Yang's Group

Inspired by the broadband anti-reflection of structured nipple array of the moth eye, sub-wavelength anti-reflective (AR) coatings have been extensively exploited. Recently, surface structures with low reflection have been fabricated by various methods and their broadband anti-reflection properties presented. Textured surface with nanostructures can be applied to solar cells to reduce the reflection between the interfaces with different refractive index.

In the world of information explosion, everybody is looking for a way to decrease the physical volume of data. Among all the efforts in this area, ReRAM is one of the non-volatile memories which seem promising to make terabyte memories. The basic idea of ReRAM is that the resistivity of a dielectric insulator can be changed by application of a sufficiently high voltage, and it is possible to set and reset the memory between two different resistivity states by applying an appropriate voltage. The picture below shows a beautiful C-AFM image of two distinct resistivity states in STO which is switched between two resistivity states by applying ± 6 V¹. Among all the tree branch of the memories based on resistance changes, including (i) phase-change memory in chalcogenides, (ii) programmable- metallization-cell memory in solid electrolytes, and (iii) resistance-change memory in transition-metal oxides, the last group which is also called resistive switching in literatures, is more of our group’s interest. The mechanism of this phenomenon is not well known and there is still a lot of controversy over the subject, but this memory seems promising since “the transition- metal oxide concept offers compelling advantages in terms of compatibility with microprocessor fabrication technology” ².

¹Szot, K., et al., Nanoscale resistive switching in SrTiO3 thin films. Physica Status Solidi - Rapid Research Letetrs, 2007. 1(2): p. R86-R88.
²Meijer, G.I., Materials science: Who wins the nonvolatile memory race? Science, 2008. 319(5870): p. 1625-1626.

Study of forming mechanisms of metal nanoparticles on top of single graphene and few layer graphene flakes. Graphene, a single layer of graphite, is a source of remarkable electrical, optical and other properties. Through study the size and surface coverage of nanoparticles formed on top of single and few layer graphene under different conditions, the physical characteristic of it can be investigated.

The discovery of ultrafast magnetization dynamics in novel magnetic materials suggested an ultrafast scheme for writing and reading data in magneto-optical recording applications. In particular, using a pump-probe Time-Resolved Magneto-Optical Kerr Effect system (TR-MOKEs), one can observe magnetization dynamics with a pico-second time resolution. The majority of samples in this project are opaque, and the experiments are on micro and nanowires, nanodots of ferromagnetic materials (like as Ni, NiFe, CoFe). The geometry of our experiments is, therefore, applying the Kerr effect in a pump-probe set-up to measure the TR-MOKE response of the samples.

We use pico-second time resolved optical techniques to explore these magnetization dynamics at low (1.5 K) and room temperatures. The magnetization dynamics will be induced by applying a pulsed magnetic field (Pump) that has arise time from 10 to 100 ps, and detected by using the Magneto-Optical Kerr effect (Probe) made with a short pulsed laser. This Pump-Probe technique is very helpful in understanding of magnetic precession and damping parameters in magnetic multilayer structures like Giant Magneto Resistance (GMR) and Tunnel Magneto Resistance (TMR) structures.

The Polar and Longitudinal components of reflected light can also be utilized to develop a Time and Vector-Resolved Scanning MOKEs. These studies of high frequency magnetization dynamics are expected to be used in next generation of hard-disk read sensors.