Epitaxial single-crystal rare-earth oxide in horizontal slot waveguide for silicon-based integrated active photonic devices.

We have epitaxially grown high-quality single-crystal rare-earth oxide thin films, including Gd2O3 and erbium-incorporated (ErGd)2O3, on silicon-on-insulator substrate, and investigated their optical properties when embedded in horizontal slot waveguides. (ErGd)2O3 with an erbium concentration in the mid-1021 cm-3 range shows well-resolved Stark-split photoluminescence emission peaks in the telecommunications band and a photoluminescence lifetime-concentration product as large as 2.67×1018 s·cm-3 at room-temperature. Using these materials, horizontal slot waveguides with strong optical confinement in low-refractive-index rare-earth oxide layers, have been fabricated for silicon-based integrated active photonic devices. Thanks to the strong light-matter interaction, a large waveguide modal absorption of 88 dB/cm related to erbium ions is achieved, leading to a large potential optical gain. Intense emissions from the waveguides are also observed, with a radiation efficiency on the order of 10-4. These results indicate that a combination of epitaxial rare-earth oxide thin films and horizontal slot waveguides provides a promising platform for light amplification and generation on silicon.

Highly sensitive transient reflection measurement in extreme ultraviolet region for tracking carrier and coherent phonon dynamics.

A highly sensitive method for detecting transient reflection in the extreme ultraviolet (XUV) region was developed on the basis of high-order harmonics for tracking carrier and coherent phonon dynamics. The use of lock-in detection and boxcar integration enables us to observe optical modulation (ΔR/R) as high as 1 × 10-4, and the data acquisition takes only four minutes. XUV transient reflections of bismuth exhibited exponential decay originating from excited carriers and periodic oscillation originating from A1g optical phonons. The linear power dependence of the electronic and phonon amplitudes indicated that one-photon excitation occurred under the experimental conditions. The cosine of the initial phase of the phonon oscillation revealed that a displacive excitation mechanism contributed to phonon generation. The phonon parameters obtained by the XUV and NIR probes were consistent even though their penetration depths were different. The result indicated that the XUV and NIR pulses probe the same excited region, which should be near the surface due to the short penetration depth of the NIR pump pulses. The present highly sensitive means of detecting XUV transient reflections in solid-state materials could be utilized for detecting attosecond dynamics in the future.

Optical coherent transients in 167Er3+ at telecom-band wavelength.

We demonstrate optical coherent transients in a Λ-like hyperfine energy-level system of Er1673+ in yttrium orthosilicate (Y2SiO5) with telecom-band photons at a zero magnetic field. Spectral hole burning was used to study the temperature dependence of the induced spectral antihole. We find that temperatures below 3.0 K suppress population dissipation induced by electron-phonon interactions sufficiently to enable population initialization in the Λ-like system. Further, the pulse area dependence of photoluminescence (PL) from the Λ-like system was measured at 2.2 K. An optical pump power dependence of PL intensity shows Rabi oscillations that contain two full Rabi cycles at the frequency of 2π×810 kHz. A two-pulse photon echo measurement reveals an optical coherence time of 12 µs. To date, this measured optical coherence time is the longest observed for Er3+ in solids at zero magnetic field. These findings will facilitate optical coherent manipulation of Λ-like Er1673+ electronic states as a quantum memories operating at telecom-band wavelengths.

Scandium effect on the luminescence of Er-Sc silicates prepared from multi-nanolayer films.

