Silicon nitride is a promising wave-guiding material for integrated photonics applications with a wide transparency bandwidth from visible to mid-infrared, with a superior performance in fiber-coupling and propagation losses, more tolerant fabrication process to the structure parameters variation and compatible with the CMOS technology. Directional coupler (DC) is very popular for realizing beam splitter because of its structural simplicity and no excess loss intrinsically. Here, a conventional silicon nitride directional coupler, three-dimensional vertical coupler, and grating waveguide assisted coupler are designed and fabricated, and compared with each other. A grating waveguide based coupler with a period of 300 nm and coupling length of 26 um, can realize a wideband 3-dB splitter for the wavelength in the range from 1540 to 1620 nm, for a transverse electric (TE) polarized wave. With further optimization of the grating period and duty cycle, the device performance can be further improved with a wider bandwidth.
Low threshold current BeZnCdSe single quantum-well (SQW) laser diodes (LDs) have been developed. The waveguide
was formed of a ridge structure with etching away the top p-type BeMgZnSe/ZnSe:N short-period superlattice cladding
layer, and then covered with a thick SiO2 layer and planarized with chemical-mechanical polishing and reactive ion
etching process. Three type LDs with different SQW thickness and Cd content were developed and compared at varying
waveguide width and length. Lasing wavelength of 535, 563, and 567 nm were realized respectively, at room-temperature
continuous-wave condition with the laser cavity formed by the cleaved waveguide facets coated with high-reflectivity
dielectric films. Compared with our previously developed gain-guided diode structure for a 5-μm-wide, 800-
μm-long gain-guided green laser with a threshold current and voltage of 68 mA and 10.4 V, a 535-nm green laser with 7-
nm-thick SQW can realize a threshold current and voltage of 7.07 mA and 7.89 V, respectively, for a cavity width of 4
μm and length of 300 μm. A 563-nm yellow LD with 4-nm-thick SQW was also developed with 7.4-mA and 8.48-V
threshold current and voltage for a 3-μm-wide, 300-μm-long cavity. A 567-nm yellow LD with 7-nm-thick SQW can
achieve a threshold current and voltage of 10.8 mA and 8.4 V, respectively, for a cavity length of 300 μm and width of 7
μm. The threshold current and voltage were decreased due to the reinforced confinement of carriers in cavities. The
device performance can be significantly improved with much lower power consumption. The threshold current and
power consumption is also sufficiently low compared with that of InGaN/GaN green LDs, which will benefit the
potential application for ZnSe-based LDs as light sources in full-color display as well as some biomedical devices.
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