Ph. D. Oral Defense: Yiyang Gong, Tuesday 11/16, 10:30am Nanoscience 232

[Yiyang Gong]

*Silicon-based nanobeam photonic crystal light emitting devices*
Tuesday, November 16^th , 2010 10:30 am (refreshments 10:15 am)

Ph. D. Candidate: Yiyang Gong

Research Advisor: Jelena Vuckovic
Committee: Mark Brongersma, Shanhui Fan, David Miller

Location: Center for Nanoscience 232

Silicon compatible light emitting materials can enable a new class of 
cost-efficient opto-electronic devices, as optical devices and 
electronic devices can be integrated on the same platform. However, the 
low emission efficiency of such materials has hampered the development 
of silicon light emitting devices. We explore the use of photonic 
cavities to enhance the emission properties of silicon nano-crystals 
(Si-NCs) in oxide and Er-doped silicon nitride, as high Q, low mode 
volume cavities enhance the light-matter interaction.

Two dimensional photonic crystal (2D- PC) cavities have been developed 
for the enhancement of emission for various materials, and cavities with 
quality (Q-) factors greater than 10^5 have been experimentally 
demonstrated in materials with index of refraction n = 3.5. However, 
materials such as Si-NC doped oxide and nitride have low index of 
refraction (n ? 2.0), and 2D PC cavities have been experimentally 
demonstrated with Q-factors up to only 3,400 in such low index systems. 
We investigate one dimensional nanobeam PC cavities, which are versatile 
and enable high Q cavities for various indices of refraction. We 
describe the design and fabrication of nanobeam cavities in silicon 
dioxide (n = 1.5), with experimental Qs over 5,000 in the visible 
wavelengths. We also study nano-optomechanical effects in passive 
Si-based nanocavities. We then fabricate nanobeam cavities in silicon 
oxide with embedded Si-NCs and Er-doped amorphous silicon nitride. We 
demonstrate Q > 9,000 for the Si-NC material, and analyze the signature 
of free carrier absorption for this type of material. In addition, we 
demonstrate nanobeam cavities in the Er-doped nitride material with Q > 
15,000. We observe linewidth narrowing in the Er material with 
increasing pump power, which is a signature of absorption saturation and 
differential gain, at both room temperature and cryogenic temperatures. 
Compared to previous designs using high index Si as part of the cavity, 
we observe a reduction of absorption losses arising from the material, 
and correspondingly larger decreases in linewidth. By using time 
resolved measurements, we calculate that optical transparency of the 
material is reached at high pump powers.


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