How power system is designed for extreme small system, such as google contact lens?
Although the article you linked to doesn't mention it, other articles like this one do confirm that the device gets its power using RFID technology.  That is the way a passive RFID employee ID badge, for example, can work without needing a battery installed.

Besides being used to receive and transmit the RF frequencies used for communication with the tiny transceiver IC, the antenna is also used to receive a relatively higher-powered RF signal that is rectified and used to power the circuit (this article mentions a capacitor).  I have no idea what the power requirement is for the Google contact lens device, but a typical power level for a passive RFID tag at maximum range is 1 µW -- 1 V at 1 µA.