The National Science Foundation's (NSF) Tokyo Office periodically receives and disseminates reports on research developments in Japan that are related to the Foundation's mission. NSF-sponsored researchers currently working in Japan prepare many of these reports. These reports provide information for use by the global science and engineering community.
The following report was prepared by Dr. William E. Torruellas, Assistant Professor in Physics and Materials Sciences, Department of Physics, Washington State University, Pullman, Washington and Dr. Akira Otomo, Kansai Advanced Research Center, Communications Research Laboratory, 588-2, Iwaoka, Nishi-ku, Kobe, 651-24, Japan. Dr. Torruellas visited Japan from June 26 - Aug 31, 1998 as a Visiting Research Fellow (short-term) under the sponsorship of the Science and Technology Agency of Japan (STA). Dr. Akira Otomo served as host scientist for Dr. Torruellas. Dr. Torruellas may be reached via email at: williamt@wsu.edu
Abstract
During the tenure of the STA fellowship we have continued working on measuring electrical fields with nonlinear optical polymers. We have concentrated our efforts in trying to understand the signals we have observed as a function of position across the gap between two electrodes covered with a thin layer of PMMA doped with DR1 molecules. We have showed that the signals are reproducible and that the shapes of our scans can be understood by simple electrodynamic considerations. We estimate that an enhancement of a factor of 150 will be achieved just by reducing the spatial resolution to 100 nm or less with thin films thinner than 100nm. Average fields less than 0.25 mV/mm near the edge of conductors will be easily detectable with our approach.
Introduction and statement of the problem
New developments in the semiconductor industry are driven by two trends: reducing the device dimensions and further increase of the switching speeds or electrical bandwidths. As a consequence, the electronics industry anticipates average feature sizes of integrated circuits (ICs) to be of the order of 100nm or less by the year 2010. Devices at the experimental stage are currently fabricated with features less than 50 nm and switching speeds of less than 5 psec. However, even at lower speeds and higher feature sizes, very few techniques exist which can probe the electrical performance, in particular the electrical fields generated in modern electronic devices. For instance, currently produced MOS field-effect transistors support electric fields between the source and the drain that are greater than 105 V/mm with switching speeds of 10-100 psec. Techniques that would resolve such electric fields, with the appropriate resolutions in time and space, are not only of paramount interest at the industrial level but in basic research as well. Scientific problems ranging from localized surface spectroscopy to the understanding of molecular and semiconductor quantum confined systems can be explored with these new techniques.
Experimental Procedure
Samples consist of two metallic electrodes deposited on fused silica substrates separated by 1- 40 mm gaps covered by thin polymer films of approximately 100nm-1 mm in thickness, deposited by spin coating on top of the electrodes. The polymer films were prepared from solutions of PMMA:DR1. At room temperature it can be assumed that before applying the DC field, the DR1 molecules are randomly oriented. An ultrafast laser system producing pulses of 100 fsec duration was used to investigate the possibility to detect the electric-field-induced-second-harmonic (Electrical-Field-Induced- Second-Harmonic) signal. The results obtained show that with only 1 mW of average pump power, a signal could easily be detected with a photomultiplier operated at moderate bias voltages indicating that electrical fields of a few mV/mm can be detected with our approach. Tuning the pump wavelength across the two-photon resonance has allowed us to detect the two-photon absorption spectrum of DR1 for the first time and has showed very unusual features.
Long term goal: Nanoscale Nonlinear Optics and Nonlinear Optical Materials
Whereas in the previous section we have discussed the optical and nonlinear optical interactions in the classical coherent perturbative regime, the material structures with nanoscale dimensions have unique controllable optical and nonlinear optical properties. They are ideal to investigate optical enhancement mechanisms that are only present with material processing at the nanoscale. Indeed, quantum-electrodynamic enhancements have been observed recently in monolayer samples and interfaces with second harmonic generation. These enhancements result from the light matter interaction, which are present in nanoscale samples. Of particular interest to the Washington State research group is the competition between optical diffraction in such nanoscale optical confinements and the nonlinear optical gain anticipated in such controlled nonlinear optical geometries.
In conclusion the combination of nanoscopic tools with ultrafast nonlinear laser spectroscopy covering a wide wavelength range offers enormous possibilities for studies of single quantum confined entities. In this new regime of light-matter interaction optical and electrical signals can be both applied or detected allowing for the study of spatio-temporal dynamics with combined sub-100 nm and sub-100fsec resolutions.