Nanoobjects microscopy

A dual benefit of laser ablation: periodic surface structures and nanoparticles
Dmytruk A.1, Karlash A.2, Dmitruk I.1,3, Berezovska N.3, Hrabovskyi Y.3, 
Korenyuk P.1, Kadan V.1, Rybak A.1, Blonskyi I.1
1 Institute of Physics of the NAS of Ukraine. Prospect Nauky, 46, Kyiv-03028, Ukraine. 
E-mail: admytruk@gmail.com
2 Faculty of Radiophysics, Electronics and Computer Systems, Taras Shevchenko National University of Kyiv, Kyiv 01601, Ukraine.
3 Faculty of Physics, Taras Shevchenko National University of Kyiv, Kyiv 01601, Ukraine.

Laser ablation – removal of material from a solid surface by intense laser beam – is a well-known phenomenon. It has several aspects. As for the surface, it can be just a damage – the surface becomes rough, but under particular conditions laser induced periodic surface structures (LIPSS) can occur. The removed material can be utilized as well: for pulsed laser deposition method – if the ablation takes place in vacuum or in a gas, or for production of colloidal solution of nanoparticles (NP) – in case of ablation in a liquid. A complex study of both LIPSS and NP formation may supplement knowledge on both the subjects.
For ablation, we used the facilities of the Femtosecond Laser Center for collective use of equipment of the National Academy of Sciences of Ukraine, based in the Institute of Physics [1]. Its main laser system consists of Mira 900F oscillator and Legend FE regenerative amplifier, both from Coherent, USA. The laser produced linearly polarized 150 fs laser pulses of 800 nm fundamental wavelength with 1.5 mJ per pulse energy at 1 kHz repetition rate. The second (400 nm) and the third (266 nm) harmonics of the radiation were generated by a kit of corresponding BBO crystals (Eksma Optics, Lithuania). The laser beam was focused by either spherical or cylindrical lens, of quartz or sapphire if the third harmonic radiation was used. A surface of a target material was positioned at a fixed distance in-front of the focal plane of the lens, in parallel with it. Varying the distance, power density of the radiation was adjusted for effective LIPSS and NP production. During the experiment, the target was moved along a raster trajectory in the plane by a motorized translation stage (Standa, Lithuania). The target was placed in water for NP collection.
Produced LIPSS were visualized by means of the scanning electron microscope Aura 100, Seron Technology, Korea. Morphology of the NP was studied by the transmission electron microscope TEM-100M, SELMI, Ukraine. Mean size and size distribution of the NP were also checked by a homemade dynamic light scattering setup. Absorption spectra of the colloidal solutions of NP were measured in UV-visible spectral region by the modernized SDL-2 spectrometer, LOMO, USSR.
A complementary set of the obtained experimental results on LIPSS and NP allowed us to draw a complex picture of laser ablation process depending on the used target material and laser radiation parameters, thus optimizing production of the both. 

1.	http://www.iop.kiev.ua/ua/lazernij-femtosekundnij-kompleks/