[jnbiche]

> the author refers to "harmonics" they are really talking about intermodulation.

No, he's talking about harmonics. It's a different effect from intermodulation. It's true that intermodulation involves the sum and difference two or more frequencies. Harmonics, however, involves integer multiples of a single frequency.

But the impact is the same as intermodulation in that it's really a hardware issue and cannot be countered using a simple frequency filter.

[femto]

Harmonics are multiples of the fundamental, so in this case they will also be ultrasonic.

Equation 2 in the paper and the subsequent paragraph shows what is going on. They use an ultrasonic carrier with modulation. The non-linearity causes the carrier to mix with the sidebands, the third-order intermodulation product being a copy of the modulation centred on 0Hz (ie. a baseband signal).

Edit: Figure 12 talks about harmonics, in the context of harmonics of the third order intermodulation product. What they are really refering to are the higher order: 5th, 7th, and so on intermodulation products, which in this case will be multiples of the third order product's frequency.