Hexagonal Boron Nitride for Photonic Device Applications: A Review.

Hexagonal boron nitride (hBN) has emerged as a key two-dimensional material. Its importance is linked to that of graphene because it provides an ideal substrate for graphene with minimal lattice mismatch and maintains its high carrier mobility. Moreover, hBN has unique properties in the deep ultraviolet (DUV) and infrared (IR) wavelength bands owing to its indirect bandgap structure and hyperbolic phonon polaritons (HPPs). This review examines the physical properties and applications of hBN-based photonic devices that operate in these bands. A brief background on BN is provided, and the theoretical background of the intrinsic nature of the indirect bandgap structure and HPPs is discussed. Subsequently, the development of DUV-based light-emitting diodes and photodetectors based on hBN's bandgap in the DUV wavelength band is reviewed. Thereafter, IR absorbers/emitters, hyperlenses, and surface-enhanced IR absorption microscopy applications using HPPs in the IR wavelength band are examined. Finally, future challenges related to hBN fabrication using chemical vapor deposition and techniques for transferring hBN to a substrate are discussed. Emerging techniques to control HPPs are also examined. This review is intended to assist researchers in both industry and academia in the design and development of unique hBN-based photonic devices operating in the DUV and IR wavelength regions.

Extraordinary Optical Transmission by Hybrid Phonon-Plasmon Polaritons Using hBN Embedded in Plasmonic Nanoslits.

Hexagonal boron nitride (hBN) exhibits natural hyperbolic dispersion in the infrared (IR) wavelength spectrum. In particular, the hybridization of its hyperbolic phonon polaritons (HPPs) and surface plasmon resonances (SPRs) induced by metallic nanostructures is expected to serve as a new platform for novel light manipulation. In this study, the transmission properties of embedded hBN in metallic one-dimensional (1D) nanoslits were theoretically investigated using a rigorous coupled wave analysis method. Extraordinary optical transmission (EOT) was observed in the type-II Reststrahlen band, which was attributed to the hybridization of HPPs in hBN and SPRs in 1D nanoslits. The calculated electric field distributions indicated that the unique Fabry-Pérot-like resonance was induced by the hybridization of HPPs and SPRs in an embedded hBN cavity. The trajectory of the confined light was a zigzag owing to the hyperbolicity of hBN, and its resonance number depended primarily on the aspect ratio of the 1D nanoslit. Such an EOT is also independent of the slit width and incident angle of light. These findings can not only assist in the development of improved strategies for the extreme confinement of IR light but may also be applied to ultrathin optical filters, advanced photodetectors, and optical devices.

Graphene Plasmonics in Sensor Applications: A Review.

Surface plasmon polaritons (SPPs) can be generated in graphene at frequencies in the mid-infrared to terahertz range, which is not possible using conventional plasmonic materials such as noble metals. Moreover, the lifetime and confinement volume of such SPPs are much longer and smaller, respectively, than those in metals. For these reasons, graphene plasmonics has potential applications in novel plasmonic sensors and various concepts have been proposed. This review paper examines the potential of such graphene plasmonics with regard to the development of novel high-performance sensors. The theoretical background is summarized and the intrinsic nature of graphene plasmons, interactions between graphene and SPPs induced by metallic nanostructures and the electrical control of SPPs by adjusting the Fermi level of graphene are discussed. Subsequently, the development of optical sensors, biological sensors and important components such as absorbers/emitters and reconfigurable optical mirrors for use in new sensor systems are reviewed. Finally, future challenges related to the fabrication of graphene-based devices as well as various advanced optical devices incorporating other two-dimensional materials are examined. This review is intended to assist researchers in both industry and academia in the design and development of novel sensors based on graphene plasmonics.

Low dark current and high-responsivity graphene mid-infrared photodetectors using amplification of injected photo-carriers by photo-gating.

Low dark current, high-responsivity middle-wavelength infrared (IR) graphene photodetectors using photo-gating amplification of injected photo-carriers are demonstrated. A graphene/p-indium antimonide (InSb) heterojunction and graphene/insulator region were formed. The injected photo-carriers from InSb to graphene were amplified by photo-gating induced in the graphene/tetraethyl orthosilicate (TEOS) region, resulting in the high responsivity and low dark current performance. A responsivity of 14.9 A/W and an ON/OFF ratio of 2.66×104 were achieved. The photoresponse is shown to be determined by the cross-sectional area between the graphene and the TEOS-SiO2, in which the injected photo-carriers into graphene were modulated and amplified by the photo-gating effect. Our results indicate that high-performance IR photodetectors based on the developed graphene photodetectors can be realized.

