Dr. Munechika — along with Alexander Kosh­elev and Giuseppe Calafiore at aBeam Technologies, and Stefano Cabrini at the Molecular Foundry at Lawrence Berkeley National Laboratory — is developing a process that would enable widespread adoption of nano-optic technology. The team perfected a technique for fabricating a Campanile probe, whose tapered, four-sided shape resembles the top of the Campanile clock tower on UC Berkeley’s campus. The probe focuses an intense beam of light onto a much smaller spot than is possible with current optics.

Tech Briefs: What is the importance of nano-optics?

Dr. Keiko Munechika: Many researchers are looking for ways to visualize materials and processes at nanometer scale. Optical spectroscopy is the go-to method since it provides access to a vast range of physical and chemical properties. Its main drawback is the lack of spatial resolution below the diffraction limit. Many physical processes happen in the range of individual molecules to a few nanometers. Nano-optics can help by going beyond the diffraction limit. In the case of Campanile probes, having a 3D plasmonic optical transformer with a nanoscale “gap” enables efficient “squeezing” of the light so that imaging and spectroscopy can be achieved with nanometer-scale resolution.

Tech Briefs: What are some of its applications?

Dr. Munechika: It enables you to get more information than with traditional optics. For example, photovoltaic cells and critical reactions like exciton diffusion and recombination happen at a scale of a few nanometers or tenths of nanometers. You can’t resolve this small with traditional optics — everything would be a blur. These nano-optical devices can be used to look at solar cells to explore different kinds of materials. You can view what’s happening chemically and physically at the highest possible resolution.

Tech Briefs: What problems were you trying to solve?

Dr. Munechika: Campanile was originally invented a few years back and considered to be one of the most promising probes. But fabrication was prohibitively laborious and expensive to scale up. We developed fiber imprint mold technology to solve this bottleneck. Our goal is to establish a simplified process that is reproducible, inexpensive, fast, and can achieve nanometer resolution, all at the same time.

Tech Briefs: How does the Campanile probe improve nano-optics?

Dr. Munechika: The design has a lot of advantages for these probes. For example, it can couple broader wavelengths of light — it’s designed to couple different wavelengths of light instead of just a very narrow range. It also has a very good signal-to-noise ratio. In addition, it’s more userfriendly than other available probes. Its high performance and user-friendliness will allow more people to use it and adopt the system.

Tech Briefs: Why is the probe coated with gold?

Dr. Munechika: That allows us to efficiently squeeze the light into a very small spot, which provides very good field enhancement at the tip for optimal signal collection and excitation.

Tech Briefs: How do you create the fiber imprint mold?

Dr. Munechika: The mold fabrication involves several steps, starting with making a pyramid structure in silicon. We then replicate the pyramid into a polymeric material to reverse the tone. Next, we mill a nano-gap at the tip using focused ion beam milling, which allows us to create arbitrary nanometer-scale features. We then replicate the pyramid once more to create the final imprint mold, which includes a pre-milled gap. The mold fabrication is time-consuming, but we only have to do this once, as we can replicate the original mold multiple times. After the imprint mold is made, the nano-imprinting process is very quick.

Tech Briefs: Will this molding technique have other applications?

Dr. Munechika: Yes. Based on this technology, we developed a series of photonics-on-fiber devices. Various diffractive optical elements are integrated directly onto a fiber to precisely manipulate light wavefronts without any additional assembly or alignment of optical components. Some examples are a vortex phase plate, which directly creates an optical vortex through the fiber; a lensed fiber; and beam shapers (elliptical, square, flat-top, etc).

Tech Briefs: Will your work eventually be used in industrial processes?

Dr. Munechika: Initially, it will mainly be used for research. Current interest is in using it to discover new material processes, but once it has been proven in research work, we anticipate that it will find industrial applications. In fact, chip manufacturers are interested in the probe as a characterization tool for analyzing circuit failure with a sub-19-nm length scale. It could also be used in inspection because chip designs are getting smaller and smaller, and they don’t currently have a tool for viewing at that small scale.

To learn more, read a full transcript, or listen to a downloadable podcast, visit www.techbriefs.com/podcast.

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This article first appeared in the August, 2017 issue of Tech Briefs Magazine.

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