Applied Physics PhD Oral Examination - Ragip Pala, Tomorrow, June 10, 10:00am

Ragip Pala rpala at stanford.edu
Wed Jun 9 19:49:50 PDT 2010


Department of Applied Physics
University PhD Dissertation Defense

Plasmonic Devices Employing Extreme Light Concentration

Ragip Pala

Research Advisor: Professor Mark L. Brongersma

10 June 2010 @10:15 A.M.
(Refreshments 10:00 A.M.)

Location: Allen Building (formerly CIS-X), Room 101


ABSTRACT
The development of integrated electronic and photonic circuits has led 
to remarkable data processing and transport capabilities that permeate 
almost every facet of our daily lives. Scaling these devices to smaller 
and smaller dimensions has enabled faster, more power efficient and 
inexpensive components but has also brought about a myriad of new 
challenges. One very important challenge is the growing size mismatch 
between electronic and photonic components. To overcome this challenge, 
we will need to develop radically new device technologies that can 
facilitate information transport between nanoscale components at optical 
frequencies and form a bridge between the world of nano-electronic and 
micro-photonics. Plasmonics is an exciting new field of science and 
technology that aims to exploit the unique optical properties of 
metallic nanostructures to gain a new level of control over light-matter 
interactions.  The use of nanometallic (plasmonic) structures may help 
bridge the size gap between the two technologies and enable an increased 
synergy between chip-scale electronics and photonics.

In the first part of the presentation I will analyze the performance of 
a surface plasmon-polariton all-optical switch that combines the unique 
physical properties of small molecules and metallic (plasmonic) 
nanostructures. The switch consists of a pair of gratings defined on an 
aluminum film coated with a thin layer of photochromic (PC) molecules. 
The first grating couples a signal beam consisting of free space photons 
to SPPs that interact effectively with the PC molecules. These molecules 
can reversibly be switched between transparent and absorbing states 
using a free space optical pump. In the transparent (signal "on") state, 
the SPPs freely propagate through the molecular layer, and in the 
absorbing (signal "off") state, the SPPs are strongly attenuated. The 
second grating serves to decouple the SPPs back into a free space 
optical beam, enabling measurement of the modulated signal with a 
far-field detector. We confirm and quantify the switching behavior of 
the PC molecules by using a surface plasmon resonance spectroscopy. The 
quantitative experimental and theoretical analysis of the nonvolatile 
switching behavior guides the design of future nanoscale optically or 
electrically pumped optical switches.

In the second part of my presentation I will provide a critical 
assessment of the opportunities for use of plasmonic nanostructures in 
thin film solar cell technology. Thin-film solar cells have attracted 
significant attention as they provide a viable pathway towards reduced 
materials and processing costs. Unfortunately, the materials quality and 
resulting energy conversion efficiencies of such cells is still limiting 
their rapid large-scale implementation. The low efficiencies are a 
direct result of the large mismatch between electronic and photonic 
length scales in these devices; the absorption depth of light in popular 
PV semiconductors tends to be longer than the electronic (minority 
carrier) diffusion length in deposited thin-film materials. As a result, 
charge extraction from optically thick cells is challenging due to 
carrier recombination in the bulk of the semiconductor. If light 
absorption could be improved in ultra-thin layers of active material it 
would lead directly to lower recombination currents, higher open circuit 
voltages, and higher conversion efficiencies. In this part of the talk, 
I will discuss how extreme light concentration ability of plasmonic 
structures can improve the overall performance of thin film solar cells 
with broadband absorption enhancements. I will present a combined 
computational\experimental study aimed at optimizing plasmon-enhanced 
absorption using periodic and aperiodic metal nanostructure arrays.

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