We report an ultrafast cross phase modulation (XPM) effect in intersubband transition (ISBT) of InGaAs/AlAs/AlAsSb coupled quantum wells, where the ISBT absorption of a transverse-magnetic mode pump signal induces phase modulation of a transverse-electric mode probe signal. Using waveguide-type ISBT devices, we have achieved XPM-based 10 Gbit/s wavelength conversion with a power penalty of 2.53 dB. Also, we propose XPM-based signal processing circuits for gate switching and modulation format conversion.

We investigated, by atomic force microscopy (AFM), the effect of the V/III ratio on the surface morphologies of GaN epilayers during the early stage of high-temperature (HT) GaN growth. The epilayers were grown on sapphire substrates by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). We observed that initial HT-GaN islands in the early stage of HT-GaN growth were larger when a lower V/III ratio was used during the low-temperature (LT) GaN-buffer-layer growth or during the early stage of HT-GaN growth (the HT-GaN-buffer growth). These larger islands were attributed to a reduced nuclei density when a lower V/III ratio was used during the LT-GaN-buffer-layer growth, or to an increased growth aspect ratio (the ratio of vertical and lateral growth rates) when a lower V/III ratio was used in the early stage of HT-GaN growth. The larger islands led to fewer defects when they coalesced, which demonstrated that the quality of GaN epilayers grown on sapphire substrates by LP-MOCVD can be improved by using a low V/III ratio during the LT or HT GaN-buffer-layer growth.Introduction A high density of threading dislocations, on the order of 10 10 cm ± ±2 , remains in device-quality GaN epilayers grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) because of the large differences between the lattice parameters and thermal expansion coefficients of the GaN and the sapphire substrate [1]. The dislocation density must be reduced to improve the optical and electrical properties of GaN-based materials and, thus, improve the device performance. Several groups have recently demonstrated that the use of a three-dimensional (3D) growth mode in the early stage of GaN growth plays an important role in the reduction of the defect density [2 to 4]. Furthermore, a very high V/III ratio is generally required to grow high-quality GaN epilayers on sapphire substrates by MOCVD [5,6]. However, we found that the crystalline quality of GaN epilayers grown on sapphire substrates by low-pressure MOCVD (LP-MOCVD) can be effectively improved by using a low V/III ratio during the LT-GaN-buffer growth or the initial stage of HT-GaN growth (the HTGaN-buffer-layer growth) [7,8]. We attributed this improvement to enhanced island growth (increased island size) in the early stage of the HT-GaN growth; this enhanced growth was due to the lower V/III ratio and led to fewer crystalline defects [2 to 4]. Our in situ optical reflectance monitoring supported this finding: we observed that the transition from the three-dimensional (3D) to the 2D growth mode (the recovery of the reflectance) was slower with a lower V/III ratio [7,8]. To gain further insights into the effect of the V/III ratio on the GaN growth process, we have since investigated the morphology of the GaN epilayers in the first stages of HT-GaN growth since the morphology is closely related to the reflectance behavior.

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We have grown laser diode structures using highly strained BeZnCdSe quantum wells by molecular beam epitaxy and successfully obtained continuous-wave lasing in the green-to-yellow spectral region (543–570 nm) at room temperature. The green-to-yellow lasing color was tuned by simply varying the Cd content of the quaternary BeZnCdSe quantum well. The threshold current densities of 20-µm-wide lasers were found to be sufficiently low (less than 0.85 kA/cm2). This result demonstrates that BeZnCdSe is a promising material for use as the active layer in high-performance green-to-yellow laser diodes.

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We report the growth of BeZnCdSe quantum-well laser diode (LD) structures with a short-period superlattice cladding layer and demonstrate continuous-wave lasing in the pure-green spectral region (545 nm) at room temperature. The threshold current density and voltage of a 5-µm-wide gain-guided laser were found to be 1.7 kA/cm2 and 10.4 V, respectively. This threshold current density is sufficiently low compared with that of InGaN/GaN green LDs.