Enhancing the light-matter interaction in two-dimensional (2D) materials with high $Q$ resonances in photonic structures has boosted the development of optical and photonic devices. Herein we intend to build a bridge between the radiation engineering and the bound states in the continuum (BIC), and present a general method to control light absorption at critical coupling through the quasi-BIC resonance. In a single-mode, two-port system composed of graphene, coupled with silicon nanodisk metasurfaces, the maximum absorption of 0.5 can be achieved when the radiation rate of the magnetic dipole resonance equals to the dissipate loss rate of graphene. Furthermore, the absorption bandwidth can be adjusted more than two orders of magnitude, from 0.9 nm to 94 nm, by simultaneously changing the asymmetric parameter of metasurfaces, the Fermi level, and the layer number of graphene. This work reveals the essential role of BIC in radiation engineering and provides promising strategies in controlling light absorption of 2D materials for the next-generation optical and photonic devices, e.g., light emitters, detectors, modulators, and sensors.

Controlling light absorption of graphene at critical coupling through magnetic dipole quasi-bound states in the continuum resonance

Recently, based on the selective excitation of the guided mode, researchers realized quasi-bound states in the continuum (quasi-BICs) in all-dielectric compound grating waveguide structures. In this paper, we introduce a graphene layer into an all-dielectric compound grating waveguide layer supporting quasi-BIC to achieve near-infrared perfect absorption of graphene. The underlying physical mechanism of perfect absorption can be clearly explained by the critical coupling theory derived from temporal coupled-mode theory in a single-mode, one-port system. By changing the Fermi level and the layer number of the graphene, the absorption rate of the system can be flexibly tuned. In addition, by changing the geometric parameter of the compound grating waveguide structure, the radiation coupling rate of the quasi-BIC can also be flexibly tuned. Therefore, the critical coupling condition can be maintained in a broad range of the Fermi level and the layer number of the graphene. The full width at half maximum of the near-infrared perfect absorption peak can be flexibly tuned from 5.7 to 187.1 nm. This bandwidth-tunable perfect absorber would possess potential applications in the design of 2D material-based optical sensors, electrical switchers, and solar thermophotovoltaic devices.

Bandwidth-tunable near-infrared perfect absorption of graphene in a compound grating waveguide structure supporting quasi-bound states in the continuum

For an optical absorber, its efficiency and bandwidth determine ultimately its light-collection performance. A general way to realize the high-efficiency absorption and manipulate the absorption bandwidth is certainly important and highly desired. In this paper, based on the coupled-mode theory, a general strategy for tailoring the bandwidth of an absorber (to broaden or narrow it) is presented. A graphene-based absorber integrated with a lossless resonator is designed, and it is shown that by increasing the layer number of graphene and adjusting structure parameters, the broadband absorber at critical coupling could be realized. Meanwhile, by adjusting the location of graphene in the structure, an ultra-narrow-band absorber could also be realized. Present work of controlling both efficiency and bandwidth of an absorber would be particularly favorable for applications in light harvesting, light emitting devices, and optical modulators.

Manipulating bandwidth of light absorption at critical coupling: An example of graphene integrated with dielectric photonic structure

Publisher Summary This chapter discusses low-dimensional structures of II–VI diluted magnetic semiconductors (DMS) with manganese and describes the carrier concentration induced ferromagnetism in Mn-based IV–VI DMS with a particular emphasis on Mn content and conducting hole concentration dependent paramagnetic–ferromagnetic–spin glass transitions in bulk crystals and thin layers of SnMnTe and related materials. The chapter also discusses the transport, magnetic, and optical properties of bulk crystals and thick epitaxial layers; low-dimensional quantum structures of IV–VI DMS with Eu; the magnetic and electrical properties of IV–VI semiconductors with Gd; and the magnetic and optical properties of all semiconductor ferromagnetic multilayers built of ferromagnetic EuS and nonmagnetic IV–VI (PbSe and PbS) semiconductors. The chapter describes DMS containing transition metals other than manganese. These materials never attracted as much attention as the ones containing Mn, mainly because of severe difficulties in their preparation, particularly when large percentages of the magnetic components were involved.

II–VI and IV–VI Diluted Magnetic Semiconductors – New Bulk Materials and Low-Dimensional Quantum Structures

Metasurface-mediated bound states in the continuum (BIC) provides a versatile platform for light manipulation at subwavelength dimension with diverging radiative quality factor and extreme optical localization. In this work, we employ magnetic dipole quasi-BIC resonance in asymmetric silicon nanobar metasurfaces to realize giant Goos-Hanchen (GH) shift enhancement by more than three orders of wavelength. In sharp contrast to GH shift based on the Brewster dip or transmission-type resonance, the maximum GH shift here is located at the reflection peak with unity reflectance, which can be conveniently detected in the experiment. By adjusting the asymmetric parameter of metasurfaces, the $Q$-factor and GH shift can be modulated accordingly. More interestingly, it is found that GH shift exhibits an inverse quadratic dependence on the asymmetric parameter. Furthermore, we design an ultrasensitive environmental refractive index sensor based on the quasi-BIC enhanced GH shift, with a maximum sensitivity of 1.5$\times$10$^{7}$ $\mu$m/RIU. Our work not only reveals the essential role of BIC in engineering the basic optical phenomena, but also suggests the way for pushing the performance limits of optical communication devices, information storage, wavelength division de/multiplexers, and ultrasensitive sensors.

Enhancing Goos-H\"anchen shift based on magnetic dipole quasi-bound states in the continuum in all-dielectric metasurfaces

Recently, optical bound states in continuum (BICs) incorporated with optical gain have been reported to exhibit lasing. Here, it is shown that each of the four BICs supported by C 4v$_{4v}$ symmetric photonic crystal slab can be made to lase, allowing the control over the topological charge of the resulting laser beam. The type of each BIC and their topological charges are identified by imaging the far‐field polarization vortices of the lasing signal. Results are compared with experimentally obtained dispersions, finite element method simulations, and multipole decomposition method based on the microscopic polarization currents in the photonic crystal plane. A demonstration of multimode lasing of two non‐degenerate BICs with opposite topological charges is presented. The momentum space overlap of the BICs results in a unique polarization pattern. The study provides a generalizable example for engineering the topological properties of coherent light.

Controlling Topology and Polarization State of Lasing Photonic Bound States in Continuum

Photovoltaic effect in paraelectric BiVO4 film

Large room temperature electrocaloric effect in PbZrO3/Ca3Mn2O7 heterostructure

Photoluminescence of ZnSe, Zn0.84Mn0.16Se alloy, and ZnSe/Zn0.84Mn0.16Se superlattice (SL) have been measured in the temperature range from 10 to 300 K. It is found that the band gap of the ZnSe was smaller than that of the Zn0.84Mn0.16Se alloy at 10 K, but larger than that of the alloy at 300 K. Then the well and barrier layers of the ZnSe/Zn0.84Mn0.16Se SL would be expected to turn over at about 180 K. This type of turn over was observed in the SL sample. The turn over took place at 80 K, somewhat lower than the expected temperature. A calculation including the strain in the ZnSe/Zn0.84Mn0.16Se SL indicates that the heavy-hole bands begin crossing at 75 K, which agrees well with experimental results. [S0163-1829(99)13127-8].

Temperature-induced turnover of the well and barrier layers in ZnSe/Zn0.84Mn0.16Se superlattices

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