Low voltage AC to DC for energy harvesting
Starting from 0.1vpp, you will need an external power source to bias up your active rectifier / converter IC. This source can be as simple as a capacitor, which stores charge from last time the converter operated, but it does need to be charged initially. That voltage is simply not sufficient to make even sub-threshold circuits work.

There are a couple of options for a circuit topology that would convert a very low voltage up to a reasonable voltage, a boost converter, or a transformer. There are plenty of application notes for energy harvesting ICs that spell out the techniques.