Kiyofumi Chikuma

Heteroepitaxial growth of KNbO3 single‐crystal films on SrTiO3 by metalorganic chemical vapor deposition

Potassium niobate (KNbO3) thin films have been deposited on silicon (Si) and strontium titanate (SrTiO3) substrates by metalorganic chemical vapor deposition. Stoichiometric KNbO3 films were obtained by adjusting the partial pressure of precursors. The crystallinity was examined by x‐ray diffraction and reflection high‐energy electron diffraction. Single‐crystal (010) KNbO3 films with smooth surface were heteroepitaxially grown on (110)SrTiO3 substrate at 850 °C.

Correlation between Dislocation Density and the Macroscopic Properties of GaN Grown by Metalorganic Vapor Phase Epitaxy.

The correlation between the dislocation density and the electrical and optical properties of lightly Si-doped GaN films grown by metalorganic vapor phase epitaxy was investigated. Photo-electro-chemical (PEC) etching, developed by Youtsey et al.. [Appl. Phys. Lett. 73 (1998) 797], was applied to determine the dislocation density. We modified the PEC etching technique by introducing an additional pulsed sequence. Clear correlation was observed between the dislocation density and the Hall mobility and room-temperature photoluminescence intensity. The influence of the reactor pressure on the dislocation density is also discussed.

Epitaxial growth of orientation-controlled KNbO3 crystal films on MgO using KTaxNb1−xO3 intermediate layer by metalorganic chemical vapor deposition

In this letter, we report a method to obtain an orientation-controlled (010) KNbO3 crystal film on (110) MgO substrate based on the epitaxial growth by the low-pressure metalorganic chemical vapor deposition. In order to control the orientation of the KNbO3 film, an intermediate layer of KTaxNb1−xO3 was used. In the case where the Ta/(Ta+Nb) ratio of the KTaxNb1−xO3 film was over 40%, we could obtain (010)-oriented epitaxial KNbO3 films. Furthermore, the surface morphology of the deposited film, the thickness of which was about 1 μm, was smooth at the Ta/(Ta+Nb) ratio of 60%. The measurement of the refractive index dispersion of the KNbO3 crystal film was also carried out.

InGaAsP/InP Laterally Coupled Distributed Feedback Ridge Laser

A 1.3-µm-wavelength InGaAsP/InP laterally coupled distributed feedback ridge laser with improved structure and characteristics is reported in this paper. For easier fabrication of the first order grating with 198 nm period on and beside the ridge waveguide side-walls, two-step etching has been used to form a narrow ridge waveguide with a special shape. The grating is patterned by electron beam lithography and deeply transferred also by two-step etching, in order to enhance the lateral evanescent field coupling. A stable CW laser oscillation with a low threshold current of 17 mA at room temperature and a high side mode suppression ratio of 45 dB for a laterally coupled distributed feedback ridge laser has been achieved. An external slope efficiency of 0.25 W/A and an output power of about 25 mW are also available in our most recent devices with anti-reflection and high reflection facet coatings. The optical transmission of a Gbit/s order modulated signal has also been tested using our preliminary laser modules through a 1 km optical fiber.

WAVEGUIDING EPITAXIAL POTASSIUM LITHIUM NIOBATE SINGLE-CRYSTAL FILMS DEPOSITED BY METALORGANIC CHEMICAL VAPOR DEPOSITION

Epitaxial potassium lithium niobate (KLN) single-crystal films were deposited on potassium lithium tantalate (KLT) single-crystal substrates by low-pressure metalorganic chemical vapor deposition (MOCVD). Pentaethoxyniobium [Nb(OC2H5)5] and K and Li-dipivaloylmethane [K(DPM), Li(DPM)] were used as volatile metalorganic precursors. The crystallinity of the films was investigated by X-ray diffraction measurements. Reciprocal space mapping indicated that good epitaxial growth had been achieved. The film surface was smooth enough to act as a waveguide. Several guided modes were observed for both TE and TM waves by the prism coupler method. Refractive indices of the films for various wavelengths were determined and the phase matching property of the waveguide is discussed. The propagation loss, estimated by the cut-back method, was found to be 0.5 dB/cm at a wavelength of 633 nm.

Room-Temperature Pulsed Operation of GaN-Based Laser Diodes on a-Face Sapphire Substrate Grown by Low-Pressure Metalorganic Chemical Vapor Deposition

We report on the room-temperature pulsed current operation of GaN-based laser diodes on a (1120) (a-face) sapphire substrate grown by low-pressure metalorganic chemical vapor deposition (LP MOCVD). Ethyl biscyclopentadienyl magnesium (Et-Cp2Mg) and methyl silane (Me-SiH3) were used as precursors for dopants. The cavity mirror facets were formed by cleaving an a-face sapphire substrate along (1102) (r-face). The threshold current density was 41 kA/cm2 and the operating voltage at the threshold was 35 V. Above the threshold, laser emission was observed at 411 nm.

1.3-/spl mu/m wavelength InP laterally coupled distributed feedback ridge laser

In this paper, the fabrication and characterization of 1.3-/spl mu/m wavelength GaInAsP-InP laterally coupled DFB ridge laser are presented for the first time, to our knowledge. One-step MOCVD (metal organic chemical vapor deposition) has been used for the epitaxial growth of the laser structure with a separate confinement heterostructure bulk active layer.

Optical Gain and Optical Internal Loss of GaN-Based Laser Diodes Measured by Variable Stripe Length Method with Laser Processing

We demonstrate a new technique for the variable stripe length (VSL) method by which the optical gain and optical internal loss of GaN-based laser diodes (LDs) can be directly measured. In the technique, the laser processing is utilized for varying the excitation length. The excitation length of GaN-based LDs can be varied by directly processing its p-electrode with high-power laser irradiation. From the results of the measurements, it was revealed that the optical internal loss of GaN-based LDs was strongly affected by the layer structure.

Second-Harmonic Generation of Blue Light in Epitaxial K3Li2-xNb5+xO15+2x Waveguides on K3Li2-x(Nb0.98Ta0.02)5+xO15+2x Substrates by Metalorganic Chemical Vapor Deposition

We report a new second-harmonic generation blue laser using a K3Li2-xNb5+xO15+2x single crystal film waveguide on a K3Li2-x(Nb0.98Ta0.02)5+xO15+2x crystal substrate by the metalorganic chemical vapor deposition method. Second-harmonic blue light with output power of 2 mW at 487 nm was successfully generated. The phase-matched wavelength bandwidth of the device was 1.3 nm, and the temperature coefficient of the phase-matched wavelength was 0.36 nm/°C. Crystallographic evaluation was also performed for the KLN thin film crystal. The (001) plane of KLN crystal film was well orientated to the (001) plane of the KLNT substrate, and the surface morphology of the film was very smooth. Moreover, it was revealed that an as-deposited KLN film crystal has a single domain structure.

Collimating properties of an axicon for the Čerenkov-radiation-type second-harmonic light from a crystal-cored fiber

The optical second-harmonic wave generated from a crystal-cored fiber from the Čerenkov-radiation-type phase matching can be collimated by an axicon and then can be focused by a spherical lens to a diffraction-limited spot. Tolerance of the optical system to meet the Maréchal criterion is analyzed; tolerable maximum errors in alignment and fabrication of the axicon lens are evaluated.