Professor, Institute of Physics, University of Tsukuba
Quantum dots studied by optical spectroscopy
—Quantum dots behaving like molecules—
When electrons are confined in a quantum dot, their energy levels are quantized depending on the size of the quantum dot. However, it is very difficult to control precisely the size of individual quantum dots. Prof. Masumoto has investigated how ensembles of quantum dots or individual quantum dots behave.
Ensembles of molecules and ions dispersed in crystals, or glass, show a broad absorption spectrum, caused by the inhomogeneity. When the crystals, or glass, are exposed to laser light with a very narrow linewidth, molecules and ions are selectively excited. This is called site-selective excitation. Then spectral holes in the absorption spectrum are observed. When the spectral hole is observed for longer time than the excited state lifetime, the phenomenon is called as “persistent spectral hole burning”. It was considered to be inherent to molecules and ions and not to bulk crystals. However, Prof. Masumoto discovered that it also occurs in a quantum dot, which is much larger than a molecule. Electrons and holes are generated in a quantum dot that has been excited by a laser beam, and the electron-hole pairs act as excitons. When electrons, or holes, are trapped in the region surrounding the quantum dot, the quantum dot energy changes due to their Coulombic interactions. If this state is kept for long time at low temperatures, persistent holes are formed in the absorption spectrum. Persistent spectral hole burning can be applied to optical multiple memory because many persistent holes can be made in an absorption spectrum. He said, “Current CD-Rs and CD-RWs hold only a single bit of information in one area on the disk. If many spectral holes in one area can be made with various laser wavelengths, more information can be stored in that area. Therefore, the multiplicity of the memory devices will be drastically increased.”
Site-selective spectroscopy can be used to study the averaged properties of several quantum dots with the same energy. In contrast, single quantum dot spectroscopy can be used to study the properties of a single quantum dot. Prof. Masumoto determined the mechanism for “intermittent light emission,” where a quantum dot excited by a laser beam blinks, using single quantum dot spectroscopy. When InP quantum dots are excited by laser light, some of them blink. He has confirmed that quantum dots around scratches blink, and the off-state luminescence spectrum of a quantum dot is consistent with the spectrum of a quantum dot under an applied electric field. In other words, when there are defects with shallow energy levels near quantum dots, carriers generated by photoexcitation are trapped in the scratches, and thus, an electric field is applied to the quantum dots. As a result, an off-state of the fluorescence intermittency is induced due to the electric field. This intermittent light emission phenomenon, like a persistent spectral hole burning, is also observed with molecules embedded in solids. “A quantum dot is composed of 1,000 to 100,000 atoms, and its size is in between a molecule/atom and bulk. It is largely affected by external fields due to its large surface-to-volume ratio. Therefore, I was able to discover that quantum dots behave like molecules,” said Prof. Masumoto.
Prof. Masumoto started his research in the 70’s. At the time, with laser spectroscopy, only static optical analysis of matter was possible. He said, “In static spectroscopy, temporal parameters are only hypothetical. I want to see the dynamical changes occurring in matter, like the changes in the positions of the subjects in stroboscopic photography. It is technically hard to see them, but I think direct observation of dynamic phenomena is indispensable for the future.” He studied femtosecond and picosecond spectroscopy using a ultrahigh-speed laser spectroscope, of which only a few existed in the world at that time, and determined the dynamic phenomena that occurs in a semiconductor quantum structure by analyzing phenomena from the viewpoint of time and energy. He talked about the pleasures of not only determining physical phenomena but also developing cutting-edge measuring methods. “If measuring objects, or amounts, smaller than ever before becomes possible by increasing the sensitivity to its maximum, my research will expand drastically, and I can conduct top-rate research. I get excited just thinking about it,” said Prof. Masumoto.
