The propagation of electromagnetic radiation in periodically stratified media is considered. Media of finite, semi-infinite, and infinite extent are treated. A diagonalization of the unit cell translation operator is used to obtain exact solutions for the Bloch waves, the dispersion relations, and the band structure of the medium. Some new phenomena with applications to integrated optics and laser technology are presented.

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Electromagnetic propagation in periodic stratified media. I. General theory

The ability to control the modes oscillating within a laser resonator is of fundamental importance. In general, the presence of competing modes can be detrimental to beam quality and spectral purity, thus leading to spatial as well as temporal fluctuations in the emitted radiation. We show that by harnessing notions from parity-time (PT) symmetry, stable single–longitudinal mode operation can be readily achieved in a system of coupled microring lasers. The selective breaking of PT symmetry can be used to systematically enhance the maximum attainable output power in the desired mode. This versatile concept is inherently self-adapting and facilitates mode selectivity over a broad bandwidth without the need for other additional intricate components. Our experimental findings provide the possibility to develop synthetic optical devices and structures with enhanced functionality.

http://science.sciencemag.org/content/sci/346/6212/975.full.pdf

Parity-time–symmetric microring lasers

Diffraction characteristics of general dielectric planar (slab) gratings and surface-relief (corrugated) gratings are reviewed. Applications to laser-beam deflection, guidance, modulation, coupling, filtering, wavefront reconstruction, and distributed feedback in the fields of acoustooptics, integrated optics, holography, and spectral analysis are discussed. An exact formulation of the grating diffraction problem without approximations (rigorous coupled-wave theory developed by the authors) is presented. The method of solution is in terms of state variables and this is presented in detail. Then, using a series of fundamental assumptions, this rigorous theory is shown to reduce to the various existing approximate theories in the appropriate limits. The effects of these fundamental assumptions in the approximate theories are quantified and discussed.

Analysis and applications of optical diffraction by gratings

The fabrication of transistors using vertical, six-sided core–multishell indium gallium arsenide nanowires with an all-surrounding gate on a silicon substrate combines the advantages of a three-dimensional gate architecture with the high electron mobility of the III–V nanowires, drastically enhancing the on-state current and transconductance. In the continuing drive to improve and miniaturize transistors, the microelectronics industry has recently adopted three-dimensional electronic gate structures. Another way of improving transistors is to use semiconductor materials with higher electron mobility than silicon, although this presents significant fabrication challenges. Katsuhiro Tomioka et al. combine the two approaches; they grow, with high precision, vertical, six-sided core–multishell indium gallium arsenide nanowires with an all-surrounding gate on a silicon substrate. The resulting devices demonstrate superior transistor performance with excellent on/off switching behaviour and fast operation. Silicon transistors are expected to have new gate architectures, channel materials and switching mechanisms in ten years’ time1,2,3,4. The trend in transistor scaling has already led to a change in gate structure from two dimensions to three, used in fin field-effect transistors, to avoid problems inherent in miniaturization such as high off-state leakage current and the short-channel effect. At present, planar and fin architectures using III–V materials, specifically InGaAs, are being explored as alternative fast channels on silicon5,6,7,8,9 because of their high electron mobility and high-quality interface with gate dielectrics10. The idea of surrounding-gate transistors11, in which the gate is wrapped around a nanowire channel to provide the best possible electrostatic gate control, using InGaAs channels on silicon, however, has been less well investigated12,13 because of difficulties in integrating free-standing InGaAs nanostructures on silicon. Here we report the position-controlled growth of vertical InGaAs nanowires on silicon without any buffering technique and demonstrate surrounding-gate transistors using InGaAs nanowires and InGaAs/InP/InAlAs/InGaAs core–multishell nanowires as channels. Surrounding-gate transistors using core–multishell nanowire channels with a six-sided, high-electron-mobility transistor structure greatly enhance the on-state current and transconductance while keeping good gate controllability. These devices provide a route to making vertically oriented transistors for the next generation of field-effect transistors and may be useful as building blocks for wireless networks on silicon platforms.

A III–V nanowire channel on silicon for high-performance vertical transistors

Integrated optics and new wave phenomena in optical waveguides

A built-in passive waveguide mechanism is introduced in Al x Ga 1-x As injection lasers by growing planar double heterostructure (DH) layers on a grooved GaAs substrate. The lasing mode is confined to the channel region due to excess absorption loss outside the channel. Stable fundamental mode oscillation is achieved up to twice the threshold current for a channel width of 5-8 \mu . Undesirable lasing behavior usually induced by transverse mode instability, such as nonlinear kinks in the light output versus current characteristics, are significantly reduced in the present lasers. The dc threshold current is 40-90 mA at room temperature. Median lifetime of 780 hours has been obtained during preliminary aging tests at a heat sink temperature of 70°C.

Transverse mode stabilized Al x Ga 1-x As injection lasers with channeled-substrate-planar structure

Undesirable nonlinear ’’kinks’’ in light‐output–vs–current characteristics of stripe geometry double‐heterostructure (DH) injection lasers are significantly reduced by stabilizing the transverse modes along the junction plane. Built‐in passive guiding mechanism is introduced by growing planar (AlGa)As/GaAs DH layers on a grooved GaAs substrate.

Channeled‐substrate planar structure (AlGa)As injection lasers

Distributed‐feedback GaAs‐GaAlAs diode lasers with separate optical and carrier confinement have been successfully operated under dc bias up to room temperature. They lased in a single longitudinal mode with a threshold current density of 0.94 kA/cm2 at 170 K and 3.5 kA/cm2 at 300 K.

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cw operation of distributed‐feedback GaAs‐GaAlAs diode lasers at temperatures up to 300 K

We report laser oscillation at 80–100°K in electrically pumped GaAs–Ga1−xAlxAs double‐heterostructure distributed‐feedback diode lasers. The feedback for laser oscillation was provided by a corrugated interface between the active GaAs layer and the p‐Ga1−xAlxAs layer. The lowest threshold current density was 2.5 kA/cm2 in pulsed operation. The wavelength of laser emission was 8112 A at 82°K with a half‐width of less than 0.3 A. The temperature dependence of the laser wavelength was found to be smaller than that of the conventional Fabry‐Perot laser.

GaAs–Ga1−xAlxAs double‐heterostructure distributed‐feedback diode lasers

Remarkable reduction of the threshold current density is achieved in GaAs‐GaAlAs distributed‐feedback diode lasers by adopting a separate‐confinement heterostructure. The diodes are lased successfully at temperatures up to 340 °K under pulsed operation. The lowest threshold current density is 3 kA/cm2 at 300 °K.

/pdf/gaas-gaalas-distributed-feedback-diode-lasers-with-separate-2g3ruk7wip.pdf

Kunio Aiki

Papers

Transverse mode stabilized Al x Ga 1-x As injection lasers with channeled-substrate-planar structure

Channeled‐substrate planar structure (AlGa)As injection lasers

cw operation of distributed‐feedback GaAs‐GaAlAs diode lasers at temperatures up to 300 K

GaAs–Ga1−xAlxAs double‐heterostructure distributed‐feedback diode lasers

GaAs‐GaAlAs distributed‐feedback diode lasers with separate optical and carrier confinement