This lecture was a part of 59th international scientific conference for students of physics and natural sciences "Open Readings 2016".

Conference was organised by Students Scientific Association, SPIE, OSA, EPS YM students organisations of Vilnius University and Center for Physical Sciences and Technology. Organisers are thankful to Ekspla, Light Conversion, Thorlabs, Altechna, Eksma Optics, Lidaris, Ruptela, American Elements, Monospektra and Brolis Semiconductors for their support.

More information: www.openreadings.eu.