Polycrystalline Er-Sc silicates (Er x Sc2-x Si2O7 and Er x Sc2-x SiO5) were fabricated using multilayer nanostructured films of Er2O3/SiO2/Sc2O3 deposited on SiO2/Si substrates by RF sputtering and thermal annealing at high temperature. The films were characterized by synchrotron radiation grazing incidence X-ray diffraction, cross-sectional transmission electron microscopy, energy-dispersive X-ray spectroscopy, and micro-photoluminescence measurements. The Er-Sc silicate phase Er x Sc2-x Si2O7 is the dominant film, and Er and Sc are homogeneously distributed after thermal treatment because of the excess of oxygen from SiO2 interlayers. The Er concentration of 6.7 × 10(21) atoms/cm(3) was achieved due to the presence of Sc that dilutes the Er concentration and generates concentration quenching. During silicate formation, the erbium diffusion coefficient in the silicate phase is estimated to be 1 × 10(-15) cm(2)/s at 1,250°C. The dominant Er x Sc2 - x Si2O7 layer shows a room-temperature photoluminescence peak at 1,537 nm with the full width at half maximum (FWHM) of 1.6 nm. The peak emission shift compared to that of the Y-Er silicate (where Y and Er have almost the same ionic radii) and the narrow FWHM are due to the small ionic radii of Sc(3+) which enhance the crystal field strength affecting the optical properties of Er(3+) ions located at the well-defined lattice sites of the Sc silicate. The Er-Sc silicate with narrow FWHM opens a promising way to prepare photonic crystal light-emitting devices.

Depth-selective structural analysis of thin films using total-external-reflection x-ray diffraction.

Total-external-reflection (TER) x-ray diffraction is the depth-sensitive technique for evaluating layered structures, including epitaxial heterostructures, ion-doped bulk crystals and several quantum-well structures. This technique can control the depth of observation by changing both incident and exit angles of x-rays from the surface. In this review, the principle of the TER technique and measurement apparatus are briefly described, and applications of layered-semiconductor samples evaluated using the TER technique are introduced.

Optical and electrical characterization at the nanoscale with a transparent probe of a scanning tunnelling microscope.

A new type of scanning probe microscope, combining features of the scanning tunnelling microscope, the scanning tunnelling luminescence microscope with a transparent probe and the aperture scanning near-field optical microscope, is described. Proof-of-concept experiments were performed under ultrahigh vacuum conditions at varying temperature on GaAs/AlAs heterostructures.

On the role of substrate in light-harvesting experiments.

An analysis of the emitted light distribution for a single emitter located at the planar interface of two optical media was performed. The interface of a varying refractive index substrate with air was considered, which is a common case in luminescence microscopy (spectroscopy) experiments. A modification of the radiative recombination rate induced by the variation of the substrate together with the emitted radiation spatial redistribution were taken into account. Simulation results show that the collection efficiency of the emitted light can vary several times depending on the substrate choice and the emitter intrinsic quantum efficiency.

Scaling and universality of roughening in thermal oxidation of Si(001).

By analyzing atomic force microscopy images, we derive a continuum equation that quantitatively explains the roughening at the Si(001)-SiO2 interface during thermal oxidation at the temperature at 1200 degrees C in an Ar atmosphere containing a small fraction of O2. We also show that there is a phase transition in the universality class from a disordered to step-terrace structure at the interface at oxidation temperatures between 1150 and 1380 degrees C with the miscut angle of the substrate as the scaling parameter.

New types of unstable step-flow growth on Si(111)-(7 x 7) during molecular beam epitaxy: scaling and universality.

New types of unstable homoepitaxial growth of vicinal surfaces are studied using ex situ atomic force microscopy. The growth features are two types of step bunching with straight step edges between 700 and 775 degrees C and one type of simultaneous bunching and meandering at 800 degrees C. The results of a quantitative size scaling analysis of the straight steps are discussed from the perspective of universality classes in bunching theory.

Atomic structures of the Ge/Si(113)-(2 x 2) surface.

Based on scanning tunneling microscopy observations of the epitaxial growth of Ge on Si(113) and first-principles total energy and band calculations, we demonstrate that the Ge/Si(113)-(2 x 2) surface is made up of alternating [1;10]-oriented rows of rebonded atoms and tilted pentamers of five atoms, where each pentamer is stabilized by an interstitial atom at the subsurface. From the existence of stacking defects in rows of tilted pentamers observed at room temperature, we have deduced that at epitaxial temperatures the pentamers frequently change their tilting orientations between two minimum energy states.