  Integrated optical power splitter is one of the fundamental building blocks
in photonic integrated circuits (PIC). Conventional multimode interferometer
based power splitter is widely used as it has reasonable footprint and is easy
to fabricate. However, it is challenging to realize arbitrary split ratio
especially for multi-outputs. In this work, an ultra-compact power splitter
with a QR code-like nanostructure is designed by a nonlinear fast search method
(FSM). The highly functional structure is composed of a number of freely
designed square pixels with the size of 120nm x 120nm which could be either
dielectric or air. The lightwaves are scattered by a number of etched squares
with optimized locations and the scattered waves superimpose at the outputs
with the desired power ratio. We demonstrate 1x2 splitters with 1:1, 1:2, 1:3
split ratios and a 1x3 splitter with the ratio of 1:2:1. The footprint for all
the devices is only 3.6umx3.6 um. Well-controlled split ratios are measured for
all the cases. The measured transmission efficiencies of all the splitters are
close to 80% over 30 nm wavelength range.
