Herein, the research progress in these issues is reviewed and commented on to help researchers grasp the current trends. In addition, the development of emerging color centers, such as germanium vacancy defects, and rare‐earth dopants, with great potential for various applications, are also briefly surveyed.
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