Jijun Feng

A Three-Dimensional Silicon Nitride Polarizing Beam Splitter

A 3-D polarizing beam splitter based on a silicon nitride (Si3N4) vertical directional coupler is experimentally demonstrated. A new planarization technique by incorporating conventional chemical-mechanical lapping with a dry-etching process is developed, in order to obtain a flat film surface for the second Si3N4 core deposition after the first-layer waveguide is formed. Both the Si3N4 layer thicknesses are 200 nm. As there is a material refractive index mismatch between the vertically separated waveguides, the polarization splitter can be realized with a bottom waveguide width of 1.55 μm and a top core width of 1.35 μm. The transverse electric (TE) polarized light can be transmitted completely to the cross-layer output-port, whereas the transverse magnetic (TM) polarized wave outputs mostly from the port at the input layer. A high-extinction ratio and a wide operation bandwidth can be achieved. An extinction ratio of 26 dB for the cross-layer output-port at 1550-nm wavelength and that of 16 dB for the input-layer output-port are obtained. There is an excess coupling loss of for the TE light, but a 1-dB loss for the TM wave.

Low-threshold-current yellow BeZnCdSe quantum-well ridge-waveguide laser diodes under continuous-wave room-temperature operation

Low-threshold-current yellow BeZnCdSe single-quantum-well (SQW) laser diodes (LDs) have been developed by using a ridge-waveguide structure. The top p-cladding layer was etched to suppress the leakage current that flowed laterally outside of the electrode. Ridge waveguides were covered with a SiO2 layer and planarized by chemical–mechanical polishing and reactive ion etching. Room-temperature lasing under continuous-wave condition was achieved with the laser cavity formed by cleaved waveguide facets coated with high-reflectivity dielectric films. Two types of LDs with different SQW thicknesses and Cd contents were developed and compared at various waveguide widths and lengths. Yellow LDs with sub-10 mA threshold current were obtained.

Vertically Coupled Silicon Nitride Microdisk Resonant Filters

Polarization-dependent vertically coupled microdisk resonant filters are experimentally demonstrated based on a multilayer silicon nitride platform. An improved planarization technique based on conventional chemical-mechanical polishing and dry-etching is adopted to flatten the gap film surface for a different core layer deposition after the first-layer photonic circuit is formed. The resonator can realize a high quality factor (> 104) and large suppression value (>20 dB) for the transverse electric light, while the transverse magnetic wave has weak resonant coupling. Other vertically coupled multifunctional devices can be further expected based on the current integration method. A cascaded resonator filter with superior characteristics, such as a flat passband, a steep roll-off, and an enhanced suppression ratio is developed.

Ultrafast all-optical switch with cross-phase modulation by area-selective ion implantation in InGaAs/AlAsSb coupled double quantum wells.

We have developed a compact gate switch with monolithic integration of all-optical cross-phase modulation (XPM) in a Mach-Zehnder interferometer (MZI). XPM is caused by intersubband transition (ISBT) in InGaAs/AlAsSb coupled double quantum wells (CDQWs) by area-selective silicon ion implantation and rapid thermal annealing (RTA). While injecting pump light through a transverse electric/transverse magnetic (TE/TM) beam combiner, XPM is induced in one MZI arm and gating operation can be realized. The RTA condition is optimized, and the sample is annealed at 780 °C for 8 s with an implantation dose of 5 × 10(13) cm(-2). Dependence of XPM efficiency on the length of the implanted mesa is also analyzed, and there exists an optimum implantation length to fulfill both high efficiency of ISBT modulation and low loss of the probe and pump signals.

Asymmetric Silicon Slot-Waveguide-Assisted Polarizing Beam Splitter

A silicon polarizing beam splitter is designed and fabricated based on an asymmetric slot waveguide structure without adopting additional strip-slot waveguide mode converters. The asymmetric slot waveguide consists of a 150-nm-wide and a 350-nm-wide nanowires with a slot of 100 nm, which can realize a transverse magnetic light cross-coupling with a 392-nm-wide strip waveguide, while little transverse electric light coupling can happen. The fabricated device has a coupling length of ~18 μm with a coupling gap spacing of 450 nm, which can achieve a polarization extinction ratio (PER) of ~14 and 25 dB at the bar and cross ports, respectively. Low insertion loss is realized, while more than 10-dB PER can be obtained over the wavelength range of 1520-1576 nm. The device performance could be further improved with the fine optimization of device structure parameters and fabrication process.

