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Student Lecture Series: Feb. 21st 2020

2/19/2020

 
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12:00 - 1:00 PM Friday, February 21st in Engineering II 3519


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Haojun Zhang
​DenBaars group
ECE Dept, UCSB

Distributed feedback (DFB) laser diodes on GaN

I will first talk about the recent work on blue III-Nitride LDs grown on semipolar GaN substrates. Then I will analyze the main hurdles that impeded its performance, focusing on improving the operating voltage, lifetime and mode quality, and discuss the efforts and approaches to further improve the efficiency and high-speed performance.

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Changyun Yoo
Sherwin group
Physics Dept, UCSB

Terahertz Heterodyne Detector Based on the Intersubband Transition of a GaAs/AlGaAs Quantum Well ​

We are developing a new type of THz heterodyne detector based on a high-mobility 2DEG in a GaAs/AlGaAs quantum well for spectroscopic applications in deep-space and planetary missions. Named as Tunable Antenna-Coupled Intersubband Terahertz (TACIT) mixer, the detector uses intersubband transition for efficient absorption of THz radiation in a 2DEG. The dual gate structure of TACIT mixers, necessary for the precise control of the intersubband absorption characteristics, enables a high coupling efficiency at THz frequencies and tunability in the detection frequency, but also poses challenges in the fabrication, modelling, and operation of the device. In this talk, I will discuss our recent experimental results with a prototype TACIT mixer that we have fabricated with a flip-chip process that enables dual-side processing of a sub-micron thick quantum well membrane.

Student Lecture Series: Feb. 14th 2020

2/12/2020

 
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Friday, Feb 14th from 12:00 - 1:00 pm in Elings 1601


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Takako Hirokawa
Schow group
ECE Dept, UCSB

Ring-Assisted Mach-Zehnder Interferometer Switch with Multiple Rings per Switch Element

Wavelength-selective switches have been propsed for datacenter use to help meet ever-increasing traffic demands. We present a 4-port silicon photonic ring-assisted Mach-Zehnder interferometer (RAMZI) switch, fabricated in the AIM Photonics process, with multiple-sized rings per switching element in a Benes network configuration to reduce the number of electrical pads required compared to a crossbar switch. Another advantage the RAMZI switch has over the crossbar switch is that the loss through the switch is not path-dependent due to its balanced path configuration. Finally, we present results from the fabricated switch co-packaged with a custom driver for control and discuss the outlook for further scaling of the switch architecture. 

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Caroline Reilly 
DenBaars Group
Materials Dept, UCSB

Characterization of InGaN quatum dots grown by metal organic chemical vapor deposition (MOCVD)

InGaN quantum dots were grown by metal organic chemical vapor deposition and shown to exhibit a bimodal size distribution. Atom probe tomography was used to characterize the dots in conjunction with atomic force microscopy (AFM), photoluminescence (PL), and x-ray diffraction (XRD). Small dots with low indium contents were found to coexist with larger, very high indium composition dots. The dots showed abrupt interfaces with the surrounding GaN, verifying the ability to cap the dots without causing intermixing for extremely high indium content dots. 

Advanced Semiconductor Lasers - Dr. Shinji Matsuo

2/5/2020

 
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Advanced Semiconductor Lasers: Ultra-low Operating Energy and Hetergeneous Integration with Si Photonics Devices



Dr. Shinij Matsuo​
NTT Device Technology Laboratories


Friday, February 7th from 12:00 - 1:00 pm in ESB 2001 
The electrical power consumed in data transmission systems is now hampering efforts to further increase speed and capacity at various scales, ranging from data centers to microprocessors. Optical interconnects employing ultra-low-energy directly-modulated lasers will play a key role in reducing the power consumption. Since a laser's operating energy is proportional to the size of its active volume, developing high-performance laser with a small cavity is important. For this purpose, we have developed DFB and photonic crystal (PhC) lasers, in which active regions are buried with an InP layer. Thanks to the reduction of cavity size and the increase in optical confinement factor, we have achieved an extremely small operating energy of 4.4 fJ/bit by employing a wavelength-scale PhC cavity. Cost reduction is also an important issue because a larger number of transmitters are required for short-distance optical links. For this purpose, Si photonics technology is expected to be a potential solution because it can provide large-scale photonic integrated circuits (PICs). Therefore, heterogeneous integration of III-V compound semiconductors and Si has attracted much attention. To fabricate these devices, we have developed wafer-scale fabrication  that employs regrowth of III-V compound semiconductors on directly-bonded thin InP templates on an SiO2/Si substrate. 
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Dr. Matsuo discusses some of the equations related to semiconductor lasers.

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