Graphene on metal-insulator-metal-based plasmonic metamaterials at infrared wavelengths.

Metal-insulator-metal-based plasmonic metamaterial absorbers (MIM-PMAs) generate strong localized surface plasmon resonance (LSPR) on their surfaces. Therefore, MIM-PMAs are expected to enhance the absorption of graphene coated on their surfaces. Graphene-coated MIM-PMAs (GMIM-PMAs) were developed and their optical properties were investigated both experimentally and numerically at infrared wavelengths. Significant modification of the absorption of GMIM-PMAs was achieved only in the main LSPR wavelength region, where the insulator is lossless. The enhancement of the absorption of graphene could be maximized by the optimization of the insulator thickness of the MIM-PMAs. The results obtained here are expected to contribute to the development of high-responsivity graphene-based photodetectors and optoelectronic devices.

Multispectral Emissions of Lanthanide-Doped Gadolinium Oxide Nanophosphors for Cathodoluminescence and Near-Infrared Upconversion/Downconversion Imaging.

Comprehensive imaging of a biological individual can be achieved by utilizing the variation in spatial resolution, the scale of cathodoluminescence (CL), and near-infrared (NIR), as favored by imaging probe Gd2O3 co-doped lanthanide nanophosphors (NPPs). A series of Gd2O3:Ln3+/Yb3+ (Ln3+: Tm3+, Ho3+, Er3+) NPPs with multispectral emission are prepared by the sol-gel method. The NPPs show a wide range of emissions spanning from the visible to the NIR region under 980 nm excitation. The dependence of the upconverting (UC)/downconverting (DC) emission intensity on the dopant ratio is investigated. The optimum ratios of dopants obtained for emissions in the NIR regions at 810 nm, 1200 nm, and 1530 nm are applied to produce nanoparticles by the homogeneous precipitation (HP) method. The nanoparticles produced from the HP method are used to investigate the dual NIR and CL imaging modalities. The results indicate the possibility of using Gd2O3 co-doped Ln3+/Yb3+ (Ln3+: Tm3+, Ho3+, Er3+) in correlation with NIR and CL imaging. The use of Gd2O3 promises an extension of the object dimension to the whole-body level by employing magnetic resonance imaging (MRI).

Rare-earth-doped nanophosphors for multicolor cathodoluminescence nanobioimaging using scanning transmission electron microscopy.

We describe rare-earth-doped nanophosphors (RE-NPs) for biological imaging using cathodoluminescence(CL) microscopy based on scanning transmission electron microscopy (STEM). We report the first demonstration of multicolor CL nanobioimaging using STEM with nanophosphors. The CL spectra of the synthesized nanophosphors (Y2O3∶Eu, Y2O3∶Tb) were sufficiently narrow to be distinguished. From CL images of RE-NPs on an elastic carbon-coated copper grid, the spatial resolution was beyond the diffraction limit of light.Y2O3∶Tb and Y2O3∶Eu RE-NPs showed a remarkable resistance against electron beam exposure even at high acceleration voltage (80 kV) and retained a CL intensity of more than 97% compared with the initial intensity for 1 min. In biological CL imaging with STEM, heavy-metal-stained cell sections containing the RE-NPs were prepared,and both the CL images of RE-NPs and cellular structures, such as mitochondria, were clearly observed from STEM images with high contrast. The cellular CL imaging using RE-NPs also had high spatial resolution even though heavy-metal-stained cells are normally regarded as highly scattering media. Moreover, since theRE-NPs exhibit photoluminescence (PL) excited by UV light, they are useful for multimodal correlative imaging using CL and PL.

Y2O3:Tm,Yb nanophosphors for correlative upconversion luminescence and cathodoluminescence imaging.

We present a phosphor nanoparticle that shows both upconversion luminescence (UCL) and cathodoluminescence (CL). With this particle, low-autofluorescence, deep-tissue and wide-field fluorescence imaging can be achieved with nanometer-order high-spatial-resolution imaging. We synthesized Y2O3:Tm,Yb nanophosphors that emit visible and near-infrared UCL under 980 nm irradiation and blue CL via electron beam excitation. The phosphors were applied to fluorescent imaging of HeLa cells. The photostability of the phosphors was superior to that of a conventional organic dye. We show that after uptake by HeLa cells, the particles can be imaged with SEM and CL contrast in a cellular section. This indicates that correlative UCL and CL imaging of biological samples could be realized.