Optics Out of Metals at SPIE Photonics West
It was literally and commercially sunny in San Francisco at SPIE Photonics West, held in late January. Lovely weather and an enthusiastic crowd. If you want a large turnout, run a conference in San Francisco in the middle of winter. The conference exhibits took over both major floors of the conference center, and SPIE estimated that nearly 20,000 exhibitors and attendees participated.
There were several themes of interest to the patterning crowd, including surface plasmonics, patterned substrates for LEDs, thick photonic crystal structures, and electro-wetting displays. I will be writing about these over the next few weeks.
Plasmonics or the idea of making optics out of metals was a major theme this year. Optical nerds who are bored making optics out of transparent materials are challenging themselves by turning to non-transparent materials. Eli Yablonovitch — director of the NSF Center for Energy Efficient Electronics Science (E3S), a multi-University Center based at the University of California at Berkeley — gave the plenary talk on the subject and between pages of mathematics observed that if we could pattern at the molecular level then LED’s switchable at terahertz frequencies would be possible. With hard disks being patterned into 75 atom features, we are not very far away.
The physics behind the patterning and potential applications in this area are fascinating. If a light is focused on a thin metalized surface at the right angle, an intense surface wave (surface plasmon) is formed in the top layer of the metal. This surface wave can create some unique interactions, which are amplified as the metal is patterned.
Perhaps the most direct application is in Surface Enhanced Raman Spectroscopy (SERS), a form of infrared spectroscopy in which the vibrating atomic bonds produce shifts in a laser that is operating at a much shorter wavelength.
SERS works on a sample sitting on a nanometal layer of a metal. The “surface plasmon” increases the intensity of the field at the surface by three or four orders of magnitude. These nano layers can be flat or as a coating on a nanosphere, and very small amounts of material can be detected. The latest research is directed to patterning the surface to further increase the strength of the surface plasmon. The figure shows the effect of a substrate on the SERS signal (www.bioeng.nus.edu.sg).
The goal of this work is to detect protein binding without needing fluorescent tags. Virtually all the latest bioassays use DNA or protein probes attached to fluorescent tags. The flourofors enable very sensitive detection of a single molecular layer bound to an underlying protein. The problem is the tags are difficult to create and tend to disrupt the binding they are trying to detect. Researchers believe that with patterned surfaces, SERS can achieve the sensitivity to detect a single bound layer without the need for tags.
Other applications for this idea discussed at SPIE Photonics West included;
- Enhancing the output of LED’s by placing the metal close to the emitter layer
- Enhancing the efficiency of photovoltaics
- Creating sub-wavelength optical probes
- Creating nanospot heating devices to be used in heat assisted magnetic recording (HAMR)
- Color filters for displays
I believe subwavelength patterning of thin metals can have a significant commercial impact in the near future.
About the Author
Mike Watts has been patterning since 1 um was the critical barrier, in other words, for a longtime. I am a tall limey who is failing to develop a Texas accent here in Austin. I have a consulting shingle at www.impattern.com.
My blog “ImPatterning” will focus on the latest developments in the business and technology of patterning. I am particularly interested in trying to identify how the latest commercial applications will evolve.