Question: Is it valid to treat a matter wave as a light wave with wavelength equal to the de Broglie wavelength of the matter wave? Either way please can you explain why?

Answer: 
I think this is very commonly confused topic about this wave-particle thing. Let me clarify. Like the post above indicates, from Maxwell's Equations, we find evidence, in a vacuum, for the electric and magnetic fields to travel together at the speed of light, perpendicular to each other. This is simply the solution of the well known equations which govern the electric and magnetic fields. We call this an electromagnetic "light" wave, referring to light. Now, at the same time, we find evidence we don't really know where particles are, such as electrons. They actually have a probability of being in space. The evolution of this probability is governed by Schrödinger's equation. I refrain from using "matter wave" and instead use probability. Thus the evolution of the probability of a particle and a light wave are totally different things, talking about different objects, governed by different equations and unnecessarily confused in our terminology. 

P.S. When we speak of wave-particle duality, be careful to use in the right context. When we speak of this term, we mean the nature of light to behave as a particle and an electromagnetic wave, nothing more:
1. light -> evidence suggests light exists as both a particle (photon) and an electromagnetic wave. We are not talking about a probability here.
2. particles -> particles have a probability of being in space. Forget using this term "wave-particle duality" when speaking about particles. They fundamentally have a probability of existing in space and you can never know where both their position and velocity are at any moment.

Hope you have a good one!
Actually both are analogous. When you quantize a single mode which is simply an oscillation associated with a single photon, you adopt the Lorentz model where you imagine your single photon as a mass on a spring, then you translate your quantum mechanical Hamiltonian that includes both position and momentum to phase space where the electric and magnetic fields will be your new canonical variables equipped with the same uncertainty relation that governs the relation between position and momentum. You can also interpret the canonical variables associated with your single photon as an AM and FM signals with the same uncertainty relation described before, since you can always switch back and forth between them using fourier transform.