LTC3108 is e.g. working from minimum 20 mV input voltage, dedicated to thermoelectric generators and similar low voltage energy sources. I don't know which practical energy harvesting applications will work with only 1 mV? In any case, energy harvesting is bound by conservation of energy law. This means, that the input power must be higher than the deliveres output power. A low input voltage will refer to a respective high input current and low circuit impedance.
LTC3108 is using a step-up transformer and self oscillating depletion mode FET inverter to allow start-up from millivolt level.