III-nitride light-emitting devices have been subjects of intense research for the last several decades owing to the versatility of their applications for fundamental research, as well as their widespread commercial utilization. Nitride light-emitters in the form of light-emitting diodes (LEDs) and lasers have made remarkable progress in recent years, especially in the form of blue LEDs and lasers. However, to further extend the scope of these devices, both below and above the blue emission region of the electromagnetic spectrum, and also to expand their range of practical applications, a number of issues and challenges related to the growth of materials, device design, and fabrication need to be overcome. This review provides a detailed overview of nitride-based LEDs and lasers, starting from their early days of development to the present state-of-the-art light-emitting devices. Besides delineating the scientific and engineering milestones achieved in the path towards the development of the highly matured blue LEDs and lasers, this review provides a sketch of the prevailing challenges associated with the development of long-wavelength, as well as ultraviolet nitride LEDs and lasers. In addition to these, recent progress and future challenges related to the development of next-generation nitride emitters, which include exciton-polariton lasers, spin-LEDs and lasers, and nanostructured emitters based on nanowires and quantum dots, have also been elucidated in this review. The review concludes by touching on the more recent topic of hexagonal boron nitride-based light-emitting devices, which have already shown significant promise as deep ultraviolet and single-photon emitters.

Inspired by the morphology of Coscinodiscus species diatom, bi-layered photonic structures comprised of dielectric-filled nano-holes of varying diameters have been designed and analyzed to enhance and tune absorption characteristics of GaAs-based thin-film photonic devices. Finite difference time domain-based numerical analysis and effective medium approximation based theoretical calculations show that by adjusting diameter and areal density of the nano-holes of the two layers, the peak absorption wavelength can be tuned over a wide spectral range, while attaining a maximum peak-absorptance value of about 97% and a maximum absorption bandwidth of ∼ 190 nm. The maximum enhancement factor of the bi-layered structure is about 11% higher than the value obtained for its equivalent single-layered counterpart over the near-ultraviolet to visible regime of the spectra. High absorptance over a wide-angle for TM polarization and tunable angle-dependent absorption characteristics for TE polarization are also obtained for the proposed ultra-thin absorbers. It has been shown that instead of having misaligned pore-centers as in Coscinodiscus species diatoms, a bi-layered structure designed with layers of identical lattice constant offers significant flexibility in terms of design and practical realization of thin-film photonic devices.

Time-dependent dielectric breakdown (TDDB) lifetime of ultrathin (1 nm) MgO in spin-transfer torque magnetoresistive random access memory (STT-MRAM) devices has recently been shown to be driven by factors other than voltage alone. This study focuses on the specific role of asymmetry in the current flow for different polarity pulsing modes of voltage stress on the TDDB lifetime of 1-nm MgO. Numerical analysis, based on a 3-D heat-diffusion equation and spintronic simulations, has been performed to characterize the temperature rise in the devices for precise correction of self-heating to obtain a correct interpretation of MgO TDDB. It is shown that the different lifetimes for the positive and negative modes can be attributed to different temperature increases arising from self-heating. While the positive and negative modes displayed a non-Arrhenius behavior, the bipolar mode showed an Arrhenius trend in which we observed a unique bimodal behavior of TDDB activation energy ( $\textit{E}_{\text{a}}$ ) as a function of stress voltage in the ultrathin MgO stack. We discuss the role of additional driving forces, such as current, self-heating, charge trapping, and interface strain governing the breakdown mechanism along with the voltage effect.

Effect of disorder on the emission directionality of a whispering gallery mode resonator comprising of circularly symmetric nanowire array is investigated using finite-difference time-domain analysis technique. In spite of rotational symmetry breaking, whispering gallery mode-like isotropic emission characteristics are retained by the nanowire array up to a certain degree of spatial disorder. For higher degrees of randomness, Anderson localized resonant modes are obtained with unidirectional emission characteristics, though the beam direction remains unpredictable because of the underlying random-scattering process. This shortcoming is overcome by designing a system of correlated disorder where nanowire spacing is varied gradually along the preferred axis of unidirectionality. This system, in spite of its high degree of disorder, can effectively support high-Q whispering gallery modes with tunable unidirectional emission characteristics. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement