Semiconductor Photonic Nanocavities on Paper Substrates

Figure 1. Illustration of photonic crystal lasers on paper substrates

Professor Yong-Hoon Cho of the Department of Physics and his team at KAIST have developed a semiconductor photonic nanocavity laser that can operate on a paper substrate.

The researchers hope that this novel method, which involves transferring nano-sized photonic crystal particles onto a paper substrate with high absorptiveness, will enable the diagnoses of various diseases by using high-tech semiconductor sensors at low cost.

The results of this research were published in the November 17th, 2016, issue of Advanced Materials.

Photonic crystals, which utilize light as a medium to provide high bandwidths, can transfer large amounts of information. Compared with their electronic counterparts, photonic crystals also consume less energy to operate.

Normally, semiconductor photonic particles require substrates, which play only a passive role in the assembly and endurance of individual, functional photonic components. These substrates, however, are bulky and environmentally hazardous as they are made up of non-biodegradable materials.

The research team overcame these two shortcomings by replacing a semiconductor substrate with standard paper. The substrate’s mass was reduced considerably, and because paper is made from trees, it degrades. Paper can be easily and cheaply acquired from our surroundings, which drastically reduces the unit cost of semiconductors.

In addition, paper possesses superior mechanical characteristics. It is flexible and can be repeatedly folded and unfolded without being torn. These are traits that have long been sought by researchers for existing flexible substrates.

The research team used a micro-sized stamp to detach photonic crystal nanobeam cavities selectively from their original substrate and transfer them onto a new paper substrate. Using this technique, the team removed nanophotonic crystals that had been patterned (using a process of selectively etching circuits onto a substrate) onto a semiconductor substrate with a high degree of integration, and realigned them as desired on a paper substrate.

The nanophotonic crystals that the team combined with paper in this research were 0.5 micrometers in width, 6 micrometers in length, and 0.3 micrometers in height—about one-hundredth of the width of a single hair (0.1 millimeter).

The team also transferred their photonic crystals onto paper with a fluid channel, which proved that it could be used as a refractive index sensor. As can be seen in current commercial pregnancy diagnosis kits, paper has high absorptiveness. Since photonic crystal particles have high sensitivity, they are highly suitable for applications such as sensors.

Professor Cho stated that “by using paper substrates, this technology can greatly contribute to the rising field of producing environmentally-friendly photonic particles” and “by combining inexpensive paper and high-performance photonic crystal sensors, we can obtain low prices as well as designing appropriate technologies with high performance.”

Dr. Sejeong Kim of the Department of Physics participated in this study as the first author, and Professor Kwanwoo Shin of Sogang University and Professor Yong-Hee Lee of KAIST also took part in this research. The research was supported by the National Research Foundation’s Mid-Career Researcher Program, and the Climate Change Research Hub of KAIST.

Figure 2. Photonic crystal resonator laser and refractive index sensor operating on paper substrates