This Partnership for International Research and Education, led by Junichiro Kono of Rice University, investigates the ultrafast and nonlinear optical properties of quantum structures based on the 6.1-angstrom (A) III-V semiconductors (InAs, GaSb, AlSb). These semiconductors are nearly lattice-matched (with lattice constant ~ 6.1 A) and can be epitaxially grown to form heterostructures. Though they possess promising properties for application in next-generation devices, there has been little exploration of their optical properties, especially in the time domain and at high laser intensities. Various optical spectroscopic methods will be used to study charge and spin dynamics. Such studies are not only expected to increase understanding of the states and dynamics of interacting electrons but also to provide new insight into which structures are optimum for specific applications. The project establishes a long-term partnership between Rice University, University of Florida and Texas A&M University in the U.S. and Osaka Institute of Technology and Tokyo Institute of Technology in Japan. The international partnership focuses on the complementary research strengths of each side, with Japanese collaborators providing unique materials growth and processing capabilities. The project also involves a tiered program of education activities targeting students at different stages of their academic careers to attract young people to the emerging areas of electrical engineering and the physical sciences, especially nanotechnology. The cornerstone of the education activities will be a ten-week study and research internship program in Japan for graduate and undergraduate students from U.S. universities. The annual symposium INNOVATE US-Nanotechnology will reinforce the benefits of the Japan research internships for students on returning to the U.S. The annual international symposium INNOVATE Asia will provide an additional opportunity for students to examine the relationship between technology and globalization in a comparative perspective.
This U.S.-Japan PIRE partnership explores optical phenomena in semiconductors and carbon nanotubes grown by our Japanese partners, using a variety of methods available in the laboratories of PI Kono and our Japanese partners. These experiments are coupled with theoretical studies by co-PIs Belyanin and Stanton. Our education program activities were directed by co-PIs Matherly and Phillips. Our PIRE research and educational activities will continue under the "PIRE: U.S. - Japan Cooperative Research on Terahertz Dynamics in Nanostructures" through 2015. Below are highlights of our successful activities in international research and education: ACCOMPLISHMENT 1: MAGNETIC BRIGHTENING OF "DARK" EXCITONS IN CARBON NANOTUBES: Single-walled carbon nanotubes (SWNTs) exhibit non-intuitive magnetic phenomena when threaded by a magnetic flux, arising from Aharonov-Bohm (AB) physics. Kono and Maruyama have collaborated to demonstrate the existence of dark excitons in SWNTs by magnetically brightening them, as a combined result of strong Coulomb interactions and the AB effect. Kono and Yusa performed magneto-photoluminescence at mK temperatures, showing that a symmetry breaking effect brightens dark excitons without a magnetic field. These new effects are currently under study, in collaboration with Saito. Kono’s group also showed that SWNTs align in magnetic fields, and aligned SWNTs show strongly anisotropic optical properties. Kono and Imanaka have extended these studies to compare metallic and semiconducting SWNTs, discovering that the magnetic susceptibility anisotropy of metallic tubes are one order of magnitude larger than that of semiconducting tubes. ACCOMPLISHMENT 2: ULTRAFAST OPTICAL MANIPULATION OF FERROMAGNETISM: There has been much interest in optically controlling magnetization for spintronic-device development, and many studies have been performed on metallic and insulating ferromagnetic systems. (III,Mn)V ferromagnetic semiconductors are an ideal system for achieving this phenomenon, as Kono, Munekata, and Stanton have demonstrated. The carrier induced nature of ferromagnetism in these systems naturally enables optical control of magnetism. Ultrashort laser pulses can create a large density of carriers, modifying the exchange interaction between Mn ions. Mittleman, Kono, and Munekata observed coherent THz radiation from laser-excited InMnAs, whose intensity and phase strongly changed with temperature. Furthermore, Kono and Munekata found that the magnetooptical Kerr angle for GaMnAs sensitively varies with the photon energy and temperature. Through comparison with 30-band k-p calculations by Belyanin, they showed that the main spectral features can be understood as interband transitions, without any ad hoc introduction of impurity transitions as reported by other groups. ACCOMPLISHMENT 3: ULTRAFAST AND NONLINEAR OPTICS IN CARBON NANOTUBES: SWNTs provide an ideal system for the exploration of novel one-dimensional (1-D) physics. Kono and collaborators have performed a series of ultrafast and nonlinear optical studies of SWNTs, elucidating the fundamental dynamical properties of 1-D carriers, phonons, and excitons. Kono, Lim, and Yee have observed coherent phonons (CPs) in SWNTs, which were used for simultaneously determining phonon and exciton energies. They have also developed a pulse-shaping method to specifically excite single-chirality SWNTs in samples containing SWNTs of many different chiralities. The excitation spectra of such selectively-excited CPs provided time-domain evidence that band gap oscillations follow the lattice oscillations. Sanders, Stanton, and Saito have developed a microscopic CP theory, which correctly reproduced the overall trends in the observed chirality dependence of CP intensities. ACCOMPLISHMENT 4: THZ SPECTROSCOPY OF HIGHLY-ALIGNED CARBON NANOTUBES: Dynamic aspects of metallic SWNTs have been poorly explored, although novel effects are expected due to the exotic nature of interacting, 1-D electrons. Kono and Tonouchi have been studying the dynamic THz conductivity σ(ω) of metallic SWNTs. Many-body effects can affect the temperature and magnetic field dependence of σ(ω), sometimes in very specific ways, making this a useful method for comparing with theory. They have studied highly-aligned samples, ideal for polarization-dependent studies. ACCOMPLISHMENT 5: NANOJAPAN: SUMMER NANOTECHNOLOGY PROGRAM: Recognized by the Institute for International Education in 2008 as a ‘Best Practice in Study Abroad’ for expanding international research opportunities for U.S. engineering students, the NanoJapan Program annually selected 16 freshmen and sophomore undergraduates from universities and colleges nationwide to participate in 12?week research experiences at leading research laboratories throughout Japan. Of the 94 undergraduates who participated in NanoJapan from 2006 – 2011, 37 are still pursuing their undergraduate degree, 38 are currently enrolled in Master’s or Ph.D. programs in STEM fields, two have completed Master’s degrees in STEM fields, five are pursuing industry careers in an engineering, science, or technical field, and one is currently enrolled in an MBA program. We do not have updated information for eleven past participants. NanoJapan Alumni have gone to receive prestigious fellowships and scholarship such as the Barry M. Goldwater Scholarship (2), Hertz Scholarship (1), Churchill Scholarship (1), and the NSF Graduate Research Fellowship (5). Of the total program participants, 32.8% were female and three participants were community?college transfer students. To date the program has received over 425 applications from students at more than 70 universities and colleges nationwide and 33 institutions are represented among program participants.
