Professor, Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University
Unraveling optical properties of nanoparticles
—Nonlinear optical response from ultrafine particles—
Ultrafine metal particles, such as gold and silver, dispersed in glass, color glass beautifully. Conduction electrons in the metals do not generally interact with electromagnetic waves, such as light. However, when surface plasmons are generated on metal surfaces, conduction electrons interact with light, and therefore, beautiful colors appear on stained glass. Prof. Nakamura says, “Only ultrafine metal particles cause this phenomenon. The finer the particles, the more influence the surface plasmons have on the interaction between ultrafine metal particles and light.” He has been studying ultrafine particles and trying to determine their optical properties.
When a transparent insulator, such as glass, with dispersed ultrafine metal particles is irradiated with light, local electric fields around the ultrafine particles which have different dielectric constants than that of the glass are generated. Prof. Nakamura says that when there are ultrafine metal particles with a negative dielectric constant, which has a large absolute value, the electric fields around the particles are high. Therefore, surface plasmons localized on the surfaces of the ultrafine particles strongly resonate with the incident light. He has confirmed that, as the ultrafine particle size becomes larger, resonance effects, caused by the local electric field and the nonlinear susceptibility of the glass, are larger. Ultrafine particles larger than 20 nm in diameter have large optical nonlinearity.
The particle size also strongly influences the speed of the optical response. When fine metal particles are irradiated with light, the electrons go into a state called hot electrons, in which the electron temperature is high. Normally, the hot electrons and lattices gradually go into a thermal equilibrium state, due to electron-lattice interactions, and then, the thermal energy transfers to the glass around the particles. Prof. Nakamura says, “However, nanoparticles in the glass vibrate in a so-called ‘breathing mode’. The frequency of the vibration becomes higher as the particle radius becomes smaller. When the frequency is approximately equal to the rate of energy transfer from the electrons to the lattice vibration, the electron energy directly transfers to the glass, and not to the metal lattice. Therefore, the rate of energy relaxation increases.” As the energy relaxation rate increases, the response time becomes shorter. In other words, ultrafine particles with a diameter of several nanometers must have shorter response times.
For a higher nonlinear susceptibility, large particles are required, and for a shorter response time, fine particles are required. The trade-off relationship between the nonlinear susceptibility and response time for the particle size is resolved by a quantum size effect. In ultrafine semiconductor particles, dispersed in glass, a quantum size effect appears when a particle’s diameter is 1 to 10 nm, and the nonlinear susceptibility of the materials increases. It also appears in ultrafine metal particles with a diameter of 2 nm, but it was unclear whether nonlinearity would increase or not. In early 2005, Prof. Nakamura confirmed that an increase in nonlinearity occurred due to quantum size effects in glasses with ultrafine metal particles. The measured nonlinear susceptibility of synthesized gold particles with a diameter of 1.0 nm, which were dispersed in solution, was 7.2x10-15 esu•cm. This is comparable to the value for particles with a diameter of 15 nm. The nonlinear susceptibility, resulting from quantum size effects and a response time shorter than one picosecond due to a breathing mode, was obtained with these ultrafine particles.
Prof. Nakamura’s other main research field is bandgap engineering using semimetal/semiconductor hetero-structures. He fabricated quantum disks of semimetal ErP on an InP substrate by organometallic vapor phase epitaxy, in collaboration with Prof. Yoshikazu Takeda at Nagoya University. He discovered that the bandgap in the disks becomes larger due to the quantum size effect when the disk thickness is less than 3.5 nm and that an increase in the bandgap converts the disk from a semimetal to a semiconductor. It was difficult to fabricate ErP films because the crystal structure and lattice constant of InP were different from those of ErP. However, he was able to fabricate ErP films by replacing the InP substrate with a GaInP substrate and bringing the lattice constants of the substrate closer to that of ErP. He says, “When semimetallic materials are used for quantum well layers for resonant tunneling diodes, devices that switch rapidly with low driving voltage can be developed. In addition, since the bandgap becomes much larger, multiple resonant peaks are obtained. Therefore, switching based on negative resistance can also be repeated several times by changing the voltage.” Since Er compounds are magnetic, by determining their magneto-optical properties and magnetic transport properties, a basic model for a high-order functional optoelectronic integrated circuit with magneto-optical switches and semimetal-base transistors is determined.
Prof. Nakamura says to young researchers, “Although it is fine to research something popular or interesting, you should find its essential features that will lead to expansion of your research. You should not try to earn popularity. Publishing your research with accurate data and an appropriate understanding of the data is essential. That means that others can verify your research results, and thus, the results will be available as persisting data.”