Band edge tailoring of InGaAs/AlAsSb coupled double quantum wells for a monolithically integrated all-optical switch.

We demonstrate a compact all-optical Michelson interferometer (MI) gating switch with monolithic integration of two different bandgap energies. Based on the ion-induced intermixing in InGaAs/AlAsSb coupled double quantum wells, the blueshift of the band edge can be tailored. Through phosphorus ion implantation with a dose of 5 × 10(14) cm(-2) and subsequent annealing at 720 °C for 60 s, an implanted sample can acquire a high transmittance compared with the as-grown one. Meanwhile, the cross-phase modulation (XPM) efficiency of a non-implanted sample undergoing the same annealing process decreases little. An implanted part for signal propagation and a non-implanted section for XPM are thus monolithically integrated for an MI switch by an area-selective manner. Full switching of a π-rad nonlinear phase shift is achieved with pump pulse energy of 5.6 pJ at a 10-GHz repetition rate.

Silicon nitride polarizing beam splitter with potential application for intersubband-transition-based all-optical gate device

In this paper, we present the design, fabrication, and characterization of a silicon nitride polarizing beam splitter, which has potential applications in an all-optical gate device based on an intersubband-transition-induced cross-phase modulation. On the basis of a Mach–Zehnder interferometer (MZI) configuration consisting of two directional coupler structures, the splitter can realize a high extinction ratio, wide operation bandwidth, and negligible inherent excess loss. The directional coupler is designed on the basis of the coupled mode theory and supermode theory, which can work as a 3-dB coupler for transverse electrical (TE) light and realize a complete cross-coupling for a transverse magnetic (TM) wave. Thus, the TE light can couple to the cross port while the TM wave is output from the through port under the MZI configuration. An extinction ratio of 37 dB at approximately 1554 nm wavelength and a 38 nm bandwidth with an extinction ratio of higher than 20 dB can be achieved. More application prospects beyond the polarization splitting in optical communications could be expected for the current device.

BeZnCdSe quantum-well ridge-waveguide laser diodes under low threshold room-temperature continuous-wave operation

Low threshold current ridge-waveguide BeZnCdSe quantum-well laser diodes (LDs) have been developed by completely etching away the top p-type BeMgZnSe/ZnSe:N short-period superlattice cladding layer, which can suppress the leakage current that flows laterally outside of the electrode. The waveguide LDs are covered with a thick SiO2 layer and planarized with chemical-mechanical polishing and a reactive ion etching process. Room-temperature lasing under continuous-wave condition is achieved with the laser cavity formed by the cleaved waveguide facets coated with high-reflectivity dielectric films. For a 4 μm-wide green LD lasing around a wavelength of 535 nm, threshold current and voltage of 7.07 mA and 7.89 V are achieved for a cavity length of 300 μm, and the internal differential quantum efficiency, internal absorption loss, gain constant, and nominal transparency current density are estimated to be 27%, 4.09 cm−1, 29.92 (cm × μm)/kA and 6.35 kA/(cm2 × μm), respectively. This compact device can realize a ...

T-Shape Suspended Silicon Nitride Ring Resonator for Optical Sensing Applications

A T-shape suspended silicon nitride racetrack ring resonator sitting on a silicon dioxide pedestal is experimentally demonstrated. Compared with unsuspended device, it allows an enhanced interaction with the surrounded environment while maintaining a high quality factor. The resonator consists of a directional coupler with a waveguide width of 1.3 μm and a ring structure with curve radius of 20 μm. More than 1.6 × 104 quality factor at a wavelength of 1550.4 nm can be obtained, with a moderate extinction ratio of ~9.2 dB and a free-spectral range of 5.48 nm. The measured device sensitivity is ~247 nm/RIU. The device is expected to have a wide optical sensing application prospect.

All-optical XOR logic gate using intersubband transition in III-V quantum well materials

A monolithically integrated all-optical exclusive-OR (XOR) logic gate is experimentally demonstrated based on a Michelson interferometer (MI) gating device in InGaAs/AlAsSb coupled double quantum wells (CDQWs). The MI arms can convert the pump data with return-to-zero ON-OFF keying (RZ OOK) to binary phase-shift keying (BPSK) format, then two BPSK signals can interfere with each other for realizing a desired logical operation. All-optical format conversion from the RZ OOK to BPSK is based on the cross-phase modulation to the transverse electric (TE) probe wave, which is caused by the intersubband transition excited by the transverse magnetic (TM) pump light. Bit error rate measurements show that error free operation for both BPSK format conversion and XOR logical operation can be achieved.