Light-Induced Structural Phase Transition in Confining Gallium and Associated Gigantic Optical Nonlinearity
- PDF / 1,341,530 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 3 Downloads / 168 Views
V. ALBANIS*, S. DHANJAL*, V. EMELYANOV**, P. PETROPOULOS***, D.J. RICHARDSON***, N.I. ZHELUDEV* * Dep. of Physics and Astronomy, University of Southampton, S017 1BJ, UK, n.i.zheludev@soton. ac.uk ""Department of Physics, Moscow State University, Moscow, 119899, Russia. "I " Optoelectronics Research Centre, University of Southampton, S017 1B1J, UK. ABSTRACT We report a light-induced, surface-assisted structural phase transition from a common orthorhombic phase of crystalline gallium (a-gallium) to a highly reflective, metastable phase of more 'metallic' nature. The transition has been observed at the interface of gallium with fused silica at temperatures just below the metal's bulk melting point and affects only several tens of gallium atomic layers. The transition is fully reversible and occurs on a nanosecond/ microsecond time scale. The transition appears to show some characteristic features of a second order structural phase transition, including an increase of the transition relaxation times at the critical temperature (of approximately 30°C). The transition has no apparent optical intensity threshold, and is induced by radiation of very low intensity of only 10' - 10. W/[Im 2. The two gallium phases involved in the phase transition have significantly different dielectric properties which gives rise to a gigantic cubic optical nonlinearity, X(3) - I esu. The transition can be stimulated by light at any wavelength in the visible and the infrared ranges out to at least 1.55gtm. The effect is therefore of great interest for applications requiring light by light control at milliwatt power levels.
INTRODUCTION Among the best-known examples of optically induced structural phase transitions in solids, which are accompanied by noticeable changes of dielectric properties, are the thermally activated semiconductor-to-metal transition in vanadium oxide and the Curie point shift in ferroelectric materials such as BaTiO 3, SbSI and HgI2 . Laser annealing, melting and transient metallization in semiconductors and metals constitute another group of light-induced structural phase transitions. Such transitions require a threshold optical excitation of about 0.1 J/cm 2 and have been observed with nanosecond to femtosecond optical pulses. For example, an ultrafast metallization associated with destabilisation of covalent bonding has been seen in GaAs. The phase transition of confining gallium, which we report here, has some common features with the phase transitions mentioned above, but shows a number of very distinct and useful characteristics, such as the requirement for very low optical intensities, and is due to the unique atomic structure and optical properties of a-Ga.
275 Mat. Res. Soc. Symp. Proc. Vol. 543 c 1999 Materials Research Society
EXPERIMENT AND DISCUSSION The observations reported herein were made on an interface formed by a freshly cleaved silica optical fibre end immersed into a bead of molten metallic gallium of 6N purity. The temperature of the gallium bead was controlied around the gallium melting point
Data Loading...
